Synthesis and characterisation of neutral dialkylaluminium complexes stabilised by salicylaldiminato ligands, and their conversion to monoalkylaluminium cations

Treatment of the salicylaldimine ligands 3,5-Bu-2(t)-2-(OH)C6H2CHNR [R=2,6-Me2C6H3 (1a), 2,6-(Pr2C6H3)-C-i (1b), 3,5-(CF3)(2)C6H3 (1c), 4-(NO2)C6H4 (1d), 4-ClC6H4 (1e), 1-naphthyl (1f), Bu-t (1g)] with Me3Al in toluene yields, after work-up, the highly crystalline (except 2c - an oil) complexes {3,5-Bu-2(t)-2-(O)C6H2CH=NR}AlMe2 [R=2,6-Me2C6H3 (2a), 2,6-(Pr2C6H3)-C-i (2b), 3,5-(CF3)(2)C6H3 (2c), 4-(NO2)C6H4 (2d), 4-ClC6H4 (2e), 1-naphthyl (2f), Bu-t (2g)] respectively. Reaction of these systems with B(C6F5)(3) in the presence of THF leads smoothly to [{3,5-Bu-2(t)-2-(O)C6H2CH=NR}AlMe(THF)](+) [R=2,6-Me2C6H3 (3a), 2,6-(Pr2C6H3)-C-i (3b), 3,5-(CF3)(2)C6H3 (3c), 4-(NO2)C6H4 (3d), 4-ClC6H4 (3e), 1-naphthyl (3f), Bu-t (3g)], as the B(C6F5)(3)Me- salts. By contrast, the same reaction performed in dichloromethane solution without THF gives complex mixtures: the NMR spectrum of the product mixture obtained from the reaction of 2g with B(C6F5)(3) in CD2Cl2 indicated, inter alia, the presence of both {3,5-Bu-2(t)-2-(O)C6H2CH=NBut}AlMe(C6F5)] and B(C6F5)(2)Me. Compounds 2a and 2b have been characterised by single crystal X-ray structure determinations and shown to have virtually identical conformations. In both systems there is a marked distortion in the tetrahedral geometry at the aluminium centre.