Kinetic analysis of the conversion of nonheme (alkylperoxo)iron(III) species to iron(IV) complexes

Low-spin mononuclear (alkylperoxo)iron(III) complexes decompose by peroxide O-O bond homolysis to form iron-(IV) species. We examined the kinetics of previously reported homolysis reactions for (alkylperoxo)iron(III) intermediates supported by TPA (tris(2-pyridyImethyl)amine) in CH₃CN solution and promoted by pyridine N-oxide, and by BPMCN (N,N-bis(2-pyridylmethyl)-N,N- dimethyl-trans-1,2-diaminocyclohexane) in its cis-β configuration in CH₃CN and CH₂-Cl₂, as well as for the previously unreported chemistry of TPA and 5-Me₃TPA intermediates in acetone. Each of these reactions forms an oxoiron(IV) complex, except for the β-BPMCN reaction in CH₂Cl₂ that yields a novel (hydroxo)(alkylperoxo)iron(IV) product. Temperature-dependent rate measurements suggest a common reaction trajectory for each of these reactions and verify previous theoretical estimates of a ca. 60 kJ/mol enthalpic barrier to homolysis. However, both the tetradentate supporting ligand and exogenous iigands in the sixth octahedral coordination site significantly perturb the homolyses, such that observed rates can vary over 2 orders of magnitude at a given temperature. This is manifested as a compensation effect in which increasing activation enthalpy is offset by increasingly favorable activation entropy. Moreover, the applied kinetic model is consistent with geometric isomerism in the low-spin (alkylperoxo)iron(III) intermediates, wherein the alkylperoxo ligand is coordinated in either of the inequivalent cis sites afforded by the nonheme ligands.