Polynuclear CdII Polymers: Crystal Structures

Apr 23, 2014 - coordination complexes containing CdII clusters, formulated as. {[Cd3L2(H2O)5]·H2O}n (1), {[Cd3L2(hbmb)(H2O)2]·2.5H2O}n (2),. {[Cd 3 ...
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Polynuclear CdII Polymers: Crystal Structures, Topologies, and the Photodegradation for Organic Dye Contaminants Lu Liu, Jie Ding,* Chao Huang, Ming Li, Hongwei Hou,* and Yaoting Fan The College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou, Henan 450052, P. R. China S Supporting Information *

ABSTRACT: To systematically explore the effect of polynuclear complexes on photocatalytic degradation of the organic dyes, a series of coordination complexes containing CdII clusters, formulated as {[Cd3L2(H2O)5]·H2O}n (1), {[Cd3L2(hbmb)(H2O)2]·2.5H2O}n (2), {[Cd3L2(btbb)(H2O)2]·2EtOH·1.5H2O}n (3), and {[Cd6L4(bipy)2(H2O)6]·3H2O}n (4) (H3L = 3,4-bi(4-carboxyphenyl)benzoic acid, hbmb = 1,1′-(1,6-hexane)bis(2-methylbenzimidazole), btbb = 1,4-bis(2-(4-thiazolyl)benzimidazole-1-ylmethyl)benzene, 4,4′-bipy = 4,4′-bipyridine), have been designed and synthesized. Complex 1 based on trinuclear CdII clusters exhibits a new (3,3,6)-connected 3D framework. 2 belongs to a (3,3,8,8)-connected tfz-d topology net with pillar-layered frameworks assembled by two kinds of trinuclear CdII clusters. 3 is a 3D pillar-layered framework, which features a (3,8)connected tfz-d net based upon one kind of trinuclear CdII cluster. 4 presents a new 3D (3,6,10)-connected framework with dinuclear and tetranuclear clusters. The photocatalytic properties of complexes 1−4 have been studied in detail. Remarkably, 1− 4 all reveal good photocatalytic activity in MB/MO degradation. The optical energy gap calculated by the diffuse reflectivity spectra of 1−4 are consistent with their degradation rates. Moreover, the experimental results further demonstrate that the cluster complexes containing different kinds of nuclei may exert different impact on the decomposition of disparate organic dyes.

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INTRODUCTION At present, in view of the intense desire for “green life”, the notable promotion has been on acquiring highly effective and ambitious light-driven catalysis to treat the pollution of the environment, particularly organic dye pollution.1 The vast majority of the colored effluents detected in wastewater are a result of organic dyes in textiles, dyestuffs, and dyeing industries.2 It is quite difficult to deal with them by the method of conventional contaminated water treatment because of the high solubilities of these contaminants in water.3 Previous investigators have reported that some coordination complexes could serve as photocatalytic materials in the green decomposition of organic dye pollutants.4,5 We consider that polynuclear complexes could exhibit better photocatalytic activity than that of common coordination complexes. The reasons can be generalized as follows: (a) metal-cluster-based complexes frequently exhibit charming frameworks and chic network topologies;6 (b) metal-cluster-based complexes probably display better functions on account of the collaborative activity among ions.7 Therefore, achievement of cluster complexes with specific photocatalysis properties has always been our pursuit. However, a profoundly challenging task is the design and synthesis of cluster complexes with our desired properties. A variety of multicarboxylates have been documented as admirable constructors of polynuclear metal polymers.8 Herein, we employ asymmetric multicarboxylates 3,4-bi(4-carboxyphenyl)-benzoic acid (H3L) (Scheme S1) as © 2014 American Chemical Society

bridging ligands to build cluster complexes, which have been anticipated to possess outstanding photocatalytic activity. As to the H3L ligand, the tricarboxyl oxygen-donor linker with diversified chelating/bridging functions could have uncertain coordination capability, and the coexistence of three benzene rings may make H3L turn into a highly conjugated organic connector, which is likely to make the formed complexes carry potential optical properties. Additionally, the upsurge in research on cluster complexes as photocatalysts in photocatalytic decomposition of organic dyes could furnish a clean strategy to deal with refractory pollutants and would also enrich the catalysis field. In this article, the reactions of CdII salts and H3L were executed in the presence/absence of auxiliary ligands 1,1′-(1,6hexane)bis(2-methylbenzimidazole) (hbmb), 1,4-bis(2-(4thiazolyl)benzimidazole-1-ylmethyl)benzene (btbb), and 4,4′bipyridine (4,4′-bipy). Four different structural polynuclear CdII complexes, {[Cd3L2(H2O)5]·H2O}n (1), {[Cd3L2(hbmb)(H 2 O) 2 ]·2.5H 2 O} n (2), {[Cd 3 L 2 (btbb)(H 2 O) 2 ]·2EtOH· 1.5H2O}n (3), and {[Cd6L4(bipy)2(H2O)6]·3H2O}n (4), have successfully been acquired to conduct the photocatalysis experiments on MB/MO degradation. The optical energy Received: February 27, 2014 Revised: April 15, 2014 Published: April 23, 2014 3035

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gap, fluorescence, and thermal stability have also been surveyed on complexes 1−4.

