Reactivity of an NHC-stabilized pyramidal hydrosilylene with electrophilic boron sources

Abstract

Silylenes have become an indispensable tool for molecular bond activation. Their use for the construction of silicon–boron bonds is uncommon in comparison to the numerous studies on silylene-derived silicon-element bond formations. Herein we investigate the reactivity of the pyramidal NHC-coordinated hydrosilylene tBu₃SiSi(H)L^Me4 (1; NHC = N-heterocyclic carbene, L^Me4 = 1,3,4,5-tetramethylimidazolin-2-ylidene) with various boron-centered electrophiles. The reaction of 1 with THF·BH₃ or H₃N→BH₃ afforded the silylene complex 1→BH₃ or the product of insertion of the silicon(II) atom into an N–H bond with concomitant dehydrogenation along the HN–BH moiety (2). The respective conversion of 1 with BPh₃ yields 1→BPh₃ which readily reacts with excess L^Me4 to form the more stable complex L^Me4→BPh₃ with release of 1. Treatment of 1 with the haloboranes Et₂O→BF₃, BCl₃, BBr₃ and Me₂S→BBr₃ resulted in the formation of the Lewis acid base adducts 1→BX₃ (X = F, Cl, Br) and an equilibrium with their auto-ionization products [1₂BX₂]⁺[BX₄]⁻ slowly develops. The ratio of 1→BX₃ significantly increases with rising atomic number of the halide, thus 1→BF₃ majorly transforms within hours while 1→BBr₃ is near-quantitatively retained over time. Accordingly, the complex 1→BPhBr₂ was isolated after conversion of 1 with PhBBr₂.