Solvent-Dependent Conformational Switching of <i>N</i>-Phenylhydroxamic Acid and Its Application in Crystal Engineering

N-Phenylbenzohydroxamic acid (3) showed solvent-dependent conformational alteration. Thus, compound 3 exists predominantly in cis form (> 98%) in CD2Cl2, whereas the percentage of the cis isomer decreased to 49% in methanol-d4, and the trans conformer was major (77%) in acetone-d6. Solvent-dependent alteration of the amide conformation was observed only in the hydroxamic acids, and the ratio of the cis conformer (98% in CD2Cl2) of N-phenylbenzhydrazide (6) was little affected by the solvent. Two unique crystal structures were obtained depending on the recrystallization solvent, and each crystal corresponded in structure to the major conformer in the solvent. These two crystals of 3 were distinguished by IR spectroscopy and DSC analysis. Although the origin of the solvent-dependent conformational switching of 3 is unclear, the results could be applied to design and development of solvent- or external-stimuli-responsive molecular machines.