A Series of Compounds Based on [P2W18O62]6â and Transition

Article pubs.acs.org/IC

Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

A Series of Compounds Based on [P2W18O62]6− and Transition Metal Mixed Organic Ligand Complexes with High Catalytic Properties for Styrene Epoxidation Ying Lü,†,‡ Xiao Zhang,§ Xiao-Bing Cui,*,† and Ji-Qing Xu†

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College of Chemistry and State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun, Jilin 130023, People’s Republic of China ‡ College of Chemistry, Tonghua Normal University, Tonghua, 134002, People’s Republic of China § MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin, People’s Republic of China S Supporting Information *

ABSTRACT: Seven compounds based on [P2W18O62]6− ({P2W18}) were successfully prepared and carefully characterized. [HC5H5N][Cu(2,2′-bpy)2]2[HP2W18O62]·2H2O (bpy = bipyridine) (1) is constructed from {P2W18} bridged by [Cu(2,2′-bpy)2]2+. [HC5H5N][Zn(2,2′-bpy)2]2[HP2W18O62]·4H2O (1a) is isostructural and isomorphous with compound 1. [Cu4(2,2′bpy)3(nic)3(OH)2(H2O)][H3P2W18O62]·0.5H2O (nic = nicotinic acid) (2) is formed by {P2W18} and tetracopper transition metal mixed organic ligand complexes (TMMCs). [Cu2(2,2′bpy)2(C2O4)]3[P2W18O62]·3H2O (3) is made up of {P2W18} and bicopper TMMCs, [Cu6(2,2′bpy)6(OH)6][P2W18O62]·2H2O (4) is built up from {P2W18}, and hexacopper complexes of 2,2′bpy and hydroxyls. [Cu(2,2′-bpy)(hnic)0.5][Cu3(2,2′-bpy)3(hnic)2(H2O)2][H3P2W18O62] (hnic = hydroxyl nicotinic acid) (5a) contains two different TMMCs. In addition, compound 5a is the first example of a compound that contains Cu−π interactions. [Cu2(2,2′-bpy)2(hnic)][H4P2W18O62]· xH2O (x ≈ 50) (5b) is based on {P2W18} and [Cu2(2,2′-bpy)2(hnic)]2+. We discuss the mechanisms for the formations of these compounds. All the catalytic performances of the compounds for styrene epoxidation to styrene oxide are high.

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INTRODUCTION Polyoxometalates (POMs), with diverse structures and versatile properties, have attracted significant attention due to their applications in analysis, nanotechnology, catalysis, magnetism, biochemistry, and so on.1 Over the past few decades, the chemistry of POM-based materials derived from the combination of POMs and organic N-containing species or metal complexes of organic N-containing species has developed greatly.2 On the basis of the analysis of literature reports, it is found that POM-based compounds containing transition metal mixed organic ligand complexes (TMMCs) are really scarce. Much attention has been focused on the chemistry of metal complexes of mixed organic ligands because they exhibit a large number of structures and properties, and can be applied in fields as photochemistry, magnetism, conducting materials, biochemistry, optical materials, and catalysis.3 The introduction of TMMCs to POMs may generate compounds with fascinating properties and interesting structures. According to the three-category model to categorize POM + TMMC types proposed by our group,4 POM + TMMC compounds can be classified into three types based on organic species (Scheme 1): (a) type A includes TMMCs of two or more organic Ncontaining ligands;5 (b) type B contains TMMCs of two or more carboxylates; (c) type C comprises TMMCs of two or © XXXX American Chemical Society

Scheme 1. Three Types of POM + TMMCs

more organic ligands composed of both organic N-containing ones and carboxylates.6,4,7,8 Except for type B, several type A and type C compounds were reported. POM + TMMC hybrids of Wells−Dawson polyanions were scarcely reported. To our knowledge, only several such compounds have been reported.6d,e,8b It will be attractive to explore the compounds of WellsDawson POMs and TMMCs. In this paper, Cu was selected as the metal of the TMMC because of its various coordination Received: June 21, 2018

A

DOI: 10.1021/acs.inorgchem.8b01705 Inorg. Chem. XXXX, XXX, XXX−XXX

a

B

compound 1a C45H47Zn2N9O66P2W18 5271.89 monoclinc P21/m 13.8342(6) 22.8992(11) 14.7578(6) 90 110.9970(10) 90 4364.7(3) 2 4.011 24.295 4660 2.48 to 25.12 28701 7991 0.0628 99.6 689 1.057 R1 = 0.0363 ωR2 = 0.0970

compound 1

C45H43Cu2N9O64P2W18 5232.20 monoclinc P21/m 13.8225(3) 22.7652(4) 14.8050(3) 90 111.032(2) 90 4348.32(16) 2 3.996 24.321 4616 2.94 to 25.34 22254 8168 0.0537 99.8 670 1.066 R1 = 0.0443 ωR2 = 0.1213

compound 2 C48H44Cu4N9O71.5P2W18 5516.32 monoclinic P21/m 16.2503(3) 19.1379(2) 16.8556(2) 90 106.7644(16) 90 5019.26(13) 2 3.650 21.494 4890 2.97 to 29.05 27909 11902 0.0352 99.8 769 1.076 R1 = 0.0396 ωR2 = 0.0947

R1 = ∑||F0| − |Fc||/∑|F0|. bωR2 = {∑ [w(F02 − Fc2)2]/∑[w(F02)2]}1/2.

empirical formula formula weight crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Z DC (Mg·m−3) μ (mm−1) F(000) θ range reflns collected reflns unique R(int) completeness to θ parameters GOF on F2 Ra [I > 2σ(I)] Rb (all data)

C66H54Cu6N12O77P2W18 5999.69 orthorhombic Pnma 17.8871(10) 23.8968(14) 26.5663(15) 90 90 90 11355.6(11) 4 3.509 19.382 10744 2.85 to 28.49 91030 14560 0.0907 98.8 847 1.008 R1 = 0.0457 ωR2 = 0.1109

compound 3

Table 1. Crystal Data and Structural Refinements for Compounds 1, 1a, 2−4, 5a, and 5b compound 4 C60H58Cu6N12O70P2W18 5819.66 triclinic P1̅ 14.0742(10) 14.2783(11) 15.8615(12) 64.730(8) 64.333(8) 80.905(6) 2596.6(4) 1 3.772 21.179 2598 2.87 to 29.03 25524 11845 0.0320 99.4 766 1.043 R1 = 0.0843 ωR2 = 0.1818

compound 5a C55H46.5Cu4N10.5O71.5P2W18 5623.92 triclinic P1̅ 13.7980(6) 15.9617(6) 25.4633(8) 87.060(3) 87.311(3) 72.503(4) 5338.6(4) 2 3.499 20.212 5000 2.99 to 29.15 51911 24443 0.0475 99.8 1516 1.009 R1 = 0.0752 ωR2 = 0.2275

compound 5b C26H23Cu2N5O65P2W18 4943.81 monoclinic P21/n 16.6532(7) 22.5194(13) 26.3509(14) 90 97.524(4) 90 9797.0(9) 4 3.352 21.578 8616 2.96 to 29.14 57009 22704 0.1165 99.8 1051 1.044 R1 = 0.1162 ωR2 = 0.3039

