Polyoxometalate-Induced New Self-Assemblies Based on Copper

Jul 3, 2013 - ... ligands (bis(3-(2-pyridyl)pyrazole-1-ylmethyl)benzene, bppmb) led to the ... [CuIILo]2H[BW12O40]·4H2O (1), [CuILo]4[SiW12O40]·5H2O (...
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Polyoxometalate-Induced New Self-Assemblies Based on Copper Ions and Bichelate-Bridging Ligands Xin Wang, Mao-Mao Zhang, Xiu-Li Hao, Yong-Hui Wang,* Ying Wei, Fu-Shun Liang,* Li-Jie Xu, and Yang-Guang Li* Key Laboratory of Polyoxometalate Science of Ministry of Education, Department of Chemistry, Northeast Normal University, Renmin Street No. 5268, Changchun, Jilin 130024, People’s Republic of China S Supporting Information *

ABSTRACT: The introduction of polyoxometalates (POMs) into the metal− organic complex synthetic system containing copper ions and bichelatebridging ligands (bis(3-(2-pyridyl)pyrazole-1-ylmethyl)benzene, bppmb) led to the isolation of three new organic−inorganic hybrid compounds [CuIILo]2H[BW12O40]·4H2O (1), [CuILo]4[SiW12O40]·5H2O (2), and [CuLp]3H2[BW12O40]·5H2O (3) (Lo = 1,2-bppmb and Lp = 1,4-bppmb). All three compounds were hydrothermally synthesized and characterized by elemental analyses, IR spectra, TG analyses, powder X-ray diffraction, and single-crystal X-ray diffraction analyses. In these compounds, the use of Keggintype POMs induces the formation of a series of new metal−organic secondary building units based on the copper ions and bppmb ligands such as helical chain and chiral Möbius Strip, which further form interesting supramolecular self-assemblies with the polyoxoanions. The degradation of Rhodamine-B (RhB) under UV irradiation with compounds 1−3 as the heterogeneous photocatalysts was investigated, and these compounds show good photocatalytic properties for RhB degradation.

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INTRODUCTION Polyoxometalate(POM)-based organic−inorganic hybrid compounds have attracted great interest in crystal engineering. In the last two decades, the introduction of POMs into the metal− organic coordination polymer systems has led to abundant and changeable structural topologies. Furthermore, the combination of POMs and metal−organic units has imparted the hybrid compounds “added-values”, which exhibit potential applications in catalysis, adsorption, luminescent, electronic, and magnetic materials.1−3 In this research field, the N-donor organic ligands, transition metal ions, and various kinds of polyoxoanions constitute one of the typical reaction systems to assemble POM-based hybrid compounds.4−10 An important issue in this subfamily is the design and use of organic N-donor ligands. According to the current known reports, N-donor ligands are generally classified into two types, the chelate and the bridging ligands. The latter have been extensively explored to build extending hybrid coordination polymers (CPs) or metal− organic frameworks (MOFs),4−8 whereas the former show no advantages in this respect because they usually inhibit the extension of metal−organic units.9,10 Nevertheless, these chelate ligands still exhibit some extra properties in contrast to the bridging ligands. First, most bi- or multidentate chelate ligands display stronger coordination ability to transition metal (TM) ions than do the bridging ligands with monodentate Ndonor groups (for example, the pyridine group). Second, the chelate ligands possess definite coordination modes with TM ions, such as the {ML3} hexa-coordinated octahedral geometry © XXXX American Chemical Society

and {ML2} tetra-coordinated distorted tetrahedral geometry (L represents the bidentate chelate ligands), which are suitable for the design and synthesis of desirable assemblies. Third, the above {ML3} and/or {ML2} metal−organic units exhibit potential chiral and/or helical properties and can serve as important nodes to design new chiral composite materials.11 If things go further that the chelate and bridging functions can be combined into the same organic N-donor ligand, it may show potential advantages to build new POM-based hybrid compounds with chiral, helical, cage-like, or even porous structural features. On the basis of aforementioned considerations, we choose a type of ligands combining both chelate and bridging functions, bis(3-(2-pyridyl)pyrazole-1-ylmethyl)benzene (abbreviated bppmb) (as shown in Scheme 1), as the building blocks to explore new POM-based hybrid compounds.12 In comparison to the common chelate (such as 2,2-bipyridine and 1,10phenanthroline) or bridging ligands (exemplified by 4,4′bipyridine and biimidazole ligands), the POM-based hybrid compounds by using such bichelate-bridging ligands have been far unexplored.13,14 Ward et al. made an important contribution to the TM-bppmb complex system. Currently, the bppmb ligands are mainly employed in the design and synthesis of discrete metal−organic cage-like complexes.15 In these Received: March 7, 2013 Revised: July 1, 2013

