Reaction of Al(tBu)₃ with Ethylene Glycol:  Intermediates to Aluminum Alkoxide (Alucone) Preceramic Polymers

The reaction of Al( t Bu) 3 with ethylene glycol (HOCH 2 CH 2 OH) in hexane yields “alucone” polymers, [Al( t Bu) 2 x (OCH 2 CH 2 O) 1.5 - x ] n (0.3 ≤ 2 x ≤ 0.8), with the concurrent formation of [Al 2 ( t Bu) 3 (OCH 2 CH 2 O)(OCH 2 CH 2 OH)] ( 1 ). If the reaction is carried out in Et 2 O solution, then [Al 3 ( t Bu) 5 (OCH 2 CH 2 O) 2 ] ( 2 ) may be isolated. The molecular structure of compound 1 consists of a dimer formed by the alkoxide termini of two ligands bridging the Al( t Bu) and Al( t Bu) 2 units. The Al( t Bu) moiety is chelated by the nonbridging oxygens of the glycolate ligands, one of which remains protonated. Compound 2 consists of two Al( t Bu) 2 units and two glycolate ligands forming a cryptand-like 10-membered heterocycle; the Al( t Bu) unit is positioned capping the four oxygen atoms. As with the previously reported ethoxy-substituted alucone polymers, [Al(OR) 2 x (OCH 2 CH 2 O) 1.5 - x ] n, thermolysis of [Al( t Bu) 2 x (OCH 2 CH 2 O) 1.5 - x ] n, results in the formation of η-alumina (η-Al 2 O 3 ) as determined by XRD. This is in contrast to the formation of γ-Al 2 O 3 from the thermolysis of alumoxanes, [Al(O)(OR) x (OH) 1 - x ] n, and may be rationalized by the structural relationship of compounds 1 and 2, and hence the alucones, to Bayerite rather than the boehmite core observed for alumoxanes.