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

Abstract

New triangular Mo(IV) and W(IV) incomplete cuboidal cluster complexes containing three η²-diethyldithiophosphates (dtp, one per metal centre) and a η²(μ)-AcO or η²(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(η²-dtp)₃(η²(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 η²-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(η²-dtp)₃(η²(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(η²-dtp)₃(η²(μ)-AcO)}₂(μ-pyrazine)], has been structurally characterised.