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irradiation. At given time intervals, aqueous samples with a fixed volume of 3 mL were taken out from the quartz glass tubes, centrifuged, and then filtered. The solution was monitored by a SP1900 UV−vis spectrophotometer. Synthesis of 1. A mixture of Cu(NO3)2·3H2O (0.24 g, 1 mmol), Lo (0.039 g, 0.1 mmol), and K5[BW12O40]·15H2O (0.332 g, 0.1 mmol) was dissolved in 10 mL of distilled water and stirred for 30 min. The pH of above mixture was adjusted to ca. 4.6 with 0.1 M HNO3, and then the suspension was transferred into a Teflon-lined autoclave and kept under autogenous pressure at 180 °C for 5 days. After the autoclave was slowly cooled to room temperature, green block crystals of 1 were obtained in 43% yield (based on W). Anal. Calcd for C48H49BCu2N12O44W12: C, 14.99; H, 1.28; N, 4.37; B, 0.29; W, 57.46; Cu, 3.28. Found: C, 15.21; H, 1.40; N, 4.30; B, 0.33; W, 57.50; Cu, 3.31. IR (KBr pellet, cm−1): 3436(s), 3137(w), 1614(m), 1503(w), 1443(m), 1335(w), 1234(m), 1086(w), 1002(w), 960(m), 911(m), 819(s), 529(w), 421(w). The TG analysis of 1 reveals that the first weight loss (obsd 2.00%) in the temperature range of 90−120 °C is attributed to the loss of all lattice water molecules (calcd 1.88%). The purity of the crystalline compound 1 is confirmed by the PXRD measurement. Synthesis of 2. A mixture of CuCl2·2H2O (0.17 g, 1 mmol), Lo (0.039 g, 0.1 mmol), and K4[SiW12O40]·15H2O (0.330 g, 0.1 mmol) was dissolved in 10 mL of distilled water and stirred for 30 min. The pH of above mixture was adjusted to ca. 4.5 with 0.1 M HCl, and then the suspension was transferred into a Teflon-lined autoclave and kept under autogenous pressure at 180 °C for 5 days. After the autoclave was slowly cooled to room temperature, yellow block crystals of 2 were obtained with the yield of 19% based on W. Anal. Calcd for C96H90Cu4N24O45SiW12: C, 24.07; H, 1.90; N, 7.02; Si, 0.58; W, 46.12; Cu, 5.26. Found: C, 24.39; H, 2.10; N, 7.16; Si, 0.52; W, 46.19; Cu, 5.20. IR (KBr pellet, cm−1): 3437(s), 1636(m), 1533(m), 1430(w), 1236(s), 1070(w), 961(w), 918(s), 807(s), 520(w), 427(w). The TG analysis of 2 reveals that the first weight loss (obsd 2.00%) in the temperature range of 100−105 °C is attributed to the loss of all lattice water molecules (calcd 1.88%). The purity of the crystalline compound 1 is confirmed by the PXRD measurement. When K5[BW12O40]·15H2O (0.332 g, 0.1 mmol) was used to substitute the K4[SiW12O40]·15H2O in above reaction system, an isostructural compound [CuILo]4H[BW12O40]·5H2O 2a was also prepared with the yield of 12% based on W (see the Supporting Information). Synthesis of 3. CuCl2·2H2O (0.17 g, 1 mmol), Lp (0.039 g, 0.1 mmol), and K5[BW12O40]·15H2O (0.332 g, 0.1 mmol) was dissolved in 10 mL of distilled water and stirred for 30 min. When the pH of above mixture was adjusted to ca. 4.4 with 0.1 M HCl, the suspension was transferred into a Teflon-lined autoclave and kept under autogenous pressure at 180 °C for 5 days. After the autoclave was slowly cooled to room temperature, red block crystals of 3 were isolated with the yield of 70% (based on W). Anal. Calcd for C72H72BCu3N18O45W12: C, 20.02; H, 1.68; N, 5.84; B, 0.26; W, 51.15; Cu, 4.37. Found: C, 20.31; H, 1.85; N, 5.69; B, 0.22; W, 51.21; Cu, 4.43. IR (KBr pellet, cm−1): 3422(s), 3128(w), 1603(m), 1499(w), 1433(m), 1355(w), 1294(w), 1239(m), 1156(w), 1066(w), 998(w), 951(m), 907(m), 822(s), 511(w), 421(w). The TG analysis of 3 reveals that the first weight loss (obsd 2.11%) in the temperature range of 90−120 °C is attributed to the loss of all lattice water molecules (calcd 2.09%). The purity of the crystalline compound 1 is confirmed by the PXRD measurement. X-ray Crystallography. The suitable single crystal of each compound was glued on a glass fiber. Data collection was performed on a Rigaku RAXIS RAPID IP diffractometer with Mo Kα (λ = 0.71073 Å) at room temperature. A multiscan correction was applied. The structure was solved by the direct method and refined by fullmatrix least-squares on F2 using the SHELXL 97 program.19 During the refinement of these crystal structures, all non-hydrogen atoms were refined anisotropically. The H atoms on their mother carbon atoms were located in calculated positions. During the refinement, the restrained command “ISOR” was used to restrain some non-hydrogen atoms with ADP or NPD problems in the crystal structures of 1−3. Moreover, some six-member and five-member rings of the organic

