Dye Molecule-Based Porous Organic Materials
Dissertation
Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy
in the Graduate School of The Ohio State University
By
Grace M. Eder, B.S.
Graduate Program in Chemistry
The Ohio State University
2018
Dissertation Committee:
Prof. Psaras L. McGrier, Advisor
Prof. J. Parquette
Prof J. Badjic
Copyrighted by
Grace M. Eder
2018
Abstract
Porous materials are an ever-expanding area of materials science well known for their
highly porous structures, which are well suited to hosting a variety of guests from small
compounds such as gasses to large complex molecules. Additionally, porous materials can
be rationally designed prior to synthesis to incorporate monomers with desired
functionalities, which makes them amenable to a variety of applications including gas
storage and uptake, sensing, catalysis, and optoelectronics.
Our interest has been to bring functional monomers, such as dye molecules, into porous
polymers for applications geared toward alternative energy. One class of dye we found
intriguing were subphthalocyanines (SubPcs), which have an unusual 3-dimensional shape
and excellent optoelectronic properties. SubPcs are often incorporated into organic
photovoltaics in the small molecule form, and have achieved good power conversion
efficiencies up to 7%. We envisioned that incorporation of a SubPc into a polymer might
have useful function in an optoelectronic application. We synthesized a boronate ester
linked SubPc-based polymer, fully characterized its structure and tested its ability in
optoelectronic applications.
ii
The boronate ester linkage has been a mainstay in the field of porous polymers, especially
covalent organic frameworks (COFs). Despite its success in the early development of
COFs, the boronate ester linkage suffers from a significant drawback of instability to small
amounts of water. The field of COFs has been shifting away from the boronate ester linkage
in favor of more chemically stable linkages that can still afford crystalline materials. One
such linkage has been developed in our laboratory, the benzobisoxazole (BBO) linkage.
We have shown the BBO linkage yields crystalline materials with good stability in water.
However since the BBO linkage is a relatively new development in the field of COFs not
much is known about how this linkage promotes crystallinity in COF materials. We
undertook a comprehensive study of the different factors at work during the formation of
BBO linked COF materials to gain some insights on this process.
With the development of this stable BBO linkage, we wanted to show its utility by
incorporating it into a functional material. We targeted a heterogeneous catalysis
application to aid in the development of an important reaction, the reduction of CO2 to
useful compounds. We decided to incorporate another dye functionality, a porphyrin, to
act as a catalytic site for the hydrosilylation of CO2. We created porphyrin-based porous
polymers both with, and without a metal ion in the central cavity of the porphyrin, then
characterized these materials and evaluated their efficacy in catalyzing the hydrosilylation
of CO2 to transform it into more useful chemical feedstocks.
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This work is dedicated to our incredible planet earth.
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Acknowledgements
First and foremost, I have to thank my wonderful friends and family for their continuous
and unwavering support throughout my education. I want to thank my parents who have
always believed in my ability, and my sisters who both inspire me by their excellent
examples and who can always make me laugh. I have to thank Tom- I don’t deserve the
help you’ve given me. I’m sure I haven’t been the easiest to deal with during this process,
but you’ve always been there for me through some of the most difficult times in my life.
Thank you for being my best friend and constant support through every day, good and bad.
I would also like to thank my good friends TJ and Ashley for being there through the
process with me, always ready to share in our complaints and celebrate our victories, and
of course always ready to play a board game and have a good time.
I have also had a lot of support from my academic family. I have to thank Psaras for
mentoring me through the process, and for the innumerable number of pep talks I’ve
received from him along the way. I would also like to thank Luke Baldwin and Jon Crowe
for helping me find my voice, and giving me the motivation to speak up for what I believe.
I’d like to thank David Pyles for being a great friend, and for always being a huge help in
the lab. I also have to thank Ben Walker for being my partner in crime during his
undergraduate research career. I couldn’t have gotten through these projects without your
help and moral support. I’d like to thank Ben Clark for bringing some goofiness and levity
v
to the lab- don’t ever stop laughing Ben. I’d also like to thank the remaining members past
and present of the McGrier group for their support, insights, and sense of humor that helped
see me through my career. I would also like to thank Dr. Sauvé for her mentorship during
my undergraduate career. Thank you for introducing me to organic and materials
chemistry, and showing me the power it has to benefit our planet.
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Vita
2009…………………………………………………….….Ann Arbor Huron High School
2013…………………………………. B.Sc Chemistry, Case Western Reserve University
2013-2018……….……………………Graduate Program in Chemistry and Biochemistry,
The Ohio State University
Publications
The Excited State Intramolecular Proton Transfer Properties of Three Imine-Linked TwoDimensional Porous Organic Polymers. Jagadesan, P.; Eder, G.; McGrier, P. L. J. Mat.
Chem. C, 2017, 5, 5555-5832.
Subphthalocyanine-based Porous Organic Polymers, Eder, G. M.; Walker, B. R.;
McGrier, P. L., RSC Advances, 2017, 7, 29271.
Mechanistic Investigations into the Cyclization and Crystalization of Bisbenzoxazolelinked 2-Dimensional Covalent Organic Frameworks. Pyles, D. A.; Coldren, W. H.;
Eder, G. M.; Hadad, C. M.; McGrier, P. L. Submitted.
Fields of Study
Major Field: Chemistry
Specializing in: Organic Chemistry, Materials Chemistry, Dyes
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Table of Contents
Abstract ............................................................................................................................... ii
Acknowledgements ............................................................................................................. v
Vita.................................................................................................................................... vii
List of Tables ................................................................................................................... xiv
List of Figures .................................................................................................................. xvi
Chapter 1: Porous Polymers............................................................................................... 1
1.1 Introduction to porous polymers ............................................................................... 1
1.2 History of porous polymers ....................................................................................... 2
1.3 Covalent organic frameworks (COFs) ...................................................................... 4
1.4 Porous organic polymers (POPs) .............................................................................. 8
Chapter 2: Synthesis of novel subphthalocyanines (SubPcs) and SubPc-based porous
organic polymers (POPs) .................................................................................................. 12
2.1 Dyes in photovoltaic applications ........................................................................... 12
2.1.1 Dyes in organic photovoltaic devices (OPVs) .................................................. 12
2.1.2 OPV morphology and extension to porous materials ....................................... 16
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2.2 Background on SubPcs............................................................................................ 19
2.2.1 History of SubPcs ............................................................................................. 19
2.2.2 Applications of SubPcs ..................................................................................... 20
2.3 Design of SubPc POPs ............................................................................................ 24
2.3.1 Design and synthesis of hexahydroxy SubPcs ................................................. 24
2.3.2 Synthesis of SubPc-POPs ................................................................................. 30
2.3.3 Characterization of SubPc-POPs ...................................................................... 30
2.4 Potential use in optoelectronics ............................................................................... 36
2.5 Experimental section ............................................................................................... 39
2.5.1 Instrumentation and methods............................................................................ 39
2.5.2 Synthetic methods............................................................................................. 40
2.5.3 Infrared spectroscopy ....................................................................................... 48
2.5.4 PXRD profiles .................................................................................................. 53
2.5.5 Solid-state NMR spectra ................................................................................... 54
2.5.6 TGA profile ...................................................................................................... 56
2.5.7 Surface area analysis ........................................................................................ 57
2.5.8 UV-Vis and fluorescence.................................................................................. 61
ix
2.5.9 SubPc-POP hydrolysis 1H NMR ...................................................................... 65
2.5.10 1H and 13C NMR spectra ................................................................................ 67
Chapter 3: Benzoxazole formation studies ...................................................................... 81
3.1 Benzoxazole background ........................................................................................ 81
3.2 Proposed mechanisms for the formation of benzoxazoles ...................................... 84
3.3 Small molecule analog studies ................................................................................ 87
3.3.1 p-tolylbenzoxaole ............................................................................................. 87
3.3.2 BBO-1 ............................................................................................................... 90
3.4 BBO-COF 3............................................................................................................. 91
3.4.1 Synthesis and design of BBO-COF 3 ............................................................... 91
3.4.2 Characterization of BBO-COF 3 ...................................................................... 93
3.5 Reaction time trials of BBO-COF 3 ........................................................................ 97
3.6 Nucleophile trials of BBO-COFs 2 and 3 ............................................................. 101
3.7 Computational experiments................................................................................... 104
3.8 Experimental section ............................................................................................. 107
3.8.1 Instrumentation and methods.......................................................................... 107
3.8.2 Synthetic methods........................................................................................... 109
x
3.8.3 BBO-1 analog trials ....................................................................................... 111
3.8.4 BBO-COF 2 formation studies ....................................................................... 113
3.8.5 BBO-COF 2 nucleophile trials ....................................................................... 114
3.8.6 BBO-COF 2 deoxygenated trial ..................................................................... 117
3.8.7 BBO-COF 3 formation studies ....................................................................... 120
3.8.8 BBO-COF 3 nucleophile trials ....................................................................... 122
3.8.9 BBO-COF 3 reaction time trials ..................................................................... 128
3.8.10 BBO-COF 3 deoxygenated trial ................................................................... 132
3.8.11 BBO-COF 3 experimental & simulated PXRD data .................................... 135
3.8.12 Solid-state NMR spectra ............................................................................... 139
3.8.13 TGA profile .................................................................................................. 140
3.8.14 Surface area analysis .................................................................................... 141
3.8.15 Scanning electron microscopy (SEM) images.............................................. 147
3.8.16 Computational studies .................................................................................. 147
Chapter 4: Porphyrin-based benzobisoxazole and benzobisthiazole-linked porous organic
polymers for hydrosilylation of CO2............................................................................... 151
4.1 Porphyrins as catalysts .......................................................................................... 151
xi
4.2 Porous materials as heterogeneous catalysts ......................................................... 153
4.3 Synthesis and design of porphyrin POPs .............................................................. 158
4.3.1 Synthesis of monomers ................................................................................... 158
4.3.2 Synthesis of free-base porphyrin polymers .................................................... 161
4.4 Characterization of H2P POPs ............................................................................... 162
4.5 Design of metalloporphyrin polymers for CO2 conversion .................................. 165
4.5.1 Hydrosilylation of CO2....................................................................................... 165
4.5.2 Synthesis and design of small molecule ruthenium porphyrin catalyst.......... 166
4.5.3 RuTPP catalytic performance ......................................................................... 168
4.6 Synthesis of 1-Ru and metallated porphyrin POPs............................................... 170
4.7 Characterization of ruthenium porphyrin POPs .................................................... 172
4.8 Future directions and outlook................................................................................ 175
4.8.1 BBO-RuP POP and BBT-RuP POP as hydrosilylation catalysts ................... 175
4.8.2 Recyclability ................................................................................................... 176
4.8.3 Outlook ........................................................................................................... 176
4.9 Experimental Section ............................................................................................ 177
4.9.1 Instrumentation and methods.......................................................................... 177
xii
4.9.2 Synthetic methods........................................................................................... 178
4.9.3 FT-IR spectra .................................................................................................. 183
4.9.4 PXRD profiles ................................................................................................ 186
4.9.5 Solid-state NMR spectra ................................................................................. 188
4.9.6 Surface area analysis ...................................................................................... 190
4.9.7 UV-Vis and fluorescence spectra ................................................................... 198
4.9.8 XPS spectra..................................................................................................... 200
4.9.9 1H and 13C NMR spectraFigure 180. 1H NMR spectrum of RuTPP .............. 202
References ....................................................................................................................... 205
xiii
List of Tables
Table 1. Summary of screening of SubPc-POP reaction conditions ................................ 47
Table 2. Resonances and assignments for SubPc-POP 1 .................................................. 51
Table 3. Resonances and assignments for SubPc-POP 2 .................................................. 52
Table 4. BET values derived from Roquerol BET analysis for SubPc-POP 1 ................. 57
Table 5. BET values derived from Roquerol BET analysis for SubPc-POP 2 ................. 59
Table 6. Ratio of p-tolylbenzoxazole product to imine-linked product ............................ 89
Table 7. Isolated yields of BBO-1 synthesized with different nucleophiles..................... 91
Table 8. B3LYP/6-31+G* NPA population and spin density analysis for BBO-COF 2.104
Table 9. B3LYP/6-31+G* NPA population and spin density analysis for BBO-COF 3.105
Table 10. Summary of BBO-COF 2 synthesis with different nucleophilic catalysts ..... 113
Table 11. FT-IR peak assignments for BBO-COF 2 synthesized with NaN3 ................. 114
Table 12. FT-IR peak assignment for BBO-COF 2 synthesized with NaSCH3 ............. 115
Table 13. FT-IR peak assignments for BBO-COF 2 synthesized without a catalyst ..... 116
Table 14. FT-IR peak assignments for BBO-COF 2 deoxygenated ............................... 118
Table 15. Summary of trials for BBO-COF 3 synthesized with different nucleophiles or
over different reaction times ........................................................................................... 121
Table 16. FT-IR peak assignments for BBO-COF 3 synthesized with NaCN ............... 123
Table 17. FT-IR peak assignments for BBO-COF 3 synthesized with NaN3 ................. 124
xiv
Table 18. FT-IR peak assignments for BBO-COF 3 synthesized with NaSCH3 ............ 125
Table 19. FT-IR peak assignments for BBO-COF 3 synthesized without a catalyst ..... 126
Table 20. FT-IR peak assignments for a ground mixture of BBO-COF 3 monomers.... 128
Table 21. FT-IR peak assignments for BBO-COF 3 after 1 day of heating ................... 129
Table 22. FT-IR peak assignments for BBO-COF 3 after 2 days of heating.................. 130
Table 23. FT-IR peak assignments for BBO-COF 3 after 3 days of heating.................. 131
Table 24. FT-IR peak assignments for BBO-COF 3 deoxygenated ............................... 133
Table 25. Fractional atomic coordinates for the P6 unit cell of BBO-COF 3 ................ 137
Table 26. Optimization of reaction conditions for hydrosilylation of CO2 by RuTPP ... 170
Table 27. Optimization of the synthesis of BBO-H2P POPs .......................................... 181
Table 28. Optimization of the synthesis of BBT-H2P POPs........................................... 181
Table 29. BET values derived from Roquerol BET analysis of BBO-H2P POP ............ 190
Table 30. BET values derived from Roquerol BET analysis of BBT-H2P POP ............ 192
Table 31. BET values derived from Roquerol BET analysis of BBO-RuP POP ........... 194
Table 32. BET values derived from Roquerol BET analysis of BBT-RuP POP ............ 196
xv
List of Figures
Figure 1. MOF-5 ................................................................................................................. 2
Figure 2. Structure of COF-1 and COF-5 ........................................................................... 3
Figure 3. COF linkers and vertexes of different symmetry, and a selection of COF pore
shapes and topologies ......................................................................................................... 4
Figure 4. COF linkages based on reversible reactions ........................................................ 6
Figure 5. TT-COF pictured with PC61BM guest ................................................................. 7
Figure 6. Bulk structure of CTF-1 ...................................................................................... 8
Figure 7. Schematic representation of Fe-POP-1 catalyzing a Knoevenagel condensation
and oxidation reactions ....................................................................................................... 9
Figure 8. COF-LZU1 pore shown doped with Pd(OAc)2 and side view depicting
interaction between Pd and imine nitrogen atoms ............................................................ 10
Figure 9. Structure of early photovoltaics......................................................................... 12
Figure 10. Morphologies of OPV devices ........................................................................ 13
Figure 11. Structure of P3HT and PC61BM ...................................................................... 14
Figure 12. Orbital mixing diagram for donor-acceptor (D-A) copolymers ...................... 15
Figure 13. Structure of electron donor and electron acceptor moieties incorporated into
D-A copolymers ................................................................................................................ 16
Figure 14. Structure of semiconductive TP-COF ............................................................. 17
xvi
Figure 15. Schematic of TT-COF based photovoltaic device........................................... 18
Figure 16. Structure of phthalocyanine (Pc) and subphthalocyanine (SubPc) ................. 19
Figure 17. UV-Vis spectra and orbital diagrams of CuPc and SubPc paired with C60
electron acceptor ............................................................................................................... 20
Figure 18. C60 trapped inside a SubPc cage reproduced ................................................... 22
Figure 19. Synthesis of SubPc polymer, poly(4-methlystyrene)-copoly(phenoxyboronsubphthalocyanine) ............................................................................ 23
Figure 20. Synthesis of 4,5-dihydroxyphthalonitrile (5) .................................................. 24
Figure 21. Protection of 5 with triisopropylsilyl (TIPS) protecting groups ...................... 25
Figure 22. Synthesis of hexahydroxy-Cl-SubPc (1b) ....................................................... 26
Figure 23. Synthesis of hexahydroxy-SubPcs 1b, 2b, and 3b from 4 .............................. 27
Figure 24. UV-Vis absorbance and fluorescence spectra for hexahydroxy-SubPcs 1b, 2b,
and 3b................................................................................................................................ 28
Figure 25. Preliminary crystallographic data for hexaTIPS-OPh-SubPc (2a) .................. 29
Figure 26. Solvothermal synthesis of SubPc-POPs .......................................................... 31
Figure 27. Scanning electron microscopy images of SubPc-POP 1 and SubPc-POP 2 ... 32
Figure 28. Porosity data for SubPc-POPs: N2 adsorption isotherms for SubPc-POP 1 and
SubPc-POP 2, pore size distribution for SubPc-POP 1 and SubPc-POP 2....................... 33
Figure 29. Kubelka-Munk function diffuse reflectance spectra for SubPc-POP 1 and
SubPc-POP 2..................................................................................................................... 35
xvii
Figure 30. Solution-state UV-Vis absorbance and fluorescence spectra for 1a, 2a, and 3a
upon titration with excess C60 ........................................................................................... 36
Figure 31. N2 adsorption isotherms and pore size distributions for SubPc-POP 1 before
and after doping with C60 .................................................................................................. 37
Figure 32. Kubelka-Munk diffuse reflectance spectra for SubPc-POP 1 before and after
doping with C60 ................................................................................................................. 38
Figure 33. FT-IR spectrum of 1a ...................................................................................... 48
Figure 34. FT-IR spectrum of 1b ...................................................................................... 48
Figure 35. FT-IR spectrum of 2a ...................................................................................... 49
Figure 36. FT-IR spectrum of 2b ...................................................................................... 49
Figure 37. FT-IR spectrum of 3a ...................................................................................... 50
Figure 38. FT-IR spectrum of 3b ...................................................................................... 50
Figure 39. Stacked FT-IR spectra of SubPc-POP 1, 2b, and BDBA ................................ 51
Figure 40. Stacked FT-IR spectra of SubPc-POP 2, 3b, and BDBA ................................ 52
Figure 41. PXRD profile of SubPc-POP 1........................................................................ 53
Figure 42. PXRD profile of SubPc-POP 2........................................................................ 53
Figure 43. 13C CP-MAS of SubPc POP 1 ......................................................................... 54
Figure 44. 13C CP-MAS of SubPc-POP 2......................................................................... 55
Figure 45. TGA profile of SubPc-POP 1 .......................................................................... 56
Figure 46. Roquerol BET analysis for SubPc-POP 1 ....................................................... 57
xviii
Figure 47. BET surface area plot for SubPc-POP 1.......................................................... 58
Figure 48. Roquerol BET analysis of SubPc-POP 2......................................................... 59
Figure 49. BET surface area plot of SubPc-POP 2 ........................................................... 60
Figure 50. UV-Vis and fluorescence spectra of 1a in toluene (λexcitation= 574 nm) .......... 61
Figure 51. UV-Vis and fluorescence spectra of 1b in acetone (λexcitation= 564 nm) .......... 61
Figure 52. UV-Vis and fluorescence spectra of 2a in toluene (λexcitation= 522 nm) .......... 62
Figure 53. UV-Vis and fluorescence spectra of 2b in acetone (λexcitation= 561 nm) .......... 62
Figure 54. UV-Vis and fluorescence spectra of 3a in toluene (λexcitation= 578 nm) .......... 63
Figure 55. UV-Vis and fluorescence spectra of 3b in acetone (λexcitation= 570 nm) .......... 63
Figure 56. Kubelka-Munk diffuse reflectance spectrum of 2b and SubPc-POP 1 ........... 64
Figure 57. Kubelka-Munk diffuse reflectance spectrum of 3b and SubPc-POP 2 ........... 64
Figure 58. 1H NMR spectrum of SubPc-POP 1 after hydrolysis ...................................... 65
Figure 59. 1H NMR spectrum of SubPc-POP 2 after hydrolysis ...................................... 66
Figure 60. 1H NMR spectrum of 4 .................................................................................... 67
Figure 61. 13C NMR spectrum of 4 ................................................................................... 68
Figure 62. 1H NMR spectrum of 1a .................................................................................. 69
Figure 63. 13C NMR spectrum 1a ..................................................................................... 70
Figure 64. 1H NMR spectrum of 1b.................................................................................. 71
Figure 65. 13C NMR spectrum of 1b ................................................................................ 72
Figure 66. 1H NMR spectrum of 2a .................................................................................. 73
xix
Figure 67. 13C NMR spectrum of 2a ................................................................................. 74
Figure 68. 1H NMR spectrum of 2b.................................................................................. 75
Figure 69. 13C NMR spectrum of 2b ................................................................................ 76
Figure 70. 1H NMR spectrum of 3a .................................................................................. 77
Figure 71. 13C NMR spectrum of 3a ................................................................................. 78
Figure 72. 1H NMR spectrum of 3b.................................................................................. 79
Figure 73. 13C NMR spectrum of 3b ................................................................................ 80
Figure 74. Structure of oxazole and its benzo-fused derivatives ...................................... 81
Figure 75. Synthesis of COP-93 and COP-94 .................................................................. 82
Figure 76. Synthesis of BBO-COF 1 and 2 ...................................................................... 83
Figure 77. Synthesis of BOLPs and BTLPs ..................................................................... 84
Figure 78. Synthesis of benxozazole by Cheon's proposed 2 electron pathway............... 85
Figure 79. Calculated energy levels of intermediates and transiton states for radical
oxidative dehydrogenation and cyclization to form a benzoxazole .................................. 86
Figure 80. Examples of radicals stabilized by the captodative effect ............................... 87
Figure 81. Synthesis of p-tolylbenzoxazole ...................................................................... 87
Figure 82. Stacked 1H NMR spectra of the crude reaction forming p-tolylbenzoxazole . 88
Figure 83. Synthesis of BBO-1 with different nucleophiles ............................................. 90
Figure 84. Synthesis of BBO-COF 3 from DABD and TFPT .......................................... 92
Figure 85. N2 adsorption isotherm and pore size distribution for BBO-COF 3 ............... 95
xx
Figure 86. Experimental PXRD data for BBO-COF 3 ..................................................... 97
Figure 87. N2 adsorption isotherms for BBO-COF 3 at various points during the 4-day
reaction period .................................................................................................................. 98
Figure 88. Pore size distributions for BBO-COF 3 at various points during the 4-day
reaction period .................................................................................................................. 99
Figure 89. FT-IR spectra of BBO-COF 3 at various points during the 4-day reaction
period .............................................................................................................................. 100
