Structural Diversity of Cadmium(II) Coordination Polymers Induced by Tuning the Coordination Sites of Isomeric Ligands

Article abstract of DOI:10.1021/acs.inorgchem.6b01416

When the coordination sites of ligands were shifted, the solvothermal reactions of four positional isomeric asymmetrical pyridyldicarboxylatic acids with Cd(NO₃)₂ generated four new coordination polymers, [Cd(L1)(DMF)₃]·DMF·H₂O (1), [H₂N(CH₃)₂]₂[Cd(L2)₂]·3DMF·H₂O (2), [Cd(L3)(H₂O)₂] (3), and [Cd(L4)]·1.5DMF (4), where DMF = N,N-dimethylformamide, H₂L1 = 2-(3′-carboxylphenyl)isonicotinic acid, H₂L2 = 2-(4′-carboxylphenyl)isonicotinic acid, H₂L3 = 5-(3′-carboxylphenyl)nicotic acid, and H₂L4 = 2-(3′-pyridyl)terephthalic acid. 1 shows a rare 2D fabric structure. 2 discloses a grid-layer structure with heterochiral helical chains and in which three sets of layers stack in different directions, affording an unprecedented 2D + 2D + 2D → 3D polycatenating framework with 3D intersecting porous systems. 3 also displays a 2D layer possessing strong intralayer π···π interactions and interlayer hydrogen bonds. 4 contains a rare Cd₂(COO)₄ paddle-wheel unit and forms a 3D framework with 1D open channels. The carboxyl and pyridyl groups of the positional isomeric H₂L1-H₂L4 ligands show distinct bridging fashions, which leads to the production of versatile architectures of 1-4, and their effects on the crystal structures are discussed. 1-4 reveal solid-state photoluminescence stemming from intraligand charge transfer. 2 and 4 show high selectivity for CO₂ over CH₄ but with different CO₂ adsorption enthalpies. Grand canonical Monte Carlo simulations identified the multiple adsorption sites in 2 for CO₂.

Full text of DOI:10.1021/acs.inorgchem.6b01416

Article
pubs.acs.org/IC
Structural Diversity of Cadmium(II) Coordination Polymers Induced
by Tuning the Coordination Sites of Isomeric Ligands
Bo Liu,†,‡ Hui-Fang Zhou,‡ Lei Hou,*,‡ Jian-Ping Wang,† Yao-Yu Wang,‡ and Zhonghua Zhu§
†College of Science, Northwest A&F University, Yangling 712100, People’s Republic of China
‡Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education, College of Chemistry &
Materials Science, Northwest University, Xi’an 710069, People’s Republic of China
§School of Chemical Engineering, The University of Queensland, Brisbane 4072, Australia
S
* Supporting Information
ABSTRACT: When the coordination sites of ligands were
shifted, the solvothermal reactions of four positional isomeric
asymmetrical pyridyldicarboxylatic acids with Cd(NO3)2
generated four new coordination polymers, [Cd(L1)-
(DMF)3]·DMF·H2O (1), [H2N(CH3)2]2[Cd(L2)2]·3DMF·
H2O (2), [Cd(L3)(H2O)2] (3), and [Cd(L4)]·1.5DMF (4),
where DMF = N,N-dimethylformamide, H2L1 = 2-(3′-
carboxylphenyl)isonicotinic acid, H2L2 = 2-(4′-
carboxylphenyl)isonicotinic acid, H2L3 = 5-(3′-
carboxylphenyl)nicotic acid, and H2L4 = 2-(3′-pyridyl)-
terephthalic acid. 1 shows a rare 2D fabric structure. 2 discloses a grid-layer structure with heterochiral helical chains and in
which three sets of layers stack in different directions, affording an unprecedented 2D + 2D + 2D → 3D polycatenating
framework with 3D intersecting porous systems. 3 also displays a 2D layer possessing strong intralayer π···π interactions and
interlayer hydrogen bonds. 4 contains a rare Cd2(COO)4 paddle-wheel unit and forms a 3D framework with 1D open channels.
The carboxyl and pyridyl groups of the positional isomeric H2L1−H2L4 ligands show distinct bridging fashions, which leads to
the production of versatile architectures of 1−4, and their effects on the crystal structures are discussed. 1−4 reveal solid-state
photoluminescence stemming from intraligand charge transfer. 2 and 4 show high selectivity for CO2 over CH4 but with different
CO2 adsorption enthalpies. Grand canonical Monte Carlo simulations identified the multiple adsorption sites in 2 for CO2.
systems9 or their mixed-ligand systems,10 which not only
illustrate the exploration of the essential factors of positional
isomeric ligands for regulating structural assembly but also
provide further insight into the design of new functional
crystalline materials.
