Aluminium compounds containing bidentate ligands: chelate ring size and rigid conformation effects

Reaction of Al(tBu)3 with 2-hydroxypyridine, 2-pyridinemethanol, 8-hydroxyquinoline and 8-quinolinemethanol, yielded [(tBu)2Al(µ-O-2-C5H4N)]2 1, (tBu)2Al(OCH2-2-C5H4N) 2, (tBu)2Al(O-8-C9H6N) 3 and (tBu)2Al(OCH2-8-C9H6N) 4, respectively. The isobutyl derivative, [(iBu)2Al(µ-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 and 3) and six (4) membered chelate heterocyclic rings. In contrast, compound 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 in the solid state and solution. The activation energy for the interconversion of the two anti enantiomers has been determined [ΔG ‡ = 68.5(4) kJ mol–1]. The formation of monomeric chelate compounds for compounds 2–4, rather than alkoxide bridged dimers as is found for (iBu)2Al(OCH2-2-C5H4N) and 5 is found to be due to a combination of the steric bulk of the aluminium alkyl and the rigid conformation of the pyridine and quinoline ligands.