Scheme 1. Schematic Views of the Ligand Configurations: (a) “syn”-Lo, (b) “anti”-Lo, (c) “syn”-Lp, and (d) “anti”-Lp

complexes, BF4− and ClO4− inorganic ions with a tetrahedral geometry play an important template role in assembling the final metal−organic cages. Enlightened by the above reports, we presume that if simple inorganic anions can be substituted by various polyoxoanions to induce the self-assembly between TM ions and the bppmb ligands, POM units with different structural types, charges, and sizes may result in new effects on the assembling process and the isolation of new hybrid complexes. In this work, we report the first examples of POMinduced hybrid complexes based on the copper ions and bppmb ligands, [Cu I I Lo] 2 H[BW 1 2 O 4 0 ]·4H 2 O (1), [CuILo]4[SiW12O40]·5H2O (2), and [CuLp]3H2[BW12O40]·5H2O (3) (Lo = 1,2-bppmb and Lp = 1,4-bppmb). It is noteworthy that the introduction of Keggintype polyoxoanions into the Cu-bppmb reaction systems led to the isolation of a series of new coordination units in POMbased hybrids such as helical chains and discrete chiral Möbius Strip. These metal−organic secondary building units are further stacked into 3-D supramolecular assemblies with the Keggintype polyoxoanions. The photocatalytic properties of compounds 1−3 have been investigated.

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EXPERIMENTAL SECTION

Materials and Methods. All reagents and solvents for the synthesis were purchased without further purification. The Keggintype polyoxometalates H4[SiW12O40]·19H2O and K5[BW12O40]·15H2O were synthesized according to the literature method and characterized by the IR and TG analysis.16,17 The bichelate-bridging ligands Lo and Lp were prepared by the reaction of 1,2-bis(bromomethyl)benzene and 1,4-bis(bromomethyl)benzene with 3-(2-pyridyl)pyrazole according to literature methods, respectively.12,18 Elemental analyses (C, H, and N) were performed on a Perkin-Elmer 2400 CHN elemental analyzer. B, Si, W, and Cu were analyzed on a Leaman inductively coupled plasma (ICP) spectrometer. The FT-IR spectra were recorded from KBr pellets in the range 4000− 400 cm−1 on a Mattson Alpha-Centauri spectrometer. Thermogravimetric analysis (TGA) was performed on a Perkin-Elmer TG-7 analyzer heated from 40 to 600 °C under nitrogen. Powder X-ray diffraction (PXRD) studies were performed with a Rigaku D/max-IIB X-ray diffractometer at a scanning rate of 1° per minute ranging from 5° to 50°, using Cu Kα radiation (λ = 1.5418 Å). The solid-state emission/excitation spectra of compounds 1−3 were measured on a SPEX FL-2T2 spectrofluorimeter equipped with a 450 W xenon lamp as the excitation source. Photocatalysis experiments were investigated as follows: a sample (10.0 mg) was mixed with 400 mL of 20.0 mg L−1 Rhodamine-B (RhB) solution, followed by magnetically stirring in the dark for about 10 min. The solution was then exposed to UV irradiation from a 500 W Hg lamp at a distance of 4−5 cm between the liquid surface and the lamp. The solution was kept stirring during B

