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

The effects of 10 paramagnetic metal complexes (Fe^IIIEDTA(H ₂O)^-, Fe^IIIEDTA(OH)^2-, Fe ^IIIPDTA^-, Fe^IIIDTPA^2-, Fe ^III₂O(TTHA)^2-, Fe^III(CN) ₆^3-, Mn^IIEDTA(H₂O)^2-, Mn^IIPDTA^2-, Mn^IIβ-EDDADP^2-, and Mn^IIPO₄^-) on F^- ion ¹⁹F NMR transverse relaxation rates (R₂ = 1/T₂) were studied in aqueous solutions as a function of temperature. Consistent with efficient relaxation requiring formation of a metal/F^- bond, only the substitution inert complexes Fe^III(CN)₆^3- and Fe^IIIEDTA(OH)^2- had no measured effect on T₂ relaxation of the F^{- 19}F resonance. For the remaining eight complexes, 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 (k_app,F-) spanned 5 orders of magnitude. In addition, we measured the rates at which O₂^•- reacts with Fe^IIIPDTA^-, Mn^IIEDTA(H₂O) ^2-, Mn^IIPDTA^2-, and Mn^IIβ- EDDADP^2- by pulse radiolysis. Although no intermediate is observed during the reduction of Fe^IIIPDTA^- by O₂ ^•-, each of the Mn^II complexes reacts with formation of a transient intermediate presumed to form via ligand exchange. These reactivity patterns are consistent with literature precedents for similar complexes. With these data, both k_app,O2- and k_app,F- are available for each of the eight reactive complexes. A plot of log(k _app,O2-) versus log(k_app,F-) for these eight showed a linear correlation with a slope ≈ 1. This correlation suggests that rapid metal/O₂^•2 reactions of these complexes occur via an inner-sphere mechanism whereas formation of an intermediate coordination complex limits the overall rate. This hypothesis is also supported by the very low rates at which the substitution inert complexes (Fe^III(CN) ₆^3- and Fe^IIIEDTA(OH)^2-) are reduced by O₂^•-. These results suggest that F^{- 19}F NMR relaxation can be used to predict the reactivities of other Fe^III complexes toward reduction by O₂^•-, a key step in the biological production of reactive oxygen species.