Hydrothermal synthesis of two supramolecular inorganic–organic hybrid phosphomolybdates based on Ni(ii) and Co(ii) ions: structural diversity and heterogeneous catalytic activities

Abstract

Two new hybrid compounds based on phosphomolybdates, namely (NHEPH₂)₅[Ni^II(P₄Mo₆O₃₁)₂]·6H₂O (1) and {(NHEPH₂)₂[Co^II(H₂O)₆]}[P₂Mo₅O₂₃]·2H₂O (2) (NHEP = N-(2-hydroxyethyl)-piperazine), have been synthesized under hydrothermal conditions and characterized by elemental analyses, IR, TGA, single crystal X-ray diffraction and PXRD studies. In compound 1, Ni(II) is sandwiched between two {P₄Mo₆O₃₁}⁶⁻ clusters and the residual negative charges are neutralized by NHEPH₂²⁺. In compound 2, the [P₂Mo₅O₂₃]⁶⁻ ion exists as a discrete moiety and the negative charges are compensated by both [Co(H₂O)₆]²⁺ and NHEPH₂²⁺. The extensive hydrogen bonding involving the organic cations and water molecules yielded three-dimensional (3D) open framework structures in these systems. These phosphomolybdate based compounds showed good catalytic efficiency in heterogeneous oxidation of styrene in the presence of an environmentally benign oxidant, H₂O₂, under mild conditions.