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In the title compound, C12H18N4OS2+center dot Cl-center dot ClO4-, the thiaminium cations form `head-to-tail' N-H center dot center dot center dot O hydrogen-bonded dimers about inversion centres. N-H center dot center dot center dot Cl and O-H center dot center dot center dot Cl hydrogen bonds further stabilize the structure, forming a one-dimensional chain running along the diagonal of the ac plane. Both anions are located adjacent to `anion holes' in the thiaminium cation. The perchlorate anions show rotational disorder about a Cl-O bond.

The ambident anion character of thiamin ylide was demonstrated by alkylation reactions of the «isolated thiamin ylide» under neutral or basic conditions. Alkylation with alkyl halides yielded S-alkyl derivatives of the yellow form of thiamin (pyrimidopyrimidine derivatives) instead of 2-alkylthiamin. These and previous results provide chemical evidence that thiamin ylide and thiaminium neothiaminthiolate undergo a facile interconversion in EtOH

The ambident anion character of thiamin ylide was demonstrated by alkylation reactions of the «isolated thiamin ylide» under neutral or basic conditions. Alkylation with alkyl halides yielded S-alkyl derivatives of the yellow form of thiamin (pyrimidopyrimidine derivatives) instead of 2-alkylthiamin. These and previous results provide chemical evidence that thiamin ylide and thiaminium neothiaminthiolate undergo a facile interconversion in EtOH

The reaction of mercuric chloride with thiamine has been explored. Two distinct salts, [ThH]2+[HgCl3]−2 and [ThH]2+ [HgCl42− (Th = thiaminium = protonated thiamine), were isolated at pH 3.5. The structure of the latter compound was solved with X-ray methods and shows the presence of discrete thiaminium and tetrachloromercurate ions. At a higher pH (5.5), a third compound, not yet fully characterized, was isolated having the empirical formula (Hg2Cl5(Th). Crystals details on [ThH]2+ [HgCl4]2−·H2O: space group P21/c (monoclinic); a = 17.189(17) A, b = 8.817(5) A, c = 13.971(6) A, β = 106.21(6)° , V = 2033(2) A3, Z = 4. Final R factor = 7.3% for 2013 reflections.