Electrochemistry and catalytic chemistry of ruthenium-substituted Keggin type heteropolytungstates in the presence of nitrite ion

Abstract

The electrochemical behaviour of ruthenium-substituted Keggin type heteropolytungstates, [H₂ORu¹¹¹XW₁₁O₃₉]^n- (X=B, n=6; X=Si, Ge, n=S; X=P, n=4), have been investigated in acetonitrile and aqueous solutions. In acetonitrile solutions, these ruthenium- based heteropolyanions exhibit two-step one-electron reduction processes, just like their corresponding unsubstituted parent compounds. These redox waves are ascribed to the W(VI®V) processes of the heteropolytungstate framework. The slope of a plot of formal redox potentials against the ionic charge is evaluated. In acid buffer solutions (PH 2.0~6.0). all these anions exhibit one pair of reversible, one- electron redox peaks. This pair is assigned to the Ru(I1I) center. These ruthenium-substituted heteropolyanions are shown to be excellent catalysts for the electroreduction of nitrite ion. The unsubstituted parent and lacunary anions show no catalytic activity. The rate constants of the catalytic reaction at differen pH values are measured. For the case of [H₂ORu¹¹¹SiW ₁₁O₃₉)^5-, the maximum rate constant of the catalytic reaction is obtained at pH 3.0, K_obsd = 4.2 x10³ M^-1s^-1. The catalytic mechanism involves the rapid formation of a nitrosyl complex with the Ru(II) form of the catalyst, which seems to depend on the electron configuration of Ru(II) and the inorganic structure of the ruthenium-substituted polyoxotungstates. The dependence of the redox potentials of the Ru(III)/Ru(II) couple on the identity of the central heteroatoms is also explored.