## 3 Results
### Powder and Single Crystal X-ray Diffraction Studies of (Ca/Sr)-RIS Compounds  
Bulk solid products of the (Ca/Sr)-RIS compounds were studied by powder X-ray diffraction to ensure that they were pure and monophasic. Comparison of the calculated (from the crystal structure determination, see below) and measured X-ray diffraction diagrams, ensured that the samples were pure (Figure 3).  
Figure 2: SEM images of single crystals of metal-BPs.  
Suitable crystals for single crystal X-ray diffraction and structure determination were obtained by hydrothermal syntheses (see above). The crystallographic data of the compounds Ca-RIS and Sr-RIS are summarized in Table 2 and their cit files are provided as Supplementary Materials.  
The RIS ligand exhibits a total charge of \"\(-\)2\" to counterbalance the \"\(+\)2\" charge of the metal cation in both structures of Ca and Sr-RIS. This means that one of the phosphonate groups (P2) is mono-deprotonated, whereas the other (P1) is bis-deprotonated. The N of the pyridine ring is protonated. Hence, RIS behaves as a zwitterion in the structure. The asymmetric unit of the two compounds Ca- and Sr-RIS is shown in Figure 4.  
Figure 3: Calculated (red) and measured (blue) powder X-ray diffraction diagrams of Ca–RIS (**upper**) and Sr–RIS (**lower**).  
In compound Ca-RIS, the coordination sphere of the Ca\({}^{2+}\) center can be described as a pentagonal bipyramid (\(D_{\text{Sh}}\), based on SHAPE analysis), while it forms six coordination bonds with oxygen atoms of three different RIS ligands and one bond with a water molecule. Each RIS ligand coordinates to three metals, and hence, Ca-RIS is a 1D coordination polymer. The Ca\({}^{2+}\) cation chains are arranged is such a way that they lie parallel to the a-axis (Figure 5a). The Ca-O bond distances (Figure 5b) are between 2.292 A and 2.633 A. There are H bonds between the chains: one O(phosphonate)\(\cdots\)O(P-hydroxy) interaction [2.454 A], one O(hydroxy)\(\cdots\)O (water) interaction [2.743 A], one O(phosphonate)\(\cdots\)O(water) interaction [2.827 A] and one N(1)\(\cdots\)O(phosphonate) [2.652 A].  
\begin{table}
\begin{tabular}{c c c} \hline  & **Ca-RIS** & **Sr-RIS** \\ \hline
**Empirical formula** & C\({}_{7}\)H\({}_{11}\)CaNO\({}_{8}\)P\({}_{2}\) & C\({}_{7}\)H\({}_{11}\)SrNO\({}_{8}\)P\({}_{2}\) \\ \hline \(M_{\text{r}}\) & 339.19 & 386.73 \\ \hline
**Crystal system** & orthorhombic & monoclinic \\ \hline
**Space group** & P\({}_{21}\)2\({}_{1}\)2\({}_{1}\) & P2\({}_{1}\)/n \\ \hline
**a (Å)** & 6.7990(9) & 13.679(5) \\ \hline
**b (Å)** & 13.091(2) & 12.101(3) \\ \hline
**c(Å)** & 13.446(2) & 6.987(4) \\ \hline
**\(\alpha\) (°)** & 90 & 90 \\ \hline \(\beta\) (°) & 90 & 98.170(15) \\ \hline \(\gamma\) (°) & 90 & 90 \\ \hline
**V (Å\({}^{3}\))** & 1196.77 & 1144.82 \\ \hline
**Z** & 4 & 4 \\ \hline
**R factor (\%)** & 5.71 & 3.19 \\ \hline
**CCDC code** & 2118543 & 2118546 \\ \hline \end{tabular}
\end{table}
Table 2: Crystal data for compounds Ca–RIS and Sr-RIS.  
Figure 4: Asymmetric unit of Ca- and Sr–RIS. Color codes: metal centers, green; P, orange; O, red; C, black; N, blue; H, white.  
Compound Sr-RIS is a 2D coordination polymer. Its layers are constructed along the b- and c-axes (Figure 6a). Each Sr2+ center is coordinated by four different RIS ligands through seven oxygen atoms and is also coordinated by a water molecule. The geometry of Sr1 in compound (II) is a biaugmented trigonal prism (\(C_{\mathrm{2v}}\), based on SHAPE analysis, Figure 6c). The structure does not display any inter-layer H bonds, but there are some intra-layer H bonds, as well as \"\(\pi\)-\(\pi\)\" stacking interactions between the pyridyl rings (Figure 6b).