Syntheses, Structures, and Photoluminescence of Three Coordination

Department of Technology, Guangdong Police Officers College, Guangzhou 510232, China, and Key. Laboratory of Bioinorganic and Synthetic Chemistry of ...
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CRYSTAL GROWTH & DESIGN

Syntheses, Structures, and Photoluminescence of Three Coordination Polymers of Cadmium Dicarboxylates Ling-Yun

Zhang,†,‡

Jie-Peng

Zhang,‡

Yan-Yong

Lin,‡

and Xiao-Ming

Chen*,‡

Department of Technology, Guangdong Police Officers College, Guangzhou 510232, China, and Key Laboratory of Bioinorganic and Synthetic Chemistry of MOE, School of Chemistry and Chemical Engineering, Sun Yat-Sen UniVersity, Guangzhou 510275, China

2006 VOL. 6, NO. 7 1684-1689

ReceiVed April 5, 2006; ReVised Manuscript ReceiVed May 17, 2006

ABSTRACT: Solvothermal reactions of 2,2′-dihydroxy-[1,1′]-binaphthalene-3,3′-dicarboxylate (H4bna), 4,4′-bipyridine (bpy), and Cd2+ salts at different conditions yielded three new one- to three-dimensional coordination polymers, ∞1[Cd(H2bna)(bpy)1.5(H2O)2] (1), ∞3[Cd6(MeCO2)4(bpy)2(H2bna)4]‚3.5H2O (2), and ∞2[Cd6(bna)(H2bna)(bpy)(C2H5OH)] (3). Crystal structural analyses revealed that 1 is an unusual bpy-bridged ladder structure with carboxylate-end coordinated H2bna ligands as lateral arms, 2 is a threedimensional R-Po network structure comprised of new hexanuclear [Cd6(µ-MeCO2)2(µ3-MeCO2)2]8+ clusters as the nodes and multiple H2bna and bpy as the linkers, whereas 3 exhibits a layer structure featuring bna-, H2bna-, and bpy-bridged infinite Cd-O ribbons. These compounds also displayed structure-related photoluminescence properties in the solid state. Introduction Recently much interest has been focused on the design and synthesis of coordination polymers not only for their interesting molecular topologies but also for the fact that they may be designed with specific functionalities.1-6 For effective control of the coordination modes of individual building blocks during the self-assembly process, chemists also use metal clusters as the molecular building blocks.6 Compared with individual metal ions, cluster-based building blocks provide not only much more predicable coordination modes and robustness to the coordination networks but also novel properties such as magnetism and photoluminescence. Meanwhile, the rational design of coordination polymers also depends on the judicious selection of the organic ligands. In this context, dicarboxylic acid,7-10 4,4′bipyridine (bpy), and their analogues have been extensively utilized to bridge the metal centers, leading to interesting metalorganic framework (MOF) structures and properties.11-13 In addition to the functionalities of the metal ions and the organic ligands, the reaction conditions are another important factor affecting the structures of the final self-assembly products. For example, we have illustrated that reaction temperature can play an important role in controlling the final structure of copper(I,II) imidazolate frameworks under solvothermal reactions.13 2,2′-Dihydroxy-[1,1′]-binaphthalene-3,3′-dicarboxylic acid (H4bna) is a multifunctional ligand containing both carboxylic and phenoxy groups, which can potentially afford various coordination modes and diverse MOF architectures. Meanwhile, it also possesses both rigidity and flexibility, since the naphthyl rings can be severely twisted at different degrees across the C-C single bond due to steric effect. Free H4bna displays very weak luminescence in the solid state, while its d10 metal complexes display strong photoluminescent properties in the blue/green region. We have also found that incorporation of d10 metal clusters is useful to enhance the luminescence intensity and lifetime of these complexes.14 As an extension of our previous investigations, bpy is now introduced into this cadmium carboxylate system providing the opportunity for the controlled * Corresponding author. E-mail: [email protected]. Fax: +86 20 8411-2245. † Guangdong Police Officers College. ‡ Sun Yat-Sen University.

