Article
pubs.acs.org/crystal
Two New Series of Coordination Polymers and Evaluation of Their
Properties by Density Functional Theory
Fredrik Lundvall,*,† Ponniah Vajeeston,† David S. Wragg,†,‡ Pascal D. C. Dietzel,§ and Helmer Fjellvåg†,‡
†
SMN - Centre for Materials Science and Nanotechnology, Department of Chemistry, University of Oslo, P.O. Box 1126, N-0318
Oslo, Norway
‡
inGAP − Innovative Natural Gas Processes and Products, Department of Chemistry, University of Oslo, P.O. Box 1033, N-0315
Oslo, Norway
§
Department of Chemistry, University of Bergen, P.O. Box 7803, N-5020 Bergen, Norway
S Supporting Information
*
ABSTRACT: Five new coordination polymers (CPs), CPO68-M (M = Zn, Mn, and Co) and CPO-69-M (M = Ca and
Cd), were synthesized by solvothermal methods using 4,4′dimethoxy-3,3′-biphenyldicarboxylic acid as the organic linker.
The three-dimensional frameworks are formed by metal
carboxylate chains that are separated by the linker. Structural
analysis reveals dense networks with narrow rhombic channels
and sra topologies for both CPO-68-M and CPO-69-M. The major structural difference between the two series of CPs is in the
metal coordination polyhedra, which are four- and eight-coordinated in CPO-68-M and CPO-69-M, respectively. The CPs are
highly crystalline, robust, and have good thermal stability (> 350 °C). On the basis of the topological similarities with MIL-53, we
tested whether the CPs would exhibit a similar flexible structure response to gas stimulus. Density functional theory (DFT)
modeling was used to evaluate the CPs’ potential as gas adsorption materials over a large range of pressures. The DFT analysis
concluded that the CPs are ill-suited for gas adsorption due to their structural rigidity. However, electronic structure calculations
reveal that CPO-68-M and CPO-69-M are indirect band gap semiconductors with an estimated band gap between 2.49 and 2.98
eV.
■
INTRODUCTION
Custom compounds with potential applications in varied fields
such as gas sorption, catalysis, luminescence, or photovoltaics
can be created by combining organic and inorganic building
blocks.1−4 For this intriguing class of materials called
coordination polymers (CPs), the vast range of interchangeable
organic linkers and metal secondary building units (SBUs) give
a unique flexibility in the synthesis and tailoring of new
materials, not only to satisfy scientific curiosity and understanding, but also to develop functional materials with
interesting properties for industrial applications.
Large scale CO2 capture from the flue gas of power plants
requires efficient and robust functional materials, among which
metal−organic frameworks (MOFs) have been and are still
being heavily explored. For large scale application, good
diffusion properties are important to increase efficiency and
minimize the pressure drop over the adsorption material.
