Anion-Dependent Crystallization of Four Supramolecular Cadmium

(10) We focused our work on the Cd(II) and H4mbna system, with the variety anions of SO42−, NO3−, and Cl−. Herein, we will report four anion dep...
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Anion-Dependent Crystallization of Four Supramolecular Cadmium Complexes: Structures and Property Studies Qian Shi,* Yunti Sun, Lizi Sheng, Kefang Ma, Maolin Hu, Xingen Hu, and Shaoming Huang

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 9 3401–3407

Nanomaterials & Chemistry Key Laboratory, AdVanced Materials Research Center of Wenzhou, College of Chemistry and Materials Engineering, Wenzhou UniVersity, Wenzhou, 325000, China ReceiVed April 5, 2008; ReVised Manuscript ReceiVed May 28, 2008

ABSTRACT: Solvothermal reactions of 4,4′-methylene-bis(3-hydroxy-2-naphthalene carboxylic acid) (H4mbna), heterocyclic bidentate ligands, and Cd(II) salts yielded four zero- to two-dimensional coordination complexes, [Cd(phen)2(H2O)2](H2mbna) · (H2O) (1), {[Cd4(H2mbna)8/2(phen)4] · 2DMF}n (2), [Cd(H2mbna)(bipy)(H2O)]n (3), and (∆,∆)- [Cd(H2mbna)(bipy)2]n (4). Crystal structural analyses revealed that the zero-dimensional units of 1 are connected by hydrogen-bond interactions forming a two-dimensional double-layer with hollow structure. The dinuclear units in 2 are bridged by the carboxylate groups of H2mbna producing a twodimensional nanoporous layer with the size about 11 × 15 Å2. The zigzag chains in 3 are linked by hydrogen bonds constructing the nanotube structure with the size about 11 × 13 Å2. The chiral chain in 4 is formed by the introduce of the ∆-configuration 2,2′-bipyridine ligands. The differences in the structures of the four complexes indicate that the anion of the metal salts might act as a template in the formation of the solid product. The thermal property studies show that all of the four complexes are thermally stable. The title complexes also displayed structure-related photoluminescence properties in the solid state. Introduction Recently, much attention has been focused on the design and synthesis of supramolecular networks with nanoporous structure. A novel class of self-assembling nanonetworks has recently emerged in which the molecular components are held together by reversible interactions, such as hydrogen bonds, metal-ligand interaction, CH/π, and van der Waals forces.1–3 4,4′-Methylenebis(3-hydroxy-2-naphthalenecarboxylic acid) (H4mbna) is a well-known substance and is used in salt formulations of several drugs.4 It might be extensively utilized to construct networks with nanoscale because of its structural character. First, it has a long-range distance between two coordination points of the carboxylate group, which make it convenient to act as a spacer in building a nanoscale network. Meanwhile, it possesses both rigidity from the naphthyl rings and flexibility from the twisted C-CH2-C single bond. It often acts as a guest molecule in the enantioselective discrimination system of cyclophanes.5 Furthermore, it can act not only as hydrogen bond donor but also as acceptor because of the existence of both a carboxylate group and hydroxy group, which makes it a wonderful candidate for the construction of supramolecular networks. Because of the limited solubility of H4mbna, which is insoluble in almost all solvents except for nitrobenzene, pyridine, dimethylformamide, and aqueous solution at pH >7, studies about the coordination behavior of H4mbna are limited. 6,7 The role of anions in self-assembly processes has emerged as an increasingly active theme in the recent literature.8 In our previous studies on the coordination chemistry of cadmium complexes with diamino-binaphthyl Schiff bases, it has been found that the presence of the anions play an important role in the syntheses of the complexes with different structures and properties through engaging in the coordination. 8a Anions chosen for their “relative innocence” in terms of coordinating ability can nevertheless direct the course of an assembly process via noncovalent interactions.9 With the aim of understanding the coordination behavior of H4mbna, we began studies on the * Corresponding author. E-mail: [email protected]. Tel: 86 577 88373112. Fax: 86 577 88373111.

