Aufbau Principle of Complex Open-Framework Structures of Metal Phosphates with Different Dimensionalities C. N. R. RAO,* ,† SRINIVASAN NATARAJAN, AMITAVA CHOUDHURY, † S. NEERAJ, AND A. A. AYI Chemistry and Physics of Materials Unit and CSIR Centre of Excellence in Chemistry, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bangalore, 560 064, India Received July 25, 2000 ABSTRACT Open-framework metal phosphates occur as one-dimensional (1D) chains or ladders, two-dimensional (2D) layers, and complex three- dimensional (3D) structures. Zero-dimensional monomers have also been isolated recently. These materials are traditionally prepared by hydrothermal means, in the presence of organic amines, but the reactions of amine phosphates with metal ions provide a facile route for the synthesis, and also throw some light on the mode of formation of these fascinating architectures. Careful studies of the transformations of monophasic zinc phosphates of well-characterized structures show that the 1D structures transform to 2D and 3D structures, while the 2D structures transform to 3D structures. The zero-dimensional monomers transform to 1D, 2D, and 3D structures. There is reason to believe that the 0D mono- mers, comprising four-membered rings, are the most basic struc- tural units of the open-framework phosphates and that after an optimal precursor state, such as the ladder structure, is formed, further building may occur spontaneously. Evidence for the oc- currence of self-assembly in the formation of complex structures is provided by the presence of the structural features of the one- dimensional starting material in the final products. These observa- tions constitute the beginning of our understanding of the building- up principle of such complex structures. 1. Introduction While supramolecular chemistry of organic compounds has developed to maturity in the past few years, 1 supra- molecular inorganic chemistry is somewhat at a nascent stage. Supramolecular design provides a means to gener- ate a variety of novel inorganic materials with complex, unusual structural features in areas such as host -guest chemistry, open-framework structures, and the like. In these compounds, one can visualize structures of different dimensionalities, the dimensionality varying from zero to three. Such complex structures would be expected to have subunits, which not only are structural motifs but also act as building blocks in the building up of the complex structures. Such building units can be considered to be synthons of complex structures. The synthons would be simple geometrical figures such as squares, cubes, or polyhedra, their corners acting as linkage points. The challenge that one faces in supramolecular inorganic materials chemistry is to establish whether such synthons exist in reality and, if so, whether one can demonstrate how they are involved in the formation of the larger structures. Mu ¨ller et al. 2 have made use of the concepts of supramolecular inorganic chemistry to provide a beau- tiful description of large assemblies based on the chem- istry of polyoxometalates. Recently, Fe ´rey 3 has described the concept of building units to understand inorganic solid-state structures, to visualize new topologies, and to conceive the formation of new solids with novel designs. On the basis of such building units, Fe ´rey also defines the notion of scale chemistry, which is concerned with the edification of the solids with building units and the consequences it has on the structure of the framework and the voids in them. Of particular interest is the fact that not only are the structures of open-framework compounds topologically interesting, but also the cavities present in them have potential applications. When one looks at the complexity as well as the beauty of the myriad of structures of both open-framework and host -guest compounds, one cannot escape the feeling that the building-up process cannot occur by conventional chemi- cal means alone, involving simple making and breaking of chemical bonds. The formation of such organized structures would be expected to involve self-assembly or some such spontaneous process at a certain stage. In this Account, we demonstrate how the formation of complex open-framework metal phosphates with open architec- tures involves a building-up process from simple building units, possibly leading to an ultimate step where sponta- neous self-assembly occurs. Several classes of inorganic open-framework structures have been synthesized and characterized in the past several years. While zeolitic aluminosilicates constitute the best-known class of open-framework structures, 4 metal phosphates have been gaining considerable importance, and a variety of metal phosphates with open architectures have been reported in the past decade. 5 The open- framework phosphates are generally synthesized under hydrothermal conditions in the presence of organic *To whom correspondence should be addressed. E-mail: cnrrao@ jncasr.ac.in. Fax: 91-80-8462766. † Also at the Solid State & Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India. C. N. R. Rao obtained his Ph.D. degree from Purdue University and a D.Sc. degree from the Mysore University. He is Linus Pauling Research Professor at the Centre and Honorary Professor at the Indian Institute of Science. He is a member of several academies, including the Royal Society, London, the U.S. National Academy of Sciences, and the Pontifical Academy of Sciences. Srinivasan Natarajan obtained his M.Sc. degree from the Madurai Kamaraj University and his Ph.D. from the Indian Institute of Technology, Madras. He did postdoctoral work at the Royal Institution London and at the University of California, Santa Barbara, and is a Faculty Fellow at the Centre. Amitava Choudhury obtained his M .Sc. degree from North Bengal University and is now a Ph.D. scholar at the Indian Institute of Science. S. Neeraj obtained his M.S. degree at the Centre and has just completed his Ph.D. work. A. A. Ayi has an M .Sc. degree from the University of Calabar, Nigeria, where he is a Lecturer. He is on a visiting fellowship of the Third World Academy of Sciences. Acc. Chem. Res. 2001, 34, 80-87 80 ACCOUNTS OF CHEM ICAL RESEARCH / VOL. 34, NO. 1, 2001 10.1021/ar000135+ CCC: $20.00  2001 American Chemical Society Published on Web 11/15/2000