2-(4-Bromophenyl)-1,2-dihydro-pyrimido[1,2-a]benzimidazol-4(3H)-one and 4-(4-methylphenyl)-3,4-dihydropyrimido[1,2-a]benzimidazol-2(1H)-one form hydrogen-bonded base-paired dimers

The title compounds, 2-(4-bromophenyl)-1,2-dihydropyrimido[1,2-a]benzimidazol-4-(3H)-one, C16H12BrN3O, (IVa), and 4-(4-methylphenyl)-3,4-dihydropyrimido[1,2-a]benzimidazol-2-(1H)-one, C17H15N3O, (Vb), both form R-2(2)(8) centrosymmetric dimers via N-H...N hydrogen bonds. The N...N distance is 2.943 (3) Angstrom for (IVa) and 2.8481 (16) Angstrom for (Vb), with the corresponding N-H...N angles being 129 and 167degrees, respectively. However, in other respects, the supramolecular structures of the two compounds differ. Both compounds contain different C-H...pi interactions, in which the C-H...pi(centroid) distances are 2.59 and 2.47 Angstrom for (IVa) and (Vb), respectively (the latter being a short distance), with C-H...pi (centroid)angles of 158 and 159degrees, respectively. The supramolecular structures also differ, with a short Br...O distance of 3.117 (2) Angstrom in bromo derivative (IVa), and a C-H...O interaction with a C...O distance of 3.2561 (19) Angstrom and a C-H...O angle of 127degrees in tolyl system (Vb). The dihydropyrimido part of (Vb) is disordered, with a ratio of the major and minor components of 0.9:0.1. The disorder consists of two non-interchangeable envelope conformers, each with an equatorial tolyl group and an axial methine H atom.