Electronic and steric factors affecting the formation of four or five-coordinated aluminum complexes: syntheses and crystal structures of some aluminum alkoxides

The reaction of 2-phenoxyethanol with AlX3 affords a five-coordinated dimer ic product [(mu-OCH2CH2OPh)AlX2](2) (1, X = Cl; 2, X = Br) in high yields. However, the reaction of 2-phenoxyethanol with Al(Bu-i)(3), yields a four-c oordinated dimeric product [(mu-O(CH2)(2)OPh)Al(Bu-i)(2)](2), (3). A mixtur e, [(mu-O(CH2)(2)OPh)Al(Et)(0.75)(Cl)(1.25)](2) (4) is Obtained from the re action of 2-phenoxyethanol with one equivalent of Et2AlCl. The reaction of 2-methoxybenzyl alcohol with AlR3 gives a semi-pentacoordinated dimeric pro duct [(mu-OCH2C6H4OMe)AlR2](2) (5, R = Me; 6, X = Bu-i) in high yields. Cry stal structure studies of these compounds reveal that the distance between Al and the phenoxy oxygen or the methoxy oxygen is 3 > 6 > 5 much greater t han 4 > 1 similar to 2. Although both the electronic and steric effect infl uence the nature of the O,O'-bifunctional ligand chelating on the aluminum center to generate a five- or four-coordinated aluminum, the electronic eff ect is considered to play a major role. The interaction of aluminum and eth ereal oxygen can be monitored by C-13-NMR spectroscopic studies. (C) 1999 E lsevier Science S.A. All rights reserved.