ARTICLE
pubs.acs.org/crystal
Tuning the Formations of MetalOrganic Frameworks by
Modification of Ratio of Reactant, Acidity of Reaction System,
and Use of a Secondary Ligand
Qian Gao,†,‡ Ya-Bo Xie,*,† Jian-Rong Li,‡ Da-Qiang Yuan,‡ Audrey A. Yakovenko,‡ Ji-Hong Sun,*,† and
Hong-Cai Zhou*,‡
†
‡
College of Environmental and Energy Engineering, Beijing University of Technology, Beijing, 100124, P. R. China
Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, Texas 77842, United States
bS Supporting Information
ABSTRACT: Four porous coordination networks (PCNs), {[Zn 3 O(H2O)3(adc)3] 3 2(C2H6NH2) 3 2(DMF) 3 3(H2O)}n (PCN-131), Zn2(DMA)2(adc)2] 3 2(DMA)}n (PCN-132), {[Zn3O(DMF)(adc)3(4,40 -bpy)] 3 2(C2H6
NH2) 3 S}n (PCN-1310 ), and {[Zn(adc)(4,40 -bpy)0.5] 3 S}n (PCN-1320 ), have
been synthesized by the assembly of anthrancene-9,10-dicarboxylic acid
(H2adc) with Zn(II) under different reaction conditions, including modifications of reactant ratio, acidity variations, and the use of a secondary ligand.
Single-crystal X-ray diffraction studies reveal that PCN-131, obtained from
the dimethylformamide (DMF) solution under acid condition, has a threedimentional (3D) framework structure with one-dimensional (1D) honeycomb channels. PCN-132 isolated from dimethylacetamide (DMA) solution
without adding acid in synthesis is a two-dimensional (2D) layer compound.
By employing 4,40 -bipyridyl (4,40 -bpy) as a secondary ligand, PCN-1310 and
PCN-1320 were synchronously synthesized as a mixture outcome with more PCN-1310 than PCN-1320 . In PCN-1310 , 4,40 -bpy
acting as a secondary ligand is arranged inside the honeycomb channel of the 3D PCN-131, resulting in an effective improvement of
thermal stability of the network, while in PCN-1320 , 4,40 -bpy ligands link 2D layers of PCN-132 to form a pillared-layer 3D
framework. Gas adsorption has been performed for selected materials. The results show that the framework of PCN-131 is thermally
unstable after removing the solvent molecules coordinated to their metal sites. While PCN-1310 is stable for gas uptake, with an
evaluated Langmuir surface area of 199.04 m2 g1, it shows a selective adsorption of CO2 over CH4.
’ INTRODUCTION
Metalorganic frameworks (MOFs) or porous coordination networks (PCNs) have attracted much attention because of their
intriguing structural architectures and topology,1 as well as their
potential applications in many fields such as gas storage,2 gas
separation,3 and drug delivery.4 These materials usually have a threedimensional (3D) open framework constructed from the combination
of multidentate organic ligands with metal ions or clusters also known
as secondary building units (SBUs). The approach of utilizing SBUs
developed by Yaghi et al.5 has been proven to be a powerful strategy in
designing functional MOFs. Inorganic building blocks, such as μ4-oxotetrametal basic carboxylate SBUs ([M4O(CO2)6]), are observed in
several famous MOFs, including MOF-56 and MOF-177,7 μ3-oxotrimetal basic carboxylate SBUs ([M3O(CO2)6]) seen in MIL-1018
and other MOFs,9 and dimetal-paddle-wheel SBUs ([M2(CO)4]).10
Selecting the proper metal ions and ligands, one expects to synthesize
the prospective SBUs and sequentially design and synthesize the
prospective MOFs. However, some metal ions such as Zn(II) can
form multiple possible SBUs with the same ligand; thus, it is difficult to
predict both the SBUs and the structures of its MOFs.
