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

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.