syntheses of three new MOFs, ∞1[Cd(H2bna)(bpy)1.5(H2O)2] (1), 3 2 ∞ [Cd6(MeCO2)4(bpy)2(H2bna)4]‚3.5H2O (2), and ∞ [Cd3(bna)(H2bna)(bpy)(C2H5OH)] (3). Experimental Section Materials and Physical Measurements. H4bna is synthesized according to methods in the literature.15 All other chemicals were purchased and used as received. Elemental analyses (carbon, hydrogen, and nitrogen) were performed with a Perkin-Elmer 240 elemental analyzer. IR spectra were measured from KBr pellets on a Nicolet 5DX FT-IR spectrometer. The emission spectra were recorded on an Edinburgh FLS920 spectrometer equipped with a µF900 microsecond flash lamp. In all cases, single crystalline samples were used for the photoluminescence measurements, and the lifetime data were fitted with single or double exponential decay functions. Synthesis of [Cd(H2bna)(bpy)1.5(H2O)2] (1). A mixture of Cd(MeCO2)2‚2H2O (0.266 g, 1 mmol), H4bna (0.094 g, 0.25 mmol), bpy (0.040 g, 0.25 mmol), and NaOH (0.020 g, 0.50 mmol) in waterethanol (1:4) (10 mL) was heated for 3 days at 130 °C in a Parr Teflonlined stainless steel vessel (23 mL), cooled to 100 °C at a rate of 5 °C h-1, and held for 10 h, followed by further cooling to room temperature. Pale-yellow crystals of 1 were collected, washed with water, and dried in air (yield 50%). IR data (KBr, cm-1): 3199m, 3056m, 1942w, 1818w, 1637s, 1608m, 1580m, 1544s, 1456s, 1393s, 1336s, 1304m, 1239s, 1096w, 1071m, 1006m, 935m, 910w, 883m, 810s, 744s, 630m, 599m, 440w. Anal. Calcd (%) for C37H28CdN3O8: C, 58.86; H, 3.74; N, 5.57. Found: C, 58.32; H, 3.82; N, 5.20. Synthesis of [Cd6(MeCO2)4(bpy)2(H2bna)4]‚3.5H2O (2). A mixture of Cd(MeCO2)2‚2H2O (0.266 g, 1 mmol), H4bna (0.094 g, 0.25 mmol), bpy (0.040 g, 0.25 mmol), and NaOH (0.020 g, 0.50 mmol) in waterethanol (1:4) (10 mL) was heated for 3 days at 160 °C in a Parr Teflonlined stainless steel vessel (23 mL), cooled to 100 °C at a rate of 5 °C h-1, and held for 10 h, followed by further cooling to room temperature. Colorless crystals of 2 were collected, washed with water, and dried in air (yield 60%). IR data (KBr, cm-1): 3407m, 3050m, 2963m, 2927m, 1947w, 1821w, 1639s, 1607m, 1548s, 1508m, 1460s, 1394s, 1338s, 1307s, 1249s, 1220m, 1151m, 1096m, 1044w, 1007m, 940m, 914w, 884m, 810s, 743s, 679m, 627m, 599m, 531w, 474w, 438w. Anal. Calcd (%) for C136H97Cd6N8O34.5: C, 53.21; H, 3.18; N, 3.65. Found: C, 53.01; H, 3.44; N, 3.58. Synthesis of [Cd3(bna)(H2bna)(bpy)(C2H5OH)] (3). A mixture of Cd(NO3)2‚4H2O (0.308 g, 1 mmol), H4bna (0.094 g, 0.25 mmol), bpy (0.040 g, 0.25 mmol), and NaOH (0.020 g, 0.50 mmol) in waterethanol (1:4) (10 mL) was heated for 3 days at 160 °C in a Parr Teflonlined stainless steel vessel (23 mL), cooled to 100 °C at a rate of 5 °C h-1, and held for 10 h, followed by further cooling to room temperature. Colorless crystals of 3 were collected, washed with water, and dried in

10.1021/cg060194f CCC: $33.50 © 2006 American Chemical Society Published on Web 06/17/2006

Coordination Polymers of Cadmium Dicarboxylates

Crystal Growth & Design, Vol. 6, No. 7, 2006 1685

Table 1. Crystallographic Data of 1, 2, and 3 complex

1

2

3

molecular formula formula weight temp, K crystal color and form crystal system space group a, Å b, Å c, Å R, deg β, deg γ, deg vol, Å3 Z Dcalcd, Mg m-3 µ(Mo KR), mm-1 F(000) crystal size, mm3 θmax, deg reflns collected independent reflns no. of params GOF on F2 final R indices [I g 2σ(I)]a,b

