Crystal Structure of Tetrachloroferrate(III) Complex with Methylene Blue

The crystal structure of tetrachloroferrate(III) complex with methylene blue, [Mb]⁺[FeCl₄]⁻ (where [Mb]⁺ methylthioninium or 3,7-bis(dimethylamino)-phenothiazine-5-ium cation), has been prepared by mechanochemical way and studied by the single crystal X-ray crystallography. The crystal structure of title compound is built from by [FeCl₄]⁻ tetrahedral ion and planar [Mb]⁺ counter ions. The [Mb]⁺ cation is linked in the 3D network by the C–H···Cl hydrogen bonds and is stacked in an antiparallel fashion with the sulfur atom disposed alternatively on an opposite sides of the stacking. The interplanar distance between two neighboring aromatic cycles is 3.431 Å, and a centroids–centroids distance between thiazine rings is 3.975 Å. Dihedral angle between aromatic rings is 1.6°. Intermolecular short contacts were analyzed by 3D Hirshfeld surfaces method and 2D fingerprint plots. Intermolecular interaction energy between two neighboring Mb⁺ cation pair was calculated by using of CE-B3LYP/6-31G(d,p) theoretical level.

The π···π stacking interactions were found plays an important role in the crystal structure formation of the FeCl₃ complex with methylene blue, and calculation by the CrystalExlorer17.5 program indicates that π···π interaction energy between two neighboring methylene blue cations is −33.5 kJ/mol.