Inorganic Chemistry Article


Article

Inorganic Chemistry geometries and its close affinity for diverse organic ligands.2b,f,9 2,2′-Bipyridine was chosen as the organic N-containing ligand because it is a good chelating ligand for copper ions.2b,10 Oxalic, picolinic, nicotinic, and isophthalic acids were selected as the carboxylates. Fortunately, seven new hybrid compounds based on {P2W18} were reported. [HC5H5N][Cu(2,2′-bpy)2]2[HP2W18O62]·2H2O (1) exhibits a one-dimensional (1-D) chain structure in which {P2W18} was linked by two [Cu(2,2′bpy)2]2+ . [HC5H5N][Zn(2,2′-bpy)2]2[HP2W18O 62]·4H2O (1a) is isostructural and isomorphous with compound 1. The difference between compounds 1 and 1a is that copper of compound 1 was replaced by zinc in compound 1a. [Cu4(2,2′bpy)3(nic)3(OH)2(H2O)][H3P2W18O62]·0.5H2O (2) shows a 1-D chain structure formed by {P2W18} and tetracopper TMMCs. [Cu2(2,2′-bpy)2(C2O4)]3[P2W18O62]·3H2O (3) is a 1-D sinusoidal chain structure constructed from {P2W18} but bicopper TMMCs. The structure of compound [Cu6(2,2′bpy)6(OH)6][P2W18O62]·2H2O (4) is based on {P2W18} linked by a hexacopper complex of 2,2′-bpy and hydroxyls. [Cu(2,2′-bpy)(hnic) 0.5 ][Cu 3 (2,2′-bpy) 3 (hnic) 2 (H 2 O) 2 ][H3P2W18O62] (5a) (2,2′-bpy = 2,2′-bipyridine) contains two different TMMCs: the tetra-copper TMMC connects {P2W18} to form a pseudo-1-D chain structure, whereas the tricopper TMMC was supported by {P2W18}. [Cu2(2,2′-bpy)2(hnic)][H4P2W18O62]·xH2O (5b) is 1-D chain structure which is generated from {P 2 W 1 8 } and bicopper [Cu 2 (2,2′bpy)2(hnic)]2+. We also discuss the mechanism for the formation of these structures. The catalytic and photocatalytic performances of all the hybrids were also carefully investigated.

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[Cu4(2,2′-bpy)3(nic)3(OH)2(H2O)][H3P2W18O62]·0.5H2O (2). Compound 2 was prepared using a procedure which is similar to that of 1, except that nicotinic acid (123.00 mg,1.00 mmol) was used to replace picolinic acid and Pr(NO3)3·6H2O (443.00 mg, 1.00 mmol) was added, and the pH was adjusted to 4 using 1.00 M NH3·H2O solution. Dark blue opacity block crystals were isolated with 53% yield (based on W). Anal. calcd for C48H44Cu4N9O71.5P2W18: W, 59.99; Cu, 4.61; P, 1.12; C, 10.45; H, 0.81; N, 2.29%. Found: W, 59.73; Cu, 4.56; P, 1.01; C, 10.69; H, 0.91; N, 2.19%. [Cu2(2,2′-bpy)2(C2O4)]3[P2W18O62]·3H2O (3). The synthesizing procedure of 3 was similar to that of compound 1, except that CuSO4 (499.00 mg, 2.00 mmol) replaced CuCl2, oxalic acid (126.00 mg, 1.00 mmol) was used to replace picolinic acid, and the pH was adjusted to 2.5 using 1.00 M HCl solution. Blue block crystals were obtained in 42% yield (based on W). Anal. calcd for C66H54Cu6N12O77P2W18: W, 55.16; Cu, 6.35; P, 1.03; C, 13.21; H, 0.91; N, 2.80%. Found: W, 54.93; Cu, 6.42; P, 1.10; C, 12.92; H, 0.71; N, 2.69%. [Cu6(2,2′-bpy)6(OH)6][P2W18O62]·2H2O (4). 4 was prepared using a procedure that is similar to that of 3, except that isophthalic acid 166.00 mg, 1.00 mmol) replaced oxalic acid, and the pH was adjusted to 3.5 using 1 M NH3·H2O solution. Dark blue block crystals were obtained with 47% yield (based on W). Anal. calcd for C60H58Cu6N12O70P2W18: W, 56.86; Cu, 6.55; P, 1.06; C, 12.38; H, 1.00; N, 2.89%. Found: W, 56.59; Cu, 6.35; P, 0.96; C, 12.56; H, 1.04; N, 2.93%. Mixture of [Cu(2,2′-bpy)(hnic)0.5][Cu3(2,2′-bpy)3(hnic)2(H2O)2][H3P2W18O62] (5a) [Cu2(2,2′-bpy)2(hnic)][H4P2W18O62]·xH2O (x ≈ 50) (5b) and 1. The synthesizing procedure of the mixture is very similar to that of compound 2, except that nicotinic acid (123.00 mg, 1.00 mmol) replaced oxalic acid, and the pH was 2.5. A mixture of three different types of crystals was obtained together with a total weight of about 125.00 mg: the first was blue block crystals (compound 5a), the second was hyaline green block crystals (compound 1), and the third was opaque dark green block crystals (compound 5b). Unfortunately, the ratio of the three compounds is hard to determine, for the three are similar to one another, so the elemental analyses of these compounds were not carried out. Crystallography. The data of compounds 1a and 2 were measured on a Bruker D8 QUEST ECO diffractometer equipped with a Mo Kα radiation (λ = 0.71073 Å) and the data of compounds 1, 3, 4, 5a, and 5b were measured on an Agilent Technology SuperNova Eos Dual system with a Mo−Kα (λ = 0.71073 Å) microfocus source and focusing multilayer mirror optics. The structure solutions and full matrix least-squares refinements on F2 were performed with SHELXS-2014/712 and SHELXL-2014/712 using the WinGX program. Anisotropic parameters were refined to the non-hydrogen atoms. Hydrogen atoms of organic ligands were calculated in their ideal positions in all compounds, and the hydrogen atoms attached to water molecules in all compounds were not located. As for compound 3, Cu(1), Cu(3), O(41), N(3) 2,2′-bpy, and N(1) nic− are all statically disordered, and the site occupancy factors of them are 0.5 except those of the N(3) pyridine ring of N(3) 2,2′-bpy. Positions of C(12), C(13) and N(3) of N(3) 2,2′-bpy are occupied by 1.0 C(12), 1.0 C(13) and 0.5 N(3) + 0.5 C(103), respectively. As for compound 4, the two sets of three polar tungstens of {P2W18} are statically disordered with occupancy factors of 0.5. As for compound 5a, the positions of Cu(4), Cu(5), the carbons and nitrogens of N(9), N(11) 2,2′-bpy, and N(13) hnic2− are all disorderedly occupied with occupancy factors of 0.5, too. CCDC number: 1587946 for 1, 1587947 for 1a, 1587948 for 2, 1587949 for 3, 1587950 for 4, 1587951 for 5a, and 1587952 for 5b. A summary of the crystallographic data and refinements of all the compounds are given in Table 1. Preparations of 1-, 1a-, 2-, 3-, 4-, and M5-CPEs. The 1-CPE was fabricated as below: the mixture of Nujol (1 μL), graphite powder (6 mg), and compound 1 (3 mg) were transferred in an agate mortar and grounded homogeneously, then packed into a poly(tetrafluoroethylene) tube with a 1.5 mm inner diameter, and the tube surface was carefully wiped with paper. In a similar manner, 1a-,

