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 5 -C 5 Me 5 )R(μ-S)] 2 [R = Me, n Bu ( 1 ), Et, CH 2 SiMe 3 , C 3 H 5 , Ph, CH 2 Ph ( 2 ), p -MeC 6 H 4 CH 2 ( 3 )] has led quantitatively to the Ta(III) tetrametallic sulfide cluster [Ta(η 5 -C 5 Me 5 )(μ 3 -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(η 5 -C 5 Me 5 )Ph(μ-S)] 2 , which proceeds through a stepwise hydrogenation process, disclosing the formation of the intermediate tetranuclear hydride sulfide [Ta 2 (η 5 -C 5 Me 5 ) 2 (H)Ph(μ-S)(μ 3 -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(η 5 -C 5 Me 5 )(η 3 -C 3 H 5 )(μ-S)] 2 and [Ta(η 5 -C 5 Me 5 )(CH 2 Ph)(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(η 5 -C 5 Me 5 )(η 3 -C 3 H 5 )}(μ-S) 2 {Ta(η 5 -C 5 Me 5 )(C 3 H 7 )}] ( 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 η 5 -cyclohexadienyl complex [Ta 2 (η 5 -C 5 Me 5 ) 2 (μ-CH 2 C 6 H 6 )(μ-S) 2 ] ( 7 ). The mechanistic implications of the latter hydrogenation process are discussed by means of DFT calculations.