The geometry of a ligand is a crucial aspect in directing the
INTRODUCTION
■
Increasing interest has been focused on the design and
construction of novel functional coordination polymers (CPs)
because of their fascinating architectures and potential
applications in the areas of luminescence,1 gas storage/
separation,2 heterogeneous catalysis,3 and molecular recogni-
structures of CPs. Highly symmetrical bridging polycarboxylate
tion.4 Accordingly, numerous CPs with excellent performance
or pyridyl organic linkers are commonly used to prepare porous
were assembled by various approaches. However, it remains
CPs, while little attention has been focused on the investigation
very challenging to control CPs with the desired structures,
of elongated asymmetrical pyridyldicarboxylate bridging
which significantly depend on not only synthetic factors, such
ligands,11 although these ligand-based CPs revealed unique
as solvents, pH values, and temperatures,5 but also the nature of
frameworks and properties.11,12 In fact, the employment of low-
the building blocks, such as metal ions, ligands, and possible
templates.6 From a design perspective, the ligand is the most
symmetry ligands has recently been proposed as a promising
synthetic strategy to construct CPs,13 which with increasing
important building block because of the fact that a large
number of ligands with diverse structures and properties can be
synthesized by the repertoire of synthetic chemistry. Through
topological complexity offers a less-investigated direction
toward CP fabrication.
Along with our recent research in the construction of CPs
the design of an appropriate ligand, it is possible to tune the
based on unsymmetrical pyridylcarboxylate ligands,14 with the
aim of a comparative study, this work designed four
asymmetrical isomeric tritopic pyridyldicarboxylic acid ligands,
H2L1 = 2-(3′-carboxylphenyl)isonicotinic acid, H2L2 = 2-(4′-
structures and functionality of CPs.7 In contrast to the fruitful
research in effect from the length, flexibility, and substituent
groups of organic ligands,8 a particular series of CP species that
can be systematically tuned by the positional isomeric effect
and undergo structural transformations remain largely unex-
plored thus far. Investigations on the positional isomeric effect
have been scarcely reported in polycarboxylate or pyridyl ligand
Received: June 12, 2016
© XXXX American Chemical Society
A
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Luminescent spectra were obtained on a PerkinElmer LS55
luminescence spectrometer.
carboxylphenyl)isonicotinic acid, H2L3 = 5-(3′-
carboxylphenyl)nicotic acid, and H2L4 = 2-(3′-pyridyl)-
terephthalic acid (Scheme 1). When these are employed with
Synthesis of [Cd(L1)(DMF)3]·DMF·H2O (1). A mixture of Cd-
(NO3)2·4H2O (30.8 mg, 0.1 mmol), H2L1 (24.3 mg, 0.1 mmol), and 2
drops of HNO3 (0.1 M) in DMF (5 mL) was sealed in a 10 mL screw-
capped vial. The vial was placed in an oven at 105 °C for 72 h and
gradually cooled to room temperature at a rate of 5 °C min−1.
Colorless rod crystals of 1 were collected by filtration and washed with
DMF. The yield was ca. 29.8 mg (44.9%, based on H2L1). Anal. Calcd
for C25H37CdN5O9: C, 45.22; H, 5.62; N, 10.55. Found: C, 45.26; H,
5.71; N, 10.49. IR (KBr, cm−1): 3460(m), 3066(w), 2927(m),
2809(w), 1657(s), 1542(s), 1394(s), 1252(m), 1108(m), 910(w),
864(m), 774(s), 696(s), 530(w), 426(w).
Scheme 1. Four Positional Isomeric Pyridyldicarboxylic
Acids Used in This Work
[H2N(CH3)2]2[Cd(L2)2]·3DMF·H2O (2). 2 was prepared with a
procedure similar to that for 1, except that H2L1 was replaced by
H2L2. Colorless block crystals of 2 were collected by filtration and
washed with DMF. The yield was ca.19.6 mg (42.5%, based on H2L2).
Anal. Calcd for C39H53CdN7O12: C, 50.68; H, 5.78; N, 10.61. Found:
C, 50.56; H, 5.62; N, 10.48. IR (KBr, cm−1): 3433(w), 3064(w),
2799(w), 2471(w), 1955(w), 1823(w), 1665(s), 1614(s),1547(s),
1466(m), 1391(s), 1255(w), 1094(m), 1017(m), 854(m), 772(s),
693(m), 524(w), 477(m), 415(w).