EXPERIMENTAL SECTION

Materials and Methods. K6[P2W18O62]·19H2O was synthesized according to a published procedure and confirmed by IR analysis.11 Infrared spectra were obtained on a PerkinElmer Spectrum One FTIR spectra-photometer using KBr pellets. UV−vis spectra were analyzed on a Shimadzu UV-3100 spectrophotometer. Powder X-ray diffraction (PXRD) patterns were recorded with a Scintag X1 powder diffractometer system using Cu Kα radiation with a variable divergent slit and a solid-state detector. The electrochemical measurements were carried out on a CHI 660B electrochemical workstation. Synthetic Procedures. [HC5H5N][Cu(2,2′-bpy)2]2[HP2W18O62]· 2H2O (1). The mixture of distilled water (20.00 mL), CuCl2 (342.00 mg, 2.00 mmol), picolinic acid (123.00 mg, 1.00 mmol), and 2,2′-bpy (78.00 mg, 0.50 mmol) was stirred continuously for about 1 h. After the addition of K6[P2W18O62]·19H2O (300.00 mg, 0.06 mmol), the mixture was further stirred for 2 h, and the pH was adjusted to 3 using 1.00 M HCl solution. The final suspension was transferred in a Teflon-lined autoclave and allowed to react to 160 °C for 3 days under autogenous pressure. After the autoclave was cooled to room temperature, blue block crystals were obtained with 42% yield (based on W). Anal. calcd for C45H43Cu2N9O64P2W18: W, 63.25; Cu, 2.43; P, 1.18; C, 10.33; H, 0.83; N, 2.41%. Found: W, 63.12; Cu, 2.66; P, 1.03; C, 9.96; H, 0.61; N, 2.14%. [HC5H5N][Zn(2,2′-bpy)2]2[HP2W18O62]·4H2O (1a). The mixture of distilled water (20.00 mL), ZnCl2 (272.00 mg, 2.00 mmol), 2,6dipicolinic acid (160.00 mg, 1.00 mmol), and 2,2′-bpy (78.00 mg, 0.50 mmol) was stirred for 1 h. K6[P2W18O62]·19H2O (300.00 mg, 0.06 mmol) was then added. After the mixture was stirred for another 2 h, the pH became 1.5. The final suspension was transferred in a Teflon-lined autoclave and heated to 165 °C for 3 days. The autoclave was finally cooled to room temperature. Green block crystals were obtained with 40% yield (based on W). Anal. calcd for C45H47Zn2N9O66P2W18: W, 62.77; Zn, 2.48; P, 1.18; C, 10.26; H, 0.90; N, 2.39%. Found: W, 62.11; Zn, 2.46; P, 1.13; C, 10.51; H, 0.80; N, 2.33%. C


Article

Inorganic Chemistry 2-, 3-, 4-, and M5-CPEs were prepared with compounds 1a, 2, 3, 4, and M5. CV measurements were carried out using a CHI 660b electrochemical workstation. A three-electrode system was used with Ag/AgCl as a reference electrode and Pt wire as a counter electrode. CPEs were used as the working electrodes. CV measurements were carried out in a 1 mol·L−1 H2SO4 solution.

compounds is directly introduced by using imidazole as the starting material, but the pyridine in our compounds is transformed via an in situ decarboxylation reaction. Crystal Structure of Compound 2. The asymmetric unit of compound 2 consists of four Cu2+, two nic−, three halves of 2,2′-bpy, and half a {P2W18}. As shown in Figure 2b, Cu(1) is

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RESULTS AND DISCUSSION Crystal Structure of {P2W18}. The results of bond valence sum calculations of tungstens of {P2W18} in compound 1 give the average value 6.11, confirming that the tungstens are in the +6 oxidation state.13 The tungstens can be divided into two types: six “polar” ones, grouped into two sets of three, and 12 “equatorial” ones, grouped into two sets of six. Accordingly, surface oxygens can be grouped into two kinds: polar and equatorial ones. Crystal Structure of Compounds 1 and 1a. The asymmetric unit of compound 1 is composed of half a [Cu(2,2′-bpy)2]2+, a {P2W18}, a water molecule, and a pyridine. Cu(1) is in an octahedral environment with four 2,2′-bpy nitrogens and two oxygens from two {P2W18}. Two Cu(1) serve as two bridges connecting {P2W18} to produce a 1-D straight chain structure propagating along the [0 0 1] direction. That is to say, {P2W18} was linked to one another in a 1-D structure by the Cu(1)-complex double-bridge (Figure 1). The pyridine, as the secondary organic species,14 does not coordinate to any metal.

Figure 2. (a) Combined polyhedral and wire representation of the 1D chain formed by the pseudo-tetracopper TMMC and {P2W18} in compound 2; (b) the triangle motif of the TMMC; (c) the horseshoe motif of the TMMC. Some of the disordered atoms are halftransparent. a: x, 1.5 − y, z.