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Table 1. Crystal Data and Structure Refinement for 1−3 compound formula Mr T/K cryst syst space group a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 Z μ/mm−1 F(000) reflns Rint data/restraints/parameters GOF R1 [I > 2σ(I)]a wR2 (all data)b Δρmax,min/e Å−3 a

1

2

3

C48H49N12O44BW12Cu2 3842.08 150(2) triclinic P1̅ 11.2615(6) 11.9601(6) 14.7931(6) 78.121(4) 84.854(4) 88.142(4) 1941.74(16) 1 18.325 1724 12 210 0.0496 6823/120/568 1.044 0.0526 0.1051 1.616 and −2.185

C96H90N24O45SiW12Cu4 4788.37 150(2) monoclinic P21/c 15.919(3) 16.025(3) 27.627(8) 90 123.20(2) 90 5897(2) 2 12.457 4424 41 340 0.1261 10 197/648/773 1.052 0.0725 0.1368 2.086 and −1.440

C72H72N18O45BW12Cu3 4317.11 296(2) cubic P213 21.6328(9) 21.6328(9) 21.6328(9) 90 90 90 10 123.7(7) 4 14.281 7880 51 672 0.1183 5991/254/388 0.953 0.0415 0.0844 1.048 and −0.910

R1 = ∑∥Fo| − |Fc∥/∑|Fo|. bwR2 = ∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]1/2.

ligands were also restrained with the “AFIX” and “DELU” commands. All above restrained refinements led to relatively high restraint values of 120, 648, and 254 for crystal structures 1−3, respectively. Furthermore, compounds 2 and 3 exhibit extra solvent accessible voids in the final refinement, but the residual peaks are too weak to be confirmed as solvent molecules. Thus, the SQUEEZE program20 was used to further estimate the possible solvent accessible voids and the number of solvent water molecules in the crystal structure. It is worth mentioning that the new 2.hkl file generated by the SQUEEZE program cannot be used to further refine the crystal structure of 2 due to the unreasonable final R value. Thus, the extra five solvent water molecules were directly included in the final molecular formula of 2 based on the elemental analysis and TG analysis. The new 3.hkl file generated by the SQUEEZE program was used to further refine the whole crystal structure of 3, and the extra five solvent water molecules were directly included in the final molecular formula of 3 based on the elemental analysis and TG analysis as well as the SQUEEZE calculation result. The crystal data and structure refinement of compounds 1−3 are summarized in Table 1. Selected bond lengths and angles for 1−3 are listed in the Supporting Information, Tables S1−S3. The CCDC reference numbers are 901992, 901993, and 901995 for the title compounds.

the isolation of compound 2 with a relatively low yield. It is presumed that the Keggin-type POM salts with different negative charges may exhibit somewhat different solubilities and pH in the hydrothermal environment; thus the synthetic conditions (including temperature, pH, and time) should be precisely modulated. Furthermore, it is noteworthy that the Ndonor ligand bppmb possesses some reductive property and can reduce Cu2+ into Cu+ ions in the hydrothermal conditions exemplified by compounds 2 and 3. Nevertheless, when a suitable oxidant is introduced into the reaction system, the above redox reactions can be effectively inhibited. In the synthesis of compound 1, the NO3− ions in the acidic reaction system act as an oxidant and prevent the reduction of Cu2+ into Cu+ ions.8 Structural Description. The single crystal X-ray diffraction analysis shows that compound 1 crystallizes in the triclinic space group P1̅. The basic structural unit contains two [CuIILo] moieties, one [BW12O40]5− cluster, and four lattice water molecules. The whole compound was further charge-balanced by an extra proton, considering the weak acidic reaction environment. In the [CuLo] unit (Figure 1a and Figure S1), the Cu(II) center is five-coordinated with four N atoms derived from the Lo ligand and one terminal aqua-ligand, exhibiting a distorted tetragonal pyramid coordination geometry. The bond lengths of Cu−N are in the range of 1.941(1)−2.012(1) Å, and that of Cu−Oauqa is 2.665(1) Å. The oxidation state of Cu(II) center was confirmed by the bond-valence sum (BVS) calculation (Table S3),21 five-coordinated environment, and the green coloration of the crystalline compound 1. Similar to a few mononuclear Cu-bppmp complexes,22 the Lo ligand adopts a “syn”-style configuration and acts as a four-dentate chelate ligand to coordinate with one Cu center. The two bppmb planes in the [CuLo] unit are not in the same plane, and the dihedral angel is 42.207(5)° (Figure S1). It seems that these butterfly-type moieties are rather stable as they cannot only