[Cd(L3)(H2O)2] (3). 3 was prepared with a procedure similar to that
for 1, except that H2L1 was replaced by H2L3. Colorless block crystals
of 3 were collected by filtration and washed with DMF. The yield was
ca. 11.7 mg (30.2%, based on H2L3). Anal. Calcd for C13H11CdNO6:
C, 40.07; H, 2.85; N, 3.60. Found: C, 40.16; H, 2.77; N, 3.53. IR (KBr,
cm−1): 3396(w), 3074(w), 2928(w), 1907(w), 1608(s), 1550(s),
1512(m), 1455(w), 1411(s), 1374(s), 1276(m), 1172(m), 1133(m),
1061(m), 909(m), 856(m), 764(s), 710(m), 630(w), 543(w), 435(m).
[Cd(L4)]·1.5DMF (4). 4 was prepared with a procedure similar to
that for 1, except that H2L1 was replaced by H2L4. Colorless block
crystals of 4 were collected by filtration and washed with DMF. The
yield was ca. 21.9 mg (47.6%, based on H2L4). Anal. Calcd for
C17.5H16.5CdN2.5O5.5: C, 45.47; H, 3.60; N, 7.57. Found: C, 45.42; H,
3.51; N, 7.63. IR (KBr, cm−1): 3385(m), 2928(w), 1660(s), 1589(s),
1488(m), 1403(s), 1367(s), 1268(w), 1195(m), 1100(s), 1061(m),
1030(m), 919(w), 838(m), 775(s), 705(m), 662(m), 556(m), 515(w),
482(m).
Cd2+ ions, four new CPs, [Cd(L1)(DMF)3]·DMF·H2O (1),
[H2N(CH3)2]2[Cd(L2)2]·3DMF·H2O (2), [Cd(L3)(H2O)2]
(3), and [Cd(L4)]·1.5DMF (4), were prepared. 1−4 exhibit
a variety of architectures from 1D chain to 2D layer to 3D
framework. The carboxyl and pyridyl groups in the H2L1−
H2L4 ligands show obviously distinct bridging fashions. The
structures, luminescence, and gas-adsorption properties were
investigated.
EXPERIMENTAL SECTION
■
Materials and General Methods. All reagents were purchased
commercially and used as-received without further purification.
Detailed procedures for the synthesis of H2L1−H2L4 are listed in
Schemes S1−S4. Elemental analyses for carbon, hydrogen, and
nitrogen were performed on a PerkinElmer 2400C elemental analyzer.
IR spectra were recorded using KBr pellets on a Nicolet Avatar 360
Fourier transform infrared (FTIR) spectrometer. Powder X-ray
diffraction (PXRD) data were collected on a Bruker D8-ADVANCE
diffractometer using Cu Kα radiation (λ = 1.5418 Å). Thermogravi-
metric analyses (TGA) were measured under a nitrogen stream using a
heating rate of 5 °C min−1 on a Netzsch TG209F3 instrument. Gas-
sorption isotherms were measured using an ASAP 2020 M adsorption
instrument with an automatic volumetric sorption apparatus.
X-ray Crystallography. The diffraction experiments for 1−4 were
conducted on a Bruker SMART APEX II CCD detector at 296(2) K
using Mo Kα radiation (λ = 0.71073 Å) and ω rotation scans at a
width of 0.3°. The structures were solved by direct methods and
refined by full-matrix least-squares refinements based on F2.15
Table 1. Crystallographic Data and Structural Refinement for 1−4
1
2
3
4
formula
Mr
C25H37CdN5O9
664.00
orthorhombic
Pbcn
C30H30CdN4O8
686.99
hexagonal
P6122
C13H11CdNO6
389.63
monoclinic
P21/c
C13H6CdNO4
352.59
orthorhombic
Fdd2
cryst syst
space group
a (Å)
9.4050(10)
20.389(3)
31.799(4)
90
18.635(19)
18.635(19)
21.563(4)
90
13.678(2)
12.028(2)
7.9500(10)
90
15.775(6)
16.035(6)
33.845(14)
90
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
90
90
98.861(2)
90
90
90
120
90
6097.7(13)
8
6484.5(16)
6
1292.3(3)
4
8561(6)
16
D
calc(g cm−3
)
1.447
1.056
2.003
1.097
F(000)
2736
2100
768
2752
Rint
0.0460
1.074
0.0852
0.987
0.0216
1.161
0.0626
1.047
GOF on F2
a
R1 [I > 2σ(I)]
0.0600
0.1772
0.0725
0.2117
0.0260
0.1173
0.0786
0.2129
b
wR2 (all data)
a
b
2
2
R1 = ∑|Fo| − |Fc|/∑|Fo|. wR2 = [∑w(Fo − Fc2)2/∑w(Fo )2]1/2
.
