Construction and property investigation of transition-metal complexes modified octamolybdate hybrid materials based on V-shaped organic ligands

Abstract

Six new transition-metal complexes modified octamolybdate compounds have been successfully synthesized with two kinds of V-shaped flexible organic ligands under hydrothermal conditions, namely, [Cu^II(HL¹)₂(H₂O)₂(Mo₈O₂₆)] (1), [Cu^II(L¹)₂(Mo₈O₂₆)_0.5] (2), [Cu^II(HL²)₂(Mo₈O₂₆)]·2H₂O (3), [Cu^II₂(L²)₄(Mo₈O₂₆)]·3H₂O (4), [Cu^I₃(L²)₂(Mo₈O₂₆)_0.5Cl] (5), [Cu^I₄(L²)₄(Mo₈O₂₆)] (6) where L¹ = 3-((1H-imidazol-1-yl)methyl)pyridine, L² = 3-((1H-1,2,4-triazol-1-yl)methyl)pyridine. Their crystal structures have been determined by X-ray single-crystal diffraction, elemental analyses, IR spectra, and thermogravimetric analyses (TGA). With L¹ ligand, two compounds were obtained: compound 1 is a 1D β-octamolybdate based chain and compound 2 is a 2D 4-connected network most strikingly, whose octamolybdate anion shows a θ-isomer. With L² ligand, compounds 3, 4, 5 and 6 were obtained, respectively. In compound 3, 1D β-octamolybdate-based metal–organic chains are further extended by hydrogen bonds to build a 3D supramolecular structure, while in compound 4, every two adjacent double-helical metal–organic chains are bridged by β-octamolybdate anions to generate a 2D 3-connected network. When CuSO₄·5H₂O was substituted by CuCl, two Cu^I compounds were attained. Interestingly, in compound 5 the octamolybdate anion threads through the metal–organic loop presenting a fascinating 2D polythreading topology, and in compound 6 the octamolybdate anions pillar the railroad-like metal–organic strands to generate a 2D 3-connected network. The structural differences among compounds 1–6 indicate the importance of the different coordination mode of organic ligands and various polyoxoanions, and of the pH values of the systems for the framework formation. Furthermore, the semiconducting properties of compounds 1–5 were investigated, and photoluminescence as well as photoconductivity for compound 5 were also studied. Compound 5 has a comparatively lower band gap and is photoconductive in the UV-vis range, so it can be potentially applied as a semiconductor of photoelectronic detectors.