Furthermore, strong interactions between the gas and a large
surface area of the adsorbent are desired to increase the amount
of gas adsorbed per gram material. Additional parameters for
commercialization are naturally connected to costs of
production, toxicity of chemicals, and not least, mechanical
strength and potential formation of byproducts and materials
loss. Our group and others have previously published several
articles describing the family of materials formed by CPO-27
and its isostructural MOF-74 analogue.5−13 These are based on
© 2015 American Chemical Society
divalent metal cations (Zn, Co, Ni, Mg, Mn, Fe, and Cu) and
2,5-dihydroxyterephtalic acid (DHTP) and feature coordinatively unsaturated sites, so-called open metal sites, upon
activation. These open metal sites create a strong host−guest
interaction toward a number of gases (CO2, CO, NO, H2, and
CH4).13−20 Recreating similarly strong host−guest interactions
in a new series of MOFs would be highly interesting in the
context of gas sorption/separation. More efficient gas diffusion
into the pores compared to CPO-27/MOF-74 would also be
beneficial. CPO-27/MOF-74 are based on a substituted
benzene linker. The logical step is hence to improve the gas
diffusion by increasing the length of the linker and thereby
create larger pores in the structure. This way of tuning the pore
dimensions (and functional properties) of known MOFs is
commonly referred to as reticular design or the isoreticular
approach21 and has already successfully been applied to create
isoreticular CPO-27/MOF-74 type MOFs.22,23
In the current work, the 4,4′-dimethoxy-3,3′-biphenyldicarboxylic acid linker has been explored along with a series of
divalent cations. Isomers of this linker have produced highly
crystalline CPs with interesting topologies,24−28 albeit none
being isostructural to CPO-27/MOF-74. Nevertheless, 4,4′Received: September 8, 2015
Revised: November 16, 2015
Published: November 23, 2015
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DOI: 10.1021/acs.cgd.5b01302
Cryst. Growth Des. 2016, 16, 339−346
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Article
Table 1. Summary of Synthesis Details for CPO-68-M and CPO-69-M
name
CPO-68-Zn
CPO-68-Mn
CPO-68-Co
CPO-69-Ca
CPO-69-Cd
metal source
metal amount
linker amount
solvent 1
solvent 2
temperature (°C)
reaction time (h)
Zn(NO3)2·6H2O
59.5 mg, 0.2 mmol
60.4 mg, 0.2 mmol
DMF, 2.0 mL
H2O, 0.1 mL
120
48
MnCl2·4H2O
39.6 mg, 0.2 mmol
60.4 mg, 0.2 mmol
DMF, 2.0 mL
H2O, 0.1 mL
100
8
Co(NO3)2·6H2O
58.2 mg, 0.2 mmol
60.4 mg, 0.2 mmol
DMF, 2.0 mL
H2O, 0.1 mL
100
48
Ca(NO3)2·4H2O
47.2 mg, 0.2 mmol
60.4 mg, 0.2 mmol
DMF, 2.0 mL
none
120
24
Cd(NO3)2·4H2O
61.7 mg, 0.2 mmol
60.4 mg, 0.2 mmol
DMF, 2.0 mL
H2O, 0.1 mL
120
48
confirm the absence of other crystalline phases in the samples (see
Figures S3−S7 in the SI). Because of fluorescence in the case of CPO68-Co, this sample was measured on the Swiss-Norwegian Beamline
(BM01A) at the European Synchrotron Radiation Facility using
monochromatic synchrotron radiation (λ = 0.69396 Å). The setup
uses a Huber goniometer and Dectris Pilatus 2M photon counting
pixel area detector.35 The 2D diffraction data from a 90 s exposure
were converted to a 1D P-XRD pattern using FIT2D.36
Additional P-XRD data on selected samples were collected at 20 bar
pressure of CO2 and ambient temperature on a Bruker D8 instrument
with monochromatic CuKα1 radiation (λ = 1.5406 Å) and a LynxEye
XE position sensitive detector operated in transmission geometry.
Diffuse reflectance UV−vis spectroscopy (DRS) was performed on
a Shimadzu UV-3600 instrument with an integrating sphere using
compacted BaSO4 as reference. The optical band gaps were estimated
by making a Tauc plot of [F(R)hν]1/2 versus the photon energy hν. To
this end, the Kubelka−Munk function F(R) = (1 − R)2/2R was
calculated from the reflectance data obtained through the DRS UV−
vis measurements.
Synthesis of CPO-68-M and CPO-69-M. The synthesis
procedure for CPO-68-Zn described below can be regarded as a
general procedure for all members of the CPO-68-M and CPO-69-M
series. The individual synthesis parameters are summarized in Table 1.
Synthesis of CPO-68-Zn. Zn(NO3)2·6H2O (59.5 mg, 0.2 mmol)
and 4,4′-dimethoxy-3,3′-biphenyldicarboxylic acid (60.4 mg, 0.2
mmol) was weighed in and dissolved in a mixture of DMF (2.0
mL) and deionized water (0.1 mL). The mixture was heated in a 5 mL
glass vial at 120 °C for 48 h and then cooled to room temperature.
