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.