Acta Crystallographica Section B

Structural Science

Volume 57, Part 4 (August 2001)

research papers

Acta Cryst. (2001). B57, 551-559 [ doi:10.1107/S0108768101006838 ]

Direct observation of deuterium migration in crystalline-state reaction by single-crystal neutron diffraction. III. Photoracemization of 1-cyanoethyl cobaloxime complexes

T. Ohhara, H. Uekusa, Y. Ohashi, I. Tanaka, S. Kumazawa and N. Niimura

Abstract: The H atoms bonded to the chiral C atoms (stereogenic center) of the 1-cyanoethyl groups in two cobalt complexes, [(R)-1-cyanoethyl]bis(dimethylglyoximato)(pyridine)cobalt(III) (2) and [(R,S)-1-cyanoethyl]bis(dimethylglyoximato)(piperidine)cobalt(III) (3), were replaced with D atoms, such as Co-C*D(CH₃)CN. The crystals of the two cobalt complexes were irradiated with a xenon lamp for 72 h and 27 d, respectively. The unit-cell dimensions were gradually changed with retention of the single-crystal form. The crystal structures after irradiation were determined by neutron diffraction. In each crystal the chiral 1-cyanoethyl group of one of the two crystallographically independent molecules was partly inverted to the opposite configuration, whereas that of the other molecule kept the original configuration. The C*-D bond in the inverted group was completely conserved in the process of the inversion of the chiral alkyl group. This suggests that the inversion of the chiral 1-cyanoethyl group proceeds with the rotation of the cyanoethyl radical after the Co-C bond cleavage by photo-irradiation so that the opposite side of the radical faces the Co atom. This is followed by recombination of the Co-C bond to form the inverted 1-cyanoethyl group.

Keywords: deuterium migration; photoracemization; neutron diffraction.