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, (Me Si) SiB(Cl)NHI (2, NHI=bulky N-heterocyclic imine) was prepared and treated with KOtBu to achieve formal extrusion of ClSiMe . The formation of an elusive borasilene (3 ) is postulated and it was verified by isolation of the N-heterocyclic carbene adduct (Me Si) SiB(I )NHI (4, I =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 (Me Si) Si(Bpin) and (NHI)BH (I ).