Coordination Polymers from Coligands: Effect of the Coexistent Groups

[Cd(bimb)0.5(O2N-bdc)(H2O)]n (2), and {[Cd(bimb)0.5(HO-bdc)]‚(H2O)}n (3), has been ... chain stretched by HO-bdc and bimb, and the intramolecular hy...
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CRYSTAL GROWTH & DESIGN

A New Family of Cadmium(II) Coordination Polymers from Coligands: Effect of the Coexistent Groups (R ) H, -NO2, -OH) on Crystal Structures and Properties

2005 VOL. 5, NO. 4 1651-1656

Xiaoju Li, Rong Cao,* Wenhua Bi, Yanqin Wang, Yuling Wang, Xing Li, and Zhengang Guo State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fujian, Fuzhou, 350002, China Received April 5, 2005

ABSTRACT: A new family of three-dimensional cadmium(II) coordination polymers, [Cd(bimb)0.5(bdc)(H2O)]n (1), [Cd(bimb)0.5(O2N-bdc)(H2O)]n (2), and {[Cd(bimb)0.5(HO-bdc)]‚(H2O)}n (3), has been solvothermally or hydrothermally prepared by the assembly of bimb, CdII with H2bdc, and its two derivatives, O2N-H2bdc and HO-H2bdc, respectively, [bimb ) 1,2-bis(imidazol-1′-yl)butane, H2bdc ) m-isophthalic acid, O2N-H2bdc ) 5-nitroisophthalic acid, HO-H2bdc ) 5-hydroxylisophthalic acid]. Both 1 and 2 have similar supramolecular assemblies with large channels from two-dimensional (2D) layers extended by hydrogen bonds; the 2D layers consist of arranged eight-membered, 16membered, and 40-membered rings with different sizes owing to the presence of nitryl groups in 2. Compound 3 is a porous framework with guest water molecules, which arises from a one-dimensional (1D) cadmium-carboxylate chain stretched by HO-bdc and bimb, and the intramolecular hydrogen bonds further stabilize the structure. As compared to H2bdc, the coexistence of electron-donating (-OH) and electron-withdrawing (-NO2) groups in dicarboxylate derivatives has a key effect on the different cavity sizes of the structures and their fluorescent properties. Scheme 1a

Introduction The observation of coordination polymers constructed from organic ligands and transition metal ions through a self-assembly route has undergone rapid development in recent years. Architectures with desirable structures and physical properties heavily depend on the organic ligands and metal centers.1-4 Accordingly, the configuration of ligands with certain functional groups and spacers is critical. The functional groups such as carboxylates,5 phosphates,6 and N-donor groups7 or their mixtures8 have been adequately considered and widely explored. In addition to coordination functional groups, coexistent electron-donating or electron-withdrawing substituents in the conjugated organic ligands have been proposed to result in unique frameworks with different sizes or channels, as can be represented an unusual variety of physical properties, such as the remarkable lower energy shift of intense luminescence from donor-acceptor alkynyl-bridged heterometallic or homometallic complexes.9,10 Metal-organic polymers from carboxylate ligands have been extensively studied because of the flexible coordination modes and sensitivity to pH values of carboxylate groups. However, in the transition metal-carboxylate system, the effect of coexistent noncoordinated electron-donating or electronwithdrawing groups on the self-assembly process is seldom investigated simultaneously, which is expected to provide a new method for the fabrication of structures with changeable sizes and diverse properties. On the basis of this point, we chose m-isophthalic acid and its two derivatives 5-nitroisophthalic acid and 5-hydroxylisophthalic acid (Scheme 1) as starting materials. The basic concept of our synthetic aim is to study the influence of coexistent groups (-NO2 and -OH) on the structures and properties of the compounds. In * Corresponding author. E-mail: [email protected]. Tel: +86591-83796710. Fax: +86-591-83714946.

a R ) H, m-isophthalic acid; R ) -NO , 5-nitroisophthalic acid; 2 R ) -OH, 5-hydroxylisophthalic acid.

Scheme 2.

