A New Photoisomerization Process of the 4-Cyanobutyl Group in a Cobaloxime Complex Crystal Observed by Neutron Diffraction

Abstract

The 4-cyanobutyl group of (4-cyanobutyl) [3,4-lutidine](dimethylglyoximato) [O-(diphenylboryl)dimethylglyoximato]cobalt(III) was isomerized to the 1-cyanobutyl group with retention of the single-crystal form, although the corresponding cobaloxime complex without diphenylboryl in the equatorial ligands did not show such a photoisomerization. The trans-cis-cis conformation of the 4-cyanobutyl group caused by the steric repulsion from the diphenylboryl group is favorable to the crystalline-state reaction. In order to elucidate the detailed isomerization mechanism, two hydrogen atoms of the 4-cyanobutyl group were replaced with the deuterium atoms such as -CH 2 CH 2 CH 2 CD 2 CN. After the photoisomerization, the crystal structure with deuterated 4-cyanobutyl group was analyzed by neutron diffraction. Only one of the two deuterium atoms of the 4-cyanobutyl group was transferred to the C 1 atom such as -CD(CN)CH 2 CH 2 CDH 2 . This result made clear that the photo-produced cyanobutyl radical turned upside down after homolytic cleavage of the Co-C bond, and then the Co(II) atom and the radical made a bond to form the 1-cyanobutyl group.

Dynamic motion and various reaction paths of cobaloxime complexes in crystalline-state photoreaction

“…One of the D atoms is D4 0 bonded to the chiral carbon, C4 0 , and another one is distributed in the methyl hydrogen atoms with the occupancy of 1/3. Reproduced from Hosoya et al [152] with permission from The Chemical Society of Japan.…”

“…Since neutron diffraction can discriminate between deuterium and hydrogen atoms, it may be clarified which hydrogen atoms are replaced with deuterium atoms during the reaction. The crystal of III of size of 4.5 Â 1.5 Â 1.4 (ca 9.5) mm 3 was exposed to a xenon lamp for 2 days at 273 K. The wavelengths were selected around 700 nm using a pair of filters [152]. A small part of the crystal after photoirradiation was cut from the mother crystal and the structure was analysed by X-rays.…”

“…The conformation of the 4-cb group is different from those of I and II. Reproduced from Hosoya et al [152] with permission from The Chemical Society of Japan.…”

“…Reaction pathway from 4-to 1-cb in the reaction cavity for the initial 4-cb group. Reproduced from Hosoya et al [152] with permission from The Chemical Society of Japan.…”

Measurements of small organic molecules on the single crystal neutron diffractmeters for biomolecules at JAERI

“…The measurement time was 1 week for either cases. In those measurements, even though the d-minimum values, 0.87 for 1 and 1.00 for 2, respectively, were relatively larger because of the highly disordered structure of the reactive part, the migration of the deuterium atoms according to the photo-isomerization were clearly observed and the reaction mechanisms were elucidated [2,7]. The sample 3 is a self-assembled coordination cage of palladium complex including water cluster.…”