self-assembly of H4mbna with metal ions via the solvothermal synthetic method in the presence of the auxiliary ligands. In particular, we were interested in such reactions in the presence of the different anions to an identical metal ion. The fact that cadmium(II) can readily vary its coordination number from 4 to 8 by merely changing the size or concentration in solution of a ligating species makes it an appealing candidate for use in deliberately designed or “tailored” polymeric coordination complexes. 10 We focused our work on the Cd(II) and H4mbna system, with the variety anions of SO42-, NO3-, and Cl-. Herein, we will report four anion dependent supramolecular cadmium complexes, [Cd(phen)2(H2O)2] (H2mbna) · (H2O) (1), {[Cd4(H2mbna)8/2(phen)4] · 2DMF}n (2), [Cd(H2mbna)(bipy)(H2O)]n (3), and (∆,∆)-[Cd(H2mbna)(bipy)2]n (4) in the presence of the auxiliary ligands of 1,10-phenanthroline and 2,2′bipyridine. Experimental Section Materials and Physical Measurements. All chemicals were purchased and used as received. Elemental analyses (carbon, hydrogen, and nitrogen) were performed with a Thermo 1112 elemental analyzer. IR spectra were measured from KBr pellets on a Bruker-Tensor 27 FT-IR spectrometer. Differential scanning calorimetric thermal analysis at low temperature was performed under a nitrogen atmosphere at the heating rate of 20 °C/min using a TA-Q1000 instrument. Thermal gravimetric analysis coupling with differential scanning calorimetric thermal analysis at the temperature range of 20-800 °C was carried out under a nitrogen atmosphere at a heating rate of 10 °C/min using a TA-Q600 instrument. Photoluminescence spectra of single-crystalline samples were recorded on a Thermo Aminco-Bowan2 spectrometer with 450-W xenon lamp monochromatized by double grating (1200 gr/mu) at room temperature. Synthesis of [Cd(phen)2(H2O)2](H2mbna) · (H2O) (1) and {[Cd4(H2mbna)8/2(phen)4] · 2(DMF)}n (2). A mixture of CdSO4 · 10H2O (0.390 g, 1 mmol), H4mbna (0.388 g, 1 mmol), and phen (1,10-phenanthroline) (0.198 g, 1mmol) in H2O-EtOH-DMF (N, N′-dimethylformamine) (3:1: 1) 15 mL was heated for 2 days at 110 °C in a Parr Teflon-lined stainless steel vessel (25 mL), cooled to room temperature at a rate of 5 °C/h. Yellow crystals of 1 were collected, washed with water, and dried in air (yield 70%). IR data (KBr, cm-1): 3450m, 3080s, 1645s, 1550s, 1455s, 1385m, 1230m, 1090w, 940m, 920w, 845s, 800m, 760s, 750s, 720s, 570m, 500w,

10.1021/cg8003504 CCC: $40.75  2008 American Chemical Society Published on Web 07/25/2008

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Table 1. Crystallographic Data of 1, 3, and 4

molecular formula fw temp (K) cryst color and form cryst syst space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V, Å3 Z Dcalcd (Mg m-3) F(000) crystal size (mm3) θmax (deg) no. of reflns collected no. of independent reflns no. of params Flack param GOF on F2 final R indices [I > 2σ(I)]a,b R indices (all data)a,b largest peak and hole (e Å-3) a

1

3

4

C47H36CdN4O9 913.21 298(2) yellow blocks monoclinic C2/c 15.732(2) 23.025(3) 11.563(1) 90 115.547(2) 90 3779.0(8) 4 1.605 1864 0.23 × 0.18 × 0.12 25.20 9982 3392 293

C33H24CdN2O7 672.95 298(2) pale yellow blocks triclinic P1j 10.0688(11) 11.7024(12) 12.9457(16) 104.929(2) 96.122(2) 112.896(2) 1320.6(3) 2 1.692 680 0.28 × 0.22 × 0.18 25.21 7074 4679 396

1.168 R1 ) 0.0518 wR2 ) 0.1159 R1 ) 0.0585 wR2 ) 0.1191 0.843 and -0.330

1.169 R1 ) 0.0436 wR2 ) 0.1061 R1 ) 0.0474 wR2 ) 0.1079 0.734 and -0.736

C43H30CdN4O 811.11 298(2) yellow blocks orthorhombic P212121 9.3082(5) 14.1780(7) 26.0421(1) 90 90 90 3436.8(3) 4 1.568 1648 0.21 × 0.15 × 0.10 25.23 18343 6191 489 0.01(2) 1.092 R1 ) 0.0323 wR2 ) 0.0756 R1 ) 0.0338 wR2 ) 0.0764 0.529 and -0.286