1,2-Bis(imidazol-1′-yl)butane

comparison to extensively investigated compounds from m-isophthalic acid,11 only a few compounds from 5-nitroisophthalic acid12 and 5-hydroxylisophthalic acid13 have been successfully isolated. Generally, the nitryl group (-NO2) and hydroxyl group (-OH) in carboxylate ligands do not engage in coordination to transition metal ions. However, the hydroxyl group (-OH) can serve as a hydrogen bond donor to stabilize supramolecular assemblies,13 while the nitryl group (-NO2) not only can act as a hydrogen bond acceptor, but also it can take on some spatial effects in the formation of polymeric networks.12c On the other hand, compared to m-isophthalic acid, coexistent noncoordinated groups (-NO2 and -OH) as electron-withdrawing and electron-donating groups, respectively, have profound impacts on the electron density of whole ligands, and thereby different physical phenomena can be produced. By comparison with pyridine-containing ligands,7,14 imidazole-containing 1,2-bis(imidazol-1′-yl)butane (Scheme 2) is seldom used, even though it exhibits the special ability to formulate a large membered ring

10.1021/cg050126d CCC: $30.25 © 2005 American Chemical Society Published on Web 06/16/2005

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because of its flexibility and conformational freedom from a (-(CH2)4-) spacer, as manifested in a series of transition metal compounds reported by Ma et al.15 From its mixture with m-isophthalic acid, one copper compound with an unprecedented 86 topology was also prepared .16 However, exploring this long, flexible ligand with the precise properties and design of coordination polymers is still a challenge. In this paper, we report a new family of cadmium(II) coordination polymers: [Cd(bimb)0.5(bdc)(H2O)]n (1), [Cd(bimb)0.5(O2N-bdc)(H2O)]n (2), and {[Cd(bimb)0.5(HO-bdc)]‚(H2O)}n (3) built from the mixed ligands 1,2-bis(imidazol-1′-yl)butane (bimb) and m-isophthalic acid (H2bdc), 5-nitroisophthalic acid (O2N-H2bdc), and 5-hydroxylisophthalic acid (HO-H2bdc), respectively. The three compounds have been characterized by fluorescent spectroscopy, IR spectrum, elemental analysis, and thermogravimetric analysis, as well as by single-crystal X-ray diffraction analysis. Differences in their structures and physical behaviors result from the presence of coexistent noncoordinated groups in dicarboxylate ligands. Discussions of their subtleties are addressed below. Experimental Section Materials and General Methods. 1,2-Bis(imidazol-1′-yl)butane was synthesized by the literature method.16 All the other chemicals were commercially available and used as purchased. Thermogravimetric experiments were performed using a TGA/SDTA851 instrument (heating rate of 15 °C/min, air stream). IR spectra using KBr pellets were recorded on a Magna 750 FT-IR spectrophotometer. Elemental analyses of C, H, and N were determined using a Perkin-Elmer 240C elemental analyzer. Fluorescence spectroscopy was performed on an Edinburgh Analytical instrument FLS920. Synthesis of [Cd(bimb)0.5(bdc)(H2O)]n 1. Cd(NO3)2‚4H2O (0.20 mmol, 61.7 mg), bimb (0.20 mmol, 38.0 mg), and H2O (10 mL) were placed in a Teflon-lined stainless steel vessel, and then H2bdc (0.20 mmol, 33.2 mg) and NaOCH3 (0.40 mmol, 21.6 mg) in CH3OH (8 mL) were added. The mixture was sealed and heated to 160 °C for 72 h, and then the reaction system was cooled to room temperature during 48 h. Light yellow prism crystals of 1 were obtained (yield: 27.3 mg, 35% based on Cd). Elemental analysis (%): calcd. for C13H13CdN2O5 (389.65): C 40.07, H 3.36, N 7.19; found: C 39.95, H 3.47, N 7.08. IR (KBr, cm-1): 3443 (bs), 3132 (w), 1616 (vs), 1563 (s), 1400 (vs), 1345 (s), 1236 (m), 1088 (m), 937 (w), 750 (s), 625 (s), 516 (w). Synthesis of [Cd(bimb)0.5(O2N-bdc)(H2O)]n 2. A mixture of Cd(NO3)2‚4H2O (0.20 mmol, 61.7 mg), bimb (0.20 mmol, 38.0 mg), and O2N-H2bdc (0.20 mmol, 42.2 mg) in 18 mL of H2O was stirred for 20 min, and then the pH value was adjusted to 5 by NaOH (1 M). After the sample was stirred for another 20 min, the mixture was transferred to a Teflon-lined stainless steel vessel and heated to 110 °C for 72 h; then the reaction system was cooled to room temperature during 48 h. Colorless block crystals of 2 were obtained (yield: 35.6 mg, 41% based on Cd). Elemental analysis (%): calcd. for C13H12CdN3O7 (434.66): C 35.92, H 2.78, N 9.67; found: C 35.83, H 2.91, N 9.55. IR (KBr, cm-1): 3443 (bs), 1622 (s), 1568 (s), 1365 (s), 1240 (m), 1088 (s), 929 (w), 789 (m), 734 (s), 652 (m), 516 (w). Synthesis of {[Cd(bimb)0.5(HO-bdc)]‚(H2O)}n 3. The process was similar to 2 except that O2N-H2bdc was replaced by HO-H2bdc (0.20 mmol, 36.4 mg), and the pH value was adjusted to 4 by NaOH (1 M). Colorless prism crystals of 3 were obtained (yield: 23.5 mg, 29% based on Cd). Elemental analysis (%): calcd. for C13H13CdN2O6 (405.65): C 38.49, H 3.23, N 6.90; found: C 38.36, H 3.41, N 6.82. IR (KBr, cm-1): 3427 (bs), 3139 (bs), 1614 (s), 1575 (s), 1396 (vs), 1276 (m), 1088 (m), 995 (w), 777 (s), 723 (s), 653 (m), 614 (m). X-ray Crystallographic Studies. Measurements of 1-3 were conducted on a Rigaku Mercury CCD diffractometer with

