ALUMINUM COMPOUNDS CONTAINING BIDENTATE LIGANDS - CHELATE RING SIZE AND RIGID CONFORMATION EFFECTS

Reaction of Al(Bu-t)(3) with 2-hydroxypyridine, 2-pyridinemethanol, 8- hydroxyquinoline and 8-quinolinemethanol, yielded [(Bu-t)(2)Al(mu-O-2- C5H4N)](2) 1, (Bu-t)(2)Al(OCH2-2-C5H4N) 2, (Bu-t)(2)Al(O-8-C9H6N) 3 an d (Bu-t)(2)Al(OCH2-8-C9H6N) 4, respectively. The isobutyl derivative, [(Bu-t)(2)Al(mu-O-8-C9H6N)](2) 5, was prepared in an analogous manner. The molecular structures of compounds 1, 3 and 5 have been determined by X-ray crystallography. Compounds 2-4 are monomeric with five (2 an d 3) and six (4) membered chelate heterocyclic rings. In contrast, com pound 1 and 5 exist as bridged dimers. The isobutyl groups on each of the aluminium centers in compound 5 are in an anti conformation both i n the solid state and solution. The activation energy for the intercon version of the two anti enantiomers has been determined [Delta G doubl e dagger = 68.5(4) kJ mol(-1)]. The formation of monomeric chelate com pounds for compounds 2-4, rather than alkoxide bridged dimers as is fo und for (Bu-t)(2)Al(OCH2-2-C5H4N) and 5 is found to be due to a combin ation of the steric bulk of the aluminium alkyl and the rigid conforma tion of the pyridine and quinoline ligands.