Chapter 11
Identification and Characterization of Aqueous Ferryl(IV) Ion
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Oleg Pestovsky* and Andreja Bakac* Ames Laboratory, 26 Spedding Hall, Iowa State University, Ames, IA 50011 The reaction between ferrous ions and ozone in acidified aqueous solution generates a short-lived species ( t 7 sec), which was identified as high-spin pentaaquairon(IV) oxo dication (ferryl) by UV-Vis, Mossbauer, XAS spectroscopies, DFT calculations, O isotopic labeling, and conductometric kinetic studies. Kinetic and O isotopic labeling studies were used to determine the rate constant for the oxo group exchange between ferryl and solvent water, k = 1400 s . Oxidation of alcohols, aldehydes, and ethers by ferryl occurs by simultaneous hydrogen atom and hydride transfer mechanisms. Ferryl was also found to be an efficient oxygen atom transfer reagent in the reactions with sulfoxides, a water soluble phosphine, and a thiolato-complex of cobalt(III). A quantitative and fast reaction between ferryl and DMSO (k = 1.3 x 10 M- s-) produces methyl sulfone. This and some other findings unambiguously rule out ferryl as a Fenton intermediate. -
/ 2
18
18
-1
eX
5
1
1
DMS0
Nonheme iron(IV) complexes have attracted considerable attention since the discovery that such species serve as reactive intermediates in a number of enzymatic processes (1-3). Attempts to unravel and mimic the chemistry of both heme and nonheme iron centers in enzyme active sites have focused on structures, mechanisms, and intermediates (4-12). Of particular interest are iron(IV) oxo intermediates, such as that in a-ketoglutarate-dependent taurine dioxygenase (13), which are capable of C-H bond activation. A number of studies directed at preparation of synthetic iron(IV) oxo species resulted in moderately stable complexes with amino-, amido-, and pyridine ligands, which were characterized by X-ray crystallography (1,14,15). Similar to their naturally occurring analogs, these complexes were shown to be able to carry out efficient C-H bond hydroxylation (16). © 2008 American Chemical Society
In Ferrates; Sharma, V.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.
167
168 Despite all the synthetic and mechanistic effort toward generating nonheme iron(IV) oxo complexes, the chemistry in the absence of stabilizing ligands remains largely unexplored. Among such species, one of the simplest and perhaps most reactive is aqueous ferryl(IV) ion, which was proposed as an alternative to hydroxyl radical in Fenton chemistry (77-24), or as an intermediate in some reactions in the atmospheric and environmental chemistry (25,26). In acidic and neutral solutions, the existence of a ferryl intermediate, most probably (H 0) Fe =0 , has not been established independently. Such a complex is, however, believed to be generated in the Fe(H 0) /ozone reaction (27,28), eq 1,which simultaneously yields an equivalent of 0 , which rules out the description of the intermediate as an ozonide complex. IV
2
2+
5
2+
2
6
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2
Fe
2+ aq
+0 -Fe 3
a q
0
2 +
+0
(1)
2
Oxidations of a limited number of organic substrates by aqueous ferryl were reported to take place by a hydrogen atom transfer mechanism (29). However, no product analyses were carried out to confirm this assignment, which was based on the kinetics alone. Another report provided kinetic evidence for a 2electron oxidation of substituted phenols via an Fe(IV)-substrate complex (30). In our own work on the chemistry of the closely related aqueous chromyl(IV), it was demonstrated that the opposite trends in reactivity exists, that is, 2-electron oxidation of alcohols and 1-electron oxidation of phenols (57). Intuitively, these finding are in contradiction with the thermodynamics of these reactions. The standard reduction potentials for the M / M couples are -0.41 V for chromium and 0.77 V for iron, demonstrating the thermodynamic preference for the formation of C r (1-electron product) and Fe (2-electron product). Therefore, some other, unknown factors must determine the outcome of these reactions, and such knowledge can be used in development of catalytic systems utilizing 0 and H 0 , as the successful catalysis often requires the chemistry to take place in 2-electron steps. Here we report identification and characterization of aqueous ferryl as a pentaaquairon(IV) oxo dication by a number of spectroscopic and kinetic techniques. The study of reactivity of ferryl in hydrogen atom, hydride, and oxygen atom transfer reactions is also presented. In addition, the unique knowledge of reactivity of genuine aqueous ferryl allows to rule out its participation as an intermediate in Fenton chemistry in aqueous solution. III
aq
11
aq
3+
aq
2
2
2+
aq
2
Preparation and Characterization of Aqueous Ferryl(IV) Ion 2
The Fe 70 reaction was investigated by conventional and stopped-flow UV-Vis spectrophotometry, as well as a stopped-flow conductometric technique in 0.10 M HC10 at 25 °C (32). With a moderate to large excess of ozone over Fe , the disappearance of ozone dominated the absorbance changes below 290 aq
3
4
2+
aq
In Ferrates; Sharma, V.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.