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RESULTS AND DISCUSSION Synthesis. During the synthesis in the {Cu/bppmb/BW12} reaction system, it is interestingly found that all crystalline compounds were isolated in a pH = 4.4−4.6 aqueous media at 180 °C for 5 days, which seems to be a suitable conditions for the crystallization of this system. Use of K5BW12O40 as the precursor is due to the fact that this Keggin-type polyoxoanion possesses a relatively high negative charge and structural stability in weak acidic media, which is favorable for introducing more TM cations as nodes into the composite compounds and increasing the complexity of the final structural topologies.8 Simultaneously, other Keggin-type POMs such as H3PW12O40 and H4SiW12O40 were also employed as the precursors under similar synthetic conditions; however, only the latter resulted in C

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Figure 1. (a) Structure of [CuIILo] unit in 1. (b) Structure of Keggintype [BW12O40]5− polyoxoanion unit in 1. (c) 3-D supramolecular framework of 1 based on [CuIILo] and [BW12O40]5− units viewed along the a axis. The yellow tunnels represent the interspaces of the supramolecular framework of 1. The H atoms and lattice water molecules are omitted for clarity.

exist at room temperature22 but also in hydrothermal conditions. The polyoxoanion [BW12O40]5− exhibits the typical α-Keggin structural feature that the central {BO4} tetrahedron is surrounded by 12 {WO6} octahedra. Moreover, these {WO6} octahedra can be further assigned into four {W3O13} fragments that are corner-linked with each other and also with the central {BO4} tetrahedron. The central B atom is encompassed by eight half-occupied oxygen atoms, suggesting that the {BO4} group in the polyoxoanion possesses a 2-fold disorder. The cationic [CuLo] units and the negative Keggin-type polyoxoanions stack together into a Coulombic aggregation (Figure 1c) in which the adjacent [CuLo] units possess weak intermolecular C−H···π interactions between the C−H group of pyrazole ring and the adjacent plane of benzene ring with the C−H···π distance of ca. 3.076(3) Å (Figure S2). Interestingly, based on such weak intermolecular interactions, these [CuLo] units not only form 1-D supramolecular chains well separated by the Keggin-type clusters (Figure 1c), but also further constitute the wall of 1-D “water tunnels” along the a axis (see Figure 2a and b). Within the tunnels reside “water-chains” that consist of four lattice water molecules via hydrogen-bonding interactions. The typical hydrogen bond lengths are O1W···O4W′ 2.923(1) Å, O2W···O4W′ 2.880(1) Å, O3W···O4W 2.700 Å, O1W···O3W 3.010(1) Å, O1W···O2W 3.020(1) Å, and O2W···O2W″ 2.922(1)Å (Figure 2c). Compound 2 crystallizes in monoclinic space group P21/c. The basic structural unit contains a classical α-Keggin-type [SiW12O40]4− unit, two types of 1-D helical chains [CuILo]∞, and five lattice water molecules. In the 1-D helical chain (Figure 3), there are two Cu(I) centers, and both exhibit the tetracoordinated environment with four N atoms derived from two different Lo ligands. Different from the limited double-helix dinuclear copper species reported before,22 the helical chain in 2 extends along the b axis to form an infinite chain. The bond distances of Cu−N vary from 2.961(1) to 2.137(1) Å. The oxidation states of both Cu(I) centers were confirmed by the BVS calculations21 and the typical coordination environments of Cu(I) ions as well as the yellow coloration of the crystalline products. It is worth mentioning that the Lo ligands in 2 display two types of configuration, that is, Lo1 and Lo2 (Figure 3a and

Figure 2. (a) 3-D supramolecular framework of 1 viewed along the b axis. (b) 1-D “water tunnel” formed by the [CuLo] units. (c) “Waterchain” residing in 1-D tunnels of 1.

