Investigation of the sol-gel chemistry of ethylacetoacetate modified aluminum sec-butoxide

Hydrolysis of aluminum sec-butoxide leads usually to precipitation; however, modification of the Al center with one ethylacetoacetate gives a new precursor, Al(OBu^s)₂(etac). Hydrolysis of Al(OBu^s)₂(etac) leads to transparent, homogeneous gels rather than precipitates and thus appears as an interesting precursor for the sol-gel synthesis of alumina-containing ceramics. The investigation of the sol-gel chemistry of Al(OBu^s)₂(etac) by Nuclear Magnetic Resonance and infrared techniques provides a detailed understanding of the effects of the ethylacetoacetate group on the chemistry at the Al center. ²⁷Al NMR shows that in solution Al(OBu^s)₂(etac) exists as oligomeric species that contain hexa-, penta- and tetra-coordinated Al. When dissolved in ethanol, Al(OBu^s)₂(etac) undergoes exchange reactions with solvent as shown by ¹³C NMR, which strongly influence the nature of the species in equilibrium, favoring the formation of pentacoordinated Al sites. The effects of changes in reaction conditions on the species formed on hydrolysis were followed by ²⁷Al, ¹³C NMR and infrared spectroscopies. These techniques indicate that the etac groups are much less susceptible to hydrolysis than the butoxy groups. Some of the etac groups survive to hydrolysis procedure, thus preventing complete condensation of the oxide network.