Chinese Journal of Chemistry, 2008, 26, 2039—2044 Full Paper

* E-mail: mx_li@mail.shu.edu.cn; Tel.: 0086-021-66132401; Fax: 0086-021-66132797 Received February 25, 2008; revised April 17, 2008; accepted July 2, 2008. Project supported by the Leading Academic Discipline Project of Shanghai Municipal Education Commission (J50102), and the Research Founda-

tion for Returned Overseas Chinese Scholars of Chinese Education Ministry (7A14219).

© 2008 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Synthesis, Structure and Luminescence of Two Coordination Polymers Based on 1,4-Benzenedicarboxylate and

2-(3-Pyridyl)benzimidazole Ligands

LI, Weia(李玮) LI, Ming-Xing*,a(李明星) HE, Xianga(何翔) SHAO, Minb(邵敏) AN, Bao-Lia(安保礼)

a Department of Chemistry, College of Science, Shanghai University, Shanghai 200444, China b Instrumental Analysis and Research Center, Shanghai University, Shanghai 200444, China

Hydrothermal reactions of Cd(II)/Zn(II) nitrates with 1,4-benzenedicarboxylic acid (H2Bdc) and 2-(3-pyridyl)- benzimidazole (3-PyHBIm) afforded complexes {[Cd(3-PyHBIm)(Bdc)(H2O)2](H2Bdc)1/2}n (1) and [Zn(3-PyHBIm)2- (Bdc)(H2O)2]n (2). X-ray structural analysis reveals that both complexes are 1D coordination polymers. In complex 1, 3-PyHBIm coordinates to Cd(II) unidentately via a pyridyl N-donor. Bdc2－ anion acts as a tetradentate ligand and coordinates to Cd(II) to form a zigzag chain where two H2O coordinate in cis-fashion. H2Bdc guest molecule is involved in the hydrogen-bonding network, which leads to a 3D supramolecular architecture. In complex 2, Bdc2－ acts as a bidentate ligand and coordinates to Zn(II) to form a linear infinite chain where two H2O molecules coordi-nate in trans-fashion. The complexes are thermally stable and exhibit luminescence in the solid state.

Keywords coordination polymer, 1,4-benzenedicarboxylic acid, 2-(3-pyridyl)benzimidazole, crystal structure, thermal stability, luminescence

Introduction

The construction of coordination polymers and su-pramolecular architectures has become a quite active research field, due to the crystal engineering of func-tional materials involving potential applications to opti-cal, electronic, magnetic and porous materials.1,2 Much effort has been focused on the rational design and con-trolled synthesis of coordination polymers using mul-tidentate ligands. Polycarboxylate and N-heterocyclic ligands have received considerable attention, owing to the variety of their coordination modes and structural motifs as well as functional properties. Numerous coor-dination polymers have been constructed where inter-molecular hydrogen-bonding and π-π stacking interac-tions play important roles in the formation of su-pramolecular architectures.3 Recently, there is a grow-ing interest in the construction of coordination polymers based on N-heterocyclic ligands such as triazole, tetra-zole, and imidazole derivates. 2-(3-Pyridyl)benzimida- zole (Scheme 1) possesses three N-donors, which can act as a uni- or bidentate neutral ligand, or tridentate anionic ligand when deprotonated. 1,4-Benzenedicarb- oxylate (terephthalate) has bifunctional carboxyl groups and exhibits varied coordinate modes, which has been widely used in the assembly of coordination poly-mers.4,5 In our continuing interest in the construction of

coordination polymers and supramolecular systems, two 1,4-benzenedicarboxylate Cd(II)/Zn(II) coordination polymers with 2-(3-pyridyl)benzimidazole as co-ligand have been prepared. Usually, d10 metal complexes dis-play varied luminescence in the solid state.6 Herein, we report their synthesis, crystal structure, thermal stability and luminescent properties.

