Syntheses and characterization of four 2D metal-organic networks based on rigid imidazolate/carboxylate functionalized ligand - Effect of the torsion of the ligands on crystal structures and properties

Article abstract of DOI:10.1016/j.ica.2012.07.031

A series of rigid imidazolate/carboxylate functionalized ligands, X-(5,7-dioxopyrrolo[3,4-f]benzimidazole-6-yl)benzoic acid (X = 4, H ₂4-DPBB; X = 3, H₂3-DPBB; X = 2, H₂2-DPBB) were designed and used for the assemble reactions with Zn²⁺/Cd ²⁺/Cu²⁺ ions. Four compounds, [Zn(4-DPBB)(H ₂O)]·DMSO·0.5MeOH (1), [Cd(3-DPBB)(DMA) _1.8(H₂O)_0.2] (2), [Cu(3-DPBB)(DMSO)(DMA)] (3) and [Cd(2-DPBB)(DMA)₂] (4) have been synthesized and structurally characterized (where DMSO = dimethylsulfoxide, DMA = dimethyl acetylamide). Single crystal X-ray analyses show that the four compounds are all based on 3-connected single metal nodes and 3-connected spacers. Compounds 1 and 2 bear a 2D (6, 3)-net structure, while 3 and 4 have a 2D (4, 8)-net. Compared with 2, 3 and 4, 1 exhibits a relatively high thermal stability which is up to ～550 °C. Luminescent analysis indicates that 1 and 2 exhibit linker-localized emissions while 4 has ligand-to-metal charge transfer (LMCT) emission. The effect of the torsion of the ligands on the crystal structures and properties of the four compounds is discussed.

Full text of DOI:10.1016/j.ica.2012.07.031

Inorganica Chimica Acta 394 (2013) 117–126
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Inorganica Chimica Acta
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Syntheses and characterization of four 2D metal–organic networks based on rigid
imidazolate/carboxylate functionalized ligand – Effect of the torsion of the ligands
on crystal structures and properties
Jing-Si Yang a, Jiang Zhu b, Rui-Bin Liu a, Jun Ni a, Zhi-Duo Chang a, Tao Hu a, Jian-Jun Zhang a,
,
⇑
Chang-Gong Meng a
a Chemistry College, Dalian University of Technology, Dalian 116024, China
b School of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian 116029, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of rigid imidazolate/carboxylate functionalized ligands, X-(5,7-dioxopyrrolo[3,4-f]benzimid-
azole-6-yl)benzoic acid (X = 4, H24-DPBB; X = 3, H23-DPBB; X = 2, H22-DPBB) were designed and used
for the assemble reactions with Zn2+/Cd2+/Cu2+ ions. Four compounds, [Zn(4-DPBB)(H2-
Received 25 March 2012
Received in revised form 10 July 2012
Accepted 25 July 2012
O)]ꢀDMSOꢀ0.5MeOH (1), [Cd(3-DPBB)(DMA)1.8(H2O)0.2] (2), [Cu(3-DPBB)(DMSO)(DMA)] (3) and [Cd
Available online 11 August 2012
(2-DPBB)(DMA)2] (4) have been synthesized and structurally characterized (where DMSO = dimethylsulf-
oxide, DMA = dimethyl acetylamide). Single crystal X-ray analyses show that the four compounds are all
based on 3-connected single metal nodes and 3-connected spacers. Compounds 1 and 2 bear a 2D (6, 3)-
net structure, while 3 and 4 have a 2D (4, 8)-net. Compared with 2, 3 and 4, 1 exhibits a relatively high
thermal stability which is up to ꢁ550 °C. Luminescent analysis indicates that 1 and 2 exhibit linker-local-
ized emissions while 4 has ligand-to-metal charge transfer (LMCT) emission. The effect of the torsion of
the ligands on the crystal structures and properties of the four compounds is discussed.
Ó 2012 Elsevier B.V. All rights reserved.
Keywords:
Supramolecular
Self-assembly
Carboxylate
Coordination polymer
Thermal stability
1. Introduction
type and the potential of exploiting properties of the MOMs thus
formed [6,7]. On the other hand, imidazole and its derivations
In the past two decades great progress has been made in the
development of metal organic materials (MOMs) that incorporate
are characterized by their special N, N-donor set. Coordinated by
tetrahedral metal ions (e.g., Zn, Co), the ligands can be used to con-
struct a special type of MOMs with zeolite-type tetrahedral topol-
ogies [17–22]. At the same time, MOMs based on imidazolate/
carboxylate ligand were also well studied. A typical example is 4,
5-imidazoledicarboxylic acid which has been used for the con-
struction of 3D anionic frameworks through a single-metal-ion
based molecular building block (MBB) in the presence of different
cationic structure direction agents [23,24]. However, in marked
contrast to the extensive studies of imidazole and its short carbox-
ylate-substituted derivations, few long rigid imidazolate/carboxyl-
ate ligands have been reported in MOMs synthesis [23–27].
