Hydrogen bonds and π–π interactions in two new crystalline phases of methylene blue

Two unprecedented solid phases involving the 3,7-bis(dimethylamino)phenothiazin-5-ium cation, i.e. methylene blue ( MB + ), have been obtained and structurally characterized. In the crystals of 3,7-bis(dimethylamino)phenothiazin-5-ium chloride dihydrate, C 16 H 18 N 3 S + ·Cl − ·2H 2 O ( I ) and 3,7-bis(dimethylamino)phenothiazinium bisulfite, C 16 H 18 N 3 S + ·HSO 4 − ( II ), the cationic dye molecules are planar and disposed in an antiparallel mode, showing π–π stacking interactions, with mean interplanar 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 molecules allows a network of hydrogen bonds involving MB + as both a donor and an acceptor, whereas in compound ( II ), the homo-interaction of the anions causes an effective absence of classical hydrogen-bond donors. This substantial difference has important consequences for the stacking geometry and supramolecular interactions of the MB + cations, which are analysed by Hirshfeld fingerprint plots and subsequently discussed.

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