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

Scheme 1. Schematic Drawing of the Reactions between H4mbna and Cd(II) Salts

420w. Anal. Calcd (%) for C47H36CdN4O9: C, 61.82; H, 3.97; N, 6.13. Found: C, 61.86; H, 3.95; N, 6.21. Using CdCl2 · 2.5H2O (0.229 g, 1mmol) instead of CdSO4 · 10H2O in the above system, pale-yellow crystals of 2 were collected, washed with water, and dried in air (yield 50%). IR data (KBr, cm-1): 3430m, 3300m, 3150s, 1715s, 1620s, 1555s, 1395m, 1250m, 1070w, 950m, 930w, 865s, 820m, 780s, 740s, 715s, 560m, 510w, 450w. Anal. Calcd (%) for C146H102Cd4N10O26: C, 61.27; H, 3.59; N, 4.89. Found: C, 61.32; H, 3.57; N, 4.92. Synthesis of [Cd(H2mbna)(bipy)(H2O)]n (3) and (∆,∆)-[Cd(H2mbna)(bipy)2]n (4). Using bipy (2,2′-bipyridine) (0.156 g, 1mmol) instead of phen, H4mbna reacted with CdCl2 · 2.5H2O or CdSO4 · 10H2O in the same reaction condition as described above. Pale-yellow crystals of 3 (yield 65%) and yellow crystals of 4 (yield 40%) were obtained. IR data (KBr, cm-1) for 3: 3500s, 3120m, 1745s, 1650s, 1385s, 1360m, 1280m, 1190w, 970m, 910w, 855s, 820m, 750s, 740s, 720s, 580m, 550w, 410w. For 4: 3050s, 1745s, 1650s, 1425s, 1355m, 1130m, 1060w, 930m, 910w, 875s, 850m, 780s, 740s, 710s, 530m, 510w, 450w. Anal. Calcd (%) for C33H24CdN2O7 (3): C, 58.90; H, 3.59; N, 4.16. Found: C, 58.94; H, 3.61; N, 4.18. For C43H30CdN4O6 (4): C, 63.67; H, 3.73; N, 6.91. Found: C, 63.70; H, 3.69; N, 6.94. X-ray Crystallography. Diffraction intensities for the complexes were collected at 298 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.11,12 The structures

were solved with direct methods and refined with full-matrix leastsquares technique using the SHELXTL program.13 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. 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 1, 3, and 4 are provided in Table 2.

Results and Discussion Syntheses. Solvothermal reaction is preference to the other methods in this system because of the consideration of the limited solubility about H4mbna. A series of experiments were carried out in order to get the suitable reaction temperature. The ligand decomposed when the temperature is above 110 °C mainly because of the weak single bond strength between naphthyl-methylene-naphthyl of H4mbna. Meanwhile, the reaction could not take place in lower temperatures. It has been found that the best temperature for the crystal growth of the Cd(II) complexes is at 110 °C in the mixed solvent of H2O, DMF, and EtOH.

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Table 2. Selected Bond Lengths (Å) and Angles (deg) for 1,3, and 4a Complex 1 Cd1-O1 O1-Cd1-O1A N1-Cd1-N1A N1-Cd1-N2

2.258(3) 77.73(19) 103.25(15) 70.75(11)

Cd1-N1 O1-Cd1-N1 O1-Cd1-N2 N1-Cd1-N2A

Cd1-O6A Cd1-N1 O6-Cd1B O7-Cd1-O2 O2-Cd1-N1 O2-Cd1-N2 O7-Cd1-O1 N2-Cd1-O1

2.219(3) 2.327(4) 2.219(3) 145.59(13) 101.76(13) 93.05(14) 92.84(12) 84.08(13)

Cd1-O7 Cd1-N2 O6A-Cd1-O7 O6A-Cd1-N1 O6A-Cd1-N2 N1-Cd1-N2 O2-Cd1-O1

Cd1-O1 Cd1-N2 Cd1-O6A O1-Cd1-N1 O1-Cd1-N2 O1-Cd1-N4 N2-Cd1-N4 O5A-Cd1-N3 O1-Cd1-O6A N2-Cd1-O6A

2.292(3) 2.436(3) 2.567(3) 126.92(12) 162.53(12) 85.16(11) 84.81(10) 146.63(10) 79.52(11) 96.14(10)

Cd1-N1 Cd1-N4 O5-Cd1B O1-Cd1-O5A N1-Cd1-N2 N1-Cd1-N4 O1-Cd1-N3 N2-Cd1-N3 N1-Cd1-O6A N4-Cd1-O6A

2.332(3) 156.25(11) 90.53(12) 96.23(11)