This procedure yielded colorless needle crystals of sufficient quality for
S-XRD analysis.
Computational Details. The quantum-mechanical calculations
were performed in the framework of density functional theory (DFT)
using the generalized gradient approximation (GGA) 37,38 as
implemented in the VASP code.39,40 The interaction between the
ion and electron is described by the projector augmented wave
method.41,42 For the calculations presented here we have used planewave cutoff energy of 600 eV which give well converged results with
respect to the basis set. The k-points were generated using the
Monkhorst−Pack method with a grid size of 2 × 6 × 6, 2 × 6 × 8, and
2 × 8 × 4 for the CPO-68/69 materials, low temperature MIL-53(Cr)
_lt and high temperature MIL-53(Cr)_ht, respectively. Iterative
relaxation of atomic positions was stopped when the change in total
energy between successive steps was less than 1 meV/cell. With this
criterion, the maximum forces generally acting on the atoms were
found to be less than 10 meV/Å. The initial atomic coordinates of the
CPO-68/69 were taken from the presented refined X-ray structures
and for the MIL-53(Cr)_lt and MIL-53(Cr)_ht framework the
coordinates are taken from work by Serre et al.43 In our theoretical
simulation, we have relaxed the atomic positions and cell parameters
globally using force-minimization techniques fixed to the experimental
volume. Then the theoretical equilibrium volume is determined by
varying the cell volume within ±10% of the experimental volume.
Finally the calculated energy versus volume data are fitted into the
universal-equation-of-state fit, and the equilibrium cell parameters are
extracted. The theoretically obtained structural parameters and the
positional parameters are in very good agreement with the
corresponding experimental findings. The calculated cell parameters
are within 2.1% of the experimental values.
dimethoxy-3,3′-biphenyldicarboxylic acid has the potential to
mimic the DHTP linker used in CPO-27/MOF-74 with regard
to forming one-dimensional chain SBUs and thereby provides a
basis for open metal sites in novel porous MOFs.
In this work we report the solvothermal synthesis and
structure determination of five new CPs that form two new
series of CPs named CPO-68-M (M = Zn, Mn, and Co) and
CPO-69-M (M = Cd and Ca). Although their structures are of
a different type than targeted, they are nevertheless highly
interesting. The topological similarities with flexible MOFs such
as MIL-53 led to an investigation into whether or not the CPs
would exhibit a flexible behavior when subjected to pressurized
gas. These issues were additionally evaluated by DFT modeling,
with a focus on gas adsorption at high pressures, as well as the
electronic properties of the CPs.
■
EXPERIMENTAL SECTION
Materials and Methods. All starting materials and solvents were
obtained from commercial suppliers (Sigma-Aldrich and VWR) and
were used without additional purification. The 1H NMR spectra were
recorded on a Bruker DPX 300 MHz spectrometer at room
temperature in the indicated solvents. Chemical shifts are expressed
in parts per million (δ) using residual solvent protons as internal
standards (1H: CDCl3: δ 7.26 ppm; DMSO-d6: δ 2.49 ppm). The
synthesis of the linker, 4,4′-dimethoxy-3,3′-biphenyldicarboxylic acid,
is based on the procedure described by Wang et al.,29 and the
experimental details are described in the Supporting Information (SI).
Thermogravimetric analysis (TGA) was performed on a
Perkin−Elmer TGA 7 under a flow of N2-gas. The samples were
heated from 30 to 800 °C in alumina crucibles, with a ramp rate of 2
°C/min. The TGA results are displayed in Figures S1 and S2 in the SI.
