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.