Systematic Electronic Tuning on the Property and Reactivity of Cobalt–(Hydro)peroxo Intermediates

A series of cobalt(III)-peroxo complexes, [CoIII(R2-TBDAP)(O2)]+ (1R2; R2 = Cl, H, and OMe), and cobalt(III)-hydroperoxo complexes, [CoIII(R2-TBDAP)(O2H)(CH3CN)]2+ (2R2), bearing electronically tuned tetraazamacrocyclic ligands (R2-TBDAP = N,N'-di-tert-butyl-2,11-diaza[3.3](2,6)-p-R2-pyridinophane) were prepared from their cobalt(II) precursors and characterized by various physicochemical methods. The X-ray diffraction and spectroscopic analyses unambiguously showed that all 1R2 compounds have similar octahedral geometry with a side-on peroxocobalt(III) moiety, but the O-O bond lengths of 1Cl [1.398(3) Å] and 1OMe [1.401(4) Å] were shorter than that of 1H [1.456(3) Å] due to the different spin states. For 2R2, the O-O bond vibration energies of 2Cl and 2OMe were identical at 853 cm-1 (856 cm-1 for 2H), but their Co-O bond vibration frequencies were observed at 572 cm-1 for 2Cl and 550 cm-1 for 2OMe, respectively, by resonance Raman spectroscopy (560 cm-1 for 2H). Interestingly, the redox potentials (E1/2) of 2R2 increased in the order of 2OMe (0.19 V) < 2H (0.24 V) < 2Cl (0.34 V) according to the electron richness of the R2-TBDAP ligands, but the oxygen-atom-transfer reactivities of 2R2 showed a reverse trend (k2: 2Cl < 2H < 2OMe) with a 13-fold rate enhancement at 2OMe over 2Cl in a sulfoxidation reaction with thioanisole. Although the reactivity trend contradicts the general consideration that electron-rich metal-oxygen species with low E1/2 values have sluggish electrophilic reactivity, this could be explained by a weak Co-O bond vibration of 2OMe in the unusual reaction pathway. These results provide considerable insight into the electronic nature-reactivity relationship of metal-oxygen species.