Direct observation of a borane–silane complex involved in frustrated Lewis-pair-mediated hydrosilylations

Perfluorarylborane Lewis acids catalyse the addition of silicon–hydrogen bonds across C=C, C=N and C=O double bonds. This 'metal-free' hydrosilylation has been proposed to occur via borane activation of the silane Si–H bond, rather than through classical Lewis acid/base adducts with the substrate. However, the key borane/silane adduct had not been observed experimentally. Here it is shown that the strongly Lewis acidic, antiaromatic 1,2,3-tris(pentafluorophenyl)-4,5,6,7-tetrafluoro-1-boraindene forms an observable, isolable adduct with triethylsilane. The equilibrium for adduct formation was studied quantitatively through variable-temperature NMR spectroscopic investigations. The interaction of the silane with the borane occurs through the Si–H bond, as evidenced by trends in the Si–H coupling constant and the infrared stretching frequency of the Si–H bond, as well as by X-ray crystallography and theoretical calculations. The adduct's reactivity with nucleophiles demonstrates conclusively the role of this species in metal-free 'frustrated-Lewis-pair' hydrosilylation reactions. Comprehensive solution and solid-state characterization of an adduct between a Lewis acidic perfluoroaryl borane and an electron-rich silane is demonstrated. This has long been proposed as an intermediate in the 'frustrated' Lewis-pair hydrosilylation of C=C, C=O and C=N double bonds, but its existence has, so far, only been inferred from indirect experimental evidence.