Photocatalytic H-Evolution by Homogeneous Molybdenum Sulfide Clusters Supported by Dithiocarbamate Ligands.

Irradiation at 460 nm of [Mo(μ-S)(μ-S)(SCNR)]I ([]I, R = Me; []I, R = Et; []I, R = Bu; []I, R = CHCH) in a mixed aqueous-polar organic medium with [Ru(bipy)] as photosensitizer and EtN as electron donor leads to H evolution. Maximum activity (300 turnovers, 3 h) is found with R = Bu in 1:9 HO:MeCN; diminished activity is attributed to deterioration of [Ru(bipy)]. Monitoring of the photolysis mixture by mass spectrometry suggests transformation of [Mo(μ-S)(μ-S)(SCNR)] to [Mo(μ-S)(μ-S)(SCNR)] via extrusion of sulfur on a time scale of minutes without accumulation of the intermediate [MoS(SCNR)] or [MoS(SCNR)] species. Deliberate preparation of [MoS(SCNEt)] ([]) and treatment with EtNCS yields [MoS(SCNEt)] (), where the fourth dithiocarbamate ligand bridges one edge of the Mo triangle. Photolysis of leads to H evolution but at ∼25% the level observed for [MoS(SCNEt)]. Early time monitoring of the photolyses shows that [MoS(SCNEt)] evolves H immediately and at constant rate, while [MoS(SCNEt)] shows a distinctive incubation prior to a more rapid H evolution rate. This observation implies the operation of catalysts of different identity in the two cases. Photolysis solutions of [MoS(SCNBu)] left undisturbed over 24 h deposit the asymmetric Mo cluster [(BuNCS)(μ-S)(μ-S)Mo](μ-S)(μ-η,η-S',η-S″-S)[Mo(μ-S)(μ-S)(SCNBu)(μ-SCNBu)] in crystalline form, suggesting that species with this hexametallic composition and core topology are the probable H-evolving catalysts in photolyses beginning with [MoS(SCNR)]. When used as solvent, ,-dimethylformamide (DMF) suppresses H-evolution but to a greater degree for [MoS(SCNEt)] than for [MoS(SCNEt)]. Recrystallization of [MoS(SCNEt)] from DMF affords [MoS(SCNEt)(η,κ-DMF)] (), implying that inhibition by DMF arises from competition for a Mo coordination site that is requisite for H evolution. Computational assessment of [MoS(SCNMe)] following addition of 2H and 2e suggests a Mo(H)-μ(SH) intermediate as the lowest energy species for H elimination. An analogous pathway may be available to the Mo cluster via dissociation of one end of the μ-SCNR ligand, a known hemilabile ligand type, in the [MoS] fragment.