Chemical design of materials: A case study of inorganic open-framework materials

Inorganic open-framework materials are discussed as a case study in the chemical design of materials. Investigations of open-framework zinc phosphates have demonstrated that the formation of the complex 3D architectures may involve a process wherein 1D ladders or chains, and possibly 0D monomers comprising four-membered rings, transform to the higher-dimensional structures. The 1D ladder and the four-membered rings appear to be important building units of these structures. At one stage of the building-up process, spontaneous self-assembly of a low-dimensional structure such as the ladder could occur, followed by the crystallization of a 3D structure. Accordingly, many of the higher-dimensional structures retain structural features of the 0D or 1D structure. It is significant that a four-membered ring zinc phosphate spontaneously yields a linear chain phosphate at room temperature, on addition of piperazine, the chain transforming to a sodalite-type 3D structure under mild conditions. The occurrence of a hierarchy of structures from 0-3 dimensions is found in open-framework metal oxalates as well. Interestingly, the 3D sodalite structure is generated readily by the assembly of metal squarates, possessing the four-membered ring motif. It is noteworthy that open-framework structures are also formed by oxyanions such as sulfate, selenite, and selenate. Transformations of molecules to complex architectures are a worthy area of study, defining a new direction in the chemistry of materials.