Compared with other properties, thermal stability of MOMs is
less concerned because only a few reported MOMs remain stable
up to 450 °C, even though this property is very important for
practical applications [28–31]. Herein we present the design and
synthesis of a series of new long rigid ligands containing imidazo-
late/carboxylate groups (Scheme 1). The assemble reactions with
different metal ions were well investigated and four new 2D
frameworks possessing (6, 3) or (4, 8) net structure were obtained:
[Zn(4-DPBB)(H2O)]ꢀDMSOꢀ0.5MeOH (1), [Cd(3-DPBB)(DMA)1.8
(H2O)0.2] (2), [Cu(3-DPBB)(DMSO)(DMA)] (3) and [Cd(2-DPBB)
(DMA)2] (4). The series of ligands provide a good model to
inorganic and organic components within
through supramolecular interaction such as coordination bond,
hydrogen bond and stacking interaction [1–7]. Most com-
a single material
p–p
pounds have been prepared for developing new porous materials
and structures with potential applications traditionally observed
in inorganic zeolites, such as size- and shape-selective uptake,
catalysis, separation, and gas storage [1–7]. However, better com-
prehension of the correlation between syntheses, structure and
application remains a great challenge due to some uncertained ef-
fect factors, including experimental conditions, geometries of
molecular building blocks (MBBs), bridging modes of the ligands
etc. [8–10].
Most synthesized MOMs are based on polycarboxylates, poly-
pyridinates/imidazolates, pyrazolate derivatives and phosphonates
ligands [11–16]. The coordination chemistry of aromatic polycar-
boxylate ligands has been well explored owing to their relatively
strong coordination ability and a variety of bridging modes coordi-
nating to almost all the metal ions, as well as the diverse structure
⇑
Corresponding author. Tel.: +86 411 84986036.
E-mail address: zhangjj@dlut.edu.cn (J.-J. Zhang).
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2012.07.031

118
J.-S. Yang et al. / Inorganica Chimica Acta 394 (2013) 117–126
O
COOH
COOH
N
N
-2H O
2
N
+
H N
2
COOH
N
H
COOH
N
H
O
Scheme 1. Syntheses of the ligands.
invistigate the effect of the torsion of the ligands (the substituted
site of the carboxylate group on benzene ring) on the crystal struc-
tures and properties of the MOMs based on the ligands. Details of
crystal structures and properties are discussed.
adding equal amount of water to the filtrate solution. The product
was filtrated, washed several time with hot DMF and EtOH, then
dried in air. Yield: 56%. Element analysis (%): Anal. Calc. for
C
16H9N3O4: C 62.54, H 2.95, N 13.68. Found: C 62.59, H 2.93, N
13.70%. IR (KBr pellet, cmꢂ1): 3340(m), 3050(s), 1768(m),
1710(vs), 1600(m), 1511(m), 1383(vs), 1112(m), 835(w), 767(m),
745(s), 695(s). Negative ESI-MS (m/z): 305.85 (H22-DPBB-H+). 1H
NMR (DMSO-d6): d: 7.527 (J = 8.0, 1.2 Hz, dd, 1H, Ph-H), 7.622
(J = 16.0, 1.2 Hz, td, 1H, Ph-H), 7.773 (J = 16.0, 1.2 Hz, td, 1H, Ph-
H), 8.045 (J = 8.0, 1.2 Hz, dd, 1H, Ph-H), 8.105 (s, 1H, Ph-H), 8.183
(s, 1H, Ph-H), 8.598 (s, 1H, Im-H), 13.305 (s, 1H, COOH). Melting
points: 295.4–296.7 °C.
2. Experimental
2.1. Materials and instrumentation
Starting materials were of reagent grade and used without fur-
ther purification. X-ray powder diffraction patterns were collected
on
a
D/MAX-2400 diffractometer using Cu
Ka radiation
0
(k = 1.54056 ÅA) under ambient conditions. FT-IR spectra were re-
corded in the range of 4000–400 cmꢂ1 on a Centauri spectrometer
with KBr pellets. Elemental analyses were determined using a Var-
io EL III elemental analyzer. Thermogravimetric analysis (TGA) was
recorded with a NETZSCH STA 449F3 unit at a heating rate of 10 °C/
min under nitrogen atmosphere. Electrospray ionization-mass
spectroscopy (ESI-MS) analysis was carried out with HP 1100LC/
MSD liquid chromatograph-mass spectrometer. 1H nuclear mag-
netic resonance (NMR) were recorded on a Bruker AvanceII400M
spectrometer. Melting points analyses were carried out with micro
melting point analysis X-6 in the range of 25–310 °C.
2.2.3.1. [Zn(4-DPBB)(H2O)]ꢀDMSOꢀ0.5MeOH (1). Equimolar amounts
of H24-DPBB (0.0061 g, 0.02 mmol) and Zn(NO3)2ꢀ6H2O (0.0059 g,
0.02 mmol) in a solvent mixture of DMSO/DMF/MeOH (1 ml/
0.5 ml/0.5 ml) were placed in a 20 ml scintillation vial, heated to
85 °C for 24 h, then cooled to room temperature. The colorless
crystals were collected, washed with DMSO and dried in air (53%
yield). Element analysis (%): Anal. Calc. for C18.5H17N3O6.5SZn: C
46.02, H 3.55, N 8.70. Found: C 45.95, H 3.82, N 8.59%. IR (KBr pel-
let, cmꢂ1): 3440(m), 3114(w), 1772(s), 1708(vs), 1608(vs),
1538(m), 1409(m), 1357(vs), 1187(vs), 1022(s), 846(s), 784(m),
744(s), 613(w), 460(m).