Figure 90. N2 adsorption isotherms, FT-IR spectra, and PXRD data for BBO-COF 2, and
BBO-COF 3, with different nucleophiles ....................................................................... 102
Figure 91. Proposed mechanism for the stepwise oxidative dehydrogenation pathway to
form the BBO-linkage using NaCN as a catalyst ........................................................... 106
Figure 92. FT-IR spectrum of BBO-COF 2 synthesized with NaN3 .............................. 114
Figure 93. FT-IR spectra of BBO-COF 2 synthesized with NaSCH3............................. 115
Figure 94. FT-IR spectrum of BBO-COF 2 synthesized without a catalyst ................... 116
Figure 95. Normalized pore size distributions of BBO-COF 2 synthesized with different
nucleophiles .................................................................................................................... 117
Figure 96. FT-IR spectrum of BBO-COF 2 deoxygenated............................................. 118
Figure 97. N2 adsorption isotherm of BBO-COF 2 deoxygenated ................................ 119
Figure 98. Pore size distribution of BBO-COF 2 deoxygenated .................................... 119
xxi
Figure 99. FT-IR spectra of BBO-COF 3 synthesized with NaCN compared with the
monomers........................................................................................................................ 122
Figure 100. FT-IR spectrum of BBO-COF 3 synthesized with NaCN ........................... 123
Figure 101. FT-IR spectrum of BBO-COF 3 synthesized with NaN3 ............................ 124
Figure 102. FT-IR spectrum of BBO-COF 3 synthesized with NaSCH3 ....................... 125
Figure 103. FT-IR spectrum of BBO-COF 3 synthesized without a catalyst ................. 126
Figure 104. Normalized pore size distributions BBO-COF 3 synthesized with different
nucleophiles .................................................................................................................... 127
Figure 105. FT-IR spectrum of a ground mixture of BBO-COF 3 monomers ............... 128
Figure 106. FT-IR spectrum of BBO-COF 3 after 1 day of heating............................... 129
Figure 107. FT-IR spectrum of BBO-COF 3 after 2 days of heating ............................. 130
Figure 108. FT-IR spectrum of BBO-COF 3 after 3 days of heating ............................. 131
Figure 109. Normalized PXRD of BBO-COF 3 at various points during the 4-day
reaction period ................................................................................................................ 132
Figure 110. FT-IR spectrum for BBO-COF 3 deoxygenated ......................................... 133
Figure 111. N2 adsorption isotherm for BBO-COF 3 deoxygenated .............................. 134
Figure 112. Pore size distribution for BBO-COF 3 deoxygenated ................................. 134
Figure 113. Precursor used to construct the hexagonal unit cell for BBO-COF 3 ......... 136
Figure 114. Simulated PXRD of BBO-COF 3 modeled using a gra unit cell ................ 138
Figure 115. 150.9 MHz 13C CP-MAS solid-state NMR spectra of BBO-COF 3 ........... 139
xxii
Figure 116. TGA profile for BBO-COF 3 ...................................................................... 140
Figure 117. BET surface area plot for BBO-COF 2 synthesized with NaN3 ................. 141
Figure 118. BET surface area plot for BBO-COF 2 synthesized with NaSCH3............. 141
Figure 119. BET surface area plot for BBO-COF 2 synthesized without a catalyst ...... 142
Figure 120. BET surface area plot for BBO-COF 2 deoxygenated ................................ 142
Figure 121. BET surface area plot for BBO-COF 3 synthesized with NaCN ................ 143
Figure 122. BET surface area plot BBO-COF 3 synthesized with NaN3 ....................... 143
Figure 123. BET surface area plot for BBO-COF 3 synthesized with NaSCH3............. 144
Figure 124. BET surface area plot for BBO-COF 3 synthesized without a catalyst ...... 144
Figure 125. BET surface area plot for BBO-COF 3 after 1 day of heating with NaCN. 145
Figure 126. BET surface area plot for BBO-COF 3 after 2 days of heating with NaCN 145
Figure 127. BET surface area plot for BBO-COF 3 after 3 days of heating with NaCN 146
Figure 128. BET surface area plot for BBO-COF 3 deoxygenated ................................ 146
Figure 129. SEM images of BBO-COF 3 at various magnifications ............................. 147
Figure 130. Benzobisoxazole modeling representative of BBO-COF 2......................... 149
Figure 131. Benzobisoxazole modeling representative of BBO-COF 3......................... 149
Figure 132. Spin density maps for BBO-COF 2 with NaCN, NaN3, and NaSCH3 ........ 150
Figure 133. Spin density maps for BBO-COF 3 with NaCN, NaN3, and NaSCH3 ........ 150
Figure 134. Structure of meso-substituted porphyrins .................................................... 151
Figure 135. Structure of chlorophyll and Heme B porphyrins ....................................... 151
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Figure 136. Cyclopropanation of diazo reagents by metalloradical catalysis performed by
a cobalt porphyrin ........................................................................................................... 152
Figure 137. Structure of BF-COF-1 showing porous space available for catalysis........ 154
Figure 138. Reduction of CO2 to cyclic carbonates by cCTF......................................... 156
Figure 139. Structure of Ir-NHC-CTF catalyzing reduction of CO2 to formate ............. 157
Figure 140. Structure of tetraformylphenylporphyrin (1) ............................................... 159
Figure 141. Two strategies to create 1 ............................................................................ 159
Figure 142. Synthesis of 1 from commerically available precursors.............................. 160
Figure 143. Synthesis of BBO-H2P POP and BBT-H2P POP ........................................ 161
Figure 144. N2 adsorption isotherms and pore size distributions for BBO-H2P POP and
BBT-H2P POP................................................................................................................. 162
Figure 145. CO2 adsorption isotherms for BBO-H2P POP and BBT-H2P POP ............. 163
Figure 146. Kubelka-Munk diffuse reflectance spectra of 1, BBO-H2P POP and BBT-H2P
POP ................................................................................................................................. 164
Figure 147. Structure of meso-tetraphenylporphyrin (TPP) ........................................... 167
Figure 148. Synthesis of RuTPP ..................................................................................... 168
Figure 149. Reaction scheme for the hydrosilylation of CO2 with RuTPP .................... 169
Figure 150. Synthesis of 1-Ru from 1 ............................................................................ 171
Figure 151. Synthesis for BBO-RuP POP and BBT-RuP POP ...................................... 172
xxiv
Figure 152. N2 adsorption isotherms and pore size distributions for BBO-RuP POP and
BBT-RuP POP ................................................................................................................ 173
Figure 153. CO2 adsorption isotherms for BBO-RuP POP and BBT-RuP POP ............ 174
Figure 154. Kubelka-Munk diffuse reflectance spectra for BBO-RuP POP and BBT-RuP
POP ................................................................................................................................. 175
Figure 155. FT-IR spectrum of RuTPP........................................................................... 183
Figure 156. FT-IR spectrum of 1-Ru .............................................................................. 183
Figure 157. FT-IR spectrum of BBO-H2P POP .............................................................. 184
Figure 158. FT-IR spectrum of BBT-H2P POP .............................................................. 184
Figure 159. FT-IR spectrum of BBO-RuP POP ............................................................. 185
Figure 160. FT-IR spectrum of BBT-RuP POP .............................................................. 185
Figure 161. PXRD profile of BBO-H2P POP ................................................................. 186
Figure 162. PXRD profile of BBT-H2P POP.................................................................. 186
Figure 163. PXRD profile of BBT-RuP POP ................................................................. 187
Figure 164. 13C CP-MAS spectrum of BBT-H2P POP ................................................... 188
Figure 165. 13C CP-MAS spectrum of BBT-RuP POP .................................................. 189
Figure 166. Roquerol BET analysis of BBO-H2P POP .................................................. 190
Figure 167. BET surface area plot for BBO-H2P POP ................................................... 191
Figure 168. Roquerol BET analysis of BBT-H2P POP................................................... 192
Figure 169. BET surface area plot for BBT-H2P POP.................................................... 193
xxv
Figure 170. Roquerol BET analysis of BBO-RuP POP.................................................. 194
Figure 171. BET surface area plot for BBO-RuP POP................................................... 195
Figure 172. Roquerol BET analysis of BBT-RuP POP .................................................. 196
Figure 173. BET surface area plot for BBT-RuP POP ................................................... 197
Figure 174. UV-Vis spectrum of 1 in toluene................................................................. 198
Figure 175. Fluorescence spectrum of 1 in toluene ........................................................ 198
Figure 176. UV-Vis spectrum of 1-Ru in acetone .......................................................... 199
Figure 177. Fluorescence spectrum of 1-Ru in acetone ................................................. 199
Figure 178. XPS spectra of the Ru 3P for RuTPP, 1-Ru, BBT-RuP POP, and BBO-RuP
POP ................................................................................................................................. 200
Figure 179. XPS spectra of the Ru 3D and C 1S for RuTPP, 1-Ru, BBT-RuP POP, and
BBO-RuP POP ................................................................................................................ 201
Figure 180. 1H NMR spectrum of RuTPP ...................................................................... 202
Figure 181. 1H NMR spectrum of 1-Ru ......................................................................... 203
Figure 182. Example quantitative 1H NMR spectrum for the hydrosilylation of CO2 by
RuTPP ............................................................................................................................. 204
xxvi
Chapter 1: Porous Polymers
1.1 Introduction to porous polymers
In recent years porous materials have jumped quickly to the forefront of materials science
as promising substrates for a wide variety of uses. Their cornerstone characteristic, their
porosity, can be viewed as an empty space just waiting to be filled by guests. Gases are by
far the most commonly used guests; in part to measure the extent of the porosity of the
material, but the materials can also act as an excellent storage medium for gases such as
nitrogen (N2) or carbon dioxide (CO2). This is of great interest to researchers working to
combat the effects of climate change by sequestering and converting CO2 into valuable
chemical feedstocks. Other guests, such as metal atoms tightly bound inside porous
materials can prove useful as heterogeneous catalysts, which can be re-isolated and
recycled numerous times. Larger, more complex guests, such as C60 can be employed as
electron acceptors and cause the material to have charge transfer properties for
optoelectronics. This ability to incorporate guests is a significant advantage over linear
polymers, which share many similarities to porous materials, and expands their utility in a
wide variety of applications. The great range of diversity in these materials erupts outward
from this point when chemists use their synthetic knowledge to design and synthesize
1
complex and functional compounds which can be incorporated into these porous materials
and tailor their function to a specific application.
1.2 History of porous polymers
The field of porous materials began in the mid 70’s starting from hyper crosslinked
polymers. Researchers found that polystyrene polymers crosslinked to different degrees
yielded different porosities and pore
sizes.1 The next major advancement in
porous materials was brought about by
metal-organic
frameworks
(MOFs).
Under a few different names, MOFs were
brought to the forefront of the literature by
Hoskins and Robson in the late 1980’s.2
MOFs are created by combining a metal
salt or cluster with a coordinating organic
Figure 1. MOF-5 reproduced from Science,
linker to create crystalline frameworks 2003, 300, 1127-1129. Reprinted with
permission from AAAS
that extend outwards in three dimensions
(see Figure 1). The expansive range of components for MOFs has given rise to hundreds
of unique MOF structures, which can be devised a priori based on the known geometry of
metals, clusters, and organic linkers. This rational design strategy helped propel the porous
2
polymer field forwards by giving a basis upon which to design and select monomers to
give predictable MOF structures with pre-designed characteristics. These rationallydesigned MOF materials have demonstrated
their utility in a range of applications
A
including gas storage3–5 and separations,6 fuel
cell applications,7,8 heterogeneous catalysis,9–
11
even
water
harvesting12
and
drug
delivery.13–15 In the early 2000’s a new type
B
of porous polymer was reported that used a
unique type of spiro-cyclic monomer to give
a rigid and contorted polymer backbone that
promotes the microporosity of the material by
inefficient packing. Since the initial report of
these polymers of intrinsic microporosity
(PIMs) by Budd and McKeown in 2003,16 a
variety
of
spiro-cyclic
and
otherwise Figure 2. Structure of A) COF-1 and B)
COF-5 Reprinted from Science, 2005,
sterically bulky monomers have been 310, 1166-1170 with permission from
AAAS
employed to create new PIMs. PIMs are
unique in the field of porous materials because they do not form extended network solids
3
and thus retain good solubility in organic solvents. Their solubility lends them to
applications in porous polymer membranes that can be easily cast from solution.17–19
1.3 Covalent organic frameworks (COFs)
Shortly thereafter in 2005, covalent organic frameworks (COFs) were introduced by Omar
Yaghi as the all-organic counterpart to MOFs. The seminal example, COF-5, was the first
Td
C2
+
C2
C3
Linkers
C3
C4
C6
Vertexes
Figure 3. COF linkers and vertexes of different symmetry, and a selection of COF pore
shapes and topologies
4
material to take advantage of reversible organic reactions, as a form of error correction
during the formation of the bulk material, leading to highly-ordered, layered structures.20
Since the report of COF 1 and COF 5 (see Figure 2), an incredible range of new COF
materials have been reported using a wide variety of linkers and vertexes to give diverse
pore shapes and sizes, gas uptake and selectivities, and potential applications. COFs, much
like MOFs, benefit immensely from their designability; rational design of a COF
framework a priori can lead to unique COF materials with designed properties. There are
a number of possible pore shapes that have thus far been reported (see Figure 3). With the
symmetry of the monomeric units in mind, a suitable type of linkage must be chosen. The
classic boronate ester linkage, formed by condensation of a catechol moiety with a boronic
acid, provides an excellent balance between stability of the covalent bond formed, and the
ease of reversibility of the reaction. Reversibility is a key component to the COF formation,
because it allows the opportunity for error correction during the formation of the material.
If a component is misaligned or bonded in the wrong position, it can be reversed to allow
for the proper positioning to occur, and result in a more thermodynamically stable
arrangement. With the initial understanding of how COFs can form through reversible
reactions, the Yaghi group expanded on the idea of rational design of COFs by
incorporating different C3 and C2 monomers using the same boronate ester and boroxine
linkages, to tune the properties of the bulk material including surface area and pore size.21
Incorporating new monomers with extended conjugation led to a novel COF material
5
which exhibited semiconducting properties.22 A significant advancement in the field was
made when the first COF created by an imine linkage was reported in 2009.23 This opened
the field up to not only a range of new functional monomers, but also to the development
of new linkages to vastly expand the possibilities for new COF structures. Since 2009, a
Figure 4. COF linkages based on reversible reactions
variety of linkages have been shown to have good reversibility to form COF materials (see
Figure 4). COFs have been used in a variety of applications, including the usual gas storage
and uptake applications. Due to their highly ordered porous nature, they also typically
display good conductivity, which lends them to optoelectronic applications. After the initial
discovery of the semiconductive properties of COFs in 2008 by the Jiang group,22 a number
of new COF structures incorporating large π-aromatic systems such as pyrene,24
porphyrins,25 and phthalocyanines.26 Interest in this property of COFs has led to the
theoretical and computational studies on the delocalization of charge in COFs,27,28 as well
as synthetic efforts to tune these properties by the creation of donor-acceptor (D-A)
COFs.29–32 Another strategy involves doping the COF material with iodine,33 PEDOT,34 or
C60 derivatives35–37 to further increase the conductivity (see Figure 5). Because of the
6
potential use of COFs in conductive devices, the next logical step was to grow oriented
thin films of COFs on substrates for subsequent device fabrication.38–42 COFs have been
incorporated into both photovoltaic,35,36 and field effect transistor devices,41 which
demonstrates the utility of COF materials for new, inexpensive, and lightweight materials
based on largely organic components. This work details the incorporation of novel
subphthalocyanine (SubPc) chromophores into porous materials for optoelectronic
applications.
Figure 5. TT-COF pictured with PC61BM guest reproduced from Angew. Chem. Int. Ed.,
2013, 52, 2920-2924 with permission of John Wiley and Sons
7
1.4 Porous organic polymers (POPs)
Porous organic polymer (POP) is an umbrella term, which encompasses any porous
polymer based on organic subunits; this includes COFs, and other less crystalline materials
such as PIMs. Many new sub-groups of POPs have developed in parallel with COFs, for
example porous aromatic frameworks (PAFs) are another subgroup of POPs comprised
largely of aromatic components43–46 but lacking the crystallinity of COFs. Conjugated
microporous polymers (CMPs), known for their
high degree of conjugation and potential for
optoelectronic applications,47 and covalent
triazine frameworks (CTFs) which specifically
incorporate a triazine motif and are known for
their high CO2 uptake (see Figure 6).48–50 The
common characteristic among POPs is their
porosity, however in structure, linkage, and Figure 6. Bulk structure of CTF-1
reprinted with permission from Nano
potential applications POPs are very diverse. Lett., 2010, 10, 537-541. Copyright
2010 American Chemical Society
Their porous nature allows them to adsorb a
variety of guests including ammonia,51 molecular iodine,52 and dye compounds.53 When
extended π-conjugated systems with restricted rotation are incorporated into porous
polymers, their luminescent properties lend them toward optoelectronic applications in
photovoltaics and organic light emitting devices.54–57
8
POPs have demonstrated enormous
potential in the field of catalysis.58
Incorporation of catalyst motifs, by a
variety of methods, can create a POP
that functions as a heterogeneous
catalyst for reactions of interest in
organic synthesis. This offers the
advantages of easy re-isolation of the
Figure 7. Schematic representation of Fe-POP-1
catalyst, and the potential for catalyzing a Knoevenagel condensation (left) and
oxidation reactions (right) reproduced from
recyclability and reuse; this is of great Applied Catalysis A, 2013, 459, 41-51 with
permission from Elsevier
benefit for catalysts that employ
expensive and rare metals. Additionally, the rational design of these porous materials can
produce materials with tuned pore sizes, which can yield heterogeneous catalysts that are
also size selective.59 The pores can also be tailored toward a stereospecific catalytic
reaction by incorporating chiral moieties into the POP structure.60 There are two main
strategies for the design of a POP based catalyst; first, a known catalyst motif as used in
homogeneous catalysis can be incorporated into a porous polymer. This strategy benefits
from the understanding of the catalytic moiety outside the context of polymers, and better
control over incorporation of metals. Primary among these homogeneous catalyst moieties
9
are porphyrins and phthalocyanines,58 due to their ability to strongly bind metal ions inside
their macrocyclic core (see Figure 7). Other catalytic moieties with weaker metal binding
are better suited to the second method of catalyst-POP formation; The second strategy
allows for a POP to be post-synthetically modified with metal ions or nanoparticles to
A
B
Figure 8. A) COF-LZU1 pore shown doped with Pd(OAc)2 and B) side view depicting
interaction between Pd and imine nitrogen atoms. Adapted with permission from J. Am.
Chem. Soc., 2011, 133, 19816-19822. Copyright 2011 American Chemical Society
impart it with catalytic activity61 (see Figure 8). This second strategy benefits from
potential ease of synthesis of simpler components compared to the known homogeneous
catalyst moieties,62 however suffers from inexact doping of metals and debate about what
species performs the catalysis, and if catalysis is occurring deep in the porous cavities of
the material or just upon the surface of the material.
10
POP-based catalysts have been employed successfully in a variety of reaction types
including oxidations,63–65 aminations,66,67 reductions,68 and organo-69 and photocatalysis.70
Another catalytic reaction of intense interest is the reduction of CO2 to other value-added
C1 compounds. POPs are already well known for their excellent CO2 uptake and storage
capabilities,71,72 so the incorporation of catalytic functionality into these materials makes
them a powerful tool to harness one of the most abundant carbon sources available, and
provide an environmental benefit along the way. This work also details the study of new,
more stable, reversible linkages for the creation of porous materials to improve their
robustness in catalysis, as well as the development of a new porphyrin-based catalyst for
the hydrosilylation of CO2.
11
Chapter 2: Synthesis of novel subphthalocyanines (SubPcs) and SubPc-based porous
organic polymers (POPs)
Portions of this chapter are adapted from the following publications:
Eder, G. M.; Walker, B. R.; McGrier, P. L. Subphthalocyanine-based Porous
Organic Polymers, RSC Advances, 2017, 7, 29271. Used with permission.
Copyright The Royal Society of Chemistry
2.1 Dyes in photovoltaic applications
2.1.1 Dyes in organic photovoltaic devices (OPVs)
Dyes have long been studied for applications in photovoltaic technology. Since the
groundbreaking results of the dye-sensitized solar
cell (DSSC) pioneered by Grätzel achieving a
power conversion efficiency (PCE) of 7%,73 dyes
have been employed in a variety of new methods
Active Layer
to create effective photovoltaics. In DSSCs, the
dye is solely responsible for absorbing light and Figure 9. Structure of early
photovoltaics. Adapted with
transferring an electron to the semiconductive permission from Chem. Rev., 2014,
114, 8943-9021. Copyright 2014
TiO2, which in conjunction with the inorganic American Chemical Society
electrodes, are responsible for the bulk of
12
electrical conduction within the cell. A different strategy uses dyes to not only absorb the
incident light, but also help conduct charge to the electrodes. This class of organic
photovoltaics (OPVs) emerged in the 1980’s employing all organic-based light absorbing
materials. When the organic light absorbing active layer was simply sandwiched between
transparent electrodes (see Figure 9), the early cells had very limited function because
organic compounds are often non-conductive.74 These OPVs were then redesigned in much
the
same
way
as
traditional
silicon
cells,
where
an
electron
rich
Donor
Acceptor
Planar Heterojunction
Bulk Heterojunction
Ideal Heterojunction
Figure 10. Morphologies of OPV devices. Adapted with permission from Chem. Rev.,
2014, 114, 8943-9021. Copyright 2014 American Chemical Society
donor component was paired with an electron poor acceptor component and arranged in
two layers. This electronic bias of the bilayer architecture led to more efficient devices (see
Figure 10).75 Initially small molecule dyes, such as phthalocyanines, perylenediimides
(PDIs), and pentacenes76 among others were employed in bilayer OPV cells. In order to
increase the semiconductive nature of these materials, the dyes in question began to be
designed to have large delocalized aromatic systems through which electrons are more
mobile. Moving to even longer conjugated polymers for the bilayer heterojunction gave
13
PCE’s of up to 5%.77 The bilayer architecture has a few drawbacks, including requiring
vacuum deposition methods to achieve the layered structure of donor and acceptor.
Fortunately it was discovered that even a physical mixture of donor and acceptor
components in a bulk heterojunction (BHJ) could effectively generate charge carriers to
create
a
photocurrent.78,79
The
BHJ
architecture has several advantages over the
bilayer cell- primary among these is the ease
of manufacture. If the components can be
physically mixed then deposited at the same
Figure 11. Structure of P3HT and
PC61BM
time, a number of facile techniques for mass
manufacture become available including spin
coating and even roll-to-roll or inkjet printing. Additionally, the physical mixing of the
components increases the surface area between the two components, which facilitates the
separation of free charges and leads to generally higher PCEs than the bilayer architecture.
Increasing the conductivity of the active layer was essential to improving the efficiency of
these devices. Switching to a polymeric strategy allows organic molecules to mimic tiny
nano-wires along which charge carriers could be conducted. One of the most well studied
polymers for OPVs is poly-3-hexylthiophene (P3HT), which has found great success in
polymer OPVs due to its relatively high conductivity and ease of synthesis. P3HT, acting
14
as the donor component is often paired with acceptors derived from C60, yielding BHJ cells
with PCEs around 5%80 (see Figure 11). P3HT has also been paired with polymeric
acceptors,81 which aim to capture a larger portion of the solar spectrum than is possible
with C60 derivatives. A vast range of
polymers have been created for BHJ
photovoltaics, which improve upon the
efficiency of P3HT by employing a donoracceptor (D-A) copolymer system. This
modular approach allows for tuning the
frontier orbital levels by changing the
Figure 12. Orbital mixing diagram for
properties of the donor or acceptor donor-acceptor (D-A) copolymers showing
decreased band gap reprinted with
components (see Figure 12). Common permission from Chem. Rev., 2009, 109,
5868-5923. Copyright 2009 American
donor components such as fluorene,82 Chemical Society
carbazole,83 and benzodithiophene units,84 and common acceptor components such as
benzothiadiazole,82 diketopyrrolopyrole,85 and napthalenediimide86 units have all been
incorporated into D-A copolymers (see Figure 13).
15
Figure 13. Structure of electron donor (red) and electron acceptor (blue) moieties
incorporated into D-A copolymers
2.1.2 OPV morphology and extension to porous materials
With a vast library of excellent OPV materials, much research effort has been put towards
engineering the cell manufacturing method to create more efficient cells from known
polymeric and/or small molecule components. One major area of work within OPVs has
focused on analyzing the morphology of the donor-acceptor active layer.87 The ideal
morphology for a BHJ consists of long, thin, interpenetrated domains of donor and
acceptor, (see Figure 10). With a physical mixture of donor and acceptor components, it’s
difficult to obtain such a morphology. The use of processing additives88 and annealing
procedures89 have been shown to improve the morphology considerably, however without
predictable control over the morphology of BHJ cells, PCEs will always be limited based
16
on the quality of the morphology. One method to overcome the unpredictable morphology
of polymer OPVs could involve the predesigning of a polymer with the desired
morphology. This can be possible to achieve with porous polymeric materials. Early in the
field of covalent organic frameworks (COFs) researchers noticed that the ordered stacking
of layers in the material imbued it with semiconductive properties22 (see Figure 14). The
benefit of the ordered nature of
COFs compared to linear organic
polymers is clear in the difference
in conductivity between the two.
P3HT being one of the most
successful linear polymers in
OPV devices has a conductivity
of 0.1 cm2/Vs90 whereas COF Figure 14. Structure of semiconductive TP-COF
reproduced from Angew. Chem. Int. Ed., 2008, 47,
materials have been measured to 8826-8830 with permission from John Wiley and
Sons
have conductivities as high as 8
cm2/Vs.25
17
Not long after this initial discovery researchers employed COF materials in photovoltaic
devices. The Jiang research group reported a highly stable phenazine-linked COF with a
high degree of conjugation and a conductivity of 4.2 cm2/Vs.36 They combined this COF
with C60 to fill the open pores of
the material, then fabricated the
bulk powder into a photovoltaic
cell with a PCE of 0.9%. The
Bein research group created a
photovoltaic COF device by a
different strategy; using the
precedent of growing COFs on
surfaces as demonstrated by
Figure 15. Schematic of TT-COF based photovoltaic
Dichtel,38 the Bein group grew a device reproduced from Angew. Chem. Int. Ed., 2013,
52, 2920-2924 with permission of John Wiley and
benzodithiophene COF on ITO Sons
to ensure optimal stacking of layers.39 Then doping this COF thin film with C60 and creating
an overlayer of C60 to avoid electrical shorts they created a cell with a PCE of 0.05%.35
With these initial examples in mind, we believe that with the incorporation of different
functional monomers and careful tuning of the frontier molecular orbitals, the field of
porous polymers has considerable room to grow towards applications in OPVs.
