Acta Crystallographica Section C

Crystal Structure Communications

Volume 68, Part 1 (January 2012)

organic compounds

sk3425 scheme

Acta Cryst. (2012). C68, o28-o32    [ doi:10.1107/S0108270111052012 ]

Tizanidine and tizanidine hydro­chloride: on the correct tautomeric form of tizanidine

S. L. Bekö, S. D. Thoms, M. U. Schmidt and M. Bolte

Abstract: A crystallization series of tizanidine hydro­chloride, used as a muscle relaxant for spasticity acting centrally as an [alpha]2-adrenergic agonist, yielded single crystals of the free base and the hydro­chloride salt. The crystal structures of tizanidine [systematic name: 5-chloro-N-(imidazolidin-2-yl­idene)-2,1,3-benzothia­diazol-4-amine], C9H8ClN5S, (I), and tizanidine hydro­chloride {systematic name: 2-[(5-chloro-2,1,3-benzothia­diazol-4-yl)amino]­imidazolidinium chloride}, C9H9ClN5S+·Cl-, (II), have been determined. Tizanidine crystallizes with two almost identical mol­ecules in the asymmetric unit (r.m.s. deviation = 0.179 Å for all non-H atoms). The mol­ecules are connected by N-H...N hydrogen bonds forming chains running along [2\overline{1}1]. The present structure determination corrects the structure determination of tizanidine by John et al. [Acta Cryst. (2011), E67, o838-o839], which shows an incorrect tautomeric form. Tizanidine does not crystallize as the usually drawn 2-amino-imidazoline tautomer, but as the 2-imino-imidazolidine tautomer. This tautomer is present in solution as well, as shown by 1H NMR analysis. In tizanidine hydro­chloride, cations and anions are connected by N-H...Cl hydrogen bonds to form layers parallel to (100).

Formula: C9H8ClN5S and C9H9ClN5S+·Cl-

hkldisplay filedownload file

Structure factor file (CIF format)
[ doi:10.1107/S0108270111052012/sk3425Isup2.hkl ]
Contains datablock I

hkldisplay filedownload file

Structure factor file (CIF format)
[ doi:10.1107/S0108270111052012/sk3425IIsup3.hkl ]
Contains datablock {vcif ERROR: unable to open file /journals/webproduction/sites/cjo-test/c/issues/2012/01/00/sk3425/sk3425IIsup3.hkl - exiting}


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