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

Abstract

The mixed N-heterocyclic carbene (NHC) complexes NHCAlHxI3-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)AlHxI3-x], with x = 0-3, with another equivalent of a 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.