Electrochemical reduction of trans-[Rh(en)2Cl2]+ and subsequent reactions with oxygen

Abstract

The final product from controlled-potential reduction of aqueous solutions of trans-[Rh(en)₂Cl₂]⁺(where en = ethylenediamine), under an argon atmosphere, has been shown to be trans-[Rh(en)₂H(OH)]⁺. The pseudo-acid–base equilibrium (i) is shown to operate in solutions which are strongly alkaline (pH > 12). In the absence of Rh^III–H + OH^–⇌ Rh^I+ H₂O (i), oxygen, aqueous solutions of trans-[Rh(en)₂H(OH)]⁺ are stable toward both acid and base (0 < pH < 12) at room temperature : in the presence of oxygen, no reaction is observed in acidic solutions whereas rapid reactions are observed in basic solutions, giving initially the hydroperoxorhodium(III) complex [Rh(en)₂(OH)(OOH)]⁺, which on concentration yields the blue superoxo-bridged rhodium(III) dimer [(H₂O)(en)₂RhO₂Rh(en)₂(H₂O)]⁵⁺. Mechanisms of these reactions are discussed.