March 30, 2004Ryan Hutcheson University of Idaho 1 Dependence of the Fe II/III EDTA complex on pH Ryan Hutcheson and I. Francis Cheng* Department of Chemistry, University of Idaho Moscow, ID

March 30, 2004Ryan Hutcheson University of Idaho 2 Importance First study of the pH dependence of Fe II/III EDTA Green chemistry – optimization of O 2 activation and pH dependence of the Fenton Reaction Antioxidants : Fe II/III EDTA is a good model for low molecular weight biological ligands

March 30, 2004Ryan Hutcheson University of Idaho 3 Fe III EDTA Speciation Diagram Fe III EDTA Fe III HEDTA Fe III (OH)EDTA Fe III (OH) 2 EDTA

March 30, 2004Ryan Hutcheson University of Idaho 4 Fe II EDTA Speciation Diagram Fe II EDTA Free Fe +2 Fe II HEDTA Fe II H 2 EDTA Fe II (OH) 2 EDTA Fe II (OH)EDTA

March 30, 2004Ryan Hutcheson University of Idaho 5 Electrocatalytic (EC’) Mechanism and Cyclic Voltammetry Fe III -L + e-  Fe II -L Fe II -L +H 2 O 2  Fe III -L OH +OH- E: O + ne- = R C’: R + Z = O + Y Regeneration of the Fe III EDTA within the vicinity of the electrode causes amplification of the CV wave

March 30, 2004Ryan Hutcheson University of Idaho 6 Conditions All scans –10mL aqueous sol’n purged w/ N 2 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 Fe III EDTA –1mM Fe III EDTA Catalytic scans (Fenton Reaction) –0.1mM Fe III EDTA catalytic scans –20mM H 2 O 2

March 30, 2004Ryan Hutcheson University of Idaho 7 Cyclic Voltammagrams of Fe II/III EDTA pH 2 pH 11 pH 5.5 Fe III EDTA + e - → Fe II EDTA Fe III EDTA + e - ← Fe II EDTA 1mM Fe III EDTA 0.1M buffer 5mV/s scan rate

March 30, 2004Ryan Hutcheson University of Idaho 8 E 1/2 vs. pH (Fe III EDTA) Fe III EDTA Fe III HEDTA Fe III (OH)EDTA Fe III (OH) 2 EDTA E 1/2

March 30, 2004Ryan Hutcheson University of Idaho 9 E 1/2 vs. pH (Fe II EDTA) Fe II EDTA Free Fe +2 Fe II HEDTA Fe II H 2 EDTA Fe II (OH) 2 EDTA Fe II (OH)EDTA E 1/2

March 30, 2004Ryan Hutcheson University of Idaho 10 O 2 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),

March 30, 2004Ryan Hutcheson University of Idaho 11 Reaction Vessel 0.5g Fe; 20 or mesh 0.44mM Xenobiotic 10.0 mL water Air flow 2.0 mL 50/50 hexane/ethyl acetate (extraction only) Stir bar 0.44mM EDTA pH 5.5 – 6.5, unbuffered. Noradoun,C., Industrial and Engineering Chemistry Research, (2003), 42(21),

March 30, 2004Ryan Hutcheson University of Idaho 12 Xenobiotic Oxidation Studies Iron particles mm Fe 2+ O 2 + 2H + H2O2H2O2 EDTA Fe II EDTA + Fe III EDTA + HO - + HO. Aqueous Xenobiotic LMW acids Noradoun,C., Industrial and Engineering Chemistry Research, (2003), 42(21),

March 30, 2004Ryan Hutcheson University of Idaho 13 Proposed O 2 Reduction Mechanism by Van Eldik Van Eldik, R. Inorg. Chem, 1997, 36, Fe II EDTAH(H 2 O) + O 2  Fe II EDTAH(O 2 ) + H 2 O Fe II EDTAH(O 2 )  Fe III EDTAH(O 2 - ) Fe III EDTAH(O 2 - ) + Fe II EDTAH(H 2 O)  Fe III EDTAH(O 2 2- )Fe III EDTAH + H 2 O Fe III EDTAH(O 2 2- )Fe III EDTAH + H 2 O + 2H +  2Fe III EDTAH(H 2 O) + H 2 O 2 2Fe II EDTAH(H 2 O) + H 2 O 2  2Fe III EDTAH(H 2 O) + H 2 O *Proposes H 2 O 2 as intermediate *Saw no evidence of H 2 O 2

March 30, 2004Ryan Hutcheson University of Idaho 14 Van Eldik’s O 2 Reduction Van Eldik, R. Inorg. Chem, 1997, 36,

March 30, 2004Ryan Hutcheson University of Idaho 15 Structures Fe III EDTA (CN = 7) Fe III HEDTA (CN = 6) Fe II EDTA Fe II HEDTA CN = 7 Octahedral Square Pyramidal Monocapped trigonal prismatic (MCP) Pentagonal-bipyramidal (PB) N O N O O O Miyoshi, K., Inor. Chem. Acta., 1995, 230, Heinemann, F.W., Inor. Chem. Acta., 2002, 337,

