Dependence of the Fe II/III EDTA complex on pH

March 30, 2004 Ryan Hutcheson University of Idaho

1

Dependence of the FeII/IIIEDTA complex on pH

Ryan Hutcheson and I. Francis Cheng*Department of Chemistry, University of Idaho

Moscow, ID [email protected]

mailto:[email protected]


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

0.00E+00

1.00E-04

2.00E-04

3.00E-04

4.00E-04

5.00E-04

6.00E-04

7.00E-04

8.00E-04

9.00E-04

1.00E-03

2 3 4 5 6 7 8 9 10 11

pH

Co

nc

en

tra

tio

n (

M)

FeIIIEDTA Speciation DiagramFeIIIEDTA

FeIIIHEDTA

FeIII(OH)EDTA

FeIII(OH)2EDTA


4

0.00E+00

1.00E-04

2.00E-04

3.00E-04

4.00E-04

5.00E-04

6.00E-04

7.00E-04

8.00E-04

9.00E-04

1.00E-03

2 3 4 5 6 7 8 9 10 11

pH

Co

nc

etr

ati

on

(M

)

FeIIEDTA Speciation DiagramFeIIEDTA

Free Fe+2

FeIIHEDTA

FeIIH2EDTA

FeII(OH)2EDTA

FeII(OH)EDTA


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Electrocatalytic (EC’) Mechanismand Cyclic Voltammetry

FeIII-L + e- FeII-L

FeII-L +H2O2 FeIII-L OH• +OH-

E: O + ne- = RC’: 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 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


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Cyclic Voltammagrams of FeII/IIIEDTA

-0.000005

-0.000004

-0.000003

-0.000002

-0.000001

0

0.000001

0.000002

-0.7-0.5-0.3-0.10.10.3

Potential (V)

Cu

rren

t (A

)

-0.000005

-0.000004

-0.000003

-0.000002

-0.000001

0

0.000001

0.000002

-0.7-0.5-0.3-0.10.10.3

Potential (V)

Cu

rren

t (A

)

pH 2

pH 11

pH 5.5

FeIIIEDTA + e- → FeIIEDTA

FeIIIEDTA + e- ← FeIIEDTA

1mM FeIIIEDTA0.1M buffer5mV/s scan rate


8

0.00E+00

1.00E-04

2.00E-04

3.00E-04

4.00E-04

5.00E-04

6.00E-04

7.00E-04

8.00E-04

9.00E-04

1.00E-03

2 3 4 5 6 7 8 9 10 11

pH

Co

nc

en

tra

tio

n (

M)

-0.35

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

Po

ten

tial (V

)

E1/2 vs. pH (FeIIIEDTA)FeIIIEDTA

FeIIIHEDTA

FeIII(OH)EDTA

FeIII(OH)2EDTA

E1/2


9

0.00E+00

1.00E-04

2.00E-04

3.00E-04

4.00E-04

5.00E-04

6.00E-04

7.00E-04

8.00E-04

9.00E-04

1.00E-03

2 3 4 5 6 7 8 9 10 11

pH

Conc

etra

tion

(M)

-0.35

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

Pote

ntia

l (V)

E1/2 vs. pH (FeIIEDTA)FeIIEDTA

Free Fe+2

FeIIHEDTA

FeIIH2EDTA

FeII(OH)2EDTA

FeII(OH)EDTA

E1/2


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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.


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Reaction Vessel

0.5g Fe; 20 or 40-70 mesh

0.44mM Xenobiotic

10.0 mL water

Air flow

2.0 mL 50/50 hexane/ethyl acetate(extraction only)

Stir bar

0.44mM EDTApH 5.5 – 6.5, unbuffered.

Noradoun,C., Industrial and Engineering Chemistry Research, (2003), 42(21), 5024-5030.


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Xenobiotic Oxidation Studies

Ironparticles0.1-1 mm

Fe2+

O2 + 2H+ H2O2

EDTAFeIIEDTA

+

FeIIIEDTA + HO- + HO.

Aqueous Xenobiotic

LMW acidsNoradoun,C., Industrial and Engineering Chemistry Research, (2003), 42(21), 5024-5030.