C37H28CdN3O8 755.06 293(2) pale-yellow blocks monoclinic P21/c (No. 14) 11.7434(7) 9.4784(5) 27.812(2) 90 97.003(1) 90 3072.7(3) 4 1.632 0.774 1532 0.04 × 0.13 × 0.23 27.5 25551 6986 442 1.005 R1 ) 0.0416 wR2 ) 0.0874 R1 ) 0.0566 wR2 ) 0.0928 0.939 and -0.623

C136H97Cd6N8 O34.5 3069.62 293(2) colorless blocks monoclinic C2/c (No. 15) 31.100(2) 24.046(1) 23.006(1) 90 131.420(1) 90 12901(1) 8 1.580 1.054 6132 0.03 × 0.30 × 0.50 27.5 53054 14613 852 1.044 R1 ) 0.0371 wR2 ) 0.16 R1 ) 0.0479 wR2 ) 0.1110 1.396 and -0.494

C56H36Cd3N2O13 1282.07 293(2) colorless blocks triclinic P1h (No. 2) 12.8165(8) 14.9807(9) 14.9864(9) 118.800(1) 101.904(1) 95.427(1) 2403.3(3) 2 1.772 1.387 1268 0.07 × 0.08 × 0.18 27.5 20737 10603 667 1.002 R1 ) 0.0366 wR2 ) 0.0720 R1 ) 0.0519 wR2 ) 0.0763 0.866 and -0.483

R indices (all data)a,b largest peak and hole, e‚Å-3 a

R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) [∑w(|Fo| - |Fc|)2/∑w|Fo|2]1/2; w ) [σ2(Fo)2 + (0.1(max(0,Fo2) + 2Fc2)/3)2]-1.

Figure 1. Perspective view of the coordination environment in 1 (symmetry codes, A ) 1+ x, y, z; B ) 1 - x, 1 - y, -z). air (yield 65%). IR data (KBr, cm-1): 3429m, 3050m, 2919m, 1947w, 1815w, 1725w, 1637s, 1605s, 1550s, 1503s, 1460s, 1415m, 1392s, 1415m, 1392s, 1335s, 1303m, 1241m, 1219m, 1146w, 1096w, 1069m, 1003s, 933m, 881m, 858w, 812s, 742m, 625m, 594m, 568w, 501w, 433w. Anal. Calcd (%) for C56H36Cd3N2O13: C, 52.46; H, 2.83; N, 2.18. Found: C, 52.30; H, 3.01; N, 2.45. X-ray Crystallography. Diffraction intensities for 1, 2, and 3 were collected at 293 K on a Bruker Smart Apex CCD area-detector diffractometer (Mo KR, λ ) 0.710 73 Å). Lorentz polarization and absorption correction were applied using SADABS.16,17 The structures were solved with direct methods and refined with full-matrix leastsquares technique using the SHELXTL program.18 Anisotropic thermal parameters were applied to all non-hydrogen atoms. The organic hydrogen atoms were generated geometrically (C-H, 0.96 Å); the aqua hydrogen atoms were located from difference maps and refined with isotropic temperature factors. Analytical expressions of neutral-atom scattering factors were employed, and anomalous dispersion corrections were incorporated.19 Crystal data as well as details of data collection and refinement for the complexes are summarized in Table 1. Selected bond lengths and angles for complexes 2 and 3 are provided in Table 2.

Results and Discussion Crystal Structures. As illustrated in Figure 1, the Cd(II) atom in 1 is coordinated by three nitrogen atoms from different bpy

Figure 2. The 1D ladder with the H2bna ligands decorated on both sides in 1.

ligands [Cd-N 2.338(2)-2.415(3) Å], two oxygen atoms from one chelating carboxylate end of H2bna [Cd-O 2.450(2) and 2.620(2) Å] with a dihedral angle of 82.7° between its two naphthyl rings, and two oxygen atoms from two aqua ligands [Cd-O 2.331(2) and 2.340(2) Å] to furnish a pentagonalbipyramidal geometry. The extended structure of 1 features ladders with Cd(II) ions as the nodes, bpy ligands as the inner rungs, and H2bna as lateral arms (Figure 2). To the best of our knowledge, molecular ladders having different inner rungs and lateral arms have seldom reported.11c Although both carboxylate groups of each H4bna ligand are deprotonated in 1, only one of them is coordinated to the Cd(II) ion in a chelating bidentate

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Zhang et al.