r 2011 American Chemical Society
The formation of MOFs is highly influenced by various
factors, such as the molar ratio of reactant reagents, solvent used,
pH value of the solution, and the selection of a secondary
ligand.11 In these factors, the secondary ligand plays a crucial
role in extending and reconstructing the structure. On the other
hand, 4,40 -bipyridyl (4,40 -bpy) is frequently introduced as a
secondary ligand to extend two-dimensional (2D) sheets into threedimensional networks by displacing coordinated solvent molecules through the “pillar-and-layer” method.12,13 This kind of
method has been proved to be a quite useful strategy. Additionally, 4,40 -bpy can also take part in channel modification to
stabilize the structure and conveniently arrange the surface
features of the channel in some MOFs.14
Gas adsorption selectivity is an important property of
MOFs which has potential applications in gas separation and
purification.3 Gas adsorption selectivity is determined not only
by the pore size and shape, but also by the channel surface feature
Received: August 14, 2011
Revised:
October 23, 2011
Published: November 04, 2011
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of MOFs. One effective way to increase the gas selectivity of
MOFs is the modification of the inner channel, by changing the
surface polarity and the acidbase property.15 Recent research
has shown that ionic MOFs have a good behavior in selectively
adsorbing high quadrupole moment molecules (for example
CO2) through the electric field in the pores of MOFs.16 In
previous works, our group successfully synthesized a 3D structural complex, PCN-13, and investigated its gas adsorption
properties.9a As a continuing effort of our systematic investigation, we report herein the syntheses, crystal structures, and gas
adsorption of four related MOFs, PCN-131, PCN-132, PCN1310 , and PCN-1320 . The effects of mole ratios of metal ion and
ligand on the formation of SBU, as well as the function of the
secondary ligand for extending and modifying the structures of
these MOFs, are established.
for 2 days, and then cooled to room temperature. Many yellow block
crystals (PCN-1310 ) and a few colorless block crystals (PCN-1320 ) were
obtained with a yield of 46% and 18% based on Zn, respectively. The two
compounds were separated manually. FT-IR (cm1) for PCN-1310 : 3388
m, 3061 m, 2786w, 1617s, 1431w, 815s, 777s, 738w, 674s, 643s, 612s, 581s.
FT-IR (cm1) for PCN-1320 : 3389 m, 3060 m, 2788w, 1618s, 1434w, 815s,
778s, 745w, 673s, 644s, 613s, 582s.
X-ray Crystallography. Single crystal X-ray data were collected on
an Apex-II diffractometer equipped with a low-temperature device.
Single crystals were picked directly from the mother liquor, attached
to a glass loop, and transferred to a cold stream of liquid nitrogen
(163 °C) for data collection. Raw data collection and refinement was
carried out using SMART. Data reduction was performed using SAINT
and corrected for Lorentz and polarization effects.18 Adsorption corrections were applied using the SADABS routine. The structure was solved
by direct methods and refined by full-matrix least-squares on F2 with
anisotropic displacement using the SHELXTL software package.19 Nonhydrogen atoms (except some in coordinated solvents) were refined
with anisotropic displacement parameters during the final cycles.
Hydrogen atoms on carbon were calculated in ideal positions with
isotropic displacement parameters. In PCN-1310 and PCN-1320 , free
solvent molecules were highly disordered, and attempts to locate and
refine the solvent peaks were unsuccessful. The diffused electron
densities resulting from the these residual solvent molecules were
removed from the data set using the SQUEEZE routine of PLATON
and further refined using the data generated.20 The contents of the
solvent region are not represented in the unit cell contents in crystal
data. Attempts to determine the final formula of such compounds from
the SQUEEZE results combined with elemental analysis and TGA data
were not successful because the volatility of the crystallization solvents
during measurements prevented accurate data from being obtained.
Crystallographic data and experimental details for structural analyses are
summarized in Table 1. The selected bond lengths and angles of all
complexes are listed in Table S1 of the Supporting Information.
’ EXPERIMENTAL SECTION
Materials and General Methods. Commercially available reagents were used as received without further purification. H2adc was
synthesized according to a literature procedure.17 Elemental analyses
(C, H, and N) were obtained by Canadian Microanalytical Service Ltd.