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technique using the SHELXL-97 crystallographic software package.10 The hydrogen atoms were fixed at calculated positions, and afterward, refined as riding atoms using isotropic displacement parameters. In 3, the atom C8, C9, C10, C11, C12, C13, C15, C16, C17, C18, C19, and C20 are disordered with two positions and are refined as two atoms with the occupancy of 0.5, viz., C8 and C8′, C9 and C9′, C10 and C10′, C11 and C11′, C12 and C12′, C13 and C13′, C15 and C15′, C16 and C16′, C17 and C17′, C18 and C18′, C19 and C19′, C20 and C20′, respectively. Besides, the position of the disordered solvent and water molecules could not be resolved from Fourier maps in the crystals of 2 and 3. No satisfactory disorder model could be achieved, and therefore the SQUEEZE program implemented in PLATON was used to remove these electron densities. The final chemical formula of 2 and 3 were calculated from SQUEEZE results combined with the TGA and elemental analysis data. Crystallographic crystal data and structure processing parameters for 1−4 are summarized in Table S1 (in the Supporting Information). Selected bond lengths and bond angles of 1−4 are listed in Table S2 (in the Supporting Information). Crystallographic data for 1−4 have been deposited at the Cambridge Crystallographic Data Centre with CCDC reference numbers 980666−980669.

EXPERIMENTAL SECTION

Materials and Physical Measurements. All of the chemicals were commercially available in addition to hbmb and btbb, which were prepared on the basis of the literature.9 The Fourier transform infrared (FT-IR) spectra were measured on a Bruker-ALPHA spectrophotometer adopting KBr pellets in the scale of 400−4000 cm−1. Thermogravimetric analyses were performed on a Netzsch STA 449C thermal analyzer with a 10 °C min−1 heating rate in the air flow. Elemental analyses were implemented using a FLASH EA 1112 analyzer. Powder X-ray diffraction patterns were carried out on a PANalytical X’Pert PRO diffractometer using Cu Kα1 radiation. UV− vis absorption spectra were put into effect with a TU-1901 doublebeam UV−vis spectrophotometer. The measurements of the luminescent spectra of the powdered solid samples were conducted on a Hitachi 850 fluorescence spectrophotometer at ambient temperature. A Cary 500 spectrophotometer equipped with a 110nm-diameter integrating sphere is used for collecting diffuse reflectivity spectra of the samples from 200 to 800 nm. In the measurement process, BaSO4 is selected as a standard with 100% reflectance. Synthesis. Synthesis of {[Cd3L2(H2O)5]·H2O}n (1). A mixture of Cd(NO3)2·4H2O (0.1 mmol), L (0.05 mmol), and EtOH and H2O in the molar ratio of 1:1 was heated at 170 °C for 3 days in a 25 mL Teflon-lined stainless steel vessel. After the reaction system was cooled to room temperature at a rate of 5 °C h−1, faint yellow block-shaped crystals of 1 were collected. (yield, 58% based on Cd). Anal. Calcd for C42H34Cd3O18 (%): C, 43.33; H, 2.94. Found: C, 43.26; H, 2.99. IR (KBr, cm−1): 3404(m), 1580(m), 1519(s), 1402(s), 1185(w), 1107(w), 1037(w), 1016(w), 1004(w), 893(w), 863(m), 780(m), 734(w), 719(w), 691(w). Synthesis of {[Cd3L2(hbmb)(H2O)2]·2.5H2O}n (2). A mixture of Cd(NO3)2·4H2O (0.1 mmol), L (0.05 mmol), hbmb (0.05 mmol), EtOH (7 mL) and H2O (3 mL) was placed in a 25 mL Teflon-lined stainless steel container. The mixture was sealed and then heated at 120 °C for 3 days, followed by slow cooling to ambient temperature at a rate of 5 °C h−1; pale yellow crystals of 2 were obtained in a yield of 67% (based on Cd). Anal. Calcd for C64H57Cd3N4O16.5 (%): C, 51.81; H, 3.87; N, 3.77. Found: C, 51.85; H, 3.84; N, 3.82. IR (KBr, cm−1): 3418(m), 3055(w), 2934(m), 2860(w), 1583(m), 1531(m), 1479(w), 1461(w), 1405(s), 1294(w), 1179(w), 1033(w), 1016(w), 932(w), 863(m), 816(w), 781(w), 751(m), 717(w), 687(w). Synthesis of {[Cd3L2(btbb)(H2O)2]·2EtOH·1.5H2O}n (3). The procedure is similar to that of 2, except that btbb was used instead of hbmb (0.05 mmol). Yield, 75% (based on Cd). Anal. Calcd for C74H61N6O17.5S2Cd3 (%): C, 51.80; H, 3.58; N, 4.90. Found: C, 51.86; H, 3.50; N, 4.85. IR (KBr, cm−1): 3423(s), 1583(s), 1537(s), 1478(w), 1397(s), 1292(w), 1180(w), 1016(w), 1005(w), 931(w), 891(w), 857(w), 783(w), 751(w), 718(w), 685(w), 657(w). Synthesis of {[Cd6L4(bipy)2(H2O)6]·3H2O}n (4). A mixture of Cd(NO3)2·4H2O (0.1 mmol), L (0.05 mmol), 4,4′-bipy (0.05 mmol), EtOH (7 mL), and H2O (3 mL) was placed in a 25 mL Teflon-lined stainless steel container. The mixture was sealed and heated at 140 °C for 4 days. After cooling to ambient temperature at a rate of 5 °C h−1, leaf-shaped crystals of 4 were obtained with a yield of 55% (based on Cd). Anal. Calcd for C104H78Cd6N4O33 (%): C, 48.29; H, 3.03; N, 2.16. Found: C, 48.23; H, 3.01; N, 2.21. IR (KBr, cm−1): 3416(s), 1935(w), 1815(w), 1578(s), 1520(s), 1377(s), 1222(m), 1180(m), 1141(w), 1103(w), 1068(w), 969(w), 889(w), 860(m), 810(m), 790(m), 777(s), 735(m), 716(m), 689(w), 629(m), 590(w), 568(w). Crystal Data Collection and Refinement. The data of the 1−4 were collected at room temperature on a Rigaku Saturn 724 CCD diffractomer with Mo Kα radiation (λ = 0.71073 Å). Absorption corrections were applied by utilizing multiscan program. The data were corrected according to Lorentz and polarization effects. The structures of complexes 1 and 4 were solved by direct methods and then refined on the basis of F2 with a full-matrix least-squares

