Light-driven hydrogen evolution reactivity of molecular thiooxomolybdate catalysts

Abstract

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 (turnover number 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 revealed 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 additional insights into the speciation and suggest 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.