We are investigating the crystal structures of pairs of molecules we define as “bridge-flipped isomers.” These molecules differ structurally only in the orientation of an atom bridge connecting two major components of the molecules, such as aryl groups. Examples can be found among the benzylideneanilines, where the isomerism is Ar1-CH=N-Ar2 vs. Ar1-N=CH-Ar2, and among the phenylhydrazones, where the isomerism is Ar1-NH-N=CH-Ar2 vs. Ar1-CH=N-NH-Ar2 (Ar = aryl). Using single-crystal X-ray diffraction, we are determining how frequently such pairs are isomorphous, crystallizing into the same molecular packing arrangement in the solid state. Isomorphous pairs may form interesting solid solutions. Based on the premise that the occurrence of similar molecular packing motifs in at least one dimension in both members of a bridge-flipped isomeric pair might encourage their isomorphism, we have begun the synthesis and crystallization of a 4-pyridine-derived system of isomers with iodine substituents; the anticipated shared motif would be close intermolecular N…I Lewis acid-base contacts. We now have successfully grown crystals of the benzylideneaniline formed by reaction of 4-iodoaniline with 4-pyridinecarboxaldehyde and have determined the crystal structure. As predicted, molecules are linked into chains in the solid state by close N…I contacts. We are pursuing the synthesis and crystallization of the bridge-flipped isomer of this compound while concurrently beginning the synthesis and crystallization of analogous diimines based on glyoxal. Crystal structures of these derivatives will be compared to their bridge-flipped analogues, the corresponding diimines based on hydrazine. Samples are now in preparation for submission for X-ray crystal structure analysis.

Toward isomorphous “bridge-flipped” isomers:  Investigations of 4-pyridyl and glyoxal-based systems and the crystal structure of a pyridyl benzylideneaniline derivative

2019 Spring National Meeting

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