Barium-Catalysed Dehydrocoupling of Hydrosilanes and Borinic Acids: A Mechanistic Insight.

Two very rare cases of barium boryloxides, the homoleptic [Ba(OB{CH(SiMe3 )2 }2 )2 ⋅C7 H8 ] and the heteroleptic [{LONO4 }BaOB{CH(SiMe3 )2 }2 ] stabilised by the multidentate aminoetherphenolate {LONO4 }- , are presented, and their structural properties are discussed. The electron-deficient [Ba(OB{CH(SiMe3 )2 }2 )2 ⋅C7 H8 ] shows, in particular, resilient η6 -coordination of the toluene molecule. Together with its amido parents [Ba{N(SiMe3 )2 }2 ⋅thf2 ] and [Ba{N(SiMe3 )2 }2 ]2 , this complex catalyses the fast and chemoselective dehydrocoupling of borinic acids R2 BOH and hydrosilanes HSiR'3 , yielding borasiloxanes R2 BOSiR'3 in a controlled fashion. The assessment of substrate scope indicates that, for now, the reaction is limited to bulky borinic acids. Kinetic analysis shows that the rate-limiting step of the catalytic manifold traverses a dinuclear transition state. A detailed mechanistic scenario is proposed on the basis of DFT computations, the results of which are fully consistent with experimental data. It consists of a stepwise process with rate-determining nucleophilic attack of a metal-bound O-atom onto the incoming hydrosilane, involving throughout dinuclear catalytically active species.