1
H NMR data were collected on a Mercury 300 spectrometer. FT-IR data
were recorded on an IRAffinity-1 instrument. TGA data were obtained on
a TGA-50 (Shimadzu) thermogravimetric analyzer with a heating rate of
2 °C min1 under N2 atmosphere. The powder X-ray diffraction patterns
(PXRD) were recorded on a Bruker D8-Focus BraggBrentano X-ray
powder diffractometer equipped with a Cu sealed tube (λ = 1.541 78 Å) at
a scan rate of 0.2 s deg1. Simulation of the PXRD spectrum was carried
out by the single-crystal data and diffraction-crystal module of the Mercury
program available free of charge via Internet at http://www.iucr.org.
ASAP 2020 surface area analyzer was used to measure gas adsorption.
Syntheses of Complexes. {[Zn3O(H2O)3(adc)3] 3 2(C2H6NH2) 3
2(DMF) 3 3(H2O)}n (PCN-131). PCN-131 was prepared by the solvothermal reaction. N,N-Dimethylformamide (DMF, 1.5 mL) solution containing anthrancene-9,10-dicarboxylic acid (H2adc) (26.2 mg, 0.1 mmol) was
mixed thoroughly with DMF (1.5 mL) solution containing Zn(NO3)2 3
6H2O (59.4 mg, 0.2 mmol), and then five drops of HBF4 were added. The
mixture was sealed in a Pyrex tube, heated at 120 °C for 1 week, and then
cooled to room temperature. The light-yellow block crystals were obtained
with a yield of 56% based on Zn. FT-IR (cm1): 3273 m, 2920 m, 2779 m,
1609s, 1428s, 1381w, 1318s, 1280 m, 1091s, 815s, 777 m, 683s, 604s, 581s.
Anal. Calcd for C60.5H73.5Zn3N4.5O25.5: C, 50.95; H, 4.87; N, 4.10. Found:
C, 51.63; H, 4.78; N, 4.15.
{[Zn2(DMA)2(adc)2] 3 2(DMA)}n (PCN-132). Complex PCN-132 was
prepared by the solvothermal reaction. An N,N-dimethylacetamide (DMA,
1.5 mL) solution containing H2adc (26.6 mg, 0.1 mmol) was mixed
thoroughly with a DMA (1.5 mL) solution containing Zn(NO3)2 3 6H2O
(59.4 mg, 0.2 mmol). The mixture was sealed in a Pyrex tube, heated at
60 °C for 3 days, and then cooled to room temperature. Colorless block
crystals were obtained with a yield of 32% based on Zn. FT-IR (cm1):
3278 m, 2921 m, 2781 m, 1607s, 1427s, 1381w, 1319s, 1282 m, 1091s,
816s, 778 m, 683s, 606s, 582s. Anal. Calcd for C48H52Zn2N4O12:
C, 57.21; H, 5.20; N, 5.56. Found: C, 57.34; H, 5.18; N, 5.53.
{[Zn3O(DMF)(adc)3(4,40 -bpy)] 3 2(C2H6NH2) 3 S}n (PCN-1310 ) and {[Zn(adc)(4,40 -bpy)0.5] 3 S}n (PCN-1320 ) (S = unassigned solvent molecule).
