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

Abstract

Treatment of the salicylaldimine ligands 3,5-Bu^t₂-2-(OH)C₆H₂CHNR [R = 2,6-Me₂C₆H₃ (1a), 2,6-Prⁱ₂C₆H₃ (1b), 3,5-(CF₃)₂C₆H₃ (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^t₂-2-(O)C₆H₂CHNR}AlMe₂ [R = 2,6-Me₂C₆H₃ (2a), 2,6-Prⁱ₂C₆H₃ (2b), 3,5-(CF₃)₂C₆H₃ (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^t₂-2-(O)C₆H₂CHNR}AlMe(THF)]⁺ [R = 2,6-Me₂C₆H₃ (3a), 2,6-Prⁱ₂C₆H₃ (3b), 3,5-(CF₃)₂C₆H₃ (3c), 4-(NO₂)C₆H₄ (3d), 4-ClC₆H₄ (3e), 1-naphthyl (3f), Bu^t (3g)], as the B(C₆F₅)₃Me⁻ 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^t₂-2-(O)C₆H₂CHNBu^t}AlMe(C₆F₅)] and B(C₆F₅)₂Me. 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.