Cd1-N2 O1-Cd1-N1A O1-Cd1-N2A N2-Cd1-N2A

2.386(3) 92.95(12) 105.55(12) 159.49(16)

2.311(4) 2.349(4) 86.67(14) 87.24(12) 155.40(14) 70.46(13) 53.99(11)

Cd1-O2 Cd1-O1 O6A-Cd1-O2 O7-Cd1-N1 O7-Cd1-N2 O6A-Cd1-O1 N1-Cd1-O1

2.317(3) 2.493(3) 101.82(13) 112.00(14) 92.06(15) 120.51(12) 144.40(12)

2.370(3) 2.468(3) 2.415(2) 87.11(10) 68.56(10) 140.61(10) 97.18(11) 92.14(10) 82.96(10) 129.93(9)

Cd1-O5A Cd1-N3 O6-Cd1B N1-Cd1-O5A O5A-Cd1-N2 O5A-Cd1-N4 N1-Cd1-N3 N4-Cd1-N3 O5A-Cd1-O6A N3-Cd1-O6A

2.415(2) 2.481(3) 2.567(3) 119.05(9) 77.09(9) 80.01(9) 84.48(10) 67.49(10) 52.06(9) 161.25(10)

Complex 3

Complex 4

a Symmetry operations. For 1: A ) -x + 1, y, -z + 1/2. For 2: A ) -x + 1, y, -z - 1/2. For 3: A ) x + 1, y + 1, z; B ) x - 1, y - 1, z. For 4: A ) -x + 3/2, -y + 2, z - 1/2; B ) -x + 3/2, -y + 2, z + 1/2.

Figure 1. (a) ORTEP view of the Cd(II) coordination environment in 1 (50% probability level ellipsoids, symmetry code, A ) -x + 1, y, -z + 1/2). (b) Perspective view of the bowl-shaped network fragment along [101].

In the reaction of CdNO3, H4mbna, and phen, interesting phenomena were observed. Yellowish crystals [Cd(phen)2(H2O)2] (H2mbna) · (H2O) (1) and pale yellowish crystals {[Cd4(H2mbna)8/2

(phen)4] · 2DMF}n (2) were formed simultaneously with low yields. The H2mbna2- ligand just acts as an antianion in 1, whereas it coordinated to Cd(II) with covalent interactions in 2. The NO3- anion was not present in in both of the complexes.

3404 Crystal Growth & Design, Vol. 8, No. 9, 2008 Scheme 2. Schematic Drawing of Nanoporous Structure in 2; Phen Ligands Are Omitted for Clarity

To know more about the anion-dependent procedure, we used small anion SO42- or Cl- instead of NO3- in the reaction. The major product was 1 with high yield (70%) for SO42-, whereas that was 2 (yield 50%) for Cl- as shown in Scheme 1. Because the sequence of the acidity is H2SO4 > HNO3 > HCl, the pH value of the aqua solution of the metal salt is CdCl2 > Cd(NO3)2 > CdSO4. It is shown that the deprotonation rate and coordination mode of H4mbna might dependent sensitively on the pH value of the system as our previous work. 3b The higher pH value might be benefit to form 2. The presumption was

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confirmed by the experiment below. Equal molar NaOH was added in the system of CdSO4, H4mbna, and phen, and complex 2 was formed with high yield (90%). 2,2′-bipyridine (bipy) is a usual chelate ligand as phen. The results we got are somewhat out of our expectation when using bipy instead of phen in the reactions. When the anions are NO3and Cl-, the major product is an one-dimensional chainlike structural complex [Cd(H2mbna)(bipy)(H2O)]n (3), but not a two-dimensional structure as 2. The yield of 3 is higher (65%) in the presence of Cl- than that of 25% in the presence of NO3-. Using CdSO4 in the reaction, unpredictable result, (∆,∆)[Cd(H2mbna)(bipy)2]n (4), was obtained. In complex 4, except for the coordination of two bipy ligands to the Cd(II) ion similar to that of phen in 1, the H2mbna2- ligand also bonds to the Cd(II) ion with the same mode in 3. It might be because of the small size of bipy compared to that of phen. It is interesting to find that two pyridine rings in the bipy ligand are twisted with ∆-configuration, which induced the chirality into the molecule. The result indicates that SO42- plays an important role in the formation of 4, probably through acting as a reaction template or deprotonation reagent. Structures. As illustrated in Figure 1a, the Cd atom in complex 1 is surrounded by four N atoms from two phen ligands and two O atoms of two aqua molecules, and the related bond lengths angles are listed in Table 2. Both of the two carboxylate groups in H4mbna are deprotronated and the dihedral angle of the naphthyl rings (mean deviation: 0.0172 Å) is 72.8°. It is also parallel to the adjacent phenanthroline ring (mean deviation: 0.0387 Å) with a dihedral angle of about 1.0°. The [Cd(phen)2(H2O)2]2+ unit connected to the H2mbna2- anion with the face to face π-π interactions (distance: 3.4340 Å) forming a one-

