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[MoV6O13(OH)2(HPO4)3(H2PO4)]2}2·13H2O [1; bpp = 1,3-bis­(4-pyridyl)­propane], (H2bpp)5{Cd­[MoV6O15(HPO4)3(H2PO4)]2}­{Cd­[MoV6O15(HPO4)4]2}·10H2O (2), (H2bpp)2[Cd­(H2O)­Cd­(H2O)2]2{Cd­[MoV6O12(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[P4MoV6X31]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 (FeIII) to ferrocyanide (FeII) 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.