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, C16H18N3S+·Cl−·2H2O (I) and 3,7-bis(dimethylamino)phenothiazinium bisulfite, C16H18N3S+·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.

Keywords

• crystal structure
• methylene blue
• chloride
• bisulfite
• hydrogen bonding
• π–π interactions
• Hirshfeld surface analysis