b), both belonging to the “anti”-style. The distances between two Cu(I) centers in Lo1 and Lo2 are 10.157(5) and 7.707(6) Å, respectively. In addition, Lo1 and Lo2 ligands are alternately linked by the Cu(I) centers to form the final helical chains along b axis with a screw pitch of 16.03(5) Å (Figures 3c and 4a). It is also interesting that the left- and right-helical chains are alternately arranged on the bc plane and the central cavities are occupied by the α-Keggin-type polyoxoanions (Figure 4b and Figure S3). Although no obvious H-bonding or π···π interactions exist between the adjacent helical chains, these cationic chains stack together with the polyoxoanions via the electrostatic forces. Compound 2a is isostructural with compound 2 except that the polyoxoanion is substituted by [BW12O40]5−. Compound 3 crystallizes in the cubic chiral space group P213. The basic structural unit contains a trinuclear ring-type [CuLp]3 metal−organic complex unit, an α-Keggin-type [BW12O40]5− unit, and five lattice water molecules. Given the weak acidic reaction environment, the whole compound is further charge-balanced by two extra protons. In the ring-type [CuLp]3 unit (Figure 5), all three Cu(I) centers exhibit the four-coordination environment with four N atoms derived from two different Lp ligands. The bond distances of Cu−N are in D

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Figure 5. Structure of the [CuLp]3 “Möbius Strip” unit in 3.

the range of 2.052(1)−2.198(9) Å. The oxidation states of Cu(I) centers were confirmed by the BVS calculations21 and the typical coordination environments of Cu(I) ions. In such a unit, all three Lp ligands show the “anti”-configuration and are connected to each other by the Cu(I) centers, forming a wellknown “Möbius Strip” (abbreviated MS) that possesses the chiral structural feature. It is noteworthy that a similar segment has been observed in a Cd-containing cage-like complex, in which the metal center possesses hexa-coordinated environment.23 In this case, the MS unit was isolated in a discrete mode with three tetra-coordinated Cu centers. Furthermore, only one type of MS exists in compound 3, and hence the chirality of the whole compound derives from these chiral units. To our knowledge, such an isolated trinuclear metal−organic MS has been observed for the first time in metal−organic complex systems. It is interesting that three adjacent MSs are closely linked to each other via strong π···π interactions between neighboring pyridine and pyrazole planes with a distance of 3.652(1) Å (Figures 6a and S4). Via this intermolecular connection mode, each MS unit is surrounded

Figure 3. Structure of Lo ligands with (a) Lo 1 and (b) Lo2 configurations in 2. (c) 1-D helical chain in 2.

Figure 4. (a) One pair of left- and right-helical chains in 2 arranged on the bc plane; the central cavities are occupied by the α-Keggin-type polyoxoanions. (b) Schematic view of the 3-D supramolecular framework of 2 based on the helical chains and POM units. The helical chains are represented by the imaginary helical lines, and the H atoms and lattice water molecules are omitted for clarity.

Figure 6. (a) View of the strong π···π interactions among three adjacent MS units and the schematic view; (b) schematic view of the chiral 3-D supramolecular framework of 3 exhibiting a SrSi2 structural topology (103). E

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with six other adjacent MS units, forming a chiral 3-D supramolecular framework with a SrSi2 structural topology (103) (Figures 6b and S5). Furthermore, the central large cavities in the 3-D framework are occupied by the α-Keggintype [BW12O40]5− polyoxoanions and lattice solvent water molecules. It is also noteworthy that the chiral compound 3 was isolated from the achiral precursors. Thus, the left-hand and right-hand enantiotropic single crystals of 3 may coexist in the final reaction system. To confirm this conjecture, the solid-state circular dichroic (CD) spectra of compound 3 have been measured to check the chirality of the crystalline product. During the measurement, one single crystal is too small to measure the CD signal. Therefore, the crystalline sample derived from one pot was divided into three equal masses of sections, named groups A, B and C, and their CD spectra were measured. As shown in Figure 7, all three CD spectra possess

Scheme 2. Schematic View of Effects of POMs, TM Ions, and Ligand Configurations on the Metal−Organic Structural Topologies

of TM ions so as to modulate the final TM-bppmb secondary building blocks. Third, the configuration of bppmb ligands also play an important role in constructing the TM-bppmb complex units. In the Lo (1,2-bppmb) ligand, the two chelate groups are close enough to be shared by one TM center, tending to form the isolated mononuclear metal−organic unit. In the Lp (1,4bppmb) ligand, the two chelate groups are well separated by the bridging groups, which is more suitable for assembling extending and ring-like structures. On the basis of the current compounds 1−3, Lm (1,3-bppmb) ligand may be also a good precursor to construct the new POM-based TM-bppmb hybrid compounds. This work is still ongoing in our group.