2.2. Synthesis
2.2.3.2. [Cd(3-DPBB)(DMA)1.8(H2O)0.2
]
(2). H23-DPBB (0.0246 g,
0.08 mmol) and Cd(NO3)2ꢀ4H2O (0.0247 g, 0.08 mmol) in a solvent
mixture of DMA/H2O (3 ml/0.5 ml) were placed in a 20 ml scintil-
lation vial, heated to 125 °C for 12 h, then cooled to room temper-
ature. The colorless crystals were collected, washed with DMA and
dried in air (49% yield). Element analysis (%): Anal. Calc. for C23.2-
H23.6CdN4.8O6: C 48.20, H 4.11, N 11.63. Found: C 48.25, H 4.26,
N 11.87%. IR (KBr pellet, cmꢂ1): 3454(w), 1766(m), 1706(vs),
1603(vs), 1547(m), 1455(m), 1400(vs), 1308(m), 1308(m),
1021(m), 853(m), 758(m), 745(m), 650(m),630(m), 616(m).
2.2.1. H24-DPBB
A mixture of benzimidazole-5,6-dicarboxylic acid (2.0616 g,
10 mmol) and 4-aminobenzoic acid (1.3714 g, 10 mmol) in 30 ml
DMF was heated at 135 °C for 10 h, then filtrated. The product
was washed several time with hot DMF and EtOH, then dried in
air. Yield: 70%. Element analysis (%): Anal. Calc. for C16H9N3O4: C
62.54, H 2.95, N 13.68. Found: C 62.55, H 2.91, N 13.65%. IR (KBr
pellet, cmꢂ1): 3340(m), 3050(s), 1770(m), 1713(vs), 1605(m),
1509(m), 1371(vs), 1264(vs), 1118(m), 865(s), 835(s), 610(s). Neg-
ative ESI-MS (m/z): 305.98 (H24-DPBB-H+). 1H NMR (DMSO-d6): d:
7.630 (J = 4.8 Hz, d, 2H, Ph-H), 8.097 (J = 1.2 Hz, d, 2H, Ph-H), 8.114
(s, 1H, Ph-H), 8.208 (s, 1H, Ph-H), 8.602 (s, 1H, Im-H), 13.320 (s, 1H,
COOH). Melting points: above 300 °C.
2.2.3.3. [Cu(3-DPBB)(DMSO)(DMA)] (3). Equimolar amounts of H23-
DPBB (0.0246 g, 0.08 mmol) and Cu(NO3)2ꢀ3H2O (0.0193 g,
0.08 mmol) in a solvent mixture of DMSO/DMA/H2O (2 ml/1 ml/
0.5 ml) were placed in a 20 ml scintillation vial, heated to 85 °C
for 24 h, then cooled to room temperature. The green crystals were
collected, washed with DMA and dried in air (57% yield). Element
analysis (%): Anal. Calc. for C22H22N4O6SCu: C 49.48, H 4.15, N
10.49. Found: C 49.25, H 4.15, N 10.47%. IR (KBr pellet, cmꢂ1):
1760(m), 1700(vs), 1610(vs), 1480(m), 1360(s), 1310(m),
1190(m), 1110(m), 996(s), 959(m), 851(m), 760(m), 671(m),
627(m), 453(m).
2.2.2. H23-DPBB
The above mentioned synthesis procedure was repeated except
3-aminobenzoic acid was used. Yield: 65.5%. Element analysis (%):
Anal. Calc. C16H9N3O4: C 62.54, H 2.95, N 13.68. Found: C 62.60, H
2.92, N 13.69%. IR (KBr pellet, cmꢂ1): 3310(m), 2520(m), 1780(vs),
1670(vs), 1600(m), 1550(m), 1480(vs), 1090(vs), 840(m), 776(m),
673(s), 543(s). Negative ESI-MS (m/z): 305.88 (H23-DPBB-H+). 1H
NMR (DMSO-d6): d: 7.669 (J = 1.6 Hz, t, 1H, Ph-H), 7.747
(J = 7.2 Hz, d, 1H, Ph-H), 7.995 (J = 8.0 Hz, d, 1H, Ph-H) 8.057
(s,1H, Ph-H) 8.104 (s, 1H, Ph-H) 8.201 (s, 1H, Ph-H), 8.596 (s, 1H,
Im-H), 13.293 (s, 1H, COOH). Melting points: above 300 °C.
2.2.3.4. [Cd(2-DPBB)(DMA)2] (4). Equimolar amounts of H22-DPBB
(0.0061 g, 0.02 mmol) and Cd(OAC)2ꢀ2H2O (0.0213 g, 0.08 mmol)
in a solvent mixture of DMA/EtOH (3 ml/0.1 ml) were placed in a
20 ml scintillation vial, heated to 105 °C for 24 h, then cooled to
room temperature. The colorless crystals were collected, washed
with DMA and dried in air (39% yield). Element analysis (%): Anal.
Calc. for C24H25N5O6Cd: C 48.70, H 4.26, N 11.83. Found: C 48.24, H
2.2.3. H22-DPBB
The above mentioned synthesis procedure was repeated except
2-aminobenzoic acid was used. White solid was obtained upon

J.-S. Yang et al. / Inorganica Chimica Acta 394 (2013) 117–126
119
4.27, N 11.91%. IR (KBr pellet, cmꢂ1): 1774(m), 1712(vs), 1656(m),
3. Results and discussion
1588(m),1541(m), 1487(w), 1397(vs), 1373(vs), 1306(m), 1193(w),
1121(s), 955(m), 884(m), 847(m), 782(m), 758(m), 744(m),
710(m), 666(m), 628(m), 420(m).