Scheme 1

Experimental

Reagents and instruments

3-PyHBIm was prepared according to the literature method.7 All other chemicals were of reagent grade and used without further purification. Elemental analyses (C, H, N) were carried out on a Vario EL III elemental ana-lyzer. FT-IR spectra were recorded on a Nicolet A370 spectrometer by KBr pellets in 4000—400 cm－1. TG- DSC analyses were completed on a Netzsch STA 449C thermal analyzer at a heating rate of 10 ℃•min－1 in air.

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Luminescent spectra of crystalline samples were re-corded on a Shimadzu RF-5301 spectrophotometer.

Synthesis of {[Cd(3-PyHBIm)(Bdc)(H2O)2](H2Bdc)1/2}n (1)

A mixture of Cd(NO3)2•4H2O (0.20 mmol), 3-Py- HBIm (0.10 mmol) and H2Bdc (0.10 mmol) in 8 mL of H2O was sealed into a 15 mL Teflon-lined stainless steel reactor. The mixture was allowed to remain at 160 ℃ for 72 h. Upon cooling to room temperature at a rate of 10 ℃•h－1, light yellow rod-like crystals were har-vested in 42% yield based on 3-PyHBIm. IR (KBr) ν: 3509 (m), 3451 (m), 3172 (w), 3138 (w), 1662 (s), 1551 (s), 1395 (s), 1256 (s), 817 (m), 713 (m) cm－1. Anal. calcd for C24H20CdN3O8: C 48.79, H 3.41, N 7.11; found C 49.67, H 3.40, N 6.49.

Synthesis of [Zn(3-PyHBIm)2(Bdc)(H2O)2]n (2)

A mixture of Zn(NO3)2•6H2O (0.10 mmol), 3-Py- HBIm (0.10 mmol) and H2Bdc (0.10 mmol) in 8 mL of H2O was sealed into a 15 mL Teflon-lined stainless steel reactor. The mixture was allowed to remain at 160 ℃ for 72 h. Upon cooling to room temperature at a rate of 10 ℃•h－1, yellow-green block crystals were har-vested in 50% yield based on 3-PyHBIm. IR (KBr) ν: 3423 (m), 3173 (m), 1595 (s), 1446 (m), 1374 (s), 817 (m), 752 (m), 697 (m), cm － 1. Anal. calcd for C32H26N6O6Zn: C 58.54, H 3.96, N 12.81; found C 57.90, H 4.14, N 12.72.

X-ray crystallography

Single crystal X-ray diffraction data for 1 and 2 were collected on a Bruker Smart Apex-II CCD diffractome-ter with graphite-monochromated MoKα radiation (λ＝0.071073 nm) at 296(2) K. Empirical absorption correc-tions were applied using the SADABS software. The structures were solved by the direct method and refined by a full-matrix least squares method on F2 with SHELXL-97 program.8 All non-hydrogen atoms were refined anisotropically, whereas hydrogen atoms were added geometrically and refined with a riding model. The crystallographic data and refinement results are summarized in Table 1. Selected bond lengths and an-gles are listed in Table 2.

Results and discussion

Synthesis and IR spectra

The hydrothermal reactions of Cd(II)/Zn(II) nitrates with 3-PyHBIm and H2Bdc afforded the complexes 1 and 2. Bdc2－ ligand and neutral H2Bdc guest molecule are involved in the structure of 1. However, the complex 2 contains only Bdc2－ anion but no H2Bdc. Although no base was added into the reacting system, excessive 3-PyHBIm may act as an organic base to neutralize the protons of 1,4-benzenedicarboxylic acid. The composi-tion of both complexes has been confirmed by elemental analysis, IR spectra and X-ray structural analysis.

