On the hydrolytic stability of organic ligands in Al-, Ti- and Zr-alkoxide complexes

Abstract

The complexation degrees of Al-, Ti- and Zr-butoxide (M) with unsaturated and saturated β-diketones (3-allylpentane-2.4-dione-APD, acetylacetone-ACAC) and β-ketoesters (methacryloxyethyl-acetoacetate-MEAA, allylacetoacetate-AAA, ethylacetoacetate-EAA) as organic ligands (L) were examined by IR and 13C NMR spectroscopy and were found to be L : M ≥ 1.5. The hydrolytic stability of the ligands of the metal alkoxide complexes (L : M = 1) during hydrolysis/condensation reactions at the molar ratio h (H2O : OR) = 0.5-2.0 decreases with increasing H2O : complex ratio. Furthermore, the ligand stability depends on the type of metal in the complexes and decreases in the order Al- > Zr- > Ti-butoxide complexes at h = 1. The ACAC ligand likewise shows in the Al-, Ti-and Zr-butoxide complexes a high hydrolytic stability (95-100%) at h = 1 within 7 days. The Ti- and Zr-butoxide complexes with β-ketoesters as ligand show at h = 1 a release to a different extent e.g., up to 60% in the case of the MEAA-ligand in the Ti-butoxide complex after 2 days. In general, the hydrolytic stability of the ligands in the Ti-butoxide complexes (L : M = 1, h = 1) decreases in the order ACAC > APD > AAA > EAA ≥ MEAA. The hydrolysis/condensation reaction of complexes having a weak ligand stability leads to larger particle sizes in the sols than those with stable ACAC ligands. The results contribute to a more controlled synthesis of sols and of new inorganic-organic hybrid polymers via the sol-gel process.