A Hybrid Density Functional Study of O−O Bond Cleavage and Phenyl Ring Hydroxylation for a Biomimetic Non-Heme Iron Complex

Abstract

Density functional calculations using the B3LYP functional have been used to study the reaction mechanism of [Fe(Tp^Ph2)BF] (Tp^Ph2 = hydrotris(3,5-diphenylpyrazol-1-yl)borate; BF = benzoylformate) with dioxygen. This mononuclear non-heme iron(II) complex was recently synthesized, and it proved to be the first biomimetic complex reproducing the dioxygenase activity of α-ketoglutarate-dependent enzymes. Moreover, the enthalpy and entropy of activation for this biologically interesting process were derived from kinetic experiments offering a unique possibility for direct comparison of theoretical and experimental data. The results reported here support a mechanism in which oxidative decarboxylation of the keto acid is the rate-limiting step. This oxygen activation process proceeds on the septet potential energy surface through a transition state for a concerted O−O and C−C bond cleavage. In the next step, a high-valent iron−oxo species performs electrophilic attack on the phenyl ring of the Tp^Ph2 ligand leading to an iron(III)−radical σ-complex. Subsequent proton-coupled electron-transfer yields an iron(II)−phenol intermediate, which can bind dioxygen and reduce it to a superoxide radical. Finally, the protonated superoxide radical leaves the first coordination sphere of the iron(III)−phenolate complex and dismutates to dioxygen and hydrogen peroxide. The calculated activation barrier (enthalpy and entropy) and the overall reaction energy profile agree well with experimental data. A comparison to the enzymatic process, which is suggested to occur on the quintet surface, has been made.