By employing bpy as a secondary ligand, 3D metalorganic frameworks of
PCN-1310 and PCN-1320 were obtained as a mixture. An N,N-dimethylformamide (DMF, 1.5 mL) solution containing H2adc (26.2 mg,
0.1 mmol) was mixed thoroughly with a DMF (1.5 mL) solution containing Zn(NO3)2 3 6H2O (59.4 mg, 0.2 mmol). 4,40 -bpy (15.6 mg,
0.1 mmol) and three drops of HBF4 were added to this solution to give
an acid solution. The mixture was sealed in a Pyrex tube, heated at 120 °C
’ RESULTS AND DISCUSSION
Syntheses and General Characterizations. Besides metal
ions and ligands, the formation of MOFs is highly influenced by
various factors, such as solvent used, pH value of solvent, ratio of
reactants, reaction time, temperature, and so on.2125 In our case,
we try to evaluate the most significant factors that drive the
formation of three primary building units, μ4-oxo-tetrazinc basic
carboxylate, μ3-oxo-trizinc basic carboxylate, and dizinc-paddlewheel carboxylate SBU. In the three SBUs, the ratios of metal and
carboxylate ligand are 2:3, 1:2, and 1:2. In terms of a dicarboxylate ligand, such as adc2‑ herein (the mole ratios of metal and
ligand in complexes are 4:3, 1:1, and 1:1), we can suppose that
the ratio of metal and ligand may be the key factor to control the
formation of different SBUs. In a previous report, a Zn-MOF
named as PCN-139a was constructed with ligand adc2‑. In PCN13, a distorted Zn4O(COO)6 cluster as SBU was observed.
Although this SBU is different from the regular μ4-oxo-tetrazinc
basic carboxylate SBU observed in one of IRMOF series,26 it still
has the same ratio of metal and ligand, 4:3. The unusually
distorted SBU implies the possibility to get some new forms of
Zn clusters with this ligand if the reaction condition changes. On
the basis of the above assumption, we performed a systematic
experiment in the syntheses of Zn(II)adc2‑ MOFs only by
decreasing the mole ratio of metal salt and ligand from 5:1 to 1:1.
Two new MOFs, PCN-131 and PCN-132, were obtained. The
details are shown in the Supporting Information (Table S2 and
Figures S1 and S2). As shown in Figure S1, when the mole ratio
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Table 1. Crystal Data and Data Collection Parameters for PCN-131, PCN-132, PCN-1310 , and PCN-1320
PCN-131
PCN-132
PCN-1310
PCN-1320
formula
C48H30Zn3O16
C48H52Zn2N4O12
C65H55Zn3N5O14
C21H12ZnNO5
formula weight
1058.83
1007.68
1326.25
423.69
crystal system
trigonal
monoclinic
monoclinic
tetragonal
space group
P31c
P21/n
P21/n
I4/mcm
a, Å
15.58(3)
15.28(11)
15.81
15.44(7)
b, Å
15.58(3)
19.70(14)
26.42
15.44(7)
c, Å
16.52(3)
15.59(11)
16.43
28.03(13)
a, deg
b, deg
90
90
90
90
90
90
90
90
g, deg
120
90
90
90
V, Å3
3472.0(12)
4692.1
6870.0
6680.0
Z
2
4
4
8
D, g/cm3
1.013
1.426
1.283
0.842
μ, mm1
1.076
1.089
1.102
0.752
independent reflections
2265
8693
11168
2451
Rint
R [I/σ(I) > 2]
0.0862
0.0421
0.0696
0.0838
0
0.0635
0.0361
0.0614
Rw [I/σ(I) > 2]
0.1195
0.2162
0.1883
0.2529
goodness-of-fit on F2
1.063
1.050
0.970
1.114
no. of reflection used
2265
8693
11168
2451
no. of parameters refined
103
607
755
77
ΔFmax, e Å3
1.127
4.034
1.458
0.873
ΔFmin, e Å3
0.337
2.023
0.755
0.649
of metal salt and ligand is above 4:1, the PXRD patterns of the
productions match well the simulated pattern (from single crystal
data) of PCN-13. When the ratio reaches 3:1, in PXRD some
new peaks are observed, which indicates a new compound
(PCN-132), except PCN-13 starts to form in this condition.
When the ratio reduced to 2:1, besides the above two compounds, a new compound (PCN-131) can also be obtained.
However, when the ratio is less than 2:1, other unassigned new
phases are formed.
