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

Abstract

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.