Measured rates of fluoride / metal association correlate with rates of superoxide / metal reactions for FeIIIEDTA(H2O)- and related complexes

Complete text of publication follows. The effects of ten paramagnetic metal complexes (FeIIIEDTA(H2O)- , FeIIIEDTA(OH)2-, FeIIIPDTA- , FeIIIDTPA2-, FeIII2O(TTHA)2-, FeIII(CN)63-, MnIIEDTA(H2O)2-, MnIIPDTA2-, MnII-EDDADP2-, and MnIIPO4-) on F- ion 19F NMR transverse relaxation rates (R2 = 1/T2) were studied in aqueous solutions as a function of temperature. The kinetic parameters (apparent second order rate constants and activation enthalpies) for metal / F- association were determined from the dependence of the observed relaxation enhancements on complex concentration and temperature. Apparent metal / F- association rate constants for these complexes (kapp,F- ) spanned five orders of magnitude. In addition, we measured the rates at which O2·- reacts with FeIIIPDTA- , MnIIEDTA(H2O)2-, MnIIPDTA2-, and MnII-EDDADP2- by pulse radiolysis. While FeIIIPDTA- is directly reduced by O2·- to the corresponding Fe2+ complex, each of the Mn2+ complexes reacted with formation of an intermediate complex, presumably having a Mn2+-O2· bond. These reactivity patterns are consistent with literature data for similar complexes. With this data both kapp,O2- and kapp,F- available for each of the eight reactive complexes. A plot of log(kapp,O2- ) versus log(kapp,F- ) for these eight showed a linear correlation with a slope ∼1. This correlation suggests that rapid metal / O2·- reactions of these complexes occur via an inner-sphere mechanism where formation of an intermediate coordination complex limits the overall rate.