Figure 1. Combined polyhedral and wire representation of the 1-D chain in compound 1.

five-coordinated in a pyramidal geometry by O(40) from N(1) nic−, O(41) of a hydroxyl, O(35a, a: x, 1.5 − y, −1 + z)) from a {P2W18} and N(1), N(2) from N(1) 2,2′-bpy. Cu(3) adopts a pyramidal geometry with O(39) from N(1) nic−, the same hydroxyl oxygen O(41), O(38) of a coordinated water molecule, O(37a) from another {P2W18}, and N(5) from N(5) nic−. Cu(2) displays a pyramidal geometry with two O(42) from two N(5) nic− and two N(4) from N(4) 2,2′-bpy forming its square plane and O(2w) of a water molecule located at its apical position, while Cu(4) exists in a pyramidal geometry, of which the square plane comprises two N(6) from N(6) 2,2′-bpy and two O(43) from two N(5) nic− and the apical position is occupied by O(10) from the same {P2W18} involving Cu(3). Obviously, Cu(1) and Cu(3) are joined by the hydroxyl O(41) and the bis-monodentate coordination bridging N(1) nic−, Cu(2) and Cu(4) are respectively linked by N(5) and N(5a) nic− with a bis-monodentate coordination mode, while Cu(2), Cu(3), and Cu(4) are joined by N(5) nic− using a tri-monodentate coordination mode. Alternatively, N(5) nic− serving as a μ3-bridge joins Cu(2), Cu(3), and Cu(4), N(5a) and N(1) nic− as μ2-bridges connect Cu(2), Cu(4), and Cu(1), Cu(3), respectively. Therefore, a tetracopper TMMC was formed. For Cu(1), Cu(3), O(41), N(3)

Compounds 1 and 1a are isostructural and isomorphous. The main difference between compounds 1 and 1a is that the metal of the transition metal complex (TMC) is Zn in compound 1a but Cu in compound 1. It should be noted that the pyridine in compound 1 is transformed from the picolinic acid via in situ decarboxylation. It has been reported that more than 10 different in situ organic ligand formation reactions were previously investigated.15 Hydrothermal decarboxylation reactions has been observed several times in preparations of coordination polymers.15,16 In complete contrast to compound 1, the pyridine in compound 1a is transformed from 2,6dipicolinic acid via a more complex in situ reaction, for two carboxyl groups were in situ eliminated to yield the pyridine. To our knowledge, in situ decarboxylation with the elimination of two carboxyl groups has never been reported before. Two very similar compounds (Himi)2[{M(2,2′-bpy)2}2(P2W18O62)]·H2O (M = Mn and Cu) have been reported by Zhou et al. recently.17 The main difference between our compounds and Zhou’s compounds is that the isolated organic moieties in our compounds are pyridine but imidazole in Zhou’s compounds. More importantly, the imidazole in Zhou’s D


Article

Inorganic Chemistry 2,2′-bpy, and N(1) nic− are all disordered, the TMMC is thus a pseudo-tetracopper one. It is interesting to see that the TMMC shows a horseshoe shape (Figure 2c). If we do not omit the disordered atoms, the TMMC exhibits a triangle shape (Figure 2b). In addition, the two chelating 2,2′-bpy of Cu(2) and Cu(4) are arranged parallel with the plane to plane distance and dihedral angles of ca. 3.9 Å and 12.1°, indicating the existence of the weak π···π interaction between the two, which can lead to an increase of the stability of the TMMC. One {P2W18} using its two terminal oxygens interacts with Cu(3) and Cu(4) of TMMC with Cu−O bond lengths of 2.22(1)−2.287(9) Å, while Cu(1) of TMMC further interacts with a terminal oxygen from another {P2W18} with the Cu−O bond length of 2.303(8) Å, meaning that TMMC acts as a bridge joining {P2W18} to give rise to a 1-D straight chain structure (Figure 2a). To our knowledge, such a complicated structure containing POMs and TMMCs was not reported previously. Crystal Structure of Compound 3. There are three Cu2+, three 2,2′-bpy, three halves of ox2−, two water molecules, and half a {P2W18} in the asymmetric unit of compound 3. Cu(1) is six-coordinated in an octahedral geometry by two 2,2′-bpy nitrogens, two oxygens from ox2−, and two oxygens from two {P2W18}. Two Cu(1) are linked by the bis-bidentate coordination ox2− to form the Cu(1) TMMC which bridges adjacent {P2W18} to generate a zigzag chain structure along the [0 1 0] direction. In contrast, both Cu(2) and Cu(3) are fivecoordinated in a pyramidal geometry by two 2,2′-bpy nitrogens, two oxygens from ox2−, and one oxygen from {P2W18}. Two Cu(2) and Cu(3) are respectively joined by one ox2− using a bis-bidentate coordination mode to form Cu(2) and Cu(3) TMMCs, both of which are supported by one {P2W18} via Cu−O contacts to give rise to a POM supported complex. In conclusion, compound 3 is built on {P2W18} connected by Cu(1) TMMC via axial Cu···O−W bounds, leading to a {[P2W18][Cu2(2,2′-bpy)2(C2O4)]}n 1-D zigzag chain. Two additional TMMCs (Cu(2) and Cu(3) ones) are also connected to each {P2W18}, ensuring the electroneutrality of compound 3 (Figure 3). What makes Cu(1) TMMC different from Cu(2) and Cu(3) TMMCs? Detailed analysis of the two kinds of TMMCs is shown below: the dihedral angles between the two planes

respectively through copper-two-ox2−-oxygens and coppertwo-2,2′-bpy-nitrogens of Cu(2) and Cu(3) TMMCs are in the range of 37.452° and 38.105°, whereas the corresponding dihedral angle of the Cu(1) TMMC is 21.822°. The dihedral angle in the Cu(1) TMMC is obviously smaller, indicating the weak steric hindrance between the Cu(1) TMMC and {P2W18}. Perhaps this is the main reason why only the Cu(1) TMMC serves as the bridge linking {P2W18}. To our knowledge, compound 3 is the first extended structure of {P2W18} and TMMC of ox2−. [Cu2(2,2′-bpy)2(ox)]2+, which was first synthesized by Gutiérrez-Zorrilla, bridges Keggin POMs in extended structures.6a,b The first difference between Gutiérrez-Zorrilla’s compounds and compound 3 is that the size of {P2W18} is obviously bigger than that of the Keggin POM in GutiérrezZorrilla’s compounds; the second is that there are two additional different TMMCs supported by {P2W18} in compound 3, but Gutiérrez-Zorrilla’s compounds contain only [Cu2(2,2′-bpy)2(ox)]2+ bridges. In 2007, Xu and Liu prepared two compounds which were similar to GutiérrezZorrilla’s compounds but based on Anderson POMs with a smaller size.18 Very recently, we also reported a bicapped Keggin Mo−V cluster with the intermediate size linked by the identical TMMC.19 The difference between our recently reported compound and compound 3 is that, except for the TMMC linker, there are two different dissociated TMCs in our recently reported compound, but there are two different supported TMMCs in compound 3. The analogous TMMC of cobalt and phen has been prepared by us recently, too, but the cobalt analogue did not serve as the bridge, and the involved POM was arsenic−vanadium cluster.20 Both compounds 1 and 2 are examples of {P2W18} with its polar terminal oxygens interacting with copper complexes. The reason can be concluded as follows: (1) more negative charges are distributed on the surface of the polar area of {P2W18}, indicating that the polar oxygens are more nucleophilic and easy to bind to metals. (2) Another reason is that TMMC has two or more organic ligands and metals, indicating that the size of TMMC is big and the steric hindrance between any two TMMCs is strong. Therefore, the polar oxygens are more suitable for two TMMCs to coordinate to so the two TMMCs can be positioned farther away from each other. That is the reason why 1-D chain structures were formed in compounds 1, 1a, and 2. (3) The M−M distance of TMMC and the Ot−Ot (Ot: terminal orygen) distance of {P2W18} can fit into each other or not. As for TMMCs of compound 2, Cu(1)−Cu(3) distance is 3.42 Å, and Cu(3)−Cu(4) distance is 7.29 Å. Ot− Ot distances between any two polar oxygens are 5.53−5.54 Å. Therefore, Cu(1)−Cu(3) cannot, but Cu(3)−Cu(4) can simultaneously interact with two polar Ot. As for compound 3, Cu(1) TMMC interacts with two polar Ot, while Cu(2) and Cu(3) TMMCs interact with two equatorial Ot. M−M distances of all the three TMMCs are 5.09−5.16 Å. Ot−Ot distances between any two polar Ot are 5.53−5.54 Å, whereas Ot−Ot distances between any two equatorial Ot can be grouped into two sets: one is in the range of 4.82−4.85 Å, and the other is in the 5.48−5.52 Å range. Clearly, Ot−Ot distances between two polar Ot and one set equatorial Ot are comparable, and thus all these involved pairs of Ot can show interactions with the coppers of TMMCs simultaneously. (4) Sometimes, the above-mentioned speculations cannot fully explain the bonds of some structures; for example, the equatorial Ot has the priority over the polar ones to coordinate

