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

A series of host-guest complex crystals has been formed between (2-cyanoeth yl)(isonicotinic acid)cobaloxime as a guest and dicyclohexylamine, cyclohep tylcyclohexylamine and cyclohexylcyclooctylamine as hosts. The X-ray analys es 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 gro up 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 crys tals were irradiated with a xenon lamp, the 2-cyanoethyl groups were isomer ized to the 1-cyanoethyl group. The reaction rate was estimated from the ch ange in the FT-IR spectra of the stretching vibration mode of the C-N tripl e bond. The reaction rates of the 2-cyanoethyl groups with gauche and trans conformations were estimated separately from the spectra. The rate constan t of the 2-cyanoethyl group with the gauche conformation is significantly g reater than that with the trans conformation in each crystal. The isomeriza tion rate was accelerated as the size of the cycloalkane ring increased. Th e reaction cavity for the 2-cyanoethyl group expands as the ring size incre ases. This suggests that it is possible to control the reaction rate if a s eries of the host-guest complexes are formed and the size of the host molec ule is systematically changed.