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PHOTOCATALYTIC PROPERTIES The use of photocatalysts to decompose waste organic molecules so as to purify the water resources has attracted great attention in recent years. In this research field, Rhodamine-B (RhB) is often employed as a typical model dye contaminant to evaluate the photocatalytic effectiveness in the purification of wastewater.24 Recently, POMs have been explored as one type of new potential photocatalysts in the degradations of organic dyes.25 However, the typical POM salts are generally water-soluble and may induce the secondary pollution if they are directly used as photocatalysts for the above purpose. Consequently, exploring heterogeneous POMbased photocatalysts is one of the key issues in this research field. POM-based inorganic−organic hybrid compounds prepared in the hydrothermal or solvothermal environments usually show a very low solubility or even insolubility in water, which reveals an ideal route to develop new photocatalysts for the degradations of organic dyes in aqueous media.26 Herein, all three compounds are insoluble in water, and therefore their heterogeneous photocatalytic performances were investigated via the experimental model of RhB photodegradation under UV irradiation. To further confirm the insolubility of all three compounds, B, Cu, Si, and W in the solution of RhB were analyzed on ICP spectrometer at the end of photocatalytic reaction. No signals of above elements can be detected, indicating that all compounds are insoluble or decomposed in the photocatalytic reaction. The solid-state UV−vis spectra of three compounds were also checked, showing absorption bands in the range of 200−350 nm (Figure S16). These characteristic peaks are mainly attributed to the O→W charge transfer transition of POM units, and such absorption bands will not overlap the UV−vis absorption peaks of RhB species during the photocatalytic experiments. Moreover, the photocatalytic properties of the [CuLo]Cl2,27 [Bu4N]5BW12 salt, and the

Figure 7. Solid-state CD spectra of compound 3 with the one-pot sample separated by three equal-mass groups (groups A, B, and C).

similar peak positions but different peak intensities, suggesting that both enantiotropic crystals of 3 may coexist in the system. Otherwise, all three CD spectra should possess the same peak intensity. Furthermore, all three CD spectra possess similar peak shape while not the opposite one, indicating that one kind of enantiomer should be obviously excessive. The Effect of POMs, TM Ions, and Ligand Configuration on the TM-bppmb Structural Units. In comparison with the known TM-bppmb complexes templated by simple anions such as BF4− and ClO4−,15 compounds 1−3 exhibit some different structural features (Scheme 2). First, the use of polyoxoanions with nanoscale sizes and high negative charges leads to new TM-bppmb complex units, such as helical chain and chiral Möbius ring. It is obvious that the introduction of POMs into the TM-bppmb reaction system can further modulate and extend the structural systems of such metal− organic coordination system. Second, under similar reaction conditions, the TM ions with different coordination modes can form different TM-bppmb units. In 1, Cu2+ ions adopt the fivecoordinated tetragonal pyramid environment, leading to the tetra-dentate Lo ligands chelated with one Cu(II) ions and hence the formation of mononuclear metal−organic units. In 2 and 3, Cu(I) ions adopt the distorted tetra-coordinated tetragonal coordination geometry, inducing more complicated connection modes with Lo and Lp ligands. Thus, such a reaction system can also be adjusted by utilizing different kinds F

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Figure 8. Absorption spectra of the RhB aqueous solution during the photodegradation under 500 W Hg-lamp irradiation with (a) 1, (b) 2, (c) 3, (d) [Bu4N]5BW12 powder, (e) no catalyst, and (f) [CuLo]Cl2 complex.

accelerate the photodegradation reaction of RhB in aqueous solutions. Moreover, the hybrid compounds containing [BW12O40]5− and [SiW12O40]4− polyoxoanions show similar photocatalytic activities. However, the photocatalytic activities of the [Bu4N]5BW12 powder are obviously lower than those of crystalline compounds 1−3, and the [CuLo]Cl2 complex shows no obviously catalytic activity in RhB degradation. On the basis of the above experiments, it is presumed that the photocatalytic active species should be POM units. The Cu-bppmpb units in compounds 1−3 may play the role in separating the POM units in a uniform mode, which can promote the photocatalytic activities of POM units in the crystalline hybrid compounds. In comparison with the recently reported POM-based hybrid catalysts in photocatalytic RhB degradation with a 500 W high pressure Hg lamp (as shown in Table S4),28 it is also found that the Keggin-type POM species display the best catalytic activities in RhB degradation. Additionally, after four cycles of the RhB degradation experiments, no significant loss in the photocatalytic activity was observed for all three compounds (Figure S17), suggesting that compounds 1−3 may be potentially good catalysts for photocatalytic RhB degradation.

