We designate as “bridge-flipped isomers” pairs of organic molecules differing only in the orientation of a bridge of atoms connecting two major molecular moieties, as among benzylideneanilines, Ar1-CH=N-Ar2 vs. Ar1-N=CH-Ar2, and phenylhydrazones, Ar1-CH=N-NH-Ar2 vs. Ar1-NH-N=CH-Ar2 (Ar = aryl). To determine whether intermolecular interactions possessed by both isomers might facilitate their isomorphism, we have determined the X-ray crystal structure of the 4-cyanophenylhydrazone of 5-bromo-2-pyridinecarboxaldehyde looking for a possible cyano…bromo interaction. Cyano…bromo contacts are actually absent; instead, the water molecule acts as an H-bond donor to the cyano group and to the ring nitrogen atom of two different molecules while acting as an H-bond acceptor from the bridge N-H moiety of a third molecule. The bromine atom participates in an R22(8) motif with a pyridine ring hydrogen atom. The participation of the bridge N-H group in this extensively H-bonded network suggests that bridge reversal in the isomer will be disruptive and structure-differentiating.

Toward isomorphous bridge-flipped isomers:  Crystal structure of a hydrated phenylhydrazone

251st National Meeting (2016)

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