B
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Figure 1. (a) Coordination environment of CdII ions in 1. (b) 1D undulating chain. (c) View of the fabric structure formed by 1D undulating chains.
(d) 3D supramolecular structure of 1. The interlayer spaces are occupied by DMF molecules.
Figure 2. (a) Coordination environment of CdII ions in 2. (b) Grid layer structure in 2. (c and d) Polycatenation of three sets of grids in different
directions with a d−d−d interpenetration mode. (e) 3D framework of 2.
C
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Figure 3. (a) Coordination environment of CdII ions in 3. (b and c) 2D layer and its simplified network. (d) 3D supramolecular structure of 3, in
which the purple dotted lines indicate the hydrogen bonds.
pyridine N atom is uncoordinated. So, each L12− acts as a 2-
Absorption corrections were applied by utilizing the SADABS routine.
Non-H atoms were refined with anisotropic temperature parameters.
H atoms of water and organic molecules were placed in calculated
connected node to bridge the adjacent Cd2+ ions to give a 1D
undulating chain along the [110] or [110] directions,
̅
positions and refined with a riding model. Because of the low residual
electron density and heavy disorder, it failed to locate the guest
molecules in the crystal structures of 2 and 4. Application of the
SQEEZE routine in the PLATON16 software package produced a new
intensity data set excluding the intensity contribution from disordered
solvent molecules. The final formulas were determined by combining
single-crystal structure, elemental microanalysis, IR, and TGA. The
crystal data and details of data collection and refinement for 1−4 are
summarized in Table 1. CCDC reference numbers for 1−4 are
1482250−1482253 (see the Supporting Information).
Grand Canonical Monte Carlo (GCMC) Simulation Method-
ology. GCMC simulations were performed for the adsorption of CO2
in 2 by the Sorption module of Material Studio.17 The partial charges
for the C and O atoms of CO2 molecules were 0.576e and −0.288e,
respectively.18 The partial charges for atoms of 2 were derived from
the Qeq method and QEq_neutral1.0 parameter (Table S1). The
interaction energies between CO2 and the framework were computed
through the Coulomb and Lennard-Jones (LJ) 6−12 potentials. All
parameters for the CO2 molecule and atoms of 2 were modeled with
the universal force field embedded in the MS modeling package. A
cutoff distance of 12.5 Å was used for LJ interactions, and the
Coulombic interactions were calculated using the Ewald summation.
For each run, 5 × 106 maximum loading steps and 1 × 107 production
steps were employed.
respectively (Figure 1b). Strikingly, the chains in different
directions cross one over the other to form a 1O/1U (one
over/one under) interwoven layer (Figure 1c), which is
stabilized by interchain H2O molecules that form strong O−
H···O (O···O = 2.946 and 2.999 Å) double hydrogen-bonded
interactions with carboxylate O atoms from two adjacent chains
(Figure S1). To the best of our knowledge, only five 2D
interwoven structures constructed from 1D chains, such as
[Cu(2,2′-bpy)(azpy)(H2O)]·NO3·H2O,19a [HgI2(bpmbpt)],19b
[Zn(bac)2(bpp)]·1.5H2O,19c [Pb(bpe)(CH3COO)-
(CF3COO)],19d and [Zn(phen)(sdc)],19e have been reported.
1 presents the second example of a molecular fabric structure
based on a single linker.20 Furthermore, in 1, the neighboring
layers are stacked in an ABAB fashion by weak C−H···O
interactions (C21···O2 = 3.429 Å) to produce a 3D
supramolecular network (Figure 1d), in which the interlayer
spaces are occupied by DMF molecules. The removal of DMF
and H2O molecules provides an accessible void volume of
22.0% (1341.9 Å3 out of the 6097.7 Å3 per unit cell volume), as
calculated by PLATON.
Crystal Structure of 2. With 2-(4′-carboxylphenyl)-
isonicotinic acid instead of 2-(3′-carboxylphenyl)isonicotinic
acid, 2 with a 3D polycatenating framework was obtained. 2
crystallizes in the chiral hexagonal space group P6122 and
contains half a Cd2+ ion and one L22− in the asymmetric unit.
The pyridine N atom of H2L2 locates at the same position as
that of H2L1, which is also uncoordinated in 2. Each Cd2+ ion
coordinates with eight carboxylate O atoms from four L22− to
form a distorted quadrangular prismatic geometry (Figure 2a),
with two Cd−O weak bonds (Cd−O = 2.685 Å). Each L22−
RESULTS AND DISCUSSION
■
Crystal Structure of 1. 1 crystallizes in the orthorhombic
space group Pbcn and shows an unusual 2D fabric structure.