18
2.2 Background on SubPcs
2.2.1 History of SubPcs
Subphthalocyanines (SubPcs) are a curious group of dyes initially synthesized by Meller
and Ossko in 1972.91 SubPcs are a lower homologue of metallophthalocyanines (MPcs)
which contain 4 isoindole units arranged around a central cavity (see Figure 16). Meller
and Ossko were originally attempting to create boron phthalocyanine when they discovered
the boron SubPc instead. The smaller atomic radius of boron is best encapsulated by only
3 isoindole units, causing the macrocycle of the SubPc to take on a distorted ‘bowl’ shape
which still maintains its
aromaticity in the 14 π
electron system. The boron
atom centered at the apex of
this bowl also binds an axial
Figure 16. Structure of phthalocyanine (Pc) and
subphthalocyanine (SubPc)
ligand, projecting outwards
from the opposite side of the
bowl. This distorted SubPc geometry is unique to boron, as every other central metallic
atom will yield a Pc instead of a SubPc, additionally the boron atom and axial ligand must
stay in place to prevent degradation of the macrocyclic core. Despite the wide use of Pcs
in POPs,92 SubPcs have been entirely overlooked as potential functional monomers for the
creation of POPs with interesting new properties.
19
2.2.2 Applications of SubPcs
Due to their excellent light absorbance and frontier orbital energies, SubPcs have found
much success in the field of organic photovoltaics (OPVs). SubPcs as small molecule
electron donor materials are an excellent counterpart to the ubiquitous electron acceptor,
C60. Not only do the two
share in their non-planar
aromatic
nature
and
complementary ball and
bowl shapes, but they also
have a large frontier orbital
overlap. This enhanced
overlap has been shown to
lead to higher open circuit
Figure 17. UV-Vis spectra and orbital diagrams of CuPc and
SubPc paired with C60 electron acceptor reprinted with
voltages and consequently
permission from J. Am. Chem. Soc., 2006, 128, 8108-8109.
Copyright 2006 American Chemical Society
better PCEs than MPcs93
(see Figure 17). Interestingly, the frontier molecular orbitals of SubPcs allows them not
only to act as electron donors when paired with C60, but also to act as electron acceptors
when paired with donors having higher lying LUMO orbitals.94 This ambipolar nature of
SubPcs allows them to function as an alternative class of electron acceptors to enhance the
open circuit voltage and light absorption over cells which use C60 as the electron acceptor.95
20
The interesting geometry of SubPcs has lead to several creative SubPc-based light
absorbing compounds. One example incorporates a fused SubPc dimer96 as electron
acceptor in combination with the simple SubPc as electron donor to create and all-SubPc
OPV which achieved a PCE of 2.5% by optimizing the counter electrodes.97 When
incorporating an electron transport layer comprised of C60 the PCE jumped to 4%. Another
example from the Fréchet research group incorporated an electron donating thiophene
moiety into their OPV device by appending it to the axial ligand of a SubPc to create a
donor-acceptor dyad. Devices based on this D-A dyad with C60 as an electron acceptor
achieved PCEs above 1%.98 SubPcs have been optimized for use in OPVs to achieve PCEs
of nearly 7%,95 which is competitive with other polymeric OPVs and DSSCs.
SubPc-based materials outside of OPVs are relatively uncommon. SubPcs have been
studied for their ability to organize on surfaces,99 which is interesting considering their
nonplanar geometry. SubPc-based thin films could prove useful for information storage or
liquid crystalline technologies, however only slow progress has been made in this direction.
The unusual geometry of SubPcs lends them well to creating small molecular cages.
Incorporating a pyridyl moiety around the periphery of the SubPc and coordination to
palladium afforded a small cage created by two SubPcs with their bowls oriented toward
the interior of the cage.100 Exposing this cage to a solution of C60 afforded capture of a
molecule of C60 inside of the SubPc cage101 (see Figure 18). On a larger scale, SubPcs
21
decorated at the periphery with alkenes were linked via alkene metathesis to afford SubPcbased nanospheres, which demonstrated antibacterial properties.102
SubPc polymers are yet rarer, with a single example from Bender’s research group using
SubPcs to post-functionalize a polystyrene-based polymer. Initially- a SubPc with a styrene
moiety in the axial position was targeted for subsequent radical polymerization. They found
this was unsuccessful due to the SubPc acting as a radical scavenger,103 thus shutting down
Figure 18. C60 trapped inside a SubPc cage reproduced from Chem. Commun. 2004,
11, 1298-1299 with permission of The Royal Society of Chemistry
the polymerization. Incorporation of the SubPc moiety was afforded by attaching pendant
SubPcs to the phenol groups on the polymer backbone through axial reactivity of the
SubPc104 (see Figure 19). Unfortunately this method did not afford a high incorporation of
SubPc into the polymer. Nonetheless these polystyrene-SubPc polymers did show potential
in organic light emitting diodes (OLEDs).105 Despite the lack of precedent of SubPcs in
22
polymers, we envisioned that the SubPc core could be amenable to porous organic
polymers due to its C3 symmetry. We also imagined that the non-planar geometry of
SubPcs could lead to interesting packing of porous polymers in the solid state. Outside of
the interesting structural features, we also envisioned a SubPc polymer could have unique
optoelectronic properties in comparison to the well-studied Pc porous polymers. This led
us to design a mild co-condensation polymerization protocol to avoid the problems of
Figure 19. Synthesis of SubPc polymer, poly(4-methlystyrene)-copoly(phenoxyboronsubphthalocyanine)
radical polymerization of SubPcs, and gain access to SubPc-based POPs with interesting
optoelectronic properties.
23
2.3 Design of SubPc POPs
2.3.1 Design and synthesis of hexahydroxy SubPcs
Subphthalocyanines are synthesized by a straightforward trimerization reaction of any
phthalonitrile at high temperatures in the presence of BCl3, which results in the chloroaxial SubPc. For the design of SubPc POPs, we required a phthalonitrile peripherally
substituted with a catechol moiety for subsequent boronate ester formation. Fortunately,
such a phthalonitrile had already been reported by Torres et al.106 The route from
commercially available veratrole consists of simple aromatic substitution reactions, and
OMe
OMe
Br2, I2 (cat.)
DCM RT, 3h
95%
Br
Br
OMe
9
Br
BBr3
DCM 0oC, 4h
97%
OMe
Br
Cl
K2CO3
NC
NC
OH
5
OH
Pd/C, H2 (1 atm)
EtOAc, RT, 4h
97%
NC
NC
OBn
6
OBn
CuCN
DMF ∆ 24h
58%
OH
OH
8
EtOH, ∆, 8h
83%
Br
Br
OBn
7
OBn
Figure 20. Synthesis of 4,5-dihydroxyphthalonitrile (5)
protections and deprotections of the catechol moiety (see Figure 20). Veratrole is first
brominated to yield 4,5-dibromoveratrole 9, then the methyl protecting groups are removed
and subsequently replaced with benzyl groups, which can later be cleaved under mild
conditions. The phthalonitrile 6 is formed by substitution of the bromine atoms for nitriles
via Rosenmund-von Braun reaction. This step was fairly troublesome in large part due to
the required double substitution, and often separating the mono- and di- substituted
24
products could not be achieved cleanly. Once the benzyl-protected phthalonitrile was
obtained, the benzyl groups could be easily cleaved by hydrogenolysis with Pd/C and
hydrogen to yield the free catechol phthalonitrile 5.
With the catechol phthalonitrile in hand, we were nearly ready to synthesize the desired
SubPc. After initial screening of protecting groups we decided to target a robust protecting
group, and settled on triisopropylsilyl (TIPS) groups for improved stability of the
protecting group during trimerization. The TIPS groups could be installed by a typical silyl
protection procedure with mild heat (see Figure 21). With the TIPS groups in place, the
cyclotrimerization proceeded smoothly, affording at first a very low yield of TIPS-Cl-
NC
NC
OH
5
OH
TIPSCl, Imidazole
DMAP (cat.)
DMF, 55oC, 24h
82%
NC
NC
OTIPS
4
SubPc (1a). The
OTIPS TIPS
Figure 21. Protection of 5 with triisopropylsilyl (TIPS) protecting
groups
groups
protecting
had
the
beneficial effect of
greatly improving the solubility of the SubPc product, which aided in purification. After
significant optimization of the reaction conditions, good yields of 1a were achieved by
heating the phthalonitrile 4 in 1M BCl3 solution in p-xylenes with no external solvent
added. This synthesis benefits from the very small amount of solvent required, which can
be easily removed by high vacuum. The crude solids can be simply ground in a mortar and
pestle and crude 1a can be extracted with hexanes. After a hot wash with acetonitrile, the
25
pure 1a can be isolated with no column chromatography required. The largest drawback of
this synthesis is the scale; the reaction was never effectively scaled up past 0.5 g without a
significant drop in yield. Removal of the TIPS protecting groups in the presence of excess
CsF in a mixture of acetone and hexanes was straightforward, yielding the hexahydroxyCl-SubPc (1b) after crashing out in water, and purifying by simple washing with water and
TIPSO
OTIPS
HO
Cl
NC
NC
OTIPS
4
OTIPS
1. BCl3
1M in p-xylenes
∆, 1 h
16%
N
N
TIPSO
TIPSO
N
B
N
OTIPS
N
1a
Cl
CsF
Acetone
48oC, 18h
67%
N
OTIPS
OH
N
N
HO
HO
N
B
N
1b
N
N
OH
OH
Figure 22. Synthesis of hexahydroxy-Cl-SubPc (1b)
organic solvents. While this final step posed no major challenges, the early synthesis of
these SubPcs was plagued with the problems of working with very small amounts of
material. The isolated SubPcs were also found to be very unstable in solution when exposed
to light for even short amounts of time. We believe this stems from the high number of
peripheral oxygen atoms on the SubPc core, which are electron donating, and contribute to
the already fairly labile nature of the B-Cl bond. These two effects in concert may lend the
Cl-SubPcs to easier degradation, and possibly lower isolated yields after the
cyclotrimerization. A similar trend has been observed in hybrid subporphyrins.107 To
investigate this, an in-situ axial ligand exchange from -Cl to –OPh was performed. SubPcs
are often substituted with phenol, or related derivatives for a variety of reasons, but partly
26
to improve their stability by employing the more covalent B-O axial bond. The hexaTIPSOPh-SubPc was synthesized by a similar procedure- an initial heating period of the
phthalonitrile with 1M BCl3, followed by treatment with a solution of phenol in o-xylenes
and further heating (see Figure 23). HexaTIPS-OPh-SubPc could be isolated and purified
in
much
the
same
way
as
the
Cl-axial
TIPSO
N
NC
OTIPS
NC
OTIPS
4
1. BCl3
1M in p-xylenes
∆, 1 h
2. PhOH, o-xylenes
∆, overnight
18%
1. BCl3
1M in p-xylenes
∆, 1 h
2. F5PhOH, o-xylenes
∆, 2 h
28%
N
N N
B
N N
1b
HO
OTIPS
OH
OH
OH
OPh
N
TIPSO
N
OTIPS
TIPSO
N N
B
N N
N
TIPSO
3a
N N
B
N N
OTIPS
OTIPS
OH
N
OH
2b
HO
OF5Ph
TIPSO
N
OTIPS
2a
N
OPh
CsF
Acetone/Hex.
o
HO
OTIPS 48 C, 18h
70%
HO
N N
B
N N
TIPSO
in
OH
N
HO
OTIPS
1a
TIPSO
isolated
Cl
CsF
Acetone/Hex.
o
OTIPS 48 C, 18h HO
68%
N N
B
N N
TIPSO
was
HO
Cl
N
but
OTIPS
TIPSO
1. BCl3
1M in p-xylenes
∆, 1 h
16%
version,
OH
OF5Ph
CsF
Acetone
48oC, 18h
84%
N
HO
HO
N N
B
N N
N
3b
OH
OH
Figure 23. Synthesis of hexahydroxy-SubPcs 1b, 2b, and 3b from 4
better yields, and seemed to have greater benchtop stability. A third SubPc, with an axial
pentafluorophenol (F5OPh) was created by the same method to investigate any effect the
electron withdrawing axial ligand might have on the monomeric SubPcs, and on the
27
resultant polymers. These axially substituted SubPcs could be deprotected to the free
catechol SubPcs by the same CsF conditions with little issue (see Figure 23).
This suite of 3 novel hexahydroxy-SubPcs were characterized by UV-Vis spectroscopy
(see Figure 24). The absorbance of these compounds follows the typical band structure of
SubPcs; and a more intense Q-band with λmax around 570 nm for all three compounds. The
weak Soret bands at ~ 368 nm are possibly attributed to an n-π* transition between the lone
pair of the oxygen atoms and the peripheral aromatic ring.108 The fluorescence was also
typical, mirroring the absorption Q-band with small Stokes shift of around 10 nm.
Normalized Intensity (a.u.)
1.0
0.8
Hexahydroxy Cl-SubPc (1b)
Hexahydroxy OPh-SubPc (2b)
0.6
Hexahydroxy OF5Ph-SubPc (3b)
Hexahydroxy Cl-SubPc (1b)
0.4
Hexahydroxy OPh-SubPc (2b)
Hexahydroxy OF5Ph-SubPc (3b)
0.2
0.0
350
400
450
500
550
600
Wavelength (nm)
650
700
750
800
Figure 24. UV-Vis absorbance (solid lines) and fluorescence (dashed lines) for
hexahydroxy-SubPcs 1b, 2b, and 3b
28
To help verify the structure of the monomers and lend insight into the SubPc polymers,
crystals of the TIPS-SubPcs were grown from supersaturated solutions. Crystals of 1a and
2a were analyzed by single crystal X-Ray diffraction, yielding tentative crystal structures
which corroborated the formation of the expected SubPcs. Unfortunately, there was
significant disorder amongst the greasy TIPS groups, so an exact structure for either could
not be definitely produced. Because the bowl portion of the SubPc could be resolved
Figure 25. Preliminary crystallographic data for hexaTIPS-OPh-SubPc (2a)
without much disorder, we believe this crystal structure can provide some insight into the
packing mode of the SubPcs in the solid state. The packing of hexa-TIPS-OPh-SubPc
shows a head-to-head packing mode with the SubPc bowls oriented in opposite directions
(see Figure 25).
29
2.3.2 Synthesis of SubPc-POPs
The SubPc POPs were synthesized by classic solvothermal synthesis in a vaccum sealed
ampule by heating in an oven for a period of days. The resulting powders were filtered and
washed with dry acetonitrile, then soaked in acetonitrile for 2 days to help leech out any
unreacted monomers. Polymers were dried on high vacuum prior to porosity analysis.
Several trials were performed to optimize the reaction conditions (see Table 1). Ratios of
polar to nonpolar solvents dioxane and mesitylene respectively were altered, with the best
being a 1:1 mixture, potentially due to the relatively low solubility of hexahydroxy SubPcs
in any but the most polar of solvents.
2.3.3 Characterization of SubPc-POPs
The SubPc-POPs were characterized by Fourier transform infrared (FT-IR) and 13C crosspolarization magic angle spinning (CP-MAS) spectroscopies. The
FT-IR spectra
of
SubPc-POP 1 and 2 both displayed the characteristic C=N stretching mode of the SubPc
monomer at 1465 and 1468 cm-1, and the B-O stretch from the axial ligands at 1046 and
1039 cm-1, respectively (Figures 38 and 39). The B-O stretching modes at 1347 and 1352
cm-1 are indicative of the boronate ester linkages for SubPc-POP 1 and 2, respectively. The
30
connectivity of the SubPc-POPs was verified by solid-state
13
C CP-MAS NMR, which
contained all of the expected resonances for the materials (Figures 42 and 43). In addition,
the incorporation of the SubPc 2b and 3b monomers was further confirmed by 1H NMR
digestion experiments (Figures 57 and 58). Thermogravimetric analysis (TGA) revealed
that SubPc-POP 1 maintains ~ 85% of its weight up to 400 °C (Figure 44). In contrast,
O
N
N
O
B
B O
O
OH
HO
R
N
HO
R
N
N
HO
B
dioxane/mesitylene
105 oC, 3 d
+
N
N
OH
HO
B
O
Cl
O
B O
F
O
F
O
O B
N
O
B
O
N
O
N
N
N
N
B
O
O
SubPc POPs
O
O
B
N
O B
O
R
N
N
B
B
N
O
O
B
O
B
R
O
O
N
O
N
N
F
O
O
R
N
N N
NBN
B
F
O
F
SubPc POP-1
R
N
N
R
N
Axial Ligands
R=
B
OH
OH
HO
N
B
OH
O
N
N
B
B
N
O
O B
O
N
N
B
B
N
N N
N BN
N
O
O
O B
B
O
N
N
SubPc POP-2
O
B
O
Figure 26. Solvothermal synthesis of SubPc-POPs
SubPc-POP 2 failed to give conclusive TGA data at high temperatures. Scanning electron
microscopy (SEM) revealed two different bulk phase morphologies for both SubPc-POPs
(see Figure 27).
31
The permanent porosities of the SubPc-POPs were evaluated by nitrogen gas adsorption
isotherms at 77 K (Fig. 3). SubPc-POP 1 and 2 both exhibit reversible type I isotherms
with a small but noticeable hysteresis. Application of the Brunauer-Emmett-Teller (BET)
model over the low-pressure region (0.001-0.005 < P/P0 < 0.13-0.20) provided surface
areas of 231 and 93 m2 g-1 for SubPc-POP 1 and 2, respectively (see Figure 28). Nonlocal
density functional theory (NLDFT) was used to estimate the pore size distributions of
a.
b.
c.
d.
Figure 27. Scanning electron microscopy (SEM) images of SubPc-POP 1 (a and b) and
SubPc-POP 2 (c and d)
32
A
100
90
Quantity Adsorbed (cm³/g)
80
70
60
50
40
30
20
10
0
0
B
0.1
0.2
0.3
0.4
0.5
0.6
Relative Pressure (P/P°)
C
0.05
0.7
0.8
0.9
1
0.08
0.07
0.06
dV (cm³/g)
dV (cm³/g)
0.04
0.03
0.02
0.05
0.04
0.03
0.02
0.01
0.01
0
0
1
2
3
Relative Pressure (p/p°)
1
4
2
3
4
Relative Pressure (p/p°)
Figure 28. Porosity data for SubPc-POPs. A) N2 adsorption isotherms for SubPc-POP 1
(pink) and SubPc-POP 2 (purple), B) pore size distribution for SubPc-POP 1 and C) pore
size distribution for SubPc-POP 2
33
SubPc-POP 1 and 2 yielding values of 1.7 and 1.5 nm, respectively, which is indicative
of the microporosity of the materials. The total pore volumes were calculated from the
single point value of P/P0 = 0.90 to provide values of 0.131 and 0.087 cm3 g-1 for SubPcPOP 1 and 2, respectively. The pore size data showed that the measured pore size was
smaller than the theoretical value of the ideal hexagonal-pored structure. The small pore
size in combination with low surface areas indicated that the SubPc-POPs were potentially
interpenetrated, and not creating the hexagonal columnar stacked structure we originally
envisioned. When SubPc-POP powder was analyzed by PXRD, the results showed no
crystallinity to the POP material, confirming the SubPc POPs to be an amorphous polymer
with no long or short-range order (see Figures 41 and 42). Initially, we anticipated that a
SubPc-POP substituted with chlorine atoms in the axial position would exhibit a concaveto-ligand solid-state packing motif similar to the trans binuclear SubPc systems previously
reported by Kobayashi and Durfee.109 However, these polymeric systems could not
constructed due to the chemical instability of 1b. As a consequence, we believe the larger
axial phenoxy and pentafluorophenoxy substitutents of 2b and 3b prevent the monomers
from forming the concave-to-ligand stacking interactions and instead lead to the formation
of disordered POPs. We believe this is supported by the crystal data showing the packing
of 2a (see Figure 25).
34
The SubPc-POPs optoelectronic properties were examined using solid-state UV-Vis and
fluorescence spectroscopy. The solid state UV-Vis spectrum revealed a similar absorption
to the monomeric SubPcs, with a significant red shift (~50-60 nm), which may be due to
disordered J-aggregates in the solid state (see Figure 29). The solids exhibited no solidstate fluorescence upon irradiation with a long wave UV lamp when observed with the
naked eye, and the solid-state fluorescence spectrum showed essentially no emission after
irradiation at 580 nm. We believe that the disordered packing of the material leads to
aggregation-caused quenching (ACQ) due to the rapid thermal decay of the photoexcited
state.110
6
5
F(R)
4
3
2
1
0
200
400
600
800
1000
Wavelength (nm)
1200
1400
Figure 29. Kubelka-Munk function diffuse reflectance spectra for SubPc-POP 1 (pink) and
SubPc-POP 2 (purple)
35
2.4 Potential use in optoelectronics
Despite the non-emissive character of the SubPc-POPs, we hoped to investigate the
possibility of employing the SubPc-POPs as a donor type material for photovoltaic
A
6
5
4
3
D
700
SubPc (1a)
600
4 eq.
8 eq.
500
8 eq.
16 eq.
400
16 eq.
32 eq.
2
64 eq.
200
128 eq.
128 eq.
100
256 eq.
B
32 eq.
300
64 eq.
1
800
4 eq.
SubPc (1a)
0
6
256 eq.
0
600
SubPc (2a)
5
E
SubPc (2a)
500
4
4 eq.
8 eq.
Intensity (a.u.)
Absorbance (a.u.)
4 eq.
16 eq.
3
32 eq.
2
64 eq.
128 eq.
1
C
SubPc (3a)
5
F
4 eq.
8 eq.
4
2
1
400
500
600
Wavelength (nm)
700
32 eq.
200
64 eq.
128 eq.
256 eq.
0
800
700
SubPc (3a)
600
4 eq.
32 eq.
400
64 eq.
300
128 eq.
200
256 eq.
100
0
300
16 eq.
8 eq.
500
16 eq.
3
8 eq.
300
100
256 eq.
0
6
400
16 eq.
32 eq.
64 eq.
128 eq.
256 eq.
0
800
500
550
600
650
700
Wavelength (nm)
750
800
Figure 30. Solution-state UV-Vis absorbance for A) 1a, B) 2a, and C) 3a, and fluorescence
spectra for D) 1a, E) 2a, and F) 3a upon titration with excess C60
devices. We first examined this possibility by combining the monomeric SubPcs with C60,
a well-studied electron acceptor, in solution state titrations to determine if charge transfer
36
100
B
90
0.1
0.09
80
0.08
70
0.07
60
0.06
dV (cm³/g)
Quantity Adsorbed (cm³/g)
A
50
40
0.05
0.04
30
0.03
20
0.02
10
0.01
0
0
0
0.2
0.4
0.6
Relative Pressure (P/P°)
0.8
1
1
2
3
Pore Width (nm)
4
Figure 31. A) N2 adsorption isotherms and B) pore size distributions for SubPc-POP 1
before (pink) and after (black) doping with C60
between the SubPc and C60 could occur. The three TIPS-protected SubPcs were dissolved
in toluene and titrated with increasing equivalents of C60. All three monomeric SubPcs
showed the same trend: Upon increasing concentration of C60, the absorption bands of the
SubPc increased in intensity, and the fluorescence of the SubPc decreased in intensity until
becoming fully quenched at 256 equivalents (see Figure 30). This trend was also visible by
the naked eye; solutions with more C60 present appeared darker purple in color, and
exhibited less fluorescence than the neat SubPc solution. Absent from these titration studies
however, was the presence of a new band in the UV-Vis absorbance spectra, which might
indicate the occurrence of charge transfer. Since this was not observed, we conclude that
the SubPcs in solution were not undergoing charge transfer to the C60. Though quenching
of fluorescence was observed, this may consist of an energy transfer between the SubPc
37
and the C60, which has been previously observed for similarly donor-substituted SubPcs in
solution with C60.108 To confirm these results, and act as simple proof of concept, the best
performing SubPc-POP was doped with C60 and the doped polymer’s properties were
examined. The SubPc-POP was dried on high vacuum and analyzed by N2 adsorption
5
4.5
4
3.5
F(R)
3
2.5
2
1.5
1
0.5
0
200
400
600
800
1000
Wavelength (nm)
1200
1400
Figure 32. Kubelka-Munk diffuse reflectance spectra for SubPc-POP 1 before (pink) and
after (black) doping with C60
isotherms as usual. The POP powder was then transferred to a small flask and submerged
in a 5M solution of C60 in dry toluene and allowed to stir in the Ar glovebox overnight.
