Crystal structures of hybrid completely reduced phosphomolybdates and catalytic performance applied as molecular catalysts for the reduction of chromium(VI).

The exploration of highly efficient and low-cost catalysts for the treatment of hexavalent chromium CrVI in environmental remediation is currently one of the most challenging topics. Here, three phosphomolybdate hybrid compounds have been successfully isolated by the hydrothermal method and been applied as supramolecular catalysts for the reduction of CrVI. Single-crystal X-ray diffraction revealed their formulae as (H2bpp)2[Fe(H2O)][Sr(H2O)4]2{Fe[Mo6O12(OH)3(H2PO4)(HPO4)(PO4)2]2}·5H2O (1), (H2bpp)2[Na(H2O)(OC2H5)][Fe(H2O)2][Ca(H2O)2]2{Fe[Mo6O12(OH)3(H2PO4)(HPO4)(PO4)2]2}·4H2O (2) and (H2bpe)3{Fe[Mo6O12(OH)3(HPO4)3(H2PO4)]2}·8H2O (3) [bpp is 1,3-bis(pyridin-4-yl)propane (C13H14N2) and bpe is trans-1,2-bis(pyridin-4-yl)ethylene (C12H10N2)]. The three hybrids consist of supramolecular networks built up by noncovalent interactions between {Fe[P4Mo6VO31]2}22- polyanions and protonated organic cations. This kind of hybrid polyoxometalate could be applied as heterogeneous molecular catalysts for the reduction of CrVI. It was found that the organic moiety plays a vital role in influencing the catalytic activity of the polyanions. Organic bpp-containing hybrids 1 and 2 are highly active in the catalytic reduction of heavy metal CrVI ions using HCOOH as reductant, while bpe-containing hybrid 3 is inactive to this reaction. This work is significant for the design of new catalysts, as well as the exploration of reaction mechanisms at a molecular level.