Reaction of the diimine pyridine ligand with aluminum alkyls: An unexpectedly complex reaction

The diimine pyridine ligand 2,6-{2,6-(Pr2C6H3N)-Pr-i=CMe}(2)C5H3N (1) was reacted with a series of aluminum alkyls (Me3Al, Et3Al, (Bu3Al)-Bu-i, (Bu2AlH)-Bu-i, Et2AlCl). Depending on the choice of alkyl, addition to the imine carbon and the pyridine C2 and C4 positions was observed. Addition to C2 usually dominates but is reversible; the C4 alkylation product eventually dimerizes via double C-C coupling. Reaction of 1 with AlCl3 gave the ionic complex [1 center dot AlCl2](+)[AlCl4](-). DFT calculations were used to support NMR assignments of the various addition products and also to study alkyl transfer from Et2AlCl to simplified model ligand 1'. Direct alkyl transfer from coordinated Et2AlCl to the ligand C4 position is not possible. Introduction of a second molecule of Et2AlCl results in formation of the ion pair [1'center dot AlEtCl](+)[Et3AlCl](-), from which alkyl transfer to any position of the ligand is relatively easy. The dimerization of the C4-alkylated product is symmetry-forbidden and was calculated to follow a stepwise biradical path with an unusually low barrier.