Modified Photoreactivity Due to Mixed Crystal Formation. III. Decelerated Photoisomerization upon Topochemically Unfavorable Conformational Change

The cobaloxime complexes of (2-cyanoethyl)(aniline)dimethylglyoximatocobalt(III) and (2-cyanopropyl)(aniline)dimethylglyoximatocobalt(III) formed mixed crystals with (4-cyanobutyl)(aniline)dimethylglyoximatocobalt(III), Mix-I and Mix-II, respectively, although the component molecules have considerably different structures. The
conformations of Co−C−C−CN of the 2-cyanoethyl and 2-cyanopropyl groups in the respective pure crystals are
gauche, but they changed to trans in the mixed crystals. Because the conformations of Co−C−C−C and C−C−C−CN of the 4-cyanobutyl group are trans in both the pure crystal and the mixed crystals, structural mimicry or
conformational adoption occurred at the Co−C−C−CN group of each minor component in the mixed crystal formation.
The 2-cyanoethyl and 2-cyanopropyl groups of the cobaloxime complexes were isomerized to 1-cyanoethyl and
1-cyanopropyl groups on exposure to a xenon lamp. The isomerization rates were estimated from the changes of the
FT/IR spectra. The rate constants of the two groups in both pure crystals and mixed crystals were calculated assuming
first-order kinetics. Since the conformations of the reactive 2-cyanoethyl and 2-cyanopropyl groups were changed to
topochemically unfavorable trans conformations on mixed crystal formation, the rate constants of isomerization of
the 2-cyanoethyl and 2-cyanopropyl groups in the Mix-I and Mix-II crystals, were reduced to about one-third and
one-fourth of the original values, respectively, compared to the corresponding rates in the pure crystals.