and NHC coordination in cationic hydride iodide complexes of aluminium

The mixed N -heterocyclic carbene (NHC) complexes NHCAlH x I 3− x , where NHC is IDip or IMes ((HCNAr) 2 C:, Ar = 2,6- i Pr 2 C 6 H 3 = Dip (IDip); or 2,4,6-Me 3 C 6 H 2 = Mes (IMes)), x = 1 or 2, were either prepared from NHCAlH 3 and NHCAlI 3 or by halogenation of NHCAlH 3 with MeI. Reaction of [(IDip)AlH x I 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)AlH 2 ( a IDip)]I ( 9 ) and [(IDip)AlHI( a IDip)]I ( 10 ), where a IDip 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( a IDip) than Al-C(IDip) distances. In addition, a complex containing [(IDip)AlI 2 ( a IDip)]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. Sterically demanding NHC aluminium hydride iodide complexes react with one equivalent of NHC to cationic mixed normal - abnormal NHC Al III complexes.