Li et al. Table 1. Crystal Data and Structure Determination Summary for 1-3 formula fw cryst size (mm) cryst syst space group a/Å b/Å c/Å R/° β/° γ/° V/Å3 Z Dc/g cm-3 µ/mm-1 F(000) T/K λ(Mo KR)/ Å reflns collected unique reflns Rint parameters S on F2 R1 (I > 2σ(I))a wR2 (I > 2σ(I))b R1 (all data) wR2 (all data)b ∆Fmin and 3 max [e/Å ] a

1

2

3

C13H13CdN2O5 389.65 0.28 × 0.20 × 0.04 triclinic

C13H12CdN3O7 434.66 0.30 × 0.20 × 0.10 triclinic

C13H13CdN2O6 405.65 0.42 × 0.32 × 0.10 monoclinic

P1h

P1 h

C2/c

8.542 9.24300(10) 10.1091(2) 70.035(10) 67.654(12) 79.818(14) 692.835(16) 2 1.868 1.599 386 173(2) 0.71073

8.411(3) 10.050(3) 10.664(3) 91.273(2) 103.991(3) 113.770(3) 793.2(4) 2 1.820 1.418 430 173(2) 0.71073

15.187(4) 14.094(3) 13.069(3) 90 109.117(2) 90 2643.1(11) 8 2.039 1.686 1608 173(2) 0.71073

5365

6086

9901

3124

3554

3001

0.0214 194 0.840 0.0344

0.0326 225 0.676 0.0320

0.0150 207 0.847 0.0205

0.1027

0.0865

0.0897

0.0408

0.0355

0.0217

0.1121

0.0901

0.0921

1.032 and -0.611

0.787 and -0.661

1.180 and -0.560

R ) ∑||F0| - |Fc||)/∑|F0|. b wR ) [∑w(F02 - Fc2)2/∑w(F02)2]1/2.

