Benzonitrile oxides dimerize in solution to yield three possible products, depending on the reaction conditions: a furoxan, a dioxadiazine, or a 1,2,4-oxadiazole-N-oxide. We are using single-crystal X-ray diffraction to examine the solid-state chemistry of variously substituted benzonitrile oxides in order to determine how the crystal structure of a given benzonitrile oxide might influence the outcome of solid-state dimerization. Our concurrent interest in comparing the crystal structures of chemically different but sterically similar molecules has led us to examine a set of bis(4-substituted phenyl)furoxans. In previous work we determined the crystal structures of two polymorphs of bis(4-chlorophenyl)furoxan (an orthorhombic form, space group Pbcn, and a monoclinic form, space group C2/c) and the crystal structure of bis(4-methylphenyl) furoxan (a monoclinic form, space group P21/c). The chloro furoxan undergoes a solid-state phase transition from the orthorhombic form to the monoclinic form on heating. The methyl compound undergoes a solid-state phase transition to a new, high-temperature polymorph which we have yet to characterize fully. We have meanwhile obtained a new polymorph of the methyl furoxan independently by crystallization from solution and have now determined its crystal structure to be orthorhombic, space group Pca21. We are currently determining whether this polymorph obtained from solution is the same as the one obtained by heating the monoclinic polymorph and whether the transformation occurs through an intermediate liquid phase involving loss of order in the molecular packing. Although the chloro substituents are sterically similar to the methyl substituents, none of the four molecular packing arrangements are identical. On the other hand, a definite packing similarity along one direction exists between the thermally unstable polymorphs, the orthorhombic chloro structure and the monoclinic methyl structure. We are currently attempting to co-crystallize the chloro furoxan with the methyl furoxan to examine the solid-state transformation behavior of the resulting solid solution.

Molecular structure similarities vs. crystal structure similarities: bis(4-Chlorophenyl)furoxan vs. bis(4-methylphenyl)furoxan

255th National Meeting (2018)

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