Four hybrid compounds based on a new type of molybdates and a flexible tripodal ligand: synthesis, structures, photochemical and electrochemical properties

Abstract

The hydrothermal reaction of a mixture of (NH₄)₆Mo₇O₂₄·4H₂O, a flexible tripodal ligand 1,3,5-tris-(imidazol-1-ylmethyl)-2,4,6-trimethylbenzene (TITMB) and metal salts at different temperatures resulted in four novel compounds, namely [H₂TITMB]₂[Mo₁₂O₃₈]·4H₂O (1), [Zn(Mo₂O₇)(TITMB)] (2), [H₃TITMB]₂[Cd(β-Mo₈O₂₆)(H₂O)₂(α-Mo₈O₂₆)]·4H₂O (3) and [H₃TITMB]₂[Co(β-Mo₈O₂₆)(H₂O)₂(α-Mo₈O₂₆)]·4.5H₂O (4). The structures of compounds 1, 2, 3 and 4 were explored with IR spectroscopy, thermal gravimetric analysis (TGA), PXRD, UV-vis diffuse reflectance spectra and single-crystal X-ray diffraction in the solid state. In 1 the new type of isopolymolybdate unit [Mo₁₂O₃₈]⁴⁻ shared ten distorted {MoO₆} octahedra and two distorted {MoO₅} tetragonal pyramid with four kinds of O atoms. And the unprecedented bimetallic oxide chain [Zn(Mo₂O₇)]_n in compound 2 consisted of trigonal bipyramids and octahedra through sharing edges. Both compounds 3 and 4 are 3-D octamolybdate-based inorganic–organic hybrids consisting of the polyacid [α-Mo₈O₂₆]⁴⁻ anions and novel infinite chains of [β-Mo₈O₂₆]⁴⁻ anions covalently linked through [Cd(H₂O)₂]²⁺ groups or [Co(H₂O)₂]²⁺ groups. In addition, the electrochemical behavior and photocatalytic activities of the title compounds were also investigated.

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Introduction

The rational design and assembly of inorganic–organic hybrid materials based on polyoxometalate (POMs) clusters with transition metal ions or organic ligands is of great interest, not only in terms of structural diversity but also because of potential applications for photochemistry and electrical conductivity.^1–6 Octamolybdates (Mo₈) are a subclass of the POMs family and have been extensively studied due to their diverse structures and intriguing properties, and they can be easily obtained under hydrothermal conditions by (NH₄)₆Mo₇O₂₄·4H₂O with controlled pH values.⁷ To date, nine isomeric forms of octamolybdates have been prepared, that is, the α-, β-, γ-, δ-, ε-, ζ-, η-, θ- and ι-isomers.^8–12 On one hand, many research efforts have been focused on the assembly of polyoxometalates with transition metal–organic units or organoammonium cations (OACs) as organic templates with the structure-directing and charge-compensating functions to form extended metal–organic hybrid materials.^13–16 On the other hand, POM building blocks can also take on efficient inorganic ligands to coordinate with various transition metal for their high electronic density, which opens up the new way to design a new family of hybrid organic–inorganic materials.^17,18 Hence, the design and synthesis of POM-based hybrid materials with abundant structures and diverse properties has become more attractive from both practical and fundamental points of view.^19–22

