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 dimeric product [(μ-OCH₂CH₂OPh)AlX₂]₂ (1, X=Cl; 2, X=Br) in high yields. However, the reaction of 2-phenoxyethanol with Al(Buⁱ)₃ yields a four-coordinated dimeric product [(μ-O(CH₂)₂OPh)Al(Buⁱ)₂] ₂ (3). A mixture, [(μ-O(CH₂)₂OPh)Al(Et)_0.75(Cl) _1.25]₂ (4) is obtained from the reaction of 2-phenoxyethanol with one equivalent of Et₂AlCl. The reaction of 2-methoxybenzyl alcohol with AlR₃ gives a semi-pentacoordinated dimeric product [(μ-OCH₂C₆H₄OMe)AlR₂]₂ (5, R=Me; 6, X=Buⁱ) 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.