Small-Molecule Models of Hydrogen-Evolving MX₂ (M = Mo, W; X = S, Se) Bulk Solids: Composition-Activity Relationships

Triangular metal chalcogenide clusters of the form [M₃Q₇L₃]An (M = Mo or W; Q = S or Se; L = Bu₂NCS₂^-, (CF₃CH₂)₂NCS₂^-, Bu₂NCSe₂^-, or Bu₂PS₂^-; An = Cl^- or I^-) have been investigated as molecular analogues of layered metal dichalcogenide (MX₂) H₂-evolution catalysts. These clusters have been evaluated for their relative H₂-evolving ability under a common photolysis protocol implementing [Ru(bpy)₃]²⁺ as chromophore and Et₃N as sacrificial electron donor. With M constant as Mo and with constant supporting ligand, clusters with an all-sulfide core enable greater H₂-TON than clusters with an all-selenide core. A more active catalyst is produced by [Mo₃S₇(S₂CNBu₂)₃]⁺I^- than its W₃ analogue with the same core sulfide composition and supporting dithiocarbamate ligands. Dichalcogenocarbamate ligands provide more active catalysts than dialkyldithiophosphate ligated clusters, and within the dichalcogenocarbamate set, greater H₂-turnovers correlate with more-electron-donating ligands (i.e., Bu₂NCS₂^- > (CF₃CH₂)₂NCS₂^- > Bu₂NCSe₂^-). Cluster cations with Cl^- as counteranion are very similar in activity H₂-evolving levels to identical clusters with I^-, ruling out any significant interfering effect by I^- upon the electron transfer relay between Et₃N and catalyst. In the aggregate, observations are consistent with a mechanism for H₂ evolution that involves reductive extrusion of H₂ from a metal hydride intermediate.