Figure 2. (a) ORTEP view of the Cd(II) coordination environments in 3 (50% probability level ellipsoids, symmetry code, A ) x + 1, y + 1, z). (b) Perspective view of the double chain supramolecular structure in 3. (c) Perspective view of the nanotubal structure along the b axis in 3.

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Figure 4. DSC curves for complexes 1 (s, red), 2 (- -, blue), 3 (- · -, black), and 4 ( · · · , pink) at low temperature.

Figure 3. (a) ORTEP view of the Cd(II) coordination environments in 4 (50% probability level ellipsoids, symmetry code, A ) -x + 3/2, -y + 2, z - 1/2). (b) Perspective view of the helix chains along the a axis in 4.

dimensional chain. The adjacent chains link to each other with numerous hydrogen-bond interactions between O(aqua) · · · O(aqua) (2.665 and 2.708 Å), O(carboxylate) · · · O(hydroxyl) (2.495 Å), and O(carboxylate) · · · O(aqua) (2.876 Å), forming two-dimesional double layers with bowl-shaped parts as shown in Figure 1b. The structure of complex 2 {[Cd4(H2mbna)8/2(phen)4] · 2DMF}n, is similar to that of {[Cd(H2mbna)4(phen)] · DMF}n reported by Du.7a The coordination number of Cd atom is also six, and the coordination environment were completed by four O atoms from three H2mbna2- ligands and two N atoms from phen. There are two coordination modes of the carboxylate groups in H2mbna2-, one is chelate and the other is bidentate bridging. Two Cd2+ ions are bridged by two carboxylate groups forming a dinuclear secondary building unit (SBU). The Cd2 SBUs are at four corners of a rectangle while the H2mbna2ligands just act as the wall, forming a two-dimensional nanotype porous structure with a size of about 11 × 15 Å2 (Scheme 2). Unanticipated phenomena were observed in the synthesis of complexes 3 and 4 when we use 2,2′-bipyridine replacing 1,10phenanthroline in the reaction. There are few differences in the structure between the two auxiliary ligands except in the stereo effect. In complex 3, the coordination environment of Cd2+ ion is completed by one aqua O atom, two N atoms from phen and three carboxylate O atoms from two H2mbna2- ligands (Figure 2a). The carboxylate groups in one H2mbna2- ligand have two coordination fashions, monodentate and chelate modes. The metal ions are connected by the carboxylate groups into a zigzag chain. The dihedral angle of the naphthyl rings is 76.5°, similar to those in 1 and 2. The adjacent chains are linked by the hydrogen bond interactions between O7 · · · H-O1 (symmetry code: -x + 1, -y + 2, -z + 2) with a distance of 2.720 Å,

