Isolation of a New C s ‐Symmetrized Mo 3 (μ 3 ‐S)(μ‐S)(μ‐S 2 ) 2 Structural Type Through Complementary Association with a Cubane‐Type Mo 3 NiS 4 Cluster

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

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 = ⁱBu; [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 = ⁱBu 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(S₂CNⁱBu₂)₃]⁺ left undisturbed over 24 h deposit the asymmetric Mo₆ cluster [(ⁱBu₂NCS₂)₃(μ₂-S₂)₂(μ₃-S)Mo₃](μ₃-S)(μ₃-η²,η¹-S',η¹-S″-S₂)[Mo₃(μ₂-S)₃(μ₃-S)(S₂CNⁱBu₂)₂(μ₂-S₂CNⁱBu₂)] 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.

Tungsten and molybdenum incomplete cuboidal clusters; kinetico-mechanistic studies and association in dimers

New triangular Mo(IV) and W(IV) incomplete cuboidal cluster complexes containing three η(2)-diethyldithiophosphates (dtp, one per metal centre) and a η(2)(μ)-AcO or η(2)(μ)-dtp ligand bridging two of these centres have been prepared as mono-substituted derivatives and characterized by the usual techniques plus single crystal X-ray diffraction (XRD) analysis in some cases. In the compounds, the single substitutionally available unlocked position is coordinated to different N-donor ligands (1,2-bis(pyridyl)ethylene, pyrimidine, pyrazine) or to the residual solvent. The compounds are very labile and the N-donor ligands are only weakly coordinated, being easily substituted by other donors or coordinating solvents such as acetonitrile. This lability has been explored kinetico-mechanistically by stopped-flow techniques at varying temperatures for the compound [Mo3S4(η(2)-dtp)3(η(2)(μ)-dtp)(H2O)]. By doing so, the nature of the activation process leading to substitution reactions on these complexes has been established as associatively activated and dominated by a very low formation equilibrium constant, that is, by the substitution of the incoming N-donor ligand. Furthermore, the existence of a non-dissociatively activated trigonal 60° fluxional twist has also been established for the η(2)-dtp ligand on the single unlocked position. The lability of the system has also enabled the association in dimers of the acetato derivative, i.e. [Mo3S4(η(2)-dtp)3(η(2)(μ)-AcO)(H2O)], via a pyrazine bridging ligand. After a very careful tuning of the reaction conditions, as established from the kinetico-mechanistic studies, the first single-bridged unsupported dimeric association of {Mo3S4} units, i.e. [{Mo3S4(η(2)-dtp)3(η(2)(μ)-AcO)}2(μ-pyrazine)], has been structurally characterised.