A series of metal–organic loops templated by [SiMo12O40]4− and [β-Mo8O26]4− anions using double chelating ligands: amperometric sensing and selective photocatalytic properties

Abstract

By using the double chelating ligands 2,3-di-pyridin-2-yl-pyrazine (L¹) and 2,3-di-pyridin-2-yl-quinoxaline (L²), six new polyoxometalate-based compounds, namely [Ag^I₄(L¹)₄(SiMo₁₂O₄₀)] (1), [Cu^II₆(L¹)₆(H₂O)₆(SiMo₁₂O₄₀)₃]·6H₂O (2), [Cu^II(L¹)(H₂O)(β-Mo₈O₂₆)_0.5] (3), [Ag^I₂(L²)(β-Mo₈O₂₆)_0.5] (4), [Cu^II(L²)(β-Mo₈O₂₆)_0.5] (5) and [Zn^II(L²)(β-Mo₈O₂₆)_0.5] (6), have been hydrothermally synthesized and characterized by single-crystal X-ray diffraction analysis, IR spectra and elemental analyses. Using the double chelating ligand L¹, compounds 1–3 were obtained. In compound 1, there is a tetra-nuclear Ag loop with four Ag atoms linked by four L¹ ligands. The [SiMo₁₂O₄₀]⁴⁻ anions are sandwiched by these loops alternately. Compound 2 has a hexa-nuclear shield-like Cu cluster, in which six Cu atoms are connected by six L¹ ligands. The channel formed by the packing hexa-nuclear loops accommodates the [SiMo₁₂O₄₀]⁴⁻ anions. By using [β-Mo₈O₂₆]⁴⁻ instead of [SiMo₁₂O₄₀]⁴⁻, compound 3 was obtained containing a zigzag metal–organic chain. The chains are linked by [β-Mo₈O₂₆]⁴⁻ anions to construct a layer. By changing L¹ to L², compounds 4–6 containing metal–organic loops were built. In compound 4, two L² ligands are linked by two Ag atoms to form a bi-nuclear loop. The [β-Mo₈O₂₆]⁴⁻ anions as tetra-dentate inorganic subunits link bi-nuclear loops to build a 2D layer. Compounds 5 and 6 are isostructural, with bi-nuclear Cu/Zn–L² loops connected by [β-Mo₈O₂₆]⁴⁻ anions alternately to construct a 1D chain. In addition, we also studied the electrochemical properties and selective photocatalytic properties of compounds 1–6.