Modification of Metal Alkoxide Precursors by Organofunctional Bidentate Ligands: Chemical Problems and Opportunities for
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Modification of Metal Alkoxide Precursors by Organofunctional Bidentate Ligands: Chemical Problems and Opportunities for Materials Syntheses Ulrich Schubert, Ulrike Bauer, Helmut Fric, Michael Puchberger, Wolfgang Rupp and Viktoria Torma Institute of Materials Chemistry, Vienna University of Technology Getreidemarkt 9/165, A-1060 Wien, Austria ABSTRACT Bidentate ligands are potentially useful to modify metal alkoxides precursors for sol-gel processing. However, anionic bidentate ligands are needed to get a strong coordination, while diamines as neutral bidentate ligand form coordination polymers [Ti2(OR)8(diamine)2]∞ when reacted with Ti(OR)4. The Lewis acidity of metal alkoxides may cause organic side reactions. The first is the formation of ester and water in the reaction with carboxylic acids, and therefore carboxylate-substituted metal oxide clusters of the general composition MnOx(OH/OR)y(OOCR)z are obtained instead of substituted monomeric precursors. The second side reaction discussed is the cleavage of ß-diketones by metal alkoxides. Despite this side reaction, 3-acetyl-6-trimethoxysilyl-hexane-2-one is a useful compound to prepare single-source precursos for silica/metal oxide composites.
INTRODUCTION Anionic bidentate (bridging or chelating) organic ligands, such as carboxylate, ß-diketonate, ß-alkoxy alkoxide or related ligands, are routinely used to modify the reactivity of metal alkoxides [1]. They can also be employed to modify metal alkoxides with functional organic groups [2], such as unsaturated groups that allow subsequent polymerization reactions. The modified precursors are prepared by reaction of the corresponding protonated compounds (carboxylic acids, ß-diketones, ß-alkoxy alcohols, etc) with metal alkoxides. This results in the substitution of one or more alkoxo ligands. Bidentate ligands are more strongly bonded than monodentate ligands because of the chelate effect. Although the modified metal alkoxides are invaluable to control the reactivity of metal-based sol-gel processes and to introduce functionalities, the Lewis acidity of metal alkoxides may cause chemical problems which have to be dealt with. In this contribution we mainly address two issues, for titanium alkoxides as an example, (i) the formation of coordination polymers instead of molecular compounds, and (ii) Lewis base-catalyzed side reactions of the organic entities.
RESULTS AND DISCUSSION Chelating coordination vs. formation of coordination polymers Metal alkoxides have a high tendency to increase their coordination numbers. In the absence of an external Lewis base the coordination number is increased by aggregation to larger units via
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alkoxo bridges. When the metal alkoxides are dissolved in polar solvents or in the presence of other Lewis bases, coordination of the Lewis base competes with association and reduces the degree of association. Several adducts of the type M2(OR)8L2 (M = Ti, Zr, Hf) were isolated, where each metal atom is six-coordinate owing to both the formation of alkoxide bridges and the
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