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 vitamin B 12 (cyanocobalamin), and its photochemical and electrochemical properties were investigated and compared to those of the cobester ( C -Co(II) ), the cobalt-corrin complex. The UV-vis absorptions of P -Co(II) in CH 2 Cl 2 , ascribed to the π-π* transition, were red-shifted compared to those of C -Co(II) due to the π-expansion of the macrocycle in the pyrocobester. The reversible redox couple of P -Co(II) was observed at E 1/2 = -0.30 V vs Ag/AgCl in CH 3 CN, which was assigned to the Co(II)/Co(I) redox couple by UV-vis, ESR, and molecular orbital analysis. This redox couple was positively shifted by 0.28 V compared to that of C -Co(II) . This is caused by the high electronegativity of the dehydrocorrin macrocycle, which was estimated by DFT calculations for the free-base ligands. The reactivity of the Co(I)-pyrocobester ( P -Co(I) ) was evaluated by the reaction with methyl iodide in CV and UV-vis to form a photosensitive Co(III)-CH 3 complex ( P -Co(III)-CH3 ). The properties of the excited state of P -Co(I) , *Co(I), were also investigated by femtosecond transient absorption (TA) spectroscopy. The lifetime of *Co(I) was estimated to be 29 ps from the kinetic trace at 587 nm. The lifetime of *Co(I) became shorter in the presence of Ar-X, such as iodobenzonitrile ( 1a ), bromobenzonitrile ( 1b ), and chlorobenzonitrile ( 1c ), and the rate constants of electron transfer (ET) between the *Co(I) and Ar-X were determined to be 2.9 × 10 11 M -1 s -1 , 4.9 × 10 10 M -1 s -1 , and 1.0 × 10 10 M -1 s -1 for 1a , 1b , and 1c , respectively.