Syntheses, structures and catalytic properties of organic–inorganic hybrid materials constructed from Evans–Showell-type polyoxometalates and zinc–organic coordination units

Abstract

Four new organic–inorganic hybrid compounds based on the polyoxoanion [Co₂Mo₁₀H₄O₃₈]⁶⁻, namely [Zn₂(H₂O)₅(4,4′-bipy)₃]H₂[Co₂Mo₁₀H₄O₃₈]·5H₂O 1, [Zn₂(H₂O)₂(bpe)₃]H₂[Co₂Mo₁₀H₄O₃₈] 2, [Zn(H₂O)₄(H₂bpp)₂][Co₂Mo₁₀H₄O₃₈]·2H₂O 3 and [Zn₃(Trz)₆(H₂O)₆][Co₂Mo₁₀O₃₈H₆]·12H₂O 4 (4,4′-bipyridine = 4,4′-bipy, 1,2-bis(4-pyridyl)ethane = bpe, 1,3-di(4-pyridyl)propane = bpp, 1,2,4-triazole = Trz), have been synthesized and characterized by elemental analysis, IR spectroscopy, TG analysis, solid diffuse reflective spectroscopy, powder X-ray diffraction and single crystal X-ray diffraction. Crystal analysis reveals that compound 1 exhibits a 1D staggered ladder-like chain constructed from [Co₂Mo₁₀H₄O₃₈]⁶⁻ and zinc-4,4′-bipy complexes. Compound 2 shows a 1D zig-zag polymeric chain by the linkage of zinc–bpe complexes and [Co₂Mo₁₀H₄O₃₈]⁶⁻. In compound 3, [Co₂Mo₁₀H₄O₃₈]⁶⁻ units are linked by zinc ions to form a 1D inorganic chain, protonated H₂bpp molecules can be connected together by directional hydrogen-bonding interactions to construct a 3D supramolecular framework. Compound 4 consists of linear trinuclear zinc units bridging six Trz ligands and a [Co₂Mo₁₀H₄O₃₈]⁶⁻ unit as the counter anion, extensive hydrogen bonds exist in the clusters forming a 3D supramolecular framework. Compounds 1–2 represent the first extended organic–inorganic hybrid structures based on Evans–Showell-type POMs. These compounds act as heterogeneous Lewis acid catalysts not only for the cyanosilylation of different carbonyl compounds with trimethylsilyl cyanide but also for recovery and reuse without displaying any significant loss of activity.