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

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