3.1. Crystal structure of 1
The structure of 1 is a 2D coordination polymer that based on 3-
connected Zn2+ nodes and 4-DPBB ligands (Fig. 1). Each Zn2+ ion is
four-coordinated with 2 N atoms from 2 ligands, 1 O atom from
carboxylate group of one ligand and 1 O atom from one coordi-
2.2.3.5. X-ray structure determinations. Intensity data were mea-
sured at 293(2) K on a Bruker SMART APEX II CCD area detector
system. Data reduction and unit cell refinement were performed
with Smart-CCD software [32]. The structures were solved by di-
rect methods using SHELXS-97 and were refined by full-matrix least
squares methods using SHELXL-97 [33].
nated water molecule. The Zn—OH bond distance is 1.973(6) Å,
2O
a little longer than that for Zn–OCOOH (1.960(6) Å). The Zn–Nim
bond distances are in the range of 1.976(6)–1.991(6) Å. To Zn(1),
small deviations of bond angles from the idealized tetrahedronal
geometry are found for angles O(1A)-Zn(1)–O(5) [107.6(3)°],
O(1A)–Zn(1)–N(1) [124.2(2)°], O(5)–Zn(1)–N(1) [108.2(2)°], O(5)–
Zn(1)–N(2B) [106.6(3)°] and N(1)–Zn(1)–N(2B) [112.4(3)°], imply-
For 1, all nonhydrogen atoms were refined anistropically. No at-
tempts were made to locate the hydrogen atoms on the half occu-
pied MeOH molecules. The hydrogen atoms related to carbon
atoms were generated geometrically. The maximum residual peak
and hole on the final difference electron density map were found to
be 1.276 eꢂ/Å3 (0.30 Å from S1) and ꢂ1.000 eꢂ/Å3 (1.14 Å from S1),
respectively. For 2, all nonhydrogen atoms were refined anisotrop-
ically. The hydrogen atoms related to carbon atoms were generated
geometrically. The coordinated water molecule and one of the
coordinated DMA molecules share the same oxygen atom. Partial
occupancy refinement was performed and the best refinement re-
sult is 0.8DMA and 0.2H2O. The maximum residual peak and hole
on the final difference electron density map were found to be
0.942 eꢂ/Å3 (0.84 Å from N4) and ꢂ0.407 eꢂ/Å3 (0.27 Å from N4),
respectively. For 3, all nonhydrogen atoms were refined anisotrop-
ically. The hydrogen atoms related to carbon atoms were generated
geometrically. The maximum residual peak and hole on the final
difference electron density map were found to be 0.513 eꢂ/Å3
(1.05 Å from C19) and ꢂ0.512 eꢂ/Å3 (0.47 Å from C18), respec-
tively. For 4, all non hydrogen atoms were refined anisotropically.
The hydrogen atoms related to carbon atoms were generated geo-
metrically. The maximum residual peak and hole on the final dif-
ference electron density map were found to be 0.404 eꢂ/Å3
(1.09 Å from Cd1) and ꢂ0.281 eꢂ/Å3 (0.65 Å from H17B),
respectively.
ing the metal ion in
a distorted tetrahedronal coordination
polyhedron.
Two imidazole nitrogen atoms in each ligand coordinate to two
metal ions and one carboxylate oxygen atom coordinates to the
third Zn2+ ion. A 46.6° dihedral angle is observed between the
benzimidazole and benzene rings for the ligand.
The combination of the 3-connected Zn(II) nodes and the li-
gands lead to the neutral 2D layer with a thickness of 6.39 Å and
the structure may be described as a (6, 3) honeycomb-type net.
The layers are packed along the ac plane and separated by free
DMSO and disordered water solvent molecules. Inside the layer,
the Zn. . .Zn distances are 5.82 (bridged by the imidazole group),
15.04 and 15.13 Å, respectively. Due to the torsion of the ligand,
the 2D net bears a wave-like structure motif with a corresponding
angle of 56.07°. The surface of the net are decorated with oxygen
atoms, either from the coordinated water molecules or uncoordi-
nated carboxylate or carbonyl groups, which makes the net
hydrophilic. Hydrogen bonds between the coordinated water
molecules and oxygen atoms link the 2D nets into a 3D frame-
works (O5. . .O2 = 2.63(1) Å, \O5–H5B. . .O2 = 173(9)°, symmetry
code for O2: ꢂx + 2, ꢂy, ꢂz + 2). Hydrogen bonds also help to
stabilize the free DMSO molecules (O5. . .O6 = 2.63(1) Å,
\O5–H5A. . .O6 = 139(8)°, symmetry code for O6: ꢂx + 2, ꢂy,
ꢂz + 2).
A summary of the most important crystal and structure refine-
ment data is given in Table 1.
Table 1
Crystal data and structure refinement for the complexes.
