Osmabenzenes from the Reactions of a Dicationic Osmabenzyne Complex

Treatment of the osmabenzyne Os(⋮CC(SiMe₃)C(Me)C(SiMe₃)CH)Cl₂(PPh₃)₂ (1) with 2,2‘-bipyridine (bipy) and thallium triflate (TlOTf) produces the thermally stable dicationic osmabenzyne [Os(⋮CC(SiMe₃)C(Me)C(SiMe₃)CH)(bipy)(PPh₃)₂](OTf)₂ (2). The dicationic osmabenzyne 2 reacts with ROH (R = H, Me) to give osmabenzene complexes [Os(C(OR)CHC(Me)C(SiMe₃)CH)(bipy)(PPh₃)₂]OTf, in which the metallabenzene ring deviates significantly from planarity. In contrast, reaction of the dicationic complex 2 with NaBH₄ produces a cyclopentadienyl complex, presumably through the osmabenzene intermediate [Os(CHC(SiMe₃)C(Me)C(SiMe₃)CH)(bipy)(PPh₃)₂]OTf. The higher thermal stability of [Os(C(OR)CHC(Me)C(SiMe₃)CH)(bipy)(PPh₃)₂]OTf relative to [Os(CHC(SiMe₃)C(Me)C(SiMe₃)CH)(bipy)(PPh₃)₂]OTf can be related to the stabilization effect of the OR groups on the metallacycle. A theoretical study shows that conversion of the dicationic osmabenzyne complex [Os(⋮CC(SiMe₃)C(Me)C(SiMe₃)CH)(bipy)(PPh₃)₂](OTf)₂ to a carbene complex by reductive elimination is thermodynamically unfavorable. The theoretical study also suggests that the nonplanarity of the osmabenzenes [Os(C(OR)CHC(Me)C(SiMe₃)CH)(bipy)(PPh₃)₂]OTf is mainly due to electronic reasons.