Mechanism of Diiron Hydrogenase Complexes Controlled by Nature of Bridging Dithiolate Ligand

Bio-inorganic complexes inspired by hydrogenase enzymes are designed to catalyze the hydrogen evolution reaction (HER). A series of new diiron hydrogenase mimic complexes with one or two terminal tris(4-methoxyphenyl)phosphine and different μ-bridging dithiolate ligands and show catalytic activity towards electrochemical proton reduction in the presence of weak and strong acids. A series of propane- and benzene-dithiolato-bridged complexes was synthesized, crystallized, and characterized by various spectroscopic techniques and quantum chemical calculations. Their electrochemical properties as well as the detailed reaction mechanisms of the HER are elucidated by density functional theory (DFT) methods. The nature of the μ-bridging dithiolate is critically controlling the reaction and performance of the HER of the complexes. In contrast, terminal phosphine ligands have no significant effects on redox activities and mechanism. Mono- or di-substituted propane-dithiolate complexes afford a sequential reduction (electrochemical; E) and protonation (chemical; C) mechanism (ECEC), while the μ-benzene dithiolate complexes follow a different reaction mechanism and are more efficient HER catalysts.