Photocatalytic H2‑Evolution by Homogeneous Molybdenum Sulfide Clusters Supported by Dithiocarbamate Ligands

• Published in: Inorganic chemistry 2019-12, Vol.58 (24), p.16458-16474
• Main Authors: Fontenot, Patricia R, Shan, Bing, Wang, Bo, Simpson, Spenser, Ragunathan, Gayathri, Greene, Angelique F, Obanda, Antony, Hunt, Leigh Anna, Hammer, Nathan I, Webster, Charles Edwin, Mague, Joel T, Schmehl, Russell H, Donahue, James P
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.9b02252

Irradiation at 460 nm of [Mo3(μ3-S)­(μ2-S2)3(S2CNR2)3]I ([2a]­I, R = Me; [2b]­I, R = Et; [2c]­I, R = i Bu; [2d]­I, R = CH2C6H5) in a mixed aqueous–polar organic medium with [Ru­(bipy)3]2+ as photosensitizer and Et3N as electron donor leads to H2 evolution. Maximum activity (300 turnovers, 3 h) is found with R = i Bu in 1:9 H2O:MeCN; diminished activity is attributed to deterioration of [Ru­(bipy)3]2+. Monitoring of the photolysis mixture by mass spectrometry suggests transformation of [Mo3(μ3-S)­(μ2-S2)3(S2CNR2)3]+ to [Mo3(μ3-S)­(μ2-S)3(S2CNR2)3]+ via extrusion of sulfur on a time scale of minutes without accumulation of the intermediate [Mo3S6(S2CNR2)3]+ or [Mo3S5(S2CNR2)3]+ species. Deliberate preparation of [Mo3S4(S2CNEt2)3]+ ([3]+) and treatment with Et2NCS2 1– yields [Mo3S4(S2CNEt2)4] (4), where the fourth dithiocarbamate ligand bridges one edge of the Mo3 triangle. Photolysis of 4 leads to H2 evolution but at ∼25% the level observed for [Mo3S7(S2CNEt2)3]+. Early time monitoring of the photolyses shows that [Mo3S4(S2CNEt2)4] evolves H2 immediately and at constant rate, while [Mo3S7(S2CNEt2)3]+ shows a distinctive incubation prior to a more rapid H2 evolution rate. This observation implies the operation of catalysts of different identity in the two cases. Photolysis solutions of [Mo3S7(S2CN i Bu2)3]+ left undisturbed over 24 h deposit the asymmetric Mo6 cluster [( i Bu2NCS2)3(μ2-S2)2(μ3-S)­Mo3]­(μ3-S)­(μ3-η2,η1-S′,η1-S″-S2)­[Mo3(μ2-S)3(μ3-S)­(S2CN i Bu2)2(μ2-S2CN i Bu2)] in crystalline form, suggesting that species with this hexametallic composition and core topology are the probable H2-evolving catalysts in photolyses beginning with [Mo3S7(S2CNR2)3]+. When used as solvent, N,N-dimethylformamide (DMF) suppresses H2-evolution but to a greater degree for [Mo3S4(S2CNEt2)4] than for [Mo3S7(S2CNEt2)3]+. Recrystallization of [Mo3S4(S2CNEt2)4] from DMF affords [Mo3S4(S2CNEt2)4(η1,κO-DMF)] (5), implying that inhibition by DMF arises from competition for a Mo coordination site that is requisite for H2 evolution. Computational assessment of [Mo3S4(S2CNMe2)3]+ following addition of 2H+ and 2e– suggests a Mo­(H)−μ2(SH) intermediate as the lowest energy species for H2 elimination. An analogous pathway may be available to the Mo6 cluster via dissociation of one end of the μ2-S2CNR2 ligand, a known hemilabile ligand type, in the [Mo3S4]4+ fragment.