Modified photoreactivity due to mixed crystal formation. II. Enhanced reactivity upon conformational mimicry and cavity enlargement

Abstract

The 2-cyanopropyl (beta-cyanopropyl) group in the cobaloxime complex of (2-cyanopropyl)(3-methylpyridine)bis(dimethylglyoximato)cobalt(III) takes a trans conformation around the Co-C-C-CN bond in the crystal and undergoes isomerization to the 1-cyanopropyl (alpha-cyanopropyl) group with a low reaction rate when the powdered crystals are irradiated with a xenon lamp. When the complex was mixed with (2-cyanoethyl)(3-methylpyridine)bis(dimethylglyoximato)cobalt(III) or (2-cyanoethyl)(3-ethylpyridine)bis(dimethylglyoximato)cobalt(III), a mixed crystal, Mix-I or Mix-II, was obtained. The cell parameters of Mix-I and Mix-II were considerably different. When the crystals of Mix-I and Mix-II were irradiated with the xenon lamp, the 2-cyanopropyl group and the 2-cyanoethyl group isomerized to the 1-cyanopropyl and 1-cyanoethyl groups, respectively, in the crystalline state as well as in the solid state. The isomerization rates of the 2-cyanopropyl and 2-cyanoethyl groups of the mixed crystals became significantly higher than the corresponding rates in the component crystals. For the 2-cyanopropyl group, the conformational change from trans to cis in the mixed crystals caused reaction rates to be enhanced, and the expanded volume of the reaction cavity in the mixed crystals increased the reactivity for the 2-cyanoethyl group, when compared with the reactivity of each component crystal. A quantitative discussion of the increased rate constants is presented, which is based on the crystal structures. The shapes of the reaction cavities for the reactive groups undergoing isomerization control the conformation and configuration of the produced 1-cyanoethyl and 1-cyanopropyl groups.

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

“…The most striking point, however, is the size increase in the unit cell volume by ∼308 Å 3 upon mixed crystal formation. There are some examples of mixed crystal formation despite vast differences in the unit cell parameters and molecular structures between the component pair, 6,20 but this seems to be the first observation revealing such a large unit-cell enlargement on mixed crystal formation.…”

“…13,17,18 Generally, in 2ce and 2cp complexes, about 40% of the reactants are converted to the products after irradiation of the sample in powdered form for about 2 h. However, in 4cb complexes, about a 40% conversion can be obtained after about 120 h of irradiation, under the same experimental conditions. 19 When the nonisomorphous 2ce and 2cp cobaloxime complexes in gauche and trans conformations, respectively, in their pure crystals, were cocrystallized to obtain the mixed crystals 20 to modify the photoreactivity, the reactivity of the 2cp cobaloxime in the mixed crystal was enhanced. X-ray structure analysis divulged a conformational change only in the 2cp group from trans to gauche in the mixed crystal form.…”

“…When the nonisomorphous 2ce and 2cp cobaloxime complexes in gauche and trans conformations, respectively, in their pure crystals, were cocrystallized to obtain the mixed crystals to modify the photoreactivity, the reactivity of the 2cp cobaloxime in the mixed crystal was enhanced. X-ray structure analysis divulged a conformational change only in the 2cp group from trans to gauche in the mixed crystal form.…”

Crystalline state photochromism of 3-furylfulgides: impact of size and bond flexibility of the non-aromatic alkylidene group

“…In fact, if there is more space around the reaction moiety of (4E) to twist the cyclohexyl group to become co-planar with the anhydride ring more easily, although the topochemical factors considered above are less favourable, it must have the ability to react faster. Further, it was reported that reactivity can be enhanced in the mixed crystal formation of cobaloxime complexes due to the enhancement of reaction cavity size in the crystalline state (Vithana et al, 2002).…”