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

A series of cobalt(III)-peroxo complexes, [Co^III(R₂-TBDAP)(O₂)]⁺ (1; R₂ = Cl, H, and OMe), and cobalt(III)-hydroperoxo complexes, [Co^III(R₂-TBDAP)(O₂H)(CH₃CN)]²⁺ (2), 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 1 compounds have similar octahedral geometry with a side-on peroxocobalt(III) moiety, but the O-O bond lengths of 1 [1.398(3) Å] and 1 [1.401(4) Å] were shorter than that of 1 [1.456(3) Å] due to the different spin states. For 2, the O-O bond vibration energies of 2 and 2 were identical at 853 cm^-1 (856 cm^-1 for 2), but their Co-O bond vibration frequencies were observed at 572 cm^-1 for 2 and 550 cm^-1 for 2, respectively, by resonance Raman spectroscopy (560 cm^-1 for 2). Interestingly, the redox potentials (E_1/2) of 2 increased in the order of 2 (0.19 V) < 2 (0.24 V) < 2 (0.34 V) according to the electron richness of the R₂-TBDAP ligands, but the oxygen-atom-transfer reactivities of 2 showed a reverse trend (k₂: 2 < 2 < 2) with a 13-fold rate enhancement at 2 over 2 in a sulfoxidation reaction with thioanisole. Although the reactivity trend contradicts the general consideration that electron-rich metal-oxygen species with low E_1/2 values have sluggish electrophilic reactivity, this could be explained by a weak Co-O bond vibration of 2 in the unusual reaction pathway. These results provide considerable insight into the electronic nature-reactivity relationship of metal-oxygen species.