1
2
3
4
Formula
C
18.5H17ZnN3O6.5
S
C23.2H23.6CdN4.8O6
578.07
monoclinic
P2(1)/n
14.601(2)
7.721(1)
22.842(3)
90
C22H22CuN4O6S
534.04
C24H25CdN5O6
591.89
monoclinic
P2(1)/c
15.286(2)
9.216(1)
18.137(2)
90
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
482.78
monoclinic
P2(1)/c
15.13(3)
13.94(2)
10.34(2)
90
monoclinic
P2(1)/n
11.7035(6)
8.6041(5)
23.7693(11)
90
a
(°)
b (°)
103.61(2)
90
2121(6)
4
95.446(2)
90
2563.6(6)
4
91.709(4)
90
2392.5(2)
4
104.793(1)
90
2470.3(5)
4
c
(°)
V (Å3)
Z
Dcalc (g/cm3)
1.512
1.498
1.483
1.591
F(000)
988
1170
1100
1200
h (°)
2.01–25.00
9934
2.79–25.00
12285
1.71–25.00
15630
2.32–25.50
11820
Reflections collected
Unique
3711
4494
3965
4576
Rint
0.1279
0.993
0.0330
0.980
0.0454
1.086
0.0270
1.045
Goodne⁄ss-of-fit (GOF) on F2
⁄
R1 wR2
I > 2
All data
r
(I)
R1 = 0.0807, wR2 = 0.1699
R1 = 0.1744, wR2 = 0.1966
0.000/0.000
R1 = 0.0391, wR2 = 0.1037
R1 = 0.0524, wR2 = 0.1124
0.000/0.000
R1 = 0.0434, wR2 = 0.0987
R1 = 0.0659, wR2 = 0.1061
0.001/0.000
R1 = 0.0282, wR2 = 0.0687
R1 = 0.0377, wR2 = 0.0727
0.001/0.000
Maximum/mean shift in final cycle
*
R1 = (||Fo|–|Fc||)/|Fo|, ⁄⁄wR2 = {w [(F2o–F2c)]/w [(F2o)2]}0.5, w = [ 2(F2o) + (aP)2 + bP]ꢂ1, where P = (F2o + 2 F2c)/3]. 1: a = 0.0375, b = 0.1804; 2: a = 0.0690, b = 1.1851; 3:
r
a = 0.0516, b = 0.5409; 4: a = 0.0789, b = 0.0000.

120
J.-S. Yang et al. / Inorganica Chimica Acta 394 (2013) 117–126
Fig. 1. The structure of compound 1. (a) The coordination environment of the Zn2+ ion. Symmetry code: A: ꢂx + 3/2, y ꢂ 1/2, ꢂz + 3/2; B: x + 1, y, z; (b) the 2D layer structure
viewed along the b direction; (c) the packing diagram viewed along the a direction; the atoms of framework are shown in wires mode while those of free DMSO molecules are
shown in ball and stick mode; (d) The framework topology of the 2D net.
3.2. Crystal structure of 2
Each ligand is bound to three Cd2+ ions and each Cd2+ ion is also
linked by three ligands. These give rise to a neutral 2D (6, 3) net
perpendicular to the c direction. When viewed along the a direc-
tion the 2D net is ruffled to create a zigzag structural motif and
the coordinated DMA molecules are used to fill the breach of the
net. Inside the layer, the Cd. . .Cd separations are 6.23 (bridged by
the imidazole group), 12.59 and 14.60 Å, respectively. The 2D nets
are packed along the c direction to form the overall structure and a
schematic representation of the 2D honeycomb-type net is shown
in Fig. 2d.
2 Features two-dimensional (6, 3) network with 3-connected
Cd2+ and 3-DPBB ligands as nodes. Fig. 2a shows the coordination
environment of Cd2+ ion. The 6-coordinated Cd2+ center is in a
slightly distorted octahedral coordination environment that is
coordinated by 2 N atoms from 2 ligands, 2 O atoms from carbox-
ylate group of one ligand and 2 O atoms from 2 coordinated solvent
molecules. The bond angles around Cd2+ ion range from 54.95(9) to
178.57(1)°. The Cd–O and Cd–N bond distances are in the range of
2.250(3)–2.485(5) and 2.211(3)–2.261(3) Å, respectively. All are
consistent with the values reported for Cd-carboxylate and Cd-
imidazole complexes [27,28]. Three Cd2+ ions are connected by a
3.3. Crystal structure of 3
3 Possesses 2D nets built of 3-connected Cu2+ nodes and 3-DPBB
ligands (Fig. 3). Each Cu2+ ion has a {N2O3} donor set and square
plane is composed of 2 N atoms from 2 ligands, 1 O atom from
the carboxylate group of the third ligand and another oxygen
atom from a coordinated DMSO molecule. The Cu–O and Cu–N
bond distances are in the range of 1.945(2)–2.009(2) and
2
1
1
3-DPBB ligand with l3-gO : gN : gN coordination mode
(Scheme 1b), in which the carboxylate group chelate one Cd2+
ion in monobidentate mode and each ligand also uses two imidaz-
ole nitrogen atoms to coordinate to two other metal ions. A 46.14°
dihedral angle is also observed between the benzimidazole and
benzene rings.

J.-S. Yang et al. / Inorganica Chimica Acta 394 (2013) 117–126
121
Fig. 2. The structure of compound 2. (a) The coordination environment of the Cd2+ ion. Symmetry code: A: ꢂx + 3/2, y ꢂ 1/2, ꢂz + 3/2; B: x + 1, y, z; (b) the 2D layer structure
viewed along the b direction. The atoms of framework are shown in wires mode while those of coordinated solvent molecules are shown in ball and stick mode; (c) the 2D
layer structure viewed along the a direction; (d) the framework topology of the 2D net.
1.991(2)–2.002(3) Å, respectively.
A
slightly square-pyramidal
the breach of the net (Fig. 3). Inside the layer, the Cu. . .Cu separa-
tions are 5.98 (bridged by the imidazole group), 12.49 and 13.45 Å,
respectively. The packing of the 2D nets and a schematic represen-
tation of the net is shown in Fig. 3d.
configuration around the Cu2+ ion is completed by the additional
binding of one DMA molecule in axial position (Cu1–
O6 = 2.486 Å). The Cu2+ ion deviates by about 0.03 Å from the least
squares plane defined by N1, N2, O3 and O5 toward the coordi-
nated DMA molecule. The bond angles around Cu2+ ion range from
86.2(1) to 177.95(9)°. Each ligand uses two imidazole nitrogen
atoms to coordinate to two metal ions and one carboxylate oxygen
atom to coordinate to the third Cu2+ ion. A 68.95° dihedral angle is
observed between the benzimidazole and benzene rings.
