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

Triangular metal chalcogenide clusters of the form [MQL]An (M = Mo or W; Q = S or Se; L = BuNCS, (CFCH)NCS, BuNCSe, or BuPS; An = Cl or I) have been investigated as molecular analogues of layered metal dichalcogenide (MX) H-evolution catalysts. These clusters have been evaluated for their relative H-evolving ability under a common photolysis protocol implementing [Ru(bpy)] as chromophore and EtN as sacrificial electron donor. With M constant as Mo and with constant supporting ligand, clusters with an all-sulfide core enable greater H-TON than clusters with an all-selenide core. A more active catalyst is produced by [MoS(SCNBu)]I than its W 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 H-turnovers correlate with more-electron-donating ligands (i.e., BuNCS > (CFCH)NCS > BuNCSe). Cluster cations with Cl as counteranion are very similar in activity H-evolving levels to identical clusters with I, ruling out any significant interfering effect by I upon the electron transfer relay between EtN and catalyst. In the aggregate, observations are consistent with a mechanism for H evolution that involves reductive extrusion of H from a metal hydride intermediate.