Table 1 Crystal data and structure refinement for 1 and 2

Compound 1 2

Empirical formula C24H20CdN3O8 C32H24N6O6Zn

Formula weight 590.83 653.94

Crystal system Monoclinic Orthorhombic

Space group P21/c Pca21

a/nm 0.60824(4) 1.14930(13)

b/nm 2.03385(13) 1.6582(2)

c/nm 1.90247(13) 1.46145(17)

β/(°) 96.332(1) 90

V/nm3 2.3391(3) 2.7853(6)

Z 4 4

Dc/(Mg•m－3) 1.678 1.559

µ/mm－1 0.990 0.941

Crystal size 0.30 nm×0.20 nm×0.20 nm 0.20 nm×0.20 nm×0.14 nm

θ/(°) 2.00—25.05 2.16—25.05

F(000) 1188 1344

Parameters 342 394

Reflections collected 12095 13909

Unique reflections 4140 [Rin＝0.0275] 4778 [Rin＝0.0621]

GOF 1.037 0.991

R, wR2 [I＞2σ(I)] 0.0269, 0.0571 0.0679, 0.1643

R, wR2 (all data) 0.0355, 0.0608 0.0950, 0.1881

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Table 2 Selected bond lengths (nm) and angles (°) for 1 and 2

1a

Cd(1)—O(1w) 0.2252(2) Cd(1)—O(1) 0.2325(2)

Cd(1)—O(2w) 0.2312(2) Cd(1)—O(2) 0.2442(2)

Cd(1)—O(3)#1 0.2319(2) Cd(1)—O(4)#1 0.2616(2)

C(7)—O(1) 0.1248(3) Cd(1)—N(1) 0.2345(2)

C(7)—O(2) 0.1266(3) C(24)—O(5) 0.1311(3)

C(8)—O(3) 0.1265(3) C(24)—O(6) 0.1210(3)

C(8)—O(4) 0.1256(3)

O(1w)-Cd(1)-O(2w) 86.47(9) O(1)-Cd(1)-O(3)#1 136.34(7)

N(1)-Cd(1)-O(2w) 175.91(9) O(2)-Cd(1)-O(4)#1 131.49(6)

O(1)-Cd(1)-O(2) 54.87(7) O(3)#1-Cd(1)-O(4)#1 52.26(6)

2b

Zn(1)—N(4) 0.2092(6) Zn(1)—O(1w) 0.2175(6)

Zn(1)—O(4)#1 0.2100(5) Zn(1)—O(2w) 0.2257(7)

Zn(1)—O(1) 0.2116(5) Zn(1)—N(1) 0.2133(6)

O(1w)-Zn(1)-O(2w) 177.1(3) O(1)-Zn(1)-N(4) 91.4(2)

N(1)-Zn(1)-N(4) 175.8(3) O(1)-Zn(1)-O(1w) 90.4(3)

O(1)-Zn(1)-O(4)#1 178.3(4) N(4)-Zn(1)-O(1w) 86.8(2) a #1 －x, y＋1/2, －z＋3/2; #2 －x, －y＋1, －z＋1; #3 －x, y－1/2, －z＋3/2; b #1 x－1, y, z; #2 x＋1, y, z.

In the IR spectrum of 1, strong absorptions at 1551, 1395 cm－1 are assigned to carboxylate asymmetric and symmetric stretching vibrations, respectively. The fre-quency difference between νas(COO－) and νs(COO－) is 156 cm－1, which indicates that the carboxylates of Bdc2－ coordinate to Cd(II) in a bidentate chelating mode.9 Absorption at 1662 cm－1 is assigned to carboxyl vibra-tion of H2Bdc, which indicates that the complex 1 con-tains the neutral H2Bdc molecule. The complex 2 con-sists of 3-PyHBIm, Bdc2－ and H2O. Its IR spectrum is obviously different from the spectrum of 1. Absorption peaks at 1595, 1374 cm－1 are assigned to νas(COO－) and νs(COO－) vibrations, respectively. The larger fre-quency difference of 221 cm－1 indicates that the car-boxylates of Bdc2－ coordinate to Zn(II) in a unidentate mode. The absence of characteristic absorption of νas(COOH) indicates that the complex 2 has no H2Bdc guest molecule.

