Racemic-to-chiral transformation in cobaloxime complex crystals only by photoirradiation

The cobaloxime complex crystal with the racemic-1-cyanoethyl group and piperidine as axial ligands, which has a chiral space group of P2(1)2(1)2(1), changed its cell dimensions with retention of the single crystal form on exposure to visible light. The chloroform solution of the crystals after irradiation showed the optical rotation ([alpha](D) = 30 degrees). This indicates the racemic-to-chiral transformation occurred in the crystal only by photoirradiation. The crystal before irradiation, analyzed by X-rays, has two complexes with R- and S-1-cyanoethyl groups in the asymmetric unit. After irradiation, one of the complexes with the S group, which has a larger reaction cavity than the R group, was partly inverted to create a disordered structure. The R:S ratio in the crystal changed from 50:50 to 70:30. The final ratio of 70:30 is well explained by the symmetric shape of the reaction cavity after the change. For the cobaloxime complex crystal with the racemic-1-cyanoethyl group and pyrrolidine as axial ligands, a similar transformation was observed. The racemic crystal with the space group P1 has four complexes in a unit cell, two of which have a R-1-cyanoethyl group while the ether two complexes have a S-1-cyanoethyl group. The complexes with the S group, which have larger cavities than those with the R group, were partly inverted to create the opposite configuration. This caused optical enrichment in the crystal. Such a racemic-to-chiral transformation will be observed not rarely but commonly if the racemic crystals with chiral space groups will be examined cautiously.