Figure 3. Combined polyhedral and wire representation of the 1-D chain in compound 3. E


Article

Inorganic Chemistry

by Wang et al. Wang’s complex is a bicopper hydroxyl cluster.7d Obviously, Wang’s compound and compound 4 are significantly different. Compound 4 here is the first structure constructed from POMs and high-nuclear copper (>2) hydroxyl 2,2′-bpy complexes. Crystal Structure of Compounds 5a and 5b. The asymmetric unit of compound 5a is composed of five Cu2+, five 2,2′-bpy, three hnic2− (hnic2− = hydroxyl nicotinic acid), a {P2W18} and three coordination water molecules. Cu(1) presents an octahedral geometry with O(103), O(104) from N(5) hnic2−, O(1w), O(2w) of water molecules and N(1), N(2) form N(1) 2,2′-bpy. Cu(2) is in a pyramidal environment with O(105) of N(5) hnic2− hydroxyl, O(104) from N(5) hnic2−, O(101) from N(6) hnic2− and N(3), N(4) from N(3) 2,2′-bpy. Cu(3) is five-coordinated by O(100) and O(101) from N(6) hnic2−, O(26a, a: 1 + x, −1 + y, z) from {P2W18} and N(7), N(8) from N(7) 2,2′-bpy. N(5) hnic2− in a bis-bidentate coordination mode respectively chelates Cu(1) and Cu(2), while N(6) hnic2− adopting the bidentate and monodentate coordination mode coordinates to Cu(2) and Cu(3) (Figure 5c). Thus, Cu(1), Cu(2), and Cu(3) were linked by N(5) and N(6) hnic2− to form a tricopper TMMC, which interacts with {P2W18} via a Cu−O contact with the Cu−O bond length of 2.50(1) Å, generating a {P2W18} supported complex. Hnic2− is transformed from the nic− via in situ hydroxylation, which is first reported by our group

to the metal of TMMC in some structures. The reason is that, with the priority of the equatorial Ot, there will be more Cu−O interactions formed between TMMCs and {P2W18} in those structures, and consequently, more Cu−O interactions will significantly increase the stability of the crystals. As for compound 3, only the Cu(2) TMMC interacts with a pair of polar Ot, but the Cu(1) and Cu(3) TMMCs interact with two pairs of equatorial Ot. If the Cu(3) TMMC was supported by two polar Ot, there would be strong steric hindrance between the Cu(3) and Cu(1) TMMCs, so the Cu(1) TMMC cannot be coordinated by the equatorial Ot, and then the Cu(1) TMMC will be dissociated. If the Cu(1) TMMC bridges two pairs of polar Ot from two adjacent {P2W18}, there will be a dimer of {P2W18} joined by the Cu(1) TMMC with two Cu(2) TMMCs supported by each {P2W18}, and the Cu(3) TMMC will be dissociated. Comparing the structure of compound 3 and the two kinds of speculations, we can see that there are more Cu−O interactions in compound 3, and consequently, the structure of compound 3 is more reasonable. Crystal Structure of Compound 4. The asymmetric unit of compound 4 is comprised of half a {P2W18}, three Cu2+, three 2,2′-bpy, and three hydroxyl atoms. The three Cu2+ adopt almost the same pyramidal geometry. Cu(1) is fivecoordinated by a terminal oxygen from {P2W18}, two hydroxyl oxygens, and two 2,2′-bpy nitrogens, while Cu(2) and Cu(3) are respectively bonded to three hydroxyl oxygens and two 2,2′-bpy nitrogens. As shown in Figure 4, Cu(1) shares two

Figure 4. Combined polyhedral and wire representation of {P2W18} and hexacopper TMC in compound 4. a: −x, 1 − y, 1 − z.

hydroxyls and one hydroxyl with Cu(2) and Cu(3), respectively, Cu(2) respectively shares two hydroxyls and one hydroxyl with Cu(3) and Cu(3a), while Cu(3) and Cu(3a) are further linked by sharing two hydroxyls. Therefore, two Cu(1), two Cu(2), and two Cu(3) complexes are join to form a novel hexacopper unit. The plane-to-plane distances of adjacent 2,2′-bpy are in the range of 3.5−3.7 Å, and the corresponding dihedral angles are in the range of 1.014− 13.969°, indicating that π···π interactions exist between any two of the 2,2′-bpy ligands. The apical position of Cu(1) pyramid is occupied by a terminal oxygen from {P2W18} with a strong Cu−O covalent interaction (Cu−O bond: 2.28(3) Å), indicating that the hexacopper cluster acts as a bridge linking adjacent {P2W18} to produce a straight chain structure (Figure 4). Copper mixed-ligand complexes of hydroxyl and 2,2′-bpy have been intensively studied.21 and the biggest cluster reported was just {Cu6(OH)6},21i,j which is identical to the copper hydroxyl cluster in compound 4. The biggest difference of the reported compounds and compound 4 is that the copper hydroxyl cluster here is not discrete but acts as an inorganic ligand rigidly linked to {P2W18}. A compound constructed from complexes of hydroxyl and Keggin POMs was reported

Figure 5. (a) Combined ellipsoid (50% probability), polyhedron, and wire representation of the 1-D chain in compound 5; Combined ellipsoid (50% probability) and wire representation of pseudotetracopper TMMCs (b) and the tricopper (c) in compound 5. a: 2 − x, 1 − y, −z. F


