An Effective Strategy To Construct Novel Polyoxometalate-Based

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An Effective Strategy To Construct Novel Polyoxometalate-Based Hybrids by Deliberately Controlling Organic Ligand Transformation In Situ Xiu-Li Wang,* Rui Zhang, Xiang Wang, Hong-Yan Lin, and Guo-Cheng Liu *

Department of Chemistry, Bohai University, Jinzhou, 121000, People’s Republic of China S Supporting Information *

ABSTRACT: Deliberately controlling organic ligand transformation in situ has remained a challenge for the construction of polyoxometalate (POM)-based inorganic−organic hybrids. In this work, four POM-based hybrids assembled from an in situ bifurcating organic ligand[Cu2(DIBA)4](H3PMo12O40)·6H2O (1), [Cu2(DIBA)4](H4SiW12O40)·6H2O (2), [Ag(HDIBA)2](H2PMo12O40)·2H2O (3), [Ag3(HDIBA)2(H2O)][(P2W18O62)1/2]·4H2O (4) (DIBAH = 3,5-di(1H-imidazol-1yl) benzoic acid)have been designed and obtained under hydrothermal conditions. Compounds 1 and 2 are isostructural, displaying a three-dimensional (3D) 2-fold interpenetrating framework with two types of channels, and the bigger channels are occupied by Keggin polyoxoanions and crystallization water molecules, but only crystallization water molecules in the smaller ones. Compound 3 displays a 3D supramolecular structure constructed from {Ag(HDIBA)2} segments and PMo12O403− polyoxoanions through hydrogen bonding interactions. Compound 4 shows a 3D 2fold interpenetrating framework based on (3, 3, 4)-connected network, which is constructed from {Ag3(HDIBA)2}n chains and P2W18O626− polyoxoanions as linkers. The DIBAH ligand was generated in situ from 3,5-di(1H-imidazol-1-yl)benzonitrile by deliberate design, which illustrates that the strategy to construct novel POM-based hybrids by controlling ligand transformation in situ is rational and feasible. In addition, the effects of the central metal and POMs on the structures of the target compounds were discussed. Finally, the electrochemical and photocatalytic properties of compounds 1−4 have been investigated in this paper.

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hydrolysis of cyano,15 tetrazole formation,16 and so on. An early review about in situ ligand synthesis was organized by Zhang in 2005,17 which not only expatiated on the types of in situ ligand syntheses, but also indicated that the best of a method for the synthesis of in situ ligands is hydro(solvo)thermal reactions. In fact, in situ ligand transformation is usually somewhat accidental. To the best of our knowledge, reports on the deliberate design and control of in situ ligand transformation are very scarce in the construction of POM-based hybrids. The bifurcating ligand 3,5-di(1H-imidazol-1-yl)benzonitrile (DICN) can usually be hydrolyzed into the corresponding carboxylic acid under hydrothermal conditions (Scheme 1),18

INTRODUCTION Nowadays, research on organic−inorganic hybrids based on polyoxometalates (POMs) has been of considerable interest, because of their versatile properties and potential application in many fields, including catalysis,1 proton conduction,2 magnetism,3 electrochemistry,4 and luminescence,5 as well as their fascinating structures.6 In the construction of POM-based hybrids with diverse structures, the choice and design of organic ligands is very crucial. Several types of organic ligands, including not only N-donor ligands and carboxylic ligands, but also the ligands combining carboxyl with N-donor groups, have been utilized in the self-assembly process of constructing POM-based frameworks. A substantial amount of POM-based hybrids assembled from versatile organic ligands have been reported in recent years, including pyridine derivatives,7 imidazole derivatives,8 triazole derivatives,9 tetrazole derivatives,10 pyridyl-tetrazole derivatives,11 poly(carboxylic acid)s,12 pyridine carboxylate derivatives,13 imidazole carboxylate derivatives,14 and so on, most of which were presynthesized and used directly in the construction of the target POM-based hybrids. In recent years, in situ ligand transformation has been considered to be a very important strategy in the construction of coordination complexes. So far, more than 10 types of in situ ligand synthesis routes have been successfully found, such as © XXXX American Chemical Society

Scheme 1. View of the In Situ Ligand Hydrolysis from DICN to DIBAH

Received: January 4, 2016

A

DOI: 10.1021/acs.inorgchem.6b00009 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Crystal Data for Compounds 1−4 formula formula weight, Fw temperature (K) crystal system space group a (Å) b (Å) c (Å) α (°) β (°) γ (°) V (Å3) Z density, Dc (g cm−3) μ (mm−1) F(000) final R1,a wR2b [I > 2σ(I)] final R1,a wR2b (all data) goodness on F2 a