Another important reason for the synthesis of novel POMs-supported inorganic–organic hybrid materials comes from that the organic moieties may have significant effects on the final structures.²³ Accordingly, tripodal ligands with N donors are good candidates for the construction of coordination architecture,^24,25 and recently a flexible tripodal ligand 1,3,5-tris-(imidazol-1-ylmethyl)-2,4,6-trimethylbenzene (TITMB), has been employed to collaborate with several transition metal salts to assemble diverse POMs-supported inorganic–organic hybrid materials. There are two different conformations (cis, cis, cis and cis, trans, trans) when TITMB reacts with metal ions (Scheme 1).^26,27 In this paper we chose TITMB and (NH₄)₆Mo₇O₂₄·4H₂O to react with PbI₂, Zn(NO₃)₂·6H₂O, CdI₂ and CoCl₂·6H₂O under hydrothermal conditions to assemble four new POM-based inorganic–organic hybrids, [H₂TITMB]₂[Mo₁₂O₃₈]·4H₂O (1), [Zn(Mo₂O₇)(TITMB)] (2), [H₃TITMB]₂[Cd(β-Mo₈O₂₆)(H₂O)₂(α-Mo₈O₂₆)]·4H₂O (3) and [H₃TITMB]₂[Co(β-Mo₈O₂₆)(H₂O)₂(α-Mo₈O₂₆)]·4.5H₂O (4), respectively. In the preparation of compound 1, PbI₂ was chosen to react with (NH₄)₆Mo₇O₂₄·4H₂O and TITMB under hydrothermal conditions to form a new POM-based inorganic–organic structure with the first example of isopolymolybdate unit [Mo₁₂O₃₈]⁴⁻. It is interesting that PbI₂ as a temporary template was not contained in compound 1 on final composition and architecture. To the best of our knowledge, the unprecedented bimetallic oxide chain [Zn(Mo₂O₇)]_n in compound 2 consisted of trigonal bipyramid and octahedron through sharing edges. Both compounds 3 and 4 are 3-D octamolybdate-based inorganic–organic hybrids consisted of the polyacid [α-Mo₈O₂₆]⁴⁻ anions and novel infinite chain of [β-Mo₈O₂₆]⁴⁻ anion covalently linked through [Cd(H₂O)₂]²⁺ group and [Co(H₂O)₂]²⁺ group. In addition, their optical band gaps, electrochemistry, and photocatalytic behaviors have also been described in detail.

Scheme 1

Experimental

Materials and methods

The ligand 1,3,5-tris-(imidazol-1-ylmethyl)-2,4,6-trimethylbenzene (TITMB) was synthesized according to the reported literature method.^28a Other chemicals were of reagent grade and used as purchased without further purification. The IR spectra were recorded on a Shimadzu IR435 spectrometer as KBr disk (4000–400 cm⁻¹). Elemental analyses (C, H, and N) were carried out on a FLASH EA 1112 elemental analyzer. The purity of the bulk microcrystalline materials obtained from the syntheses was checked by powder X-ray diffraction analysis. Powder X-ray diffraction (PXRD) patterns were recorded using Cu Kα1 radiation on a PAN analytical X'Pert PRO diffractometer. Thermogravimetric analyses (TGA) were carried out on a model NETZSCHTG209 thermal analyzer in flowing N₂ atmosphere of 20 mL min⁻¹ at a heating rate of 10 °C min⁻¹ in the temperature range 0–800 °C using platinum crucibles. The UV-vis diffuse reflectance spectra (DRS) were recorded on a Cary 5000 UV-vis-NIR at the speed of 300 nm min⁻¹ from 800 to 200 nm. UV-vis absorption spectra were obtained using UV-5500 PC spectrophotometer. Electrochemical measurements were performed with a CHI660 electrochemical workstation. A Ag/AgCl electrode was used as a reference electrode, and a Pt wire as a counter electrode. Chemically bulk-modified carbon-paste electrode (CPEs) was used as the working electrode.

Compound synthesis

[H₂TITMB]₂[Mo₁₂O₃₈]·4H₂O (1). A mixture of (NH₄)₆Mo₇O₂₄·4H₂O (61.8 mg, 0.05 mmol), TITMB (13.7 mg, 0.025 mmol), PbI₂ (46.1 mg, 0.10 mmol) and H₂O (10 mL) was stirred at room temperature until the reaction mixture was homogeneous, and adjusted by HCl to pH = 2.5–3.0. Then the mixture was sealed in a 25 mL Teflon-lined stainless steel container, which was heated to 170 °C under autogenously pressure for 6 d. After slow cooling to room temperature with the rate 10 °C h⁻¹, the resulting green rodlike crystals of 1 were formed (45% yield based on Mo). Anal. calcd (%) for 1: C, 22.36; H, 2.69; N, 7.45. Found: C, 22.43; H, 2.64; N, 7.54. IR (KBr, cm⁻¹): 3454(m), 3125(m), 1622(m), 1577(m), 1543(m), 1517(m), 1448(m), 1400(m), 1384(w), 1292(m), 1232(m), 1164(s), 1096(s), 950(m), 913(m), 878(w), 719(m), 660(m), 620(m), 560(w), 522(w), 467(w).

