N-Heterocyclic Carbene-Assisted Reversible Migratory Coupling of Aminoborane at Magnesium.

A combined synthetic and theoretical investigation of N-heterocyclic carbene (NHC) adducts of magnesium amidoboranes is presented, which involves a rare example of reversible migratory insertion within a normal valent s-block element. The reaction of (NHC)Mg(N(SiMe 3 ) 2 ) 2 (1) and dimethylamine borane yields the tris(amide) adduct (NHC-BN)Mg(NMe 2 BH 3 )(N(SiMe 3 ) 2 ) (2; NHC-BN = NHC-BH 2 NMe 2 ). In addition to Me 2 N=BH 2 capture at the NHC C-Mg bond, mechanistic investigations suggest the likelihood of aminoborane migratory insertion from an RMg(NMe 2 BH 2 NMe 2 BH 3 ) intermediate. To elucidate these processes, the carbene complexes (NHC)Mg(NMe 2 BH 3 ) 2 (8) and (NHC)Mg(NMe 2 BH 2 NMe 2 BH 3 ) 2 (9) were synthesized, and a dynamic migration of Me 2 N=BH 2 between Mg-N and NHC C-Mg bonds was observed in 9. This unusual reversible migratory insertion is presumably induced by dissimilar charge localization in the - {NMe 2 BH 2 NMe 2 BH 3 } anion, as well as the capacity of NHCs to reversibly capture Me 2 N=BH 2 in the presence of Lewis acidic magnesium species.