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RESULTS AND DISCUSSION Crystal Structure of {[Cd3L2(H2O)5]·H2O}n (1). Single crystal X-ray diffraction analysis reveals that 1 crystallizes in monoclinic space group P21/c. As shown in Figure S1a, the asymmetric unit of 1 contains three crystallographically unique Cd ions, two L ligands, five coordinated water molecules, and one lattice water. All Cd2+ ions display distorted octahedral coordination geometry. Cd1 ion is surrounded by two oxygens (O2, O8C) from two different μ2-η1:η1-bridge carboxylates, two oxygens (O16C, O17C) from one μ1-η1:η1-chelate carboxylate, one oxygen (O10B) from one μ2-η2:η1-chelate/bridge carboxylate, and one water oxygen atom (O1). Cd2 ion is coordinated with two oxygens (O3D, O9E) from two different μ2-η1:η1bridge carboxylates, two oxygens (O6, O7) from one μ1-η1:η1chelate carboxylate, and two oxygen atoms (O4, O5) from two coordinated water molecules. Cd3 ion is ligated by two oxygen atoms (O10, O11) from one μ2-η2:η1-chelate/bridge carboxylate, two oxygen atoms (O12, O13) from one μ1-η1:η1-chelate carboxylate, as well as two oxygen atoms (O14, O15) from two coordinated water molecules. The Cd(1)−O(9), Cd(2)−O(2), and Cd(3)−O(8) distance are 2.765 Å, 2.671 Å, and 2.757 Å, respectively, suggesting a non-negligible interaction with the uncoordinated carboxylate oxygen atom, which can be described as a semichelating coordination mode.11 All of the other Cd−O bond lengths fall in the range of 2.207(7)− 2.503(7) Å, similar to the values found in other cadmium complexes.12 The bond angles around Cd(II) ions are in the range of 54.1(2)−165.2(3)°. There are two different types of L3−, which adopt the two coordination modes as follows: the first type of L3− acts as a pentaconnector to link five Cd atoms with the (κ2-κ1-μ2)-(κ2-κ1-μ2)-(κ2)-μ5 coordination mode (Mode I of Scheme 1), whereas the second type of L3− adopts (κ2)-(κ2-κ1-μ2)-(κ2)-μ5 coordination mode connecting four Cd atoms (Mode II of Scheme 1). Cd1 and Cd2A ions are held together by the carboxylate O atoms to form a binuclear unit [Cd2(CO2)4]. The Cd−Cd distance across the binuclear unit is 3.803 Å. Cd3B ion is connected to the adjacent binuclear unit [Cd2(CO2)4] by the carboxylate O atoms resulting in [Cd3(CO2)6] unit (yellow polyhedron) (Figure S1b). The trinuclear [Cd3(CO2)6] units are bridged by the L3− ligands adopting the coordination Mode I to form a 2D layer, which is further linked by the L3− ligands adopting the coordination Mode II to complete the overall 3D 3036