Article

Inorganic Chemistry recently.4 In addition, the bis-bidentate coordination mode of N(5) hnic2− was not reported previously.4 Except for the tricopper TMMC, there is a pseudotetracopper TMMC (Figure 5b). Cu(4) is seven-coordinated by two 2,2′-bpy nitrogens, two carboxylic oxygens from N(13a, a: 2 − x, 1 − y, −z) hnic2−, one terminal oxygen form a {P2W18}, one heterocyclic nitrogen and carbon from N(13) hnic2−. The short Cu−N (2.06(8) Å) and Cu−C (2.11(9) Å) distances between the Cu(4) and N(13) pyridine ring of N(13) hnic2− indicate that there is a special cation−π interaction between Cu(4) and the N(13) pyridine ring with Cu−N−C and Cu−C−N angles of 74.5° and 66.7°. The coordination mode can be regarded as η2. Cu(5) displays a square pyramidal geometry with two 2,2′-bpy nitrogens, one carboxylic oxygen, and the hydroxyl oxygen from N(13) hnic2− and a terminal oxygen from another {P2W18}. Cu(5) and Cu(4a, a: 2 − x, 1 − y, −z) are linked by the bis-bidentate coordination N(13) hnic2−, while Cu(4) is linked to the N(13) hnic2− via the special Cu-π interaction. Therefore, two Cu(4) and two Cu(5) are joined by two N(13) hnic2− to form a tetracopper TMMC. For Cu(4), Cu(5), N(9), N(11) 2,2′-bpy, and N(13) hnic2− are all disorderedly occupied, and the TMMC here is a pseudo-tetracopper TMMC. Some complexes of Ag−π and Au−π interactions were reported and studied.22,23 Theoretical study about Cu−π interactions of Cu+ has been reported too.24 However, to our knowledge, still no complexes of Cu−π interactions have been reported up to now. Compound 5a is the first example of complexes containing Cu−π interactions. The existing of cation−π interaction or not is important in a number of supramolecular systems.25 And recently, it has been proposed that the interaction of Cu−π plays a significant role in the aromatic cross-coupling reaction.26 Therefore, one needs to know more about the Cu−π interaction. Compound 5a here provides a remarkable example. Both Cu(4) and Cu(5) have strong Cu−O covalent interactions with equatorial Ot from {P2W18} (Cu−O distances: 2.41(2)−2.55(3) Å). That is to say, each pseudotetracopper TMMC is linked to four {P2W18}, while each {P2W18} is connected to two pseudo-tetracopper TMMCs. Thus, {P2W18} and pseudo-tetracopper TMMCs are connected to form a pseudo-1-D double chain structure. To our knowledge, such an interesting compound was not reported previously (Figure 5a). Cu(2)−Cu(3) and Cu(1)−Cu(3) distances of the tricopper TMMC are 5.39 and 7.02 Å, indicating that both Cu(2)− Cu(3) and Cu(1)−Cu(3) can be coordinated simultaneously by two Ot. But there is only one copper of the tricopper TMMC coordinated by a polar Ot, and the reason is that the steric hindrance between the tricopper TMMC and {P2W18} cannot let any two of its coppers to be coordinated by two Ot simultaneously. The Cu(4)−Cu(5) distance of the pseudotetracopper TMMC is 5.59 Å, but Cu(4)−Cu(5) were not coordinated by any two Ot simultaneously, too, also for the strong steric hindrance between the pseudo-tetracopper TMMC and {P2W18} . The tricopper TMMC was supported by {P2W18} using one of its polar Ot. There is a question: why the pseudotetracopper TMMC interacts with {P2W18} via equatorial Ot atoms but not polar ones. As shown in Figure 5, the pseudotetracopper TMMC can interact with four equatorial Ot atoms from four {P2W18} (all the metals of the TMMC show interactions with {P2W18}). If the pseudo-tetracopper TMMC

interacts with four polar Ot atoms from four {P2W18}, for the four {P2W18} each have already supported a tricopper TMMC. That is to say, the two sides of each {P2W18} will be one pseudo-tetracopper TMMC and one tricopper TMMC, respectively. So, no 1-D extended structure mentioned above will be formed, and also not so many Cu−O contacts will be formed too. As mentioned above, more Cu−O contacts will increase the stability of the crystal structure. Therefore, the tetracopper TMMC has to be coordinated by equatorial Ot. We have focused on the preparations and characterizations of POM-TMMCs recently. A series of POM-based hybrids of TMMCs have been synthesized by us.4,8 To our knowledge, POM-based hybrids based on two TMMCs like compound 5a have never been reported before. Compound 5b with compound 5a was synthesized together. The structure of compound 5b is much simpler than that of compound 5a. In compound 5b, there is only one TMMC [Cu2(2,2′-bpy)2(hnic)]2+, which consists of a hnic2−, two coppers, and two 2,2′-bpy. Cu(1) is six-coordinated in an octahedral geometry by the hydroxyl oxygen O(65) and one carboxylic oxygen of N(5) hnic2−, two oxygens from two {P2W18}, and two nitrogens from a 2,2′-bpy. Cu(2) is fivecoordinated by the other carboxylic oxygen of N(5) hnic2−, two oxygens from two {P2W18}, and two nitrogens from a 2,2′bpy, adopting a pyramidal geometry. N(5) hnic2− adopts a monodentate and bidentate coordination mode to connect the two coppers in the TMMC. Both coppers show covalent interactions with the same two {P2W18}, indicating that the two coppers serve as bridges linking {P2W18} in a 1-D chain structure (Figure 6).

Figure 6. Combined ellipsoid (50% probability), polyhedron, and wire representation of the TMMC and {P2W18} in compound 5b.

It is interesting to discuss the interactions between the TMMC and the its two adjacent {P2W18}. The involved Cu− O contacts of Cu(2) (1.99(3) and 2.37(3) Å) in TMMC are much stronger than those of Cu(1) (2.5282(1) and 2.7190(1) Å), indicating that the driving force that causes the formation of compound 5b is Cu−O interactions between Cu(2) and the two equatorial Ot atoms from two neighboring {P2W18}. It G


