*3.4. Stability Studies in Solution and Solid State*

In order to understand the phase stability of the multicomponent forms in 1:1 molar ratio in solution, the process was investigated over time by PXRD. After 2 h, either using water or acetonitrile resulted to be almost undistinguished products, as the XRD patterns were quite similar. However, the slurry in water after 24 h contained a mixture of the previous phase and a new one, which resulted the unique after 48 h and matched with the diffractogram obtained for compound **1** (Figure 5, below). For the mixture in acetonitrile, the phase transformation was also detected by the presence of new peaks in the powder pattern after stirring for 24 h, which after 48 h resulted in the same PXRD as for compound **2**.

**Figure 5.** Phase transformations in slurry experiments in (**a**) water or (**b**) in acetonitrile.

Furthermore, the physical stability of the anhydrous oxalate salts against humidity was studied. These solids were stored in a desiccator at 75% RH using a saturated NaCl salt solution for 7 days at 40 ◦C and for 1 month at room temperature. After these periods the solids were again analyzed by PXRD to confirm the phase of the solids. The analysis of the solids indicated that both anhydrous oxalate salts were stable under the two stability conditions tested (Figure S9).
