A Mechanistic Study of the Reaction between a Diiron(II) Complex [Fe^II₂(μ-OH)₂(6-Me₃-TPA)₂]²⁺ and O₂ to Form a Diiron(III) Peroxo Complex

A kinetic study of the reaction between a diiron(II) complex [Fe^II₂(μ-OH)₂(6-Me₃-TPA)₂]²⁺ 1, where 6-Me₃-TPA = tris(6-methyl-2-pyridylmethyl)amine, and dioxygen is presented. A diiron(III) peroxo complex [Fe^III₂(μ-O)(μ-O₂)(6-Me₃-TPA)₂]²⁺ 2 forms quantitatively in dichloromethane at temperatures from −80 to −40 °C. The reaction is first order in [Fe^II₂] and [O₂], with the activation parameters ΔH^‡ = 17 ± 2 kJ mol^-1 and ΔS^‡ = −175 ± 20 J mol^-1 K^-1. The reaction rate is not significantly influenced by the addition of H₂O or D₂O. The reaction proceeds faster in more polar solvents (acetone and acetonitrile), but the yield of 2 is not quantitative in these solvents. Complex 1 reacts with NO at a rate about 10³ faster than with O₂. The mechanistic analysis suggests an associative rate-limiting step for the oxygenation of 1, similar to that for stearoyl-ACP Δ⁹-desaturase, but distinct from the probable dissociative pathway of methane monoxygenase. An η¹-superoxo Fe^IIFe^III species is a likely steady-state intermediate during the oxygenation of complex 1.