Because PCN-13, PCN-131, and PCN-132 were simultaneity
produced at the same reaction conditions, additional experiments were carried out for clarifying the particular synthetic
conditions of the three compounds. In this case, the mole ratio of
metal/ligand is kept at 2:1 and just changes the acidity of the
solvent system. The results showed that PCN-131 is the only
product obtained in acid condition (by adding 48% HBF4 in
water). Under low acidity conditions, the crystal size formed
is too tiny to determine its structure by a single-crystal X-ray
crystallographic study (the PXRD results showed that the product is still PCN-131), while in high acidity the crystal quality is
very good. The results were confirmed by the PXRD patterns
shown in Figure S2 (Supporting Information). The PXRD
patterns show all samples have a slightly front shift of the peaks
after 15° for 2θ that is probably due to the unstableness of PCN131 without solvent. It is interesting that when two drops of
HBF4 acid were added to the system, some new peaks can be
observed in the PXRD pattern (green) and the new shape
crystals are found in the system. Those new shape crystals were
manually picked up and their unit cell parameters were checked
out several times by the single-crystal diffraction. The results
show that they are all PCN-131. The new shape crystal is just a
little bit of a twin crystal to the old one, which may cause the
differences shown in PXRD patterns. However, pure phase of
PCN-132 was found at the temperature of 60 °C without any
HBF4 acid added.
In order to extend layered PCN-132 into 3D networks, bpy
acting as a secondary ligand was introduced in the synthesis
process using the “layer and pillar” method. However, two new
compounds, PCN-1310 and PCN-1320 , were found in the same
reaction system. Pure PCN-1310 can be obtained under
optimized conditions of metal salt mole ratio (6:5:3) of
H2adc and 4,40 -bpy. Unfortunately, we could not get a pure phase
of PCN-1320 either by changing the pH value of the solvent or by
using different solvent systems or even by modifying the ratios of
reagent.
In the IR spectra of all complexes, the peaks at 29212787 cm1
belong to the CH3 stretching of solvent molecules. The Deacon
Philips rule is helpful to determine the coordination mode between
carboxylate groups and center metal ions, by calculating the
frequency separation (Δν) between the asymmetric (νas) and
symmetric stretching (νs) modes of the carboxylate unit.27 The Δν
for PCN-131 provides an indication of the bridging coordination
mode [Δν = 181 cm1 < 200 cm1, νas(COO) = 1609 cm1,
νs(COO) = 1428 cm1]. Similar characteristics are also observed in PCN-132 [Δν = 180 cm1 < 200 cm1, νas(COO) =
1607 cm1, νs(COO) = 1427 cm1), PCN-1310 (Δν = 186 cm1
< 200 cm1, νas(COO) = 1617 cm1, νs(COO) = 1431 cm1]
and PCN-1320 [Δν = 184 cm1 < 200 cm1, νas(COO) =
1618 cm1, νs(COO) = 1434 cm1]. For PCN-131, TGA
studies show that a mass loss of 8.36% corresponds to the leaving
of uncoordinated and coordinated water molecules (calcd 8.01%)
in a temperature range of 6394 °C, while a mass loss of 13.17%
corresponds to the leaving of uncoordinated DMF molecules
(calcd 13.49%) in a temperature range of 157241 °C.
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Figure 2. (a) SBU structure of PCN-132. (b) The 2D square-grid net
structure of PCN-132. (c) The packing style of PCN-132. (d) The
hydrogen bonds between adjacent layers.
