Kinetics and mechanism of the reduction of a macrocyclic Rh(iii) complex by chromium(ii) ions: pH-controlled selectivity to rhodium(ii)vs.rhodium(iii) hydride

Abstract

Aqueous chromium(II) ions reduce a macrocyclic Rh(III) complex L¹(H₂O)₂Rh³⁺ (L¹ = 1,4,8,11-tetraazacyclotetradecane) to the hydride L¹(H₂O)RhH²⁺ in two discrete, one-electron steps. The first step generates L¹(H₂O)Rh²⁺ with kinetics that are first order in each rhodium(III) complex and Cr(H₂O)₆²⁺, and inverse in [H⁺], k/M⁻¹s⁻¹ = 0.065/(0.0031 + [H⁺]). Further reduction of L¹(H₂O)Rh²⁺ to L¹(H₂O)RhH²⁺ is kinetically independent of [H⁺], k/M⁻¹s⁻¹ = 0.30. The difference in [H⁺] dependence allows relative rates of the two steps to be manipulated to generate either L¹(H₂O)Rh²⁺ or L¹(H₂O)RhH²⁺ as the final product.