Inorganic oxides possess a remarkable diversity of chemical composition and architecture, giving rise to a vast domain of solid state properties with applications ranging from heavy construction to molecular electronics. Despite such inherent fundamental and practical significance, their structural design, in the sense of controlled synthesis of metastable but persistent extended structures, remains a challenge in solid state chemistry. It is also noteworthy that, while some naturally occurring oxide minerals possess complex crystal structures, the majority are of simple composition and have highly symmetrical structures with rather small unit cells. Although such simple oxides can possess unique and specific properties, such as piezoelectricity, ferromagnetism or catalytic activity, as a general rule there is a correlation between the complexity of the structure of a material and its functionality. However, the occurrence of complex inorganic oxides in the geosphere and the biosphere suggests that Nature may provide useful guidelines for the preparation of synthetic phases and for the modification of oxide microstructures. Thus, a powerful approach to the design of novel oxide materials mimics Nature's use of organic molecules to modify inorganic architectures. In such materials, the inorganic oxide contributes to the increased functionality via assimilation as one component in a hierarchical structure where there is a synergistic interaction between organic material and the inorganic oxide. Synthetic studies of materials possessing such an interface, coupled with the acquisition of the appropriate structural information, should contribute to the development of an increased understanding of methods to control the structure-property relationships within these hybrid materials. Since the structural characteristics of the organic component will not be retained under the conventional high temperature methods of solid state synthesis, low temperature hydrothermal techniques have been adopted for the preparation of organic-inorganic hybrid materials with retention of the structural elements of the reactants in the final products. Consequently, the general synthetic approach employs an organic component at low temperature to modify or control the surface of growing oxide crystals in a hydrothermal medium.
This approach derives from the concepts of supramolecular self-assembly, as reflected in the exploitation of molecular building blocks in a "bottom up" design of the extended material. Consequently, the syntheses generally proceed from the self-assembly of multicomponent building blocks: a chelating or di- or multitopic organoamine ligand, a "secondary" metal cation, and the inorganic oxide sources. While organoamine constituents have been conventionally introduced as charge-balancing counterions in zeolite synthesis, in our application the organic component serves as a ligand to the secondary metal site. Consequently, a coordination complex cation is assembled which serves to provide charge-compensation, space-filling and structure-directing roles. The interplay of the coordination preferences of the secondary metal site and the geometric constrains imposed by the ligand provides considerable structural flexibility, as well as an effective subunit for the spatial transmission of structural information. One strategy adopts appropriate stoichiometric control to form mononuclear metal-ligand chelate complex cations which are coordinatively unsaturated and hence capable of bonding to the oxo-groups of the oxide substructure, so as to provide linkage between the oxide subunits. Alternatively, a ligand which adopts a bridgingmodality between secondary metal sites will produce a one-, two- or three-dimensional matrix for the modification and entrainment of the oxide structure. Examples of materials constructed following these general guidelines will be presented.