Description of crystal structure 1

X-ray structural analysis reveals that the complex 1 is a 3D supramolecular framework, which is comprised of 1D [Cd(3-PyHBIm)(Bdc)(H2O)2]n zigzag chains and H2Bdc guest molecules. The perspective view of an asymmetric unit and packing diagram are shown in Fig-ure 1. The Cd(II) center is seven-coordinated by two chelating carboxylates, two H2O ligands and a pyridyl N-donor of 3-PyHBIm to form a distorted pentagon- bipyramidal geometry. Bdc2－ anion acts as a tetraden-tate ligand and chelates to two Cd(II) ions through both carboxylate terminals, leading to the formation of a zigzag infinite chain. Cd(1)—O(1) bond length 0.2325(2) nm is similar to Cd(1)—O(2) 0.2442(2) nm,

whereas Cd(1)—O(4) 0.2616(2) nm is obviously longer than Cd(1)—O(3) 0.2319(2) nm. Two coordinated H2O molecules bind to Cd(II) in a cis-fashion with a bond angle of 86.47(9)°. 3-PyHBIm acts as a neutral uniden-tate ligand and coordinates to Cd(II) through a pyridyl N-donor. Cd(1)—N(1) is 0.2345(2) nm. Other bond lengths and angles are normal as expected.

As shown in the packing diagram of 1, a grid su-pramolecular framework can be observed along the a-axis. Interestingly, the framework contains uncoordi-nated 1,4-benzenedicarboxylic acid as guest molecule, which is incorporated into the square tunnel. The H2Bdc guest molecule links the coordinated water and car-boxylates through strong hydrogen bonds, leading to the formation of a 3D supramolecular architecture. So far, 1,4-benzenedicarboxylate has been frequently used as a uni-, bi- or multidentate ligand in metal complexes. However, H2Bdc guest molecules were rarely observed in coordination systems,10,11 although a number of H2Bdc-organic base co-crystals have been docu-mented.12, 13

Description of crystal structure 2

The complex 2 is a 1D linear chain coordination polymer. As shown in Figure 2, the Zn(II) center is six-coordinated by four oxygen and two nitrogen donors to form an octahedral geometry. 1,4-Benzenedicar- boxylic acid is completely deprotonated, and the car-boxylate groups coordinate to Zn(II) in a unidentate mode. Each Bdc2－ anion as a bidentate bridge links Zn(II) ions to form a 1D linear infinite chain. Zn(1)—O(1) bond length 0.2116(5) nm is similar to Zn(1)—O(4A) 0.2100(5) nm. The bond angle of O(1)-Zn(1)-O(4A)

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Figure 1 View of asymmetric unit and packing diagram of 1.

Figure 2 View of 1D linear chain and packing diagram of 2.

is 178.3(4)°. 3-PyHBIm is neutral and coordinates to Zn(II) in a unidentate mode through a pyridyl N-donor. Each Zn(II) ion is coordinated by two 3-PyHBIm ligands, which is obviously different with the structure of 1 where the Cd(II) ion is coordinated by one 3-PyHBIm. The average Zn—N bond length is 0.2113 nm. The N(1)-Zn(1)-N(4) bond angle is 175.8(3)°. Two coordinated H2O molecules bind to Zn(II) in a trans-fashion. The O(1W)-Zn(1)-O(2W) bond angle is 177.1(3)°. The average Zn—O(W) bond length is 0.2216 nm.

Along the 1D chain, the Zn…Zn separation is 1.1493 nm. As shown in packing diagram of 2, the polymeric chains are packed in a parallel fashion to form the crystal structure. The carboxylate groups are coplanar with the phenyl ring of Bdc2－. The pyridyl ring is almost coplanar with the benzimidazole with a dihe-dral angle of 4.03°. In the crystal structure, the aromatic plane of 3-PyHBIm is perpendicular to the phenyl ring of Bdc2－. There are some π-π stacking interactions be-tween adjacent 3-PyHBIm aromatic planes with per-pendicular distances from 3.657(5) to 3.768(5) Å. There are two types of intermolecular hydrogen bonds O(2W)

— H(2WB) … O(3)#1 (2.742(8) Å) and O(1W) — H(1WB)…N(2)#3 (2.992(8) Å), making the crystal structure more stable.

