The crystalline oxozirconium methacrylate clusters Zre(O-Zre(OHj404(0Mc)12 form an octahedron, the cluster having Hj404(OMc)12 and Zr402(OMc)12 were obtained by reaction crystallographic C,, symmetry. Each metal atom is squareof Zr(OnPr), with an excess of methacrylic acid and analysed antiprismatically coordinated by eight oxygen atoms. In by X-ray diffraction. The oxide and hydroxide groups are in Zr402(OMc)12, the zirconium atoms have a distorted butterfly a p..-bridging mode in both structures, and the methacrylate arrangement; their coordination geometry is square-antiprisligands are chelating or bridging. The zirconium atoms in matic or capped octahedral.The high reactivity of metal alkoxides, E(OR),,, towards water often necessitates the use of chemical additives (mostly bidentate ligands L, such as acetate or acetylacetonate) to moderate their reactivity in sol-gel processcs. By substitution of some of the alkoxide groups, a new molecular precursor, E(OR),-,L,, is formed which exhibits a different reactivity. Upon addition of water, the E-OR groups are primarily hydrolyzed"]. The bidentate ligands inevitably lower the degree of cross-linking in the thus obtained oxide materials E0~,,-,)/2L,, because they block condensation sites. Deliberate adjustment of the E(OR),/L ratio therefore also allows control of the size of thc primary building blocks (i.e,, the primarily obtained 0x0 clusters) and thus the architecture of the final materialsr21.The potential for synthesizing novel materials originating from the presence of organic groups in the modifying ligands L was hardly realized until recently. Organofunctionul ligands L not only block condensation sites like their nonfunctional counterparts, but additionally introduce chemical functions. They allow extensive chemical modifications of the whole system and the development of materials with new properties L 3 4 .The most obvious choices for L are functionalized carboxylates, because they are readily available. Reaction of carboxylic acids with metal alkoxides provides derivatives E(OR),-JOOC-X-A], where the functional group A is linked via the carboxylate group and some spacer X to the metal alkoxide moiety.In thc reaction of Ti(ORj4 with carboxylic acids, the maximum degree of substitution achieved to date is 1.33. An excess of carboxylic acid results in partial hydrolysis, due to the in situ production of water by the esterification of the carboxylic acid with the cleaved alcohol. The structural information from these clusters allows conclusions to be made on how the building blocks in the (amorphous!) gels, obtained after complete hydrolysis of the organically modified alkoxide precursors, are composed and interconnected.In contrast to the reactions of Ti(ORk with carboxylic acids, no corresponding crystalline derivative has hitherto been obtained from Zr(OR)+ A different structural chemistry was expected, because of the preference of Zr for coordination numbers higher than 6. Furthermore, earlier experiments had already shown that more than 1.33 OR group...

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Reaction of titanium butoxide and zirconium butoxide with methacrylic acid in different molar ratios results in the formation of the mixed-metal clusters Ti 4 Zr 4 O 6 (OBu) 4 (OMc) 16 , Ti 2 Zr 4 O 4 (OBu) 2 (OMc) 14 , Ti 4 Zr 2 O 4 (OBu) 6 (OMc) 10 , and Ti 2 Zr 6 O 6 (OMc) 20 (OMc = methacrylate). The molecular structures of these clusters have been determined by single-crys- [a]

Hybrid materials represent one of the most growing new material classes at the edge of technological innovations. Unique possibilities to create novel material properties by synergetic combination of inorganic and organic components on the molecular scale makes this materials class interesting for application-oriented research of chemists, physicists, and materials scientists. The modular approach for combination of properties by the selection of the best suited components opens new options for the generation of materials that are able to solve many technological problems. This review will show in selected examples how science and technological driven approaches can help to design better materials for future applications.

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