3.4. Crystal structure of 4
The structure of 4 may be described as a 2D net based on 3-con-
nected Cd2+ nodes and 2-DPBB ligands (Fig. 4). Selected bond dis-
tances and angles are shown in Table 2. In the structure each
Cd2+ ion is 6-coordinated with 2 imidazole N atoms from 2 ligands,
2 O atoms from carboxylate group of one ligand and 2 O atoms
from 2 coordinated disordered DMA molecules respectively. The
Each ligand is bound to three Cu2+ ions and each Cu2+ ion is also
linked by three ligands. Thus a neutral 2D (4, 8) distorted network
is formed. Coordinated DMA and DMSO molecules are used to fill

122
J.-S. Yang et al. / Inorganica Chimica Acta 394 (2013) 117–126
Fig. 3. The structure of compound 3. (a) The coordination environment of the Cu2+ ion. Symmetry code: A: x ꢂ 1/2, ꢂy + 3/2, z ꢂ 1/2; B: ꢂx + 1/2, y + 1/2; (b) the 2D layer
structure viewed along the a direction. The atoms of framework are shown in wires mode while those of coordinated DMSO and DMA molecules are shown in ball and stick
mode; (c) the 2D layer structure viewed along the b direction; (d) the framework topology of the 2D net.
Cd–ODMA bond distances are 2.488(2) Å and 2.352(2) Å, respec-
tively, a little longer than that of Cd–OCOOH (2.270(2) Å and
2.463(2) Å). Additionally, the average Cd–Nim bond distance is
2.24(1) Å. To Cd(1), small deviations of bond angles from the ideal-
ized octahedral geometry are found for angles N(2A)-Cd(1)–N(1)
[97.00(8)°], N(2A)–Cd(1)–O(2B) [145.14(8)°], N(1)–Cd(1)–O(2B)
[117.47(8)°], N(2A)–Cd(1)–O(6) [88.98(8)°], N(1)–Cd(1)–O(6)
[94.04(8)°], which implies that the metal ion has a distorted octa-
hedral coordination polyhedron. Each ligand uses two imidazole
nitrogen atoms to coordinate to two metal ions and the carboxyl-
ate group to chelate the third Cd2+ ion. Maybe due to the coordina-
tion to the metal ions, the ligand is distorted and a 79.76° dihedral
angle is observed between the benzimidazole and benzene rings.
The combination of the 3-connected Cd(II) nodes and the li-
gands provides a 2D (4, 8) distorted network extending along the
bc plane. As shown in Fig. 4b, the four-membered and eight-mem-
bered rings are almost parallel and perpendicular to the a direction
respectively. The coordinated DMA molecules are captured in the
eight-membered rings thus formed. Inside the layer, the Cd. . .Cd
separations are 6.37 (bridged by the imidazole group) and 7.85 Å,
respectively. A schematic representation of the 2D net is shown
in Fig. 4c.

J.-S. Yang et al. / Inorganica Chimica Acta 394 (2013) 117–126
123
Fig. 4. The structure of compound 4. (a) The coordination environment of the Cd2+ ion. Symmetry code: A: ꢂx + 1, y + 1/2, ꢂz + 3/2; B: ꢂx + 1, ꢂy + 2, ꢂz + 1; (b) the 2D layer
structure viewed along the a direction. The atoms of framework are shown in wires mode while those of coordinated DMA molecules are shown in ball and stick mode; (c)
packing diagram of the 2D nets view along the b direction and (d) the framework topology of the 2D net.
3.5. The effect of torsion of the ligands on the structure of the
compounds
of 4-DPBB ligand, the shape of the connector is more like a ‘‘Y’’ con-
figuration. Strong hindrance effect prevents it from being used to
build (4, 8) 2D net but (6, 3) 2D net. When the substituted carbox-
ylate group is on the meta-position, the connector is in an interme-
diate state and can be used for the assembling of either (4, 8) or (6,
3) 2D net, by which the structures of 2 and 3 can be formed. These
results clearly demonstrate that the torsion angle of the ligands
plays an important role in the design of supramolecular structures.
1
1
1
2
Although the three ligands adopt l3-gO : gN : gN ; l3-gO :
1
1
1
1
1
2
1
gN
: gN ; l3-gO : gN : gN and l3-gO : gN :
gN1-coordination
modes in the four compounds respectively, all of them can be trea-
ted as 3-connected nodes (Scheme 2). However, depending on the
substituted sites of the carboxylate group on the benzene ring, the
torsion angles of the three ligands are different and this does have
an interesting effect on the final topology of the compounds. After
coordination with the metal ion, the shape of 2-DPBB ligand can be
described as trigonal pyramidal. This configuration makes it a good
candidate for the construction of (4, 8) 2D net. However, in the case
3.6. Syntheses and characterization of the compounds
Through the condensation reaction between benzimidazole-
5,6-dicarboxylic acid and aminobenzoic acid in DMF solvent, three

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J.-S. Yang et al. / Inorganica Chimica Acta 394 (2013) 117–126
Table 2
Bond lengths (Å) and angles (°) for the compounds.