169 2+
nm, and the formation of F e 0 was observed above 290 nm with a maximum absorbance changes at 320nm. Global fitting of the kinetic data to a monoexponential kinetic model afforded a series of pseudo-first-order rate constants, which varied linearly with [0 ] and yielded k, = (8.30 ± 0.10) x 10 M" s' . These data are in good agreement with an earlier determination (28). At longer times, an approximately exponential decay of F e 0 was detected above 300 nm, k = 0.10 s' at pH 1.0, eq 2. Solutions of F e 0 in D 0 (7.6 % H) at pD 1.0 were more stable, and decayed with a rate constant k = 0.040 s", yielding a solvent kinetic isotope effect of 2.85. aq
5
3
1
1
2+
aq
1
2+
2H
aq
2
1
2D
Fe 0
2+
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aq
+H
+
Fe
3+ aq
+ V4 0 + Vi H 0 2
(2)
2
2+
In experiments using a large excess (20-30 fold) of Fe over 0 , the disappearance of ferryl, eq 3, was accompanied by hydrolysis of Fe (OH) Fe , eq 4 (33). aq
3
4+
aq
Fe 0
2+
2+
+ Fe
aq
Fe (OH) Fe
aq
aq
aq
2
4+
-> 2 Fe
3+ aq
/ Fe (OH) Fe aq
2
+
+ 2 H + -> 2 Fe
3+ aq
2
4+
aq
(3)
aq
+2H 0
(4)
2
4
A global fit to a biexponential kinetic model afforded k = (4.33 ± 0.01) x 10 M" s and = 0.79 s . A series of stopped-flow conductivity measurements was carried out to determine the number of proton equivalents consumed during F e 0 decay. Figure 1 shows conductivity changes accompanying the decay of 0.18 mM Fe 0 . Also shown is a trace obtained by mixing 0.20 mM Fe and 9.7 mM H 0 , eq 5. Nearly identical amplitudes in the two kinetic traces demonstrate that the same number of proton equivalents was consumed in reactions 2 and 5. Since the only persistent charged species in these reactions are H , Fe , Fe , and ferryl itself, this result establishes a 2+ charge for ferryl. 3
1
l
l
2+
aq
2+
2+
aq
2
aq
2
+
2+
aq
Fe
2+ aq
+
+ 14 H 0 + H - Fe 2
2
3+ aq
H 0 2
3+
aq
(5)
Mossbauer and XAS Spectroscopies Samples of aqueous ferryl(IV) were prepared by stopped-flow mixing of equimolar concentrations of Fe(H 0) and 0 , and freeze-quenching the sample in a rapidly-rotating liquid nitrogen-cooled home-made brass receptacle. The Mossbauer spectra were recorded at 4.2 K in magnetic fields of 0.05 T to 8.0 T. The spectrum in the 0.05 T exhibits a doublet with 5 (isomer shift) = 0.38(2) mm/s, and AE (quadrupole splitting) = 0.33(3) mm/s. The data are consistent with a high-spin (S=2) ferryl species (34). In a DFT-optimized structure, the Fe-oxo bond length is 1.63 A, similar to Fe=0 distances in other 57
2+
2
6
3
q
In Ferrates; Sharma, V.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.