Single-crystal X-ray diffraction data (S-XRD) were recorded at
ambient temperature on a Bruker D8 instrument fitted with an APEX2
CCD area-detector and using monochromatic MoKα1 radiation (λ =
0.7093 Å) from a sealed tube source. The crystals were extracted from
the mother liquor and mounted on thin glass rods with a small amount
of epoxy glue. Data reduction was performed using SAINT, and
absorption correction was performed using SADABS.30 The structures
of CPO-68-Zn/Mn/Co and CPO-69-Ca were solved by direct
methods using SIR9231 and refined with SHELXL-201232 as
implemented in the WinGX33 program suite. The structure of CPO69-Cd was solved from a nonmerohedral twin and absorption
correction was performed with TWINABS.30 This structure was
solved with SHELXS-9732 and refined with SHELXL-201232 as
implemented in the WinGX33 program suite. Hydrogen atoms were
positioned geometrically at distances of 0.93 (CH) and 0.96 Å (CH3)
and refined using a riding model with Uiso (H) = 1.2 Ueq (CH) and
Uiso (H) = 1.5 Ueq (CH3).
Powder X-ray diffraction data (P-XRD) collected at ambient
atmosphere and temperature were recorded on a Siemens D5000
instrument with monochromatic CuKα1 radiation (λ = 1.5406 Å) and
a Braun position sensitive detector operated in transmission geometry.
Crystals of CPO-68-M and CPO-69-M were gathered, ground to a
powder in a mortar, and sealed in glass capillaries for measurement. A
Pawley fit was performed on the recorded patterns using the TOPAS34
software with the unit cell parameters determined by S-XRD to
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Table 2. Crystallographic Data for CPO-68-M and CPO-69-Ma
name
CPO-68-Zn
CPO-68-Mn
CPO-68-Co
CPO-69-Ca
CPO-69-Cd
formula
formula weight
T (K)
crystal system
space group (#)
Z
a (Å)
b (Å)
c (Å)
β (deg)
V (Å3)
Dc (g cm−3)
μ (mm−1)
reflns collected
reflns unique
Rint
crystal size (mm3)
crystal color
crystal shape
F(000)
residual density max/min (e·Å−3)
GOF
final R indices [I > 2σ(I)]
C16H12O6Zn
365.63
296 (2)
monoclinic
C2/c (15)
4
23.596 (2)
8.2484 (8)
7.6112 (7)
98.7920 (10)
1464.0 (2)
1.659
1,707
6139
1766
0.0143
0.50 × 0.20 × 0.15
colorless
needle
744
0.274/−0.295
1.128
R1 = 0.0260
wR2 = 0.0649
R1 = 0.0292
wR2 = 0.0664
C16H12O6Mn
355.20
296 (2)
monoclinic
C2/c (15)
4
24.026 (11)
8.565 (4)
7.214 (3)
96.093 (6)
1476.0 (12)
1.598
0.923
2773
1104
0.054
0.15 × 0.08 × 0.05
brown
plate
724
0.795/−0.977
1.152
R1 = 0.0621
wR2 = 0.1818
R1 = 0.0851
wR2 = 0.2105
C16H12O6Co
359.19
296 (2)
monoclinic
C2/c (15)
4
23.388 (3)
8.5369 (9)
7.3474 (8)
99.3740 (10)
1447.4 (3)
1.648
1.215
6082
1759
0.0125
0.28 × 0.15 × 0.09
purple
plate
732
0.796/−0.453
1.081
R1 = 0.0273
wR2 = 0.0770
R1 = 0.0300
wR2 = 0.0785
C16H12O6Ca
340.34
296 (2)
monoclinic
C2/c (15)
4
24.721 (6)
8.000 (2)
7.5253 (19)
90.949 (2)
1488.1 (6)
1.519
0.451
5430
1463
0.0415
0.09 × 0.08 × 0.03
colorless
needle
704
0.413/−0.224
1.091
R1 = 0.0505
wR2 = 0.1293
R1 = 0.0698
wR2 = 0.1400
C16H12O6Cd
412.66
296 (2)
monoclinic
C2/c (15)
4
24.514 (6)
8.401 (2)
7.300 (2)
92.548 (3)
1501.8 (7)
1.825
1.483
3986
3474
0.0416
0.50 × 0.20 × 0.16
yellow
needle
816
1.188/−0.752
1.093
R1 = 0.0376
wR2 = 0.1107
R1 = 0.0411
wR2 = 0.1135
R indices (all data)
a
Calculated standard deviations in parentheses.
