Hydrogen-bonding patterns in enamino­nes: (2Z)-1-(4-bromo­phen­yl)-2-(pyrrolidin-2-yl­idene)ethanone and its piperidin-2-yl­idene and azepan-2-yl­idene analogues

The title compounds, namely (2Z)-1-(4-bromo­phenyl)-2-(pyrrolidin-2-yl­idene)ethanone, C₁₂H₁₂BrNO, (I), (2Z)-1-(4-bromo­phenyl)-2-(piperidin-2-yl­idene)ethanone, C₁₃H₁₄BrNO, (II), and (2Z)-2-(azepan-2-yl­idene)-1-(4-bromo­phenyl)­ethan­one, C₁₄H₁₆BrNO, (III), are characterized by bifurcated intra- and inter­molecular hydrogen bonding between the secondary amine and carbonyl groups. The former establishes a six-membered hydrogen-bonded ring, while the latter leads to the formation of centrosymmetric dimers. Weak C—H...Br inter­actions link the individual mol­ecules into chains that run along the [011], [101] and [101] directions in (I)–(III), respectively. Additional weak Br...O, C—H...π and C—H...O inter­actions further stabilize the crystal structures.