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.