1

2

3

4

C52H51N16O54Cu2PMo12 3073.38 293(2) tetragonal P42/n 18.001(5) 18.001(5) 12.946(5) 90.000(5) 90.000(5) 90.000(5) 4195(3) 2 2.421 2.358 2938.0 0.0381, 0.0966 0.0665, 0.1125 0.955

C52H52N16O54Cu2SiW12 4126.30 293(2) tetragonal P42/n 18.120(5) 18.120(5) 12.905(5) 90.000(5) 90.000(5) 90.000(5) 4237(3) 2 3.222 16.831 3704.0 0.0311, 0.0672 0.0567, 0.0764 1.004

C26H26N8O46AgPMo12 2476.63 293(2) triclinic P1̅ 13.246(5) 13.603(5) 16.304(5) 87.400(5) 74.351(5) 88.125(5) 2825.3(17) 2 2.904 3.053 2332 0.0444, 0.1089 0.0580, 0.1167 1.010

C26H28N8O40Ag3PW9 3101.67 293(2) monoclinic C2/c 39.804(5) 13.587(5) 23.074(5) 90.000(5) 119.014(5) 90.000(5) 10913(5) 8 3.766 20.076 10992.0 0.0449, 0.1114 0.0840, 0.1351 1.030

R1 = ∑∥F0| − |Fc∥/∑|F0|. bwR2 = [∑w(F02 − Fc2)2/∑[w(F02)2]1/2. temperature, and carbon paste electrodes modified with corresponding compounds, Pt wire, and Ag/AgCl electrode were used as a working, counter and referenced electrode, respectively. Thermogravimetric (TG) analyses were recorded on a Pyris-Diamond TG instrument. Xray photoelectron spectroscopy (XPS) was measured on a spectrometer (Escalabmkii , Model 250) with an Al K (1486.8 eV) achromatic X-ray source. A diffuse reflectivity spectrum was obtained with a spectrophotometer (Lambda, Model 750). Ultraviolet−visiblelight (UV/vis) absorption spectra were recorded with a Model SP1901 UV/vis spectrophotometer. X-ray Crystallographic Study. X-ray diffraction (XRD) analysis data for compounds 1−4 were collected with a Bruker SMART APEX II system with Mo Kα radiation (λ = 0.71073 Å) by ω and θ scan mode. All structures were solved by direct methods and refined on F2 with full-matrix least-squares methods, using the SHELXTL package.20 The hydrogen atoms attached to water molecules were not located, but were included in the structure factor calculations. A summary of the crystallographic data and structural determination for compounds 1−4 are given in Table 1. Selected bond lengths and angles are listed in Table S1 in the Supporting Information. Crystallographic data of 1−4 have been deposited in the Cambridge Crystallographic Data Center (CCDC No. 1432032−1432035). Preparation of Compounds 1−4. Synthesis of [Cu2(DIBA)4](H3PMo12O40)·6H2O (1). A mixture of Cu(CH3COO)2·H2O (0.20 g, 1 mmol), DICN (0.05g, 0.21 mmol), H3PMo12O40·12H2O (0.30 g, 0.15 mmol) and 10 mL of deionized (DI) water, was stirred for 1 h at room temperature. The pH value was adjusted to 4.7 with 1.0 mol L−1 NaOH. The resulting solution was transferred to a Teflon-lined autoclave and kept at 160 °C for 5 days. After slow cooling to room temperature, green crystals were filtered and washed with deionized water (35% yield, based on Cu). The final pH is 4.03. Anal. Calcd for C52H51N16O54Cu2PMo12 (3073.38): C 20.32, H 1.67, N 7.29. Found: C 20.22, H 1.73, N 7.21. IR (solid KBr pellet, cm−1): 3425 (w), 3138 (s), 1676 (w), 1591 (s), 1516 (s), 1076 (s), 938 (s), 873 (s), 807 (s). Synthesis of [Cu2(DIBA)4](H4SiW12O40)·6H2O (2). The synthetic method was similar to that of compound 1, except that H4SiW12O40· 12H2O (0.30 g, 0.14 mmol) was used instead of H3PMo12O40·12H2O. Green crystals were filtered and washed with distilled water (28% yield, based on Cu). The final pH is 4.10. Anal. Calcd for C52H52N16O54Cu2SiW12 (4126.30): C 15.13, H 1.27, N 5.43. Found: C 15.24, H 1.34, N 5.36. IR (solid KBr pellet, cm−1): 3433 (w), 3142 (s), 1673 (w), 1597 (s), 1515 (s), 1081 (s), 967 (s), 923 (s), 795 (s).

