Salt forms of a thioamide: protonation of 1-(2,6-dimethylphenyl)thiourea

Treating the thioamide 1-(2,6-dimethylphenyl)thiourea (C₉H₁₂N₂S, DMPT) with concentrated aqueous strong acids gave three salt forms with protonation at the thioamide S atom. The structures of 1-(2,6-dimethylphenyl)thiouronium chloride, C₉H₁₃N₂S⁺·Cl⁻ or [DMPT(H)][Cl], 1-(2,6-dimethylphenyl)thiouronium bromide, C₉H₁₃N₂S⁺·Br⁻ or [DMPT(H)][Br], and 1-(2,6-dimethylphenyl)thiouronium hydrogen sulfate, C₉H₁₃N₂S⁺·HSO₄⁻ or [DMPT(H)][HSO₄], are compared with that of neutral DMPT. Protonation at the S atom results in a systematic lengthening of the C=S bond and a shortening of the C-N bonds, with a larger effect found for the bonds involving the secondary amine rather than the primary amine function. The [DMPT(H)]⁺ cations are found to adopt one of two possible conformations, with the halide species having secondary N-H syn to S-H, whilst in the hydrogen sulfate structure, this relationship is anti. The change in cation conformation is related to a change in the hydrogen-bonding motif, but all three salt forms form layered structures with two-dimensional hydrogen-bonded units separated by layers of hydrophobic arene rings. An alternative product type was isolated on adding DMPT to concentrated sulfuric or tetrafluoroboric acid. Structures of the dicarbocationic disulfide species (disulfane-1,2-diyl)bis[N-(2,6-dimethylphenyl)methanediaminium] bis(tetrafluoroborate), C₁₈H₂₄N₄S₂²⁺·2BF₄⁻ or [Dimer][BF₄]₂, and (disulfane-1,2-diyl)bis[N-(2,6-dimethylphenyl)methanediaminium] bis(hydrogen sulfate) monohydrate, C₁₈H₂₄N₄S₂²⁺·2HSO₄⁻·H₂O or [Dimer][HSO₄]₂·H₂O, are also described.