The asymmetric unit of 1 contains one Cd2+ ion, one L12−, and
three coordinated DMF molecules. Each Cd2+ ion coordinates
with seven O atoms from two L12− and three DMF molecules
(Figure 1a). Each L12− links two Cd2+ ions, in which two
carboxylate groups adopt the same chelating modes, while the
D
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Figure 4. (a) Coordination environment of CdII ions in 4. (b) 2D square layer constructed from the Cd2(COO)4 units and terephthalate groups. (c)
3D framework of 4 (the purple plane is the pyridine units). (d) Topology network.
bridges two Cd2+ ions through the carboxylate groups with a
chelating mode, forming a 2D grid layer parallel to the ac
crystal plane (Figure 2b). The layer contains two symmetry-
related left- and right-handed 21 helical chains along the a and c
axes, respectively. The most striking feature of 2 is the very
uncommon type of 2D + 2D + 2D → 3D polycatenation in that
three sets of layers exist and stack in three coplanar directions
of [100], [010], and [110] (Figure 2c,d), with a relative
rotation angle of 60°. Each window in one layer is catenated by
the other two different directions of layers with a diagonal−
diagonal (d−d) type. The interpenetration mode in 2 can be
described as d−d−d, with a density of catenation of (2/2/2).21
To date, very few polycatenating frameworks containing three
sets of layers in three coplanar directions are known.22
Meanwhile, most of the reported species display parallel−
parallel−parallel (p−p−p) interpenetration modes with
densities of catenation of (2/2/2)22a−c and (2/4/4),22d and
only one case shows d−d−d interpenetration modes but with a
different density of catenation of (1/1/1)22e compared to 2.
Thereby 2 represents the first example of a 2D + 2D + 2D →
3D polycatenating framework by interlayer inclined inter-
penetration with d−d−d mode and a density of catenation of
(2/2/2). Such a unique interpenetration in 2 produces trigonal
(3.6 × 3.6 Å2, excluding van der Waals radii of the atoms) and
hexagonal (5.2 × 5.2 Å2) channels along the c axis. Meanwhile,
relatively narrow channels also exist along the a and b axes
(Figure S2). The three channels are intersected to form a 3D
continuous porous space with 56.0% free voids, which is filled
by guest molecules and H2N(CH3)2 cations derived from the
decomposition of DMF molecules. The uncoordinated pyridine
groups point toward the inner surfaces of the pores.
conditions similar to those of 1, 3 was obtained, which
crystallizes in the monoclinic space group P21/c and shows a
2D layer. The asymmetric unit is composed of one Cd2+ ion,
one L32−, and two coordinated H2O molecules (Figure 3a).
Different from 1 and 2, the pyridine N atom of L32− in 3 is
coordinated. The Cd2+ ion shows a distorted octahedral
geometry defined by three carboxylate O atoms and one N
atom of three L32− and two water O atoms. Because of the axial
positions of Cd2+, the coordination sphere is occupied by H2O
molecules, and each Cd2+ ion links three 3-connected L32− to
form a layer parallel to the ab crystal plane (Figure 3b). Strong
intralayer π···π interactions exist (the vertical distance between
phenyls is 3.299 Å; Figure S3). The adjacent 2D layers stack in
an AAA fashion and form hydrogen bonds between the
coordinated H2O molecules (O5 and O6) and two carboxylate
O2 and O4 atoms (O2···O5 = 3.006 Å, O4···O5 = 2.828 Å,
O2···O6 = 2.761 Å, and O4···O6 = 2.751 Å), giving rise to a 3D
supramolecular framework (Figure 3d).
Crystal Structure of [Cd(L4)]·2DMF (4). Compared to
H2L3, the position of one carboxyl group is changed to form
H2L4. The reaction of H2L4 with a Cd2+ ion yields a 3D
framework of 4. 4 with the orthorhombic space group Fdd2
consists of two Cd2+ ions and two L42− in the asymmetric unit.
As shown in Figure 4a, four carboxylate groups of four different
L42− connect two Cd2+ ions to generate a paddle-wheel unit
with a Cd···Cd separation of 3.442 Å, in which the axial
positions of Cd2+ coordination spheres are occupied by two
pyridine N atoms of two different L42−. It should be noted that
the M2(COO)4 paddle-wheel units are widely found in Cu2+,
Co2+, Zn2+, and Mn2+ complexes; however, the Cd2+ case
remains rarely known,23 especially in 3D CPs.23f−h
Crystal Structure of 3. Compared to H2L1, the position of
the pyridine N atom of H2L3 is changed. Under synthetic
For a clearer understanding on the structure of 4, the
bridging fashion of L42− can be segmented as the combination
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of terephthalate and pyridine units. The Cd2(COO)4 units are
connected by terephthalates to furnish a 2D square layer
parallel to the ab crystal plane (Figure 4b). The layers are
pillared by pyridine units of L42− to form a 3D porous
framework with solvent-accessible voids of 41.7% (Figure 4c).
