Aluminium Complexes of a Phenoxyimine Ligand with a Pendant Imidazolium Moiety: Synthesis, Characterisation and Evidence for Hydrogen Bonding in Solution

Novel alkylaluminium complexes (phim)AIMe(2) (1) and (phimid)AIR(2)(+)Br(-) [R = Me (2), R = iBu (3)] bearing the Schiff base ligands 3,5-tBu(2)-2-(OH)C(6)H(2)CH=NiPr (phim-H) and 3,5-tBu(2)-2-(OH)C(6)H(2)CH=NCH(2)CH(2)[CH(NCHCHNiPr)]Br (phimid-H center dot Br) have been prepared and fully characterised. Complexes 1-3 each have a tetrahedral structure, with the aluminium atom surrounded by the oxygen and nitrogen atoms of the chelating ligand and two alkyl groups. The structures of phimid-H center dot Br and of complex 1 have been determined by X-ray diffraction studies. Investigation of the solution structures of 1-3 by (1)H NMR spectroscopy revealed that the coordinated phimid ligand is involved in hydrogen bonding with bromide anion. Treatment of 1 with B(C(6)F(5))(3) led smoothly to (phim)Al(C(6)F(5))Me (4) by transfer of a C(6)F(5) group from MeB(C(6)F(5))(3)(-) to the initially formed coordinatively unsaturated cationic intermediate. In contrast, treatment of 2 with one equiv. of B(C(6)F(5))(3) afforded the cabonic monomethyl species (phimid)AIMeBr(+) MeB(C(6)F(5))(3)(-) (5), stabilised by the coordination of the bromide anion acting as a Lewis base.