blank RhB aqueous solution without any catalyst were also checked as the control experiments. The RhB solutions were monitored by UV−vis spectra, and the decolorization speeds under different conditions are displayed in Figure 8. It is found that the maximum absorbance at 550 nm of the blank RhB aqueous solution decreases from 2.59 to 0.22 after 135-min UV irradiation (Figure 8e), while those in the presence of 1−3 decrease to the same level only within 40min irradiation (Figure 8a−c). In comparison, the one with [Bu4N]5BW12 decreases from 2.80 to 0.22 after 120-min irradiation (Figure 8d), and the one with [CuLo]Cl2 complex decreases from 2.86 to 1.25 after 120-min irradiation (Figure 8f). In addition, the conversion of RhB (K) versus reaction time (t) is also plotted in Figure 9. The decomposition rate of RhB

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CONCLUSION In summary, three new organic−inorganic hybrid compounds based on the copper ions, bichelate-bridging ligands, and Keggin-type polyoxoanions were successfully prepared. The use of POMs in the metal−organic complex synthetic system containing copper ions and bppmb ligands led to the isolation of various new metal−organic secondary building units, which further construct a series of interesting supramolecular selfassemblies with the polyoxoanions. These POM-based hybrid materials exhibit good photocatalytic activities toward the model degradation experiment of RhB under UV irradiation. The preparation of compounds 1−3 may reveal a new route to explore novel metal−organic secondary building units based on TM ions and bichelate-bridging ligands by the use of POMs with different types, charges, sizes, and shapes so as to construct

Figure 9. Conversion rate of RhB (K) with the reaction time (t).

(K) can be expressed as K = (I0 − It)/I0, where I0 represents the UV−vis absorption intensity of RhB at the initial time (t = 0) and It the intensity at a given time (t). On the basis of the experimental results, the decomposition rates of RhB without catalyst and with [Bu4N]5BW12 reach 91% (135-min irradiation) and 92% (120-min UV irradiation), respectively, while those in the presence of 1−3 reach 91% only in ca. 40 min. These results demonstrate that the use of 1−3 can dramatically G

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new functional organic−inorganic hybrid compounds. This work is ongoing in our group.

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ASSOCIATED CONTENT

S Supporting Information *

Single-crystal data of 1−3, additional structure figures, selected bond lengths and angles, physical characterizations including IR, TG, PXRD, and solid-state UV−vis spectra, four-cycle photocatalytic experiment results, BVS calculations, and photocatalytic activity of relevant reported POM compounds. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y.-G.L.); wangyh319@nenu. edu.cn (Y.-H.W.); [email protected] (F.-S.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 91027002, 21271039, 21201032) and the Fundamental Research Funds for the Central Universities.

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(27) [CuLo]Cl2 is synthesized by CuCl2·3H2O and Lo in methanol solution with mole ratio 1:1, and the IR spectrum is shown in Figure S15. (28) (a) Niu, J.; Zhang, S.; Chen, H.; Zhao, J.; Ma, P.; Wang, J. Cryst. Growth Des. 2011, 11, 3769. (b) Shi, D. Y.; Zhao, J. W.; Chen, L. J.; Ma, P. T.; Wang, J. P.; Niu, J. Y. CrystEngComm 2012, 14, 3108. (c) Wang, K.; Zhang, D.; Ma, J.; Ma, P.; Niu, J.; Wang, J. CrystEngComm 2012, 14, 3205. (d) Zhao, J.; Shi, D.; Chen, L.; Cai, X.; Wang, Z.; Ma, P.; Wang, J.; Niu, J. CrystEngComm 2012, 14, 2797. (e) Liu, Y.; Shi, D.; Zhao, J.; Chen, L.; Wang, Z.; Ma, P.; Niu, J. Inorg. Chem. Commun. 2011, 14, 1178. (f) Luo, J.; Chen, L. J.; Shi, D. Y.; Li, Y. Y.; Zhao, J. W. Russ. J. Coord. Chem. 2013, 39, 141. (g) Shi, D.; Wang, Z.; Xing, J.; Li, Y.; Luo, J.; Chen, L.; Zhao, J. Synth. React. Inorg. Met. 2012, 42, 30. (h) Shi, D.; Shang, S.; Chen, L.; Cai, X.; Wang, X.; Zhao, J. Synth. Met. 2012, 162, 1030.

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