resulting in a tridentate ligand containing two N-donor groups and one carboxylate group, 3,5-di(1H-imidazol-1-yl)benzoic acid (DIBAH). Inspired by this report, in this work, we selected the DICN as the reactant to assemble with different transition metals and POMs under hydrothermal conditions, in order to investigate whether the DICN can be transformed in situ to DIBA in the POMs system, and to construct novel POM-based inorganic−organic hybrids. To the best of our knowledge, the POM-based hybrids assembled from such ligand have never been reported up to now. Here, Cu2+ and Ag+ ions were selected as the central metals, because of their versatile coordination geometries, such as linear, “seesaw”, T-type, and “square-pyramidal” geometries,19 which are usually employed to construct novel topological structures in POM systems. Keggin-type PMo 12 O40 3−/ SiW12O404−, Wells−Dawson-type P2W18O626− polyoxoanion were selected as the representative POMs to assemble with Cu 2+ /Ag + ions and DICN ligand under hydrothermal conditions. As is expected, four POM-based hybrids with different structures have been obtained: [Cu2(DIBA)4](H3PMo12O40)·6H2O (1), [Cu2(DIBA)4] (H4SiW12O40)· 6H2O (2), [Ag(HDIBA)2](H2PMo12O40)·2H2O (3), and [Ag3(HDIBA)2(H2O)][(P2W18O62)1/2]·4H2O (4). The DICN has been transformed in situ to DIBA in the POM system, which confirms that our strategy is effective. Moreover, the electrochemical and photocatalytic properties of compounds 1−4 have been investigated.

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EXPERIMENTAL SECTION

Materials and Methods. All reagents were purchased commercially, and used without further purification. DICN was synthesized according to the literature.18 Physical Measurements. Elemental analyses (C, H, and N) were performed on an elemental analyzer (PerkinElmer, Model 2400C). Fourier transform infrared (FT-IR) spectra were given on a spectrometer (Shimadzu, Model FT-IR 8400) (KBr pellets). Powder X-ray diffraction (PXRD) investigations were recorded with an Ultima IV with D/teX Ultra diffractometer at 40 kV, 40 mA with Cu Kα radiation. Cyclic voltammograms were performed using an electrochemical workstation (CH Instruments, Model 440) at room B

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Inorganic Chemistry Synthesis of [Ag(HDIBA)2](H2PMo12O40)·2H2O (3). The synthetic procedure was similar to that of compound 1, except that AgNO3 (0.166 g, 1 mmol) was used instead of Cu(CH3COO)2·H2O and the pH value was then adjusted to ∼2.1 with 1.0 mol L−1 HNO3. After the reaction, the pH is 2.19. Orange crystals were filtered and washed with distilled water (23% yield, based on Ag). Anal. Calcd for C26H26N8O46AgPMo12 (2476.63): C 12.61, H 1.06, N 4.52. Found: C 12.54, H 1.11, N 4.47. IR (solid KBr pellet, cm−1): 3439 (w), 3130 (s), 1722 (w), 1613 (s), 1575 (s), 1515 (s), 1066 (s), 958 (s), 875 (s), 805 (s). Synthesis of [Ag3(HDIBA)2(H2O)][(P2W18O62)1/2]·4H2O (4). The synthetic method was similar to that of compound 3, except that αK6P2W18O62·14H2O (0.48 g, 0.1 mmol) was used instead of H3PMo12O40·12H2O and the pH was adjusted to ∼1.5. Yellow crystals were filtered and washed with distilled water (27% yield, based on Ag). The final pH is 1.74. Anal. Calcd for C26H28N8O40Ag3PW9 (3101.67): C 10.07, H 0.91, N 3.61. Found: C 10.01, H 0.87, N 3.54. IR (solid KBr pellet, cm−1): 3498 (w), 3130 (s), 1578 (w), 1512 (s), 1395 (s), 1089 (s), 962 (s), 914 (s), 775 (s).

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RESULTS AND DISCUSSION Synthesis. The hydrothermal technique is not only an effective and widely used method to synthesize POM-based

Figure 2. (a) The tetradentate windmill-like building block constructed from one Cu1 atom and four DIBA ligands. (b) View of the 4-connected 3D metal−organic framework in compound 1.