The doped polymer was filtered and gently rinsed with dry toluene. The doped polymer
was further dried under high vacuum and the porosity was reanalyzed. The porosity of the
doped polymer followed the same type I isotherm, but the overall porosity was much lower38
indicating that C60 is likely occupying the pores of the POP (see Figure 31). This was
difficult to characterize by NMR hydrolysis experiments, owing to the very different
solubilities of the polar POP monomers, and the very non- polar C60. The doped POP was
compared to the pre-doped POP by solid state UV-Vis to determine if charge transfer could
happen between SubPcs in the polymer, and C60 guests within the pores. After doping with
C60 the solid state UV-Vis absorbance was exactly the same as the pre-doped POP,
indicating that no charge transfer was occurring between the POP and the C60 guest (see
Figure 32). Inducing charge transfer between this particular SubPc and C60 seemed
challenging, thus no further studies were performed to this end.
2.5 Experimental section
2.5.1 Instrumentation and methods
Unless stated otherwise all reagents were purchased from commercial sources and used
without further purification. Dioxane, mesitylene, 3-pentanone, and acetonitrile were
distilled over CaH2. Infrared spectra were recorded on a Thermo Scientific Nicolet iS5 with
an iD7 diamond ATR attachment and are uncorrected. UV-Vis absorbance spectra were
recorded on a Cary 5000 UV-Vis/NIR spectrophotometer using an internal DRA with stock
powder cell holder to record the % reflectance spectra. Emission spectra were recorded on
a Cary Eclipse Fluorescence spectrophotometer equipped with a xenon flash lamp. Surface
area measurements were conducted on a Micromeritics ASAP 2020 Surface Area and
39
Porosity Analyzer using ca. 15 mg samples. Nitrogen isotherms were generated by
incremental exposure to ultra high purity nitrogen up to ca. 1 atm in a liquid nitrogen (77
K) bath. Surface parameters were determined using BET adsorption models in the
instrument software. Pore size distributions were determined using the non-local density
functional theory (NLDFT) model (cylinder pore, N2-cylindrical pores-oxide surface with
high regularization) in the instrument software (Micromeritics ASAP 2020 V4.02). 1H
NMR spectra were recorded in deuterated solvents on a Bruker Avance DPX 400 (400
MHz). Chemical shits are reported in parts per million (ppm, δ) using the solvent as the
internal standard.
13
C NMR spectra were recorded on a Bruker Avance DPX 400 (100
MHz) using the solvent as an internal standard.
2.5.2 Synthetic methods
Compounds 5-9 have been synthesized previously by Torres et al.106
Synthesis of 4,5-bis(triisopropylsilyloxy)phthalonitrile (4)
To a dry flask was added 5 (1.5 g, 9.36 mmol), imidazole (3.8 g, 56.2 mmol, 6 eq), DMAP
(0.034 g, 0.28 mmol, 0.03 eq), and TIPSCl (8 mL, 7.22 g, 37.44 mmol, 4 eq). The mixture
was degassed with N2 while dry DMF (30 mL, 0.3 M) added via syringe. The mixture was
further degassed with N2 before submerging in a 55˚C oil bath. The mixture was allowed
to react for 24 h under N2. After cooling, the mixture was quenched by pouring into
40
saturated NH4Cl solution. Product was extracted into ether, dried over Na2SO4 and reduced
on the rotovap to a yellow oil. Overnight the oil yields needle shaped crystals, which were
swamped with MeOH and cooled in the fridge before filtering, washing with additional
MeOH. Product isolated in 82% yield as white needly solids. 1H NMR (400 MHz CDCl3)
δ = 7.10 (s, 2H), 1.31 (m, 6H), 1.11 (d, 36H)
13
C NMR (100 MHz CDCl3) δ = 151.72,
123.77, 115.78, 108.29, 17.76, 13.04. HRMS (ESI-MS) m/z calculated for C26H44N2O2Si2
[M+Na]+, 495.2834 Found 495.2835. IR (FT-IR, ATR): 2946, 2867, 2229, 1513, 1342 cm1
. Melting point: 159-160 ˚C.
Synthesis of 1a
To a dry 2 neck flask was added 4 (0.5 g, 1.05 mmol) and the flask was fitted with reflux
condenser and septa. The apparatus was degassed with N2. BCl3 (1.05 mL, 1.05 mmol, 1M
solution in p-xylenes) was added to the solids at room temperature under 1 ATM N2. The
mixture was submerged in a 150˚C oil bath and allowed to react for 1 h. After cooling,
solvents were removed via hivac. Product was extracted from the crude solids by grinding
in a mortar and pestle, then washing with hexanes. The red filtrate was reduced on the
rotovap. Product was crashed out from hot DCM with acetonitrile. Solids filtered to yield
the desired product in 16% yield as dark purple solids. 1H NMR (400 MHz CDCl3) δ =
8.16 (s, 6H), 1.51 (m, 18H), 1.19 (t, 108H) 13C NMR (100 MHz CDCl3) δ = 151.14, 149.97,
125.04, 110.86, 17.99, 13.26. HRMS (ICR MALDI) m/z calculated for C78H132BClN6O6Si6
41
[M]+ 1462.82 Found 1463.86. IR (FT-IR, ATR): 2938, 2862, 1457, 1145 cm-1. Melting
point: 271 ˚C
Synthesis of 1b
CsF (0.429 g, 2.83 mmol, 18 eq) was added to a dried flask and held under N2. Solutions
of 1a (0.23 g, 0.157 mmol) were created in acetone (~0.007 M), then added to the CsF via
syringe. The mixture was degassed with N2 briefly before heating in a 40˚C oil bath
overnight (~16 h). The mixture was further cooled in an ice bath before quenching with
TFA (20 eq) and diluting with water. Volatile solvents removed via rotovap and solids
were isolated via filtration, washing with water, DCM and hot hexanes. Product isolated in
68% yield as plum-colored solids. 1H NMR (400 MHz D6 acetone) δ = 9.03 (s, 6H), 8.13,
(s, 6 H) 13C NMR (100 MHz D6-acetone) δ = 149.94, 149.18, 124.17, 106.71. HRMS (ESIMS) m/z calculated for C24H12BN6O6 [M-Cl]+ 491.09 Found 491.09. IR (FT-IR, ATR):
3053, 1469, 1128, 1043 cm-1. Melting point: 266 ˚C (decomposed without melting)
Synthesis of 2a
To a dry 3 neck flask was added 4 (0.5 g, 1.05 mmol), one neck fitted with an addition
funnel, one with reflux condenser, and the third a septa. The addition funnel was loaded
with phenol (0.98 g 10.5 mmol, 10 eq), and o-xylenes (10 mL, 1.05 M). The apparatus was
capped and degassed with N2. BCl3 (1.05 mL, 1.05 mmol, 1M solution in p-xylenes) was
42
added to the solids at room temperature under 1 ATM N2. The mixture was submerged in
a 150˚C oil bath and allowed to react for 1 h before the phenol solution was added slowly
to the hot mixture. The mixture was further heated while shielded from light overnight
(~16h). After cooling, high boiling solvents were removed on rotovap and the dark residue
was further dried on hivac. Product extracted from the crude solids by grinding in a mortar
and pestle, then washing with hexanes. The red filtrate was reduced on rotovap, and the
resulting residue was crashed out from minimal hot DCM with rt MeCN. The suspension
was cooled in the fridge several hours before filtering to isolate solids. Product was isolated
in 18% yield as a reddish-pink solid. 1H NMR (400 MHz CDCl3) δ = 8.09 (s, 6H), 6.77 (t,
2H), 6.58 (t, 1H) 5.42 (d, 2H) 1.49 (m, 18H) 6.58 (t, 108H) 13C NMR (100 MHz CDCl3) δ
=152.30, 150.67, 149.11, 127.74, 124.01, 119.62, 117.66, 109.74, 16.98, 12.24. MS
(MALDI) m/z calculated for C84H137BN6O7Si6 [M]+, 1522.35 Found 1522.11. IR (FT-IR,
ATR): 2944, 2866, 1463, 1146 cm-1. Melting point: 271 ˚C (decomposed without melting)
Synthesis of 2b
To a dry flask was added CsF (0.52 g, 3.43 mmol, 18 eq). The flask was capped and
degassed with N2. Solutions of 2a (0.29 g, 0.19 mmol) in 5:1 acetone: hexanes (~0.007 M)
were added to the flask via syringe under degassing conditions. The mixture was heated to
45˚C under 1 ATM N2 and allowed to react in the dark overnight (~16h). After cooling,
low boiling solvents were removed via rotovap. The mixture was further diluted with water,
43
and a few drops of 1M HCl were added to protonate. The mixture was filtered to isolate
solids, which were further washed with water, DCM, and hot hexanes. Product was isolated
in 70% yield as plum-colored solids. 1H NMR (400 MHz D6-acetone) δ = 9.09 (s, 6H),
8.16 (s, 6H), 6.74 (t, 2H), 6.56 (t, 2H), 5.33 (d, 2H) 13C NMR (100 MHz D6-acetone) δ =
153.58, 150.67, 148.46, 128.58, 125.02, 120.64, 118.97, 106.30. MS (MALDI) m/z
calculated for C30H17BN6O7 [M]+, 584.30 Found 584.14. IR (FT-IR, ATR): 3196, 2946,
1474, 1143 cm-1. Melting point: 254 ˚C (decomposed without melting)
Synthesis of 3a
To a dry 3 neck flask was added 4 (0.5 g, 1.05 mmol), one neck fitted with an addition
funnel, one with reflux condenser, and the third a septa. The addition funnel was loaded
with pentafluorophenol (1.93 g, 10.5 mmol, 10 eq) and o-xylenes (10 mL, 1.05 M). The
apparatus was capped and degassed with N2. BCl3 (1M solution in p-xylenes) was added
to the solids at room temperature under 1 atm N2. The mixture was submerged in a 150˚C
oil bath and allowed to react for 1 h before the phenol solution was added slowly to the hot
mixture. The mixture was further heated while shielded from light 2h. After cooling, high
boiling solvents were removed on rotovap and the dark residue was further dried on hivac.
Product extracted from the crude solids by grinding in a mortar and pestle, then washing
with hexanes. The red filtrate was reduced on rotovap, and the resulting residue was
crashed out from minimal hot DCM with rt MeCN. The suspension was cooled in the fridge
44
several hours before filtering to isolate solids. Product was isolated in 28% yield as dark
colored solids. 1H NMR (400 MHz CDCl3) δ = 8.10 (s, 6H), 1.49 (m, 18H), 1.17 (m, 108H)
13
C NMR (100 MHz CDCl3) δ = 151.28, 150.39, 124.96, 110.73, 17.99, 13.22. MS
(MALDI) m/z calculated for C84H132BF5N6O7Si6 [M]+, 1612.30 Found 1611.05. IR (FTIR, ATR): 2945, 2868, 1467, 1149 cm-1. Melting point: 284 ˚C (decomposed without
melting)
Synthesis of 3b
To a dry flask was added CsF (0.339 g, 2.23 mmol, 18 eq). The flask was capped and
degassed with N2. Solutions of 3a (0.2 g, 0.124 mmol) in 5:1 acetone: hexanes (~0.007 M)
were added to the flask via syringe under degassing conditions. The mixture was heated to
45˚C under 1 atm N2 and allowed to react in the dark overnight (~16h). After cooling, low
boiling solvents were removed via rotovap. The mixture was further diluted with water,
and a few drops of 1M HCl were added to protonate. The mixture was filtered to isolate
solids, which were further washed with water, DCM, and hot hexanes. Product was isolated
in 84% yield as plum-colored solids. 1H NMR (400 MHz D6-acetone) δ = 9.08 (br s, 6H),
8.08 (s, 6H) 13C NMR (100 MHz D6-DMSO) δ = 169.85, 151.33, 150.19, 149.81, 125.19.
124.25, 109.82, 106.88. MS (MALDI) m/z calculated for C30H12BF5N6O7 [M]+, 674.26
Found 674.07. IR (FT-IR, ATR): 3028, 1463, 1275, 1133, 986 cm-1. Melting point: 239 ˚C
(Decomposed without melting)
45
SubPc POP synthesis (SubPc-POP 1, 2)
To a dry ampule was added 1b, 2b, or 3b (0.03 mmol) and BDBA (0.05 mmol, 1.5 eq).
Solvent mixture (0.05 M) was added, and the mixture was sonicated for 1 minute. Mixture
was freeze-pump-thawed for 3 cycles before ampule was sealed under vacuum. The sealed
ampule was placed in a closed oven for 3 d. POP powder was isolated by filtration. Powder
was suspended in dry acetonitrile to remove residual monomers over a period of 2 days,
periodically changing out the acetonitrile. Powder was dried on high vacuum ~8 h then
degassed 12 h at 75 °C on the porosity analyzer. SubPc-POP 1: 17 mg, 67% yield. SubPcPOP-2: 17.4 mg, 58% yield.
46
Table 1. Summary of screening of SubPc-POP reaction conditions
Rax=
Cl
OPh
(POP-1)
OF5Ph
(POP-2)
Eq
SubPc
1
Eq
BDBA
1.5
Solvents
Time
Temp
Porosity
2:1 Diox:Mes
72 h
105 °C
Nonporous
1
1.5
1:1 Diox:Mes
72 h
105 °C
Nonporous
1
1.5
1:1 Diox:Mes
72 h
105 °C
231 m /g
1
1.5
1:2 Diox:Mes
72 h
120 °C
123 m /g
1
1.5
1:2 Diox:Mes
72 h
95 °C
160 m /g
1
1.5
1:1 Diox:Mes
72 h
120 °C
103 m /g
1
1.5
1:1 Pent:Mes
72 h
105 °C
66 m /g
1
1.5
1:2 Pent:Mes
72 h
105 °C
93 m /g
1
1.5
1:1 Pent:Mes
72 h
105 °C
24 m /g
47
2
2
2
2
2
2
2
2.5.3 Infrared spectroscopy
4000
3500
3000
2500
2000
1500
1000
500
1500
1000
500
Wavenumber (cm-1)
Figure 33. FT-IR spectrum of 1a
4000
3500
3000
2500
2000
Wavenumbers (cm-1)
Figure 34. FT-IR spectrum of 1b
48
4000
3500
3000
2500
2000
1500
1000
500
1500
1000
500
Wavenumbers (cm-1)
Figure 35. FT-IR spectrum of 2a
4000
3500
3000
2500
2000
Wavenumbers (cm-1)
Figure 36. FT-IR spectrum of 2b
49
4000
3500
3000
2500
2000
1500
1000
500
Wavenumbers (cm-1)
Figure 37. FT-IR spectrum of 3a
4000
3500
3000
2500
2000
Wavenumbers (cm-1)
Figure 38. FT-IR spectrum of 3b
50
1500
1000
500
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1)
Figure 39. Stacked FT-IR spectra of SubPc-POP 1 (black, top), hexahydroxy-OPh-SubPc
(2b) (pink, middle), and BDBA (green, bottom)
Table 2. Resonances and assignments for SubPc-POP 1
Resonance
Assignment
1465 cm-1
C=N
-1
1347 cm
B-O (boronate ester)
-1
1046 cm
B-O (axial ligand)
51
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1)
Figure 40. Stacked FT-IR spectra of SubPc-POP 2 (black, top), hexahydroxy-OF5PhSubPc (3b) (purple, middle), BDBA (green, bottom)
Table 3. Resonances and assignments for SubPc-POP 2
Resonance
Assignment
1468 cm-1
C=N
-1
1352 cm
B-O (boronate ester)
-1
1039 cm
B-O (axial ligand)
52
2.5.4 PXRD profiles
3500
3000
2500
2000
1500
1000
500
0
0
5
10
15
20
25
2Θ
Figure 41. PXRD profile of SubPc-POP 1
18000
16000
14000
12000
10000
8000
6000
4000
2000
0
0
5
10
15
20
2Θ
Figure 42. PXRD profile of SubPc-POP 2
53
25
2.5.5 Solid-state NMR spectra
Figure 43. 13C CP-MAS of SubPc POP 1
54
Figure 44. 13C CP-MAS of SubPc-POP 2
55
2.5.6 TGA profile
100
90
80
% Weight lost
70
60
50
40
30
20
10
0
0
100
200
300
400
500
Temperature (°C)
Figure 45. TGA profile of SubPc-POP 1
56
600
700
800
2.5.7 Surface area analysis
60
Q (1 - p°) (cm³/g STP)
50
40
30
20
10
0
0
0.2
0.4
0.6
Relative Pressure (p/p°)
0.8
1
Figure 46. Roquerol BET analysis for SubPc-POP 1
Table 4. BET values derived from Roquerol BET analysis for SubPc-POP 1
(P/P°)
0.005939-0.139089
0.005939-0.106749
0.005939-0.084445
BET (m
231
231
229
2
SubPc-POP 1
/g) Correlation Coefficient
0.9999
0.9998
0.9997
57
C
321.478
320.559
339.447
0.0025
1/[Q(p°/p - 1)]
0.002
0.0015
y = 0.0187x + 6E-05
R² = 0.99971
C=321.478
0.001
0.0005
0
0
0.02
0.04
0.06
0.08
Relative Pressure (p/p°)
Figure 47. BET surface area plot for SubPc-POP 1
58
0.1
0.12
25
Q (1 - p°) (cm³/g STP)
20
15
10
5
0
0
0.2
0.4
0.6
Relative Pressure (p/p˚)
0.8
1
Figure 48. Roquerol BET analysis of SubPc-POP 2
Table 5. BET values derived from Roquerol BET analysis for SubPc-POP 2
(P/P°)
0.001086-0.200654
0.001086-0.167247
0.001086-0.133936
SubPc-POP 2
BET (m2/g)
Correlation coefficient
93
0.9988
93
0.9981
91
0.9977
59
C
136.591
139.102
170.091
0.012
0.01
1/[Q(p°/p - 1)]
0.008
0.006
y = 0.0461x + 0.0003
R² = 0.99773
C=136.591
0.004
0.002
0
0
0.05
0.1
0.15
Relative Pressure (p/p˚)
Figure 49. BET surface area plot of SubPc-POP 2
60
0.2
0.25
2.5.8 UV-Vis and fluorescence
Normalized Absorbance and Intensity
1.2
1
0.8
1a Normalized
Absorbance
1a Normalized
Fluorescence
0.6
0.4
0.2
0
400
450
500
550
600
650
Wavelength (nm)
700
750
800
Figure 50. UV-Vis and fluorescence spectra of 1a in toluene (λexcitation= 574 nm)
Normalized Absorbance and Intensity
1.2
1
0.8
1b Normalized
Absorbance
1b Normalized
Fluorescence
0.6
0.4
0.2
0
250
350
450
550
Wavelength (nm)
650
750
Figure 51. UV-Vis and fluorescence spectra of 1b in acetone (λexcitation= 564 nm)
61
Normalized Absorbance and Intensity
1.2
1
0.8
2a Normalized
Absorbance
0.6
2a Normalized
Fluorescence
0.4
0.2
0
200
300
400
500
600
Wavelength (nm)
700
800
Figure 52. UV-Vis and fluorescence spectra of 2a in toluene (λexcitation= 522 nm)
Normalized Absorbance and Intensity
1.2
1
0.8
2b Normalized
Absorbance
2b Normalized
Fluorescence
0.6
0.4
0.2
0
250
300
350
400
450
500
550
600
Wavelength (nm)
650
700
750
800
Figure 53. UV-Vis and fluorescence spectra of 2b in acetone (λexcitation= 561 nm)
62
Normalized Absorbance and Intensity
1.2
1
0.8
0.6
3a Normalized Absorbance
3a Normalized Fluorescence
0.4
0.2
0
325
375
425
475
525
575
625
Wavelength (nm)
675
725
775
Figure 54. UV-Vis and fluorescence spectra of 3a in toluene (λexcitation= 578 nm)
Normalized Absorbance and Intensity
1.2
1
0.8
3b Normalized Absorbance
0.6
3b Normalized Fluorescence
0.4
0.2
0
325
375
425
475
525
575
625
Wavelength (nm)
675
725
775
Figure 55. UV-Vis and fluorescence spectra of 3b in acetone (λexcitation= 570 nm)
63
6
5
F(R)
4
3
2
1
0
200
400
600
800
1000
Wavelength (nm)
1200
1400
Figure 56. Kubelka-Munk function diffuse reflectance spectrum of 2b (pink) and SubPc
POP 1 (black)
6
5
F(R)
4
3
2
1
0
200
400
600
800
1000
Wavelength (nm)
1200
1400
Figure 57. Kubelka-Munk function diffuse reflectance spectrum of 3b (purple) and
SubPc-POP 2 (black)
64
2.5.9 SubPc-POP hydrolysis 1H NMR
A small sample (~5 mg) of POP powder was hydrolyzed by addition of D2O and the
monomers were redissolved in deuterated acetone. Samples were sonicated briefly to help
solubilize monomers. This technique is used to confirm the presence of both monomers in
the polymer.
Figure 58. 1H NMR spectrum of SubPc-POP 1 after hydrolysis
65
Figure 59. 1H NMR spectrum of SubPc-POP 2 after hydrolysis
66
2.5.10 1H and 13C NMR spectra
Figure 60. 1H NMR spectrum of 4
67
Figure 61. 13C NMR spectrum of 4
68
Figure 62. 1H NMR spectrum of 1a
69
Figure 63. 13C NMR spectrum 1a
70
Figure 64. 1H NMR spectrum of 1b
71
Figure 65. 13C NMR spectrum of 1b
72
Figure 66. 1H NMR spectrum of 2a
73
Figure 67. 13C NMR spectrum of 2a
74
Figure 68. 1H NMR spectrum of 2b
75
Figure 69. 13C NMR spectrum of 2b
76
Figure 70. 1H NMR spectrum of 3a
77
Figure 71. 13C NMR spectrum of 3a
78
Figure 72. 1H NMR spectrum of 3b
79
Figure 73. 13C NMR spectrum of 3b
80
Chapter 3: Benzoxazole formation studies
Portions of this chapter are adapted from the following publications:
Pyles, D. A.; Coldren, W. H.; Eder, G. M.; Hadad, C. M.; McGrier, P. L.
Mechanistic
investigations
into
the
cyclization
and
crystallization
of
benzobisoxazole-linked two-dimensional covalent organic frameworks. Submitted
Used with permission.
3.1 Benzoxazole background
Oxazoles are aromatic 5-membered heterocycles incorporating an oxygen atom and a
nitrogen atom (see Figure 74). Oxazoles are found frequently as ligands in catalysis, and
are a functional group often appended to molecules of interest in drug discovery. Oxazoles
are most widely used when fused to a benzene ring to form benzoxazoles, which have been
extensively studied in small molecule and polymeric materials applications.
Benzoxazoles were entirely
unknown
Figure 74. Structure of oxazole and its benzo-fused
derivatives
in
the
porous
material literature until the first
report by Yavuz et. al. in 2014
showing that a silyl-protected bisbenzoxazole derivative could be combined with an acyl
81
O
H
TBS N
H
N TBS
TBS O
O TBS
+
Cl
Cl
O
N
N
O
O
5-25˚C
O
Cl
O
O
N
H
N
O
TBS
O
O
TBS
1. Thin film
casting
COP-93
N
H
N
COP-94
2. Annealing
400˚C
N
N
O
O
O
Figure 75. Synthesis of COP-93 and COP-94 by thermal annealing of amide-linked
prepolymers
chloride to form a pre-polymer, then annealed at 400˚C to yield a covalent organic polymer
(BOX-COP) with high CO2 uptake111 (see Figure 75). Their good CO2 uptake is a valuable
asset; unfortunately the method of annealing at high temperatures did not lend itself to a
wide variety of functional monomers. As such, there were no reports of new
benzobisoxazole (BBO) porous polymers until work in the McGrier Group produced 2 new
BBO polymers that were not only porous, but also crystalline, allowing them to be
catergorized as COFs112 (see Figure 76). The McGrier method for synthesizing BBO-COFs
employed a much milder method that lends itself better to the reversible formation of
ordered systems. Shortly thereafter the El-Kaderi group published a report of BBO linked
porous polymers using a similar synthetic method and extending the methodology from 2dimensional polymers up to 3-dimensional polymers, as well as the analogous version of
82
Figure 76. Synthesis of BBO-COF 1 and 2
the polymer using sulfur in place of oxygen to yield benzobisthiazole (BBT) linked POPs113
(see Figure 77). Very recently, Wei Wang’s group reported a series of BBO-linked COFs
with high stability and photocatalytic properties.114 This increased stability of a COF
linkage is incredibly important due to increasing interest in using COFs and porous
materials for applications in devices or chemical catalysis. In this area the BBO linkage
surpasses many others; It is very stable to water, even at 100˚C for 24h.112 Additionally
BBO-linked COFs have been shown to be stable even to 9M acid or base. 114 In order for
this relatively new linkage to gain traction in the world of catalysis, it would be beneficial
to understand this linkage in depth in order to bypass the tedious task of screening
numerous reactions conditions (e.g., solvent concentrations, reaction times, temperature,
etc.) to produce a desired framework, but also to establish a rational experimental protocol
83
for generating high quality COF structures with improved efficiency and controlled
morphologies to help advance the field of porous materials in heterogeneous catalysis.
