Chapter 14: Structural Diversity in Metal Phosphonate Frameworks: Impact on Applications Check Access

A great number of metal phosphonate materials have been synthesized and structurally characterized. The family of phosphonic acids (and their deprotonated forms) includes monophosphonates, bis-phosphonates, tris-, and tetrakis-phosphonates, with higher analogs as well. The structural literature reveals that although structurally characterized metal phosphonates exist with phosphonate ligands possessing up to four phosphonate moieties, no such materials exist with pentakis-phosphonates or higher analogs. Apart from the importance metal phosphonates posses from a structural point of view, relatively few have been used in (potential) applications. This chapter attempts to explore the structural features of metal phosphonate chemistry and their impact on certain applications, such as ion exchange, intercalation, gas sorption, and metal ion absorption. The structural and chemical features of the phosphonate moiety (R-PO₃H_x^−(2–x) (x = 0, 1, 2) are discussed in the framework of pH-dependent deprotonation behavior, bridging and chelation capability, charge delocalization, features of the metal ion (charge, ionic radius), etc. These salient features are then reflected on reported metal phosphonate crystal structures. Furthermore, a number of functional metal phosphonate materials are discussed and an attempt is made to relate their activity with their specific structural features. Finally, structural correlations between the phosphonate and carboxylate groups are made, with emphasis on how their differences (and similarities) may affect the crystal structures of end inorganic–organic hybrid products.