Figure 1. Ball/stick/polyhedral view of the coordination fashions of Cu atoms and polyoxoanion in compound 1. The crystal water and hydrogen atoms are omitted for clarity. Symmetry operation: #1, −0.5 + x, 0.5 + y, 1 − z.

hybrids, but it also can provide hydrolytic conditions for the in situ hydrolysis of DICN. Here, to investigate the effect of detailed synthetic conditions on the crystallinity of the title compounds, many parallel experiments were performed via adjusting the pH, reaction time, and temperature to synthesize target compounds. First, by changing the pH of initial reactants in the range of 1.0−5.0, compounds 1 and 2 can be obtained in the narrow range of 4.5−4.8, while 3 and 4 were obtained in the pH range of 1.9−2.2 for 3 and 1.4−1.6 for 4, respectively. In addition, parallel experiments reveal that the best pH value is 4.7 for 1 and 2, 2.1 for 3, and 1.5 for 4. Second, the reaction time is also very important in the formation of compounds 1− 4. The parallel experiments show that a reaction time of 5 days is perfect, while only precipitates can be obtained when the reaction time is 4 days. If the reaction time is over 6 days, only a small amount of crystalline product can be observed. Third, we investigated the influence of reaction temperature on constructing compounds 1−4 in the range of 150−180 °C.

Figure 3. (a) View of the 2-fold interpenetrating 3D framework in compound 1. (b) Schematic view of the 3D framework with two types of channels.

The results indicated that the temperature of 160 °C was rational, while less or no products could be formed if the reaction temperature was altered. Crystal Structure of 1 and 2. Single-crystal X-ray diffraction (XRD) analysis reveals that compounds 1 and 2 are isostructural. Thus, the structure of compound 1 is described here as an example. Compound 1 consists of two copper atoms, C

DOI: 10.1021/acs.inorgchem.6b00009 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. (a) View of 1D {Ag3(HDIBA)2}n chain in compound 4. (b) View of 2D gridlike layer constructed from {Ag3(HDIBA)2}n chains and P2W18 polyoxoanions in compound 4.

Figure 4. (a) View of asymmetric unit in compound 3. The H atoms and crystallization water molecules are omitted for the sake of clarity. (b) View of the 3D supramolecular structure of compound 3.

Figure 5. Ball/stick/polyhedral view of the coordination fashions of AgI atoms and polyoxoanion in compound 4. The hydrogen atoms and crystallization water molecule are omitted for the sake of clarity. Symmetry operations: #1, 0.5 − x, 0.5 + y, 0.5 − z; #2, x, y, −1 + z; #3, 0.5 − x, −0.5 + y, 1.5 − z; #4, −x, y, 0.5 − z; #5, −0.5 + x, −0.5 + y, −1 + z.

Figure 7. (a) Schematic view of the 3D framework in compound 4. (b) View of the 2-fold interpenetrating 3D framework of compound 4.

An interesting structural feature of compound 1 is that four DIBA ligands are aggregated by one Cu atom center through Cu−N bonds to form a tetradentate windmill-like building block (Figure 2a), which crossed with each other along different axis to construct a 4-connected 3D framework structure (here, each Cu1 atom is considered as a fourconnected node) (see Figure 2b). Finally, two 4-connected 3D frameworks are interpenetrated with each other to form a beautiful 2-fold interpenetrating framework (Figure 3a). It is worthwhile to note that two type of channels (A and B) exist in the final framework along the baxis, as shown in Figure 3b. The dimension (ca. 15.75 Å × 12.29 Å) of hexagonal channel A is big enough to accommodate Keggin-type PMo12 polyoxoanions and water molecules. Furthermore, there exist strong interactions between the Cu centers and surface O atoms from PMo 1 2 polyoxoanions, with the Cu−O distance of 2.769 Å. The

four DIBA ligands, one [PMo12O40]3− (abbreviated to PMo12) polyoxoanion, and six crystallization water molecules. The P−O and Mo−O lengths in the PMo12 polyoxoanion are in the normal ranges.21 Bond valence sum (BVS) calculations show that all Mo atoms are in the +6 oxidation state, and Cu atoms are in the +2 oxidation state.22 In order to balance the charges, three H protons have been added to the molecule formula. In compound 1, each Cu center displays a four-coordinated geometry, completed by four imidazolyl donors (two N2 atoms, two N3 atoms) from four DIBA ligands (Figure 1). The corresponding bond lengths and angles are given in Table S1 in the Supporting Information. The DIBA coordinated with CuII ions with its two imidazolyl groups acting as a bridging ligand, and the carbxoyl group formed in situ is noncoordinated. D

