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

摘要

The reaction of 2-phenoxyethanol with AlX_3 affords a five-coordinated dimeric product [(#mu#-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 [(#mu#-O(CH_2)_2OPh)Al(Bu~i)_2]_2 (3). A mixture, [(#mu#-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 [(#mu#-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 approx 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 ~(13)C-NMR spectroscopic studies.