Direct observation of deuterium migration in crystalline-state reaction bysingle-crystal neutron diffraction. II. 3-1 Photoisomerization of a cobaloxime complex

Single crystal neutron diffraction analysis of photo-exposed (3-cyanopropyl -d(2)(alpha,alpha))-[(R)-1-phenylethylamine-d(11)]bis(dimethylglyoximato-d( 14))cobalt(III) was carried out in order to clarify the mechanism of the cr ystalline-state photoisomerization of the 3-cyanopropyl group bonded to the Co atom in some cobaloxime complexes. Before irradiation the two H atoms b onded to the C-1 atom of the 3-cyanopropyl group were exchanged with the D atoms such as -CH2CH2CD2CN. On exposure to a xenon lamp, the cell dimension s of the crystal were gradually changed. After 7 d exposure the change beca me insignificantly small. The structure was analyzed by neutron diffraction . The 3-cyanopropyl group was transformed to the l-cyanopropyl group such a s -CD(CN)C-(H-1/2,D-1/2)(2)CH3 with retention of the single-crystal form. T his indicates that one of the D atoms bonded to C-1 migrates to either posi tion bonded to C-2. The other atoms of the complex remained unchanged. Thes e results indicate that photoisomerization proceeded in two steps: the 3-cy anopropyl group was isomerized to the 2-cyanopropyl group in the first plac e and then the 2-cyanopropyl group was transformed to the 1-cyanopropyl gro up. Moreover, it was made clear that the second-step isomerization was irre versible, since one of the D atoms was retained. The disordered structure a t C-2 is estimated to be caused by the interconversion between the 1-cyanop ropyl group produced and its dehydrogenated olefin after the photoisomeriza tion.