[Zn(Mo₂O₇)(TITMB)] (2). A mixture of (NH₄)₆Mo₇O₂₄·4H₂O (61.8 mg, 0.05 mmol), TITMB (13.7 mg, 0.025 mmol), Zn(NO₃)₂·6H₂O (29.7 mg, 0.10 mmol) and H₂O (10 mL) was stirred at room temperature until the reaction mixture was homogeneous, and adjusted by HCl to pH = 2.5–3.0. Then the mixture was sealed in a 25 mL Teflon-lined stainless steel container, which was heated to 170 °C under autogenously pressure for 3 d. After slow cooling to room temperature with the rate 10 °C h⁻¹, the resulting light yellow bulk crystals of 2 were formed (41% yield based on Mo). Anal. calcd (%) for 2: C, 34.56; H, 3.32; N, 11.52. Found: C, 34.66; H, 3.27; N, 11.45. IR (KBr, cm⁻¹): 3444(m), 3113(m), 1730(w), 1653(w), 1534(w), 1515(m), 1474(m), 1397(w), 1339(w), 1311(m), 1279(s), 1234(s), 1110(s), 1030(m), 956(w), 917(m), 897(m), 840(m), 724(m), 653(s), 594(s), 501(m), 457(w).

[H₃TITMB]₂[Cd(β-Mo₈O₂₆)(H₂O)₂(α-Mo₈O₂₆)]·4H₂O (3). A mixture of (NH₄)₆Mo₇O₂₄·4H₂O (61.8 mg, 0.05 mmol), TITMB (13.7 mg, 0.025 mmol), CdI₂ (36.6 mg, 0.10 mmol) and H₂O (10 mL) was stirred at room temperature until the reaction mixture was homogeneous, and adjusted by HCl to pH = 2.5–3.0. Then the mixture was sealed in a 25 mL Teflon-lined stainless steel container, which was heated to 150 °C under autogenously pressure for 3 d. After slow cooling to room temperature with the rate 10 °C h⁻¹, the resulting white bulk crystals of 3 were formed (27% yield based on Mo). Anal. calcd (%) for 3: C, 15.22; H, 2.01; N, 5.07. Found: C, 15.13; H, 2.07; N, 5.11. IR (KBr, cm⁻¹): 3439(m), 3140(m), 1622(m), 1580(m), 1542(m), 1451(m), 1379(w), 1325(w), 1291(m), 1250(w), 1169(m), 1092(s), 952(m), 925(m), 906(m), 888(m), 799(s), 654(s), 619(m), 556(w), 520(w).

[H₃TITMB]₂[Co(β-Mo₈O₂₆)(H₂O)₂(α-Mo₈O₂₆)]·4.5H₂O (4). A mixture of (NH₄)₆Mo₇O₂₄·4H₂O (61.8 mg, 0.05 mmol), TITMB (13.7 mg, 0.025 mmol), CoCl₂·6H₂O (23.8 mg, 0.10 mmol) and H₂O (10 mL) was stirred at room temperature until the reaction mixture was homogeneous, and adjusted by HCl to pH = 2.5–3.0. Then the mixture was sealed in a 25 mL Teflon-lined stainless steel container, which was heated to 170 °C under autogenously pressure for 3 d. After slow cooling to room temperature with the rate 10 °C h⁻¹, the resulting light red bulk crystals of 4 were formed (39% yield based on Mo). Anal. calcd (%) for 4: C, 15.42; H, 2.07; N, 5.14. Found: C, 15.34; H, 2.16; N, 5.19. IR (KBr, cm⁻¹): 3404(m), 3111(s), 2171(w), 1610(w), 1582(m), 1544(m), 1508(w), 1476(w), 1445(w), 1385(w), 1324(m), 1298(m), 1235(m), 1185(w), 1148(w), 935(m), 902(m), 834(m), 674(s), 554(m), 514(m), 480(w).

Preparation of CPEs. The carbon paste electrode bulk-modified with compounds 1–4 (1, 2, 3 and 4-CPEs) were fabricated as follows: 0.1 g of compounds 1–4 and 0.5 g of graphite powder were mixed and grounded together by an agate mortar and pestle to achieve a uniform mixture, and then 0.05 mL of liquid paraffin was added with stirring. The homogenized mixture was packed into a glass tube with a 3 mm inner diameter, and the tube surface was wiped with weighing paper. Electrical contact was established with a copper rod through the back of the electrode.

