Formation and characterization of Co(III)-semiquinonate phenoxyl radical species

Co(III) complexes of N ₃O-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)CI] (1) and [Co(tbuL(Mepy) ₂)Cl] (2), and ternary Co(III) complexes, [Co(tbuL)-(acac)]ClO ₄ (3), [Co(tbuL)(tbu-cat)] (4), and [Co(tbuL(Mepy) ₂)(tbu-SQ)]ClO ₄ (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-N ₃O ₃ donor set, while 4 has a mer-N ₃O ₃ structure. The cyclic voltammogram (CV) of complex 3 exhibited one reversible redox wave centered at 0.93 V (vs Ag/AgCl) in CH ₃CN. 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 E _1/2 = 0.01 and E _pa = 0.92 V for 4 and E _1/2a = 0.05 and E _1/2b = 0.69 V for 5. The rest potential of 4 (-0.11 V) was lower than the E _1/2 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 ν _8a 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.