Formate oxidation induced by a copper peroxo complex produced in Fenton-like reactions

Abstract

Yields of CO₂ resulting from the oxidation of formate induced by the reaction of copper(I) 1: 10 phenanthroline [(OP)₂Cu⁺] with H₂O₂ have been studied (a) by radiolysis of (OP)₂Cu²⁺–H₂O₂–formate mixtures in which (OP)₂Cu⁺ is produced at low steady-state concentrations and (b) by mixing initially high concentrations of (OP)₂Cu⁺ with H₂O₂ and formate. The CO₂ yields per mole of (OP)₂Cu⁺ were independent of [H₂O₂], but increased with increasing [HCO^–₂] and with decreasing [(OP)₂Cu⁺] to a limiting value of 1.75, which was independent of these concentrations, and hence of dose rate during radiolysis. It is concluded that Fenton-like reactions of (OP)₂Cu⁺ produce both (OP)₂Cu³⁺ and a peroxo complex, (OP)₂CuH₂O⁺₂, but not OH˙. The peroxo complex oxidises formate with a rate constant in the region of 3 × 10⁷ dm³ mol^–1 s^–1, but (OP)₂Cu³⁺ does not. Instead it is consumed by reacting quantitatively with (OP)₂Cu⁺, thus terminating what would otherwise be an efficient chain mechanism. The peroxo complex decomposes to (OP)₂Cu³⁺, and differs from OH˙ in its reactivity with formate, (OP)₂Cu²⁺, and phenanthroline.