Formation of complex three-dimensional inorganic open-framework structures from secondary building units

Investigations of open-framework zinc phosphates have demonstrated that the formation of the complex 3D architectures may involve a process wherein one-dimensional ladders or chains, and possibly zero-dimensional monomerscomprising four-membered rings transform to the higher dimensional structures. The one-dimensional ladder and the 4-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 three-dimensional structure. Accordingly, many of the higher dimensional structures retain structural features of the OD or 1D structure. It is significant that a four-membered ring zinc phosphate has been found recently to spontaneously yield 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 zero- to three-dimensions is found in open-framework metal oxalates as well. Interestingly, the three-dimensional sodalite structure is generated readily by the assembly of metal squarates, possessing the 4-membered ring motif. It is noteworthy that open-framework structures are also formed by oxyanions such as sulphate and selenite.