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η 2 :η 0 -bridge carboxylates, and two oxygens from two coordinated water molecules. The Cd−N bond lengths vary from 2.273(6) to 2.31(6) Å, while the Cd−O bond lengths are from 2.226(4) to 2.523(5) Å, which are in the normal range.12 The hbmb ligand adopts asymmetric trans-conformation with two different Ndonor···N−Csp3···Csp3 torsion angles of 97.557° and 98.174°. The L in 2 are completely deprotonated, exhibiting one kind of coordination mode: (κ2-κ1-μ2)-(κ1-μ2)-(κ2)-μ5 (Mode III of Scheme 1). In this mode, three carboxylic groups act as μ2η2:η1, μ2-η2:η0, and μ1-η1:η1 modes in a clockwise direction, respectively, to bridge five Cd(II) ions together. On the basis of these connection modes, Cd1 atom, Cd4 atom, and symmetryrelated Cd1A (1-x, -y, 2-z) are linked together by four carboxylate groups in μ2-η2:η0 and μ2-η2:η1 to give a trinuclear [Cd3(CO2)4] unit (SBU-A) (bright green polyhedron). Cd2 atom, Cd3 atom, and symmetry-related Cd2B (2-x, 1-y, 1-z) are also linked together by four carboxylate groups in μ2-η2:η0 and μ2-η2:η1 to generate a trinuclear [Cd3(CO2)4] unit (SBU-B) (pink polyhedron). It is worth noting that these two kinds of trinuclear units are surrounded by six L ligands and two hbmb ligands; each L3− connects to three trinuclear units, while the hbmb ligands act as linkers. And interestingly, the trinuclear units encircling the L3− ligand (type I and II) are different. L3− in type I links an SBU-A and two SBUs-B; nevertheless, L3− in type II links two SBUs-A and one SBU-B. By this means, the L3− ligands connect SBUs-A and SBUs-B to form two disparate 1D double chains, respectively (Figure S2b and S2c). The two kinds of 1D double chains are interconnected in a parallel manner and united together to afford a 2D layer net (Figure S 2d). The 2D nets are pillared by ligand hbmb in a transconformation via the Cd−N connections to generate a pillarlayered 3D framework (Figure S2e). From the viewpoint of structural topology, if the trinuclear SBU-a and SBU-b are considered as 8-connected nodes, and L3− in type I and II as 3-connected nodes, singly (Figure 2a), the whole 3D structure exhibits a (3,3,8,8)-connected tfz-d topology The extended point symbol for this network is (43)2(46·618·84)2 (Figure 2b).

Scheme 1. Diverse Coordination Modes of H3L in Complexes 1−4

network. From the viewpoint of structural topology, each [Cd3(CO2)6] unit can be viewed as a 6-connected node, and two different kinds of L ligands both can be considered as 3connected nodes (Figure 1a). Therefore, the whole framework

Figure 1. Crystal structures of 1: (a) Defined 3- and 6-connected nodes. (b) Schematic representation of topological net of 1 with different nodes discriminated by colors.

of 1 can be simplified as a new (3,3,6)-connected net (Figure 1b). Topological analysis by the TOPOS program suggests that the point (Schläfli) symbol of the net is (43·66·86)(42·6)(4·62), which is a new topology. Crystal Structure of {[Cd3L2(hbmb)(H2O)2]·2.5H2O}n (2). The results of crystallographic analysis reveal that the asymmetric unit of 2 possesses four crystallographically independent CdII atoms, two L ligands, one hbmb ligand, two coordinated water molecules, and two and a half free lattice water molecules. As illustrated in Figure S2a, the four unique cadmium cations including Cd1, Cd2, Cd3, and Cd4 (occupancy rate of 1, 1, 0.5, and 0.5, singly) are all distorted octahedral coordination geometries. Both Cd1 and Cd2 are anchored by two oxygens from one μ1-η1:η1-chelate carboxylate, two oxygens from one μ2-η2:η1-chelate/bridge carboxylate, one oxygen from one μ2-η2:η0-bridge carboxylate and one nitrogen atom from one hbmb ligand. Both Cd3 and Cd4 are coordinated by two oxygens from two different μ2-η2:η1chelate/bridge carboxylates, two oxygens from two different μ2-

Figure 2. Crystal structures of 2: (a) Defined 3- and 8-connected nodes. (b) Schematic representation of topological net of 2. 3037