The framework contains rectangular channels along the [101]
direction with opening sizes of about 5.6 × 5.6 Å2. Considering
the Cd2(COO)4 unit and L42− as 6- and 3-connected nodes,
respectively, 4 forms a (3,6)-connected binodal ant net with the
topology symbol of (42·6)2(44·62·88·10) (Figure 4d).
Positional Effects of Coordination Sites on the
Structures. In 1 and 2, both L12− and L22− as 2-connected
nodes display similar coordination modes, in which the
carboxyl groups exhibit a μ1:η1,η1 coordination mode (Scheme
2a,b), while the pyridine N atom is uncoordinated because of
In addition, the four isomeric ligands show different electron-
conjugated systems with an order of H2L2 > H2L1 > H2L3 >
H2L4 (Figure S4). The electron-conjugated effect of ligand
molecules is important for their structural configurations in
frameworks, which leads to the formation of not only different
torsion angles of the ligands but also distinct structure
symmetries of the final frameworks. Obviously, because of the
small torsion angles for the two aryls of 17.06° in L12− and
14.16° in L22−, the pyridine N atoms are uncoordinated; in
contrast, the corresponding angles are bigger in L32− (39.21°)
and L42− (61.51°), leading to coordination of the pyridine
groups. Such a high torsion angle in L42− is conducive to
expanding the connection direction and to forming the 3D
porous framework of 4. The above results reveal the significant
effect of the positions of the carboxyl and pyridyl groups on the
final structures.
Scheme 2. Different Bridging Fashions of H2L in 1 (a), 2 (b),
PXRD and TGA. The experimental PXRD patterns of 1−4
are in agreement with the simulated ones from the single-crystal
structures (Figure S5), indicating the phase purity. TGA was
performed under a N2 atmosphere on the polycrystalline
samples of 1−4 (Figure S6). For 1, the TGA curve exhibits the
first weight loss of about 46.7% within the temperature range
70−325 °C, which is ascribed to the release of all DMF and
H2O molecules (calcd, 46.6%), and then the residual
framework starts to decompose. For 2, the preliminary weight
loss in the range 30−145 °C is attributed to the loss of guest
DMF and H2O molecules (found, 25.4%; calcd, 25.7%), and
then a thermally stable flat is followed before structural
decomposition at 350 °C. For 3, the weight loss of 9.5% in the
range 30−188 °C corresponds to the removal of all H2O
ligands (calcd, 9.2%). Above 360 °C, a further heating induces
decomposition of 3. For 4, the weight loss of about 23.6%
within the temperature range 30−205 °C is due to the release
of DMF molecules (calcd, 23.7%), and then there is no further
weight loss before structural decomposition at 305 °C. The
solvent molecules in 2 and 4 can be completely removed by
soaking the samples in CH3OH for 72 h and subsequently
heating at 80 °C under a vacuum for 6 h. Then guest-free
phases, 2a and 4a, were verified by TGA and IR (Figures S6
and S7). PXRD patterns demonstrate the framework integrity
of 2a and 4a (Figure S5), and the weakening and slight shifting
of some peaks for 2a should be attributed to the slight
framework distortion due to solvent release.
3 (c), and 4 (d)
steric hindrance from the H atom of the phenyl ring. The two
carboxylates of L12− in 1 form an arrangement of about 180°,
and it is about 120° for L22− in 2. This different connection
direction leads to two distinct structures, a 1D chain for 1 and a
2D layer for 2, which further form an uncommon 2D fabric
layer and 2D + 2D + 2D → 3D polycatenation, respectively.
H2L1 and H2L3 possess very similar geometry except that the
pyridine N atom from the ortho position of benzoic acid in
H2L1 is moved to the meta position of benzoic acid in H2L3,
which makes the pyridine N atom of L32− coordinate and forms
a 2D layer structure of 3 but not a 1D chain as shown in 1.
Meanwhile, the two carboxylates of L32− in 3 exhibit μ1:η1,η1
and μ1:η0,η1 coordination modes, respectively (Scheme 2c). In
contrast, the two carboxylates of L42− in 4 locate at one phenyl
unit and display a μ2:η1,η1 coordination fashion (Scheme 2d).
Sorption Properties. Gas-adsorption measurements (N2,
H2, CO2, and CH4) were performed to evaluate the porosity of
2a and 4a. As shown in Figure 5a, the N2 and H2 sorption
Figure 5. N2 and H2 sorption isotherms of 2a (a) and 4a (b) measured at 77 K.
