A second monoclinic polymorph of 2,9-dimethyl-1,10-phenanthroline dihydrate

A second monoclinic polymorph of the title compound, neocuproine dihydrate, C₁₄H₁₂N₂.2H₂O, is reported. Unlike the first polymorph [Baggio, Baggio & Mombrú (1998). Acta Cryst. C54, 1900-1902], in which the phenanthroline ring system was constrained to lie in a crystallographic mirror plane, here there is no such imposed symmetry. Consequently, the molecule shows small deviations from planarity, the outer rings being twisted slightly in opposite directions from the plane of the central ring. The hydrogen-bonding motifs remain essentially the same as in the first polymorph, involving small rings of four water molecules and large rings containing four water molecules and two neocuproine molecules, but with no H-atom disorder for the water molecules in this case. There are also aromatic π-π stacking interactions.