Oxalates, hierarchy

In our studies of the open-framework zinc oxalates, we have recently isolated monomeric, dimeric, ID linear chain, 2D layer, and 3D structures by the reaction of amine oxalates with Zn2+ ions,21 suggesting thereby that the presence of a hierarchy of structures is not unique to the phosphates alone. We believe that the evidence provided by our studies for the existence of an Aufbau principle of open-framework complex structures is of considerable significance. Many other complex inorganic structures are also likely to be formed by similar building-up processes, involving basic building units and self-assembly. 

Vaidhyanathan, R. Natarajan, S. Rao, C. N. R. Synthesis of a hierarchy of open-framework zinc oxalates from amine oxalates, communicated. 

The hierarchy of open-framework structures in metal phosphates and oxalates... 

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. 

Figure 7.34. The hierarchy of zinc oxalate structures (a) monomer, (b) dimer, (c) onedimensional chain, (d) two-dimensional layer and (e) three-dimensional structure. Note the close relationships between them (Vaidhyanathan et al. [49]).

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]. 

Several open-framework metal oxalates have been synthesized and characterized in last two to three years. They include ID, 2D and 3D structures. Following the success of the amine phosphate route to open-framework metal phosphates, reactions of organic amine oxalates with metal ions have been carried out. These reactions have yielded a hierarchy of zinc oxalates as shown in Figure 7. These different structures are mutually related. 

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