Figure 77. Synthesis of BOLPs (where X=O) and BTLPs (where X=S)
3.2 Proposed mechanisms for the formation of benzoxazoles
The most commonly accepted mechanistic pathway for the formation of benzoxazoles has
been based on a 3-step, 2 electron pathway (see Figure 78). First, condensation of a free
amine with an aldehyde reversibly affords an imine, as is commonly employed in COF
synthesis. In a second step, the hydroxyl group ortho to the imine nitrogen can perform an
intramolecular nucleophilic attack on the imine carbon creating the basis of the 5membered ring. In a third and final step, oxidation by metals115 or strong oxidants
transforms the benzoxazoline intermediate to the benzoxazole. In 2012, Cheon et. al.
proposed a catalytic version of this synthesis which employed a strong nucleophile to
initiate the nucleophilic attack of the imine carbon, then be displaced by the attack of the
ortho hydroxyl group followed by oxidation by ambient air.116 This is postulated to occur
84
by a 5-exo-tet cyclization following Baldwin’s rules,117 whereas the direct attack of the
imine carbon by oxygen would represent a 5-endo-trig cyclization which is forbidden by
Baldwin’s rules. Cheon’s group also showed that cyanide was the most effective
nucleophile to affect this transformation, whereas other strong alkoxide or thioxide
O
NH2
N
Ph
OH
Ph
Nuc
OH
H
N
O
N
Ph
H
N
[O]
Ph
Nuc
Nuc
Ph
O
O
Figure 78. Synthesis of benxozazole by Cheon's proposed 2 electron pathway
employing a nucleophilic catalyst
nucleophiles were unable to catalyze the reaction.118 In 2016 Yu et. al. put forward an
extensive computational study on the formation of benzoxazoles. This work showed that
after the initial imine formation and nucleophilic attack of cyanide, a direct nucleophilic
attack of alkoxide anion to displace the cyanide nucleophile represented an extremely high
energy transition state of 31.1 kcal/mol119 (see Figure 79) The authors further postulated
that at this point aerobic oxygen could instead homolytically remove the proton at the
newly formed SP3 carbon at a much lower 6.9 kcal/mol. They hypothesize that this radical
85
Figure 79. Calculated energy levels of intermediates and transiton states for radical
oxidative dehydrogenation and cyclization to form a benzoxazole reprinted with
permission from J. Org. Chem., 2016, 81, 10857-10862. Copyright 2016 American
Chemical Society
86
could be stabilized by the captodative effect from electron withdrawing adjacent cyanide
group. The captodative effect can occur when adjacent electron donor and electron acceptor
substituents stabilize a radical
center by resonance.120 The
Figure 80. Examples of radicals stabilized by the
captodative effect
concept of donor-acceptor (DA) resonance stabilization is
not new, and is frequently employed in the fields of polymers121 and organic devices,122
However, mechanistic studies detailing how the captodative effect and careful selection of
organic linkers can support the nucleation and growth of ordered 2D polymeric systems
has yet to be reported.
3.3 Small molecule analog studies
3.3.1 p-tolylbenzoxaole
Intrigued by the postulations set forth by Cheon et. al. and Yu et. al. we set out to investigate
the ring closure reaction of benzoxazoles experimentally. In order to test these hypotheses,
we decided to screen a variety of nucleophiles that were not tested by Cheon et. al.
including nucleophiles
capable
of
the
captodative effect, to
Figure 81. Synthesis of p-tolylbenzoxazole
determine
87
if
the
captodative effect could in fact be responsible for the excellent performance of cyanide as
a catalyst in this reaction. Our small molecule studies began with p-tolylbenzoxazole (see
Figure 81) as the target for this small molecule study in order to screen nucleophiles using
commercially available substrates. Our investigation employed a variety of strong
NaCN
NaSCH3
NaN3
NaI
DMAP
HCl
NaF
MeOH
None
H2O
8.8
8.6
8.4
8.2
8.0
7.8
7.6
7.4
7.2
7.0
6.8
6.6
6.4
6.2
ppm
Figure 82. Stacked 1H NMR spectra of the crude reaction forming p-tolylbenzoxazole. The
red band indicates the presence of the desired oxazole product, and the blue band indicates
the presence of the undesired imine linked product
nucleophiles including 2 capable of the captodative effect, NaCN and NaN3. We also tested
some of the reaction parameters to see if they had any effect on the reaction outcome, such
88
as including methanol with no catalyst, and creating the acid salt of 2-aminophenol to
mimic the dihydrochloride salt precursor to the BBO linkage (see Table 6). We evaluated
the efficacy of these nucleophiles by the ratio of peaks present in the 1H NMR (see Figure
82).
Table 6. Ratio of p-tolylbenzoxazole product to imine-linked product based on 1H NMR
integration
Additive
1 Eq CN
200 µL MeOH
4 µL H2O
Ratio Ox: Imine
1:0
1 : 12
1 : 16
NONE
1 : 13
1 Eq NaI
1 : 2.8
1 Eq DMAP
1 Eq NaF
1 Eq NaN3
1 : 3.8
1 : 8.4
1: 0.47
1 Eq NaSCH3
1 : 0.41
Aminophenol HCl Salt
1 : 4.1
We found that NaCN is still by far the most effective catalyst in this system, showing
complete conversion to the desired benzoxazole. NaN3 was also a very effective catalyst
for this system, showing good conversion to the benzoxazole with only some evidence of
residual imine product. NaSCH3 also performed fairly well in the reaction due to its strong
nucleophilicity, however it is incapable of supporting the captodative effect. Other
89
nucleophiles either gave mixtures of benzoxazole and imine, or no conversion to the
oxazole whatsoever.
3.3.2 BBO-1
These trials on p-tolylbenzoxazole gave us some interesting insights into the properties a
nucleophilic catalyst for this reaction must possess. However the electronics of ptolylbenzoxazole
differ
significantly
from
the
2,5-diamino-1,4-benzenediol
dihydrochloride (DABD) polymer precursor. We thus completed a small study on model
molecule BBO-1 (see Figure 83) using our most successful p-tolylbenzoxazole catalysts in
Figure 83. Synthesis of BBO-1 with different nucleophiles
order to get a better idea of how nucleophilic catalysts might work in the polymeric system.
Based on isolated yields of pure BBO-1, we found that NaCN still performs the best out of
the three chosen catalysts (see Table 7). Again, NaN3 also provided a good yield of BBO1. NaSCH3 was able to catalyze the reaction as well, albeit at a lower yield than the other
catalysts. Trials without the presence of a nucleophile show that background reaction can
proceed under these high temperature conditions to give a low yield of the desired product.
We believe the best performing catalysts could be able to stabilize the system by the
90
captodative effect to yield such good results. With this hypothesis, we went on to
investigate this effect in the full BBO-COF system.
Table 7. Isolated yields of BBO-1 synthesized with different nucleophiles
Nuc
% Yield (isolated)
NaCN
62%
NaN3
37%
NaSCH3
21%
No Nuc
11%
No O2
<1%
3.4 BBO-COF 3
3.4.1 Synthesis and design of BBO-COF 3
In addition to investigating the captodative effect, we were also interested in examining
what impact an electron deficient organic linker like 1,3,5-tris(4-formylphenyl)triazine
(TFPT) would have on the formation and crystallization of the BBO-linked COFs. Since
triazine-based monomers are known for exhibiting high electron mobilities (thanks to their
electron withdrawing nature)123 we hypothesized that this feature could assist with
stabilizing the radical species generated during the oxidative dehydrogenation process. In
this study, we first established the optimal cyanide-catalyzed reaction conditions for
synthesizing BBO-COF 3 using TFPT and DABD organic linkers114 (see Figure 84).
91
Afterwards, we investigated the ability of other good nucleophiles like NaN3 and NaSCH3
to initiate the formation of BBO-COF 3 and the previously synthesized BBO-COF 2. These
studies were performed in conjunction with one another to determine if the electron
withdrawing TFPT linker of BBO-COF 3 would exhibit any enhanced effect on the
formation and crystallization of the COFs in comparison to the isostructural 1,3,5-tris(4formylphenyl)benzene (TFPB) linker that was used to synthesize BBO-COF 2. We
demonstrate that NaN3 and NaSCH3 are effective at catalyzing the formation of BBOCOFs, but NaCN is the only catalyst that promotes the stabilization of radical intermediates
through the captodative effect. Interestingly, we also show that the electron withdrawing
Figure 84. Synthesis of BBO-COF 3 from DABD (blue) and TFPT (red)
92
TFPT monomer not only enhances the crystallinity of BBO-COF 3, but also plays a
significant role in stabilizing the radical intermediates generated during oxidative
dehydrogenation. These results were validated using DFT calculations, including
population and spin density analyses, along with powder X-ray diffraction (PXRD). We
expect that this work will provide a rational protocol for constructing highly ordered BBObased polymeric systems for practical applications.
BBO-COF 3 was synthesized by reacting TFPT with DABD in DMF at -15˚C for ~3h.
Afterwards, the reaction mixture was slowly warmed to room temperature overnight before
adding 1 equiv. of NaCN dissolved in 0.2 mL of methanol. The mixture was then stirred at
130˚C for four days in the presence of air. The product was obtained by filtration and
washed with acetone to afford a light brown crystalline solid. BBO-COF 3 was purified by
immersing the solids in methanol and acetone for 24 h to remove any unreacted monomers
and dried under vacuum.
3.4.2 Characterization of BBO-COF 3
Thermogravimetric analysis (TGA) revealed that BBO-COF 3 maintained more than 98%
of its weight up to 470˚C (see Figure 116). BBO-COF 3 was characterized using Fourier
transform infrared (FT-IR) and
13
C cross-polarization magic angle spinning (CP-MAS)
spectroscopic analyses. The FT-IR spectrum revealed stretching modes at 1662 (C=N) and
93
1120 cm-1 (C-O) confirming the formation of the benzoxazole ring (see Figure 100). The
TFPT linker exhibited two intense stretches at 1515 and 1360 cm-1, which correspond to
the presence of the benzene and triazine moieties respectively (see Figure 99). The triazine
ring and BBO-linkage were also confirmed by solid-state 13C CP-MAS NMR, displaying
distinct resonances at 167.7, 160.5, and 146.4 ppm (see Figure 115).
The porosity of BBO-COF 3 was evaluated using nitrogen gas adsorption measurements
at 77 K (see Figure 85). BBO-COF 3 displays a type IV isotherm exhibiting a steep uptake
at low pressure (p/p˚ < 0.06) followed by a noticeable step between p/p˚ = 0.06 and 0.22
which validates the mesoporosity of the material. The Brunauer-Emmett-Teller (BET)
method was applied over the low-pressure region (0.01 < p/p˚ < 0.22) of the isotherm to
afford a surface area of 2039 m2/g. It is worth noting that this is the highest surface area
reported to date for a benzoxazole-linked porous material.111–113 The total pore volume of
BBO-COF 3 calculated at p/p˚ = 0.989 provided a value of 1.22 cm2/g. The pore size
distribution was estimated using nonlocal density functional theory (NLDFT) to provide
an average pore size of 2.7 nm, which is close to the predicted value of 3.3 nm. In
comparison, the isostructural BBO-COF 2 exhibited an average pore size of 1.8 nm. The
enhanced pore size of BBO-COF 3 could be attributed to the planarity of the triazine TFPT,
which allows for coplanarity between the vertex and linker units (see Figures 129 and 130).
94
This is in contrast to the out-of-plane twisting of the TPFB moieties, which can hinder the
formation of vertically stacked eclipsed layers.
A
1000
900
700
600
B
500
400
dV (cm3/g)
Quantity Adsorbed (cm3/g)
800
300
200
1.4
1.2
1
0.8
0.6
0.4
0.2
0
1
100
2
3
4
Pore Width (nm)
5
0
0
0.1
0.2
0.3
0.4
0.5
0.6
Relative Pressure (p/p˚)
0.7
0.8
0.9
Figure 85. A) N2 adsorption isotherm and B) pore size distribution for BBO-COF 3
95
1
Powder X-ray diffraction (PXRD) was used to evaluate the crystallinity of BBO-COF 3
(see Figure 86). Taking into account the variations between the observed and predicted
pore size distributions, we modeled BBO-COF 3 using a P6 hexagonal unit cell wherein
the adjacent layers were offset by 6 Å. BBO-COF 3 displayed an intense peak at 2.95
followed by smaller peaks at 5.11, 5.89, 7.82, 26.2°, which correspond to the (100), (110),
(200), (210), and (001) planes, respectively. Pawley refinement of the experimental PXRD
data afforded unit cell parameters of a = b = 34.595 Å and c = 3.4 (residuals R p = 3.20%,
Rwp = 4.55%). The experimental PXRD data is consistent with the simulated hexagonal
unit cell in which the layers are slipped by 6 Å. We also considered the possibility of BBOCOF 3 forming staggered gra (P63 /mmm) stacking layers, but the simulated patterns did
not match the experimental data (see Figure 114). In contrast to the PXRD pattern of BBOCOF 2, BBO-COF 3 displays a two-fold enhancement in crystallinity (see Figure 86). We
believe this enhancement is attributed to (1) the planarity of the TFPT units allowing for
more efficient van der Waals interactions between the layers, and (2) the ability of the
electron deficient triazine units to form donor-acceptor (D-A) stacks with the adjacent
TFPT aromatic rings along the c direction. Lotsch and coworkers have also observed this
phenomenon for imine-linked COFs containing triazine moieties.124 Since DA interactions
are often more favorable than non-covalent interactions between π-electron rich aromatic
96
rings,125 this could also further explain why the offsets for BBO-COF 2 (15 Å) are more
significant compared to BBO-COF 3 (6Å).
10000
(1 0 0)
9000
8000
Intensity
7000
6000
5000
4000
3000
(1 1 0)
(2 0 0)
(2 1 0)
2000
1000
(0 0 1)
0
0
5
10
15
2θ (Degrees)
20
25
30
Figure 86. Experimental PXRD data for BBO-COF 3 (blue) compared to the Pawley
refined (red) and simulated hexagonal unit cell (green) with an offset of 6 Å and a view
along the c direction
3.5 Reaction time trials of BBO-COF 3
Before evaluating other nucleophiles as potential catalysts, we first wanted to ensure that
the optimal cyanide-catalyzed reaction conditions for constructing BBO-COF 3 were
97
established. While reaction at high temperatures for four days yielded highly ordered
crystalline materials, we were curious if that long a reaction period was really required for
the full formation of the BBO linkages. To address this issue, we monitored the growth of
BBO-COF 3 over time. After reacting for one day, the isolated solid provided only a
moderate surface area of 704 m2/g and a pore size of 1.7 nm indicating that the reaction
requires a longer reaction time (see Figures 87 and 88). The IR spectrum displayed a broad
stretch at 3343 cm-1 and a sharp stretch at 1693 cm-1, which are attributed to the OH stretch
1000
900
Quantity Adsorbed (cm3/g)
800
Four days: 2039 m2/g
700
600
Two days: 1435 m2/g
500
400
Three days: 1295 m2/g
300
200
One day: 704 m2/g
100
Overnight
0
0
0.1
0.2
0.3
0.4
0.5
0.6
Relative Pressure (p/p˚)
0.7
0.8
0.9
Figure 87. N2 adsorption isotherms for BBO-COF 3 at various points during the 4-day
reaction period
98
1
from the phenolic imine-linked intermediate and the aldehyde (C=O) stretch from the
TFPT linker, respectively (see Figure 89). Although the IR stretches for the intermediate
and starting material begin to disappear after reacting for a few additional days, the surface
area increases by two orders of magnitude to 1435 m2/g after day two, but then surprisingly
decreases to 1295 m2/g on day three. This trend is echoed in the pore size data yielding a
value of 2.7 nm on day two followed by the formation of multiple smaller pores at 2.6 and
2.1 nm on day three. Since the benzoxazole bond is irreversible, we believe that the
1.4
decreases in surface area
Normalized Pore Volume
1.2
and pore size are indicative
Before Heating
1
of
the
formation
of
1 day
0.8
0.6
2 days
crosslinked
polymeric
3 days
networks that are corrected
4 days
0.4
during the dynamic imine
0.2
exchange process prior to
the formation of the BBO
0
1
1.5
2
2.5
3
3.5
Pore Width (nm)
4
4.5
5
Figure 88. Pore size distributions for BBO-COF 3 at various
points during the 4-day reaction period
linkage.25 PXRD patterns
from the isolated solids
also indicate that a four-day reaction period is required to attain the crystalline BBO-COF
3 structure (see Figure 109). However, it is worth noting that the BBO-linkage does not
99
form without oxygen even under the optimal growth conditions (see Figures 96-98).
Before Heating
1 day
2 day
3 day
4 day
4000
3500
3000
2500
2000
Wavenumbers (cm-1 )
1500
1000
500
Figure 89. FT-IR spectra of BBO-COF 3 at various points during the 4-day reaction period
100
3.6 Nucleophile trials of BBO-COFs 2 and 3
With the optimal reaction conditions established, we examined if other nucleophiles could
be used to catalyze the formation of the BBO-COFs. Wang and Yu have shown that the
activation energy for the direct 5-exo-tet cyclization of benzoxazole in the presence of
NaCN has to overcome a high- energy barrier of 47.9 kcal/mol,119 and suggested that the
cyclization proceeds through a stepwise mechanism. DFT calculations revealed that after
the cyanide attacks the imine carbon to convert the hybridization from sp2 to sp3 tripletstate oxygen then directly abstracts a hydrogen from the newly generated sp3 α-carbon to
produce a radical that is stabilized through the captodative effect (see Figure 91).
Surprisingly, the activation energy for this process was found to be ~ 24.4 kcal/mol lower
than the suggested direct cyanide-catalyzed cyclization pathway. Intrigued by these results,
we were curious to examine if (1) BBO-COF 2 and 3 could be constructed using other good
nucleophiles like NaN3 and NaSCH3, and (2) the electron deficient TFPT linker of BBOCOF 3 would have any effect on the cyclization and crystallization of the framework
compared to BBO-COF 2, which contained the electron rich TPFB linker.
The formation of BBO-COF 2 and 3 using NaN3 and NaSCH3 as catalysts was evaluated
using nitrogen gas adsorption isotherms, IR spectroscopy, and PXRD analysis (see Figure
90). Since the direct cyclization of benzoxazoles only requires ~ 8.4 kcal/mol,119 we also
101
500
450
400
350
300
250
200
150
100
50
0
D 1000
BBO-COF 2
Quantity Adsorbed (cm3/g)
Quantity Adsorbed (cm3/g)
A
NaCN: 1106 m2g-1
NaN3: 1033 m2g-1
NaSCH3: 236 m2g-1
No catalyst: 386
0
0.2
m2g-1
0.4
0.6
0.8
Relative Pressure (p/p˚)
NaSCH3 : 1697 m2/g
NaN3 1439 m2/g
No catalyst: 386 m2/g
0.2
0.4
0.6
0.8
Relative Pressure (p/p˚)
1
E
3500
3000 2500 2000 1500
Wavenumbers (cm-1)
1000
500
4000
3500
3000
2500
2000
1500
1000
500
Wavenumbers (cm-1)
C
0
NaCN: 2039 m2/g
0
1
B
4000
BBO-COF 3
900
800
700
600
500
400
300
200
100
0
F
5
10
15
2Ө
20
25
0
5
10
15
2ϴ
20
25
Figure 90. A) and D) N2 adsorption isotherms, B) and E) FT-IR spectra, and C) and F)
PXRD data for A)-C) BBO-COF 2, and D)-F) BBO-COF 3, with different nucleophiles
NaCN (red), NaN3 (green), NaSCH3 (blue), and no catalyst (yellow)
102
monitored the formation of the BBO-COFs without the addition of a nucleophile. The
activated solids for BBO-COF 2 provided surface area values of 1033 and 236 m2/g for
NaN3 and NaSCH3, respectively, whereas the reaction without catalyst yielded a surface
area of 169 m2/g. Interestingly, NaSCH3 and NaN3 yielded pore sizes that were closer to
the predicted value than NaCN (see Figure 95). The IR spectra for BBO-COF 2 revealed
that the reaction with NaSCH3 exhibited broad OH stretches at ~3300 cm-1 indicating that
the isolated solids still contain some of the unreacted phenolic imine-linked intermediate
(see Figure 90 B). The PXRD data revealed that NaN3 and NaCN were the only catalysts
that produced crystalline samples of BBO-COF 2 while NaSCH3 afforded an amorphous
porous polymer (see Figure 90 C). In contrast to BBO-COF 2, the activated solids for BBOCOF 3 yielded surface areas of 1697, 1439, and 386 m2/g for NaSCH3, NaN3, and the
reaction with no catalyst, respectively (see Figure 90 D). The BBO-COF 3 synthesized
without a catalyst contained two pores at 1.7 and 2.6 nm, while the others displayed one
distinct pore size at ~ 2.7 nm (see Figure 104). NaSCH3 is the only nucleophile that
exhibited an OH stretch at ~3300 cm-1 signifying that a small portion of the polymer was
not fully converted to the BBO-linkage (see Figure 90). Surprisingly, all of the nucleophiles
studied provided high quality samples of BBO-COF 3 with the lone exception being the
reaction in which no catalyst was used (see Figure 90 F). Although the complete formation
of BBO-COF 2 and 3 seemed to vary upon the usage of NaSCH3 or NaN3, our studies
103
suggest that NaCN is the most effective catalyst at producing crystalline BBO-COF
materials. It should be noted that the reactions without catalyst generated amorphous
porous polymers, and were unsuccessful at promoting the cyclization of the BBO-linkage
even in the presence of oxygen. This also indicates that nucleophiles are critical for
providing the sp3 hybridized α-carbon needed to initiate the stepwise oxidative
dehydrogenation mechanistic pathway (see Figure 91).
3.7 Computational experiments
With this in mind, we wanted to validate that the radical generated at the α-carbon prior to
the cyclization of the BBO-linkage is stabilized through the captodative effect. We were
also curious to explore the role of the TFPB and TFPT linkers during the radical
delocalization process. To examine this, we performed DFT geometry optimizations
(B3LYP/6-31+G*)126–129 along with natural population analysis (NPA, B3LYP/6Table 8. B3LYP/6-31+G* NPA population and spin density analysis for BBO-COF 2. The
calculated structure with a single electron on the α-carbon is shown
X=
Group
C
N
H
W
X
Y
Z
N3
Spin %
–1
–1
0
0
100
2
0
Charges
0.43
–0.56
0.46
–0.65
–0.64
–0.02
–0.03
Group
C
N
H
W
X
Y
Z
CN
Spin %
18
25
–1
30
10
14
4
Charges
–0.02
–0.51
0.45
–0.56
–0.15
–0.13
–0.10
104
Group
C
N
H
W
X
Y
Z
SCH3
Spin %
19
26
0
21
4
24
8
O
Charges
–0.03
–0.57
0.46
–0.65
0.12
–0.19
–0.14
W
N
X
H
C
Y
Z
31+G*)130 charge density (population) and spin density calculations on the fragment cores
of BBO-COF 2 and 3, as implemented in Gaussian 16. The results of these calculations are
shown in Tables 8 and 9, respectively. Surprisingly, the calculations show that 100% of
Table 9. B3LYP/6-31+G* NPA population and spin density analysis for BBO-COF 3. The
calculated structure with a single electron on the α-carbon is shown
X=
Group
C
N
H
W
X
Y
Z
N3
Spin %
–1
–1
0
–1
100
2
1
Charges
0.42
–0.55
0.46
–0.60
–0.64
0.04
–0.12
Group
C
N
H
W
X
Y
Z
CN
Spin %
11
24
–1
34
6
14
11
Charges
–0.001
–0.50
0.46
–0.49
–0.12
–0.08
–0.26
Group
C
N
H
W
X
Y
Z
SCH3
Spin %
14
17
0
20
3
24
23
O
Charges
0.01
–0.55
0.46
–0.60
0.16
–0.14
–0.34
W
N
X
H
C
Y
N
N
Z
N
the single electron is delocalized entirely on the azide for both BBO-COF 2 and 3 indicating
that these intermediates are not stabilized by the captodative effect. In the case of cyanide,
the single electron for BBO-COF 2 is broadly delocalized on the nitrogen atom (25%),
cyanide substituent (10%), and the TFPB linker (18%). In contrast, BBO- COF 3 displays
similar percentages for the nitrogen atom and cyanide substituent, but exhibits a 7%
increase in charge delocalization for the electron deficient TFPT linker. The calculations
for cyanide confirm that (1) the radical intermediates generated are stabilized by the
captodative effect for these particular systems, and (2) electron deficient substituents can
also play a vital role in stabilizing radical intermediates that are generated during the
formation of BBO-COFs. Lastly, only a small percentage of the single electron is
105
Figure 91. Proposed mechanism for the stepwise oxidative dehydrogenation pathway to
form the BBO-linkage using NaCN as a catalyst
delocalized on the SCH3 substituent for BBO-COF 2 and 3 possibly due to its electron
donating nature, while 32% and 47% are delocalized on the TFPB and TFPT linkers,
respectively. The higher charge delocalization for the latter could explain why the NaSCH3
catalyzed reaction for BBO-COF 3 produces a crystalline solid, while the reaction for
BBO-COF 2 yields an amorphous powder.