direction (Figure 1b). PLATON calculations indicate that the
effective volume for solvent molecules is 1757.4 Å3 per unit cell,
which is 50.6% of the crystal volume.28
{[Zn2(DMA)2(adc)2] 3 2(DMA)}n (PCN-132). PCN-132 crystallized in monoclinic space groups P21/n with two Zn ions, two
adc ligands, two coordinated DMA solvent molecules, and two
free DMA molecules in the asymmetric unit. Each Zn2+ ion is
coordinated by five O atoms with four from the adc ligands and
one from the coordinated DMA solvent molecule. Four carboxylate groups bridge two Zn2+ to form a distorted Zn2(CO2)4
paddle-wheel SBU as a square-planar four-connected node
(Figure 2a). The distance of Zn 3 3 3 Zn in the paddle-wheel cluster
is 2.979 Å, which is similar to those found in another dizinc
paddle-wheel SBUs.29 The bond distance of ZnO (solvent) is
1.982 Å and ZnO (adc) bond distances range from 2.039 to
2.059 Å. The ligand adc bridges dizinc paddle-wheel SBUs to form
a two-dimensional sheet (Figure 2b). There exist three kinds of
CH 3 3 3 O hydrogen bonds between adjacent layers. The first
one is formed between the methyl H atoms of the coordinated
DMA solvent molecules and the carboxylic O atoms of ligands
from adjacent layers. The second one is formed between H atoms
of the anthrancene rings and O atoms of the free DMA solvent
molecules. And the last one is formed between the methyl H
atoms of free DMA solvent molecules and the carboxylic O atoms
of ligands. These hydrogen bonds link the complex with an
“ABAB” packing fashion, resulting in a nonporous structure
(Figure 2c,d). The hydrogen-bond parameters are presented in
Table S3 (Supporting Information).
{[Zn(adc)(4,40 -bpy)0.5] 3 S}n (PCN-1320 ). PCN-1320 crystallized in tetragonal space group I4/mcm with one-half of a Zn2+
ion, one adc ligand, and one-quarter of a 4,40 -bpy in the
asymmetric unit. The zinc ion adopts a similar coordination
geometry to that of PCN-132 with 4,40 -bpy replacing the axial
Figure 1. (a) SBU structure of PCN-131 and (b) 3D structure of PCN131 with 1D channels.
This compound starts to decompose at ca. 310 °C (see Figure S3,
Supporting Information). For PCN-132, TGA studies show that a
mass loss of 17.18% corresponds to the loss of uncoordinated
DMA molecules (calcd 17.27%) in a temperature range of
84151 °C, and the other mass loss of 17.79% corresponds to
the leaving of coordinated DMA molecules (calcd 17.27%) in a
temperature range of 262343 °C. The complex starts to
decompose at ca. 350 °C (see Figure S4, Supporting Information).
Crystal
Structures. {[Zn3O(H2O)3(adc)3] 3 2(C2H6NH) 3 2
(DMF) 3 3(H2O)}n (PCN-131). X-ray single crystal diffraction
analyses revealed that PCN-131 crystallizes in trigonal space
group P31c. In the asymmetric unit of PCN-131, there are
one-sixth of a μ3-O atom, one-half of a Zn2+ ion, a H2O
molecule, the adc2‑ ligand and one third of C2H6NH2+.
Uncoordinated C2H6NH2+ part is an NH2(CH3)2+ cation
(dimethylammonium) which is formed by the decomposed
DMF molecules during heated. For the whole framework structure, PCN-131 is built on a μ3-oxo-trizinc basic carboxylate SBU
[Zn3O(COO)6] (Figure 1a) and the adc ligand, with a Znμ3O distance of 2.003 Å and adjacent Zn 3 3 3 Zn distance of 3.470 Å,
which is isorecticular to PCN-19 (Ni-MOF).9c In this SBU, three
Zn2+ ions and a μ3-O atom are on the same plane, forming a sixconnected node. Each pair of adjacent Zn2+ ions are bridged by
two carboxylate groups from two different adc ligands, and the
coordination geometry of each Zn2+ ion is octahedral with the
ZnO (μ3-O) at a distance of 2.110 Å and ZnO (adc) bond
distances ranging from 2.071 to 2.077 Å. Each trizinc SBU
connects to six adc ligands and each adc ligand binds two SBUs
to enclose a honeycomb one-dimensional channel along the c axis
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Figure 3. (a) SBU structure of PCN-1320 , and (b) 3D structure of
PCN-1320 .
