Influence of the Dithiolate Bridge on the Oxidative Processes of Diiron Models Related to the Active Site of [FeFe] Hydrogenases

Abstract

Electrochemical studies of [Fe₂ (CO)₄ (κ² -dmpe)(μ-dithiolate)] (dithiolate=adt^Bn , pdt) and density functional theory (DFT) calculations reveal the striking influence of an amine functionality in the dithiolate bridge on their oxidative properties. [Fe₂ (CO)₄ (κ² -dmpe)(μ-adt^Bn )] (1) undergoes two one-electron oxidation steps, with the first being partially reversible and the second irreversible. When the adt^Bn bridge is replaced with pdt, a shift of 60 mV towards more positive potentials is observed for the first oxidation whereas 290 mV separate the oxidation potentials of the two cations. Under CO, oxidation of azadithiolate compound 1 occurs according to an ECE process whereas an EC mechanism takes place for the propanedithiolate species 2. The dication species [1-CO]²⁺ resulting from the two-electron oxidation of 1 has been spectroscopically and structurally characterized. The molecular details underlying the reactivity of oxidized species have been explored by DFT calculations. The differences in the behaviors of 1 and 2 are mainly due to the presence, or not, of favored interactions between the dithiolate bridge and the diiron site depending on the redox states, Fe^I Fe^II or Fe^II Fe^II , of the complexes.

Keywords: bridging ligands; density functional theory; electrochemistry; enzyme models; oxidation.