1
2
3
4
Zn(1)–O(1A)
Zn(1)–O(5)
Zn(1)–N(1)
Zn(1)–N(2B)
O(1)–Zn(1C)
N(2)–Zn(1D)
1.962(6)
1.952(8)
1.971(6)
1.988(7)
1.962(6)
1.988(7)
107.1(3)
108.3(3)
124.3(3)
106.7(3)
96.4(3)
112.5(3)
108.0(5)
121.7(5)
133.5(5)
123.1(5)
134.5(5)
111.4(6)
127.0(6)
121.6(6)
116.5(7)
Cd(1)–N(2A)
Cd(1)–O(2B)
Cd(1)–N(1)
Cd(1)–O(6)
Cd(1)–O(5)
Cd(1)–O(1B)
2.215(3)
2.248(3)
2.264(3)
2.382(3)
2.434(3)
2.486(3)
146.9(1)
93.9(1)
119.0(1)
87.4(1)
91.8(1)
94.1(1)
93.6(1)
Cu(1)–O(3A)
Cu(1)–N(1)
Cu(1)–N(2B)
Cu(1)–O(5)
Cu(1)–O(6)
1.945(2)
1.991(2)
2.002(3)
2.009(2)
2.486(2)
Cd(1)–N(2A)
Cd(1)–N(1)
Cd(1)–O(2B)
Cd(1)–O(6)
Cd(1)–O(1B)
Cd(1)–O(5)
2.235(2)
2.252(2)
2.270(2)
2.352(2)
2.463(2)
2.488(2)
97.00(8)
145.14(8)
117.47(8)
88.98(8)
94.04(8)
83.87(7)
94.88(8)
156.94(8)
55.09(7)
105.88(7)
103.18(8)
84.58(8)
86.05(7)
167.83(7)
73.49(8)
O(5)–Zn(1)–O(1A)
O(5)–Zn(1)–N(1)
O(1A)–Zn(1)–N(1)
O(5)–Zn(1)–N(2B)
O(1A)–Zn(1)–N(2B)
N(1)–Zn(1)–N(2B)
C(16)–O(1)–Zn(1C)
C(1)–N(1)–Zn(1)
C(3)–N(1)–Zn(1)
C(1)–N(2)–Zn(1D)
C(2)–N(2)–Zn(1D)
C(8)–N(3)–C(9)
C(8)–N(3)–C(10)
C(9)–N(3)–C(10)
N(1)–C(1)–N(2)
N(2A)–Cd(1)–O(2B)
N(2A)–Cd(1)–N(1)
O(2B)–Cd(1)–N(1)
N(2A)–Cd(1)–O(6)
O(2B)–Cd(1)–O(6)
N(1)–Cd(1)–O(6)
N(2A)–Cd(1)–O(5)
O(2B)–Cd(1)–O(5)
N(1)–Cd(1)–O(5)
O(6)–Cd(1)–O(5)
N(2A)–Cd(1)–O(1B)
O(2B)–Cd(1)–O(1B)
N(1)–Cd(1)–O(1B)
O(6)–Cd(1)–O(1B)
O(5)–Cd(1)–O(1B)
O(3A)–Cu(1)–N(1)
O(3A)–Cu(1)–N(2B)
N(1)–Cu(1)–N(2B)
O(3A)1–Cu(1)–O(5)
N(1)–Cu(1)–O(5)
N(2B)–Cu(1)–O(5)
C(16)–O(3)–Cu(1C)
S(1)–O(5)–Cu(1)
C(1)–N(1)–Cu(1)
C(2)–N(1)–Cu(1)
C(1)–N(2)–Cu(1D)
C(3)–N(2)–Cu(1D)
C(9)–N(3)–C(8)
172.5(1)
90.6(1)
95.8(1)
87.46(9)
86.2(1)
177.95(9)
108.1(2)
118.7(1)
122.9(2)
133.1(2)
130.6(2)
125.7(2)
112.1(3)
122.7(3)
124.9(3)
N(2A)–Cd(1)–N(1)
N(2A)–Cd(1)–O(2B)
N(1)–Cd(1)–O(2B)
N(2A)–Cd(1)–O(6)
N(1)–Cd(1)–O(6)
O(2B)–Cd(1)–O(6)
N(2A)–Cd(1)–O(1B)
N(1)–Cd(1)–O(1B)
O(2B)–Cd(1)–O(1B)
O(6)–Cd(1)–O(1B)
N(2A)–Cd(1)–O(5)
N(1)–Cd(1)–O(5)
O(2B)–Cd(1)–O(5)
O(6)–Cd(1)–O(5)
O(1B)–Cd(1)–O(5)
87.7(1)
85.0(1)
178.7(1)
93.2(1)
54.95(9)
160.6(1)
104.1(1)
76.6(1)
(9)–N(3)–C(10
C(8)–N(3)–C(10)
Symmetry transformations used to generate equivalent atoms. For 1, A: ꢂx + 3/2, y ꢂ 1/2, ꢂz + 3/2; B: x + 1, y, z; C: x ꢂ 1, y, z; D: ꢂx + 3/2, y + 1/2, ꢂz + 3/2; For 2, A: ꢂx + 3/2,
y ꢂ 1/2, ꢂz + 3/2; B: x + 1, y, z; For 3, A: x ꢂ 1/2, ꢂy + 3/2, z ꢂ 1/2; B: ꢂx + 1/2, y + 1/2, ꢂz + 1/2; C: x + 1/2, ꢂy + 3/2, z + 1/2; D: ꢂx + 1/2, yꢂ1/2, ꢂz + 1/2; For 4, A: ꢂx + 1, y + 1/
2, ꢂz + 3/2; B: ꢂx + 1, ꢂy + 2, ꢂz + 1.
