*3.2. Synthesis of 1,2-Bis[(1-methyl-1H-imidazole-2-yl)thio]ethane Dihydrochloride Tetrahydrate (2b)*

Method (A) Methimazole (100 mg, 0.87 mmol) was dissolved in 10 mL of 1,2-dichloroethane. The solution was left without stirring for 15 days in the dark at room temperature in a humidity non-controlled environment. The precipitated product was collected using vacuum suction and rinsed few times with cold 1,2-dichloroethane, yielding 1,2-bis[(1-methyl-1*H*-imidazole-2-yl)thio]ethane dihydrochloride tetrahydrate (**2b**, 53 mg, 31%) in the form of colourless plate shape crystals. Melting point (DSC, onset): 208 ◦C, MS-QTOF: [M+H-2HCl]<sup>+</sup> 255.0743, [M-2HCl] 254.066, C10H14N4S2. <sup>1</sup>H NMR (D2O, 600 MHz/ppm): N-CH<sup>3</sup> (s, 6H), 3.82, H-C4 (d, 2H) 7.46, *J* = 2.1 Hz, H-C5 (d, 2H) 7.50, *J* = 2.1 Hz, S-CH<sup>2</sup> (s, 4H) 3.26. <sup>13</sup>C NMR (D2O, 151 MHz/ppm): N-CH<sup>3</sup> 35.22, C4 120.98, C5 125.70, C2 138.52, S-CH<sup>2</sup> 34.59, Purity (HPLC): 98%, RRT: 20.518. Ionic chromatography: experimentally determined percentage of Cl ions (19%) corresponded to the theoretical value (18%) within the experimental error. The structure of **2b** was confirmed by single crystal X-ray diffraction analysis.

Method (B) Methimazole (100 mg, 0.87 mmol) was dissolved in 10 mL of 1,2-dichloroethane. The solution was left without stirring for 15 days in daylight and room temperature in a humidity non-controlled environment. The precipitated product was collected using vacuum suction and rinsed a few times with cold 1,2-dichloroethane, yielding dihydrochloride tetrahydrate **2b** (52 mg, 30%) in the form of colourless plate shaped crystals. Melting point (DSC, onset): 208 ◦C. The NMR spectra and XRPD diffractogram of the prepared sample were identical to the spectra and diffractogram of the **2b** sample obtained by Method A.
