Electron-Transfer Reactions of Nitrosyl and Superoxo Metal Complexes

Abstract

Novel chromium nitrosyl complexes L(H2O)CrNO(2+) (L = L(1) = 1,4,8,11-tetraazacyclotetradecane, L(2) = meso-Me 6-1,4,8,11-tetraazacyclotetradecane) are oxidized by Ru(bpy)3(3+) to LCr(H2O)2(3+) and NO with rate constants k = 2.22 M(-1) s(-1) (L(1)) and 6.83 (L(2)). Analogous reactions of the superoxo complexes L(H2O)CrOO(2+) are only slightly faster, k = 45 M(-1) s(-1) (L(1)) and 15 M(-1) s(-1) (L(2)). A related rhodium complex L(2)(H2O)RhOO(2+) has k = 15.8 M(-1) s(-1). These results, combined with our earlier data for the oxidation of Cr(aq)NO(2+) and Cr(aq)OO(2+), suggest only a modest role for thermodynamics in determining the kinetics of oxidation. This behavior is even more pronounced in the oxidation of rhodium hydrido and hydroperoxo complexes, with the latter reacting more than 10(5)-fold faster despite being thermodynamically less favored by more than 0.3 V. The X-ray crystal structure of [L(1)(H2O)CrNO](ClO4)2 supports the limiting Cr(III)-NO(-) description for the complex cation.