A number of 2-(n-pyridyl)benzimidazole metal co-ordination polymers have been reported, wherein n-Py- HBIm ligands are neutral or deprotonated.14,15 Neutral n-PyHBIm has two N-donors, which can act as a uni- or bidentate ligand. Deprotonated n-PyBIm anion has three N-donors, which can act as a tridentate ligand.16 In this work, 2-(3-pyridyl)benzimidazole is neutral and acts as a unidentate ligand in both complexes.

Thermal analysis

Thermal analyses of 1 and 2 were performed in the temperature range of 20—800 ℃ in air (Figure 3). The complex 1 was stable up to 160 ℃ and released two coordinated water molecules in the range of 160—240 ℃ (found 6.57%, calcd 6.10%). H2Bdc guest molecule was released in 250—320 ℃, whereas Bdc2－ ligand was decomposed in 340—360 ℃. This mixed process might lead to the intermediate [Cd(3-PyHBIm)(CO3)] (found 63.25%, calcd 62.22%).17 3-PyHBIm was re-leased in 400—620 ℃ (found 32.10%, calcd 33.04%).

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Figure 3 TG-DSC curves of 1 (left) and 2 (right).

The resulting CdCO3 was further decomposed to form 23.12% CdO residue at 800 ℃ (calcd 21.73%). The complex 2 lost coordinated water in 150—180 ℃ (found 2.02%, calcd 5.51%). 3-PyHBIm was rapidly released in 250 — 320 ℃ (found 54.06%, calcd 59.70%). Bdc2－ ligand was decomposed in 400—600 ℃. The final residue was ZnO (found 14.10%, calcd 12.45%).

Luminescence

Both complexes 1 and 2 display blue luminescence under photo-excitation (Figure 4). The complex 1 shows an emission band maximum at 441 nm with 379 nm light excitation, while the complex 2 exhibits 469 nm emission maximum with 383 nm light excitation. It was reported that H2Bdc acid and {[Zn(H2Bpz)(Bdc)]- (H2Bpz)1/2}n complex exhibited emissions at 382, 470 nm, respectively.18 However, the luminescence of 3-PyHBIm is unclear, although isomer 4-PyHBIm shows an emission band centered at 492 nm with 370 nm light excitation.19 We determined the emission bands of 3-PyHBIm under 370, 379 and 383 nm light excitations, which shows a similar emission with a band maximum at 574 nm. The energy of the luminescence strongly suggests that the emissions of 1 and 2 are Bdc2－ ligand-centered transitions rather than LMCT, espe-cially since Cd(II) and Zn(II) are not oxidizable or re-ducible ions.18

Conclusion

In summary, two 1D coordination polymers have been assembled by Cd(II)/Zn(II) ions, 1,4-benzenedicar- boxylic acid and 2-(3-pyridyl)benzimidazole where 3-PyHBIm acts as a neutral unidentate ligand. The Cd(II) complex contains 1,4-benzenedicarboxylic acid guest molecules in the supramolecular architecture. Both complexes are thermal stable and show blue lumines-cence.

Supplementary material

Crystallographic data for 1 and 2 have been deposited

Figure 4 Excitation (left) and emission (right) spectra of 1 and 2.

with the Cambridge Crystallographic Data Center, CCDC-670865 for complex 1 and CCDC-670868 for complex 2. These data can be obtained free of charge from the Cambridge Crystallographic Data Center, 12, Union Road, Cambridge CB21EZ, UK via E-mail: de-posit@ccdc.cam.ac.uk.

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