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 3 Si) 3 SiB(Cl)NHI ( 2 , NHI=bulky N‐heterocyclic imine) was prepared and treated with KO t Bu to achieve formal extrusion of ClSiMe 3 . The formation of an elusive borasilene ( 3 int ) is postulated and it was verified by isolation of the N‐heterocyclic carbene adduct (Me 3 Si) 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 (Me 3 Si) 2 Si(Bpin) 2 and (NHI)BH 2 (IMe4).