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

Abstract

The reaction of 2-phenoxyethanol with AlX 3 affords a five-coordinated dimeric product [( μ-OCH 2CH 2OPh)AlX 2] 2 ( 1, X=Cl; 2, X=Br) in high yields. However, the reaction of 2-phenoxyethanol with Al(Bu i ) 3 yields a four-coordinated dimeric product [( μ-O(CH 2) 2OPh)Al(Bu i ) 2] 2 ( 3). A mixture, [( μ-O(CH 2) 2OPh)Al(Et) 0.75(Cl) 1.25] 2 ( 4) is obtained from the reaction of 2-phenoxyethanol with one equivalent of Et 2AlCl. The reaction of 2-methoxybenzyl alcohol with AlR 3 gives a semi-pentacoordinated dimeric product [( μ-OCH 2C 6H 4OMe)AlR 2] 2 ( 5, R=Me; 6, X=Bu i ) in high yields. Crystal structure studies of these compounds reveal that the distance between Al and the phenoxy oxygen or the methoxy oxygen is 3> 6> 5⪢ 4> 1∼ 2. Although both the electronic and steric effect influence the nature of the O,O′-bifunctional ligand chelating on the aluminum center to generate a five- or four-coordinated aluminum, the electronic effect is considered to play a major role. The interaction of aluminum and ethereal oxygen can be monitored by 13C-NMR spectroscopic studies.