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

A series of cobalt(III)-peroxo complexes, [Co III (R 2 -TBDAP)(O 2 )] + ( 1 R2 ; R 2 = Cl, H, and OMe), and cobalt(III)-hydroperoxo complexes, [Co III (R 2 -TBDAP)(O 2 H)(CH 3 CN)] 2+ ( 2 R2 ), bearing electronically tuned tetraazamacrocyclic ligands (R 2 -TBDAP = N , N '-di- tert -butyl-2,11-diaza[3.3](2,6)- p -R 2 -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 R2 compounds have similar octahedral geometry with a side-on peroxocobalt(III) moiety, but the O-O bond lengths of 1 Cl [1.398(3) Å] and 1 OMe [1.401(4) Å] were shorter than that of 1 H [1.456(3) Å] due to the different spin states. For 2 R2 , the O-O bond vibration energies of 2 Cl and 2 OMe were identical at 853 cm -1 (856 cm -1 for 2 H ), but their Co-O bond vibration frequencies were observed at 572 cm -1 for 2 Cl and 550 cm -1 for 2 OMe , respectively, by resonance Raman spectroscopy (560 cm -1 for 2 H ). Interestingly, the redox potentials ( E 1/2 ) of 2 R2 increased in the order of 2 OMe (0.19 V) < 2 H (0.24 V) < 2 Cl (0.34 V) according to the electron richness of the R 2 -TBDAP ligands, but the oxygen-atom-transfer reactivities of 2 R2 showed a reverse trend ( k 2 : 2 Cl < 2 H < 2 OMe ) with a 13-fold rate enhancement at 2 OMe over 2 Cl 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 OMe in the unusual reaction pathway. These results provide considerable insight into the electronic nature-reactivity relationship of metal-oxygen species.