Crystal Structure of 1,1-Dimethyl-3-(quinolin-2-yl)urea and 3-(Isoquinolin-1-yl)-1,1-dimethylurea

The crystal structures of two isomeric urea derivatives—1,1-dimethyl-3-(quinolin-2-yl)urea (1) and 3-(isoquinolin-1-yl)-1,1-dimethylurea (2)—have been investigated by single-crystal X-ray diffraction. These compounds differ in the relative position of the heterocyclic nitrogen atom and the urea substituent. A structural study revealed that the compounds exhibit distinct molecular conformations. In 1, the molecule exists predominantly in a ureide form with a single C–N bond between the quinoline fragment and urea moiety. The ureide group is out of the plane of the quinoline ring, forming an anti-conformation. In contrast, 2 adopts an exocyclic C=N configuration with an intramolecular hydrogen bond between the heterocyclic NH group and ureide oxygen, resulting in a syn-conformation. The supramolecular organization of the compounds also differs: 1 forms one-dimensional chains through N–H···O hydrogen bonds, which further assemble into two-dimensional layers via C–H···O interactions; in 2, the molecules form dimers through intermolecular C–H···O hydrogen bonds. Thus, subtle structural differences between isomeric urea derivatives can lead to distinct molecular conformations, tautomeric forms, and supramolecular organizations.