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crystallizes in the triclinic crystal system P1.̅ As depicted in Figure S4a, the asymmetric unit consists of three independent CdII atoms, two L ligands, one 4,4′-bipy ligand, three associated water molecules, and one and a half lattice water molecules. The Cd1 atom is seven-coordinated by five carboxylate oxygen atoms (O2, O1A, O2A, O14B, O15B) from three different L3− (Cd−O distances range from 2.341 (4) to 2.454 (4) Å), and one oxygen atom (O3) of one coordinated H2O molecule (Cd−O distances are 2.320(4) Å), one 4,4′-bipy nitrogen atom (N1) with a Cd−N distance of 2.340 (4) Å to give rise to a pentagonal bipyramid coordination geometry. The Cd2 atom is six-coordinated by three carboxylate oxygen atoms (O6, O7, O10) from two L ligands (Cd2−O6 = 2.324(3), Cd2−O7 = 2.407(3), Cd2−O10 = 2.194(3) Å), one nitrogen donor (N2C) from one 4,4′-bipy ligand (Cd2−N2C = 2.317(4) Å), and two oxygen atoms (O8, O9) of two coordinated H2O molecules (Cd2−O8 = 2.307(4), Cd2−O9 = 2.279(4) Å) exhibiting a distorted octahedral coordination geometry. The Cd3 atom is five-coordinated by five carboxylate oxygen atoms (O4E, O5D, O11, O12F, O13F) provided by four different L ligands, taking a distorted square-pyramidal geometry (τ = 0.14).13 It is noted that the Cd3−O4D distance is 2.702 Å, suggesting a nonnegligible interaction between the cadmium atom and the uncoordinated carboxylate oxygen atom.11 The H3L in 4 are completely deprotonated, taking two different coordination modes: the first type of H3L acts as a pentaconnector to link five Cd atoms with the (κ2-κ1-μ2)-(κ2-κ1-μ2)-(κ2)-μ5 coordination mode (Mode I of Scheme 1), whereas the second type of H3L adopts (κ2)-(κ2)-(κ1-κ1)-μ4 coordination mode connecting four Cd atoms (Mode IV of Scheme 1). Cd1 ion and symmetry-related Cd1A (-x, −1-y, 2-z) are connected by two bidentate-chelating carboxylate groups and two μ2-η2:η1 bridging carboxylate groups to generate a [Cd2(CO2)4] binuclear SBU-a (rose polyhedron) (Figure S4b) with the Cd···Cd distance about 3.808 Å. Cd3 ion and symmetry-related Cd3G (1-x, 1-y, 1-z) are also bridged by two μ2-η2:η1 bridging carboxylate groups and two bidentatechelating carboxylate groups, giving rise to a [Cd2(CO2)4] binuclear SBU-b. Nevertheless, the separation of Cd−Cd across the binuclear unit SBU-b is 3.994 Å. Cd2 ion and symmetryrelated Cd2G (1-x, 1-y, 1-z) are ligated with SBU-b by two synanti-μ2-η1:η1 bridging carboxylate groups, resulting in the formation of the tetranuclear SBU-c (turquoise polyhedron) (Figure S4c), which is a linear linkage. The binuclear SBUs-a [Cd2(CO2)2] and the tetranuclear SBUs-c [Cd4(CO2)4] are further linked by L ligands to give a complicated 3D framework. The distance between these two kinds of SBUs (SBU-a and SBU-c) is 18.364 Å. 4,4′-bipy just plays a role in connecting SBU-a and SBU-c, which add the connecting numbers of SBU-a and SBU-c. The L3− anion in mode I acts as a μ3-bridge linking one SBUa and two SBUs-c, in which three carboxylate groups adopt μ2η2:η1, μ1-η1:η1, and μ1-η1:η1 coordination fashions, respectively. The L3− anion in mode IV also serves as a μ3-bridge linking one SBU-a and two SBUs-c, in which three carboxylate groups adopt syn-anti-μ2-η1:η1, μ1-η1:η1, and μ1-η1:η1 coordination fashions, respectively. Better insight into the nature of this intricate framework can be accessed by the topological method. As discussed above, the L3− anion in mode I and mode IV can be regarded as 3-connected nodes, separately. As for SBU-a, it links four L ligands and two 4,4′-bipy ligands; hence, SBU-a can be treated as a 6-connector. As for SBU-c, it links eight L ligands and two SBUs-a. Thereby, SBU-c can also be regarded

Crystal Structure of {[Cd3L2(btbb)(H 2O)2]·2EtOH· 1.5H2O}n (3). The crystal structure determination reveals that complex 3 crystallizes in the triclinic crystal system P1̅. As shown in Figure S3a, the asymmetric unit consists of two independent CdII atoms (the occupancy rate of Cd1 and Cd2: 1 and 0.5), one L ligand, half a btbb ligand, one associated water molecule, one free EtOH molecule, and three-quarters of lattice water molecule. The Cd1 ion is seven-coordinated by five carboxylate oxygen atoms (O1, O7B, O8B, O9C, and O10C) from three different L3− ligands (Cd1−O distances range from 2.360(4) to 2.458(5) Å) as well as two nitrogen atoms (N1 and N2) from one btbb (Cd1−N1 = 2.447(6), Cd2−N2 = 2.320(5) Å), giving rise to a pentagonal biyramid coordination geometry. The bond angles around Cd1 range from 52.9(2)° to 149.7(2)°. The Cd2 ion is in an octahedral coordination sphere, which is ligated by four oxygen atoms (O1, O1A, O9C, and O9D) from four different L3− ligands (Cd2−O1 = 2.250(4), Cd2−O1A = 2.250(4), Cd2−O9C = 2.346(5), Cd2−O9D = 2.346(5) Å) at the equatorial position, and two coordinated water O atoms (O3 and O3A) at the axial position (Cd2−O3 = 2.290(5), Cd2−O3A = 2.290(5) Å). The bond angles around Cd2 range from 79.34(16)° to 180.00(2)°. In this structure, the L3− ligand has one coordination mode to coordinate with five Cd2+ cations through three carboxyl oxygen atoms, in which one carboxyl group adopts a chelating coordination mode, another in bridging coordination mode, and the third one in chelating/bridging mode (Mode III of Scheme 1). The btbb ligand adopts symmetric transconformation with Ndonor···N−Csp3···Csp3 torsion angles of 150.099°. Cd1 ion, Cd2, and symmetry-related Cd1A (1-x, 1-y, 2-z) ion are bridged by two μ2-η2:η1 chelating/bridging carboxylate groups and two μ2-η1:η1 bridging carboxylate groups to provide a trinuclear [Cd3(CO2)4] SBU (green polyhedron) (Figure S3b) with the Cd···Cd distance about 3.716 Å. Each trinuclear SBU is surrounded by eight organic ligands (six L ligands and two btbb), and each L ligand is connected to three trinuclear [Cd3(CO2)4] units. In this way, the L anions link the trinuclear units to form a two-dimensional (2D) layer (Figure S3c), which is further extended by the btbb ligands to generate a 3D framework (Figure S3d). To better understand the nature of this intricate framework, topology analysis is provided: the L anions can be regarded as 3-connected nodes, the trinuclear [Cd3(CO2)4] SBU can be considered as 8-connected nodes, and the btbb ligands can be viewed as linkers (Figure 3a). Thus, the whole 3D framework can be represented as a (3,8)-connected tfz-d net with (43)(46· 618·84) topology (Figure 3b). Crystal Structure of {[Cd6L4(bipy)2(H2O)6]·3H2O}n (4). The crystal structure determination reveals that complex 4

