Competing inter­molecular inter­actions in some `bridge-flipped' isomeric phenyl­hydrazones

To examine the roles of competing inter­molecular inter­actions in differentiating the mol­ecular packing arrangements of some isomeric phenyl­hydrazones from each other, the crystal structures of five nitrile-halogen substituted phenyl­hydrazones and two nitro-halogen substituted phenyl­hydrazones have been determined and are described here: (E)-4-cyano­benzaldehyde 4-chloro­phenyl­hydrazone, C₁₄H₁₀ClN₃, (Ia); (E)-4-cyano­benzaldehyde 4-bromo­phenyl­hydra­zone, C₁₄H₁₀BrN₃, (Ib); (E)-4-cyano­benzaldehyde 4-iodo­phenyl­hydrazone, C₁₄H₁₀IN₃, (Ic); (E)-4-bromo­benzaldehyde 4-cyano­phenyl­hydrazone, C₁₄H₁₀BrN₃, (IIb); (E)-4-iodo­benzaldehyde 4-cyano­phenyl­hydrazone, C₁₄H₁₀IN₃, (IIc); (E)-4-chloro­benzaldehyde 4-nitro­phenyl­hydrazone, C₁₃H₁₀ClN₃O₂, (III); and (E)-4-nitro­benzaldehyde 4-chloro­phenyl­hydrazone, C₁₃H₁₀ClN₃O₂, (IV). Both (Ia) and (Ib) are disordered (less than 7% of the molecules have the minor orientation in each structure). Pairs (Ia)/(Ib) and (IIb)/(IIc), related by a halogen exchange, are isomorphous, but none of the `bridge-flipped' isomeric pairs, viz. (Ib)/(IIb), (Ic)/(IIc) or (III)/(IV), is isomorphous. In the nitrile-halogen structures (Ia)-(Ic) and (IIb)-(IIc), only the bridge N-H group and not the bridge C-H group acts as a hydrogen-bond donor to the nitrile group, but in the nitro-halogen structures (III) (with Z' = 2) and (IV), both the bridge N-H group and the bridge C-H group inter­act with the nitro group as hydrogen-bond donors, albeit via different motifs. The occurrence here of the bridge C-H contact with a hydrogen-bond acceptor suggests the possibility that other pairs of `bridge-flipped' isomeric phenyl­hydrazones may prove to be isomorphous, regardless of the change from isomer to isomer in the position of the N-H group within the bridge.