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Dependence of the Fe II/III EDTA complex on pH

Dependence of the Fe II/III EDTA complex on pH. Ryan Hutcheson and I. Francis Cheng* Department of Chemistry, University of Idaho Moscow, ID 83844 ifcheng@uidaho.edu. Importance. First study of the pH dependence of Fe II/III EDTA

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Dependence of the Fe II/III EDTA complex on pH

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  1. Dependence of the FeII/IIIEDTA complex on pH Ryan Hutcheson and I. Francis Cheng* Department of Chemistry, University of Idaho Moscow, ID 83844 ifcheng@uidaho.edu Ryan Hutcheson University of Idaho

  2. Importance • First study of the pH dependence of FeII/IIIEDTA • Green chemistry – optimization of O2 activation and pH dependence of the Fenton Reaction • Antioxidants : FeII/IIIEDTA is a good model for low molecular weight biological ligands Ryan Hutcheson University of Idaho

  3. FeIIIEDTA Speciation Diagram FeIIIEDTA FeIII(OH)2EDTA FeIII(OH)EDTA FeIIIHEDTA Ryan Hutcheson University of Idaho

  4. FeIIEDTA Speciation Diagram FeIIEDTA FeII(OH)2EDTA Free Fe+2 FeII(OH)EDTA FeIIHEDTA FeIIH2EDTA Ryan Hutcheson University of Idaho

  5. Electrocatalytic (EC’) Mechanismand Cyclic Voltammetry FeIII-L + e-  FeII-L FeII-L +H2O2 FeIII-L OH• +OH- E: O + ne- = R C’: R + Z = O + Y Regeneration of the FeIIIEDTA within the vicinity of the electrode causes amplification of the CV wave Ryan Hutcheson University of Idaho

  6. Conditions • All scans • 10mL aqueous sol’n purged w/ N2 for 10-15min • 0.1M Buffer - HOAcCl, HOAc, HEPES • 5mV/s sweep rate • BAS carbon disk electrode • BAS Ag/AgCl reference electrode • Spectroscopic graphite rod counter electrode • BAS CV-50w potentiostat • Cyclic Voltammetric scans of FeIIIEDTA • 1mM FeIIIEDTA • Catalytic scans (Fenton Reaction) • 0.1mM FeIIIEDTA catalytic scans • 20mM H2O2 Ryan Hutcheson University of Idaho

  7. Cyclic Voltammagrams of FeII/IIIEDTA 1mM FeIIIEDTA 0.1M buffer 5mV/s scan rate FeIIIEDTA + e-→ FeIIEDTA pH 5.5 FeIIIEDTA + e- ← FeIIEDTA pH 2 pH 11 Ryan Hutcheson University of Idaho

  8. E1/2 vs. pH (FeIIIEDTA) FeIII(OH)2EDTA FeIIIEDTA FeIII(OH)EDTA E1/2 FeIIIHEDTA Ryan Hutcheson University of Idaho

  9. E1/2 vs. pH (FeIIEDTA) FeII(OH)2EDTA FeIIEDTA Free Fe+2 FeII(OH)EDTA E1/2 FeIIHEDTA FeIIH2EDTA Ryan Hutcheson University of Idaho

  10. O2 Activation • First example of abiotic RTP oxygen activation able to destructively oxidize organics. • Oxygen activation is pH dependent. Noradoun,C., Industrial and Engineering Chemistry Research, (2003), 42(21), 5024-5030. Ryan Hutcheson University of Idaho

  11. Reaction Vessel Air flow 2.0 mL 50/50 hexane/ethyl acetate (extraction only) 10.0 mL water pH 5.5 – 6.5, unbuffered. 0.44mM EDTA 0.44mM Xenobiotic Stir bar 0.5g Fe; 20 or 40-70 mesh Noradoun,C., Industrial and Engineering Chemistry Research, (2003), 42(21), 5024-5030. Ryan Hutcheson University of Idaho

  12. Xenobiotic Oxidation Studies H2O2 O2 + 2H+ + EDTA Iron particles 0.1-1 mm FeIIEDTA Fe2+ FeIIIEDTA + HO- + HO. Aqueous Xenobiotic LMW acids Noradoun,C., Industrial and Engineering Chemistry Research, (2003), 42(21), 5024-5030. Ryan Hutcheson University of Idaho

  13. Proposed O2 Reduction Mechanism by Van Eldik FeIIEDTAH(H2O) + O2 FeIIEDTAH(O2) + H2O FeIIEDTAH(O2)  FeIIIEDTAH(O2-) FeIIIEDTAH(O2-) + FeIIEDTAH(H2O)  FeIIIEDTAH(O22-)FeIIIEDTAH + H2O FeIIIEDTAH(O22-)FeIIIEDTAH + H2O + 2H+ 2FeIIIEDTAH(H2O) + H2O2 2FeIIEDTAH(H2O) + H2O2 2FeIIIEDTAH(H2O) + H2O *Proposes H2O2 as intermediate *Saw no evidence of H2O2 Van Eldik, R. Inorg. Chem, 1997, 36, 4115-4120 Ryan Hutcheson University of Idaho

  14. Van Eldik’s O2 Reduction Van Eldik, R. Inorg. Chem, 1997, 36, 4115-4120 Ryan Hutcheson University of Idaho

  15. Structures FeIIEDTA FeIIHEDTA CN = 7 FeIIIEDTA (CN = 7) FeIIIHEDTA (CN = 6) Monocapped trigonal prismatic (MCP) Pentagonal-bipyramidal (PB) Octahedral O Square Pyramidal N N O O O Miyoshi, K., Inor. Chem. Acta., 1995, 230, 119-125. Heinemann, F.W., Inor. Chem. Acta., 2002, 337, 317-327. Ryan Hutcheson University of Idaho

