A DFT study of structure and electrochemical properties of diiron-hydrogenase models with benzenedithiolato and benzenediselenato ligands†
The diiron benzenedithiolato carbonyl complex is a biomimetic catalyst for proton reduction whose catalytic pathways depend on the solvent properties and the proton donor acidity. Previous studies showed that the initial steps in electrocatalytic generation of dihydrogen in acetonitrile involve a two-electron reduction followed by protonation, but the structures and physical properties of other intermediates are not known. We have performed a systematic quantum chemical analysis of the reduced and protonated complexes with benzenedithiolato and benzenediselenato ligands that can be formed by addition of up to three electrons and/or protons. The exchange of the sulfur atoms by selenium increases the basicity of the iron atoms but is not favorable for the protonation of the chalcogen atoms. Our results show that the most stable singly protonated complexes possess the proton in a bridging position between both irons, irrespective of the total complex charge. The second proton can be attached to a chalcogen atom or to an iron atom in a terminal position, depending on the complex charge. The most stable isomers of the triply protonated complexes have protons in the bridging and terminal positions as well as one proton bound to a chalcogen atom. Standard reduction potentials and acidities of all examined complexes were computed. We also discussed possible intermediates and reaction pathways in the electrocatalytic proton reduction to molecular hydrogen formation.