# Advances and Challenges in the Creation of Porous Metal Phosphonates
## 2 Synthesis of Porous Metal Phosphonates
### Layered Metal Phosphonates  
Layered metal phosphonates were synthesized by the direct precipitation between metal precursors and organophosphonic acids in a similar way as known for metal phosphates [9; 10]. The resulting materials were found to be analogous in structure to layered metal phosphates, but with the interlayer distance depending on the organic linker group [22; 23]. As in the literature \"metal organophosphonates\" are often described as \"metal phosphonates\", we will from here on leave out \"organo\" even though these materials are hybrid materials synthesized from metal and organophosphonic precursors. In this section, we will discuss the syntheses of metal phosphonates of divalent/trivalent metals and end by discussing tetravalent metal phosphonates. The key difference between those two relates to the solubility of metal ions in solutions. Transition metals strongly interact with phosphonic acid precursors and form an insoluble coordination polymer. However, a prerequisite to obtain a crystalline structure is a controlled, ordered buildup of molecular precursors into a hybrid network, requiring the metal moieties to be solubilized in sufficient quantity, allowing homogeneous interactions with the phosphonic precursors. In the case of metals that are divalent/trivalent, this is achieved by lowering the pH of the reaction medium, and thus synthesizing in an acidic condition. However, with tetravalent metal precursors, the formation of a metal phosphonate coordination network is kinetically too fast, resulting in the precipitation of the mixtures in an uncontrolled way, even in high acidic conditions and therefore hampering controlled structural properties [24]. One of the ways to overcome this has been by the introduction of HF as a solubilizing/mineralizing agent, whereby F\({}^{-}\) ions form a complex with metal ions, causing a slow release of metal ions into the solution available for interaction with the phosphonic precursor upon heating under pressure [20]. With respect to divalent metals, zinc (II) diphosphonates formed by phenylene and bisphenylene phosphonic linkers resulted in a structure that was found to be dependent on the length of the aryl linker. In the case of the monophenylene linker, the phenylene phosphonate groups functioned as pillars between the octahedral zinc oxide layers. However, in the presence of biphenylene as a linker, the inorganic unit formed chains rather than a layered structure with the zinc being coordinated tetrahedrally. The same phenomena were observed with copper (II) diphosphonates with phenylene and bisphenylene linkers [25; 26]. Moreover, when using non-aromatic linkers, it was reported that the structure of the materials was sensitive to the length of the linker/spacer in copper (II) and zinc (II) diphosphonates made from ethylene diphosphate acid or propylene diphosphonic acid. The presence of an extra methylene unit leads to a change in the coordination of copper and zinc from an octahedral layered into a tetrahedral chain structure. Hence, the length of linker moiety (alkyl or aryl) has pronounced consequences on the coordination of the linkers to the metal-oxo centers and thus dictates the final structure of the material. These key structural features in layered materials weredetermined by single-crystal X-ray diffraction and was therefore important to understand the details of the coordination geometry with different linkers [27]. Different types of precursors in combination with variations in synthesis conditions were found to influence the structural properties of layered metal phosphonates. The use of diphosphonate acids containing an amine group allows to tune the coordination with the metal, based on the reaction conditions and the choice of metal precursor, which influences the formation of the network. Zinc (II) and manganese (II) diphosphonates were synthesized with N-ethylimino bis(methylene phosphonic acid), yielding a 2D-layered structure with Mn (II) and a 3D network when Zn (II) was the metal source.