Figure 1. Structure of CPO-68-Zn, viewed along the c-axis (Zn: teal, C: gray, O: red). Hydrogen atoms are omitted for clarity.
■
CPO-68-M. S-XRD analysis of CPO-68-M (M = Zn, Mn
and Co) reveals that the CP crystallizes in the monoclinic space
group C2/c with one divalent metal and one deprotonated
linker in the asymmetric unit. The structure features metal
carboxylate chains (Figure 3a) along the c-axis that are linked
by the biphenyl linker to form three-dimensional (3D) CPs
with sra topology (Figure 5). The metal is four-coordinate with
a slightly distorted tetrahedral coordination polyhedron, which
is defined by four oxygen atoms from four different biphenyl
linkers. Thus, every linker is connected to four different metal
atoms where the carboxylate-groups provide a bridging motif
between two adjacent metal atoms (Figure 4). The M−O bond
distances in CPO-68-Zn range from 1.9468 (12) Å to 1.9835
(13) Å and are in accordance with the expected bond lengths
when applying the bond valence method.44 When the structure
RESULTS AND DISCUSSION
The solvothermal synthesis of 4,4′-dimethoxy-3,3′-biphenyldicarboxylic acid and a range of divalent cations resulted in two
new series of isostructural CPs; CPO-68-M (M = Zn, Mn, and
Co) and CPO-69-M (M = Cd and Ca) (Table 2). CPO-68-M
and CPO-69-M (Figure 1 and Figure 2) have at a first glance
very similar structures differing mainly in their metal
coordination polyhedra. Indeed, analyses of the structures
reveal that their underlying topologies are in fact the same.
Although the two series of CPs share many features, there are
also key differences. The crystallographic details of the two
series of CPs will therefore be discussed separately. Where it is
necessary for the sake of comparison, the structures of CPO-68Zn and CPO-69-Ca have been chosen as representative
examples of their series.
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Figure 2. Structure of CPO-69-Ca, viewed along the c-axis (Ca: light gray, C: gray, O: red). Hydrogen atoms are omitted for clarity.
conditions. All materials in the CPO-68-M series show good
thermal stability as well as long-term stability in ambient
conditions. The TGA data show that the main thermal
decomposition of the materials occurs above 350 °C (Figure
S1). Note that the TGA curve for CPO-68-Mn hints at a two
step decomposition, possibly due to the available oxidation
states of manganese when compared to zinc and cobalt. The
exact details of the decomposition and possible intermediates
are however yet to be determined experimentally.
CPO-69-M. S-XRD analysis of CPO-69-M (M = Ca and
Cd) shows that the CP crystallizes in the monoclinic space
group C2/c with one divalent metal and one deprotonated
linker in the asymmetric unit. The structure features metal
carboxylate chains (Figure 3b) along the c-axis that are linked
by the biphenyl linker to form 3D CPs with sra topology. The
irregular coordination polyhedron comprises eight oxygen
atoms originating from four different biphenyl linkers. The
linkers are coordinated to the metal in two different modes.
Two of the four ligands are coordinated with both oxygen
atoms of the carboxylate-group and the remaining two ligands
with oxygen atom from the carboxylate group and the other
from the methoxy group (Figure 4). As expected, the M−O
bond distances of CPO-69-M are significantly longer when
compared to CPO-68-M and range from 2.302 (2) Å to 2.589
(2) Å in the case of CPO-69-Ca. As with CPO-68-M, when the
structure is viewed along the c-axis, narrow rhombic channels
are revealed that cannot accommodate guest molecules at
ambient conditions (Figure 2). The distances between the Caatoms in the channels of CPO-69-Ca are 24.721 (6) Å along
the a-axis and 8.000 (2) Å along the b-axis. Like CPO-68-M,
the materials of the CPO-69-M series also exhibit good stability,
with thermal decomposition occurring above 350 °C (Figure
S2). The TGA curve of CPO-69-Cd also hints at a two step
decomposition, but the nature of any decomposition
intermediates is still unknown.
