*2.3. Characterization*

*Powder X-ray Diffraction (PXRD).* PXRD data were collected using a Siemens D5000 powder X-ray diffractometer (Siemens, Munich, Germany) with Cu-Kα radiation (λ = 1.5418 Å), with 35 kV and 45 mA voltage and current applied. An amount of powder was gently pressed on a glass slide to afford a flat surface and then analyzed. The samples were scanned in the 2θ range of 2–50◦ using a step size of 0.02◦ and a scan rate of 1 s/step.

*Single Crystal X-ray Diffraction (SC-XRD).* Suitable crystals of **1** and **3** were selected for X-ray single crystal diffraction experiments, covered with oil (Infineum V8512, formerly known as Paratone N) and mounted at the tip of a nylon CryoLoop on a BRUKER-NONIUS X8 APEX-II KAPPA CCD diffractometer (Bruker, Karlsruhe, Germany) using graphite monochromated MoKα radiation (λ = 0.7107 Å). Crystallographic data were collected at 300 (2) K. Data were corrected for Lorentz and polarization effects and for absorption by SADABS [20]. The structural resolution procedure was made using the WinGX package [21]. The structure factor phases were solved by SHELXT-2014/5 or SHELXT-2018/2 [22]. For the full matrix refinement SHELXL-2017/1 or SHELXL2018/3 was used [23]. The structures were checked for higher symmetry with help of the program PLATON [24]. H-atoms were introduced in calculated positions and refined riding on their parent atoms, except for the protonation sites (H1A and H1B).

In Table 1 general and crystallographic data for the two new salts described are summarized.


**Table 1.** Crystallographic data and refinement for salts **1** and **3**.

*Thermogravimetric analysis—Differential scanning calorimetry (TGA-DSC).* A simultaneous thermogravimetric analysis (TGA)—differential scanning calorimetry/differential thermal analysis (heat flow DSC/DTA) system NETZSCH -STA 449 F1 Jupiter (NETZSCH, Selb, Germany) was used to perform thermal analysis on the solids. Samples (3–8 mg) were placed in open alumina pan and measured at a scan speed of 10 ◦C min−<sup>1</sup> from ambient temperature to 250 ◦C under N2 atmosphere as protective and purge gas (their respective flow velocities were 20 and 40 mL/min).

*Attenuated Total Reflection Fourier Transform Infrared spectroscopy (ATR-FT-IR).* A Jasco 4700LE spectrophotometer (JASCO, Tokyo, Japan) with attenuated total reflectance accessory was used to record the FT-IR spectra of 9ETADE, oxalic acid dihydrate and the new compounds prepared in this work in the range from 4000 to 400 cm−<sup>1</sup> and at a resolution of 4.0 cm<sup>−</sup>1.

*Determination of approximate solubilities.* The approximate solubilities of the anhydrous 9ETADE salts were determined by the gravimetric method following the procedure described in [25,26]. To sum up, in a vial, an amount of solid (ca. 40–50 mg) was added a determined volume of Milli-Q water to obtain a supersaturated solution at room temperature. The suspensions were stirred for 2 h and then the agitation was stopped to allow slow settling of the solids in excess for at least 24 h. Samples of the supernatant liquid were taken using a syringe and filter via a nylon syringe filter (0.22 μm). The clear solutions were added to a pre-weighted vial (m1) and the vial was weighted again (m2). The solvent was allowed to evaporate in the fume hood until dry and the mass was recorded (m3). The solids were dried in an oven at 30 ◦C under vacuum for 2 h to confirm no further weight decrease. The solubility was calculated as the amount of solid recovered (m3 − m1) divided by the volume of the solution (m2 − m3). The given values are the median of three replicates. The residual solids which did not dissolve were analyzed by PXRD to check the stability of the salts.
