Photoisomerization of (2-cyanoethyl)(isonicotinic acid)cobaloxime complex in a series of isostructural host–guest complex crystals

Abstract

A series of host–guest complex crystals has been formed between (2-cyanoethyl)(isonicotinic acid)cobaloxime as a guest and dicyclohexylamine, cycloheptylcyclohexylamine and cyclohexylcyclooctylamine as hosts. The X-ray analyses revealed that the three crystals are isostructural to each other. There are two crystallographically independent molecules in an asymmetric unit of each crystal. One of the cobaloxime molecules, A, has the 2-cyanoethyl group with gauche conformation around the central C–C bond while that of the B molecule has the trans conformation. When the powdered samples of the crystals were irradiated with a xenon lamp, the 2-cyanoethyl groups were isomerized to the 1-cyanoethyl group. The reaction rate was estimated from the change in the FT-IR spectra of the stretching vibration mode of the C–N triple bond. The reaction rates of the 2-cyanoethyl groups with gauche and trans conformations were estimated separately from the spectra. The rate constant of the 2-cyanoethyl group with the gauche conformation is significantly greater than that with the trans conformation in each crystal. The isomerization rate was accelerated as the size of the cycloalkane ring increased. The reaction cavity for the 2-cyanoethyl group expands as the ring size increases. This suggests that it is possible to control the reaction rate if a series of the host–guest complexes are formed and the size of the host molecule is systematically changed.