Synthesis, Crystal Structure and Solid State Transformation of 1,2-Bis[(1-methyl-1H-imidazole-2-yl)thio]ethane

The spontaneous S-alkylation of the thyreostatic drug methimazole (1-methyl-1,3-dihydro-1H-imidazole-2-thione, 1) with 1,2-dichloroethane at room temperature, in dark or light conditions, led to the formation of its related substance 1,2-bis[(1-methyl-1H-imidazole-2-yl)thio]ethane, C₁₀H₁₄N₄S₂ (2a), primarily isolated in the form of dihydrochloride tetrahydrate [C₁₀H₁₆N₄S₂]Cl₂·4(H₂O) (2b), which crystallized in the monoclinic P2₁/c space group. Neutralization of 2b, followed by crystallization from the acetone/water mixture, produced dihydrate C₁₀H₁₄N₄S₂·2(H₂O) (2c), which crystallized in the trigonal R-3 space group. Six water molecules in 2c are H-bonded mutually and to the nitrogen atoms of six molecules of 2a. DSC and TGA showed that 2c melts at 65 °C and loses water up to 120 °C. By cooling to room temperature, anhydrous 2a was obtained. Single crystals of 2a that are suitable for X-ray structure analysis were obtained by neutralization of 2b, followed by crystallization from dry dichloromethane. Anhydrous 2a crystallizes in the monoclinic P2₁/c space group. The dehydration of 2c led to the formation of the anhydrous product 2a, which is identical to the one obtained by crystallization, as was found by complementary solid-state techniques. No intermediate monohydrate or hemihydrate phases were detected. Powder diffraction showed the same pattern of 2c via both preparation procedures. The structures of all the forms were elucidated by spectroscopy, microscopy and thermal methods and confirmed by single crystal X-ray analysis.