A continuous wave and pulse EPR and ENDOR investigation of oxygenated Co(II) corrin complexes

Heptamethyl cobyrinate perchlorate, [Cob(II)ester]ClO4, has the relevant st ructural features of base-off B-12r, the reduced Co(II) form of vitamin B-1 2. The reversible oxygenation behavior of this complex in different solvent s is investigated using continuous wave (cw) EPR at X-band and compared wit h that of Co(EF) porphyrin complexes. Furthermore, the influence of the add ition of a nitrogen base (pyridine or 1-methylimidazole) to the solutions i s investigated. To determine the electronic structure of the oxygenated com plexes, different pulse electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) techniques are applied. The g and cobalt h yperfine matrix and their principal axes are determined using a combination of cw-EPR at X- and Q-band. ESE (electron spin-echo)-detected EPR at W-ban d and Davies-ENDOR at Q-band. The experimental g and cobalt hyperfine value s are found to be sensitive to the change of solvent, addition of a nitroge n base, and change in the ring structure. From the HYSCORE (hyperfine suble vel correlation) spectra measured at X-band, the interactions with the corr in nitrogen nuclei and the nitrogens of the axial base are deduced. Compari son of these data with those of oxygenated base-on Co(II) porphyrin complex es revealed shorter axial cobalt-nitrogen bonds in the Co(II) corrin than i n the porphyrin case. On the other hand, the nitrogen atoms of the corrin a nd of the porphyrin ligands show similar, small interactions, which is due to the fact that the unpaired electron resides mainly on the O-2 fragment.