Hydrogen evolution reaction evidenced for Mo2O2S2-thiosemicarbazone coordination complexes

In this study, a family of 7 dinuclear and tetranuclar complexes formed between the thiocation [Mo₂O₂S₂]²⁺ and various bis-thiosemicarbazone ligands are studied towards their properties for the reduction of protons into hydrogen in aqueous medium. The HER properties of the complexes in the form of modified electrodes are evidenced in the 1–4 pH range and the impact of the variation of ligand as well as the nuclearity of the complexes are investigated. DFT studies allowed to elucidate the HER mechanism through a Volmer–Tafel mechanism involving a first step of reduction of the Mo cluster with 2 electrons, a first protonation leading to the formation of a molybdenum hydride, a second protonation on the thiolate of the nearby thiosemicarbazone ligand associated with the decoordination of Mo and then the combination of this proton with a hydride to form H₂ and regenerate the initial complex.