# Advances and Challenges in the Creation of Porous Metal Phosphonates
## 2 Synthesis of Porous Metal Phosphonates
### MOFs-Phosphonates
The absence of the amino group of the triazole lead to the classical pillared copper benzene diphosphonate structure with no interlayer porosity [44]. In another study, the reaction of lead (II) with organo-piperazine as a diphosphonic ligand resulted in an initial 2D layered structure. This 2D structure was then transformed into a 3D framework via supramolecular assembly during the synthesis, due to the weak interaction of Pb-O(N) contacts. The formation of the supramolecular assembly leading to the obtained framework was attributed to hydrogen bonds, van der Waals and some electrostatic interactions [45]. Modifying the phosphonate ligands into R[PO(OH)(OR)]\({}_{2}\), by derivativizing one of the acidic P-OH groups into a P-OR group, mimics a carboxylate ligand. Thus, this type of precursors is expected to allow the formation of secondary building units (SBU) that are well known from carboxylate MOFs. This concept was first employed with zinc(1,4-benzenediphosphonate-bis(monoethyl ester)), to create a pillared structure with the ester moiety pointing inwards between the layers and with permanent porosity (aperture \(\sim\)0.6 nm) due to the Van der Waal forces between the alkyl chains in the interlayer [46]. The same phosphonic linker was later used to synthesize frameworks based on copper, and in this case, the linker possessed both methyl and ethyl ester groups. The resulting structure of the copper phosphonate framework with the ethyl ester was similar to the previous example with zinc. The use of these phosphonate linkers, mimicking a carboxylate linker, has led to the creation of porous frameworks built up via SBUs. These phosphonate MOFs also have advantages in application as they, for example, showed good affinity for CO\({}_{2}\) with the high isosteric heat of adsorption of 45 kJ/mol similar to carboxylate MOFs such as HKUST-1 (35 kJ/mol), MIL-101 (44 kJ/mol), MIL-100 (63 kJ/mol) and Mg-MOF-74 (47 kJ/mol) [47]. Larger pore metal phosphonate framework synthesized from nickel (II) and copper (II) with N,N'-piperazinebis(methylenephosphonic)acid as the linker have been prepared, consisting of a hexagonal array of channels with a pore size of 1 nm that was filled with physisorbed water. However, upon dehydration, the framework distorted. When using a 2-methylated derivative of the same phosphonic linker, this structural distortion could be prevented and the framework retained its structure [48]. Reticular syntheses are described as the process of assembling rigid molecular building blocks into ordered networks, held by strong bonds [1]. Thus, to attain larger pores, isoreticular methods have been employed to synthesize large pore metal phosphonate MOFs in materials such as STA-12 with an isoreticular pore size of 0.9 nm. They were produced by the reaction of N,N'-piperazine bis(methylenephosphonic acid) with divalent metals (Mn, Fe, Co and Ni) [49; 50]. To obtain larger pores, pushing towards the mesopore region, linkers such as the N,N'-4,4'-bipiperazine bis(methylenephosphonic acid) were reacted with cobalt to obtain 3D frameworks that have pores of 1.8 nm (STA-16). This 3D framework was composed of CoON\({}_{5}\) octahedra as a spiral, which resulted in hexagonal pores with a 1.8 nm pore opening as shown in Figure 3[51]. Phosphonate MOFs stable in water were synthesized from barium and octaethyl pyrene-1,3,6,8-tetraphosphonate resulting in a permanently porous framework denoted CALF-25. It is a three-dimensional framework made up of 1D inorganic-organic chains of barium phosphonate that are cross-linked by pyrene ligands. The pores of this framework consist of 1D rectangular openings of 0.46 nm \(\times\) 0.39 nm containing hydrophobic ethyl chains [52]. Alternatively, a framework built up from tin and a triphosphonate ligand resulted in an amorphous, highly robust, and porous system having a surface area of 500 m\({}^{2}\)/g and pore width of 0.85 nm (CALF-28) [53; 54].