Crystal chemistry of sibutramine: thermal, diffractometric and spectroscopic characterization

Monohydrated sibutramine hydrochloride is a widely used active ingredient for the treatment of obesity. An anhydrous form of sibutramine hydrochloride was prepared starting from its monohydrate form upon heating it at 140 degrees C for 15 min. This dehydration process was monitored using conventional TG/DSC methods. Heated above 190 degrees C, sibutramine hydrochloride sublimes and recrystallizes on the cold walls of the test tube, giving platelet shaped crystals suitable for single crystal X-ray diffraction analysis: monoclinic, P2(1)/n, a = 7.321(2) A, b = 25.456(5) A, c = 9.750(3) A, beta = 101.60(2) degrees , V = 1779.9(8) A(3), Z = 4. At variance, sibutramine free base was typically recovered as a viscous oily material, upon treatment of its hydrochloride salt in ethyl acetate solution. Recrystallization from hexane yielded a white polycrystalline powder, the structure of which was determined by unconventional ab initio X-ray powder diffraction analysis: triclinic, P-1, a = 8.6578(3) A, b = 9.3318(3) A, c = 11.1224(4) A, alpha = 110.434(3) degrees , beta = 100.159(3) degrees , gamma = 89.201(2) degrees , V = 827.76(5) A(3), Z = 2. Sibutramine, in its different crystalline environments, was also fully characterized by solid state (13)C NMR analyses. Additional spectral information was obtained by collecting spectra of a metastable, oily sample, before it slowly recrystallizes (within hours).