graphite monochromated Mo KR radiation (λ ) 0.71073 Å) at low temperature (-100 °C).17 The structures were solved by direct methods with the SHELX-97 programs18 and refined on F2 by full-matrix least-squares using the SHELXL-97 program package.19 The positions of H atoms were generated geometrically (C-H bond fixed at 0.96 Å), assigned isotropic thermal parameters, and allowed to ride on their parent carbon atoms before the final cycle of refinement. Crystal data and structure determination summaries for 1-3 are listed in Table 1. The selected bond lengths and angles for the three compounds are listed in Table 2. CCDC number for compounds 1-3 are 267900, 267901, and 267902, respectively.

Results and Discussion Syntheses. According to our investigative aim, we selected m-isophthalic acid (R ) H) as a starting material with the aid of 1,2-bis(imidazol-1′-yl)butane to formulate a novel coordination polymer. As expected, 1 was successfully isolated under solvothermal conditions, which can be assumed as a comparable substance in this research system. However, under the same reaction conditions, the expected results for 2 and 3 were not obtained, and they were prepared at different pH values under hydrothermal conditions. We speculated that in addition to the influence of the coordinated carboxylates, the coexistent groups (-NO2, -OH) also have an important effect on the construction of the compounds in the self-assembly process, although the reactive mechanism, especially in hydrothermal or solvothermal conditions, was not clear. Description of Crystal Structures. [Cd(bimb)0.5(bdc)(H2O)]n 1. Single-crystal X-ray diffraction analysis

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Table 2. Selected Bond Lengths (Å) and Angles (°) for 1-3a Cd-N1 Cd-O4B Cd-O3A N1-Cd-O4B N1-Cd-O3A O4B-Cd-O3A N1-Cd-O1 O4B-Cd-O1 O3A-Cd-O1 N1-Cd-O2 O4B-Cd-O2

Compound 1 2.225(3) Cd-O1 2.299(3) Cd-O2 2.324(3) Cd-OW 106.66(12) O3A-Cd-O2 95.95(12) O1-Cd-O2 105.90(11) N1-Cd-OW 166.81(12) O4B-Cd-OW 83.84(10) O3A-Cd-OW 88.58(10) O1-Cd-OW 112.59(12) O2-Cd-OW 135.91(10)

2.367(3) 2.413(3) 2.416(3) 89.51(11) 54.92(10) 85.00(12) 87.14(11) 165.95(11) 87.64(11) 77.27(12)

Cd-N1 Cd-O1A Cd-O4B N1-Cd-O1A N1-Cd-O4B O1A-Cd-O4B N1-Cd-O2 O1A-Cd-O2 O4B-Cd-O2 N1-Cd-OW O1A-Cd-OW

Compound 2 2.222(2) Cd-O2 2.285(2) Cd-OW 2.334(2) Cd-O3B 124.28(9) O4B-Cd-OW 148.65(9) O2-Cd-OW 86.53(8) N1-Cd-O3B 88.48(8) O1A-Cd-O3B 100.39(7) O4B-Cd-O3B 91.56(8) O2-Cd-O3B 88.76(8) OW-Cd-O3B 84.14(8)

2.387(2) 2.388(2) 2.447(2) 88.91(8) 175.46(7) 93.91(9) 137.59(8) 55.05(7) 98.11(8) 78.48(8)

Cd-O4 Cd-N1 Cd-O1A O4-Cd-N1 O4-Cd-O1A N1-Cd-O1A O4-Cd-O2B N1-Cd-O2B O1A-Cd-O2B O4-Cd-O1C N1-Cd-O1C

Compound 3 2.1961(15) Cd-O2B 2.2043(17) Cd-O1C 2.3115(14) Cd-O2A 130.57(6) O1A-Cd-O1C 99.83(5) O2B-Cd-O1C 128.83(6) O4-Cd-O2A 91.07(5) N1-Cd-O2A 88.52(6) O1A-Cd-O2A 100.24(5) O2B-Cd-O2A 83.72(5) O1C-Cd-O2A 102.23(5)

2.4409(15) 2.4755(15) 2.5577(14) 71.29(6) 169.01(5) 140.90(5) 84.25(6) 53.51(5) 69.95(5) 108.45(5)

a Symmetry codes: for 1 A -x, -y, - z + 1; B x, y, z - 1. 2 A -x + 1, -y, -z + 1; B -x + 2, -y + 1, -z + 1. 3 A x - 1/2, y - 1/2, z; B -x + 1/2, -y + 1/2, -z + 2; C -x + 1/2, y - 1/2, -z + 3/2.

