Low‐Coordinate Barium Boryloxides: Synthesis and Dehydrocoupling Catalysis for the Production of Borasiloxanes

Abstract

The first soluble barium boryloxides [Ba]- OB{CH(SiMe ) } are presented. These mono- or dinuclear complexes feature low coordination numbers, as low as two for [Ba(OB{CH(SiMe ) } ) ], which is further stabilized by intra- and intermolecular Ba⋅⋅⋅H C agostic interactions. Barium boryloxides and the parent [Ba{N(SiMe ) } ⋅(thf) ] catalyze the dehydrocoupling of borinic acids with hydrosilanes, providing borasiloxanes under mild conditions.

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

“…Yet, in the solid state, toluene could not be removed from 2 ⋅(tol) even upon moderate heating under dynamic vacuum (10 −2 torr) for several hours. The rather stable coordination of this weak ligand (for d 0 AE complexes, unable to involve in d–π* back‐donation) highlights the very electrophilic character of 2 , in agreement with the presence of intra‐ and intermolecular agostic Ba⋅⋅⋅H−C interactions in the molecular solid‐state structure of this complex …”

“…The coupling of {(Me 3 Si) 2 CH} 2 BOH with PhSiH 3 was used as benchmark reaction. As communicated earlier, the coupling of 50–100 equiv of both substrates (1:1) vs. Ba occurred readily at 60 °C, generating PhSi(H) 2 OB{CH(SiMe 3 ) 2 } 2 in quantitative yields. The main conclusions of the initial investigation, for which the easily accessed 4 ⋅(thf) 2 precatalyst was used, can be summarised as follows: 1) the reactions are chemoselective, as for instance only one coupling occurs with PhSiH 3 even with excess {(Me 3 Si) 2 CH} 2 BOH; 2) reaction rates increase according to Ca<Sr<Ba; 3) 4 2 and 4 ⋅(thf) 2 are the most efficient precatalysts; 4) the kinetic rate law using the mononuclear 4 ⋅(thf) 2 shows first‐order dependence on [PhSiH 3 ], second‐order on [ 4 ⋅(thf) 2 ] and zeroth‐order on [{(Me 3 Si) 2 CH} 2 BOH], with ΔH ≠ =9.1(2) kcal mol −1 , ΔS ≠ =−24.2(8) cal mol −1 K −1 and ΔG ≠ =16.3(5) kcal mol −1 at 25 °C; 5) isolated data points suggested that reaction rates increases (resp.…”

“…We surmised that the partial kinetic order of two in [precatalyst] determined for the coupling of {(Me 3 Si) 2 CH} 2 BOH and PhSiH 3 catalysed by the mononuclear 4 ⋅(thf) 2 indicated the reaction traversed an associated dinuclear transition state; this was compatible with the large negative entropy of activation, ΔS ≠ =−24.2(8) cal mol −1 K −1 . This assumption proved justified since, under otherwise identical conditions, the reaction catalysed by the dinuclear precatalyst [Ba{μ 2 ‐N(SiMe 3 ) 2 }(OB{CH(SiMe 3 ) 2 } 2 )] 2 ( 1 2 ) unambiguously exhibited first‐order kinetics in [dinuclear precatalyst], see Figure S44 in the Supporting Information.…”

“…We reported in our initial communication that the dimer [Ba{μ 2 ‐N(SiMe 3 ) 2 }(OB{CH(SiMe 3 ) 2 } 2 )] 2 ( 1 2 ) could be obtained upon equimolar reaction of [Ba{N(SiMe 3 ) 2 } 2 ⋅thf 2 ] and {(Me 3 Si) 2 CH} 2 BOH, and that further reaction of 1 2 with two equivalents of {(Me 3 Si) 2 CH} 2 BOH in petroleum ether afforded the monometallic [Ba(OB{CH(SiMe 3 ) 2 } 2 ) 2 ] ( 2 ), the first formally two‐coordinate barium complex.…”

“…We have recently communicated on low‐coordinate barium boryloxides such as the dimeric [Ba{μ 2 ‐N(SiMe 3 ) 2 }(OB{CH(SiMe 3 ) 2 } 2 )] 2 ( 1 2 ) and the monomeric [Ba(OB{CH(SiMe 3 ) 2 } 2 ) 2 ] ( 2 ) . These complexes catalyse the dehydrocoupling of {(Me 3 Si) 2 CH} 2 BOH and hydrosilanes HSiR 3 to competently generate borasiloxanes {(Me 3 Si) 2 CH} 2 BOSiR 3 .…”