The title compound, C 21 H 20 ClN 5 , has been synthesized in two steps from ( RS )-4-chloro-6,11-dimethyl-6,11-dihydro-5 H -benzo[ b ]pyrimido[5,4- f ]azepine and characterized by 1 H and 13 C NMR spectroscopy and by high-resolution mass spectrometry. Crystallization from hexane–ethyl acetate yields approximately equal quantities of a monoclinic polymorph in the space group Cc , (I), and an orthorhombic polymorph in the space group Pna 2 1 , (II). The molecules in polymorphs (I) and (II) show small differences in their molecular conformations, particularly in the shape of the azepine ring and the orientation of the chlorophenyl substituent. The molecules in polymorph (I) are linked by C—H...N and C—H...π(arene) hydrogen bonds to form sheets, which are linked into a three-dimensional framework structure by C—Cl...π(arene) interactions. There are no C—Cl...π(arene) interactions between the molecules in polymorph (II) and the supramolecular assembly takes the form of sheets built from C—H...N and C—H...π(arene) hydrogen bonds. Comparisons are made with some related structures.