Figure 1. Perspective view of the asymmetric unit of 1 (30% probability ellipsoids). Symmetry codes: A -x, -y, -z + 1; B x, y, z - 1.

revealed that 1 crystallizes in the triclinic space group P1 h and has a three-dimensional (3D) supramolecular architecture with channels arising from two-dimensional (2D) layers extended by hydrogen bonds. The asymmetric unit of 1 is shown in Figure 1, which consists of one CdII, one bimb, three bdc, and one coordinated water molecule. The CdII center is sixcoordinated in a distorted octahedral environment. The equatorial plane is defined by one chelating carboxylate group, one syn-carboxylate oxygen atom, and one nitrogen atom from bimb. The CdII center is approximately coplanar with the mean plane of the four equatorial

Figure 2. The 2D layer comprises eight-membered (A), 16membered (B), and 40-membered (C) rings in 1.

Figure 3. View of a 3D supramolecular network with channels in 1.

atoms with a deviation of 0.1184 Å. One anti-carboxylate oxygen atom and one coordinated water molecule occupy the axial positions of the octahedron with a O3A-CdOW bond angle of 165.95(11)°. The Cd-OW bond length is 2.416(3) Å, and the Cd-N and Cd-O bond lengths are all within the normal ranges. The bdc ligand acts as a bridge through one chelating carboxylate group and one syn,anti-bridging carboxylate group linking CdII centers into a one-dimensional (1D) double chain motif with alternating eight-membered (A) and 16-membered (B) rings. This chain is a common arrangement that is found in other transition metal-bdc compounds.20 The bimb ligand as an exo-bidentate linkage extends the adjacent chain into a 2D layer, in which 40-membered rings (C), perpendicular to the above-mentioned eightmembered (A) and 16-membered (B) rings, are displayed (Figure 2). This may be due to the coordination performance of metal centers. Finally, the 2D layer is further extended into a 3D supramolecular architecture with large channels through hydrogen bonds involving the coordinated water molecules and carboxylate groups [OW-H‚‚‚O1i 2.725 Å; OW-H‚‚‚O4ii 2.822 Å; symmetry codes: (i) -x + 1, -y, -z; (ii) -x + 1, -y, -z + 1] (Figure 3). [Cd(bimb)0.5(O2N-bdc)(H2O)]n 2. As compared to m-isophthalic acid, 5-nitroisophthalic acid has a large nitryl group (R ) -NO2). Similar to 1, 2 is also a 3D supramolecular network. Interestingly, the presence of a nitryl group results in different cavity sizes in the structure and different fluorescent properties (described below) as compared to those of 1. In the 40-membered ring as seen in Figure 4, the metal-metal separations bridged by O2N-bdc and bimb are 10.179 and 13.124 Å, respectively, which are longer than those in 1 (10.109 and 12.250 Å). As speculated, the increase of the

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Li et al.

Figure 6. View of a 3D supramolecular network with channels in 2.

Figure 4. View of a 40-membered ring in 2 with nitryl groups of O2N-bdc inside. Symmetry codes: B -x + 1, -y, -z + 1; C -x + 2, -y + 1, -z + 1. Figure 7. View of the coordination environment of CdII center and 1D metal-carboxylate chain in 3. Symmetry codes: A x - 1/2, y - 1/2, z; B -x + 1/2, -y + 1/2, -z + 2; C -x + 1/2, y - 1/2, -z + 3/2.

