ELECTROCHEMISTRY AND CATALYTIC CHEMISTRY OF RUTHENIUM-SUBSTITUTED KEGGIN-TYPE HETEROPOLYTUNGSTATES IN THE PRESENCE OF NITRITE ION

The electrochemical behaviour of ruthenium-substituted Keggin type het eropolytungstates, [(H2ORuXW11O39)-X-III](n-) (X=B, n=6; X=Si, Ge, n=5 ; X=P, n=4), have been investigated in acetonitrile and aqueous soluti ons. In acetonitrile solutions, these ruthenium-based heteropolyanions exhibit two-step one-electron reduction processes, just like their co rresponding unsubstituted parent compounds. These redox waves are ascr ibed to the W(VI-->V) processes of the heteropolytungstate framework. The slope of a plot of formal redox potentials against the ionic charg e is evaluated. In acid buffer solutions (pH 2.0 similar to 6.0), all these anions exhibit one pair of reversible, one-electron redox peaks. This pair is assigned to the Ru(III) center. These ruthenium-substitu ted heteropolyanions are shown to be excellent catalysts for the elect roreduction of nitrite ion. The unsubstituted parent and lacunary anio ns show no catalytic activity. The rate constants of the catalytic rea ction at differen pH values are measured. For the case of [(H2ORuSiW11 O39)-Si-III](5-), the maximum rate constant of the catalytic reaction is obtained at pH 3.0, K-obsd = 4.2 x 10(3) M-1 s(-1). The catalytic m echanism involves the rapid formation of a nitrosyl complex with the R u(II) form of the catalyst, which seems to depend on the electron conf iguration of Ru(II) and the inorganic structure of the ruthenium-subst ituted polyoxotungstates. The dependence of the redox potentials of th e Ru(III)/Ru(II) couple on the identity of the central heteroatoms is also explored.