direction coordinated solvent molecules to form an octahedral sixconnected node instead of a square-planar four-connected node
(Figure 3a). It is interesting that the attendance of 4,40 -bpy relieves
the crowding among anthracene rings and breaks the weak
intermolecule interactions, forming a typical dizinc paddle-wheel
SBU with a ZnO (adc) bond distance of 2.046 Å, a ZnO (bpy)
bond distance of 2.022 Å, and a Zn 3 3 3 Zn distance of 2.925 Å,
which is shorter than that of PCN-132. With the linkage of 4,40 bpy PCN-1320 adopts a pillaredlayered structure with the
formula of {[Zn(adc)(4,40 -bpy)0.5] 3 S}n (S = unknowable solvent
molecule) (Figure 3b). However, 4,40 -bpy is disordered in this
structure due to its rotation along the symmetry axis. Compared
with PCN-132, the “layer” structure packs in an “AA” fashion,
resulting in a “pillar” effect in PCN-1320 . Unfortunately, there are
no pores in c direction because of the bulkiness of the anthracene
rings, and finally only 2D channels form in PCN-1320 . PLATON
calculations indicate that the effective volume for solvent molecules
is 3575.3 Å3 per unit cell, which is 53.5% of the crystal volume.28
Figure 4. (a) SBU structure of PCN-1310 . (b) The structure of 1D
channel and connection mode of decrated bpy. (c) The 3D packing style
structure of PCN-1310 .
{[Zn3O(DMF)(adc)3(4,40 -bpy)] 3 2(C2H6NH2) 3 S}n (PCN-1310 ).
PCN-1310 crystallizes in monoclinic space group P21/n with three
Zn2+ ions, one μ3-O atom, one 4,40 -bpy three adc ligands, one
coordinated DMF molecule, and two C2H6NH2+ cations
[C2H6NH2+ is an NH2(CH3)2+ cation (dimethylammonium)
which is formed by the decomposed DMF molecules during
heated.] in the crystallographically asymmetric unit. It is one of a
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Crystal Growth & Design
ARTICLE
few cases of using 4,40 -bpy to “arrange” a channel in a MOF.14
The Zn(II) adopts a similar coordination geometry to that of
PCN-131, with bpy replacing two coordinated water molecules,
breaking the symmetry in the μ3-oxo-trizinc carboxylate SBU to
form an eight-connected node instead of six-connected node
(Figure 4a). In PCN-1310 , the honeycomb framework maintains
the linking relationship while it exhibits significant expansion along
the diagonal of two perpendicular directions a and b, as evidenced
by the large variations in channel size parameters, from 1.66 1.66
1.66 nm to 1.74 1.59 1.56 nm (Figure 4b). This is also the
reason why only two molecules of solvent can be replaced. The
third one is just sitting along the expansion direction out of the
reach of bpy. The “arrangement” of 4,40 -bpy not only changes the
shape of the channel from honeycomb to rectanglular channel but
also increases the stability of the whole framework (Figures 4c and
5). PLATON calculations indicate that the effective volume for
solvent molecules is 2712.0 Å3 per unit cell, which is 39.5% of the
crystal volume.28
Gas Adsorption. To characterize the porosity of PCN-131
and PCN-1310 , the samples were first soaked in MeOH for
3 days and then in CH2Cl2 for 3 days. However, the different
stabilities of these compounds do not allow using the same
activation conditions. Under high vacuum, PCN-131 has already
been decomposed only by 3 h at room temperature, while the
structure of PCN-1310 has remained stable even after heating to
80 °C for 10 h. This means that PCN-131 is unstable without the
support of solvent molecules, and conversely, PCN-1310 is
stable. It is also confirmed that the arrangement of 4,40 -bpy
inside the channel can effectively increase the stability of the
frameworks. Although PCN-131 is isorecticular to PCN-19,
which is mentioned above, two MOFs have totally different
stabilities after activation. The possible reason is that PCN-19 is a
nickle-based MOF while PCN-131 is a zinc-based MOF, and the
differences between the central metal ions cause different
stabilities of their MOFs. Adsorption isotherms for N2, H2, O2,
and Ar at 77 K and CO2 and CH4 at 195 and 273 K were
measured. No adsorption was observed for PCN-131, which
means that the framework collapsed after activation, as confirmed by PXRD pattern (Figure S5, Supporting Information).
