Effect of Macrocycles on the Photochemical and Electrochemical Properties of Cobalt-Dehydrocorrin Complex: Formation and Investigation of Co(I) Species

Co(II)-pyrocobester (P ( -Co(II) )), a dehydrocorrin complex, was semisynthesized from vitaminB(12) (cyanocobalamin), and its photochemical and electrochemicalproperties were investigated and compared to those of the cobester(C ( -Co(II) )), the cobalt-corrincomplex. The UV-vis absorptions of P ( -Co(II) ) in CH2Cl2, ascribedto the pi-pi* transition, were red-shifted comparedto those of C ( -Co(II) ) dueto the pi-expansion of the macrocycle in the pyrocobester. Thereversible redox couple of P ( -Co(II) ) was observed at E (1/2) = -0.30V vs Ag/AgCl in CH3CN, which was assignedto the Co(II)/Co(I) redox couple by UV-vis, ESR, and molecularorbital analysis. This redox couple was positively shifted by 0.28V compared to that of C ( -Co(II) ). This is caused by the high electronegativity of the dehydrocorrinmacrocycle, which was estimated by DFT calculations for the free-baseligands. The reactivity of the Co(I)-pyrocobester (P ( -Co(I) )) was evaluated by the reaction withmethyl iodide in CV and UV-vis to form a photosensitive Co(III)-CH3 complex (P ( -Co(III)-CH3 )). The properties of the excited state of P ( -Co(I) ), *Co(I), were also investigatedby femtosecond transient absorption (TA) spectroscopy. The lifetimeof *Co(I) was estimated to be 29 ps from the kinetic trace at 587nm. The lifetime of *Co(I) became shorter in the presence of Ar-X,such as iodobenzonitrile (1a), bromobenzonitrile (1b), and chlorobenzonitrile (1c), and the rateconstants of electron transfer (ET) between the *Co(I) and Ar-X weredetermined to be 2.9 x 10(11) M-1 s(-1), 4.9 x 10(10) M-1 s(-1), and 1.0 x 10(10) M-1 s(-1) for 1a, 1b, and 1c, respectively. Co(II)-pyrocobesterwas semisynthesized from natural vitaminB(12) via Co(III)-pyrocobester by TiO2 photoreduction.The redox behavior of the Co(II)-pyrocobester was investigated toconfirm the formation of Co(I)-pyrocobester, and the Co(I) speciesreacted with methyl iodide to form the Co(III)-CH3 complex.An excited-state analysis of Co(I) with Ar-X was also performed bytransient absorption spectroscopy.

Automated summary

A dehydrocorrin complex, Co(II)-pyrocobester (P ( -Co(II) )), was semisynthesized from cyanocobalamin and its photochemical and electrochemical properties were compared to those of the cobester (C ( -Co(II) )). The UV-vis absorptions of P ( -Co(II) ) in CH2Cl2 were red-shifted due to the pi-expansion of the macrocycle in the pyrocobester. The redox couple of P ( -Co(II) ) was positively shifted compared to that of C ( -Co(II) ), coinciding with the high electronegativity of the dehydrocorrin macrocycle. The reactivity of P ( -Co(I) ) was evaluated by its reaction with methyl iodide.