Four 2‐amino‐6‐aryl‐4‐methoxy‐11H‐pyrimido[4,5‐b][1,4]benzodiazepines: similar molecular structures but different crystal structures

2‐Amino‐4‐methoxy‐6‐phenyl‐11H‐pyrimido4,5‐b]1,4]benzodiazepine, C₁₈H₁₅N₅O, (I), and its 6‐(2‐fluorophenyl)‐, 6‐(3‐nitrophenyl)‐ and 6‐(4‐methoxyphenyl)‐ analogues, viz. C₁₈H₁₄FN₅O, (II), C₁₈H₁₄N₆O₃, (III), and C₁₉H₁₇N₅O₂, (IV), respectively, all adopt molecular conformations which are almost identical, containing boat‐shaped seven‐membered rings. In each structure, paired N—H...N hydrogen bonds link the molecules into centrosymmetric dimers. In each of (I)–(III), the dimers are further linked, forming a different three‐dimensional framework in each case, while in compound (IV) the dimers are linked into sheets. The significance of this study lies in the observation of different crystal structures in four compounds whose molecular structures are very similar.