Open-Framework Metal Phosphates

B) Choudhury. A, Rao, C, N, R, Understanding the Building-Up Process of Three-Dimensional Open-Framework Metal Phosphates Acid Degradation of the 3D Structures to Lower Dimensional Structures, Chem, Commun. 2003, 366-367. 

Inorganic open-framework compounds constitute an important class of materials that has attracted much attention as evidenced by the vast number of research papers published in the last few years. Although work in this area started with aluminium silicates1,2 because of their important uses in sorption and catalysis, much of the recent work pertains to the structure and characterization of open-framework metal phosphates and carboxylates. Both these families of open-framework materials have been reviewed recently.3 7 Since open-framework silicates and phosphates can essentially be... 

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. 

From the above discussions it is clear that the amine has an important role in the formation of open-framework metal phosphates. Recent studies from this laboratory have shown that in the family of zinc phosphates, almost all the members in the hierarchy of open-framework structures can be synthesized using the same structure-directing agent, triethylenetetramine (TETA) [32], In Fig. 7.18 we show all the structures obtained with TETA along with the relative concentrations of the amine and the phosphoric acid employed in the synthesis. We see that the one-dimensional (ladder) and the two-dimensional (layer) structures are obtained when the relative concentration of phosphoric acid is high with the ratio of concentrations of the acid and the amine in the range 3 1-4 1. This is in contrast to the three-dimensional structures where the ratio... 

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. 

Figure 7.24. (a) The structure of an amine phosphate, (b) The structure of the final open-framework metal phosphate. Note that the structure of the metal phosphate can be generated simply by replacing the water molecules by the metal atoms. 

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]. 

Figure 1. Open-framework metal phosphates of different dimensionalities 1, ID linear chain structure with piperazine. [C4N2H, ][Zn(HP04)J-H20 2, ID ladder structure, [C6N4H22](i.5[Zn(HP04)2] with...

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. 

Among open-framework metal phosphates, zincophosphates exhibit diverse compositions and fascinating structural topologies.[212] Notable examples are zincophosphates with extra-large pores bound by 16-, 20-, and 24-rings, and helical channels. 

More recently, open-framework metal phosphates (metal = Ti, Mo, V, Fe, Co, etc.) with redox features have been continually reported. Cheetham summarized them in 1999.[11]... 

C.N.R. Rao, N. Srinivasan, and S. Neeraj, Building Open-framework Metal Phosphates from Amine Phosphates and Monomenic Four-membered Ring Phosphate. J. Solid State Chem., 2000, 152, 302-321. 

H-bonding interactions exert dominant roles in the chirality transfer of guest chiral SDAs to host open-framework metal phosphates based on their systematic work by using chiral cationic cobaltammine complexes as the SDAs.[901... 

In 1995, Morgan et al. synthesized a layered aluminophosphate compound by using a chiral cobaltammine complex as the template for the first time.[61] Recently, the Jilin group has synthesized a number of 2-D layered and 3-D open-framework metal phosphates by using a racemic mixture or an optically pure chiral metal complex as the template, and has systematically studied the chirality transfer from the guest chiral complex templates to the host inorganic open frameworks.1901 Table 7.15 lists some metal phosphates and oxides with open-framework structures templated by optically pure or racemic cobalt ammine complexes. 

Construction of Open-framework Metal Phosphates through Building-block Built-up Approach... 

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