March 30, 2004Ryan Hutcheson University of Idaho 16 Structures cont’d < pH 3 pH 3 – pH 4 > pH 4 PB MCP Free Fe +2 Fe II EDTA Fe II HEDTA Miyoshi, K., Inor. Chem. Acta., 1995, 230, Active site

March 30, 2004Ryan Hutcheson University of Idaho 17 Fenton Reaction Fe III L +e-→ Fe II L E°’=depends on ligand H 2 O 2 + e - → HO + OH - E°=0.32V 7 Fe II L + H 2 O 2 → Fe III L + HO + OH - Only iron complexes with E 0 ’ negative of 0.32 V are thermodynamically capable of hydrogen peroxide reduction. However, Fenton inactivity may result from kinetic factors as well.

March 30, 2004Ryan Hutcheson University of Idaho 18 Electrocatalytic CV pH 4 pH 3.5 pH 2 pH 2.5 pH 3 pH 4 pH 4.5 Fe III EDTA + e - → Fe II EDTA 0.1mM Fe III EDTA 20mM H M buffer 5mV/s scan rate

March 30, 2004Ryan Hutcheson University of Idaho 19 Fenton Reactivity vs. pH Fe II EDTAFree Fe +2 Fe II HEDTA Fe II H 2 EDTA Each data point was collected 9 times.

March 30, 2004Ryan Hutcheson University of Idaho 20 Conclusion E 1/2 of the Fe II/III EDTA complex depends on pH, corresponding to the pH distribution diagram. Fenton reactivity increases around pH 3.5 due to geometric rearrangement of the Fe II EDTA complex (MCP to PB).

March 30, 2004Ryan Hutcheson University of Idaho 21 Future pH dependence of Fenton reactivity at higher pH values Expand van Eldik’s O 2 activation to higher pH values

March 30, 2004Ryan 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

March 30, 2004Ryan Hutcheson University of Idaho 23 Nernst Equations E 1/2 pH 2 to pH 3.5 –E 1/2 (mV) = 83mV – 69.5mV*(pH ) pH 3.5 to 7 –E 1/2 (mV) = -89.5mV ± 5.6mV pH 7 to 9 –E 1/2 (mV) = 202.8mV – 41.8mV*(pH) pH 9 to 11 –E 1/2 (mV) = 409.1mV – 64.6mV*(pH)

March 30, 2004Ryan Hutcheson University of Idaho 24 Fe III EDTA Model EDTA -4 + H + → HEDTA -3 log β = 9.52 HEDTA -3 + H + → H 2 EDTA -2 log β = 6.13 H 2 EDTA -2 + H + → H 3 EDTA - log β = 2.69 H 3 EDTA - + H + → H 4 EDTA log β = 2.00 H 4 EDTA + H + → H 5 EDTA + log β = 1.5 H 5 EDTA + + H + → H 6 EDTA +2 log β = 0.0 EDTA -4 + Fe +3 → Fe III EDTA - log β = 25.1 Fe III EDTA - + H + → Fe III HEDTA log β = 1.3 Fe III EDTA - + H 2 0 → Fe III (OH)EDTA -2 + H + log β = Fe III (OH)EDTA -2 → Fe III 2 (OH) 2 EDTA 2 -4 log β = Fe III (OH)EDTA H 2 O → Fe III (OH) 2 EDTA H + log β = 4.26 H + + OH - → H 2 O log β = Fe +3 + OH - → Fe III (OH) +2 log β = Fe OH - → Fe III (OH) 2 + log β = 23.0 Fe OH - → Fe III (OH) 3 log β = Fe OH - → Fe III (OH) 4 - log β = Fe OH - → Fe III 2 (OH) 2 +4 log β = Fe OH - → Fe III 3 (OH) 4 +8 log β = 49.7

March 30, 2004Ryan Hutcheson University of Idaho 25 Fe II EDTA Model EDTA -4 + H + → HEDTA -3 log β = 9.52 HEDTA -3 + H + → H 2 EDTA -2 log β = 6.13 H 2 EDTA -2 + H + → H 3 EDTA - log β = 2.69 H 3 EDTA - + H + → H 4 EDTA log β = 2.00 H 4 EDTA + H + → H 5 EDTA + log β = 1.5 H 5 EDTA+ + H + → H 6 EDTA +2 log β = 0.0 EDTA -4 + Fe +2 → Fe II EDTA -2 log β = 14.3 HEDTA -3 + Fe +2 → Fe II HEDTA - log β = 6.82 H 2 EDTA -2 + Fe +2 → Fe II H 2 EDTA log β = Fe II EDTA -2 + OH - → Fe II (OH)EDTA -3 log β = Fe II (OH)EDTA -3 + OH - → Fe II (OH) 2 EDTA -4 log β = Fe +2 + OH - → Fe II (OH) - log β = 4.2 Fe OH - → Fe II (OH) 2 log β = 7.5 Fe OH - → Fe II (OH) 3 - log β = 13 Fe OH - → Fe II (OH) 4 -2 log β = 10