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(SiMe₃ )₂ )₂ (1) and dimethylamine borane yields the tris(amide) adduct (NHC-BN)Mg(NMe₂ BH₃ )(N(SiMe₃ )₂ ) (2; NHC-BN = NHC-BH₂ NMe₂ ). In addition to Me₂ N=BH₂ capture at the ^NHC C-Mg bond, mechanistic investigations suggest the likelihood of aminoborane migratory insertion from an RMg(NMe₂ BH₂ NMe₂ BH₃ ) intermediate. To elucidate these processes, the carbene complexes (NHC)Mg(NMe₂ BH₃ )₂ (8) and (NHC)Mg(NMe₂ BH₂ NMe₂ BH₃ )₂ (9) were synthesized, and a dynamic migration of Me₂ N=BH₂ 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 ^- {NMe₂ BH₂ NMe₂ BH₃ } anion, as well as the capacity of NHCs to reversibly capture Me₂ N=BH₂ in the presence of Lewis acidic magnesium species.