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-pyrimido[4,5-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 center dot center dot center dot 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.