Based on the experimental and computational data collected, we propose the following
mechanism for the cyclization of the BBO-linkage during the nucleation process (see
Figure 91). The aminophenol and formyl precursors initially undergo an imine
condensation reaction to produce (A) followed by subsequent proton transfer to form the
protonated imine intermediate (B). Then, cyanide attacks the imine α-carbon of (B) to
generate the sp3 hybridized intermediate (C). Afterwards, triplet state oxygen moves in to
106
abstract the hydrogen atom at the sp3 hybridized α-carbon of (C) to produce intermediate
(D) and generate a hydroperoxyl radical species. Later, this hydroperoxyl radical species
returns to abstract a hydrogen atom from the β-nitrogen of (D) to eliminate hydrogen
peroxide and produce intermediate (E). From here, an intramolecular cyclization followed
by elimination of the cyano group produces the BBO-linkage. Although the proposed
mechanism does shed more light on the role of cyanide during the cyclization process, it is
unclear if radical-radical interactions131 between the adjacent layers of BBO-COFs occur
or assist with the nucleation and growth process.
3.8 Experimental section
3.8.1 Instrumentation and methods
Infrared spectra were recorded on a Thermo Scientific Nicolet iS5 with an iD7 diamond
ATR attachment and are uncorrected. Powder X-Ray diffraction (PXRD) patterns were
recorded on a Bruker D8 Advance diffractometer (40 kV, 40 mA, sealed Cu X-Ray tube)
equipped with an incident beam monochromator (Johansson type SiO2-crystal) and
Lynxeye XE-T position sensitive detector. Samples were mounted on a zero background
sample holder by dropping powders from a vial and flattening them by firmly pressing the
sample with a wide-blade spatula. No sample grinding was used prior to analysis. The
holder was then placed on the mounting apparatus. Surface area measurements were
conducted on a Micromeritics ASAP 2020 Surface Area and Porosity Analyzer and a
107
Micromeritics Tristar Surface Area and Porosity Analyzer using ca. 20 mg samples.
Nitrogen isotherms were generated by incremental exposure to ultra high purity nitrogen
up to ca. 1 atm in a liquid nitrogen (77 K) bath. Surface parameters were determined using
BET adsorption models in the instrument software. Pore size distributions were determined
using the non-local density functional theory (NLDFT) model (cylinder pore, N2cylindrical pores-oxide surface with high regularization) in the instrument software
(Micromeritics ASAP 2020 V4.02). 1H NMR spectra were recorded in deuterated solvents
on a Bruker Avance DPX 400 (400 MHz). Chemical shits are reported in parts per million
(ppm, δ) using the solvent as the internal standard. 13C NMR spectra were recorded on a
Bruker Avance DPX 400 (100 MHz) using the solvent as an internal standard. Solid-state
13
C NMR spectra for BBO-COF 3 was recorded using a Bruker AVIII 600 MHz
spectrometer with wide-bore magnet (600.3 MHz) using a 3.2 mm magic angle spinning
(MAS) HXY solid-state NMR probe and running 32 k scans. Cross-polarization with MAS
(CP-MAS) was used to acquire 13C data at 150.9 MHz. The 13C cross polarization time was
2 ms at 50 kHz for 13C. 1H decoupling was applied during data acquisition. The decoupling
power corresponded to 100 kHz. The HXY sample spinning rate was 15 kHz. Elemental
analysis was performed by Galbraith Laboratories using a Thermo Finnigan FlashEA 1112
Elemental Analyzer. Thermogravimetric analysis (TGA) was performed by Galbraith
Laboratories by heating samples from 25 oC to 500 oC under air at a heating rate of 10 oC
min-1. Scanning electron microscopy (SEM) was performed on a FEI sirion FE-SEM.
108
Materials were deposited onto a film of a wet colloidal silver paint on an aluminum sample
stub and dried in a vacuum oven at 40 oC. The samples were coated with gold in a Leica
EM ACE600 coater, using rotation, to a depth of approximately 20 nm. After coating, the
samples were imaged in the SEM at 5 keV, without tilting, using both the secondary
electron (SE) detector and the through lens detector (TLD).
3.8.2 Synthetic methods
2,5-Diamino-1,4-benzenediol dihydrochloride (DABD): this compound was prepared
using procedures adapted from Strom and Jeffries-EL et al.132,133
1,3,5-Tris(4-formylphenyl)benzene (TFPB): this compound was prepared using a
procedure from Cooper et al.134
1,3,5-Tris(4-bromophenyl)triazine: this compound was prepared using a procedure from
Adachi et al.135
1,3,5-Tris(4-formylphenyl)triazine (TFPT): this compound was prepared using a procedure
from El-Kaderi et al.136
BBO-COF 2: BBO-COF 2 was synthesized as previously reported, varying the
nucleophilic catalyst’s identity.112
109
Synthesis of BBO-COF 3
A dry 25 mL vial with stirbar was loaded with 2,5-diamino-1,4-benzenediol
dihydrochloride (31.9 mg, 0.15 mmol, 1.5 eq) and DMF (5 mL) under a N2 atmosphere.
The solution was cooled to -15˚C (ethylene glycol/CO2) and 2,4,6-tris(4-formylphenyl)1,3,5-triazine (39.3 mg, 0.10 mmol, 1 eq.) in DMF (5 mL) was added to the solution over
5 minutes. The solution was stirred at -15˚C for an additional 3 h before warming to room
temperature in a water bath overnight. The reaction was opened to air and the NaCN (0.10
mmol, 1 eq) was dissolved in MeOH (0.2 mL) before adding to the solution. The reaction
was heated to 130˚C for 96 h before filtering solids and washing with acetone. The solids
were soaked in methanol for 24 h changing out the solvent 3 times over that period. An
additional soak was performed in acetone following the same procedure. The light brown
solids were filtered and dried under vacuum before characterization. (38.4 mg 74.8%). FTIR (powder, ATR): 1569, 1515, 1430, 1409, 1360, 1296, 1176, 1120, 1054, 1015, 919, 862,
843, 816, 739, 690, 512. CP-MAS 13C NMR (75.5 MHz, δ ppm): 167.8, 160.6, 146.4,
138.7, 138.4, 136.2, 128.0, 127.9. Elemental analysis for (C33H15O3N6)n: Calculated: C
(72.92%), H (2.78%), N (15.46%). Observed: C (66.43%), H (3.31%), N (14.79%).
110
3.8.3 BBO-1 analog trials
Synthesis of BBO-1
A dry 25 mL flask with stirbar was loaded with DABD (106.5 mg, 0.5 mmol, 0.5 eq), and
DMF (8.3 mL). The reaction flask was purged with nitrogen and cooled to -15 oC, in an
ethylene glycol/CO2 bath. Once cooled, p-tolualdehyde (0.11 mL, 1.0 mmol, 1 eq) was
added via microsyringe. The flask was kept under nitrogen, and stirred at -15 oC for 3h.
The reaction was removed from the cooling bath and warmed to room temperature in a
water bath overnight. The reaction was opened to air and the chosen nucleophilic catalyst
(1.0 mmol, 1 eq) was dissolved in a chosen solvent (1.3 mL) before adding to the solution.
NaCN and NaSCH3 were dissolved in methanol, but due to solubility reasons DMF was
chosen to dissolve NaN3. The mixture was heated at 130˚C for 24 hours. After, the mixture
was cooled to room temperature, and diluted with water (15 mL). A precipitate formed,
which was vacuum filtered, washing with water. The solids were collected and
recrystallized from toluene to afford BBO-1 as a brown solid. 1H-NMR (CDCl3, 400 MHz)
δ 8.16 (d, 4H); 7.88 (s, 2H); 7.34 (d, 4H). 13C-NMR (CDCl3, 150 MHz) 164.6, 148.6, 142.4,
140.5, 129.7, 127.6, 124.4, 100.7, 21.8.
Synthesis of BBO-1 under deoxygenated conditions
A dry 25 mL flask was loaded with DMF (8.3 mL) and p-tolualdehyde (0.11 mL, 1.0 mmol,
1 eq), which were degassed using the freeze-pump-thaw method 3 times and backfilled
111
with N2. The solution was transferred via syringe to a round bottom flask under inert
atmosphere containing a stirbar and DABD (106.5 mg, 0.5 mmol, 0.5 eq). The reaction
was cooled to -15 oC (ethylene glycol/CO2) while remaining under N2. The flask was kept
under nitrogen, and stirred at -15 oC for 3h. The reaction was removed from the cooling
bath and warmed to room temperature in a water bath overnight. NaCN (1.0 mmol, 1 eq)
was dissolved in a MeOH (1.3 mL) and was degassed using the freeze-pump-thaw method
three times and backfilled with N2. The NaCN solution was transferred via syringe to the
reaction flask. The mixture was heated at 130˚C for 24 hours, while remaining under
nitrogen. After, the mixture was cooled to room temperature, and diluted with water (15
mL) and prepared as above.
112
3.8.4 BBO-COF 2 formation studies
General Synthetic Procedure for BBO-COF 2
BBO-COF 2 was synthesized as previously reported, varying the nucleophilic catalyst’s
identity.112 To test the different nucleophiles for this reaction, the chosen catalyst (1.0
mmol, 1 eq) was dissolved in a chosen solvent (1.3 mL) before adding to the solution.
NaCN and NaSCH3 were dissolved in methanol, but due to solubility reasons DMF was
chosen to dissolve NaN3.
Table 10. Summary of BBO-COF 2 synthesis with different nucleophilic catalysts
Nucleophile
Temp
Reaction
(Nuc)
(˚C)
Time
NaCN
130
4d
NaN3
130
4d
Nucleophiles
NaSCH3
130
4d
None
130
4d
*NaCN
130
4d
*Sample was run without oxygen present (deoxygenated)
Trial
113
Porosity
(m2/g)
1106
1033
236
169
142
% Yield
62%
54%
40%
40%
13%
3.8.5 BBO-COF 2 nucleophile trials
4000
3500
3000
2500
2000
Wavenumbers
1500
1000
(cm-1)
Figure 92. FT-IR spectrum of BBO-COF 2 synthesized with NaN3
Table 11. FT-IR peak assignments for BBO-COF 2 synthesized with NaN3
Peak (cm-1)
1624
1118
Assignment
C=N stretch of benzoxazole
C-O stretch of benzoxazole
114
500
4000
3500
3000
2500
2000
Wavenumbers (cm-1)
1500
1000
Figure 93. FT-IR spectra of BBO-COF 2 synthesized with NaSCH3
Table 12. FT-IR peak assignment for BBO-COF 2 synthesized with NaSCH3
Peak (cm-1)
3369
1630
1120
Assignment
O-H stretch
C=N stretch of benzoxazole
C-O stretch of benzoxazole
115
500
4000
3500
3000
2500
2000
Wavenumbers (cm-1)
1500
1000
Figure 94. FT-IR spectrum of BBO-COF 2 synthesized without a catalyst
Table 13. FT-IR peak assignments for BBO-COF 2 synthesized without a catalyst
Peak (cm-1)
3360
1666
1212
Assignment
O-H stretch
C=N stretch of benzoxazole
C-O stretch of benzoxazole
116
500
1.2
Normalized Pore Volume
1
0.8
0.6
0.4
0.2
0
1
2
3
Pore Width (nm)
4
5
Figure 95. Normalized pore size distributions of BBO-COF 2 synthesized with different
nucleophiles
3.8.6 BBO-COF 2 deoxygenated trial
Synthesis of BBO-COF 2 under deoxygenated conditions
BBO-COF 2 deoxygenated was synthesized by modifying the previously reported
synthesis.112 For the deoxygenated sample, DMF, and methanol were degassed using the
freeze-pump-thaw method three times each and backfilled with N2. The reaction was
117
performed under nitrogen throughout the four day heating process. NaCN was used as the
catalyst. In the deoxygenated sample we see a peak signifying significant –OH stretching
and a shift in the C=N imine stretch that corresponds to the imine linked instead of the
benzoxazole linked polymer.
4000
3500
3000
2500
2000
Wavenumbers (cm-1)
1500
1000
Figure 96. FT-IR spectrum of BBO-COF 2 deoxygenated
Table 14. FT-IR peak assignments for BBO-COF 2 deoxygenated
Peak (cm-1)
3361
1701
1205
Assignment
O-H stretch
C=N stretch of Imine
C-O stretch
118
500
180
160
Quantity Adsorbed (cm3/g)
140
120
100
80
60
40
20
0
0
0.1
0.2
0.3
0.4
0.5
0.6
Relative Pressure (p/p˚)
0.7
0.8
0.9
Figure 97. N2 adsorption isotherm of BBO-COF 2 deoxygenated
1.2
Normalized Pore Volume
1
0.8
0.6
0.4
0.2
0
0
0.5
1
1.5
2
2.5
3
Pore Width (nm)
3.5
4
4.5
Figure 98. Pore size distribution of BBO-COF 2 deoxygenated
119
5
1
3.8.7 BBO-COF 3 formation studies
Procedure for BBO-COF 3 nucleophile trials
The nucleophile trials were run using the synthetic procedure found above for BBO-COF
3, varying the nucleophilic catalyst’s identity. The chosen catalyst (1.0 mmol, 1 eq) was
dissolved in a chosen solvent (1.3 mL) before adding to the solution. NaCN and NaSCH3
were dissolved in methanol, but due to solubility reasons DMF was chosen to dissolve
NaN3.
Procedure for BBO-COF 3 time trials
The time trials were run using the synthetic procedure found above for BBO-COF 3, with
NaCN as the catalyst. The polymer trials were stopped at various times throughout the
heating process. The before heating trial was stopped prior to the exposure to air and
addition of NaCN. The heated samples were stopped at 24h, 48h, 72h, and 96h to determine
the required length of reaction time for complete oxazole formation. All 5 different time
trials were worked up by filtering the solids and soaking them in methanol for 24 h,
changing out the solvent 3 times, followed by an additional soak in acetone for 24 h,
refreshing the solvent 3 times. The solids were filtered and dried under vacuum before
characterization.
120
Table 15. Summary of trials for BBO-COF 3 synthesized with different nucleophiles or
over different reaction times
Reaction
Time
NaCN
130
4d
NaN3
130
4d
Nucleophiles
NaSCH3
130
4d
None
130
4d
Before
NaCN
130
Heating
NaCN
130
1d
NaCN
130
2d
Reaction Time
NaCN
130
3d
NaCN
130
4d
*NaCN
130
4d
*Sample was run without oxygen present (deoxygenated)
Trial
Nucleophile
Temp (˚C)
121
Porosity (m2/g)
% Yield
2039
1439
1697
386
74.8%
71.6%
67.6%
64.5%
Nonporous
83.2%
704
1435
1295
2039
1366
79.6%
73.7%
74.1%
74.8%
47.9%
3.8.8 BBO-COF 3 nucleophile trials
BBO-COF 3
TFPT
DABD
4000
3500
3000
2500
2000
Wavenumbers (cm-1)
1500
1000
500
Figure 99. FT-IR spectra of BBO-COF 3 synthesized with NaCN compared with the
monomers
122
4000
3500
3000
2500
2000
Wavenumbers (cm-1)
1500
1000
Figure 100. FT-IR spectrum of BBO-COF 3 synthesized with NaCN
Table 16. FT-IR peak assignments for BBO-COF 3 synthesized with NaCN
Peak (cm-1)
1640
1569
1119
Assignment
C=N stretch of benzoxazole
C=N stretch of triazine
C-O stretch of benzoxazole
123
500
4000
3500
3000
2500
2000
Wavenumbers (cm-1)
1500
1000
Figure 101. FT-IR spectrum of BBO-COF 3 synthesized with NaN3
Table 17. FT-IR peak assignments for BBO-COF 3 synthesized with NaN3
Peak (cm-1)
1647
1566
1117
Assignment
C=N stretch of benzoxazole
C=N stretch of triazine
C-O stretch of benzoxazole
124
500
4000
3500
3000
2500
2000
Wavenumbers (cm-1)
1500
1000
Figure 102. FT-IR spectrum of BBO-COF 3 synthesized with NaSCH3
Table 18. FT-IR peak assignments for BBO-COF 3 synthesized with NaSCH3
Peak (cm-1)
3384
1624
1568
1120
Assignment
O-H stretch
C=N stretch of benzoxazole
C=N stretch of triazine
C-O stretch of benzoxazole
125
500
4000
3500
3000
2500
2000
Wavenumbers (cm-1)
1500
1000
Figure 103. FT-IR spectrum of BBO-COF 3 synthesized without a catalyst
Table 19. FT-IR peak assignments for BBO-COF 3 synthesized without a catalyst
Peak (cm-1)
3352
1695
1574
1149
Assignment
O-H stretch
C=N stretch of imine
C=N stretch of triazine
C-O stretch
126
500
Figure 104. Normalized pore size distributions BBO-COF 3 synthesized with different
nucleophiles
127
3.8.9 BBO-COF 3 reaction time trials
4000
3500
3000
2500
2000
Wavenumbers (cm-1)
1500
1000
500
Figure 105. FT-IR spectrum of a ground mixture of BBO-COF 3 monomers, DABD and
TFPT
Table 20. FT-IR peak assignments for a ground mixture of BBO-COF 3 monomers,
DABD and TFPT
Peak (cm-1)
2814
1696
1581
Assignment
O-H stretch of DABD monomer
C=O stretch of aldehyde
C=N stretch of triazine
128
4000
3500
3000
2500
2000
Wavenumbers (cm-1)
1500
1000
Figure 106. FT-IR spectrum of BBO-COF 3 after 1 day of heating
Table 21. FT-IR peak assignments for BBO-COF 3 after 1 day of heating
Peak (cm-1)
3343
1693
1573
1012
Assignment
O-H stretch
C=N stretch of imine
C=N stretch of triazine
C-O stretch
129
500
4000
3500
3000
2500
2000
Wavenumbers (cm-1)
1500
1000
Figure 107. FT-IR spectrum of BBO-COF 3 after 2 days of heating
Table 22. FT-IR peak assignments for BBO-COF 3 after 2 days of heating
Peak (cm-1)
3379
1698
1567
1015
Assignment
O-H stretch
C=N stretch of imine
C=N stretch of triazine
C-O stretch
130
500
4000
3500
3000
2500
2000
Wavenumbers (cm-1)
1500
1000
Figure 108. FT-IR spectrum of BBO-COF 3 after 3 days of heating
Table 23. FT-IR peak assignments for BBO-COF 3 after 3 days of heating
Peak (cm-1)
3369
1660
1572
1015
Assignment
O-H stretch
C=N stretch of benzoxazole
C=N stretch of triazine
C-O stretch of benzoxazole
131
500
Before Heating
1 day
2 days
3 days
4 days
0
5
10
15
2ϴ
20
25
Figure 109. Normalized PXRD of BBO-COF 3 at various points during the 4-day
reaction period
3.8.10 BBO-COF 3 deoxygenated trial
Synthesis of BBO-COF 3 under deoxygenated conditions
BBO-COF 3 deoxygenated was synthesized by modifying the procedure mentioned
above.6 For the deoxygenated sample, DMF, and methanol were degassed using the freezepump-thaw method three times each and backfilled with N2. The reaction was performed
under nitrogen throughout the four day heating process. NaCN was used as the catalyst. In
132
the deoxygenated sample we see a peak signifying significant –OH stretching and a shift
in the C=N imine stretch that we corresponds to the imine linked instead of the benzoxazole
linked polymer. For BBO-COF 3 we also see a 0.2 nm shift in the major pore of the
material, which can happen due to the increased slip stacking cause by the flexible imine
linkage.
4000
3500
3000
2500
2000
Wavenumbers (cm-1)
1500
1000
500
Figure 110. FT-IR spectrum for BBO-COF 3 deoxygenated
Table 24. FT-IR peak assignments for BBO-COF 3 deoxygenated
Peak (cm-1)
3359
1699
1572
1016
Assignment
O-H stretch
C=N stretch of imine
C=N stretch of triazine
C-O stretch
133
600
Quantity Adsorbed (cm3/g)
500
400
300
200
100
0
0
0.1
0.2
0.3
0.4
0.5
0.6
Relative Pressure (p/p˚)
0.7
0.8
0.9
Figure 111. N2 adsorption isotherm for BBO-COF 3 deoxygenated
1.2
Normalized Pore Volume
1
0.8
0.6
0.4
0.2
0
0
0.5
1
1.5
2
2.5
3
Pore Width (nm)
3.5
4
4.5
Figure 112. Pore size distribution for BBO-COF 3 deoxygenated
134
5
1
3.8.11 BBO-COF 3 experimental & simulated PXRD data
The simulated PXRD profiles were performed using Materials Studio 7.0 using the unit
cell precursors shown in Figure S24. Before the simulations were performed, each
precursor was optimized using the geometry optimization task and Universal Forcefield
parameters from the Forcite module. Each structure was then modeled using a primitive
hexagonal unit cell with a P6 space group. The a = b parameters were estimated by
measuring the distance between the BBO phenyl ring of the linkers for each COF. The c
parameter was arbitrarily set at 3.4 Å. The staggered molecular models were performed by
offsetting the initial structures by half of the a=b parameters using a gra (P63/mmc) space
group and c parameter of 6.7 A. Simulation of the possible structures was performed using
Reflux Plus module to produce the expected PXRD profiles. The experimental PXRDs
were then subjected to a Pawley refinement using Pseudo-Voigt peak shape function and
Berar-Baldinozzi asymmetry correction function to produce the refined PXRD profile.
