Benzonitrile oxides (Ar-C≡N+-O- where Ar = Aryl) have long been known to be useful in solution-phase 1,3-dipolar cycloaddition reactions forming heterocyclic rings, but the solid-state chemistry of the benzonitrile oxides themselves, as well as that of their dimers, has been understudied. While examining selected dimers by single-crystal X-ray diffraction, we have now determined the crystal structure of a compound that we unexpectedly obtained while attempting to prepare and crystallize one of the dimers of 4-nitrobenzonitrile oxide, 3,5-bis(4-nitrophenyl)-1,2,4-oxadiazole-N-oxide, by solution-phase dimerization of the parent benzonitrile oxide in the presence of a catalytic amount of triethylamine. The compound actually obtained proved to be simply 3,5-bis(4-nitrophenyl)-1,2,4-oxadiazole, not its N-oxide. Previous workers have established that the deoxygenated product can form upon reaction of the initially formed oxadiazole-N-oxide with the reactant benzonitrile oxide. As part of our concurrent interest in the crystal structures of strict isosteres, chemically different molecules possessing a close atom-for-atom correspondence with respect to their atomic coordinates and their van der Waals radii, we are comparing our structure to those of its corresponding strict isosteres in the crystallographic literature. Our structure has proved to be isomorphous with the ambient-pressure polymorph (one of four occurring at different pressures) of 2,5-bis(4-nitrophenyl)-1,3,4-oxadiazole (NAGHOT01 in the Cambridge Structural Database). Both structures occur in space group Pbcn, with molecules of the published 1,3,4-oxadiazole located on crystallographic twofold axes and molecules of our 1,2,4-oxadiazole disordered about the corresponding crystallographic twofold axes. This is in notable contrast to another pair of strictly isosteric five-membered-ring heterocycles, 3,4-diphenyl-1,2,5-oxadiazole and 1,5-diphenyl-1H-tetrazole (ZZZTQC/ZZZTQC01 and MOYLAP in the CSD), which occupy general positions and assume different molecular packing arrangements in their respective crystals. Why isomorphism is observed in one case and not the other (as yet) is the subject of our ongoing studies.

Isomorphism in heteropentacycles:  Crystal structure of a 1,2,4-oxadiazole and comparison to its isosteric analogues

255th National Meeting (2018)

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