To analyze the topology of CPs, it is necessary to reduce or
simplify the structure for analysis. For CPs containing finite
metal cluster SBUs, the structure is commonly simplified by
reducing the linker and the metal SBU to single points or nodes
with a defined connectivity. However, when the metal SBU is
an infinite one-dimensional (1D) chain, this method is not
suitable. To elucidate the underlying structures, we adopted the
procedure for analyzing CPs containing 1D rodlike SBUs
Figure 3. (a) The metal carboxylate chain of CPO-68-Zn (tetrahedral
coordination). (b) The metal carboxylate chain of CPO-69-Ca. (c)
The zigzag ladder formed by linking the carboxylate C atoms of CPO69-Ca (Zn: teal, Ca: light gray, C: gray, O: red).
Figure 4. Coordination modes of 4,4′-dimethoxy-3,3′-biphenyldicarboxylic acid in CPO-68-Zn (left) and CPO-69-Ca (right).
is viewed along the c-axis, narrow rhombic channels are
revealed (Figure 1). The distances between the Zn-atoms in the
channels of CPO-68-Zn are 23.596 (2) Å along the a-axis and
8.2484 (8) Å along the b-axis. These distances give an
indication of the size of the channels and suggest a cross section
sufficiently large for small guest molecules such as N2 or CO2.
However, when the atoms of the linkers are taken into account
and the structure is viewed in a space filling model, the channels
are clearly too narrow for any guest molecules at ambient
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Figure 5. sra topology formed by linking the carboxylate C atoms of CPO-68-Zn. One metal carboxylate chain is added for context (Zn: teal, C:
gray, O: red).
described by O’Keeffe and Yaghi.45 Using the TOPOS46,47
software, the carboxylate C atoms of CPO-68/69 were selected
as the nodes of extension and these were connected to create a
uninodal four-connected network of zigzag ladders and
rhombohedral channels (Figure 5). The network was
subsequently run through the classification procedure of
TOPOS to reveal the point symbol 42.63.8 and vertex symbol
4.6.4.6.6.8(2). This corresponds to the sra topology, following
the three letter codes recommended by RCSR.48
■
DFT CALCULATIONS PERFORMED ON CPO-68-M
AND CPO-69-M
In order to understand the electronic properties of the CPs we
have calculated the electronic structure of these compounds.
The calculated band structure and total density of states (DOS)
at the equilibrium volume for CPO-68-M and CPO-69-M with
GGA-PBE level is displayed in Figures S8−S12. The calculated
band gaps (Eg) of CPO-68-M and CPO-69-M series are
indirect and range between 2.49 and 2.98 eV. In general, CPO69-M seems to yield larger Eg than the CPO-68-M CPs. The
calculated band gap of CPO-68/69 is close to the IRMOF-1049
and smaller than that of MOF-5.50 Band gap (Eg) values of
solids obtained from usual DFT calculations are often
systematically underestimated (commonly 30−50%) due to
discontinuity in the exchange correlation potential. In our
recent contribution, however,51 we found that the DFT
calculations on MOF-5 gave a band gap value (3.5 eV) which
was in unexpectedly good agreement with that observed from
experimental studies.52,53 To evaluate the accuracy of the DFT
calculations in this work, we performed optical measurements
using diffuse reflectometry UV−vis spectroscopy (DRS). These
measurements were used in a Tauc plot of [F(R)hν]1/2 as a
function of photon energy to estimate the band gap of selected
samples (Figure 6).54−56 The experimentally determined band
gaps are in relatively good agreement with the calculated values,
albeit with some underestimation in the DFT values (eV calc/
exp for CPO-68-Zn: 2.49/2.95 and CPO-69-Ca: 2.98/3.04).
