Chapter 4. Metal Carboxyphosphonates

Abstract

Carboxyphosphonic or phosphonocarboxylic acids are heterofunctional organic ligands (with –PO3H2 and –CO2H groups, and they may also contain amino- and hydroxy- groups), which result in the formation of new interesting materials. These hybrid compounds can be considered as an intermediate step between carboxylate-MOFs and phosphonate-MOFs. In this chapter, we compiled the most used carboxyphosphonate ligands and we propose an acronym description that, if incorporated in the bibliography, it will allow better data mining within this fast growing field. The number of carboxyphosphonate hybrid compounds is exploding because the combination of many possible ligands and cations, plus the use of different organic templates, inorganic mineralizers and even auxiliary ligands. This revision will include the synthesis methodologies, some key crystal structures, and the main chemical and physical properties. The great majority of metal carboxyphosphonates have been prepared by hydrothermal processes, with or without template species. However, new methods such as solvothermal, ionothermal and microwave-assisted syntheses are being increasingly employed (in addition to the coupling with high-throughput methodology, whenever possible). Concerning the molecular and crystal structures, special attention is paid to the coordination modes of phosphonate and carboxylate groups within the final networks. Although these compounds usually crystallize in 2D layered or pillared structures, numerous 1D and 3D open-framework (sometimes microporous) solids are being reported. Some hybrids display helical chains/channels and others incorporate chiral phosphonate groups. Finally, examples of chemical and physical properties are discussed, including microporosity and gas adsorption, catalysis and proton conductivity, luminescent and magnetic properties, and corrosion inhibiting behaviour.