Figure 5. View of a 2D layer in 2; the bonds represent the bimb ligands.

distances can be ascribed to the existence of nitryl groups (-NO2) in the ring. In other words, the nitryl group (-NO2) can provide some spatial effects on the construction of the architecture even though it is not involved in coordination with metal centers. The CdII center, similar to that in 1, is in a distorted octahedral environment, surrounded equatorially by four atoms from one chelating carboxylate group, one bime nitrogen atom, and one syn-carboxylate oxygen atom, and axially by two atoms from one anti-carboxylate oxygen atom and one water molecule. The Cd-OW bond length [2.388(2) Å] is much shorter than that in 1. In the same way, a 1D double chain is formed from CdII centers connected by the chelating and bridging carboxylate groups of O2N-bdc, and the exo-bidentate bime ligands extend the chain into a 2D layer including 8-, 16-, and 40-membered rings (Figure 5). The 2D layer is further extended into a 3D supramolecular network through hydrogen bonds between the coordinated water molecules and the carboxylate oxygen atoms [OW-H‚‚‚O1i 2.818 Å; OW-H‚‚‚O4ii 2.705 Å; symmetry codes: (i) x + 1, y, z; (ii) x, -y - 1, z] (Figure 6). {[Cd(bimb)0.5(HO-bdc)]‚(H2O)}n 3. During our research, 3 was obtained under hydrothermal conditions. Structure determination shows that 3 crystallizes in the monoclinic space group C2/c and has a 3D porous architecture with guest water molecules, which arise from 1D cadmium-carboxylate chains extended by bimb and HO-bdc. The 1D chain and the coordination

environment of the metal ion are shown in Figure 7. Similar to those in 1 and 2, the CdII center is in a distorted octahedral environment and is coordinated by one nitrogen atom from bimb and five oxygen atoms from different carboxylate groups of HO-bdc. The Cd-N bond length is 2.2043(17) Å, and the average bond length of Cd-O is 2.397 Å, larger than those in 1. Thus, CdII and its symmetrical atoms are bridged by chelating and bridging carboxylate groups of HO-bdc to generate the 1D cadmium-carboxylate chain. Another such chain was displayed in a robust porous framework [Cd(L)‚H2O]n (H2L ) 2,2′-bipyridyl-4,4′dicarboxylic acid).21 Through two organic connectors as in the following description, the chains are further extended into a 3D framework with resultant channels, where guest water molecules reside (Figure 8). First, the HO-bdc ligand uses one monodentate and one chelating, bridging carboxylate group with a 120° separation to stretch the 1D chain along different directions to give rise to a 3D architecture with rhombic channels, where the void volume is estimated by PLATON to be 18.2% of the total volume.22 The hydroxyl group (-OH) is not in coordination, but it is involved in intramolecular hydrogen bonds with guest water molecules (O5-H‚‚‚Ow 2.670 Å). Second, the bimb ligand serves as an exo-bidentate bridge to link chains across the rhombic channel; the metal-metal distance across the bimb ligand is 13.178 Å, which is longer than that in 1. The dihedral angle between two imidazolyl rings is 35.9°. On the other hand, the intramolecular hydrogen bonds between guest water molecules and the uncoordinated carboxylate oxygen atoms [OW-H‚‚‚O3i 2.917 Å; OW-H‚‚‚O3ii 2.833 Å; symmetry codes: (i) x - 1/2, -y + 1/2, z - 1/2; (ii) -x, y, -z - 3/2] also appear to stabilize the framework. It is noticeable that, as displayed from the following thermogravimetric analysis (TGA), the open framework is still retained after losing guest water molecules, so this compound may be a possible candidate for porous material.23 IR spectra and Thermogravimetric Analyses. The IR spectrum of 1 shows a strong and broad peak at 3443 cm-1, which indicates the presence of water molecules. The peaks around 1616, 1563, 1400, and

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Figure 8. Representation of a 3D porous framework with guest water molecules along the c-axis in 3; the bonds represent the bimb ligands.