DOI: 10.1021/acs.inorgchem.6b00009 Inorg. Chem. XXXX, XXX, XXX−XXX

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Crystal Structure of 3. Compared with 1, when AgI ion was employed in place of the CuII ion, a completely different 3D supramolecular structure (3) was obtained. Interestingly, the DICN was still in situ transformed to DIBA ligand. The asymmetric unit of compound 3 consists of one Ag atom, one PMo12 polyoxoanion, two DIBA ligands, and two lattice water molecules. The Mo−O and P−O bond distances in the PMo12 polyoxoanion are in the normal ranges. BVS calculations exhibit that all Mo atoms are in the +VI oxidation state, and the Ag atom is in the +I oxidation state.22 One of the imidazolyl groups of each DIBA ligand is protonated, and additional two H protons are added to PMo12 polyoxoanion to balance the charges; thus, compound 3 is formulated as [Ag(HDIBA)2](H2PMo12O40)·2H2O. The structure of compound 3 is constructed from two types of moieties: {Ag(HDIBA)2} segments and PMo12 polyoxoanions. In the {Ag(HDIBA)2} segment, the Ag atom adopts a linearly geometry, coordinated by two N atoms (N2, N8) from two DIBA ligands, and the bond lengths are 2.135 (5) Å for Ag1−N2 and 2.139 (5) Å for Ag1−N8 (Figure 4a). Note that the DIBA acts as a monodentate ligand coordinating to an AgI ion with one imidazolyl group, and the other imidazolyl group is protonated, which is different from that in 1 and 2. Furthermore, these {Ag(HDIBA)2} segments are extended to a 3D supramolecular structure by PMo12 polyoxoanions through hydrogen bonding interactions (O1···O38, 2.966 Å; O1···O4,

Figure 8. Schematic view of the connection modes of DIBA ligands in compounds 1−4.

smaller triangular channels B are only filled with water molecules.

Figure 9. Cyclic voltammograms of (a) 1-CPE, (b) 2-CPE, (c) 3-CPE, and (d) 4-CPE in 0.1 M H2SO4 + 0.5 M Na2SO4 aqueous solution at different scan rates (from inner to outer: 60, 80, 100, 120, 140, 160, 180, and 200 mV s−1). E

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Figure 10. Cyclic voltammograms of (a) 1-CPE, (b) 2-CPE, (c) 3-CPE, and (d) 4-CPE in 0.1 M H2SO4 + 0.5 M Na2SO4 aqueous solution containing 0.0−8.0 mM of bromate.

acidic solution; thus, compound 4 is formulated as [Ag3(HDIBA)2(H2O)][(P2W18O62)1/2]·4H2O. In compound 4, all three crystallographically independent Ag atoms show three-coordinated T-type geometry (Figure 5): Ag1 is nearby two N atoms (N6, N7) of imidazolyl groups from two DIBA ligands and one O atom (O10) from one P2W18 polyoxoanion; Ag2 is coordinated by two O atoms (O33, O34) from two DIBA ligands and one water molecule; Ag3 is coordinated by two O atoms (O32, O35) from two DIBA ligands and one O14 atom from one P2W18 polyoxoanion, respectively. The P2W18 anion acting as a tetradentate building unit links four Ag atoms through Ag−O covalent bonds. The corresponding bond lengths and angles are given in Table S1. Different from those in compounds 1−3, the in situ DIBA acts as a tridentate ligand in 4: it utilizes its two carboxyl O atoms and one imidazolyl N atom to coordinate with three AgI ions. Two Ag atoms (Ag2, Ag3) are aggregated by two bridging carboxyl groups from two DIBA ligands to form a binuclear unit {Ag2(HDIBA)2}, which is further stabilized by Ag−Ag bonding interactions (the distance between Ag2 and Ag3 is 2.8123(4) Å). These binuclear units then are linked to each other by Ag1 atoms through Ag−N bonds to form a onedimensional (1D) {Ag3(HDIBA)2}n chain (Figure 6a). Furthermore, these {Ag3(HDIBA)2}n chains are connected by P2W18 polyoxoanions to generate a two-dimensional (2D) gridlike layer (Figure 6b), in which each polyoxoanion provides two O atoms (O10, O14 #5) to coordinate with two Ag atoms (Ag1, Ag3 #5) from neighboring chains, respectively. Consid-

Figure 11. Diffuse reflection spectra of Kubelka−Munk (K-M) function versus energy (eV) of compounds 1−4.