  16. Structures cont’d FeIIEDTA FeIIHEDTA > pH 4 pH 3 – pH 4 MCP PB Active site Active site Free Fe+2 < pH 3 Miyoshi, K., Inor. Chem. Acta., 1995, 230, 119-125. Ryan Hutcheson University of Idaho

  17. Fenton Reaction FeIIIL +e-→ FeIIL E°’=depends on ligand H2O2 + e- → HO• + OH- E°=0.32V SHE @pH 7 FeIIL + H2O2 → FeIIIL + HO• + OH- Only iron complexes with E0’ negative of 0.32 V are thermodynamically capable of hydrogen peroxide reduction. However, Fenton inactivity may result from kinetic factors as well. Ryan Hutcheson University of Idaho

  18. Electrocatalytic CV 0.1mM FeIIIEDTA 20mM H202 0.1M buffer 5mV/s scan rate pH 4 pH 3.5 FeIIIEDTA + e-→ FeIIEDTA pH 4 pH 4.5 pH 3 pH 2.5 pH 2 Ryan Hutcheson University of Idaho

  19. Fenton Reactivity vs. pH Free Fe+2 FeIIEDTA FeIIHEDTA FeIIH2EDTA Each data point was collected 9 times. Ryan Hutcheson University of Idaho

  20. Conclusion • E1/2 of the FeII/IIIEDTA complex depends on pH, corresponding to the pH distribution diagram. • Fenton reactivity increases around pH 3.5 due to geometric rearrangement of the FeIIEDTA complex (MCP to PB). Ryan Hutcheson University of Idaho

  21. Future • pH dependence of Fenton reactivity at higher pH values • Expand van Eldik’s O2 activation to higher pH values Ryan Hutcheson University of Idaho

  22. Acknowledgments • National Institute of Health • National Science Foundation • University of Idaho • Malcom and Carol Renfrew • Dr. Cheng Group • Dr. Mark Engelmann Ryan Hutcheson University of Idaho

  23. Nernst Equations E1/2 • pH 2 to pH 3.5 • E1/2(mV) = 83mV – 69.5mV*(pH ) • pH 3.5 to 7 • E1/2(mV) = -89.5mV ± 5.6mV • pH 7 to 9 • E1/2(mV) = 202.8mV – 41.8mV*(pH) • pH 9 to 11 • E1/2(mV) = 409.1mV – 64.6mV*(pH) Ryan Hutcheson University of Idaho

  24. FeIIIEDTA Model EDTA-4 + H+ → HEDTA-3 log β = 9.52 HEDTA-3 + H+ → H2EDTA-2 log β = 6.13 H2EDTA-2 + H+ → H3EDTA- log β = 2.69 H3EDTA- + H+ → H4EDTA log β = 2.00 H4EDTA + H+ → H5EDTA+ log β = 1.5 H5EDTA+ + H+ → H6EDTA+2 log β = 0.0 EDTA-4 + Fe+3 → FeIIIEDTA- log β = 25.1 FeIIIEDTA- + H+ → FeIIIHEDTA log β = 1.3 FeIIIEDTA- + H20 → FeIII(OH)EDTA-2 + H+ log β = 17.71 2FeIII(OH)EDTA-2 → FeIII2(OH)2EDTA2-4 log β = 38.22 FeIII(OH)EDTA-2 + 2H2O → FeIII(OH)2EDTA-3 + 2H+ log β = 4.26 H+ + OH- → H2O log β = 13.76 Fe+3 + OH- → FeIII(OH)+2 log β = 11.27 Fe+3 + 2OH- → FeIII(OH)2+ log β = 23.0 Fe+3 + 3OH- → FeIII(OH)3 log β = 29.77 Fe+3 + 4OH- → FeIII(OH)4- log β = 34.4 2Fe+3 + 2OH- → FeIII2(OH)2+4 log β = 24.5 3Fe+3 + 4OH- → FeIII3(OH)4+8 log β = 49.7 Ryan Hutcheson University of Idaho

  25. FeIIEDTA Model EDTA-4 + H+ → HEDTA-3 log β = 9.52 HEDTA-3 + H+ → H2EDTA-2 log β = 6.13 H2EDTA-2 + H+ → H3EDTA- log β = 2.69 H3EDTA- + H+ → H4EDTA log β = 2.00 H4EDTA + H+ → H5EDTA+ log β = 1.5 H5EDTA+ + H+ → H6EDTA+2 log β = 0.0 EDTA-4 + Fe+2 → FeIIEDTA-2 log β = 14.3 HEDTA-3 + Fe+2 → FeIIHEDTA- log β = 6.82 H2EDTA-2 + Fe+2 → FeIIH2EDTA log β = 13.41 FeIIEDTA-2 + OH- → FeII(OH)EDTA-3 log β = 18.93 FeII(OH)EDTA-3 + OH- → FeII(OH)2EDTA-4 log β = 13.03 Fe+2 + OH- → FeII(OH)- log β = 4.2 Fe+2 + 2OH- →FeII(OH)2 log β = 7.5 Fe+2 + 3OH- → FeII(OH)3- log β = 13 Fe+2 + 4OH- → FeII(OH)4-2 log β = 10 Ryan Hutcheson University of Idaho

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