F
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Figure 6. CO2 and CH4 sorption isotherms of 2a (a) and 4a (b) measured at 195 K.
Figure 7. Sorption isotherms of 2a (a) and 4a (b): CO2 at 273 K (a) and 298 K (b); CH4 at 273 K (c) and 298 K (d).
Figure 8. IAST adsorption selectivity of 2a (a) and 4a (b) for CO2 over CH4 at different compositions.
isotherms of 2a and 4a at 77 K show the characteristics of
microporous materials, but 2a displays obvious hysteresis
curves. The desorption hysteresis is mainly related to the
narrow channel system in 2a, which blocks the departure of
adsorbed gas molecules.24 The Brunauer−Emmett−Teller and
Langmuir surface areas are 369 and 402 m2 g−1 for 2a and 1376
and 1509 m2 g−1 for 4a, respectively. The total microporous
volumes are 0.16 and 0.59 m3 g−1 for 2a and 4a, respectively.
The measured pore-size-distribution curves based on the
density functional theory model show the main pore sizes of
3.9−5.4 and 5.0−6.8 Å for 2a and 4a, respectively (Figure S8).
In contrast to the low H2 uptake of 2a at 77 K, 4a exhibits an
excellent H2 storage capacity of 227.2 m3 g−1 (2.03 wt %) at 1
atm (Figure 5b), which is comparable to the highest capacity
values in PCN-6 (1.9 wt %),25a DUT-6 (2.02 wt %),25b DMOF-
1−NH2 (2.08 wt %),25c and SNU-4 (2.07%).25d
The CO2 and CH4 sorption isotherms of 2a and 4a at 195 K
were also measured (Figure 6). For 2a, the CO2 and CH4
uptakes at 1 atm are 83.6 cm3 g−1 (16.4 wt %) and 39.4 cm3 g−1
(2.8 wt %), respectively. For 4a, at 1 atm, the CO2 uptake
reaches 313.0 cm3 g−1 (61.5 wt %), corresponding to 5.0 CO2
molecules per formula unit, and the CH4 uptake is 127.3 cm3
g−1 (9.1 wt %). The CH4 adsorption amounts of 2a and 4a are
much lower than the values of the CO2 uptakes. The preferred
CO2 adsorption properties of 2a and 4a encourage us to
examine their potential application for CO2−CH4 separation at
ambient temperature. As shown in Figure 7, the CO2 uptakes at
1 atm are 37.8 cm3 g−1 (7.4 wt %) and 106.6 cm3 g−1 (20.9 wt
%) at 273 K and 23.0 cm3 g−1 (4.5 wt %) and 70.3 cm3 g−1
(13.8 wt %) at 298 K for 2a and 4a, respectively. The CO2
uptake of 4a at 298 K and 1 atm is slightly higher than that of
MAF-23 (11.0 wt %), which possesses multiple strong
adsorption sites,26 and comparable to some recently reported
G
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Figure 9. Simulated favorable CO2 sorption sites in 2a: (a) site I; (b) site II.
Figure 10. Solid-state excitation and emission spectra of 1−4 at room temperature.
metal−organic frameworks (MOFs) based on excellently
designed structure, such as rht-MOF-pyr (13.0 wt %),27a
Cd−L (15.5 wt %),27b and Cu4[(C57H32N12)(COO)8] (14.5 wt
%).27c The larger desorption hysteresis of CO2 in 2a suggests
stronger interactions of the framework of 2a with CO2 than
that of 4a. At 1 atm, the CH4 uptakes of 2a and 4a are 6.2 and
31.5 cm3 g−1 at 273 K and 2.1 and 18.8 cm3 g−1 at 298 K,
respectively. The CO2/CH4 selectivities of 2a and 4a under
mixture gas conditions at 298 K can be estimated by the ideal
adsorbed solution theory28 (IAST; Figure S9). For CO2/CH4
mixtures with general feed compositions of landfill gas (50:50
CO2/CH4) and natural gas (5:95 CO2/CH4), the CO2/CH4
selectivities calculated at 1 atm are 24.7 and 18.5 for 2a and 6.0
and 5.1 for 4a, respectively (Figure 8). These values in 2a are
higher than many famous MOFs such as HKUST-1 (2.28),
MIL-53(Al) (2.30), and ZIF-8 (1.32)29 and approximate with
those in MPM-1-TIFSIX (20.3)30a and [Cu3(TDPAH)(H2O)3]
(18.0).30b Notably, by a comparison of the CO2/CH4
selectivities in 2a and 4a with those known MOFs that
possessed good CO2/CH4 selectivity (Table S2), it is clearly
indicated that the CO2/CH4 selectivity in 2a is significantly
high, ranking in the top region.
