Isolation of an N-Heterocyclic Carbene Complex of a Borasilene

Borasilenes, that is complexes which contain a boron–silicon double bond, have scarcely been isolated to date. In pursuit of such species, (Me₃Si)₃SiB(Cl)NHI (2, NHI=bulky N-heterocyclic imine) was prepared and treated with KOtBu to achieve formal extrusion of ClSiMe₃. The formation of an elusive borasilene (3_int) is postulated and it was verified by isolation of the N-heterocyclic carbene adduct (Me₃Si)₂SiB(I (Formula presented.))NHI (4, I (Formula presented.) =1,3,4,5-tetramethyl-imidazolin-2-ylidene). X-ray crystallographic study and theoretical calculations on 4 diagnosed a boron–silicon double bond with marked zwitterionic character. The negative charge resides at the Si atom which marks the apex of a trigonal pyramid. Structural comparison of 4 with boron cation congeners (5⁺, 6⁺) suggests that the positive charge is mainly located at the trigonal planar-coordinated B center. The conversion of 4 with pinacolborane (HBpin, 2 equiv) resulted in cleavage of the double bond to produce (Me₃Si)₂Si(Bpin)₂ and (NHI)BH₂(I (Formula presented.)).