The N2 adsorption of PCN-1310 represents a typical type I
isotherm (Figure 6). Its BET surface area is 442.20 m2 g1(Langmuir surface area is 496.85 m2 g1), and the total pore
volume is 0.177 cm3 g1. The PXRD pattern after gas adsorption
of PCN-1310 is still matches well with the simulated pattern,
showing that the framework structure of this material is still
stable (Figure S6, Supporting Information).
Figure 5. The channel structures of PCN-131 and PCN-1310 .
Figure 6. N2 adsorption isotherms of PCN-131 and PCN-1310 at 77 K
(solid symbols stand for adsorption and open ones for desorption.
Figure 7. (a) H2 adsorption isotherm of PCN-1310 measured at 77 K and (b) N2, O2 and Ar adsorption isotherm of PCN-1310 measured at 77 K.
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Crystal Growth & Design
ARTICLE
Figure 8. (a) CO2 and CH4 adsorption isotherm of PCN-1310 measured at 195 K and (b) CO2 and CH4 adsorption isotherm of PCN-1310 measured
at 273 K.
The H2 sorption isotherm was measured at 77 K; as shown in
Figure 7a, PCN-1310 can absorb 0.84 wt % (excess) without any
hysteresis at 77 K and 800 Torr. Although the presence of bpy
reduces the porous volume, it effectively enhances the stability of
1D channel compound, and its hydrogen uptake is still comparable to that of the similar complex (PCN-199c) without bpy. The
O2 and Ar uptake was also carried out at 77 K (Figure 7b). In
addition, PCN-1310 exhibits a selective gas adsorption to CO2
over CH4 both at 195 and 273 K, as shown in Figure 8, which
is similar to several reported MOFs.16,30
’ CONCLUSIONS
Two porous coordination networks (PCNs), PCN-131 and
PCN-132, were synthesized by solvothermal reactions of Zn(II)
nitrate with anthrancene-9,10-dicarboxylic acid (H2adc) through
tuning the mole ratio of mental salt and ligand from 5:1 to 1:1.
The studies still show that PCN-131 is the only product under
acid environment. By introducing 4,40 -bpy acting as secondary
ligand, into reaction system, PCN-1310 and PCN-1320 were
obtained. Pure PCN-1310 is the primary product and can be obtained with a mole ratio of metal salt, H2adcd, and 4,40 -bpy of
6:5:3. Through the smart synthetic design, the 1D honeycomb
channel of PCN-131 was “rearranged” and modified by use of
4,40 -bpy, resulting in PCN-1310 , which is stable for gas adsorption. By use of 4,40 -bpy as secondary ligand, the 2D sheet of
PCN-132 is pillared to form a 3D framework, PCN-1320 . In
summary, judiciously tuning metalligand mole ratio could be
an effective way to form different SBUs so as to affect the
structural formation of complexes, and bpy as a good candidate
of secondary ligand not only can extend structural dimension of
complexes through the pillaredlayered method but also can
modify the structures and enhance the stability of frameworks.
’ AUTHOR INFORMATION
Corresponding Author
*Y.-B.X.: fax, +86-10-67391983; tel, +86-10-67392130; e-mail:
xieyabo@bjut.edu.cn (Y.-B.X.). H.C.Z.: e-mail: zhou@mail.chem.
tamu.edu (H.-C.Z.).
’ ACKNOWLEDGMENT
This work was supported by National Natural Science Foundation of China (No.21075114, 21076003, 20851002), the National
Basic Research Program of China (973 Program 2009CB930200),
the Special Environmental Protection Fund for Public Welfare
project (201009015), the Funding Project for Academic Human
Resources Development in Institutions of Higher Learning under
the jurisdiction of the Beijing Municipality (PHR 201107104), the
Ninth Technology Fund for Postgraduates of Beijing University of
Technology (ykj-2011-5406), and U.S. Department of Energy
(ARPA-E: AR0000073 and EFRC: DE-SC0001015).
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bS
Supporting Information. PXRD patterns of complexes
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