Figure 6. Tauc plot of [F(R)hν]1/2 as a function of photon energy in
eV. The dashed lines indicate the linear sections of the plot that were
used to estimate the band gaps of CPO-68-Zn (red) and CPO-69-Ca
(blue).
structure or “breathing” effect upon uptake or removal of
solvent guest molecules.43,57,58
The CPs reported herein were initially evaluated for gas
adsorption by analyzing them with the SOLV function in
PLATON.59 This analysis indicated that the CPs have no
solvent accessible void in the structure and that any surface area
would originate from the surface of the particles. Indeed, if the
structures are drawn with a space filling model, it becomes
obvious that the rhombic channels are too narrow to
accommodate gas or solvent at ambient conditions. However,
the rhombic nature of the channels bears great resemblance to
the MIL-53 MOF which has the ability to expand under
moderate (1−10 bar) pressure.43,60,61
Recently, Gustafsson et al. reported a family of flexible
MOFs, the SUMOF-6-Ln family, which is based on lanthanides
and bipyridine dicarboxylates.62 These MOFs differ from CPO68-M or CPO-69-M due to the more linear nature of the linker
and the trivalent metals used in the synthesis. However, the
underlying topologies are very similar, and thus a comparison is
interesting. Furthermore, the SUMOF-6-Ln MOFs demonstrate a similar reversible flexibility as MIL-53 upon desorption
and readsorption of the synthesis solvent. In this way, the
MOFs provide precedence for flexible MOFs with sra topology
and biphenyl linkers.
■
HOST−GUEST INTERACTION IN FLEXIBLE
COORDINATION POLYMERS
Several MOFs reported in the literature show interesting
properties with regard to host−guest interactions. Notable
examples are MIL-53 and MIL-88, which exhibit a flexible
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During initial tests, the CPs were subjected to moderate
pressures of N2 gas while recording the P-XRD patterns to
reveal any changes in the unit cell parameters. However,
problems with the experimental setup meant that these results
had to be regarded as inconclusive. Nonetheless, the
topological similarities of CPO-68-M and CPO-69-M with
flexible MOFs reported in the literature prompted a more
thorough investigation of the potential of these materials for gas
adsorption at high pressure. Since the materials are nonporous
at ambient conditions, it was necessary to evaluate at what
pressureif anythe materials would respond to gas stimulus.
DFT modeling has proven to be a versatile and strong method
for evaluating physical properties of solid state materials and
was therefore selected for the investigation. Furthermore,
recent publications in the literature have started to elucidate the
effects of extreme pressures on MOFs and MOF-like
materials.63−66 The results are intriguing and demonstrate
that even supposedly rigid frameworks can respond to guest
molecules. The adsorption modes are generally different from
flexible networks and phase transitions or distortions of the
network are common. In particular, the relationship between
framework compression and guest molecule inclusion is
interesting.67 Since DFT modeling is not limited by the
pressure tolerances of hardware, the method proved even more
fitting for our needs.
For the gas adsorption isotherm calculation we have used the
sorption module implemented in the Material Studio 6.068
package. Sorption is used to simulate sorption of small
molecules (guest molecules) into porous 3D frameworks. A
loading curve is generated using series of fixed pressure (grand
canonical ensemble) calculations performed over a series of
fugacities. For the sorption computation, we have used the
optimized structures obtained from the VASP calculation as
input for the starting model. The adsorption isotherm displays
the adsorption in molecules per cell at each fugacity. In a typical
adsorption isotherm the curve will rise toward a saturation
point value beyond which no more molecules can be adsorbed.
We have tested for N2, H2, CO2, CO, N2O, and CH4, and there
is no significant uptake by the CPO-68/69 materials up to the
pressure of 0.1 GPa. This finding is consistent with the
experimental gas adsorption observations where selected
samples were subjected to moderate gas pressures (∼20 bar)
of CO2, while recording the P-XRD patterns.
