Two polymorphs of 2-(4-chlorophenyl)-4-methylchromenium perchlorate.

Crystallization (from ethyl acetate solution) of 2-(4-chlorophenyl)-4-methylchromenium perchlorate, C(16)H(12)ClO(+).;ClO(4)(-), (I), yields two monoclinic polymorphs with the space groups P2(1)/n [polymorph (Ia)] and P2(1)/c [polymorph (Ib)]; in both cases, Z = 4. Cations and anions, disordered in polymorph (Ib), form ion pairs in both polymorphs as a result of Cl-O...pi interactions. Related by a centre of symmetry, neighbouring ion pairs in polymorph (Ia) are linked via pi-pi interactions between cationic fragments, and the resulting dimers are linked through a network of C-H...O(perchlorate) interactions between adjacent cations and anions. The ion pairs in polymorph (Ib), arranged in pairs of columns along the a axis, are linked through a network of C-H...O(perchlorate), C-Cl...pi, pi-pi and C-Cl...O(perchlorate) interactions. The aromatic skeletons in polymorph (Ia) are parallel in the cationic fragments involved in dimers, but nonparallel in adjacent ion pairs not constituting dimers. In polymorph (Ib), these skeletons are parallel in pairs of columns, but nonparallel in adjacent pairs of columns; this is visible as a herring-bone pattern. Differences in the crystal structures of the polymorphs are most probably the cause of their different colours.