X-ray crystallography study

Crystallographic data for the four compounds were collected at 100(2) K on a Bruker APEX-II area-detector diffractometer equipped with graphite-monochromatized Mo-Kα radiation (λ = 0.71073 Å). Their structures were solved by direct method and expanded using Fourier techniques. The non-hydrogen atoms were refined with anisotropic thermal parameters. The hydrogen atoms were assigned with common isotropic displacement factors and included in the final refinement by using geometrical constraint. The structures were refined with full-matrix least-squares techniques on F² using the OLEX-2 program package.^28b Crystal data for 1–4 were summarized in detail in Table 1. Selected bond lengths and bond angles were put in Table S1 (see the ESI).† In order to confirm the phase purity of the bulk materials, X-ray powder diffraction (XRPD) experiments were carried out for compounds 1–4. The experimental PXRD patterns of 1–4 correspond well to the simulated PXRD patterns, indicating that the bulk phase materials are isomorphous (Fig. S1–S4†). The CCDC reference numbers are 1057373 for 1, 1057374 for 2, 1057375 for 3 and 1057376 for 4.†

Table 1 Crystal data and structure refinement details for 1–4

	1	2	3	4
a R1 = ||Fo| − |Fc||/|Fo|; wR2 = [w(Fo^2 − Fc^2)^2/w(Fo^2)^2]^1/2.
Formula	C42H60Mo12N12O42	C21H24Mo2N6O7Zn	C42H66CdMo16N12O58	C42H67CoMo16N12O58.5
Formula weight	2556.30	729.71	3314.51	3270.44
Crystal system	Orthorhombic	Triclinic	Triclinic	Triclinic
Space group	Pbca	P1̄	P1̄	P1̄
a/Å	18.9997(3)	10.6380(8)	10.9486(12)	10.7586(5)
b/Å	14.97489(18)	11.4494(12)	11.2769(12)	11.3075(4)
c/Å	25.4844(5)	11.5486(13)	17.9528(19)	17.9670(9)
α/degree	90.00	65.029(11)	100.4090(10)	100.614(4)
β/degree	90.00	79.634(8)	96.4200(10)	96.093(4)
γ/degree	90.00	70.313(8)	104.2740(10)	103.887(4)
Vol./Å^3	7250.8(2)	1199.4(2)	2084.2(4)	2059.51(16)
Z	4	2	1	1
Dc/g cm^−3	2.342	2.021	2.641	2.637
μ/mm^−1	2.103	2.078	2.679	2.655
F(000)	4944.0	724.0	1586	1570.0
Rflns collected	23 355	8404	15 982	17 103
Unique rflns	7406	8404	7719	8405
GOF	1.077	1.086	1.020	1.059
R1^a (I > 2σ(I))	0.0468	0.0602	0.0270	0.0329
wR2^a (all data)	0.1056	0.2207	0.0715	0.0694
Δρmax/Δρmin (e Å^−3)	0.91/−0.65	1.64/−1.39	1.709/−1.895	0.60/−1.46

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Results and discussion

Description of crystal structures

Crystal structure of [H₂TITMB]₂[Mo₁₂O₃₈]·4H₂O (1). The single crystal X-ray diffraction analysis shows that complex 1 crystallizes in the orthorhombic with space group Pbca. As shown in Fig. 1a, the asymmetric unit of 1 consists of two double protonated H₂TITMB ligands, one [Mo₁₂O₃₈]⁴⁻ anion, and four free water molecules. The double protonated H₂TITMB ligand in compound 1 existed in a cis, trans, trans conformation, and the unprotonated N1 atoms of imidazole from two H₂TITMB ligands coordinated to two Mo1 atoms from [Mo₁₂O₃₈]⁴⁻ anion. As we know, the typical single [β-Mo₈O₂₆]⁴⁻ anion²⁹ and twin [β-Mo₈O₂₆]⁴⁻ anion^30a,b have already reported in some papers. The new type of isopolymolybdate unit [Mo₁₂O₃₈]⁴⁻ is composed of ten distorted {MoO₆} octahedra and two distorted {MoO₅} tetragonal pyramid with four kinds of O atoms, twenty O_t, eight μ₂-O, six μ₃-O, and four μ₄-O, which was firstly gained by us. And the polyanion [β-Mo₈O₂₆N₂]⁴⁻ may be also regarded as the siamesed [β-Mo₈O₂₆]⁴⁻ anion constructed by isopolymolybdate unit [Mo₁₂O₃₈]⁴⁻ and two unprotonated N1 atoms of imidazole from two H₂TITMB ligands (Fig. 1b). This crystal structure is different from the crystal [1,3,5-tris(imidazol-1-ylmethyl)-2,4,6-trimethyl benzene]₂ [β-Mo₈O₂₆][Mo₆O₁₉]^30c reported by us without PbI₂ as a temporary template, which contained two kinds of polyoxomolybdate anions in the same crystal architecture.