Article

Inorganic Chemistry

2-, 3-, 4-, and M5-CPEs) between −650 and +50 mV were recorded in Figure S5. Three pairs of redox couple peaks with E1/2 = (Epa + Epc)/2 of −571, −342, −101 mV, −590, −365, −128 mV, −585, −356, −115 mV, −567, −345, −106 mV, −572, −349, −120 mV and −578, −354, −102 mV, respectively, were observed in the CVs of 1, 1a, 2−4, and M5- CPEs. All the CVs are similar, confirming the same polyanions in them. The peaks can be associated with one twoelectron processes and two consecutive one-electron processes of {P2W18}.28 POM materials may be photocatalysts to decolorize dyes.29 Rhodamine B (RhB) is selected as model pollutant to evaluate the photocatalytic activity of photocatalysts. The performances of compounds 1, 1a, 2−4, and M5 for degrading RhB have been investigated with the process: catalyst (5 mg) was added in RhB solution (100 mL, 3.0 × 10−5 mol·L−1), and the suspension was stirred magnetically for 30 min in the dark and then were placed about 4−5 cm under the irradiation of a 500 W Hg lamp. The stirring rate was 790−800 rpm. At 60 min intervals, 4 mL of samples was taken out, centrifuged, and then analyzed by UV−vis spectroscopy. The RhB photodegradation without catalyst was carried out for comparison, and 30.1% of RhB was decolorized after 360 min (Figure S6). Changes in the Ct/C0 plot of RhB solutions versus reaction time are displayed in Figure S7. The absorption peaks of all compounds decreased more rapidly upon irradiation, indicating that these catalysts exhibit photocatalytic properties. Though compound 1 and 1a are isostructural and isomorphous, the degradation efficiency of compound 1a (69.41%) is much higher than that of compound 1 (56.57%). Compound M5 contains compounds 1, 5a, and 5b, the RhB degradation of M5 (58.12%) is very similar to the degradation of 1. Of the compounds, and compound 1a exhibits the best photocatalytic activity with the RhB degradation of 69.41%. Compound 3 based on TMMC of 2,2′-bpy and ox2− exhibits the lowest degradation efficiency, which is only 34.59%. Repeated photocatalytic experiments of compound 1 were carried out to evaluate its stability and reusability because its amount is enough. With the dose of 1 increased up to 50 mg, the conversion increased from 56.57% to 89.83%. After each cycle of photocatalytic degradation, 1 was separated, washed with water, dried, and reused directly. The degradation rates of compound 1 and the recovered 1 after three cycles decreased from 89.83% to 85.60%, indicating the good reusability of compound 1 (Figure S8). The XRD and IR analysis (Figure S9) of the recovered samples further demonstrated the stability of 1. The epoxidation of alkenes is a significant reaction because epoxides are rather important intermediates for organic synthesis.30 POMs have been paid more attention as a significant family of catalysts in the epoxidation.31 Our group also used some POM-based compounds as catalysts for styrene epoxidation, and some display higher catalytic activities.5g,20 The styrene epoxidation using aqueous tertbutyl hydroperoxide (TBHP) with catalysts 1, 1a, 2−4, and M5 was performed. The catalytic reactions were performed based on the following procedure. An acetonitrile (2 mL) solution of styrene (0.114 mL, 1 mmol) as the substrate, 2 mg catalyst (compounds 1, 1a, 2−4, and M5) were mixed to a 10 mL twoneck flask where one neck was equipped with a reflux condenser. The mixture was continuously stirred and then heated to 80 °C. After the addition of 2 mmol TBHP to start the reaction, the products of the reaction were identified and

should be noted that the two corresponding {P2W18} also sandwich Cu(1) in TMMC via one polar and one equatorial Ot atoms from the two {P2W18}, respectively (Figure S1). The Cu−O contacts of Cu(1) are weaker but also important for the structure. If one of the two equatorial Ot around Cu(2) were substituted by one polar Ot from one of the two {P2W18}, there will be no chance for Cu(1) to interact with any Ot of the same {P2W18}, because the two {P2W18} Ot atoms coordinating to Cu(2) are located in cis-positions. Thus, Cu(2) interacting with two equatorial Ot can make the interactions between Cu(1) and the two {P2W18} possible and consequently increase the number of Cu−O contacts. Syntheses and Discussion. All compounds were prepared hydrothermally at 160 °C except that compound 1a was obtained at 165 °C. According to the experiments, the combination of different Cu salts, different carboxylates and 2,2′-bpy resulted in different products. The pH has a significant influence on the preparations. The pH was adjusted in the range of 2.5−4 for all compounds. When the pH was out of the range, only unidentified precipitates were obtained. The molar ratio of {P2W18}/2,2′-bpy/carboxylate/metal is also very important for the preparations of these hybrids. We found that the molar ratio of about 3:25:50:100 is optimal for these compounds. In the synthesis of compound 2, Pr(NO3)3 was used as a starting material; unfortunately, Pr was not incorporated into compound 2. Although Pr is absent in the product, Pr(NO3)3 is necessary for the formation of compound 2. One or more starting materials that were not incorporated into the final products were often observed in hydrothermal preparations.27 In summary, the pH, reaction temperature, selection, and molar ratio of raw materials are key factors for the preparations of these compound.

■

CHARACTERIZATION OF THE COMPOUNDS As shown in Figure S2, four peaks at 1094, 961, 901, and 790 cm−1 were observed in the IR spectra of 1, ascribed to ν(P− Oc), ν(W−Ot), ν(W−Ob−W), and ν(W−Oc−W), respectively. The bands of 1618−1188 cm−1 are assigned to 2,2′-bpy in 1. IR spectra of compounds 1a, 2−4, and the mixture of compounds 5a, 5b, and 1 (the mixture of compounds 5a, 5b, and 1 = compounds M5) exhibit four similar characteristic peaks at 1093, 960, 902, 790 cm−1, 1093, 950, 906, 793 cm−1, 1092, 955, 906, 814 cm−1, 1091, 954, 910, 790 cm−1, and 1089, 954, 914, 793 cm−1, respectively, indicating the same POMs in all these compounds. The phase purity of compounds 1, 1a, and 2−4 has been confirmed by the XRD analysis (Figure S3). The electronic spectra (UV−vis) of compounds 1, 1a, and 2−4 and M5 are presented in Figure S4. There are two peaks and a shoulder peak at 258, 287, and 315 nm in the UV−vis spectrum of 1, associated with O → W charge transfer of {P2W18} and n → π* transitions of 2,2′-bpy. UV−vis spectra of compounds 1a, 2−4, and M5 present similar peaks at 256, 283, 304 nm, 256, 302, 313 nm, 257, 302, 313 nm, 257, 302, 313 nm and 256, 302, 313 nm, also due to their {P2W18} and organic ligands, respectively. The peaks due to 2,2′-bpy of compounds 1 and 1a are centered at 287 and 283 nm, whereas the corresponding peaks of compounds 2−4 and M5 were centered at 302 nm. The red shift of the peaks in the spectra of compounds 2−4 and M5 may be attributed to the relatively complex coordination environments in compounds 2−4 and M5. The cyclic voltammograms (CVs) of the carbon-paste electrodes (CPEs) modified with 1, 1a, 2−4 and M5 (1a-, H


Article

Inorganic Chemistry

Table 3. Recyclability and Reusability of Compound 3a

quantified by gas chromatography (Shimadzu, GC-8A) equipped with an HP-5 capillary column and a flame detector. The result of the styrene epoxidation experiment without catalyst showed that the styrene conversion is about zero after 8 h. The results obtained in the styrene oxidation with various catalysts is shown in Table 2. Though compound 1a displays

product selectivity (%)

Table 2. Catalytic Activity and Product Distributiona

catalysts

styrene conversion (%)

So

Bza

others

first second third

95.93 95.72 94.86

86.06 76.18 88.55

13.94 15.00 9.21

0 8.82 2.24

a

So: styrene oxide, Bza: benzaldehyde, others: including benzoic acid, phenylacetaldehyde.

