Synthesis and characterisation of neutral and cationic alkyl aluminium complexes bearing N,O-Schiff base chelates with pendant donor arms

Abstract

The Schiff base ligands [3,5-Bu^t₂-2-(OH)C₆H₂CHNL] [L = CH₂CH₂NMe₂ (1a), 2-(PhO)C₆H₄ (1b), 2-CH₂C₅H₃N (1c), 8-C₉H₆N (quinoline) (1d) and 2-(PPh₂)C₆H₄ (1e)] are accessed in good yields (>85%) via standard imine condensation reactions. Reaction of 1a–e with Me₃Al at room temperature affords the corresponding complexes [(3,5-Bu^t₂-2-(O)C₆H₂CHNL)AlMe₂] (2a–e); in the case of L = 8-quinoline, the same reaction conducted in refluxing toluene affords binuclear {[3,5-Bu^t₂-2-(O)C₆H₂CHMeN-8-C₉H₆N]AlMe}₂ (3) by methyl migration from metal to ligand. Further reaction of the dimethyl compounds with B(C₆F₅)₃ in CD₂Cl₂ or C₆D₆ affords the cationic systems [(3,5-Bu^t₂-2-(O)C₆H₂CHNL)AlMe]⁺ (4a–e). The crystal structures of 2a, 2c, 2e and 3 have been determined. In 2a and 2c the respective ligands bind to the metal centre via all three heteroatoms, the aluminium having a trigonal bipyramidal geometry, whereas in 2e coordination is via nitrogen and oxygen only, and the aluminium is tetrahedral. Complex 3 has a dimeric structure with the ligand adopting both tridentate and binucleating roles; the aluminium centres are trigonal bipyramidal.