Hydrogen bonds and π-π Interactions in two new crystalline phases of methylene blue

Abstract

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, C16 H18 N3 S +·Cl -·2H2 O (I) and 3,7-bis(dimethylamino)phenothiazinium bisulfite, C16 H18 N3 S +·HSO4 - (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.