Phosphates, four-membered monomer

After providing a brief description of zeolitic structures, we discuss the hierarchy of structures of open-framework metal phosphates ranging from zerodimensional monomeric units and one-dimensional linear chains to complex three-dimensional structures. Aspects related to the likely pathways involved in the assemblage of these fascinating structures are examined, pointing out how the formation of the complex three-dimensional structures of open-framework metal phosphates involves the transformation and assembly of smaller units. Besides the role of the four-membered monomer, the amine phosphate route to the formation of the three-dimensional structures is discussed. The last step in the formation of these structures from preformed units of the desired structure is likely to be spontaneous. Our recent studies of open-framework metal oxalates have shown the presence of a hierarchy of structures. Reactions of amine oxalates with metal ions yield members of the oxalate family with differing complexity. 

Clearly, the monomer containing the four-membered ring emerges as the primary building block of open-framework metal phosphates. The formation of the layer structure may require free Zn2+ ions in solution, and it is possible that they are produced during the reaction by the dissociation of the monomer (1). 

It is to be noted that, in spite of the advances made, there is no control over the structure of the products obtained, because of the comparable energies of the various structures. The amine phosphate route, however, gives us the hope that rational synthesis of the open-framework metal phosphates may become possible in the not too distant future. If the amine phosphate plays a crucial role in the formation of open-framework metal phosphates, it should be possible to obtain a metal phosphate with a structure even simpler than the linear chain, such as the monomeric four-membered ring metal phosphate. It is noteworthy that the zinc phosphate monomer described earlier, comprising just a four-membered ring, has been obtained by the reaction of an amine phosphate with Zn2+ ions under mild conditions. 

What is significant is that zinc oxalates with one-, two- and three-dimensional architectures, in addition to the monomeric and dimeric oxalates, could be synthesized by using amine oxalates. In Fig. 7.34, the various types of structure obtained are presented to demonstrate the similarities and relationships. As can be seen, the structure of the dimer can be derived from that of the monomer, the chain from the dimer, and the layer from the chain. Just as the four-membered ring monomeric phosphate unit plays a crucial role in the building of framework phosphates [18, 38], it is possible that the monomeric and dimeric oxalates are involved in the construction of the extended oxalate framework structures. 

Among the inorganic open-framework compounds, the family of phosphates is a large one [3]. A large variety of open-framework metal phosphates of different architectures have been synthesized in the last few years. They include one-dimensional (ID) linear chain and ladder structures, two-dimensional (2D) layer structures and three-dimensional (3D) channel structures [4]. In the linear chain and ladder structures, four-membered metal phosphate units of the type M2P2O4 share comers and edges respectively. Zero-dimensional four-membered zinc phosphates have been synthesised and characterized recently [5]. Several open-framework metal carboxylates have also been reported [6] and the presence of a hierarchy of zinc oxalates covering the monomer, dimer, chain, honeycomb-layer and 3D structures has indeed been established [7]. 

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