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Proposed O2 Reduction Mechanism by Van Eldik

Van Eldik, R. Inorg. Chem, 1997, 36, 4115-4120

FeIIEDTAH(H2O) + O2 FeIIEDTAH(O2) + H2O

FeIIEDTAH(O2) FeIIIEDTAH(O2-)

FeIIIEDTAH(O2-) + FeIIEDTAH(H2O) FeIIIEDTAH(O2

2-)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


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Van Eldik’s O2 Reduction

Van Eldik, R. Inorg. Chem, 1997, 36, 4115-4120


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Structures

O-

O-

O-

N N

O-

Fe

O

O

O

O

NH

O-

O-

O-

NH

FeOH

O

O

O

O

FeIIIEDTA (CN = 7)FeIIIHEDTA (CN = 6)

O-

O-

O-

O- N

N Fe

O

O

O

O

FeIIEDTAFeIIHEDTACN = 7

OctahedralSquare Pyramidal

Monocapped trigonal prismatic (MCP) Pentagonal-bipyramidal (PB)

NO

N

OO

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 cont’d

< pH 3

pH 3 – pH 4 > pH 4

PB MCP

Free Fe+2

FeIIEDTAFeIIHEDTA

Miyoshi, K., Inor. Chem. Acta., 1995, 230, 119-125.

Active site

Active site


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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.


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Electrocatalytic CV

0

0.00001

0.00002

0.00003

0.00004

0.00005

0.00006

0.00007

-0.7-0.5-0.3-0.10.10.3

Potential (V)

Cu

rre

nt

(A)

-0.7-0.5-0.3-0.10.10.3

pH 4pH 3.5

pH 2

pH 2.5

pH 3

pH 4

pH 4.5

FeIIIEDTA + e- → FeIIEDTA

0.1mM FeIIIEDTA20mM H202

0.1M buffer5mV/s scan rate


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Fenton Reactivity vs. pH

FeIIEDTAFree Fe+2

FeIIHEDTAFeIIH2EDTA

Each data point was collected 9 times.

0.00E+00

1.00E-05

2.00E-05

3.00E-05

4.00E-05

5.00E-05

6.00E-05

7.00E-05

8.00E-05

9.00E-05

1.00E-04

2 2.5 3 3.5 4 4.5 5

pH

Co

nc

en

tra

tio

n (

M)

0

0.00002

0.00004

0.00006

0.00008

0.0001

0.00012

0.00014

0.00016

0.00018

Cu

rren

t (A

)


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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 Eldik’s O2 activation to higher pH values


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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 ModelEDTA-4 + H+ → HEDTA-3 log β = 9.52HEDTA-3 + H+ → H2EDTA-2 log β = 6.13 H2EDTA-2 + H+ → H3EDTA- log β = 2.69H3EDTA- + H+ → H4EDTA log β = 2.00H4EDTA + H+ → H5EDTA+ log β = 1.5H5EDTA+ + H+ → H6EDTA+2 log β = 0.0EDTA-4 + Fe+3 → FeIIIEDTA- log β = 25.1FeIIIEDTA- + H+ → FeIIIHEDTA log β = 1.3FeIIIEDTA- + H20 → FeIII(OH)EDTA-2 + H+ log β = 17.712FeIII(OH)EDTA-2 → FeIII

2(OH)2EDTA2-4 log β = 38.22

FeIII(OH)EDTA-2 + 2H2O → FeIII(OH)2EDTA-3 + 2H+ log β = 4.26H+ + OH- → H2O log β = 13.76Fe+3 + OH- → FeIII(OH)+2 log β = 11.27Fe+3 + 2OH- → FeIII(OH)2

+ log β = 23.0Fe+3 + 3OH- → FeIII(OH)3 log β = 29.77Fe+3 + 4OH- → FeIII(OH)4

- log β = 34.42Fe+3 + 2OH- → FeIII

2(OH)2+4 log β = 24.5

3Fe+3 + 4OH- → FeIII3(OH)4

+8 log β = 49.7


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FeIIEDTA ModelEDTA-4 + H+ → HEDTA-3 log β = 9.52HEDTA-3 + H+ → H2EDTA-2 log β = 6.13H2EDTA-2 + H+ → H3EDTA- log β = 2.69H3EDTA- + H+ → H4EDTA log β = 2.00H4EDTA + H+ → H5EDTA+ log β = 1.5H5EDTA+ + H+ → H6EDTA+2 log β = 0.0EDTA-4 + Fe+2 → FeIIEDTA-2 log β = 14.3HEDTA-3 + Fe+2 → FeIIHEDTA- log β = 6.82H2EDTA-2 + Fe+2 → FeIIH2EDTA log β = 13.41FeIIEDTA-2 + OH- → FeII(OH)EDTA-3 log β = 18.93FeII(OH)EDTA-3 + OH- → FeII(OH)2EDTA-4 log β = 13.03Fe+2 + OH- → FeII(OH)- log β = 4.2Fe+2 + 2OH- → FeII(OH)2 log β = 7.5Fe+2 + 3OH- → FeII(OH)3

- log β = 13Fe+2 + 4OH- → FeII(OH)4

-2 log β = 10

Dependence of the Fe II/III EDTA complex on pH

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