Benzonitrile oxides bear a fulminate group (C≡N+_O-) on a benzene ring. Because nitrile oxides are useful in 1,3-dipolar cycloadditions, their solution chemistry has been studied extensively, including their potential dimerization into a furoxan (in the absence of added catalyst), a dioxadiazine (in the presence of added pyridine), or a 1,2,4-oxadiazole N-oxide (in the presence of added triethylamine); however, their solid-state chemistry has not yet been investigated in detail. We are investigating the crystal chemistry of halogenated benzonitrile oxides, motivated by the published observation that the solid-state stability of these compounds is strongly dependent on the ring substitution pattern; 2-chlorobenzonitrile oxide and 4-chlorobenzonitrile oxide possess reported stabilities of 3-6 days and 10 days, respectively, while the reported stability of 3-chlorobenzonitrile oxide is only 50-60 minutes. As part of our efforts to elucidate the role of molecular packing in the enhanced reactivity of the 3-chloro isomer and to determine the product(s) of solid-state dimerization, we have now synthesized the bis(3-chlorophenyl)furoxan from solution and have determined its crystal structure. Unlike the crystal structures of many symmetrically substituted furoxans, this structure shows little evidence of twofold disorder about the approximate molecular twofold axis, refining to R = 0.026 for all data without additional modeling. No chlorine…chlorine approaches closer than the sum of the van der Waals radii are observed in this structure. If halogen…halogen contacts play a significant role in the packing arrangement of the parent nitrile oxide, and if the crystal structure we have obtained for this furoxan is the same as that formed as the result of solid-state dimerization, it appears that halogen…halogen contacts are not so significant to the course of solid-state dimerization that they must be maintained in the final solid-state product. Future work will focus on determining the crystal structure of the parent 3-chlorobenzonitrile oxide and its remaining two dimers.

Solid-state structures and reactivity of halogenated benzonitrile oxides: Crystal structure of bis(3-chlorophenyl)furoxan

253rd National Meeting (2017)

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