Evidence for Cu-O2 intermediates in superoxide oxidations by biomimetic copper(II) complexes

The mechanism by which [Cu-II(L)](OTf)(2) and [(CuN3)-N-II(L)](OTf) (L = TEPA: tris(2-pyridyl ethyl) amine or TMPA: tris(2-pyridylmethyl)amine; OTf = trifluoromethanesulfonate) react with superoxide (O-2 center dot(-)) to form [Cu-I(L)](OTf) and O-2 is described. Evidence for a CuO2 intermediate is presented based on stopped-flow experiments and competitive oxygen (180) kinetic isotope effects on the bimolecular reactions of O-16,16(2)center dot- and O-18,16(2)center dot(-) ((16,16)k/(18,16)k). The (16,16)k/(8,16)k fall within a narrow range from 0.9836 +/- 0.0043 to 0.9886 +/- 0.0078 for reactions of copper(II) complexes with different coordination geometries and redox potentials that span a 0.67 V range. The results are inconsistent with a mechanism that involves either rate-determining O-2 center dot(-) binding or one-step electron transfer. Rather a mechanism involving formation of a CuO2 intermediate prior to the loss Of O-2 in the rate-determining step is proposed. Calculations of similar inverse isotope effects, using stretching frequencies Of CuO2 adducts generated from copper(1) complexes and O-2, suggest that the intermediate has a superoxo structure. The use of 180 isotope effects to relate activated oxygen intermediates in enzymes to those derived from inorganic compounds is discussed.