Table 2. Selected Bond Lengths [Å] and Angles [deg] for 2 and 3a Cd1-N2A Cd1-O16 Cd2-O1C Cd3-N1 Cd3-O6 N3-Cd1-O16B O16B-Cd1-O15 O15-Cd1-O16 O15-Cd1-O14 O11-Cd2-O1C O14-Cd2-N4D O1C-Cd2-O2C O1C-Cd2-C1C N1-Cd3-O15 N1-Cd3-O7 O15-Cd3-O8 O15-Cd3-O6

2.308(3) 2.437(3) 2.342(2) 2.307(3) 2.550(3) 93.7(1) 122.26(8) 53.27(8) 93.59(8) 138.92(9) 162.4(1) 55.15(8) 27.43(9) 167.45(9) 99.4(1) 92.88(9) 80.01(8)

Cd1-N3 Cd1-O14 Cd2-N4D Cd3-O15 N2A-Cd1-N3 O13-Cd1-O16B N2A-Cd1-O16 N2A-Cd1-O14 O16-Cd1-O14 O6-Cd2-O1C O1C-Cd2-N4D N4D-Cd2-O2C N4D-Cd2-C1C O10-Cd3-O5 O15-Cd3-O7 O5-Cd3-O8 O5-Cd3-O6

2.321(3) 2.500(2) 2.352(3) 2.313(2) 175.3(1) 91.59(9) 83.0(1) 4.6(1) 142.60(8) 95.52(9) 84.84(9) 96.2(1) 89.7(1) 130.79(9) 86.23(9) 90.02(9) 53.12(8)

Cd1-O5A Cd1-N2B Cd2-O2C Cd3-O4D O5A-Cd1-O5 O6-Cd1-N2B O5-Cd1-O9B O1C-Cd2-O13 O8-Cd2-N1 O13-Cd2-O2C O6-Cd3-O4D O10B-Cd3-O3

2.227(2) 2.301(3) 2.542(2) 2.314(2) 78.00(8) 99.52(9) 163.38(9) 90.9(1) 76.2(1) 74.63(9) 167.14(8) 154.4(1)

Cd1-O2A Cd2-O1C Cd2-N1 Cd3-O3 O2A-Cd1-O5 O5-Cd1-N2B N2B-Cd1-O9B O4D-Cd2-O13 O13-Cd2-N1 N1-Cd2-O2C O7-Cd3-O4D O4D-Cd3-O3

2.257(2) 2.204(2) 2.323(3) 2.319(2) 108.99(8) 89.83(9) 80.85(9) 86.32(9) 169.8(1) 115.42(9) 90.1(1) 88.26(8)

Complex 2 Cd1-O13 Cd2-O11 Cd2-O2C Cd3-O5 N2A-Cd1-O13 N2A-Cd1-O15 N3-Cd1-O16 N3-Cd1-O14 O11-Cd2-O6 O14-Cd2-O1C O11-Cd2-O2C O11-Cd2-C1C O2C-Cd2-C1C N1-Cd3-O5 O5-Cd3-O7 O7-Cd3-O8 O7-Cd3-O6

2.328(3) 2.262(2) 2.403(2) 2.320(2) 90.4(1) 96.6(1) 99.2(1) 96.0(1) 124.97(9) 108.7(1) 84.96(8) 112.05(9) 27.74(9) 86.1(1) 143.16(9) 55.50(9) 157.01(8)

Cd1-O16B Cd2-O6 Cd2-C1C Cd3-O7 N3-Cd1-O13 N3-Cd1-O15 O13-Cd1-O16 O13-Cd1-O14 O11-Cd2-O14 O11-Cd2-N4D O6-Cd2-O2C O6-Cd2-C1C O10-Cd3-N1 O15-Cd3-O5 O10-Cd3-O8 O10-Cd3-O6 O8-Cd3-O6

2.368(2) 2.287(2) 2.723(3) 2.329(3) 86.2(1) 88.07(9) 160.63(8) 53.85(8) 86.1(1) 90.9(1) 150.01(9) 122.8(1) 89.2(1) 82.72(9) 137.51(9) 79.25(8) 142.94(8)

