The effects of 10 paramagnetic metal complexes (FeIIIEDTA(H 2O)-, FeIIIEDTA(OH)2-, Fe IIIPDTA-, FeIIIDTPA2-, Fe III2O(TTHA)2-, FeIII(CN) 63-, MnIIEDTA(H2O)2-, MnIIPDTA2-, MnIIβ-EDDADP2-, and MnIIPO4-) 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 FeIII(CN)63- and FeIIIEDTA(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 (kapp,F-) spanned 5 orders of magnitude. In addition, we measured the rates at which O2•- reacts with FeIIIPDTA-, MnIIEDTA(H2O) 2-, MnIIPDTA2-, and MnIIβ- EDDADP2- by pulse radiolysis. Although no intermediate is observed during the reduction of FeIIIPDTA- by O2 •-, each of the MnII 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 kapp,O2- and kapp,F- are available for each of the eight reactive complexes. A plot of log(k app,O2-) versus log(kapp,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 (FeIII(CN) 63- and FeIIIEDTA(OH)2-) are reduced by O2•-. These results suggest that F- 19F NMR relaxation can be used to predict the reactivities of other FeIII complexes toward reduction by O2•-, a key step in the biological production of reactive oxygen species.
ASJC Scopus subject areas
- Chemistry (all)
- Colloid and Surface Chemistry