1345 cm-1 can be attributed to the asymmetric and symmetric vibrations of the carboxylate groups. For 2, similar to that in 1, the strong and broad band at 3443 cm-1 indicates the presence of water molecules. The characteristic bands of carboxylate groups are shown at 1622 and 1568 cm-1 for antisymmetric stretching vibrations and 1365 cm-1 for symmetric stretching vibrations. For 3, the characteristic peak of the hydroxyl group (-OH) is displayed at 3139 cm-1 and the peaks around 3427 cm-1 are consistent with the presence of water molecules. The strong bands of the asymmetric and symmetric vibrations of the carboxylate groups are shown at 1614, 1575, and 1396 cm-1, respectively. The thermal stability of the three compounds was measured by TGA on polycrystalline samples in an air atmosphere. For 1, the TGA curve shows two significant weight losses: 5.12% between 90 and 142 °C, and 56.42% above 315 °C. The former can be attributed to the loss of one coordinated water molecule (H2O/Cd(bimb)0.5(bdc)(H2O) calculated: 4.62%). The latter suggests the chemical decomposition of 1. The residue weight (38.46%) is attributed to the mixture of CdO and CdCO3 (CdO/Cd(bimb)0.5(bdc)(H2O) calculated: 32.95%, CdCO3/Cd(bimb)0.5(bdc)(H2O) calculated: 58.30%). For 2, the weight loss is occurred when the sample is heated above 362 °C, suggesting the decomposition of the framework. For 3, TGA curve also displays two weight losses. The first weight loss of 4.62% from 45 to 112 °C is in accordance with the loss of one free water molecule (H2O/[Cd(bimb)0.5(HO-bdc)]‚(H2O), calculated: 4.44%), while the second weight loss of 58.12% from 379 to 477 °C corresponds to the decomposition of 3. Fluorescent Properties. The emission spectra of compounds 1-3 in the solid state at room temperature are shown in Figure 9. The fluorescent properties are also affected by the different coexistent groups in dicarboxylate ligands. It can be seen that the intense

Figure 9. Photoluminescence spectra and excitation spectra (inset) of compounds 1-3 at room temperature: (a) for 1, (b) for 2, and (c) for 3.

broad photoluminescence emission at 419 nm (λex ) 314 nm) for 1, 527 nm (λex ) 448) for 2, and 450 nm (λex ) 400 nm) for 3, respectively, are exhibited. As previously reported,14b,24,25 the emission of 1 and 3 may be due to σ-donation from the coordination environments of the CdII centers and may be assigned as ligand-to-metal

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charge-transfer (LMCT). In comparison to 1, a red shift is exhibited for 3. The reason for this phenomenon may be due to the increase of electron density and the lower energy state of the ligand inflected by the electrondonating hydroxyl group. However, it is noted that there is also a remarkable lower energy emission shift for 2 relative to 1. We speculated that it can be originated from the obvious decrease of the electron density of the ligand affected by electron-withdrawing nitryl group and is in line with the energy of π* orbital. It is likely that the emission band of 2 is assigned to the metal-to-ligand charge-transfer (MLCT). This low energy shift phenomenon influenced by the coexistent groups has also been reported in other metallorganic systems.9,10 Conclusions Three cadmium(II) coordination polymers from 1,2bis(imidazol-1′-yl)butane with m-isophthalic acid, 5-nitroisophthalic acid, and 5-hydroxylisophthalic acid, respectively, have been successfully synthesized and characterized. This research reveals that (i) 1,2-bis(imidazol-1′-yl)butane as a long flexible spacer is a good candidate to build compounds containing large membered rings or channels; (ii) the coexistent groups of organic ligands have profound effects on the construction of coordination polymers with different structures and properties. Thus, considering coexistent noncoordinated groups besides coordinated functional groups may be an original and feasible way for the formation of various architectures with different physical properties. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Nos. 90206040, 20325106, 20333070), Natural Science Foundation of Fujian Province, and “One Hundred Talent Project” from CAS. Supporting Information Available: Crystallographic information files (CIF) of three compounds are available free of charge via the Internet at http://pubs.acs.org.

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