O
M
O
O
M
N
N
O
O
N
M
N
N
O
O
M
N
O
M
M
(a)
(c)
(b)
O
O
M
O
M
N
N
N
N
N
N
M
O
M
O
O
O
O
M
M
(d)
Scheme 2. Four coordination modes of the ligands.
new long rigid ligands bearing imidazolate/carboxylate groups,
H24-DPBB, H23-DPBB, H22-DPBB, are obtained (Scheme 1). The for-
mation of the coordination ploymers is not significantly affected by
the ratio of reactants. 1, 2, 3 and 4 are insoluble in water and com-
mon solvents.
The phase purity of 1–4 was confirmed by powder XRD analysis
on bulk samples. The powder XRD profile of each as-synthesized
compound is well matched with the simulated ones based on the
single crystal X-ray structure (Fig. 5). To examine the thermal sta-
bilities of the compounds, TGA experiments were performed by
heating the crystalline sample from room temperature to 800 °C
under nitrogen atmosphere (Fig. 6). In the TGA curve of 1 the first
weight loss of 24.56% in the temperature region of 55–272 °C is
due to the loss of free DMSO, MeOH and coordinated H2O mole-
cules (Calc. 23.23%). There is a long plateau in the temperature
range of 272–547 °C, indicating a relatively high thermal stability
in the absence of solvent molecular. After 550 °C, the material
shows a striking weight loss, indicating the completed decomposi-
tion of the structure. Curve 2 exhibits two distinct weight losses.
The first step (28.0%) involves the loss of all the coordinated sol-
vent molecules in the temperature region of 181–371 °C (Calc.
27.75%). In the second step, the material shows a striking weight
loss, indicating the completed decomposition of the structure. In
curve 3 the first weight loss of 29.6% in the temperature region
of 80–300 °C corresponding to the loss of coordinated DMSO and
DMA molecules (Calc. 30.9%). After 300 °C, the material exhibits a
striking weight loss, indicating the completed decomposition of
the structure. Curve 4 reveals two distinct weight losses. The first
step (27.8%) involves loss of both coordinated DMA molecules in
the temperature region of 78–291 °C (Calc. 29.40%). In the second
step, the material shows a striking weight loss, indicating the com-
pleted decomposition of the structure. These results demonstrate

J.-S. Yang et al. / Inorganica Chimica Acta 394 (2013) 117–126
125
Fig. 5. Powder X-ray diffraction patterns of compounds 1–4.
Fig. 7. The solid state fluorescence emission spectra for (a) 4 (kex = 332 nm); (b)
H22-DPBB (kex = 332 nm); (c) 2 (kex = 300 nm); (d) H23-DPBB (kex = 300 nm); (e) 1
(kex = 320 nm); (f) H24-DPBB (kex = 320 nm).
compounds. When the substituted carboxylate group is on the
meta- or para-position, only linker-localized emission is observed
because of the mismatch of the energy level of the ligand with
the energy levels of 3d10 metal ions. When the substituted carbox-
ylate group is on the ortho-position, the two energy levels are
matched and LMCT emission can be detected.
4. Conclusion
In summary, the syntheses, structures and properties of four 2D
MOMs based on a series of new rigid imidazolate/carboxylate func-
tionalized ligands are presented. The work shows that the substi-
tuted sites of the carboxylate group on the benzene ring of the
ligands can influence the torsion degree of the three ligands, which
can not only greatly affect the structures of the coordination poly-
mers and resulted in two types of topology, but also the thermal
stabilities and luminescent properties of the products. Thus the
work represents a good example to reveal the relationship be-
tween ligands, structures and properties of the MOMs.
Fig. 6. Thermal stabilities of compounds 1–4.
that 1 is much more stable than the other three compounds. Such a
high thermal stability is also well beyond those of most of the
MOMs that are generally stable below 300 °C [28–31]. Considering
the structures of the four compounds, the exceptional thermal sta-
bility of 1 can be attributed to the use of a more symmetrically ri-
gid ligand. Several thermal stable MOMs based on symmetrically
rigid ligands have been reported [28–31].
Acknowledgments
This material is based upon work supported by the National Sci-
ence Foundation of China (21071025, 21101020 and 91122031),
the Fundamental Research Funds for the Central Universities (DU-
T12YQ04 and DUT11LK22), and fund of Ministry of Education of
China (20100041120021 and ROCS).
The luminescent properties of the ligands and 1–4 were
checked in the solid state at ambient temperature (Fig. 7). The
compounds and ligands show emission at 395–460 nm when ex-
cited at about 330 nm. For 1 and H24-DPBB ligand, intense fluores-
cent emissions at 454 nm and 460 nm were observed when excited
at 320 nm. The 454 nm peak of 1 can be assigned as linker-local-
Appendix A. Supplementary material
CCDC 870398–870401 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-
ated with this article can be found, in the online version, at http://
dx.doi.org/10.1016/j.ica.2012.07.031.
ized n ? p⁄ or
p ?
p⁄ transition. 2 shows emission at 435 nm
(kex = 300 nm) which can also be attributed to the linker-localized
transition as a 440 nm (kex = 300 nm) emission was observed for
H23-DPBB. 3 shows no emission which can be explained as the
quenching effect due to the presence of transition-metal ion with
unpaired electrons [34]. Compared with the 440 nm (kex = 332 nm)
emission peak of H22-DPBB, 4 has an apparent blue shift emission
at 395 nm (kex = 332 nm), which could be assigned to the emission
of ligand-to-metal charge transfer (LMCT) [35,36]. The results indi-
cate the substituted site of the carboxylate group on benzene ring
do have some interesting effect on the fluorescent properties of the
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