2.886 Å; O1···O7, 2.993 Å; O11···O29, 2.823 Å; O5···O29, 2.902 Å; and O3···O9, 2.962 Å) (Figure 4b). Crystal Structure of 4. Compound 4 consists of three Ag atoms, two DIBA ligands, half of [P2W18O62]6− polyoxoanion (abbreviated to P2W18), one coordination water molecule, and four crystallization water molecules. The P−O and W−O lengths in the P2W18 polyoxoanion are in the normal ranges.23 BVS calculations show that all W atoms are in the +6 oxidation state, and Ag atoms are in the +1 oxidation state.22 One of the imidazolyl groups of each DIBA ligand is protonated in the F

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Figure 12. Absorption spectra of the MB solution during the decomposition reaction under UV irradiation in the presence of compounds (a) 1, (b) 2, (c) 3, and (d) 4.

ering the {Ag2(HDIBA)2} units and Ag1 atoms as threeconnected nodes, the 2D layer can be considered as a (3, 3)connected network structure. A 3D framework structure is built from the 2D networks and 1D {Ag3(HDIBA)2}n chains through Ag−O covalent bonds (see Figure 7a, as well as Figure S1 in the Supporting Information). Each P2W18 polyoxoanion from a 2D network continues to offer another two symmetric terminal O atoms (O10 #4, O14) to coordinate with Ag1 #4 and Ag3 atoms from two neighboring {Ag3(HDIBA)2}n chains, resulting in a 3D framework. For the entire framework, the {Ag2(HDIBA)2} units and Ag1 atoms can be considered as three-connected nodes, respectively, P2W18 polyoxoanions can be regarded as four-connected nodes, then the 3D structure can be considered as a (3, 3, 4)-connected 3D framework. Finally, two sets of such frameworks interpenetrate with each other to form a 3D 2-fold interpenetrating framework, as shown in Figure 7b. Effects of the In Situ Ligand, Central Metals, and POMs on the Construction and the Final Structures of the Title Compounds. In compounds 1−4, the DICN ligand was transformed in situ to the DIBAH ligand under hydrothermal conditions, and the DIBAH ligand generated in situ displays diverse coordination modes with the change of metal ions and polyoxoanions (Figure 8). In Cu/Keggin-POM reaction system, the DIBAH ligands utilize all imidazolyl groups, except carboxyl groups, to coordinate with Cu atoms in compounds 1 and 2, resulting in a 2-fold interpenetrating 3D framework, in which Keggin polyoxoanions and water

molecules occupied two types of channels (Keggin polyoxoanions and water molecules for bigger ones, other water molecules for smaller ones). Only replacing Cu atoms with Ag atoms in compound 3, each of two DIBAH ligands utilizes one imidazolyl N atom to link one Ag atom, forming a {Ag(HDIBA)2} segment, which connects with adjacent Keggin PMo12 polyoxoanions via hydrogen bonding interactions to form a 3D supramolecular structure. The other imidazolyl group of DIBA is protonated in the acidic conditions. The results suggest that metal ions show great effect on the structures of Keggin-metal-DIBA networks. However, when we replaced Keggin polyoxoanions with Wells−Dawson polyoxoanions at the presence of Ag atoms, the DIBA ligands not only utilized their carboxyl groups to link two Ag atoms, but also coordinate with another Ag atom through one imidazolyl group, constructing a 1D {Ag3(HDIBA)2}n chain in compound 4. Such chains are further linked by tetradentate P2W18 polyoxoanions into a (3, 3, 4)-connected 3D framework. Similar to that in 3, the other imidazolyl group of DIBA is also protonated in the acidic conditions. In a word, the in situ generated DIBAH ligand can exhibit various coordination modes with metal ions in different reaction system based on POMs, and the strategy of utilizing a ligand in situ is effective in the construction of POM-based complexes. Fourier Transform Infrared (FT-IR) Spectra, Powder Xray Diffraction (PXRD), and Thermogravimetric (TG) Analyses. As shown in Figure S2 in the Supporting Information, in the infrared (IR) spectra of compounds 1−3, G