Fig. 1 (a) Structure of compound 1 with atomic labeling. All H atoms were omitted for clarity. Symmetric codes: ¹1 − x, −y, 1 − z. (b) Polyhedra representation of siamesed [Mo₁₂O₃₈]⁴⁻ anion. (c) Polyhedral packing of compound 1 along the crystallographic b axis.

We can see from Fig. 1c that inorganic units [Mo₁₂O₃₈]⁴⁻, protonated H₂TITMB ligand and water molecules are combined by covalent interactions and non-covalent interactions (C–H⋯O and N–H⋯O hydrogen-bonding interactions) and static attracting forces to form a so-called organic–inorganic supramolecular structure along the crystallographic b axis.

Crystal structure of [Zn(Mo₂O₇)(TITMB)] (2). The single crystal X-ray diffraction analysis shows that complex 2 crystallizes in the triclinic with space group P1̄. As shown in Fig. 2a, the unprotonated TITMB ligand in compound 2 existed in a cis, cis, cis conformation. There are one crystallographically independent Zn atom and two crystallographically independent Mo atoms. Zn1 adopts distorted trigonal bipyramid geometry, coordinated by three O atoms from [Mo₂O₇]²⁻ anion and two N atoms from two different TITMB ligands, with Zn–O bond distances of 2.048(5)–2.115(4) Å, two equal Zn–N bond distances of 1.999(5) Å. Mo1 exhibits distorted octahedral geometry, coordinated by five O atoms from [Mo₂O₇]²⁻ anion and one N atoms from TITMB ligand, with Mo–O bond distances of 1.703(5)–2.205(4) Å, one Mo–N bond distance of 2.471(5) Å. Mo2 adopts distorted trigonal bipyramid geometry, coordinated by five O atoms from [Mo₂O₇]²⁻ anion, with Mo–O bond distances of 1.702(5)–2.083(4) Å. And then its structure is constructed from novel 1D bimetallic oxide chain [Zn(Mo₂O₇)]_n with the TITMB ligands located on either side of the chain up and down (Fig. 2b). The unprecedented bimetallic oxide chain [Zn(Mo₂O₇)]_n in compound 2 consisted of trigonal bipyramid and octahedron through sharing edges (Fig. 2c top). And the Zn atoms and Mo atoms are linked by μ₃-O atoms to form a novel ladder-like chain (Fig. 2c bottom).

Fig. 2 (a) The asymmetric unit and coordination environments around metal ions of 2 with atomic labeling. All H atoms were omitted for clarity. Symmetric codes: ¹1 − x, 3 − y, 1 − z; ²+x, 1 + y, +z; ³1 − x, 2 − y, 1 − z; ⁴+x, −1 + y, +z. (b) 1D bimetallic oxide chain [ZnMo₂O₇]_n with the TITMB ligands located on either side of the chain up and down. (c) Polyhedra representation of [ZnMo₂O₇]_n (top) and a novel ladder-like chain (bottom). (d) The packing of compound 2 along the crystallographic b axis.

We can see from Fig. 2d each [Zn(Mo₂O₇)(TITMB)] was combined by non-covalent interactions (C–H⋯O hydrogen-bonding interactions) and static attracting forces to form a so-called organic–inorganic supramolecular structure along the crystallographic b axis.

