Polyanionic Clusters [M(P4Mo6)2] (M = Ni, Cd) as Effective Molecular Catalysts for the Electron-Transfer Reaction of Ferricyanide to Ferrocyanide

Three polyanion-containing supramolecular hybrids, (H2bpp)6{Ni[Mo(V)6O13(OH)2(HPO4)3(H2PO4)]2}2·13H2O [1; bpp = 1,3-bis(4-pyridyl)propane], (H2bpp)5{Cd[Mo(V)6O15(HPO4)3(H2PO4)]2}{Cd[Mo(V)6O15(HPO4)4]2}·10H2O (2), (H2bpp)2[Cd(H2O)Cd(H2O)2]2{Cd[Mo(V)6O12(OH)3(HPO4)2(PO4)2]2}·8H2O (3), have been synthesized and characterized. The inorganic moieties in 1-3 belong to the reduced {M[P4Mo(V)6X31]2}(n-) (M = Ni, Cd) clusters. Three supramolecular hybrids exhibit reversible multiple electron-transfer behaviors and have been used as heterogeneous molecular catalysts for the reduction of ferricyanide (Fe(III)) to ferrocyanide (Fe(II)) with Na2S2O3 as a reductant. The experimental results indicate that hybrid materials 1-3 have higher catalytic activity toward the reaction. These low-cost catalysts can effectively decrease the activation energy of this reaction. These values are calculated to be 25.35 kJ·mol(-1) for hybrid 2, 84.11 kJ·mol(-1) for hybrid 1, and 83.14 kJ·mol(-1) for hybrid 3. The reaction mechanism is subjected to a surface-catalytic process.