Light-driven hydrogen evolution reactivity of molecular thio-oxomolybdate catalysts

Molybdenum sulfides are widely used noble metal-free hydrogen evolution reaction (HER) catalysts. Their molecular analogues, so-called thiomolybdates, have been developed as viable minimal models to study reactivity of Mo-S HER catalysts. Here, we explore the light-driven HER reactivity and stability of the mixed thio-oxo-molybdate prototype [Mo2O2S6]2- in homogeneous solution using experimental and theoretical methods. In combination with the photosensitizer [Ru(bpy)3]2+, [Mo2O2S6]2- shows promising HER performance (TON > 500), as well as strong reactivity dependence on the solvent mixture used (here: methanol-water mixtures). Mechanistic studies show that increasing water concentrations in the reaction solution led to a reduction of HER reactivity. Time-dependent Raman spectroscopy show, that the system undergoes exchange of the terminal disulfide ligands for solvent ligands, leading to complex, coupled speciation equilibria in solution. Analysis of turnover-frequency (TOF) time-profiles indicate initial formation of a more active species followed by catalyst deactivation. Density functional theory (DFT) calculations provide complementary information into the speciation and show that ligand-exchanged species [Mo2O2S4(L)2]0 (L = MeOH/H2O), feature favorable free-energy landscapes for proton-coupled electron transfer than the native catalyst species. In sum, combined experiment and theory provide unique molecular-level insights into the reactivity of thio-oxo-molybdate HER catalysts and shed light on the complex speciation and changes of reactivity upon ligand exchange at these species. These structure–reactivity insights outline design rules for more robust, solvent-tolerant Mo–S HER catalysts.