DOI: 10.1021/acs.inorgchem.6b00009 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry the bands in the region of 1081−795 cm−1 could be ascribed to the characteristic peaks of νas(P/Si−Oa), νas(Mo/W−Od), νas(Mo/W−Ob−Mo/W), and νas(Mo/W−Oc−Mo/W) of Keggin-type PMo12 and SiW12 anions, respectively.24 For compound 4, the bands at 1089, 962, 914, and 775 cm−1 could be ascribed to the characteristic peaks of νas(P−Oa), νas(W−Od), νas(W−Ob−W), and νas(W−Oc−W) of the P2W18 anion.25 The bands in the region of 3433−1515 cm−1 could be ascribed to the characteristic peaks of the DIBA ligand.18 Furthermore, the PXRD experiments are also performed to verify the purities of compounds 1−4 at room temperature. As shown in Figure S3 in the Supporting Information, the experimental diffraction peaks are consistent with simulated patterns, indicating the good purity of the title compounds. The TG analyses were also completed to confirm further the chemical composition of compounds 1−4 and characterize their thermal stability (Figure S4 in the Supporting Information). The TG curves of compounds 1−4 are similar and exhibit two steps of the weight loss process. The first weight loss step should be ascribed to the loss of lattice water molecules, 3.53% (calcd. 3.51%) for 1 before 258 °C, 2.68% (calcd. 2.62%) for 2 before 250 °C; 1.40%, (calcd. 1.45%) for 3 before 245 °C; 2.89% (calcd. 2.90%) for 4 before 207 °C. The second weight loss step occurs in the range of 210−450 °C, which should be attributed to the decomposition of DIBA ligands, 33.07% (calcd. 33.09%) for 1; 24.61% (calcd. 24.65%) for 2, 20.50% (calcd. 20.53%) for 3; 16.36% (calcd. 16.39%) for 4. X-ray Photoelectron Spectroscopy (XPS) Analyses. As mentioned above, the BVS calculations demonstrate that all Cu atoms, all Ag atoms, all Mo/W atoms are in the +2, + 1 and +6 oxidation state, respectively, for complexes 1−4. The oxidation states of Cu, Ag, Mo, and W are further confirmed by XPS measurements in the energy regions of Cu2p, Ag3d, Mo3d and W4f. The XPS spectra (Figure S5 in the Supporting Information) show two peaks at 934.6 and 954.6 eV for 1 and 2, attributed to Cu2+(2p3/2) and Cu2+(2p1/2); two peaks at 368.3 and 374.3 eV for 3 and 4, ascribed to Ag+(3d5/2) and Ag+(3d3/2). In the XPS spectra, two peaks at 232.4 and 235.6 eV for 1 and 3 are attributed to Mo6+(3d5/2) and Mo6+(3d3/2); two peaks at 35.3 and 37.4 eV for 2 and 4 are ascribed to W6+(4f5/2) and W6+(4f1/2). These results are consistent with BVS calculation and elemental analysis, which further confirm the structure analysis. Electrochemical Properties. Here, to investigate whether the redox abilities of polyoxoanions could be retained in the title compounds, the electrochemical properties of compounds 1−4 were studied in 0.1 M H2SO4 + 0.5 M Na2SO4 aqueous solution with 1−4 bulk-modified carbon paste electrodes (CPEs). The corresponding bulk CPE modified by compounds 1−4 are prepared according to the literature,19b which are abbreviated as 1-CPE, 2-CPE, 3-CPE, and 4-CPE, respectively. As shown in Figure 9, three pairs of redox peaks with mean peak potentials E1/2 = (Epc + Epa)/2 (scan rate: 160 mV s−1) at +231 mV (I−I′), +42 mV (II−II′), −180 mV (III−III′) for 1, + 325 mV (I−I′), + 168 mV (II−II′), −67 mV (III−III′) for 3, should belong to three consecutive two-electron redox processes of Mo centers of PMo12 polyoxoanions in compounds 1 and 3.26 The slight peak potential difference for PMo12 anions between compounds 1 and 3 may be due to their different architectures.27 Furthermore, three pairs of redox peaks with mean peak potentials at −367 mV (I−I′), −602 mV (II−II′), −795 mV (III−III′) for 2 should be attributed to two

one-electron and one two-electron redox processes of W centers of SiW12 polyoxoanions.28 Three pairs of redox peaks with mean peak potentials at −259 mV (I−I′), −541 mV (II− II′), −789 mV (III−III′) for 4 should be ascribed to three consecutive two-electron redox processes of W centers of P2W18 polyoxoanions.23 The results manifest that the redox abilities of the polyoxoanions in final structures can still be maintained. These values indicate their potential applications in the field of electrochemistry. As is well-known, POMs can usually show good electrocatalytic activities for the reductions of bromate.29 Therefore, the electrocatalytic behaviors of compounds 1−4 for the reduction of bromate were also investigated here. As shown in Figure 10, for 1-CPE, it can be clearly shown that the second and third reduction peak currents gradually increase with the addition of bromate, and the corresponding oxidation peak currents decrease, while the first reduction peak currents remain almost unchanged. The electrocatalytic activity of 1CPE toward the reduction of bromate come from the four- and six-electron reduced species of PMo12 anion and the reduction product is Br−.30 The electrocatalytic behavior of 1-CPE toward BrO3− can be explained by the following mechanism:30,31 3H4PMo4 V Mo8 VIO40 3 − + BrO3− → 3H 2PMo2 V Mo10 VIO40 3 − + 3H 2O + Br − 3H6PMo6 V Mo6 VIO40 3 − + BrO3− → 3H4PMo4 V Mo8 VIO40 3 − + 3H 2O + Br −

