Pre-organized Metal Alkoxide Precursors

Mixtures of (semi-)metal alkoxide precursors are commonly used for the preparation of mixed-oxides by sol-gel processing. This also includes organically modified precursors for the preparation of inorganic-organic hybrid materials. In the case of alkoxysilanes, the organic groups are directly bonded to the silicon atom, while bi- or tridentate ligands must be used to tether organic groups to metal alkoxide moieties. In the planned project, a first step towards a new generation of sol-gel inorganic-organic hybrid materials with new and interesting properties is planned by using new pre-organized sol-gel precursors containing two or more metal alkoxide moieties. To this end, the metal alkoxide moieties will be connected with each other by hydrolytically stable organic linkers. Molecular building blocks with different structural and chemical characteristics will thus be arranged in specific sequences. The pre-organized structures should be retained during sol-gel processing. The simplest case is precursors with two metal alkoxide moieties of the same kind connected by an organic linker. Network structures are formed upon sol-gel processing in which the organic groups become part of the network. Such network structures are known for alkoxysilane precursors, but corresponding metal alkoxide have not yet been synthesized. An even greater challenge is the synthesis of bimetallic precursors, where two different metal alkoxide moieties are connected. Sol-gel processing of the latter would allow to prepare bimetallic gels in which the two metals are deliberately positioned this respect to each other. Finally, new network structures will be obtained when chain- or sheet-like modules are subjected to sol-gel processing. The one- or two-dimensional pre- organization of metal alkoxide moieties will again be achieved by means of suitable bi- or trifunctional organic ligands.

Inorganic-organic hybrid materials had major impact on materials science in the past 15-20 years. In this class of materials, molecular or nanoscale inorganic and organic building blocks are combined with each other giving rise to new materials properties. Various types of such materials have been prepared by various preparative routes. Although the principles of the mutual arrangement of the building blocks are increasingly discerned, the deliberate arrangement of the latter according to a given blueprint is still a vision of the future. In the finished project, a first step towards a new generation of sol-gel inorganic-organic hybrid materials was done by using new pre-organized sol-gel precursors containing two or more metal alkoxide moieties of the same or different kind. To this end, molecular building blocks with different structural and chemical characteristics were connected with each other by organic linkers and thus arranged in a specific manner. The pre-organized structures were retained during sol-gel processing. The simplest case are precursors with two metal alkoxide moieties of the same kind connected by an organic linker. It was intended to create network structures upon sol-gel processing in which the organic groups become part of the network. Surprisingly, however, ring- or cage-like oligomers were observed in most cases. An even greater challenge was the synthesis of bimetallic precursors, where two different metal alkoxide moieties are connected. Sol-gel processing of such precursors allowed to prepare bimetallic gels in which the two metals are deliberately positioned with respect to each other. It was shown that this concept results in an extraordinarily homogeneous distribution of the metallic elements in the thus obtained bimetallic oxides, which can hardly be obtained by any other variation of sol-gel processing. Finally, new network structures were obtained by using chain- or sheet-like cyanometallate modules in sol-gel processing. The thus obtained novel materials combine the interesting magnetic properties of cyanometallates with that of sol-gel materials.