Oxygen activation by a macrocyclic chromium complex. Mechanism of hydroperoxo-chromium(iii) to oxo-chromium(v) transformation

Abstract

The transformation of the hydroperoxo complex L¹(H₂O)CrOOH²⁺ (L¹ = 1, 4 8, 11-tetraazacyclotetradecane) to an oxo-chromium(V) species is a first-order process throughout the pH range examined, 1.7 < pH < 9.2. The pH dependence of the rate constant (k₁) yielded an apparent pK_a of 5.6 for L¹(H₂O)CrOOH²⁺. In the acidic range, (pH <4), the value of k₁ is 0.191 s⁻¹. At the other extreme, pH >7.5, k₁ = 0.025 s⁻¹. No [H⁺]-dependence is observed within the two limiting regimes, clearly ruling out a simple attack by H⁺ at the hydroperoxo group. The temperature dependence of k₁ in 0.020 M HClO₄ yielded the activation parameters ΔH^‡ = 53.7 kJ mol⁻¹ and ΔS^‡ = −80.5 J mol⁻¹ K⁻¹.