Two dibenzodiazepinone molecules with dissimilar dimeric associations and apparent different tautomeric forms

In two dibenzodiazepinones, viz. the tricyclic core structure, 5H‐dibenzob,e]diazepin‐11(10H)‐one, C₁₃H₁₀N₂O, and an acylated derivative, 1‐(11‐hydroxy‐5H‐dibenzob,e]diazepin‐5‐yl)‐2‐{4‐3‐(1H‐imidazol‐1‐yl)propyl]piperidin‐1‐yl}ethanone ethanol monosolvate, C₂₆H₂₉N₅O₂·C₂H₅OH, dimeric association via hydrogen‐bond bridging between the cyclic amide entities is evident, but there are considerable differences between the parent compound and the amidated derivative. Highly consistent with reported structures of related tricyclic lactams, two molecules of the nonsubstituted compound are bridged through two N—H...O hydrogen bonds across a crystallographic centre of symmetry and the bond lengths of the cyclic amide entity correspond to the amino–oxo (lactam) tautomeric form. In contrast, the structure of the derivative shows two similar, but crystallographically unique, molecules hydrogen bonded into a dimeric unit exhibiting an approximate (noncrystallographic) C2 axis. The bond lengths of the two derivative cyclic amide groups support their potential presence in the hydroxyimine (lactim) tautomeric forms, with the resulting possibility of intermolecular tautomerism. Likely driving forces for the two extreme configurations are discussed.