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

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)LMe4 (1; NHC = N-heterocyclic carbene, LMe4 = 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 LMe4 to form the more stable complex LMe4→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.