Measured rates of fluoride/metal association correlate with rates of superoxide/metal reactions for Fe(III)EDTA(H2O)- and related complexes.
The effects of 10 paramagnetic metal complexes (Fe(III)EDTA(H2O)-, Fe(III)EDTA(OH)2-, Fe(III)PDTA-, Fe(III)DTPA2-, Fe(III)2O(TTHA)2-, Fe(III)(CN)6(3-), Mn(II)EDTA(H2O)2-, Mn(II)PDTA2-, Mn(II)beta-EDDADP2-, and Mn(II)PO4(-)) 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(III)EDTA(OH)2- had no measured effect on T2 relaxation of the F- 19F 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(III)PDTA-, Mn(II)EDTA(H2O)2-, Mn(II)PDTA2-, and Mn(II)beta-EDDADP2- by pulse radiolysis. Although no intermediate is observed during the reduction of Fe(III)PDTA- 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 approximately 1. This correlation suggests that rapid metal/O2*- 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(III)EDTA(OH)2-) are reduced by O2*-. These results suggest that F- 19F 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.