Crystal structure of [H₃TITMB]₂[Cd(β-Mo₈O₂₆)(H₂O)₂(α-Mo₈O₂₆)]·4H₂O (3). The single crystal X-ray diffraction analysis shows that complex 3 crystallizes in the triclinic with space group P1̄. As shown in Fig. 3a, the asymmetric unit of 3 consists of protonated H₃TITMB ligands, one [α-Mo₈O₂₆]⁴⁻ anion, novel infinite chain of [β-Mo₈O₂₆]⁴⁻ anion covalently linked through [Cd(H₂O)₂]²⁺ group and free water molecules. The protonated H₃TITMB ligand in compound 3 existed in a cis, cis, cis conformation. Each [α-Mo₈O₂₆]⁴⁻ anion consists of a ring of six edge-sharing {MoO₆} octahedral capped at the poles by two {MoO₄} tetrahedral,⁷ and the typical [β-Mo₈O₂₆]⁴⁻ anion in 3 is composed of eight distorted {MoO₆} octahedral with four kinds of O atoms, six μ₂-O, four μ₃-O, two μ₅-O, and twelve O_t.²³ Cd1 exhibits distorted octahedral geometry, coordinated by four O atoms from two [β-Mo₈O₂₆]⁴⁻ anions and two O atoms from two water molecules, with Cd–O bond distances of 2.229(4)–2.268(3) Å. The [β-Mo₈O₂₆]⁴⁻ anion acted as an inorganic ligand to bridge Cd1 to form a 1D inorganic chain (Fig. 3b) which was relatively rare.^23,31 The polyhedral ball and stick packing diagram of 3 was shown in (Fig. 3c) along the crystallographic b axis. Polyacid [α-Mo₈O₂₆]⁴⁻ anions and the novel infinite chains showed a parallel pack, and protonated H₃TITMB ligands (acted as the charge balancing component and space-filling component) and water molecules filled in cavities of anions' clusters, leading to form a 3D octamolybdate-based inorganic–organic hybrid complex by electrostatic interactions and weak hydrogen bond interactions (C–H⋯O and N–H⋯O hydrogen-bonding interactions).

Fig. 3 (a) The asymmetric unit and coordination environments around metal ions in 3 with atomic labeling. All H atoms were omitted for clarity. Symmetric codes: #1 −x + 2, −y + 1, −z + 1; #2 −x + 1, −y + 1, −z + 1; #3 −x + 2, −y + 2, −z + 2. (b) The 1D inorganic chain in 3. (c) The ball-and-stick packing of compound 3 along the crystallographic b axis.

Crystal structure of [H₃TITMB]₂[Co(β-Mo₈O₂₆)(H₂O)₂(α-Mo₈O₂₆)]·4.5H₂O (4). The single crystal X-ray diffraction analysis shows that complex 4 crystallizes in the triclinic with space group P1̄. As shown in Fig. 4a, compound 4 owned the similar structure to compound 3, however, the transition metal center Co taken the place of the Cd and added one half water molecule in the compound 4. The protonated H₃TITMB ligand in compound 4 also existed in a cis, cis, cis conformation. Co1 adopted distorted octahedral geometry, coordinated by four O atoms from two [β-Mo₈O₂₆]⁴⁻ anion and two O atoms from two water molecules, with different Co–O bond distances of 2.026(3)–2.147(3) Å in the compound 4. The analogous 1D chain existed in the compound 4, in which the [β-Mo₈O₂₆]⁴⁻ anion acted as an inorganic ligand to bridge Co1 (Fig. 4b). We can see from Fig. 4c that the polyhedral ball and stick packing diagram and supermolecular interactions of 4 were also close to that of 3.

Fig. 4 (a) The asymmetric unit and coordination environments around metal ions of 4 with atomic labeling. All H atoms were omitted for clarity. Symmetric codes: ¹−x, −y, −z; ²1 − x, 1 − y, 1 − z; ³−x, 1 − y, 1 − z. (b) The 1D inorganic chain in 4. (c) The ball-and-stick packing of compound 4 along the crystallographic b axis.

