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

Abstract

Single crystal neutron diffraction analysis of photo-exposed (3-cyanopropyl-d2(alpha,alpha))-[(R)-1-phenylethylamine-d11]bis(dimethylglyoximato-d14)cobalt(III) was carried out in order to clarify the mechanism of the crystalline-state photoisomerization of the 3-cyanopropyl group bonded to the Co atom in some cobaloxime complexes. Before irradiation the two H atoms bonded to the C1 atom of the 3-cyanopropyl group were exchanged with the D atoms such as --CH2CH2CD2CN. On exposure to a xenon lamp, the cell dimensions of the crystal were gradually changed. After 7 d exposure the change became insignificantly small. The structure was analyzed by neutron diffraction. The 3-cyanopropyl group was transformed to the 1-cyanopropyl group such as --CD(CN)C(H1/2,D1/2)2CH3 with retention of the single-crystal form. This indicates that one of the D atoms bonded to C1 migrates to either position bonded to C2. The other atoms of the complex remained unchanged. These results indicate that photoisomerization proceeded in two steps: the 3-cyanopropyl group was isomerized to the 2-cyanopropyl group in the first place and then the 2-cyanopropyl group was transformed to the 1-cyanopropyl group. Moreover, it was made clear that the second-step isomerization was irreversible, since one of the D atoms was retained. The disordered structure at C2 is estimated to be caused by the interconversion between the 1-cyanopropyl group produced and its dehydrogenated olefin after the photoisomerization.

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

“…Molecular structure of the S-1-cp with deuterium atoms after photoirradiation. Reproduced from Ohhara et al [122] with permission from the International Union of Crystallography. Scheme 14.…”

“…In order to make clear the mechanism, the crystal structures of the deuterium complex shown in the Scheme 13 before and after photoirradiation were analysed by neutron diffraction [122]. To reduce the background due to the incoherent scattering from H atoms in the neutron diffraction, all the H atoms of the equatorial ligands and axial base ligand were replaced with D atoms.…”

Photocrystallography

“…X-ray diffraction studies have concentrated on initial-state (before irradiation) and final-state (after irradiation) crystallography. Studies have involved proton transfer (Ohhara et al, 2000), formation of a heterocyclic ring (Takayama et al, 2003), asymmetry generation and chiral inversion (Sekine et al, 1997), formation of a radical pair (Kawano et al, 2002), ligand substitution in organometallic compounds (Sato et al, 2007), excited-state formation . Separately, Kawano and coworkers have also described photochemical reactions that result in the formation of heterocyclic rings (Furusawa et al, 2007), organometallic catenation (Yamashita et al, 2007) and olefin dimerization (Takaoka et al, 2006).…”

Direct Evidence of Solid‐State Tautomerism by Diffraction Methods: Isomers, Equilibria, and Kinetics

“…Provided these conditions are met, neutron diffraction can provide direct evidence and accurate positions of the proton (deuteron). As an example, neutron diffraction was employed to monitor the migration of the deuterium atom during the photoisomerization of a cobaloxime complex [24]. Neutron diffraction was also used to observe the migration along the strong O-H· · ·O hydrogen bond in ureaphosphoric acid determined at temperatures of 150-335 K by using the multiple single-crystal technique [25].…”