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)(3)SiB(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)(2)SiB(IMe4)NHI (4, IMe4=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)(2)Si(Bpin)(2) and (NHI)BH2(IMe4).