135
O
N
N
N
O
O
N
O
N
N
N
N
O
O
N
Figure 113. Precursor used to construct the hexagonal unit cell for BBO-COF 3
136
Table 25. Fractional atomic coordinates for the P6 unit cell of BBO-COF 3 calculated
using Materials Studio 7.0
BBO-COF 3
Hexagonal, P6
a=b= 34.595, c = 3.4
Atom
x
y
z
C1
0.414957
0.580507
1.203390
C2
0.439019
0.557393
1.203020
N3
0.419963
0.515597
1.204470
C4
0.454138
0.507057
1.203550
C5
0.451984
0.467082
1.204530
H6
0.419799
0.433412
1.206270
C7
0.491783
0.468837
1.203290
O8
0.499673
0.434900
1.203900
C9
0.544986
0.456600
1.202100
C10
0.569052
0.433489
1.202070
C11
0.546982
0.388605
1.203970
H12
0.508932
0.369451
1.205550
C13
0.569544
0.366349
1.204020
H14
0.550296
0.328300
1.205660
C15
0.614588
0.388606
1.202160
137
100
80
60
40
20
0
00
55
10
10
15
15
20
20
25
25
2Θ (Degrees)
2θ (Degrees)
Figure 114. Simulated PXRD of BBO-COF 3 modeled using a gra unit cell
138
30
30
3.8.12 Solid-state NMR spectra
Figure 115. 150.9 MHz 13C CP-MAS solid-state NMR spectra of BBO-COF 3
139
3.8.13 TGA profile
Figure 116. TGA profile for BBO-COF 3
140
3.8.14 Surface area analysis
0.018
y = 0.0919x + 0.0025
R² = 0.99622
0.016
0.014
1/[Q(p°/p - 1)]
0.012
0.01
0.008
0.006
SABET=1033 m2/g
C=37.9
0.004
0.002
0
0
0.02
0.04
0.06
0.08
0.1
Relative Pressure (p/p˚)
0.12
0.14
0.16
Figure 117. BET surface area plot for BBO-COF 2 synthesized with NaN3
0.003
y = 0.0183x + 7E-05
R² = 0.99995
0.0025
1/[Q(p˚/p-1)]
0.002
0.0015
0.001
SABET = 236 m2/g
C = 256.9
0.0005
0
0
0.02
0.04
0.06
0.08
0.1
Relative Pressure (p/p˚)
0.12
0.14
0.16
Figure 118. BET surface area plot for BBO-COF 2 synthesized with NaSCH3
141
0.004
0.0035
y = 0.0256x + 0.0002
R² = 0.99986
1/[Q(p˚/p-1)
0.003
0.0025
0.002
0.0015
0.001
SABET = 169 m2/g
C = 165.4
0.0005
0
0
0.02
0.04
0.06
0.08
0.1
Relative Pressure (p/p˚)
0.12
0.14
0.16
Figure 119. BET surface area plot for BBO-COF 2 synthesized without a catalyst
0.005
0.0045
y = 0.0304x + 0.0002
R² = 0.99973
0.004
1/[Q(p˚/p-1)]
0.0035
0.003
0.0025
0.002
0.0015
SABET = 142 m2/g
C = 181.5
0.001
0.0005
0
0
0.02
0.04
0.06
0.08
0.1
Relative (p/p˚)
0.12
0.14
0.16
Figure 120. BET surface area plot for BBO-COF 2 deoxygenated
142
0.0006
0.0005
y = 0.0021x + 5E-05
R² = 0.98221
1/[Q(po/p-1)]
0.0004
0.0003
SABET = 2039 m2/g
C = 45.3
0.0002
0.0001
0
0
0.05
0.1
0.15
Relative Pressure (p/p˚)
0.2
0.25
Figure 121. BET surface area plot for BBO-COF 3 synthesized with NaCN
0.0008
y = 0.003x + 5E-05
R² = 0.99017
0.0007
1/[Q(p˚/p-1)]
0.0006
0.0005
0.0004
SABET = 1439 m2/g
C = 60.5
0.0003
0.0002
0.0001
0
0
0.05
0.1
0.15
Relative Pressure (p/p˚)
0.2
0.25
Figure 122. BET surface area plot BBO-COF 3 synthesized with NaN3
143
0.0007
0.0006
y = 0.0025x + 4E-05
R² = 0.99132
1/[Q(p˚/p-1)]
0.0005
0.0004
0.0003
0.0002
SABET = 1697 m2/g
C = 59.1
0.0001
0
0
0.05
0.1
0.15
Relative Pressure (p/p˚)
0.2
0.25
Figure 123. BET surface area plot for BBO-COF 3 synthesized with NaSCH3
0.003
y = 0.0112x + 7E-05
R² = 0.99977
0.0025
1/[Q(p˚/p-1)]
0.002
0.0015
0.001
SABET = 386 m2/g
C = 168.4
0.0005
0
0
0.05
0.1
0.15
Relative Pressure (p/p˚)
0.2
0.25
Figure 124. BET surface area plot for BBO-COF 3 synthesized without a catalyst
144
0.001
y = 0.0062x + 2E-05
R² = 1
0.0009
0.0008
1/[Q(p˚/p-1)]
0.0007
0.0006
0.0005
SABET = 704 m2/g
C = 284.9
0.0004
0.0003
0.0002
0.0001
0
0
0.02
0.04
0.06
0.08
0.1
Relative Pressure (p/p˚)
0.12
0.14
0.16
Figure 125. BET surface area plot for BBO-COF 3 after 1 day of heating with NaCN
0.0008
y = 0.003x + 6E-05
R² = 0.99019
0.0007
1/[Q(p˚/p-1)]
0.0006
0.0005
0.0004
SABET = 1435 m2/g
C = 54.7
0.0003
0.0002
0.0001
0
0
0.05
0.1
0.15
Relative Pressure (p/p˚)
0.2
0.25
Figure 126. BET surface area plot for BBO-COF 3 after 2 days of heating with NaCN
145
0.0009
0.0008
y = 0.0033x + 5E-05
R² = 0.99361
0.0007
1/[Q(p˚/p-1)]
0.0006
0.0005
0.0004
0.0003
SABET = 1295 m2/g
C = 95.6
0.0002
0.0001
0
0
0.05
0.1
0.15
Relative Pressure (p/p˚)
0.2
0.25
Figure 127. BET surface area plot for BBO-COF 3 after 3 days of heating with NaCN
0.0008
0.0007
y = 0.0031x + 4E-05
R² = 0.99638
1/[Q(p˚/p-1)]
0.0006
0.0005
0.0004
0.0003
SABET = 1366 m2/g
C = 108.2
0.0002
0.0001
0
0
0.05
0.1
0.15
Relative Pressure (p/p˚)
0.2
Figure 128. BET surface area plot for BBO-COF 3 deoxygenated
146
0.25
3.8.15 Scanning electron microscopy (SEM) images
Figure 129. SEM images of BBO-COF 3 at various magnifications
3.8.16 Computational studies
Computational Methods
All calculations were performed using the Gaussian 16 suite of programs. Geometry
optimizations and frequency calculations for BBO-COF 2 and BBO-COF 3 monomers
were performed with the 6-31G(d) basis set128 in conjunction with Becke’s three-parameter
hybrid exchange functional and the LeeYang–Parr correlation functional (B3LYP) density
functional theory (DFT) method.137,138 Rotational barriers were located by torsional scans
and the located transitions states were optimized. The nature of all stationary points, either
147
minima or transition states, was confirmed by calculating the vibrational frequencies at the
corresponding level of theory. Minima were characterized by the absence of any imaginary
vibrational frequencies, while transition states possessed only one imaginary vibrational
frequency.
In order to support the presence of a captodative effect promoting radical formation and
subsequent cyclization of benzobisoxazole precursors computational methods were
employed to evaluate the thermodynamic and molecular properties of these substrates.
Both the anionic starting materials and corresponding radical anions were examined to
assess bond dissociation energies, spin distribution, and charge distribution. Spin
distribution and charge distribution were quantified using natural population analysis as
implemented by nbo version 3.1 in the Gaussian 16 suite of programs.130 Due to the
conformational flexibility of these substrates mixed torsional/Low-mode sampling Monte
Carlo conformational analyses were performed on all starting anionic compounds with the
OPLS3 forcefield139 using the MacroModel package of the Schrodinger suite of programs.
The resulting structures were further refined by reoptimization and frequency calculations
conformation at the B3LYP/6-31+G* level of theory.129 In order to calculate the geometry
of the radical anions, the labile hydrogen atom was abstracted from the anionic starting
material in silico and the structure optimized for a final time. BDEs were calculated by
comparing the energy of the starting anion to the sum of the energies of the product radical
148
and hydrogen atom. Spin density contour plots were produced using the cubegen utility of
the Gaussian 16 suite of programs.
Figure 130. Benzobisoxazole modeling representative of BBO-COF 2
Figure 131. Benzobisoxazole modeling representative of BBO-COF 3
149
Figure 132. Spin density maps for BBO-COF 2 with NaCN (left), NaN3 (center), and
NaSCH3 (right)
Figure 133. Spin density maps for BBO-COF 3 with NaCN (left), NaN3 (center), and
NaSCH3 (right)
150
Chapter 4: Porphyrin-based benzobisoxazole and benzobisthiazole-linked porous organic
polymers for hydrosilylation of CO2
4.1 Porphyrins as catalysts
Porphyrins are a class of 18 π electron aromatic organic
R
R
N
H
macrocycles comprised of four pyrrolic subunits connected by
N
methene bridges (see Figure 134). The pyrrolic subunits oriented
inwards toward the center of the macrocycle provide an excellent
N
H
N
R
R
Figure 134.
chelating pocket for the binding of metal ions. Porphyrins are quite Structure of mesosubstituted
common in biology, and thus have been extensively studied. Their
porphyrins
function in biology is often to assist in catalyzing important reactions, for example
chlorophyll is responsible for photosynthesis in plants by absorbing light, and also helps
give many plants their green color. Heme porphyrins are found in hemoglobin proteins,
H
O
CH2
CH3
H 3C
H3 C
N
N
N
O
H 3C
O
O
H3 C
R
N
CH2
Fe
N
N
CH3
O
N
CH3
Mg
H 3C
CH3
Chlorophyll
H3 C
O
are
responsible
for
oxygen fixation in
N
CH3
OH
which
O
Heme B
Figure 135. Structure of chlorophyll and Heme B porphyrins
151
OH
the blood. Outside
of biology, chemists
have been able to
design
porphyrin-
based catalysts for a variety of reactions including oxidations,140 reductions,141
fluorination,142
epoxidation,143
cyclopropanation,144145
and
photo-146
and
electrocatalysis.147 Their large aromatic macrocycle also endows porphyrins with good
light absorbing properties, and also a wide range of delightful colors. In fact the name
‘porphyrin’ comes from the Greek word for purple. This property of light absorption has
Figure 136. Cyclopropanation of diazo reagents by metalloradical catalysis performed by
a cobalt porphyrin reprinted with permission from J. Am. Chem. Soc., 2017, 139, 10491052. Copyright 2017 American Chemical Society
given porphyrins a great deal of success as dye sensitizers for dye sensitized solar cells
(DSSC),148 and other optoelectronic and photovoltaic applications. These optoelectronic
152
and catalytic properties have led to a large number of reports of porphyrins incorporated
into porous materials.
4.2 Porous materials as heterogeneous catalysts
As mentioned previously, our group has long been interested in using porous materials as
hetereogeneous catalysts. Elegant catalysts for difficult reactions are incredibly useful, but
homogenous catalysts which cannot be easily recovered or rapidly lose efficacy can
quickly become a waste of valuable resources and synthetic time. Heterogeneous catalysts
have the important functionality of ease of re-isolation, and repeated use before their
catalytic performance wanes. In this respect, porous polymers have several advantages;
incorporation of functional monomers joined by chemically stable linkages allows for
customization of catalytic sites within a material that is resistant to degradation. High
surface area allows for many catalytic sites inside the material to be accessed, as opposed
to surface catalysis (see Figure 137). Additionally, the porous nature of the catalytic
support polymer lends itself well to size selective catalysis of small molecules over larger
ones.59
The single most important catalytic reaction at this point in history is the reduction of
carbon dioxide (CO2). CO2 is widely accepted to be one of the major contributors to climate
change; after decades of unchecked CO2 emissions, the amount of CO2 in the atmosphere
153
has risen dramatically. Porous polymers have a deep history of uptake and separations of
CO2 gas, however in order to combat climate change, the sequestration and storage of CO2
gas will not be enough. It logically follows that CO2 gas must be converted into a different,
non-greenhouse gas compound in order to truly begin to relieve the effects of climate
change. Conversion of atmospheric CO2 is of the utmost importance to helping slow the
deleterious effects of human created climate change, and if it can furthermore be reduced
to a synthetically useful chemical or a simple fuel, it is not only environmentally important,
but also potentially of great value to industry. Many researchers in alternative fuels point
Figure 137. Structure of BF-COF-1 showing porous space available for catalysis (pink
sphere) reproduced from Angew. Chem. Int. Ed., 2014, 53, 2878-2882 with permission of
John Wiley and Sons
154
toward methanol as the potential replacement for traditional fossil fuels. Conveniently,
methanol and CO2 are both single carbon compounds, and thus methanol can be made
directly from CO2 using state of the art catalysis. Methanol is also a common commodity
chemical used widely in chemical and biological laboratories. Methanol is an admirable
target for the reduction of CO2, however it does have some significant drawbacks. The
toxicity of methanol is a concern for any fuel which would be readily accessible to
consumers, in addition CO2 must be reduced several times to arrive at methanol. Another
potential target for the reduction of CO2 is formic acid. Formic acid has the great advantage
of being only one reduction away from CO2. Formic acid, much like methanol is a
commodity chemical commonly used in laboratory research, which has a lower toxicity
and less harmful environmental effects than methanol. Formic acid has been postulated as
a possible basis for a new fuel system, which could incorporate fuel cell technology as well
as employing formic acid as a transportation medium for hydrogen gas.149
Porous polymers present an excellent solution to this mounting problem by not only
affording a place to adsorb and store CO2,72 but if a catalytic site can be incorporated into
the polymer as well, it can act as an all-in-one system for CO2 reduction. Researchers have
begun to investigate the potential for porous polymers to chemically convert CO2 into other
compounds. One common strategy is to employ an epoxide in the presence of a halide salt
as a substrate for CO2 conversion into cyclic carbonates (see Figure 138). This reaction has
155
been shown to be effective with MOFs,150 COFs,151 CTFs48 and POPs.152,153 This reaction
can be catalyzed by a variety of different metal ions and even carbenes.154 This strategy
has been well studied, but suffers from the drawback of the limited use of cyclic carbonates.
Electrochemical methods have also been employed for the reduction of CO2. A notable
Figure 138. Reduction of CO2 to cyclic carbonates by cCTF reprinted with permission
from ACS Appl. Mater. Interfaces, 2017, 9, 7209-7216. Copyright 2017 American
Chemical Society
156
example from the Yaghi group was their cobalt-porphyrin based COF capable of reducing
CO2 to carbon monoxide (CO) in water.155
Figure 139. Structure of Ir-NHC-CTF catalyzing reduction of CO2 to formate reprinted
with permission from Chem. Mater., 2017, 29, 6740-6748. Copyright 2017 American
Chemical Society
While electrochemistry can be a very effective tool for catalyzing difficult reactions, this
method has its drawbacks of complex electrochemical cell set up, compared to the ease of
solvothermal synthesis as employed to form cyclic carbonates. Additionally, other
electrochemical reactions such as water reduction can compete against product formation.
157
Ideally- a porous material functioning as a heterogeneous catalyst for the reduction of CO2
would be able to do so without complex electrochemical cell set up and electrode
fabrication, and would directly produce a useful product for chemical synthesis or fuels.
One such CTF material already exists, which uses a carbene functionality on the pore wall
of the material to host an iridium atom for the conversion of CO2 to formate156 (see Figure
139). Formate or it’s protonated form, formic acid has been postulated as a potential fuel
for fuel cells or as a method of hydrogen storage.149 As such, we believe further
investigation into methods to create formic acid from CO2 with heterogeneous porous
polymer catalysts is warranted, and we believe this can be effectively achieved by
employing benzobisoxazole- and benzobisthiazole-linked POPs as heterogeneous catalysts
for this important reaction.
4.3 Synthesis and design of porphyrin POPs
4.3.1 Synthesis of monomers
Precursors to the benzobisoxazole (BBO) and benzobisthiazole (BBT) linkages 2,5diamino-1,4-benzenediol dihydrochloride (DABD)112 and 2,5-diamino-1,4-benzenedithiol
dihydrochloride (DABTD)157 were created by previously reported literature procedures.
The tetraphenylporphyrin substituted with formyl groups (1) had previously been
synthesized and used in porous organic cages158 (see Figure 140). Our initial pathway to 1
aimed to use their synthetic strategy. However, column chromatography of several
158
intermediates, and further dealings with protecting
O
O
groups were not ideal, thus an alternative strategy was
sought. The difficulty in creating a formyl porphyrin
N
H
N
lies in the need of formyl groups to cyclize the
N
H
N
porphyrin ring. Thus any additional formyl groups
cannot be left free during cyclization without obtaining O
1
O
a mixture of complex products. The two strategies to
get around this challenge are firstly, to have both
Figure 140. Structure of
tetraformylphenylporphyrin (1)
formyl groups installed prior to the porphyrin condensation, but have one protected with a
group that can be easily removed from the porphyrin at a later time, or secondly, to have
only one formyl group with which to condense the porphyrin, and install the second formyl
Required for porphyrin
macrocyclization
O
Protect,
Cyclize,
then cyclize
then install
O
group after the porphyrin has
been formed (see Figure 140).
O
This second strategy faces the
challenge of installing a formyl
O
O
O
Required for polymer
condensation
Figure 141. Two strategies to create 1
Br
group on a potentially sensitive
porphyrin, and having to install
all 4 equivalents, and separate
from any di- or tri- substituted products, which could cause defects in the resultant polymer.
Fortunately, a route to create the desired formyl porphyrin by this second method had also
159
previously been published.159 This straightforward 2-step synthesis was the more appealing
of the two strategies (see Figure 141). This method condenses 4-bromobenzaldehyde with
pyrrole to create the bromotetraphenyl porphyrin. The step to install the formyl groups is
also fairly simple: a lithium-halogen reaction quenching with a formyl source to create the
formyl porphyrin. Some optimization of the reported procedure was required to get good
conversion for this step. It was noted that no lithium halogen exchange occurred unless the
reaction mixture was allowed to warm to 0˚C for 3 hours before adding DMF
O
Br
Br
O
O
+
Br
H
N
N
H
Propionic acid
∆, ~1 h
20%
N
N
H
1. BuLi -78 oC --> 0oC, 3h
2. DMF -78 oC
warm to RT
3. H3O+ workup
31%
N
H
N
Br
Br
N
N
H
N
O
1
O
Figure 142. Synthesis of 1 from commerically available precursors
as the formyl source. The eluent for column chromatography was also adjusted. An
additional cold acetonitrile wash of the purified product helped afford clean product for
polymerization. Additionally, any unreacted bromoporphyrin could be retained and reformylated, and any slightly impure formylporphyrin was retained and repurified to afford
additional product.
160
4.3.2 Synthesis of free-base porphyrin polymers
Porphyrin BBO- and BBT-linked polymers could be created through the same 2-step
process as previously reported by our group.112 DABD and 1 are combined at low
temperatures and allowed to react cold for several hours under inert atmosphere to allow
slow, reversible formation of the initial imine linkage as previously discussed (see Figure
143) The mixture is allowed to warm to RT overnight before purging with air, and adding
a catalyst (typically NaCN). The mixture is then allowed to heat at 130˚C for 4 days to
fully form the BBO or BBT linkage. After the polymerization period the polymers are
isolated by filtration, washing with organic solvents, and are allowed to soak for several
Figure 143. Synthesis of BBO-H2P POP (where X=O) and BBT-H2P POP (where X=S)
days to remove residual monomers and small oligomers. The optimal reaction conditions
were determined by screening different conditions including reaction time, temperature,
and solvents (see Table 27). From this optimization we found the BBT-linked polymers
required a longer, cooler initial reaction period to form the imine linkage than the BBO161
linked polymers. The BBT-linked polymers were also found to achieve higher porosities
when no nucleophilic catalyst was employed. We attribute this to the higher nucleophilicity
of sulfur in the BBT linkage compared to oxygen in the BBO linkage. Accordingly, the
remainder of optimization trials for the BBT linkage were performed without a catalyst
(see Table 28).
4.4 Characterization of H2P POPs
300
A
200
150
0.3
B
0.25
dV (cm³/g)
Quantity Adsorbed (cm3/g)
250
100
50
0.2
0.15
0.1
0.05
0
1
1.5
2
2.5
3
Pore Width (nm)
3.5
4
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Relative Pressure (p/p˚)
Figure 144. A) N2 adsorption isotherms and B) pore size distributions for BBO-H2P POP
(green) and BBT-H2P POP (purple)
162
BBO-H2P POP and BBT-H2P POP were characterized by Fourier transform infrared
spectroscopy (FT-IR). The FT-IR spectrum of BBO-H2P POP and BBT-H2P POP display
the characteristic C=N stretching of the porphyrin at ~1597 cm-1 and the oxazole or thiazole
stretch at ~1671 cm-1
90
with
Quantity Adsorbed (mg/g)
80
significant
70
attenuation of the C=O
60
stretch from monomer 1
50
and the OH, NH2, and
40
SH stretches of DABD or
30
DABTD,
20
indicating
complete conversion to
10
0
0
0.5
1
Relative Pressure (p/p˚)
Figure 145. CO2 adsorption isotherms for BBO-H2P POP
(green) and BBT-H2P POP (purple)
1.5
the oxazole or thiazole
(see Figures 157 and
158).
The
permanent
porosity of porphyrin
POPs was measured by nitrogen gas adsorption at 77 K (see Figure 144). The porphyrin
POPs both exhibited reversible type I isotherms with small hysteresis. Application of the
Brunauer-Emmett-Teller (BET) model over the low-pressure region (0.001<p/p˚<0.1)
provided surface areas of 760 m2/g and 535 m2/g for the BBO-H2P POP and BBT-H2P POP
respectively. Nonlocal density functional theory (NLDFT) was used to estimate the pore
163
size distribution of the BBO-H2P POP and BBT-H2P POP yielding values of 2.46 nm for
both BBO-H2P POP and BBT-H2P POP (see Figure 142 B). The CO2 uptake capacity of
BBO-H2P POP and BBT-H2P POP was measured by CO2 adsorption isotherms at 295 K
from 0 to 1.2 bar (see Figure 145). Both BBO-H2P POP and BBT-H2P POP exhibited
6
5
F(R)
4
3
2
1
0
200
400
600
800
1000
Wavelength (nm)
1200
1400
Figure 146. Kubelka-Munk diffuse reflectance spectra of 1 (black), BBO-H2P POP
(green) and BBT-H2P POP (purple)
respectable CO2 uptake at low pressures reaching capacities of 83 and 60 mg/g for BBOH2P POP and BBT-H2P POP respectively. Powder X-Ray Diffraction (PXRD) analysis of
the porphyrin POPs revealed amorphous materials with no long range order (see Figures
161 and 162). We were surprised to find these materials are completely amorphous, given
164
the large number of porphyrin-based COFs reported in the literature.92 Additionally, BBOH2P POP and BBT-H2P POP were made by the same method that produced crystalline
BBO-COF 1, 2, and 3, so it is unclear what contributes the amorphous nature of BBO-H2P
POP and BBT-H2P POP. The porphyrin optoelectronic properties were examined using
solid-state UV-Vis and fluorescence spectroscopy. The solid state UV-Vis spectrum
revealed a strong absorption with similar λmax to the monomeric porphyrin 1 (see Figure
146).
4.5 Design of metalloporphyrin polymers for CO2 conversion
4.5.1 Hydrosilylation of CO2
The hydrosilylation of CO2 catalyzed by transition metals has been known since the 1970’s
initially studied out of the interest in CO2-metal complexes;160 since the 1970’s this work
has expanded and taken on a new importance as a method to create new fuels and combat
climate change. Employing silanes as a hydride source for the reduction of CO2 is
beneficial due to their relatively low cost and ease of handling. Additionally, the
hydrosilylation of CO2 can afford numerous possible products; formic acid, a compound
that may be of interest to the fuel industry, can be created upon simple aqueous workup.
Additionally, the reaction has application to pure organic chemistry; after addition of the
hydride to CO2, a silyl formate is created, these volatile reactive intermediates can then be
converted in a one-pot process to esters or amides by condensation with the desired alcohol
165
or amine respectively. Since the 1970’s many improvements have been made to the
synthetic methodology of this important reaction, including eliminating the requirement
for high pressured CO2,161 and the use of non-noble, earth abundant metals as catalytic
species,162 and even developing metal-free hydrosilylation reactions.163 Unfortunately,
these reactions still suffer from non-recyclable catalysts, or recycling by energy intensive
processes.164 We wanted our polymeric CO2 conversion strategy to focus on
hydrosilylation of CO2 because this transformation, if completed with a porous polymer as
a heterogeneous, recyclable catalyst, could prove of great value to both the research and
industrial communities.
4.5.2 Synthesis and design of small molecule ruthenium porphyrin catalyst
In order to determine the feasibility of this reaction with our desired metalloporphyin
moiety, we synthesized a small-molecule analog of our porphyrin polymer and employed
it in our hydrosilylation reaction. For ease of synthesis, we targeted meso-tetraphenyl
porphyrin (TPP) as our analog (see Figure 147). TPP can be created by the same synthetic
protocol to create the brominated porphyrin precursor to 1 (see Figure 142). Porphyrins
can generally be metallated by a simple substitution of the desired metal salt into the cavity
of the deprotonated porphyrin in polar solvents at high temperatures. After an initial
screening of metal ions, we decided to employ ruthenium as our catalytic metal of choice
166
due to its ease of access of a variety of oxidation states and track record as an effective
metalloporphyrin catalyst across a variety of reactions including oxidations,165140
cyclopropanations,166
amidation,167
and
hydrogenation.141 There is evidence of the ability of
N
H
N
ruthenium porphyrins to create the required hydride
N
H
N
species for hydrosilation from as early as 1985.168
With the wealth of precedent of ruthenium
porphyrins in catalysis we contemplated our options
for ruthenium sources. As far as commercially
Figure 147. Structure of mesotetraphenylporphyrin (TPP)
available ruthenium sources, we had two primary
choices: ruthenium (III) trichloride (RuCl3) or
triruthenium (0) dodecacarbonyl (Ru3CO12). RuCl3 has the benefit of already being
oxidized to our desired oxidation state of 3+ as would be required for the hydride species.
Upon complexation with TPP the remaining Cl atom axial on ruthenium could endow the
complex with a higher hydricity than the analogous complex created with Ru3CO12.
Unfortunately RuCl3 would have the drawback of creating a charged complex with the TPP
ligand. As such, it would be isolated as a salt with either chloride or another anionic
counterion. This represented a significant structural change from the free-base monomer 1,
thus we turned to Ru3CO12, which would afford a neutral ruthenium porphyrin complex
with a carbonyl (CO) as an axial ligand. This axial CO could potentially hinder the
167
hydricity of a ruthenium hydride species, due to the strong pi backbonding interaction
between ruthenium and CO,169 but we envisioned that this potent metal could still catalyze
our reaction. Synthesis of RuTPP was synthetically facile following reported procedures
for the metalation of other porphyrins with Ru3CO12170 (see Figure 148).
