Hydrogen bonds and π-π inter-actions in two new crystalline phases of methyl-ene blue.

Two unprecedented solid phases involving the 3,7-bis-(di-methyl-amino)-pheno-thia-zin-5-ium cation, i.e. methyl-ene blue ( MB+ ), have been obtained and structurally characterized. In the crystals of 3,7-bis-(di-methyl-amino)-pheno-thia-zin-5-ium chloride dihydrate, C16 H18 N3 S+ ·Cl- ·2H2 O ( I ) and 3,7-bis-(di-methyl-amino)-pheno-thia-zinium bis-ulfite, C16 H18 N3 S+ ·HSO4 - ( II ), the cationic dye mol-ecules are planar and disposed in an anti-parallel mode, showing π-π stacking inter-actions, with mean inter-planar distances of 3.326 (4) and 3.550 (3) Å in ( I ) and ( II ), respectively. In compound ( I ), whose phase was found affected by merohedral twinning [BASF = 0.185 (3)], the presence of water mol-ecules allows a network of hydrogen bonds involving MB+ as both a donor and an acceptor, whereas in compound ( II ), the homo-inter-action of the anions causes an effective absence of classical hydrogen-bond donors. This substantial difference has important consequences for the stacking geometry and supra-molecular inter-actions of the MB+ cations, which are analysed by Hirshfeld fingerprint plots and subsequently discussed.