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

Hydrogenolysis of a series of alkyl sulfido-bridged tantalum(IV) dinuclear complexes [Ta(η-CMe)R(μ-S)] [R = Me, Bu (), Et, CHSiMe, CH, Ph, CHPh (), -MeCHCH ()] has led quantitatively to the Ta(III) tetrametallic sulfide cluster [Ta(η-CMe)(μ-S)] () along with the corresponding alkane. Mechanistic information for the formation of the unique low-valent tetrametallic compound was gathered by hydrogenation of the phenyl-substituted precursor [Ta(η-CMe)Ph(μ-S)], which proceeds through a stepwise hydrogenation process, disclosing the formation of the intermediate tetranuclear hydride sulfide [Ta(η-CMe)(H)Ph(μ-S)(μ-S)] (). Extending our studies toward tantalum alkyl precursors containing functional groups susceptible to hydrogenation, such as the allyl-and benzyl-substituted compounds [Ta(η-CMe)(η-CH)(μ-S)] and [Ta(η-CMe)(CHPh)(μ-S)] (), enables alternative reaction pathways en route to the formation of . In the former case, the dimetallic system undergoes selective hydrogenation of the unsaturated allyl moiety, forming the asymmetric complex [{Ta(η-CMe)(η-CH)}(μ-S){Ta(η-CMe)(CH)}] () with only one propyl fragment. Species , 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 η-cyclohexadienyl complex [Ta(η-CMe)(μ-CHCH)(μ-S)] (). The mechanistic implications of the latter hydrogenation process are discussed by means of DFT calculations.