Intramolecular coordination in compounds of aluminum

To develop an understanding of the steric and electronic factors that control the strength of the Lewis acid-base interaction of neutral donor atoms with aluminum in organoaluminum compounds, [R2Al{ m -O(CH2)nER'x}] 2 (n = 2, 3; ER'x = OR' , SR', NR'2) have been prepared from AlR3 and the appropriate substituted alcohol, thiol, or amine (R = tBu, iBu, Et, and Me). The equilibrium that exists between the four-coordinate isomer, [R2Al{ m -O(CH2)nER'x}] 2, and the five-coordinate isomer, [R2Al{ m -O(CH2)nER'x}] 2, where Keq = [4-coord.]/[5-coord.], has been probed by 13C and 27Al NMR spectroscopy using the alkoxide derivatives, [R2Al( m -OnBu)]2 R = tBu, iBu, and Et as comparisons. The solution equilibria have been determined using the 13C NMR shift of the OCH2 carbon as a measure of the Al-O-Al angle and hence coordination environment at aluminum. The measured bond dissociation energies for the intramolecular coordination were found to be surprisingly weak. Factors that control the extent of this equilibrium and hence the strength of intramolecular coordination include: the steric bulk of the substituents at aluminum and the Lewis base donor, the basicity of the neutral donor group and the chelate ring size. In addition, the sterically demanding aryloxide, 2,6-di-tert-butyl-4-methylphenol, was used as the substituent on the aluminum to investigate the effect of increased steric bulk on the degree of oligomerization. To develop an understanding of the geometric factors influencing the extent of oligomerization and coordination number at the aluminum center [(tBu)2Al( m -O-2-C5H4N)]2, (tBu) 2Al(OCH2-2-C5H4N), (tBu) 2Al(O-8-C9H6N), and (tBu) 2Al(OCH2-8-C9H6N) were synthesized. It was found that the formation of monomeric structures for compounds is associated with the rigid conformation of the pyridine and quinoline ligands and not upon the basicity of the chelating ligand. Consequently, the stability of monomeric versus dimeric structures in this system depends not only on basicity and steric factors, but also on the ligand conformation