# Advances and Challenges in the Creation of Porous Metal Phosphonates
## 2 Synthesis of Porous Metal Phosphonates
### MOFs-Phosphonates
In another study, the use of flexible tritopic phosphonate linkers provided the platform to synthesize materials of 2D and 3D frameworks or even 0D dimers with various metals (Cu, Ni, and Mn). Benzene-1,3,5-triyl-tris(methylene)triphosphonic-acid (BttP) or 2,4,6-trimethyl-BttP were used as the flexible linkers with a rigid aromatic core. It was shown that flexible tritopic phosphonate linkers allow a tunable structure of the framework, based on the choice of metal, pH, and the synthesis conditions, to end up with either a 2D or 3D architecture. The flexibility in the triphosphonate moiety allowed the linkers to adopt various coordination environments in bonding to different metals, influencing the conformation of the triphosphonate moiety and the resulting structure of the metal phosphonate framework [55]. Finally, phosphonate MOFs made from metal (IV) precursors are summarized in the following paragraphs. Phosphonate-based MOFs based on metal (IV) have been extremely difficult to prepare since they usually form a dense, amorphous layered structure. MIL-22 is a titanium (IV)-based 3D layered framework, wherein hydrous titanium oxide was reacted hydrothermally with ethylene diphosphate acid. The inorganic-organic chain built up by Ti-O-Ti and Ti-O-P bonds, containing narrow channels that are filled with water in the interlayer region, provide minimal to no porosity. However, an important property was the excellent thermal stability of MIL-22 compared to other open-frameworks [56]. These MIL-22 structures were derived from the metal phosphonate frameworks synthesized with vanadium and alkyl diphosphonic acids [57; 58; 59]. However, by employing N,N\({}^{\prime}\)-piperazine bismethyl phosphonic acid, the frameworks MIL-91 (Ti (IV) and Al (III)) were synthesized. The MIL-91-Ti framework consists of MO\({}_{6}\) octahedra and bisphosphonate groups having a surface area of \(\sim\)500 m\({}^{2}\)/g, pore volume of 0.19 cm\({}^{3}\)/g and pores of size 0.35 nm \(\times\) 0.40 nm. This was the first successful open phosphonate framework made from titanium(IV) as the metal node, shown in Figure 4[60]. Figure 3: Frameworks of STA-12 (**A**) and STA-16 (**B**) viewed along the channel axis. Color code: cobalt: purple, phosphorus: orange, oxygen: red, nitrogen: blue, carbon: black, hydrogen: pink. Reprinted with permission from [51]. Copyright (2011), American Chemical Society. In another study using the same linker, piperazine-N,N\({}^{\prime}\)-bis(methylencephalophonic acid) and zirconium(IV), two 3D frameworks, were synthesized. The structure of these frameworks was found to be dependent on the pH of the reaction medium. At higher pH, a 3D framework was created by zirconium octahedra bonded to RPO\({}_{3}\), forming composite building units (CBU), linking the inorganic-organic chains through the piperazine units resulting in the formation of a zirconium phosphonate framework. At a lower pH, the formation of the classic pillared-layered structure was observed [61]. Alternatively, a robust honeycomb-layered framework of zirconium triphosphonate was synthesized with 1,3,5-tris(4-phosphophenyl)benzene as the linker by direct precipitation in the presence of HF. Each layer consisted of zirconia octahedra linked to RPO\({}_{3}\) tetrahedra that acted as the SBU to form the honeycomb-layered framework. Even though the framework had good hydrolytic stability, it lacked porosity due to the capping of the layers [62]. Therefore, in order to induce porosity, the linker was replaced by 2,4,6-tris(4-phosphonomethyl)-phenyl 1,3,5-triazine (ttbmp), which is a flexible phosphonate ligand compared to the previous rigid linker. The resulting open framework UPG-1 was composed of 1D polymeric chains of zirconia octahedra linked by ttbmp, creating two kinds of channels: a 1 nm channel lined with free P-O groups and a smaller channel of 0.5 nm lined by uncoordinated PO\({}_{3}\)C groups as shown in Figure 5[63]. In a similar manner, an aluminum phosphonate framework with a pore size of 1.24 nm was obtained [64].