# Advances and Challenges in the Creation of Porous Metal Phosphonates
## 2 Synthesis of Porous Metal Phosphonates
### MOFs-Phosphonates  
Metal-organic frameworks are one of the prominent classes of coordination polymeric networks that are built-up from metal nodes and organo-functional units resulting in porous structures with very high surface areas. Here, we focus on a specific type of MOFs that consists of metallic units with phosphonate linkers. Frameworks based on metal phosphonates are rather an emerging class of MOFs when compared to the well established carboxylates, whereas the surge in carboxylate MOFs has been on a continuous rise ever since their inception in the late 1990s. The successful synthesis of frameworks based on metal phosphonates (albeit small in number) has only been possible in this decade. This can largely be attributed to the stronger coordination of the phosphonate moiety towards metals than the carboxylate counterparts [11]. Therefore, hindering the orderly growth of a porous metal phosphate framework. We begin this section by discussing some of the phosphonate frameworks, similar to layered metal phosphonates: divalent/trivalent metal phosphonate frameworks that have led to the development of tetravalent metal phosphonate frameworks. One can expect that similar to layered structures, the problem of solubility is very apparent and impedes creating porous architectures. One finds in the literature that MOFs that have been deemed as the 'ultra-stable' systems are mostly composed of Ti (IV) and Zr (IV) nodes [24]. Thus, it is crucial in studying how these frameworks could be obtained with phosphonic linkers. In this section, we build on the knowledge of layered metal phosphonates and describe how their lamellar motif can be altered, going towards open porous architectures. A 3D framework synthesized from zinc and 1,4-dihydroxy-2,5-benzenediphosphonic acid in DMF resulted in the formation of 1D chains of zinc tetrahedra and RPO\({}_{3}\)\({}^{2-}\) with a surface area of 209 m\({}^{2}\)/g (microporous with 1 nm \(\times\) 1 nm pores), once DMF was removed from the framework. The key component of the phosphonic linker, the dihydroxy group, prevented the formation of a dense layered motif and thus resulted in an open framework [42]. The reaction of ethylenediaminetetrakis  
Figure 2: Schematic drawing of how micropores are formed in the metal biphenylene bis(phosphonates). The horizontal rectangles represent the inorganic layers (M = Zr or Sn) which are cross-linked by the organic moieties. R stands for F\({}^{-}\), if HF is used in the preparation, and the surfaces are capped by M because an excess of metal is used in the syntheses. Reprinted with permission from [39]. Copyright (2009), Royal Society of Chemistry. (methylenphosphonic acid) with lead (II) and zinc (II) also resulted in two different types of 3D open frameworks that were microporous. This phosphonic ligand containing eight phosphonic acid sites provided myriad coordination modes that were dependent on the choice of metal and the pH of the reaction medium. The framework based on lead was made up of helical chains of square-pyramidal lead and RPO\({}_{3}\) phosphonate units. However, in the case of zinc, the amino group was not involved in binding with the metal and resulted in a square-wave type arrangement of zinc and the phosphonate layer. Both these frameworks possessed open channels in the micropore region (0.68 nm \(\times\) 1 nm) [43]. Based on the reaction conditions and the choice of metal and phosphonic precursors, a wide variation in coordination modes between the metal units to the phosphonic acid units can be obtained (see for example Figures 3 and 4 versus Figure 5). When additives are added, novel coordination modes can be formed. For example, a framework with 1D pores was synthesized from benzene diphosphonic acid and copper (II) in the presence of amino triazole as an additive. This framework features a pillared structure with apparent pores of \(\sim\)0.45 nm. It could only be formed due to the amino group of the triazole coordinating the copper atoms and producing 1D columns of 6-coordinated copper.