In order to validate our theoretical approach we have carried
out the CO2 adsorption isotherm calculation on MIL-53(Cr) as
a model with various unit-cell volumes. Our calculated CO2
isotherm for the MIL-53(Cr) MOF in two different
polymorphs (lt and ht) as a function of cell parameters vs
saturation point is displayed in Figure 7. The calculations
successfully reproduce the reported difference in CO 2
adsorption in the lt and ht polymorphs. Further details on
the calculation method are provided in the Supporting
Information. The above finding clearly demonstrates that the
presented type of approach is valid. From our theoretical
adsorption simulation and from experimental study we
conclude that the CPO-68/69 CPs are not suitable candidates
for gas adsorption applications and that they are nonporous at
all pressures. On the other hand, based on the magnitude of the
band gap as well as the high stability, these CPs might have
potential application in the photovoltaics industry. More
research is needed in this direction.
Figure 7. Calculated CO2 adsorption isotherm as a function of the cell
volume for MIL-53(Cr).
■
A POSSIBLE RATIONALE FOR THE LACK OF
FLEXIBILITY IN CPO-68/69
As discussed above, MIL-53 and CPO-68/69 share the same
sra topology. Since the difference in flexibility cannot be
attributed to the topology, it must originate from other
structural factors. In MIL-53, the O···O axis of the carboxylate
group acts as a form of hinge between the organic linker and
the metal carboxylate chain. As the structure opens up, the
dihedral angle between the Cr−O−O−Cr and O−C−O planes
changes from 139° (lt) to 180° (ht). Moreover, the expansion
of the structure is accompanied by a rotation of the benzene
moiety of the linker (Figure 8).69 This rotation has a relatively
small energetic barrier due to the single bond between the
benzene moiety and the carboxylate groups.
Figure 8. Molecular “hinges” of MIL-53(Cr)_lt (left) and CPO-69-Ca
(right) (Cr: green, Ca: light gray, C: gray O: red).
This is in contrast to CPO-68/69, where the O···O hinge is
oriented in an unfavorable direction. In fact, the hinge is off by
almost 40° relative to the c-axis. Furthermore, the necessary
rotation of the linker is severely hindered (Figure 8). We
propose that these two structural features provide a rationale
for the difference in flexibility between MIL-53 and CPO-68/
69.
■
CONCLUSIONS
Two series of coordination polymers with the sra topology,
CPO-68-M (M = Zn, Mn, and Co) and CPO-69-M (M = Cd
and Ca), were synthesized by solvothermal methods. The CPs
344
DOI: 10.1021/acs.cgd.5b01302
Cryst. Growth Des. 2016, 16, 339−346
Crystal Growth & Design
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have good thermal stability as well as good stability against
degradation in ambient conditions. The CPs were evaluated for
gas sorption by DFT, which indicated that the materials are
nonporous even at high pressures and do not have a flexible
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■
ASSOCIATED CONTENT
* Supporting Information
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.cgd.5b01302.
Synthesis details for the ligand, TGA, P-XRD of the
structures, and additional details regarding the DFT
calculation. (PDF)
Accession Codes
CCDC 1437170−1437174 contains the supplementary crystallographic data for this paper. These data can be obtained free
of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12, Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
■
AUTHOR INFORMATION
Corresponding Author
*Phone: +47 92292547; e-mail: fredrik.lundvall@smn.uio.no.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
We acknowledge the support from the Research Council of
Norway (Project No. 190980), inGAP and the Departments of
Chemistry at UiO and UiB. P.V. acknowledges the Research
Council of Norway for providing computing time at the
Norwegian supercomputer facilities. The skillful assistance from
the staff at the Swiss−Norwegian Beamlines, ESRF, Grenoble,
is highly acknowledged. We acknowledge use of the Norwegian
national infrastructure for X-ray diffraction and scattering
(RECX).
■
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