Unique reactivity characteristics of Mo-coordinated S2-2 and S2-4 ligands

Abstract

The synthesis of the new dithiolene complexes, [{(MeOOC)2C2S2}2Mo([mu]2-S)]-2 and [OMo(S2C2(COOMe)2)2]-, is reported. These complexes are obtained by the reaction of dicarbomethoxyacetylene (DMA) with either [(S4)Mo(S)([mu]2-S)2Mo(S)(S4)]2- or [(CS4)Mo(S)([mu]2-S)2Mo(S)(CS4)]2- and [OMo(S4)2]2-, respectively. The reaction of [(S4)Mo(O)([mu]2-S)2Mo(O)(S2)]2- with DMA results in the new dithiolene complex [{(MeOOC)2C2S2}Mo(O)([mu]2-S)]-2, which is the isomeric form of the vinyl disulfide complex obtained in the reaction of the [(S2)Mo(O)([mu]2-S)2Mo(O)(S2)]2- complex with DMA. The difference in reactivity between the two complexes that contain the same [Mo2O2S2]2+ core is attributed to the intrinsically different reactivity characteristics of the S2-4 and S2-2 ligands. As a result of Mo-S d[pi]-p[pi] bonding an alternation in the S---S bond lengths is observed in virtually all of the structurally characterized Mo-S4 units. The consequent weakening of the S---S bonds adjacent to the Mo---S bonds allows for the ready dissociation of S02 from the Mo-coordinated S2-4 ligands. This weakening also accounts for the facile formation of dithiolenes in cycloaddition reactions of alkynes with the Mo-S4 units. By comparison, the S---S bond in side-on Mo-coordinated S2-2 ligands is strengthened as a result of depopulation of the ligand [pi]*-orbitals. Reactions of the latter with alkynes do not proceed by cycloaddition. Instead, insertion into the Mo---O bond has been reported for at least one such reaction. The importance of activated polysulfide ligands in the hydrodesulfurization reaction is discussed.