Dependence of the FeIIIIIEDTA complex on pH

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_at_uidaho.edu

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

3
FeIIIEDTA Speciation Diagram
FeIIIEDTA
FeIII(OH)2EDTA
FeIII(OH)EDTA
FeIIIHEDTA
4
FeIIEDTA Speciation Diagram
FeIIEDTA
FeII(OH)2EDTA
Free Fe2
FeII(OH)EDTA
FeIIHEDTA
FeIIH2EDTA
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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
6
Conditions

• All scans
• 10mL aqueous soln 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

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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
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E1/2 vs. pH (FeIIIEDTA)
FeIII(OH)2EDTA
FeIIIEDTA
FeIII(OH)EDTA
E1/2
FeIIIHEDTA
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E1/2 vs. pH (FeIIEDTA)
FeII(OH)2EDTA
FeIIEDTA
Free Fe2
FeII(OH)EDTA
E1/2
FeIIHEDTA
FeIIH2EDTA
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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.
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.
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.
13
Proposed O2 Reduction Mechanism by Van Eldik
FeIIEDTAH(H2O) O2 ? FeIIEDTAH(O2)
H2O FeIIEDTAH(O2) ? FeIIIEDTAH(O2-) FeIIIEDTAH(O
2-) 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
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Van Eldiks O2 Reduction
Van Eldik, R. Inorg. Chem, 1997, 36, 4115-4120
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Structures
FeIIEDTA FeIIHEDTA CN 7
FeIIIEDTA (CN 7)
FeIIIHEDTA (CN 6)
Monocapped trigonal prismatic (MCP)
Pentagonal-bipyramidal (PB)
Octahedral
Square Pyramidal
O
N
N
O
O
O
Miyoshi, K., Inor. Chem. Acta., 1995, 230,
119-125. Heinemann, F.W., Inor. Chem. Acta.,
2002, 337, 317-327.
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Structures contd
FeIIEDTA
FeIIHEDTA
gt pH 4
pH 3 pH 4
MCP
PB
Active site
Active site
Free Fe2
lt pH 3
Miyoshi, K., Inor. Chem. Acta., 1995, 230,
119-125.
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Fenton Reaction
FeIIIL e-? FeIIL
Edepends on ligand H2O2 e- ?
HO OH- E0.32V SHE _at_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.
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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
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Fenton Reactivity vs. pH
FeIIEDTA
Free Fe2
FeIIHEDTA
FeIIH2EDTA
Each data point was collected 9 times.
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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).

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Future

• pH dependence of Fenton reactivity at higher pH
  values
• Expand van Eldiks O2 activation to higher pH
  values

22
Acknowledgments

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

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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)

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FeIIIEDTA Model
EDTA-4 H ? HEDTA-3
log ß 9.52 HEDTA-3 H ?
H2EDTA-2
log ß 6.13 H2EDTA-2 H ? H3EDTA-
log ß
2.69 H3EDTA- H ? H4EDTA
log ß 2.00 H4EDTA H ?
H5EDTA
log ß 1.5 H5EDTA H ? H6EDTA2
log ß 0.0 EDTA-4
Fe3 ? FeIIIEDTA-
log ß 25.1 FeIIIEDTA- H ?
FeIIIHEDTA
log ß 1.3 FeIIIEDTA- H20 ? FeIII(OH)EDTA-2
H log ß 17.71 2FeIII(OH)EDT
A-2 ? FeIII2(OH)2EDTA2-4
log ß 38.22 FeIII(OH)EDTA-2 2H2O ?
FeIII(OH)2EDTA-3 2H log ß 4.26 H OH-
? H2O
log ß 13.76 Fe3 OH- ?
FeIII(OH)2
log ß 11.27 Fe3 2OH- ? FeIII(OH)2
log ß
23.0 Fe3 3OH- ? FeIII(OH)3
log ß 29.77 Fe3
4OH- ? FeIII(OH)4-
log ß 34.4 2Fe3 2OH- ?
FeIII2(OH)24
log ß 24.5 3Fe3 4OH- ? FeIII3(OH)48
log ß 49.7
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FeIIEDTA Model
EDTA-4 H ? HEDTA-3
log ß 9.52 HEDTA-3 H ?
H2EDTA-2
log ß 6.13 H2EDTA-2 H ? H3EDTA-
log ß
2.69 H3EDTA- H ? H4EDTA
log ß 2.00 H4EDTA H ?
H5EDTA
log ß 1.5 H5EDTA H ? H6EDTA2
log ß 0.0 EDTA-4
Fe2 ? FeIIEDTA-2
log ß 14.3 HEDTA-3 Fe2 ? FeIIHEDTA-
log ß
6.82 H2EDTA-2 Fe2 ? FeIIH2EDTA
log ß 13.41 FeIIEDTA-2 OH-
? FeII(OH)EDTA-3 log
ß 18.93 FeII(OH)EDTA-3 OH- ? FeII(OH)2EDTA-4
log ß 13.03 Fe2 OH- ?
FeII(OH)-
log ß 4.2 Fe2 2OH- ? FeII(OH)2
log ß
7.5 Fe2 3OH- ? FeII(OH)3-
log ß 13 Fe2 4OH- ?
FeII(OH)4-2
log ß 10