Formation and Characterization of Co(III)−Semiquinonate Phenoxyl Radical Species

Co(III) complexes of N3O-donor tripodal ligands, 2,4-di(tert-butyl)-6-{[bis(2-pyridyl)methyl]aminomethyl}phenolate
(tbuL), 2,4-di(tert-butyl)-6-{[bis(6-methyl-2-pyridyl)methyl]aminomethyl}phenolate (tbuL(Mepy)2), were prepared, and
precursor Co(II) complexes, [Co(tbuL)Cl] (1) and [Co(tbuL(Mepy)2)Cl] (2), and ternary Co(III) complexes, [Co(tbuL)(acac)]ClO4 (3), [Co(tbuL)(tbu-cat)] (4), and [Co(tbuL(Mepy)2)(tbu-SQ)]ClO4 (5), where acac, tbu-cat, and tbu-SQ
refer to pentane-2,4-dionate, 3,5-di(tert-butyl)catecholate, and 3,5-di(tert-butyl)semiquinonate, respectively, were
structurally characterized by the X-ray diffraction method. Complexes 3 and 5 have a mononuclear structure with
a fac-N3O3 donor set, while 4 has a mer-N3O3 structure. The cyclic voltammogram (CV) of complex 3 exhibited one
reversible redox wave centered at 0.93 V (vs Ag/AgCl) in CH3CN. Complex 5 was converted to a phenoxyl radical
species upon oxidation with Ce(IV), showing a characteristic π−π* transition band at 412 nm. The ESR spectrum
at low temperature and the resonance Raman spectrum of 3 established that the radical species has a Co(III)−phenoxyl radical bond. On the other hand, the CVs showed two oxidation processes at E1/2 = 0.01 and Epa =
0.92 V for 4 and E1/2a = 0.05 and E1/2b = 0.69 V for 5. The rest potential of 4 (−0.11 V) was lower than the E1/2
value, whereas that of 5 (0.18 V) was higher, indicating that the first redox wave of 4 and 5 is assigned to the
tbu-cat and the tbu-SQ redox process, respectively. One-electron oxidized 4 showed absorption, resonance Raman,
and ESR spectra which are similar to those of 5, suggesting formation of a stable Co(III)−semiquinonate species,
which has the same oxidation level of 5. The resonance Raman spectrum of two-electron oxidized 4 showed the
ν8a bands of the semiquinonate and phenoxyl radical, which were absent in the spectrum of one-electron oxidized
5. Since both oxidized species were ESR inactive at 5 K, the former was concluded to be a biradical species
containing semiquinonate and phenoxyl radicals coupled antiferromagnetically and the latter to a species with a
coordinated quinone.