forming a double chain structure with nanosized cavities of about 11 × 13 Å2 as shown in Figure 2b. The face to face π-π interactions (distance: 3.5734 Å) among the phenyl rings stack the double chains into nanotubes further along the c axis, as illustrated in Figure 2c. Compared with 3, the Cd2+ ion in complex 4 is coordinated with two 2,2′-bipyridine (bipy) ligands, and the coordination number is 7 surrounded by four N atoms from two bipy ligands and three carboxylate O atoms from two H2mbna2- ligands (Figure 3a). The distances of the Cd-N bonds in 4 (with the mean value of 2.439Å) are much longer than those in 1 (mean value 2.359 Å), 2 (mean value 2.332 Å),7a and 3 (mean value 2.327 Å), which might be beneficial in avoiding the strong electronic repulsion from two bipy ligands around the Cd(II) ion. Both of the two bipy ligands in 4 have a ∆ configuration, and the dihedral angles between the pyridine rings are 12.7 and 30.6°, respectively. The space group of 4 is P212121, and the chirality of the complex is induced by the bipy ligand. The coordination modes of the carboxylate groups in H2mbna2- are similar to those of 3. Two naphthyl rings in H2mbna2- are almost vertical with the dihedral angle of 90.9°, which is much larger than those in 1, 2, and 3. The Cd ions form a chiral chain by the connection of H2mbna2- ligands as shown in Figure 3b. Studies of Thermal Property. Differential scanning calorimetric thermal analysis, thermal gravimetric analysis and differential thermal analysis are important in studying the transformation of a solid sample in a thermal process. They are commonly used in studying adsorption, sintering, calcination, phase transition, decomposition, and many other processes for different materials. To understand the thermal stability of complexes at low temperature, we studied differential scanning calorimetric thermal analyses in the range of -80 to 20 °C, and the DSC curves are shown in Figure 4. There is no obvious heat flow change observed when increasing the temperature for complexes 1, 3, and 4, suggesting that the crystal lattice is stable at low temperature. Meanwhile, the small peak shown in the DSC curve of complex 2 indicates that there might be some phase transition occurring at -44.50 °C with the heat flow about 0.5455J/g. The TGA-DSC curves for complex 1 at the temperature range of 20 to 800 °C are shown in Figure 5, whereas in complex 2, the 5.06% weight decrease found by thermogravimetric analysis in the temperature range of 22-216 °C corresponds to the loss of solvent molecule DMF (calculated weight loss is 5.10%). From 342 to 673 °C, the 53.6% weight decrease corresponds

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Figure 5. TGA (s, solid) and DSC (- -, dashed) curves for complex 1.

to the decomposition of four H2mbna2- molecules in one structure moiety of complex 2. The peaks appear in the DSC curve at 370, 531, and 601 °C, indicating that the decomposition process of four H2mbna2- motifs may have different mechanisms because of the different coordination fashions of the carboxyl groups. In complexes 3 and 4, the temperature ranges of the decomposition process to H2mbna2- are somewhat lower compared to that of 2, with values of 265-550 °C for 3 and 280-565 °C for 4. Luminescence Properties. To well-compare the luminescence intensity of the compound, we determined all of the spectra with the same voltage of about 510 V. Free H4mbna ligand displays a very weak luminescence upon excitation at 299 nm in the solid state at ambient temperature, as shown in Figure 6a. The emission maxima of H4mbna is at ca. 469 nm with some weak shoulder peaks at ca. 474, 493 and 545 nm. Complexes 2, 3, and 4 all exhibit intense blue photoluminescence, as depicted in Figure 6b. The emission spectra of 1 is similar with the emission maxima at ca. 467 nm with a weak peak at 534 nm, upon excitation at 379 nm, which is consistent with the uncoordinated behavior of H2mbna2- in 1. The emission spectra of 3 had the emission maxima at ca. 469 nm, whereas that of 4 was at ca. 464 nm upon excitation at 409 and 399 nm, respectively. Compared to the spectra of the H4mbna ligand, the emission maxima of 3 and 4 are similar because of the slight difference in the structure, except the disappearances of the shoulder peak at 545 nm in H4mbna. The enhancement of luminescence in complexes may be attributed to the ligation of the ligand to the metal center, which enhances the rigidity of the ligand and reduces the loss of energy through a radiationless pathway.14 For 2, only a broad emission band appears with the maximum at 526 nm, which is red-shifted compared to that of the free H4mbna ligand. The spectra of 2 are different from those of 3 and 4, because of the different coordination mode of the H4mbna ligand. As to the 4d10 valence electron configuration of the Cd(II) ion, the emission band of the Cd(II) complexes may be assigned to ligand-to-ligand change transfer (LLCT), admixing with ligand-to-metal change transfer (LMCT) as previously reported.15 Conclusion Four anion-dependent cadmium(II) complexes with nanoporous structures were synthesized by the solvothermal method, in a reaction with H4mbna and phen or 2,2′-bipyridine in the presence of different anions. The results indicate that the anion existing in the reaction system might play an important role in the syntheses of the complexes with different structures and

Figure 6. (a) Emission spectra of the free H4mbna ligand in the solid state. (b) Emission spectra of 1 (s, black), 2 (- -, red), 3 ( · · · , dark blue), and 4 (- · -, light blue) in solid state at room temperature.

properties. The complexes are all thermally stable and exhibit intense blue photoluminescence, especially in complex 2. Acknowledgment. This work was supported by the NSFC of China (Grant 20601021) and the Project of the Educational Department of Zhejiang Province (Grant 20060375). Supporting Information Available: X-ray data files (CIF). This information is available free of charge via the Internet at http:// pubs.acs.org.

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