Normal and abnormal NHC coordination in cationic hydride iodide complexes of aluminium

Abstract

The mixed N-heterocyclic carbene (NHC) complexes NHCAlH_xI_3−x, where NHC is IDip or IMes ((HCNAr)₂C:, Ar = 2,6-iPr₂C₆H₃ = Dip (IDip); or 2,4,6-Me₃C₆H₂ = Mes (IMes)), x = 1 or 2, were either prepared from NHCAlH₃ and NHCAlI₃ or by halogenation of NHCAlH₃ with MeI. Reaction of [(IDip)AlH_xI_3−x], with x = 0–3, with another equivalent of IDip afforded either fluxional equilibria in benzene solution for x = 0, no reaction for x = 3, or the new mixed normal–abnormal NHC-coordinated ionic complexes [(IDip)AlH₂(aIDip)]I (9) and [(IDip)AlHI(aIDip)]I (10), where aIDip is the abnormal IDip carbene tautomer bonded through its 4-position. The molecular structures of 9 and 10 were determined and show slightly shorter Al–C(aIDip) than Al–C(IDip) distances. In addition, a complex containing [(IDip)AlI₂(aIDip)]I (11) was structurally characterized though could not intentionally be synthesised. Possible formation mechanisms for 9–11 are discussed and the normal and abnormal IDip coordination to the aluminium(III) centre is believed to occur for steric reasons.