product selectivity (%) catalysts


So

Bza

others

no catalyst compound 1 compound 1a compound 2 compound 3 compound 4 compounds M5 compound 6 compound 7 K6P2W18

∼0 93.75 16.38 85.05 92.54 91.33 91.96 78.4 54.0 22.5

84.28 32.63 65.53 82.98 70.00 76.15 67.2 69.2 10.8

15.72 67.37 33.41 11.25 30.00 16.17 30.30 30.80 89.10

0 0 1.05 5.77 0 7.68 2.4 0.0 0.1

recyclability of 3. The IR and X-ray analysies of the catalyst after three reaction cycles have been further recorded (Figure S10), suggesting that the catalyst has an excellent stability during the styrene selective oxidation reactions. Though compound 3 is stable during the catalytic reaction and only a few of compound 3 were leached into the solution, there is a significant activity in the supernatant after the suspension of compound 3 is used (after a catalytic run, filter to remove the insoluble catalyst, and repeat the reaction by adding fresh oxidant and substrate to the previously used supernatant MeCN solution. The conversion of styrene is 57.5%, and the selectivity is 79.9%). Therefore, the leached compound 3 argues for the homogeneous nature of the catalytic epoxidation. To further investigate the catalytic mechanism of these compounds, we used the same procedure except that K6[P2W18O62]·19H2O was not added in the mixture to prepare the TMMC compound of compound 3, and finally after the autoclave was cooled to room temperature and blue solid crystals of 8 were obtained. Unfortunately, these crystals of 8 were not suitable for single crystal analysis. Then we used these crystals of 8 as the catalyst for styrene epoxidation. From the comparison of the catalytic properties of these crystals of 8, the mixture of the crystals of 8 and K6[P2W18O62]·19H2O, and compound 3 (Table 4), we can conclude that compound 8 is

a

So: styrene oxide, Bza: benzaldehyde, others: including benzoic acid, phenylacetaldehyde. K6P2W18K6[P2W18O62]·19H2O

the highest photocatalytic degradation efficiency, compound 1a shows the lowest activity for styrene oxidation with a conversion of 16.38% and a selectivity of 32.63%. 1 displays the best catalytic activity with the conversion and selectivity of 93.75% and 84.28%. Compounds 2, 3, 4, and M5 also show better catalytic conversions of 85.05, 92.54, 91.33, 91.96% and selectivities of 65.53, 83.98, 70.00, and 76.15%, respectively. Compared with the catalytic result of K6[P2W18O62]·19H2O, all the compounds of copper in this article were excellent catalysts for the styrene selective oxidation.10b Although compound 1 and 1a are isostructural and isomorphous, the catalytic activities of the two have a significant difference, which is mainly attributed to the distinction of the transition metals. It is still elusive why these compounds of copper are good catalysts. Very recently, we synthesized two compounds based on {P2W18} and copper TMCs [Cu(phen)2][Cu(phen)(H2O)3][Cu(phen)2(H2O)][P2W18O62]·8H2O (6) (phen = 1,10-phenanthroline) and [Cu(2,2′-bpy)(H2O)2]2[Cu(2,2′-bpy)2][P2W18O62]·3.5H2O (7).10b Compound 1 of the five compounds of copper is similar to the two reported compounds, but the catalytic performance of compound 1 is significantly higher than those of compounds 6−7. The first difference between compound 1 and compounds 6−7 is that the copper TMCs in compound 1 and the two reported compounds is different. The second and the most important difference between 1 and compounds 6−7 is that compound 1 is a 1-D extended structure, but the two are discrete. It should be noted that all the compounds here are 1-D extended structures. Is the 1-D extended structure the main reason for the high performances of compounds 1−4 and M5? We think we still need more different catalysts and catalytic results to demonstrate it. 3 was chosen to investigate its stability and recyclability, for the amount of 3 is sufficient (Table 3). The experimental conditions were the same, except that the dose of compound 3 was increased to 10 mg. With the dose increased, the conversion increased to 95.93%, and the selectivity was also higher. The styrene conversion is reduced from 95.93% to 94.86% after three cycles (Table 3), showing the good

Table 4. Comparison of Compound 8, Mixture of 8 and K6P2W18 and Compound 3a product selectivity (%) catalysts 8 (0.4 mg) 8 (0.4 mg) + K6P2W18 (1.6 mg) compound 3


So

Bza

Others

61.2 67.4

47.4 57.8

43.3 37.1

9.3 5.1

92.54

82.98

11.25

5.77

a

So: styrene oxide, Bza: benzaldehyde, others: including benzoic acid, phenylacetaldehyde. K6P2W18K6[P2W18O62]·19H2O.

very important for the epoxidation of styrene, and the final product of compound 3 is also very important for the catalytic process. The simple mixture of 8 and K6[P2W18O62]·19H2O only slightly increased the conversion, but compound 3 significantly increased the conversion of styrene. The comparison of these catalytic results further verified that the structure of the multicomponent catalyst significantly influenced their catalytic properties.32

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CONCLUSIONS Seven new compounds of Wells-Dawson POMs were synthesized and characterized. All the compounds had a 1-D chain structure. Compounds 1 and 1a are isomorphous and isostructural with each other, and both exhibit a 1-D chain I


Article

Inorganic Chemistry

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structure. It is well-known that both Cu and Zn are amphoteric cations, which are, we think, the reason why compounds 1 and 1a are isomorphous and isostructural. Compound 2 is based on a tetracopper TMMC, while compound 3 is based on a bicopper TMMC. Compound 4 is the first compound that is based on POM and hexacopper TMMC of 2,2′-bpy and hydroxyls. Compound 5a is the first compound that contains Cu···π interactions. Compound 5b is formed by {P2W18} and [Cu2(2,2′-bpy)2(hnic)]2+. We not only present several novel compounds with unprecedented structures, we but also studied the catalytic properties of these novel compounds, and all of them exhibit high catalytic performances for styrene epoxidation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01705. The coordination environments of Cu(1) and Cu(2) in compound 5b. IR spectra of compounds. XRD patterns. UV-vis spectra. Cyclic voltammograms. UV-vis absorption spectra of the RhB solutions degraded with compounds. Changes in the Ct/C0 plots of RhB solutions versus reaction time. The reproducibility of compound 1 for photodegradation of RhB for three cycles. X-ray powder diffraction patterns and IR spectra of compound 1 after each cycle of the photodegradation of RhB. X-ray powder diffraction patterns and IR spectra of compound 3 after three cycles of epoxidation of styrene to styrene oxide (PDF) Accession Codes

CCDC 1587946−1587952 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xiao Zhang: 0000-0002-3036-7001 Xiao-Bing Cui: 0000-0002-5254-7935 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This manuscript was supported by National Science Foundation of China (No. 21003056). REFERENCES

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