N
H
N
N
H
N
Ru3CO12
DMF
∆, 16 h
87%
N O
N
Ru
N
N
Figure 148. Synthesis of RuTPP
4.5.3 RuTPP catalytic performance
In order to optimize our hydrosilylation reaction conditions for the heterogeneous polymer
catalyst, we employed our small molecule RuTPP as a catalyst for the hydrosilylation of
CO2 (see Figure 149). For simplicity and ease of characterization, we decided to quench
the silyl formate intermediate with potassium fluoride (KF) to yield the potassium formate
salt alongside the fluorinated silane. The catalysis was completed by solvothermal
synthesis combining RuTPP and potassium fluoride (KF) in a dry vial. A solution of silane
168
in the desired solvent was added, and the mixture was warmed to the boiling point of the
Figure 149. Reaction scheme for the hydrosilylation of CO2 with RuTPP
solvent under an atmosphere of CO2 for the desired reaction time. This strategy affords
potassium formate, which can be easily isolated as a solid and identified by NMR. A
quantitative NMR experiment was performed for each trial to determine the yield of the
reaction (see Figure 182). The results of these trials are summarized in Table 26. From
these preliminary trials, we found that Me2PhSiH was the most effective silane for this
reaction giving an optimized yield of 19.5% in MeCN. Dioxane was our initial solvent of
choice for this reaction based on its polarity and high boiling point. While the reaction in
dioxane did allow for the formation of potassium formate, the yields were prohibitively
low (~ 2%), thus other solvents were tested. THF was employed in this reaction for its
similarity to dioxane, however it caused worse performance of our catalyst. We believe
this to be due to the low boiling point of THF, which required lower reaction temperatures.
Finally, MeCN was shown to be the solvent performing the highest in our catalytic system,
pushing the yield over 10% in as short a reaction time as 4 hours. We believe that the
169
stronger coordinating ability of MeCN as a solvent ligand on ruthenium may help improve
the efficiency of this reaction.
Table 26. Optimization of reaction conditions for the hydrosilylation of CO2 by RuTPP
Silanes
Reaction
Catalyst
Time
Loading
Et3SiH
PMHS Me2PhSiH (EtO)3SiH
4h
0.5 %
0.01%
0.23%
2.14%
1.49%
16h
0.5 %
0.32%
0.07%
3.84%
1.88%
24h
0.5 %
0%
1.09%
4.31%
1.34%
4h
0.5 %
0%
0%
0.63%
0.22%
4h
0.5 %
16.4%
1.38%
19.5%
3.95%
16h
0.5%
35.9%
6.23%
21.3%
3.95%
Solvent
Dioxane
THF
MeCN
4.6 Synthesis of 1-Ru and metallated porphyrin POPs
With the initial promising results from RuTPP as a catalyst for this reaction we went
forward with synthesis of the ruthenium-containing porphyrin polymers. In order to
achieve greater control over the amount of ruthenium incorporated into our polymers we
opted for a pre-metallation of our porphyrin to ensure full metallation in the polymer. The
desired formyl ruthenium porphyrin 1-Ru can be simply synthesized from free base
170
porphyrin 1 by heating with Ru3CO12 at elevated temperatures overnight (see Figure 150).
Simple purification by silica gel chromatography afforded 1-Ru.
The synthesis of the ruthenium porphyrin POPs was executed using the optimized
conditions for the free-base H2P POPs (see Figure 151). One modification was
O
O
O
N
H
N
N O
Ru3CO12
DMF
∆, 16 h
19%
N
H
N
O
O
1
N
N
N
O
O
Ru
1-Ru
O
Figure 150. Synthesis of 1-Ru from 1
required for the oxazole polymer; The requirement for a strong nucleophile such as cyanide
to catalyze the formation of the oxazole ring would prove problematic in the presence of
ruthenium. Cyanide is not only an excellent nucleophile, but also a strong ligand due to πbackbonding, thus the oxazole linkage must be formed without cyanide to prevent shutting
down the catalytic activity of our polymer. Without cyanide we were unsure if the oxazole
linkage would be formed, however we do not observe the strong OH stretching associated
with phenolic imine-linked polymers, which potentially indicates the formation of the
171
oxazole ring. There is some precedent in the literature for transition metals catalyzing the
formation of benzoxazoles,115 so one hypothesis is that the ruthenium porphyrin could
potentially be responsible for catalyzing its own incorporation into the BBO linked
polymer. However at this time, we do not have significant evidence to support this claim.
Figure 151. Synthesis for BBO-RuP POP and BBT-RuP POP
4.7 Characterization of ruthenium porphyrin POPs
Fourier transform infrared (FT-IR) spectroscopy of the ruthenium polymers was found to
display similar resonances to the free base polymers with the distinct addition of a strong
resonance at 1930 cm-1 assignable to the carbonyl moiety present at the axial position of
the ruthenium atom (see Figures 159 and 160). The permanent porosity of BBO-RuP POP
and BBT-RuP POP was measured by nitrogen gas adsorption at 77 K. The BBO-RuP POP
and BBT-RuP POP both exhibited reversible type I isotherms with small hysteresis (see
Figure 152). Application of the Brunauer-Emmett-Teller (BET) model over the lowpressure region (0.001<p/p˚<0.1) provided surface areas of 621 m2/g and 644 m2/g for the
172
250
A
150
0.06
B
0.05
dV (cm3/g)
Quantity Adsorbed (cm3/g)
200
100
0.04
0.03
0.02
0.01
50
0
1
1.5
2
2.5
3
Pore Width (nm)
3.5
4
0
0
0.1
0.2
0.3
0.4
0.5
0.6
Relative Pressure (p/p˚)
0.7
0.8
0.9
1
Figure 152. A) N2 adsorption isotherms and B) pore size distributions for BBO-RuP POP
(brown) and BBT-RuP POP (red)
BBO-RuP POP and BBT-RuP POP respectively. Interestingly, BBT-RuP POP showed an
increase in surface area compared to the non-metallated BBT-RuP POP. We believe that
the carbonyl axial ligand on ruthenium decreases the π-π stacking ability of 1-Ru compared
to 1, and might increase the space between porphyrins in the polymeric material. Nonlocal
density functional theory (NLDFT) was used to estimate the pore size distribution of the
BBO-RuP POP and BBT-RuP POP yielding values of 3.26 nm for both BBO-RuP POP
and BBT-RuP POP (see Figure 152). The CO2 uptake capacity of BBO-RuP POP and BBT173
Quantity Adsorbed (mg/g)
80
RuP POP was measured by CO2
70
adsorption isotherms at 295 K
60
from 0 to 1.2 bar (see Figure
50
153). Both BBO-RuP POP and
40
BBT-RuP
30
POP
exhibited
20
respectable CO2 uptake at low
10
pressures reaching capacities of
0
0
0.2
0.4
0.6
0.8
1
Relative Pressure (p/p˚)
1.2
1.4
Figure 153. CO2 adsorption isotherms for BBO-RuP
POP (brown) and BBT-RuP POP (red)
74 and 67 mg/g respectively.
The photophysical properties of
BBO-RuP POP and BBT-RuP POP were examined by solid state UV-Vis spectroscopy.
BBO-RuP POP and BBT-RuP POP both exhibit blue-shifted absorption maxima around
540 nm as expected after the incorporation of ruthenium into the porphyrin macrocycle
(see Figure 154). X-Ray photoelectron spectroscopy (XPS) analysis of the Ru-POPs and
monomers confirmed the presence of ruthenium in our RuTPP homogeneous catalyst, 1Ru, and in both BBO-RuP POP and BBT-RuP POP (see Figures 178 and 179).
174
5
4.5
4
3.5
F(R)
3
2.5
2
1.5
1
0.5
0
200
400
600
800
1000
Wavelength (nm)
1200
1400
Figure 154. Kubelka-Munk diffuse reflectance spectra for BBO-RuP POP (brown) and
BBT-RuP POP (red)
4.8 Future directions and outlook
4.8.1 BBO-RuP POP and BBT-RuP POP as hydrosilylation catalysts
BBT-RuP POP has been employed in a preliminary trial of hydrosilylation reaction and
qualitatively showed some conversion of CO2 to potassium formate. These results have yet
to be repeated and quantified to understand the capability of this polymer as a
heterogeneous catalyst. BBO-Ru-POP has yet to be employed as a hydrosilylation catalyst.
175
These polymers will additionally be characterized by ICP-MS, and thermogravimetric
analysis to quantitatively determine their composition and thermal stability.
4.8.2 Recyclability
After the initial catalytic activity of BBO-RuP POP and BBT-RuP POP are investigated in
the hydrosilylation reaction, we will investigate their catalytic activity after several cycles.
After isolating the heterogeneous catalyst by filtration, washing and drying, we envision
the polymers should maintain their catalytic activity over several uses. Additionally, we
will investigate the stability of these polymers to determine how useful the BBO and BBT
linkages can be in heterogeneous catalysis under different aqueous, acidic, or basic
conditions.
4.8.3 Outlook
We envision that reduction of CO2 to value added fuels or commodity chemicals will be
an extremely important reaction in the future, both economically, and for the environmental
benefit of the planet. Now passing the pinnacle of fossil fuels, a variety of different
alternative energy technologies will begin to take the place of traditional fuels. Between
solar, wind, and geothermal methods for generating electricity, to the reduction and use of
CO2 as a basis for a new, closed cycle fuel system, we believe that porous organic polymers
have incredible potential for use in many of these new energetic systems.
176
4.9 Experimental Section
4.9.1 Instrumentation and methods
Infrared spectra were recorded on a Thermo Scientific Nicolet iS5 with an iD7 diamond
ATR attachment and are uncorrected. UV-Vis absorbance spectra were recorded on a Cary
5000 UV-Vis/NIR spectrophotometer using an internal DRA with stock powder cell holder
to record the % reflectance spectra. Emission spectra were recorded on a Cary Eclipse
Fluorescence spectrophotometer equipped with a xenon flash lamp. Surface area
measurements were conducted on a Micromeritics ASAP 2020 Surface Area and Porosity
Analyzer using ca. 15 mg samples. Nitrogen isotherms were generated by incremental
exposure to ultra high purity nitrogen up to ca. 1 atm in a liquid nitrogen (77 K) bath.
Carbon dioxide isotherms were generated incremental exposure to ultra-high purity carbon
dioxide up to ca. 900 mmHg in a water (295 K) bath. Surface parameters were determined
using BET adsorption models in the instrument software. Pore size distributions were
determined using the non-local density functional theory (NLDFT) model (slit pore, 2DNLDFT, N2-carbon finite pores As=6) in the instrument software (Micromeritics ASAP
2020 V4.02).
1
H NMR spectra were recorded in deuterated solvents on a Bruker Avance DPX 400 (400
MHz). Chemical shits are reported in parts per million (ppm, δ) using the solvent as the
internal standard. Quantitative 1H NMR spectra were recorded in deuterated solvents on a
177
Bruker Avance III HD Ascend 600 MHz using the solvent as an internal standard. Solidstate 13C NMR spectra for BBO- and BBT-linked polymers were recorded using a Bruker
AVIII 600 MHz spectrometer with wide-bore magnet (600.3 MHz) using a 3.2 mm magic
angle spinning (MAS) HXY solid-state NMR probe and running 32 k scans. Crosspolarization with MAS (CP-MAS) was used to acquire
13
C data at 150.9 MHz. The
13
C
cross polarization time was 2 ms at 50 kHz for 13C. 1H decoupling was applied during data
acquisition. The decoupling power corresponded to 100 kHz. The HXY sample spinning
rate was 15 kHz.
X-Ray photoelectron spectroscopy (XPS) was performed on a Kratos Axis Ultra XPS
instrument. A monochromatic aluminum X-Ray (12 kV, 10 mA) source was used, with a
charge neutralizer to minimize sample charging. The carbon 1s peak was subsequently
calibrated to 284.4 eV with sample dwells ranging from 100-650 with 8 sweeps for each
element.
4.9.2 Synthetic methods
2,5-Diamino-1,4-benzenediol dihydrochloride (DABD): this compound has been
previously synthesized by Pyles et al.112
2,5-Diamino-1,4-benzenedithiol dihydrochloride (DABTD): this compound has been
previously synthesized by Wolfe et. al.157
178
Synthesis of 1
Compound 1 was synthesized by a modified procedure from Hirel et. al.159 To an ovendried 100 mL flask with stirbar was added 2 (AMT). The flask was capped and degassed
with N2. Dry Et2O (50 mL, etc) was added via syringe to the degassing mixture. The flask
was cooled in a -78˚C bath before n-BuLi ( M, mL, mmol, eq) was added via syringe. The
mixture was allowed to warm to 0˚C for 3 h. After 3 h the mixture was cooled again to 78˚C and dry DMF (5 mL, amt) was added via syringe. The mixture was allowed to warm
to RT over 3 h. After 3 h the mixture was opened to air and quenched by pouring into dilute
HCl (1.65 M, 250 mL, etc). Mixture stirred in air ~15 minutes before neutralizing with
conc. NH4OH solution. Product extracted into CHCl3 and washed with water. Crude
product purified by column chromatography on silica gel in 40:1 DCM:Et2O. Product
isolated in 31% yield as deep purple solids.
Synthesis of BBO-H2P POP and BBT-H2P POP
To an oven dried 10 mL flask with stirbar was added DABD or DABTD (DABTD stored
and weighed out in glovebox) the flask was then capped and degassed with N2 while dry
DMF (2.5 mL) was added. The mixture was cooled to -30˚C for BBO polymers and -78˚C
for BBT polymers. A solution of 1 (g, mmol) in DMF (2.5 mL) was created and added to
179
the salt via syringe under degassing conditions. The mixture was allowed to stir under N2
while cool for 3 hours for BBO polymers or 5 hours for BBT polymers. After stirring cold,
the mixture was removed from the bath and allowed to warm to room temperature and stir
under N2 overnight (~16 h). After overnight stirring the mixture was degassed with air (1
balloon) and a solution of NaCN catalyst (0.0025 g, 0.05 mmol, 1 eq) in MeOH (200 µL)
was added to BBO polymers, no additives were required for BBT polymers. The mixture
was heated in a 130˚C bath open to air for 4d. After 4d the mixture was allowed to cool,
then filtered to isolate solids, and washed with acetone. Unreacted starting materials were
removed by soaking polymer solids in MeOH for 1 d (changing MeOH 3 times) and DCM
for 1 d (changing DCM 3 times) then dried under high vacuum. BBO-H2P POP, 38 mg,
67% yield. BBT-H2P POP, 41 mg, 66% yield.
180
Table 27. Optimization of the synthesis of BBO-H2P POPs
Trial Set
Condition
Changed
1 Eq in MeOH
Eq CN
0 Eq
0 Eq W/ MeOH
1:1 DMF: O-xyl
Solvents
1:1 DMF: Mes
DMF (dilute)
100
Temperatures
80
Porosity
(m2/g)
Pore Size
(nm)
760
566
564
496
271
485
660
419
1.46
3.26
1.46
1.46
1.46
1.46
1.46
1.46
Table 28. Optimization of the synthesis of BBT-H2P POPs
Trial Set
Eq CN
Solvents
Temperatures
Condition
Changed
1 Eq in MeOH
0 Eq
0 Eq W/ MeOH
1:1 DMF: O-xyl
1:1 DMF: Mes
1:1 (dilute)
100
80
181
Porosity
(m2/g)
139
266
91
535
480
557
26
211
Pore Size
(nm)
1.74
1.46
1.74
1.46
1.46
1.46
1.46
1.46
Synthesis of BBO-RuP POP and BBT-RuP POP
BBO-RuP POP and BBT-RuP POP were synthesized by the same method as BBO-H2P
POP and BBT-H2P POP, using the same reaction times and temperatures listed previously,
and 1-Ru was used in place of 1. BBO-RuP POP, 51 mg, 77% yield. BBT-RuP POP, 50
mg, 74% yield.
Synthesis of potassium formate by hydrosilylation of CO2
An oven dried 4 mL vial with stirbar was loaded with RuTPP (4 mg, 0.005 mmol, 0.005
eq) and KF (58 mg, 1 mmol, 1 eq) and the vial was capped with a septum cap. A solution
of silane (1 mmol, 1 eq) was created in the desired solvent (2 mL) and the mixture was
added to the catalyst via syringe. The vial was placed under an atmosphere of CO2. The
vial was placed in a heating block at a temperature near the boiling point of the desired
solvent (100˚C for dioxane, 80˚C for MeCN, and 60˚C for THF). The mixture was allowed
to react for the desired reaction period. After the reaction period solvents were removed on
hivac. The isolated solids were redissolved in a water/DCM mixture, and washed with
DCM. The water was removed via hivac to afford a mixture of salts. Yields of potassium
formate were calculated by quantitative NMR: An NMR sample was created in D2O (0.7
mL), dissolving all solids by sonication, and DMSO (12 µL, 1/6 mmol) was added as an
internal standard. Integration of the DMSO peak against the potassium formate peak was
used to determine the yield of the reaction.
182
4.9.3 FT-IR spectra
4000
3500
3000
2500
2000
1500
Wavenumber (cm-1)
1000
500
Figure 155. FT-IR spectrum of RuTPP
4000
3500
3000
2500
2000
1500
-1
Wavenumber (cm )
Figure 156. FT-IR spectrum of 1-Ru
183
1000
500
4000
3500
3000
2500
2000
1500
-1
Wavenumber (cm )
1000
500
Figure 157. FT-IR spectrum of BBO-H2P POP
4000
3500
3000
2500
2000
1500
-1
Wavenumber (cm )
Figure 158. FT-IR spectrum of BBT-H2P POP
184
1000
500
4000
3500
3000
2500
2000
1500
-1
Wavenumber (cm )
1000
500
Figure 159. FT-IR spectrum of BBO-RuP POP
4000
3500
3000
2500
2000
Wavenumber (cm-1)
Figure 160. FT-IR spectrum of BBT-RuP POP
185
1500
1000
500
4.9.4 PXRD profiles
0
5
10
15
20
2Θ
25
30
35
40
Figure 161. PXRD profile of BBO-H2P POP
0
5
10
15
20
2Θ
25
30
Figure 162. PXRD profile of BBT-H2P POP
186
35
40
0
5
10
15
20
2Θ
Figure 163. PXRD profile of BBT-RuP POP
187
25
30
35
4.9.5 Solid-state NMR spectra
Figure 164. 13C CP-MAS spectrum of BBT-H2P POP
188
Figure 165. 13C CP-MAS spectrum of BBT-RuP POP
189
Q (1 - p°) (cm³/g STP)
4.9.6 Surface area analysis
200
180
160
140
120
100
80
60
40
20
0
0
0.2
0.4
0.6
0.8
Relative Pressure (p/p˚)
Figure 166. Roquerol BET analysis of BBO-H2P POP
Table 29. BET values derived from Roquerol BET analysis of BBO-H2P POP
BBO-H2P POP
BET
Correlation
(p/p˚)
2
(m /g) coefficient
0.001003-0.106287 760
0.9999
0.001003-0.009911 698
0.9998
0.001003-0.008899 694
0.9998
190
C
1097.02
2529.88
2690.75
1
0.0007
1/[Q(p°/p - 1)]
0.0006
0.0005
0.0004
0.0003
y = 0.0057x + 5E-06
R² = 0.99993
C=1097.02
0.0002
0.0001
0
0
0.02
0.04 0.06 0.08
0.1
Relative Pressure (p/p˚)
Figure 167. BET surface area plot for BBO-H2P POP
191
0.12
Q (1 - p°) (cm³/g STP)
140
120
100
80
60
40
20
0
0
0.2
0.4
0.6
0.8
Relative Pressure (p/p˚)
Figure 168. Roquerol BET analysis of BBT-H2P POP
Table 30. BET values derived from Roquerol BET analysis of BBT-H2P POP
BBT-H2P POP
BET
Correlation
(p/p˚)
2
(m /g)
coefficient
0.001045-0.090111
535
0.9999
0.001045-0.0801
535
0.9999
0.001045-0.070004
534
0.9999
192
C
886.63
889.23
904.17
1
0.0008
1/[Q(p°/p - 1)]
0.0007
0.0006
0.0005
0.0004
0.0003
y = 0.0081x + 9E-06
R² = 0.99991
C= 886.63
0.0002
0.0001
0
0
0.02
0.04
0.06
0.08
Relative Pressure (p/p˚)
Figure 169. BET surface area plot for BBT-H2P POP
193
0.1
Q (1 - p°) (cm³/g STP)
160
140
120
100
80
60
40
20
0
0
0.2
0.4
0.6
0.8
Relative Pressure (p/p˚)
Figure 170. Roquerol BET analysis of BBO-RuP POP
Table 31. BET values derived from Roquerol BET analysis of BBO-RuP POP
BBO-RuP POP
BET Correlation
(p/p˚)
(m2/g) coefficient
0.000977-0.060194
622
0.9999
0.000977-0.049623
622
0.9999
0.000977-0.039284
620
0.9999
194
C
1527.39
1548.93
1625.02
1
1/[Q(p°/p - 1)]
0.00045
0.0004
0.00035
0.0003
0.00025
0.0002
0.00015
0.0001
0.00005
0
y = 0.007x + 5E-06
R² = 0.99993
C=1527.39
0
0.01 0.02 0.03 0.04 0.05 0.06 0.07
Relative Pressure (p/p˚)
Figure 171. BET surface area plot for BBO-RuP POP
195
Q (1 - p°) (cm³/g STP)
160
140
120
100
80
60
40
20
0
0
0.2
0.4
0.6
0.8
Relative Pressure (p/p˚)
Figure 172. Roquerol BET analysis of BBT-RuP POP
Table 32. BET values derived from Roquerol BET analysis of BBT-RuP POP
BBT-RuP POP
BET Correlation
(p/p˚)
(m2/g) coefficient
0.001001-0.069851
655
0.9999
0.001001-0.060028
654
0.9999
0.001001-0.049236
652
0.9999
196
C
1138.19
1156.92
1191.37
1
1/[Q(p°/p - 1)]
0.0005
0.00045
0.0004
0.00035
0.0003
0.00025
0.0002
0.00015
0.0001
0.00005
0
y = 0.0066x + 6E-06
R² = 0.99992
C=1138.19
0
0.02
0.04
0.06
Relative Pressure (p/p˚)
Figure 173. BET surface area plot for BBT-RuP POP
197
0.08
4.9.7 UV-Vis and fluorescence spectra
6
Absorbance (a.u.)
5
4
3
2
1
0
300
350
400
450
500
550
600
Wavelength (nm)
650
700
750
800
Figure 174. UV-Vis spectrum of 1 in toluene
250
Intensity (a.u.)
200
150
100
50
0
500
550
600
650
700
Wavelength (nm)
750
800
Figure 175. Fluorescence spectrum of 1 in toluene (λexitation = 430 nm)
198
6
Absorbance (a.u.)
5
4
3
2
1
0
350
400
450
500
550
600
650
Wavelength (nm)
700
750
800
Figure 176. UV-Vis spectrum of 1-Ru in acetone
100
90
80
Intensity (a.u.)
70
60
50
40
30
20
10
0
500
550
600
650
700
Wavelength (nm)
750
800
Figure 177. Fluorescence spectrum of 1-Ru in acetone (λexitation = 415 nm)
199
4.9.8 XPS spectra
850
1000
A
750
700
650
850
800
700
550
495
485
475
465
Binding Energy (eV)
495
455
800
C
485
475
465
Binding Energy (eV)
455
D
750
Intensity (a.u.)
1100
Intensity (a.u.)
900
750
600
1150
B
950
Intensity (a.u.)
Intensity (a.u.)
800
1050
1000
950
700
650
600
550
900
500
850
495
485
475
465
Binding Energy (eV)
495
455
485
475
465
Binding Energy (eV)
455
Figure 178. XPS spectra of the Ru 3P for A) RuTPP, B) 1-Ru, C) BBT-RuP POP, and D)
BBO-RuP POP
200
A
4000
3000
2000
1000
0
290
285
280
Binding Energy (eV)
295
275
C
Intensity (a.u.)
5000
Intensity (a.u.)
B
5000
295
6000
6000
Intensity (a.u.)
Intensity (a.u.)
4500
4000
3500
3000
2500
2000
1500
1000
500
0
4000
3000
2000
1000
0
295
290
285
280
Binding Energy (eV)
275
4000
3500
3000
2500
2000
1500
1000
500
0
290
285
280
Binding Energy (eV)
275
290
285
280
Binding Energy (eV)
275
D
295
Figure 179. XPS spectra of the Ru 3D and C 1S for A) RuTPP, B) 1-Ru, C) BBT-RuP
POP, and D) BBO-RuP POP
201
4.9.9 1H and 13C NMR spectra
Figure 180. 1H NMR spectrum of RuTPP
202
Figure 181. 1H NMR spectrum of 1-Ru
203
Figure 182. Example quantitative 1H NMR spectrum for the hydrosilylation of CO2 by
RuTPP. The product at 8.376 ppm is intergrated against DMSO at 2.643 ppm
204
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