Optical absorption spectra, TGA analysis of compounds 1, 2, 3 and 4. Fig. S5† shows the UV/vis absorption spectra of the ligand TITMB and compounds 1, 2, 3 and 4. For the TITMB, the higher energy absorption peak is 238 nm, which is assigned to spin allowed π–π* transition of the ligand. And then for compounds 1, 2, 3 and 4, the higher energy absorption peaks are 238 nm, 234 nm, 237 nm and 234 nm, respectively, which can be probably attributed to the absorption peak of intraligand. Compounds 1 and 2 exhibit the lower energy absorption peaks, ranging from 400–800 nm, which may be assigned to the following: after coordinating to metal centers, neutral ligands may change their highest occupied molecular orbital (HOMO) and lowest occupied molecular orbital (LUMO) energy levels. And compound 4 exhibited an intense absorption peak at ca. 570 nm, which may be caused by d–d transition due to cobalt(II) centre. In order to investigate the relation between structures and the thermostability of compounds 1–4, the TGA experiments were carried out up to 800 °C in flowing N₂ atmosphere. As shown in Fig. S6,† compounds 1 and 3 exhibited similar thermal behavior, which didn't show obvious stage. And over the range of 45–797 °C, successive weight-loss process corresponds to the decomposition of free water molecules, organic ligand and inorganic bulks for compounds 1 and 3. As the TG curves shown, the mass decreased for 2 in range of 300 °C to the end mainly involved the decomposition of organic ligands TITMB and inorganic bulks. And then, the mass decreased for 4 in range of 190 °C to the end mainly involved the decomposition of free water molecules, organic ligand and inorganic bulks. The compounds 3 and 4 exhibited very close structures but different behaviours under heating. The thermostability of compound 4 was better than compound 3, which may be caused by the following: the Co²⁺ centre had the empty d orbit relative to the Cd²⁺ centre, when water molecules and [β-Mo₈O₂₆]⁴⁻ anion coordinated to the Co²⁺ centre, the coordinated interactions were more stable.

Study of optical band gap

To explore conductivity of the compounds 1, 2, 3 and 4, the UV-vis diffuse reflectance spectra of them were measured to achieve their band gaps (E_g). The band gaps (E_g) was determined as the intersection point between the energy axis and the line extrapolated from the linear portion of the absorption edge in a plot of Kubelka–Munk function F against energy E.³² Kubelka–Munk function, F = (1 − R)²/2R, was converted from the recorded diffuse reflectance data, where R is the reflectance of an infinitely thick layer at a given wavelength. The F versus E plot, as shown in Fig. S7,† the E_g values assessed from the steep absorption edge are 1.76 eV for 1, 1.68 eV for 2, 1.67 eV for 3 and 1.64 eV for 4, which indicates that these complexes are potential semiconductive materials and possess possible photocatalytic activity.^32b

Photocatalytic property

It is well known that a wide range of hybrids possess photocatalytic activities in the degradations of organic dyes under UV irradiation by oxidation of organic materials.³³

To research in detail the photocatalytic activity of complexes 1–4, we selected methylene blue (MB), Rhodamine B (RhB) and methyl orange (MO) as models of dye contaminant in water with a 500 W mercury vapor lamp irradiation. The photodegradation process of MB, RhB and MO without any catalyst had also been studied for the control experiment. A suspension of compounds 1, 2, 3 and 4 (100 mg) and MB (100 mL, 1.0 × 10⁻⁵ mol L⁻¹) and RhB (100 mL, 1.0 × 10⁻⁵ mol L⁻¹) and MO (100 mL, 4.0 × 10⁻⁵ mol L⁻¹) aqueous solution, magnetically stirred in the dark for 30 min to ensure the equilibrium. Then the solution was exposed to UV irradiation, kept stirring during irradiation. 3.0 mL sample was taken for analysis every appropriate time. The change of typical absorption band of MB, RhB and MO is illustrated under the different reaction times in Fig. S8–S10.† Besides, the concentrations of MB, RhB and MO (C) versus reaction time (t) of complexes 1–4 are plotted in Fig. 5 (wherein, C₀ is the initial concentration of the MB and C_t is the concentration of the dye at any given time).

Fig. 5 Photocatalytic decomposition of MB (a), RhB (b) and MO (c) solution under UV light irradiation with the use of compounds 1, 2, 3 and 4 and the control experiment without any catalyst.