Cd1-O15 Cd2-O14 Cd3-O10 Cd3-O8 N2A-Cd1-O16B O13-Cd1-O15 O16B-Cd1-O16 O16B-Cd1-O14 O6-Cd2-O14 O6-Cd2-N4D O14-Cd2-O2C O14-Cd2-C1C O10-Cd3-O15 O10-Cd3-O7 N1-Cd3-O8 N1-Cd3-O6

2.405(2) 2.306(2) 2.284(2) 2.404(2) 83.1(1) 145.98(8) 69.6(1) 143.15(9) 81.55(9) 86.03(9) 100.79(9) 107.5(1) 102.41(9) 85.90(9) 81.3(1) 97.9(1)

Complex 3 Cd1-O6 Cd2-O4D Cd3-O10B O5A-Cd1-O2A O6-Cd1-O5 O5A-Cd1-O9B O1C-Cd2-O4D O8-Cd2-O13 O1C-Cd2-O2C O6-Cd3-O7 O10B-Cd3-O4D

2.260(2) 2.253(2) 2.264(3) 79.34(8) 88.76(7) 114.95(9) 153.29(8) 94.9(1) 54.30(8) 101.4(1) 81.68(9)

Cd1-O5 Cd2-O8 Cd3-O6 O5A-Cd1-O6 O5A-Cd1-N2B O2A-Cd1-O9B O1C-Cd2-O8 O1C-Cd2-N1 O4D-Cd2-O2C O6-Cd3-O10B O6-Cd3-O3

2.296(2) 2.267(3) 2.202(2) 165.18(7) 87.21(9) 84.52(9) 107.8(1) 96.7(1) 99.56(8) 103.46(9) 82.17(8)

Cd1-O9B Cd2-O13 Cd3-O7 O2A-Cd1-O6 O2A-Cd1-N2B O6-Cd1-O9B O4D-Cd2-O8 O4D-Cd2-N1 O8-Cd2-O2C O7-Cd3-O10B O7-Cd3-O3

2.513(3) 2.315(3) 2.203(3) 99.05(8) 153.7(1) 79.33(9) 98.9(1) 89.9(1) 158.1(1) 92.3(1) 111.30(9)

a Symmetry operations. For 2: A ) -x + 1/ , y + 1/ , -z + 5/ ; B ) -x, -y + 1; -z + 2; C ) x, -y + 1, z - 1/ ; D ) x + 1/ , -y + 1/ , z + 1/ . For 2 2 2 2 2 2 2 3: A ) -x + 2, -y, -z; B ) -x + 1, -y - 1, -z - 1; C ) x - 1, y, z; D ) -x + 1, -y, -z.

Scheme 1.

Coordination Modes of the H2bna/bna Liganda

a (a) Chelating bidentate; (b) chelating/bridging and chelating bidentate; (c) bridging bis-bidentate; (d) chelating bis-bidentate; (e) bridging hexadentate.

mode (Scheme 1a), while the other forms interladder hydrogen bonds in the three-dimensional (3D) packing structure (Figure 3). There are four kinds of hydrogen bonds in 1: O-H‚‚‚O interactions between the aqua and carboxylate oxygen atoms [2.661(3)-2.710(3) Å], O-H‚‚‚N interactions between the aqua oxygen atoms and bpy nitrogen atoms [2.692(3)-3.292(3) Å], weak C-H‚‚‚O interactions between the aromatic bpy rings and the carboxylate or hydroxy oxygen atoms [3.202(4)-3.275(4) Å], and weak C-H‚‚‚π interactions between the H2bna carboxylate atoms and aromatic bpy rings [3.38-3.45 Å]. As illustrated in Figure 4, the Cd(II) atoms in 2 were bridged by acetate ligands to form an unusual hexanuclear cluster [Cd6(µ-MeCO2)2(µ3-MeCO2)2]8+, which is different from the previ-