Figure 3. Crystal structures of 3: (a) Defined 3- and 8-connected nodes. (b) Schematic representation of topological net of 3. 3038

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Analyses of Thermal Stability. To estimate the stability of the four polynuclear CdII polymers, thermogravimetric analyses (TGA) were performed as exhibited in Figure S6 (in the Supporting Information). For complex 1, the TGA displays a weight loss of 4.43% from 95 to 209 °C corresponding to the release of one lattice water molecule and two coordinated water molecules (calcd, 4.62%), and then, a plateau region is observed. The overall framework of 1 begins to decompose from 339 °C, corresponding to the decomposition of three coordinated water molecules and L3−, and the CdO residue of 33.94% (calcd, 33.10%) is observed at 478 °C. Complex 2 exhibits that the first weight loss of 5.80% in the range of 107− 186 °C is related to the losses of two and a half lattice water molecules as well as two coordinated water molecules (calcd, 5.46%). The overall framework of 2 begins to decompose from 357 to 559 °C, corresponding to the decomposition of L3− and hbmb, and the CdO residue of 27.50% (calcd, 25.97%) is observed. The TGA curve of complex 3 shows a weight loss of 6.68% in the range of 107−172 °C corresponding to two free EtOH molecules and one and a half lattice water molecules (calcd, 6.94%). The removal of the framework occurs in the range of 377−663 °C, which is related to the losses of two coordinated water molecules as well as L3− and btbb. The remaining weight corresponds to the formation of CdO (obsd, 23.32%; calcd, 22.46%). For complex 4, the first weight loss is about 2.62% from 108 to 144 °C, corresponding to the release of three lattice water molecules and one coordinated water molecule (calc. 2.79%). The second weight loss of 1.79% between 144 and 200 °C corresponds to the release of three coordinated water molecules (calc. 2.09%). Then, the framework of 4 begins to decompose at 399 °C, corresponding to the losses of two coordinated water molecules as well as the decomposition of L3− and 4,4′-bipy. Finally, the CdO residue of 30.22% (calcd, 29.79%) is obtained at 491 °C. Photophysical Properties and Optical Band Gaps. As we know, the photophysical studies of the ligands (H3L, hbmb, btbb, and 4,4′-bipy) and the complexes 1−4 at room temperature in solid state are conducive to preliminarily picking out the efficient systems for the further photocatalytic decomposition experiments of the organic dyes. UV−vis absorption spectra of the ligands and the complexes 1−4 are depicted in Figure S7. The ligand H3L presents strong absorption from 200 to 440 nm with two peaks at ca. 254 and 331 nm. The absorption bands of hbmb stretch from 200 to 350 nm with two peaks at 249 and 286 nm, while those of btbb spread from 200 to 450 nm with the peaks at 245 and 323 nm. As for 4,4′-bpy, it shows two strong absorptions at 236 and 296 nm in the range of 200−400 nm. Such absorption bands could be assigned to the π−π* transitions of the aromatic rings in the free ligands, respectively.15,16 Similarly, complex 1 shows strong absorption from 200 to 450 nm (λmax = 253 nm, 310 nm), and complex 2 displays two absorption bands with two peaks at 258 and 329 nm in the range of 200−450 nm. At the same time, complex 3 also exhibits two absorption peaks at 250 and 326 nm, while complex 4 embodies two absorption peaks at 257 and 333 nm in a similar range. In comparison with the above results, the lowest energy absorption bands in the H3L ligand are lower than that of N-donor ligands in the energy level, which indicates that H3L ligand might have more influence than the lowest energy absorption band of N-donor ligands by the coordination with the metal ion. The lowest energy absorption bands of complex 1 are distinctly blueshifted (21 nm) in comparison to H3L. For 2, 3, and 4, their

as a 10-connector (Figure 4a). According to the simplification principle, the resulting structure of complex 4 is a (3,6,10)-

Figure 4. Crystal structures of 4: (a) Defined 3-, 6-, and 10-connected nodes. (b) Schematic representation of topological net of 4.