As shown in Fig. S8–S10,† the compound 1 exhibited photocatalytic behavior to decompose the MB, RhB and MO in water using UV-vis absorption spectroscopy, in the order of MB > RhB > MO. However, the UV-vis spectra of compounds 2, 3 and 4 revealed that photocatalytic reaction of 2, 3 and 4 are slower than photocatalyst 1, and even prohibit the photodegradation of organic dyes. In addition, according to Fig. 5, the dissociations of MB, RhB and MO were 28.6%, 27.4% and 0% without any catalyst. However, in the same time scope, the systems with different photocatalysts 1, 2, 3 and 4 indicate MB decomposition (1: 76.5%, 2: 37.2%, 3: 38.1%, 4: 45.4%, respectively), RhB decomposition (1: 58.5%, 2: 12.3%, 3: 5.78%, 4: 40.1%, respectively) and MO decomposition (1: 92.5%, 2: 56.0%, 3: 21.2%, 4: 10.4%, respectively). Curiously, compound 1 is a POMs-based hybrid material without metal ion as a heterogeneous photocatalyst, with a relatively large band gap of 1.76 eV. UV-irradiation of the compound 1 leads to an O–Mo charge transfer and formation of the excited-state POM species, producing considerable holes and electrons, which can oxidize the organic dyes in the solution.³⁴ Compounds 2, 3 and 4 exhibit lower or no photocatalytic efficiency, with small band gap relative to compound 1 of 1.68 eV for 2, 1.67 eV for 3 and 1.64 eV for 4, which may be due to that compounds 2, 3 and 4 can work as absorbers of Hg lamp irradiation.³⁵ It is believed that this result will further facilitate the exploration of design and assemble new efficient POMs-based hybrid photocatalysts. Further work is under way in our lab.

Electrochemical behavior

The electrochemical studies of the 1, 2, 3 and 4-CPEs were carried out in 1 M H₂SO₄ aqueous solution.^34a,36 The electrochemical behaviors of 1, 2, 3 and 4-CPEs are similar except for some light potential shift and the cyclic voltammograms for them in H₂SO₄ aqueous solution at different scan rates in the potential range of 800 to 200 mV are shown in Fig. 6. One pair of redox peak (I–I′) is observed, with the mean peak potentials E_1/2 = (E_pa + E_pc)/2 of 452 for 1 (scan rate: 30 mV s⁻¹), 499 for 2 (scan rate: 30 mV s⁻¹), 462 for 3 (scan rate: 100 mV s⁻¹) and 457 for 4 (scan rate: 100 mV s⁻¹), respectively. The redox peaks may be ascribed to Mo^VI/Mo^V. The peak potentials changed gradually as the scan rates were varied from low to high: the anionic peak potentials shifted toward the positive direction, while the corresponding cathodic peak potentials shifted toward the negative direction with increasing scan rates.

Fig. 6 Cyclic voltammograms of 1–4 in 1 M H₂SO₄ under scan rates from inner to outer: (a) 1-CPE (scan rates: 10, 20, 30, 40, 50, 60 and 80 mV s⁻¹) (b) 2-CPE (scan rates: 10, 20, 30, 40, 60, 80, 100 and 140 mV s⁻¹) (c) 3-CPE (scan rates: 20, 40, 60, 80, 100 and 150 mV s⁻¹) and (d) 4-CPE (scan rates: 40, 60, 80, 100, 150, 200, 250 and 300 mV s⁻¹).

Conclusions

Four new octamolybdate-based inorganic–organic hybrid compounds were obtained under hydrothermal conditions. Compound 1 contained a new type of isopolymolybdate unit [Mo₁₂O₃₈]⁴⁻, while the unit [Zn(Mo₂O₇)]_n in compound 2 represented an unprecedented type of bimetallic oxide chain. Both compounds 3 and 4 are 3D octamolybdate-based inorganic–organic hybrids consisted of the polyacid [α-Mo₈O₂₆]⁴⁻ anions and novel infinite chain of [β-Mo₈O₂₆]⁴⁻ anion covalently linked through [Cd(H₂O)₂]²⁺ group or [Co(H₂O)₂]²⁺ group. In addition, their optical band gaps, electrochemistry, and photocatalytic behaviors were also discussed.

Acknowledgements

Research efforts in the Niu group are supported by the National Science Foundation of China (No. 21171148).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Selected bond distances and angles for compounds 1–4, powder X-ray diffraction (PXRD) and simulated pattern patterns for 1–4, UV/vis absorption spectra of ligand TITMB and compounds 1–4, TG plot of compounds 1–4, K–M function versus energy (eV) curve compounds 1–4. Absorption spectra of the MB, RhB, and MO aqueous solution during the decomposition reaction with the use of compounds 1–4 (d) and blank (e). CCDC 1057373–1057376. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra13007g

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