For 2-CPE, 3-CPE, and 4-CPE, with the addition of bromate, all the reduction peak currents gradually increase, while the corresponding oxidation peak currents decrease, suggesting that 2-CPE, 3-CPE, and 4-CPE exhibit good electrocatalytic activity for the bromate reduction. Optical Band Gaps. To evaluate the semiconductor behaviors and photocatalysis activities of compounds 1−4, the diffuse reflection spectra of compounds 1−4 were carried out in the crystalline state at room temperature (see Figure 11, as well as Figure S6 in the Supporting Information). The band gaps (Eg) of compounds 1−4 were obtained from the Kubelka−Munk (K-M) function F vs E,32 which are estimated to be 2.44 eV for 1, 2.54 eV for 2, 2.08 eV for 3, and 2.67 eV for 4, respectively. The band gap values indicate that compounds 1−4 may respond to UV irradiation and have the potential capacity for photocatalytic reactions.33 Photocatalytic Activity. POM-based hybrids exhibit usually photocatalytic activity for the degradation of organic dyes under UV irradiation, such as Methylene Blue (MB),34 Methyl Orange (MO),35 Rhodamine B,36 and so on. The photocatalytic activities of compounds 1−4 for degradation of MB are investigated here as an example. A typical process is carried out according to ref 37. The results are shown in Figure 12. It can be clearly observed that the absorption peak of MB decreased gradually for compounds 1−4 as photocatalysts with the increase of irradiation time, respectively, and the conversions of MB are 61.4% for 1, 64.7% for 2, 68.3% for 3, and 78.5% for 4 after 180 min. The control experiments have been performed under UV irradiation without any photocatalyst and in darkness in the presence of compounds 1−4 (see Figures S7 and S8 in the Supporting Information). The degradation ratio of MB under UV irradiation without any photocatalyst is ∼12.0%, and that in the presence of H

DOI: 10.1021/acs.inorgchem.6b00009 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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compounds 1−4 in darkness is ∼10%. The results indicate that compounds 1−4 show good photocatalytic activity for the degradation of MB under UV irradiation, compared with the parent POMs in previous works (42% for PMo12, 49% for P2W18, and 43.3% for SiW12).11,38 The possible mechanisms were that POM/organic ligands (L) were induced under UV irradiation to result in oxygen and/or nitrogen−metal charge transfer from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). The excited state (POM* and/or L*) can be deactivated during oxidizing the organic dyes, to achieve the photocatalytic process accordingly.39 In addition, the PXRD patterns of compounds 1−4 after photocatalytic reactions were also obtained, which are identical to those of the original compounds (see Figure S3 in the Supporting Information), indicating that compounds 1− 4 are still stable after the photocatalytic decomposition of MB.

CONCLUSION Compounds 1−4 have been successfully constructed from different polyoxometalates and in situ ligand under hydrothermal conditions. Both 1 and 2, based on CuII ions and Keggin anions, show a three-dimensional (3D) 2-fold interpenetrating framework containing two types of channels with different sizes. A 3D supramolecular structure based on {Ag(HDIBA)2} segments and PMo12 polyoxoanions is formed in compound 3. When a Wells-Dawson polyoxoanion is utilized, a 3D 2-fold interpenetrating framework based on {Ag3(HDIBA)2}n chains is obtained in compound 4. The DIBA ligand that is generated in situ exhibits various connecting modes under different metal/POM reaction systems and results in versatile architectures. The successful syntheses of compounds 1−4 illustrate that the strategy of constructing novel POM-based complexes via controlling ligand transformation in situ is rational and feasible, which will greatly enrich the POM family. ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00009.

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Selected bond lengths and angles, structure illustrations for compound 4, FT-IR spectra, and PXRD of compounds 1−4 (PDF) X-ray crystallographic data (CIF)

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-416-3400158. 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 of China (No. 21471021, 21401010, and 21501013), and Program for Distinguished Professor of Liaoning Province (No. 2015399) and General Program Fund for Education Department of Liaoning Province (L2014449). I

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DOI: 10.1021/acs.inorgchem.6b00009 Inorg. Chem. XXXX, XXX, XXX−XXX