ously observed mixed carboxylate/phenoxo-bridged tetranuclear and mixed carboxylate/hydroxy-bridged hexanuclear clusters containing H2bna ligands,14a,b and is unprecedented to the best of our knowledge. One of the coordination modes (Scheme 2a) of the two crystallographically independent acetate ligands in 2 is less found in the literature.20 Moreover, three crystallographically independent H2bna ligands are identified (Figure 5a), one acts in a chelating/bridging and chelating bidentate fashion (Scheme 1b) with a dihedral angle of 78.2° between its two naphthyl rings, the other acts in a bridging bis-bidentate fashion (Scheme 1c) with a dihedral angle of 86.3° between its two naphthyl rings, while the third one functions in a chelating bis-bidentate fashion (Scheme 1d) with a dihedral angle of 67.0° between its two naphthyl rings. Adjacent hexanuclear Cd(II) clusters are first quadruply interlinked by H2bna ligands into a ribbon running along the c-axis. These cadmium carboxylate ribbons are further interconnected by bpy ligands via Cd-N coordination bonds to form a 3D R-Po network, in which the linkers of the original six-connected R-Po net were replaced by totally 16 bridging ligands including two sets of four H2bna ligands and four sets of two bpy ligands (Figure 5). As illustrated in Figure 6, three crystallographically independent Cd(II) atoms exhibit two different coordination geometries in 3. Cd1 adopts a distorted octahedral arrangement, being ligated by three phenoxy oxygen atoms from three different H2bna ligands [Cd1-O 2.227(2)-2.296(2) Å], two oxygen atoms from two carboxylate ends [Cd1-O 2.257(2)-2.513(3) Å], and one bpy nitrogen atom [Cd1-N 2.301(3) Å]; Cd2 is also coordinated by four oxygen atoms from four different dicarboxylate groups [Cd2-O 2.204(2)-2.254(2) Å], one ethanol oxygen atom [Cd2-O 2.315(3) Å], and one bpy nitrogen atom [Cd2-N 2.323(3) Å] to furnish a highly distorted octahedral geometry; Cd3 is coordinated by four oxygen atoms from four carboxylate [Cd3-O 2.203(3)-2.319(2) Å] and one oxygen atom from a hydroxyl group [Cd3-O 2.202(2) Å] to furnish a tetragonal-pyramidal geometry. There is a bridging bis-bidentate H2bna ligand (Scheme 1c) with the dihedral angle of 88.2°

Coordination Polymers of Cadmium Dicarboxylates

Crystal Growth & Design, Vol. 6, No. 7, 2006 1687

Figure 3. Perspective view of the molecular packing in 1.

Figure 4. Perspective view of the [Cd6(MeCO2)4]8+ in 2 (only the bpy nitrogen atoms and H2bna carboxylate groups are shown; symmetry code A ) -x + 1/2, y + 1/2, -z + 5/2).

Scheme 2.

Coordination Modes of the Acetate Ligands in 2

between its two naphthyl rings and a fully deprotonated bna ligand (Scheme 1e) in 3, which exhibits an unusual µ6hexadentate coordination mode with a dihedral angle of 80.2° between its two naphthyl rings to link the Cd(II) atoms into CdO chains. These CdO chains are further extended into layers by the H2bna ligands and bpy ligands (Figure 7). Synthesis. We have investigated the solvothermal reactions of H4bna, bpy, and Cd(II) salts under different conditions including variations of the Cd(II) salts, reaction temperatures, and solvents. A reaction of Cd(MeCO2)2 with H4bna and bpy ligands in aqueous ethanol at 130 °C led to the formation of a ladder-like MOF 1. With the same starting materials, the clusterbased 3D MOF 2 was obtained at a higher temperature of 160 °C. When the starting reagent Cd(MeCO2)2 was replaced by Cd(NO3)2, the 2D MOF 3 with infinite CdO ribbons was synthesized at the same conditions as those for 2. According to the literature,21 higher reaction temperature usually leads to an increase of condensation and density of the MOF. The local coordination environments of 1 and 2 do show that a higher reaction temperature causes condensation between

Figure 5. Perspective views showing the hexanuclear Cd(II) clusters connected by H2bna ligand into ribbons (a) along the c-direction and (b) a 3D R-Po-like network.

the Cd(II) ions and bridging ligands and that the coordination number of the H2bna ligands of 2 is significantly larger than that of 1. However, due to the presence of lattice water in 2 and the close packing of the chains in 1, the density of 2 (1.580 Mg/m3) is smaller than that of 1 (1.632 Mg/m3). The presence of a less bulky bridging ligand, such as acetate, seems to favor the formation of Cd(II) clusters in this system. When the acetate was replaced by a weaker ligand, nitrate, in 3 (1.772 Mg/m3), the Cd(II) ions are directly linked into the CdO chains. Luminescent Properties. It should be noted that free H4bna ligand displays a very weak luminescence in the solid state at ambient temperature,11c while its complexes 1, 2, and 3 all exhibit intense blue photoluminescence upon excitation at 355