The adsorption selectivities of 2a and 4a for CO2 are closely
related to the smaller kinetic diameter of CO2 compared to that
of CH4 (CO2, 3.3 Å; CH4, 3.8 Å), as well as the larger
quadrupole moment and higher polarizability value of CO2
relative to CH4. The considerable difference of the CO2/CH4
selectivity values for 2a and 4a relies on the different structure
features. The more remarkable CO2 sorption selectivity of 2a
should be attributed to the anionic framework that generates a
significantly electric field, while the free pyridine N atoms of
L22− and H2N(CH3)2 cations can easily approach CO2
molecules.31 To estimate the strength of framework−CO2
interactions, the isosteric heat of adsorption (Qst) was
calculated via the virial method by fitting the adsorption curves
collected at 273 and 298 K (Figures S10 and S11). The Qst
value of 2a for CO2 is 37.2 kJ mol−1 at initial coverage,
indicating strong CO2−framework interactions. In contrast, 4a
H
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reveals low Qst values (5.8−18.1 kJ mol−1) of CO2 because of
the lack of classical interacting sites such as open metal sites
and/or free amine groups within the framework.32 Only the
aromatic sites in 4a afford an affinity toward CO2 molecules,
leading to a relatively low CO2/CH4 selectivity compared to 2a.
GCMC Simulations. The interaction mechanism between
the framework of 2a and CO2 can be identified by GCMC
simulations. The results indicate two main adsorption sites
existing in 2a. The first site is located at the center of two
H2N(CH3)2 cations (Figure 9a), in which the two electro-
functional crystalline materials from the positional isomeric
ligands for regulating structural assembly.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b01416.
■
S
X-ray crystallographic data of 1 in CIF format (CIF)
X-ray crystallographic data of 2 in CIF format (CIF)
X-ray crystallographic data of 3 in CIF format (CIF)
X-ray crystallographic data of 4 in CIF format (CIF)
Additional structural figures, PXRD patterns, TGA plots,
and FTIR spectra of 1−4, detailed calculations on
sorption, and solid emission spectra of free ligands
(PDF)
negative O atoms of CO2 form two pairs of C−H···OCO
2
hydrogen bonds with the four methyl H atoms of two
H2N(CH3)2 cations, and the H···OCO distances of 2.65−2.97 Å
2
are comparable to the sum of the van der Waals radii of H (1.20
Å) and O (1.52 Å) atoms, implying significant interactions.
Meanwhile, in this site, the CO2 molecule is parallel with the
phenyl rings of two L22− from two layers; consequently, the
electropositive C atom of CO2 also forms strong CCO2···π
interactions with the two phenyl rings, and the CCO2···πcentroid
distances of 3.45 and 3.47 Å are shorter than those reported in
other MOFs.33 The increase of CO2 loading makes the other
adsorption site available, which is near the middle of two
coordination motifs from two layers (Figure 9b). As a result,
AUTHOR INFORMATION
Corresponding Author
*E-mail: lhou2009@nwu.edu.cn (L.H.).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
the C atoms of the CO2 molecules form CCO ···O contacts with
the coordinated electronegative carboxylate O atoms. The
2
■
This work is supported by the National Science Foundation of
China (Grants 21471124 and 21531007), the National Science
Foundation of Shannxi Province (Grants 2013KJXX-26,
2014JQ2049, and 15JS113), a Ph.D. Start-up Fund of
Northwest A&F University (Grant 2452015359), and an
Australian Research Council Future Fellowship (Grant
FT12010072).
CCO ···O distance (3.11−3.94 Å) is close to the sum of the van
2
der Waals radii of C (1.70 Å) and O atoms, suggesting
moderate interactions. These multiple framework−CO2 inter-
actions lead to the relatively high adsorption heat of 2 for CO2
and significant CO2/CH4 selectivity.
Photoluminescence Properties. The solid-state photo-
luminescence of 1−4 (Figure 10) and free ligands H2L1−H2L4
(Figure S12) was investigated at room temperature. The
emission spectra in the blue region have broad peaks with
maxima at 439 nm (λex = 376 nm) for 1, 433 nm (λex = 338
nm) for 2, 448 nm (λex = 370 nm) for 3, and 455 nm (λex = 396
nm) for 4. These bands can be assigned to π* → π or π* → n
intraligand charge transfer because similar emissions are
observed at 432 nm for H2L1, 405 nm for H2L2, 409 nm for
H2L3, and 400 and 470 nm for H2L4.
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