Reductive Hydrogenation of Sulfido-Bridged Tantalum Alkyl Complexes: A Mechanistic Insight

Hydrogenolysis of a series of alkyl sulfido-bridged tantalum(IV) dinuclear complexes [Ta(η⁵-C₅Me₅)R(μ-S)]₂ [R = Me, nBu (1), Et, CH₂SiMe₃, C₃H₅, Ph, CH₂Ph (2), p-MeC₆H₄CH₂ (3)] has led quantitatively to the Ta(III) tetrametallic sulfide cluster [Ta(η⁵-C₅Me₅)(μ₃-S)]₄ (4) along with the corresponding alkane. Mechanistic information for the formation of the unique low-valent tetrametallic compound 4 was gathered by hydrogenation of the phenyl-substituted precursor [Ta(η⁵-C₅Me₅)Ph(μ-S)]₂, which proceeds through a stepwise hydrogenation process, disclosing the formation of the intermediate tetranuclear hydride sulfide [Ta₂(η⁵-C₅Me₅)₂(H)Ph(μ-S)(μ₃-S)]₂ (5). Extending our studies toward tantalum alkyl precursors containing functional groups susceptible to hydrogenation, such as the allyl-and benzyl-substituted compounds [Ta(η⁵-C₅Me₅)(η³-C₃H₅)(μ-S)]₂ and [Ta(η⁵-C₅Me₅)(CH₂Ph)(μ-S)]₂ (2), enables alternative reaction pathways en route to the formation of 4. In the former case, the dimetallic system undergoes selective hydrogenation of the unsaturated allyl moiety, forming the asymmetric complex [{Ta(η⁵-C₅Me₅)(η³-C₃H₅)}(μ-S)₂{Ta(η⁵-C₅Me₅)(C₃H₇)}] (6) with only one propyl fragment. Species 2, in addition to the hydrogenation of one benzyl fragment and concomitant toluene release, also undergoes partial hydrogenation and dearomatization of the phenyl ring on the vicinal benzyl unity to give a η⁵-cyclohexadienyl complex [Ta₂(η⁵-C₅Me₅)₂(μ-CH₂C₆H₆)(μ-S)₂] (7). The mechanistic implications of the latter hydrogenation process are discussed by means of DFT calculations.