connected net with point symbol of (42·68·85)(42·6)2(46·620·815· 94) calculated by the TOPOS program, which is a new topology (Figure 4b). It should be pointed out that trinodal (3,6,10)connected nets are exceedingly scarce as a consequence of the arduous formation of highly connected polynuclear units.14 Effect of Diverse Coordination Modes of H3L and NDonor Coligands on the Structures of Complexes 1−4. The aforementioned results reveal that the thaumaturgic coordination fashions of carboxylate groups from H3L ligands have important influence on the fabrication of the ultimate structures. Besides that, auxiliary N-donor ligands also play a definite role in deciding the structural diversities of complexes 2−4. In 1, the trinuclear [Cd3(CO2)6] units are linked by the L3− ligands adopting the (κ2-κ1-μ2)-(κ2-κ1-μ2)-(κ2)-μ5 coordination mode (Mode I) to yield a 2D layer, which is further bridged by the L3− ligands adopting the (κ2)-(κ2-κ1-μ2)-(κ2)-μ5 coordination mode (Mode II) to afford the overall 3D network with point symbol of (43·66·86)(42·6)(4·62). Different from 1, the L3− ligands in 2 show one kind of coordination mode: (κ2κ1-μ2)-(κ1-μ2)-(κ2)-μ5 (Mode III). The CdII ions are bridged by the carboxylate groups to produce the two kinds of trinuclear units (SBU-A and SBU-B). Ultimately, the L3− ligands connect SBUs-A and SBUs-B, singly, resulting in the formation of two 1D double chains. The two kinds of 1D double chains are interconnected in a parallel manner and united together to afford a 2D layer. The 2D sheets are joined by hbmb with asymmetric trans-conformation into a graceful 3D structure with point symbol of (43)2(46·618·84)2. The L3− ligands in 3 also adopt coordination Mode III. However, the L anions link the sole kind of trinuclear SBU to accomplish a two-dimensional (2D) layer, which are held together via btbb ligands with symmetric trans-conformation to generate a 3D framework with point symbol of (43)(46·618·84). The H3L ligands, which take two different coordination modes, (κ2-κ1-μ2)-(κ2-κ1-μ2)-(κ2)-μ5 (Mode I) and (κ2)-(κ2)-(κ1-κ1)-μ4 (Mode IV), connect the binuclear SBUs and the tetranuclear SBUs to give a complicated 3D framework with point symbol of (42·68· 85)(42·6)2(46·620·815·94) in 4. Here, 4,4′-bipy just plays a part in adding the connecting numbers of the binuclear SBUs and the tetranuclear SBUs, leading to the production of the highly connected polynuclear units. 3039

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Figure 5. Absorption spectra of the MB solution during the decomposition reaction under UV light irradiation with the presence of complexes 1−4.

were investigated through the measurements of their diffuse reflectivity. The band gaps (Eg) were defined as the intersection point between the energy axis and the line extrapolated from the linear portion of the adsorption edge in a plot of Kubelka− Munk function F versus energy E. Kubelka−Munk function, F = (1 − R)2/2R, was transformed from the recorded diffuse reflectance data, where R is the reflectance of an infinitely thick layer at a given wavelength. The F vs E plots are revealed in Figure S9, and the Eg values evaluated from the steep absorption edge are 3.08 eV for 1, 3.16 eV for 2, 3.10 eV for 3, and 3.20 eV for 4, respectively, which manifests that complexes 1−4 are underlying semiconductive materials.20 Hence, due to the smallest band gap of 1, it might represent the best performance in photocatalytic degradation of the organic dye in water. Photocatalysis. To research in detail the photocatalytic activity of complexes 1−4, methylene blue (MB) and methyl orange (MO) were selected as models of dye contaminant. In cases, 80 mg powder of the title complexes and 30% H2O2 (10 μL) were added into a 150 mL MB/MO (4 × 10−5 mol L−1) aqueous solution, magnetically stirred in the dark for 1 h to ensure the equilibrium of adsorption/desorption. Afterward, the solution was under the irradiation of a 500 W high-pressure mercury lamp. The solution was kept continuously stirring with the aid of a magnetic stirrer during the irradiation process. At ca. 20 min intervals, 5 mL transparent sample solution was obtained from the vessel, and analyzed by a UV−visible spectrometer. The control experiment was also accomplished under the uniform conditions without any catalyst. The characteristic absorption of methylene blue (MB) and methyl orange (MO) at about 664 nm (MB) and 465 nm (MO),

lowest energy absorption bands show smaller hypochromatic shift (2 nm, 5 nm, 2 nm), singly, compared with that of H3L. The situation may be vested in the coordination of the H3L to the Cd(II) ion increasing the energy gap of the intraligand (IL, π → π*) transition. Moreover, the solid-state luminescences of complexes 1−4, the free ligand H3L, and the relevant auxiliary ligands are also investigated at ambient temperature. As reported before,17 the free ligand H3L displays a relatively strong emission band at 398 nm (λex = 333 nm). Meanwhile, by the literature,18 the ligand hbmb shows one weak emission at 306 nm (λex = 275 nm), while the emission of 4,4′-bipy is 428 nm (λex = 350 nm). The photoluminescent spectrum of btbb exhibits a weak emission at 377 nm (λex = 323 nm, Figure S8-a). For complex 1, excited at 330 nm, it gives rise to an emission band at 387 nm (Figure S8b). As compared to the H3L ligand, a hypsochromic shift (11 nm) is observed. The emission spectrum of complex 2 displays an emission band centered at 374 nm with excitation maximum at 300 nm, and the emission band presents larger hypsochromic shifts (24 nm) compared with that of the ligand H3L. With respect to the ligand H3L and btbb, the emission (λem = 373 nm, λex = 333 nm) of 3 is hypsochromically shifted (∼25 and 4 nm, singly). For complex 4, the emission band (λem = 376 nm, λex = 321 nm) is blue-shifted (22 and 52 nm) corresponding to those of the H3L and 4,4′-bipy group, respectively. Obviously, illustrated by the Figure S8-b, the emission spectra of 1−4 are parallel to that of the H3L ligand, which may also be mainly ascribed to the intraligand emission of H3L.16 As shown in the reported examples,19 the optical band gap (Eg) was one principal factor affecting the speed of the photocatalysis degradation process of the organic dye. Illuminated by this, the band gaps (Eg) of complexes 1−4 3040

