3-FOLD OR 4-FOLD COORDINATION OF ALUMINUM IN ALKYLALUMINUM PHENOXIDES AND ITS DIFFERENTIATION BY MEANS OF AL-27-NMR SPECTROSCOPY

The Al-27 NMR spectra for a series of aryloxyaluminium compounds [R(n)Al(OAr)3-n]m (R = methyl, isobutyl; n = 0-2; m = 1, 2, 3) with various alkyl substituents in the 2 and 6 positions of the phenoxy rings have been measured. The delta-(Al-27) resonances give an indication of the number of aryloxy ligands and the coordination number of each Al atom. Monomeric R2AlOAr compounds have delta-(Al-27) resonances around 190 ppm whereas dimeric analogues give signals at 167 ppm. The Al-27 NMR resonances for monomeric RAl(OAr)2 are found at ca. 100 ppm while those for [Al(OAr)3]2, where the aluminium atoms are solely bonded to aryloxyl groups, lie at ca. 50 ppm. Characteristic shifts of the resonances were also observed for complexes of these compounds with THF, whereby it was found that the resulting change DELTA-delta-(Al-27) decreases with increasing number of aryloxy ligands. The crystal structures of three selected compounds, 3, 8 and 11, were determined by X-ray crystallography.