# Advances and Challenges in the Creation of Porous Metal Phosphonates
## 4 Summary and Future Outlook  
The field of porous metal phosphonates is a growing field of research and is not yet as well established as the carboxylate MOFs, organic-inorganic hybrid silicas, or layered hybrid materials. In this exploratory review, we have highlighted the different classes of porous metal phosphonates coordination networks. Based on layered metal phosphonate systems, we saw the evolution of porous metal phosphonates towards the creation of microporous phosphonate MOFs and porous hybrid inorganic-organic systems (with or without supramolecular templating). The kinetic control, in particular when aiming for mesoporous metal phosphonates, still remains a key challenge in  
Figure 10: (**a**) Proton conductivity of the single crystals of CoCa-nH2O measured along different directions at 25 \({}^{\circ}\)C upon increasing (filled) or decreasing (open) humidity. The conductivity of the pellet sample is also shown for comparison; (**b**) plots of log(\(\sigma\)T) versus 1000/T for bulk and single crystal samples of CoCa-4H2O at 95% relative humidity. The solid lines represent the best fit of the data. [PERSON] plots for CoCa-nH2O at 95% RH and various temperatures; (**c**) bulk sample; and (**d–f**) single crystal sample along the [010], [202], and [20\(\overline{1}\)] directions, respectively. Reprinted with permission from [126]. Copyright (2016), American Chemical Society. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Metal Phosphonate** & **Conductivity (S cm\({}^{-1}\))** & **Activation Energy (eV)** & **References** \\ \hline ZnCO3-2 Zn(OH)2 + BTP & \(1.0\times 10^{-5}\) (at 98\% RH, 25 \({}^{\circ}\)C) & 0.17 & [122] \\ Mg(NO3)2 + H6 DDTMP & \(1.6\times 10^{-3}\) (at 100\% RH, 19 \({}^{\circ}\)C) & 0.31 & [124] \\ \(\beta\)-PCMOF-2\({}^{1/2}\) & \(2.1\times 10^{-2}\) (at 90\% RH, 85 \({}^{\circ}\)C) & 0.21 & [123] \\ CoCa. n H2O (pellet) & \(1.55\times 10^{-5}\) (pellet) at 95\% RH, 25\({}^{\circ}\)C) & 0.98 (pellet) & [126] \\ CoCa. n H2O (SC) & \(1.00\times 10^{-3}\) (SC) (at 95\% RH, 25\({}^{\circ}\)C) & 0.90 (SC) & \\ MMM-500-Ni & \(4.5\times 10^{-4}\) (at 98\% RH, 25 \({}^{\circ}\)C) & 0.43 & [125] \\ MFM-500-Co & \(4.4\times 10^{-5}\) (at 98\% RH, 25 \({}^{\circ}\)C) & - & [65] \\ UPG-2 & \(5.7\times 10^{-4}\) (at 95\% RH, 100 \({}^{\circ}\)C) & - & [65] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Summary of proton conduction in the phosphonate frameworks. attaining interesting porous architectures. Rapid precipitation due to the heterocondensation/phase separation between metal precursors and phosphonic moieties is one of the remaining concerns. Specifically, when one considers the group IV metals, challenges to harmonize the interactions in a controlled manner are still present when designing porous systems without the use of hazardous HF. Key insights on how the different types of (chelated) metal(IV) precursors react with phosphonate moieties is still a largely unexplored domain. It can thus be envisioned that the challenge to create novel coordination networks of metal phosphonates lies in the reactivity control of the interaction between the metal and phosphonic precursors, in such a way that rapid precipitation is avoided and a controlled build-up of a hybrid porous network is achieved. This problem is further complicated when porosity needs to be induced by the use of SDAs, especially in the case of ionic templates due to their interaction with the phosphonic anion or with the metal ion, influencing reactivity and kinetics. Therefore, the role of templating in these metal phosphonate systems is often not straightforward and more challenging compared to porous metal oxide systems. To date, little studies focused on how ionic and/or polymeric SDAs interact with the phosphonic moieties during the formation of porous metal phosphonate networks.