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Figure 6. Absorption spectra of the MO solution during the decomposition reaction under UV light irradiation in the presence of complexes 1−4.

respectively were chosen to monitor the photocatalytic degradation process. The control experiments show that the photocatalytic decomposition rate of MB without catalyst is 19.8% and that of MO is 12.6% after irradiation of 180 min. Interestingly, all of complexes display the photocatalytic activities under highpressure mercury lamp irradiation. As illustrated in Figure 5 and Figure 6, the absorption peak of MB/MO decreased distinctly with the irradiation time increasing from 0 to 180 min in the presence of 1−4. In addition, the changes in the Ct/C0 plot of MB/MO solutions versus irradiation time are drawn in Figure 7 and Figure 8 (wherein C0 is the initial concentration of the MB/MO solutions and Ct is the concentration at time t of the MB/MO solutions). The calculation results show that 88.7% for complex 1, 65.9% for complex 2, 85.8% for complex 3, and 63.8% for complex 4 of the MB have been dissociated after 180 min of irradiation. Nevertheless, approximately 59.8% for complex 1, 47.3% for complex 2, 51.4% for complex 3, and 80% for complex 4 of the MO have been decomposed after 180 min. In the existence of catalyst, the increased photocatalytic decomposition rate indicates that complexes 1−4 are vigorous for the decomposition of MB/MO under UV irradiation. As mentioned in the literature,21 the photocatalytic mechanism is deduced as follows: the electrons (e−) of complexes were excited from the valence band (VB) to the conduction band (CB). The same number of holes (h+) that have oxidation remained in the valence band. Furthermore, O2 or hydroxyl (OH−) adsorbed on the surfaces of complexes could interact with the electrons (e−) on the CB or the hole (h+) on the VB, respectively, which probably formed the hydroxyl radicals (·

Figure 7. Photocatalytic decomposition of MB solution under UV light irradiation with the use of complexes 1−4 and the control experiment without any catalyst under the same conditions.

OH). Later on, the ·OH radicals could degrade the organic dye effectively to accomplish the photocatalytic process. Popularly, the narrower band gap is conducive to the separation of the charge. As calculated, the band gaps of complexes 1−4 are 3.08, 3.16, 3.10, and 3.20 eV, respectively. Distinctly, the band gaps of 1, 2, 3, and 4 defer to the order 1 < 3 < 2 < 4. For MB, the degradation rate agrees with 1 > 3 > 2 > 4, which is perfectly consistent with the theoretical result. Meanwhile, the degradation rate of MO follows the order 4 > 1 > 3 > 2. Apparently, for complexes 1, 2, and 3, the degradation 3041

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Article

ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic data, selected bond lengths and bond angles, powder X-ray patterns, thermogravimetric curve, UV− vis absorption spectra, photoluminescences, diffuse reflectance UV−vis-NIR spectra for 1−4 and parts of figures. This material is available free of charge via the Internet at http://pubs.acs. org/.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *Fax: (86) 371-67761744; E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation (Nos 21371155 and 21301157) and Research Found for the Doctoral Program of Higher Education of China (20124101110002).

Figure 8. Photocatalytic decomposition of MO solution under UV light irradiation with the use of complexes 1−4 and the control experiment without any catalyst under the same conditions.

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rate of MO matches with the order of the band gaps. Extraordinarily, complex 4 gives better photocatalytic activity in the degradation process of MO, which may be influenced by its special construction, because of the small difference of the optical band gap between 4 and other complexes (ΔEg ≤ 0.12 eV). As reported in the literature,19 multinuclear CdII clusters are possibly in favor of the degradation process of dyes. Herein, the framework of 4 is made up of the two kinds of different nuclei: dinuclear and tetranuclear CdII clusters. The cooperation of dinuclear CdII clusters and tetranuclear CdII clusters in 4 perhaps play a beneficial role in the degradation process of MO. Besides, the miraculous situation might be brought about from the different structures of MB and MO. The “V”-shaped MB involves a basic group, whereas the linear MO includes a basic group and an acidic group. The distinction probably leads to the different adsorption and desorption of O2/hydroxyl (OH−) on the surface of 4, and affects the formation of the ·OH radicals, which could further trigger the different degradation rate. Based on the above discussion, we speculate that the cluster complexes containing different kinds of nuclei may exert a different impact on the decomposition of disparate organic dyes. The more elaborate mechanism is also under investigation. In addition, the aforesaid phenomenon also makes known that 1−4 reveal laudable photocatalytic activities in the degradation of MB/MO under UV irradiation.

REFERENCES

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CONCLUSIONS In a nutshell, a series of coordination complexes containing CdII clusters have successfully been fabricated. Complexes 1−4 show strong fluorescence emissions, signifying that these complexes may be good candidates for use as optical materials. The study on optical energy gaps of complexes 1−4 attests that these complexes are latent semiconductive materials. Strikingly, complexes 1−4 present good photocatalytic activity for the decomposition of MB and MO under high-pressure mercury lamp irradiation. The cluster complexes containing different kinds of nuclei may exert different impact on the decomposition of disparate organic dyes. Currently, further research is underway to prepare other new cluster-based coordination complexes with better photocatalytic activities for the degradation of other organic dyes. 3042

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