1688 Crystal Growth & Design, Vol. 6, No. 7, 2006

Figure 6. Perspective view of the coordination environments of the Cd(II) atoms in 3 (symmetry codes, A ) -x + 1/2, y + 1/2, -z + 5/2; B ) -x, -y + 1, -z + 2; C ) x - 1, y, z; D ) -x + 1, -y, -z).

Figure 7. Perspective view of the 2D network viewed along the a-axis direction in 3 (bna ligands in red, bpy ligands in blue, H2bna ligands in green, and Cd atoms in purple).

nm, as depicted in Figure 8. There are two intense emission maxima at ca. 439 and 469 nm for 1. The fluorescence spectrum of 2 is not structureless with an emission maximum at ca. 470 nm, which is consistent with that of the [Cd6(OH)4] unit14b and a weak shoulder peak at ca. 547 nm. Complex 3 also exhibits blue photoluminescence very similar to that of 2, having an emission maximum at ca. 470 nm and a shoulder at ca. 540 nm. Similar to that found in a silver(I) complex of H4bna,14c the enhancement of luminescence in d10 complexes may be attributed to the ligation of the ligand to the metal center. The coordination enhances the “rigidity” of the ligand and thus reduces the loss of energy through a radiationless pathway. Two transition types of electronic excited states, namely, ligand-to-ligand change transfer (LLCT) and ligand-to-metal change transfer (LMCT), may be possible for such Cd(II) coordination complexes.14 However, it is hard to propose a correct mechanistic conclusion for their luminescence based only on emission spectra, although the excited-state lifetime (τ) measurements may be helpful in verifying the photoluminescence mechanisms. The lifetime of 1 is ca. 0.81 ns, which is similar to free H4bna ligand (τ ) 0.69 ns) and slightly different

Zhang et al.

Figure 8. The emission spectra of 1 (s), 2 (‚‚‚), and 3 (- - -) in the solid state at room temperature upon excitation at 355 nm.

from a related Cd(II) coordination polymer of H4bna (λmax ) 523 nm; τ ) 2.0 ns).14c By considering the fact that the molecular calculations suggested the photoemission of previously reported Cd(II) coordination polymers14b to be mainly LMCT (the HOMO is associated with the π-bonding orbital from the naphthalene rings, while the LUMOs are mainly associated with the Cd-O σ-antibonding orbital), as well as the similarity of the lifetimes of the free H4bna ligand and 1, the emission bands of 1 may be attributed to mainly LLCT transitions of π-π from the H2bna ligands, admixing LMCT transitions. Concomitant with a red-shift of the emission bands, the lifetime of 2 is slightly increased to ca. 1.37 ns, which is more similar to those of the previously reported Cd(II) complexes with H4bna ligand; hence the emission band may be assigned to LLCT, admixing with LMCT transitions. The decay profile of the 470 nm emission of 3 can be well fitted by a double-exponential function with τ1 and τ2 at 1.34 and 0.68 ns, respectively; therefore the emission bands may be also assigned to LLCT, admixing with LMCT transitions. On the other hand, the different red shifts of 1-3 compared with the free H4bna ligand may be attributed to the different coordination modes (Scheme 1) of the ligands.14 Conclusions Three new 1D-3D MOFs were obtained by solvothermal self-assembly of H4bna, bpy, and Cd2+ as the molecular building blocks. These polymers display structure-related photoluminescence properties in the solid state, which can be slightly tuned by the coordination modes of the binaphthalene-dicarboxylate liagnds. Our results revealed that temperature is an important factor in controlling the dimensionality of the resulting coordination polymers. We have also found that the presence of a less bulky bridging ligand such as acetate is very helpful for the generation of Cd(II) clusters in this MOF system. Acknowledgment. This work was supported by NSFC (Grant No. 20531070) and the Scientific and Technological Department of Guangdong Province (Grant No. 04205405). Supporting Information Available: X-ray data files (CIF). This material is available free of charge via the Internet at http://pubs.acs.org.

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