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170
10
15 20 25 Time / s
30 35
Figure 1. Conductivity changes observed during (a) self-decay ofO. 18 mM FeaqO * (obtained by mixing 0.20 mMFe * and 0.25 mM0 ) and (b) in the reaction between 0.20 mMFe and9.7 mMH 0 in 0.10MHClO at 25 °C. Adaptedfrom J. Am. Chem. Soc. 2004, 126, 13757-13764. Copyright 2004 American Chemical Society. 2
2
aq
3
2+
aq
2
2
4
non-heme oxoiron(IV) complexes (55), and significantly shorter than the five Fe-OH bond distances (2.04 to 2.09 A) in the Fe(H 0) 0 cation. In agreement with the Mrissbauer spectrum, the DFT calculations yielded a ground state with S=2. In the X-ray absorption near edge structure (XANES) of the ferryl(IV) ion the edge energy was found at 7126 eV, i.e. between the values found for Fe(H 0) (7129 eV) and for nonheme Fe =0 complexes (7123-7125 eV). This observation is consistent with the strong covalent interaction with the terminal oxo group. 2+
2
2
3+
2
5
IV
6
Reactions of Aqueous Ferryl(IV) Ion with Organic Substrates 2+
In its reactions with reducing substrates, F e 0 utilizes one-electron (hydrogen atom transfer, electron transfer) and two electron (hydride transfer, oxygen atom transfer) pathways (52). In the reactions with alcohols, the two aq
In Ferrates; Sharma, V.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.
171 pathways take place concurrently. This is best illustrated on the example of cyclobutanol, which was oxidized to a mixture of 70% cyclobutanone and 30% (by difference) of ring-opened products formed by rearrangement of the intervening radical. The H-NMR spectrum of the product mixture is shown in Figure 2. L
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b
8/ppm Figure 2. H-NMR spectrum ofproducts generatedfrom cyclobutanol (4.16 mM) and 0.18 mMFeafi ' in D 0 (0.10 MDC10 3.2 %H,3.2% CD CN), at 25 °C and 0.4 mM0 . Peak assignment: a) cyclobutanol, b) CD HCN, c) cyclobutanone. Adaptedfrom J. Am. Chem. Soc.2004,126, 13757-13764. Copyright 2004 American Chemical Society. 2
2
4)
3
2
2
Even though the rate constant for radical ring-opening is not known, all the literature precedents agree that this reaction is too fast for the cyclic radical to be captured by an external substrate and further oxidized to cyclobutanone. Thus the ratio of the two types of products reflects directly the relative contributions of the two pathways, eqs 6 and 7. c - (CH ) CHOH — c - ( C H 2
3
c-(CH ) CHOH 2
3
f a s t
) C - OH
2
3
>' C H -(CH ) -CHO 2
2
2
c - (CH ) C=0 2
(6)
(7)
3
2+
The kinetics of disappearance of F e 0 in the reaction with various substrates was determined by stopped-flow, Table I. Unlike in the case of cyclobutanol, the products of oxidation of most of the substrates in Table I do aq
In Ferrates; Sharma, V.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.
172 not distinguish between 1-electron and 2-electron mechanisms. Contribution from the individual pathways were obtained by combining the kinetics data for the disappearance of F e 0 with precise reaction stoichiometry, which changes depending on whether Fe or Fe is produced in the reduction step. Kinetic simulations, which were essential in both planning of the experiments and in data analysis, afforded the kinetics data for each individual 1-electron or 2electron pathways, Table II. 2+
aq
2+
3+
aq
aq
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Table I. Second-Order Rate Constants for Reactions of Aqueous Ferryl(IV) with Organic Substrates" Substrate CD OH
k/Af's' 1.26 x 5.74 x 5.72 x 2.51 x 3.22 x 3.07 x 7.00 x 6.60 x 1.42 x 1.00 x 1.41 x
3
CH3OH CH3OD (in D 0) 2
C H OH (CH ) CHOH (CD ) CHOH (CH ) CDOH (CD ) CDOH Ph-CH OH 4-CF -Ph-CH OH 4-Br-Ph-CH OH 2
5
3
2
3
2
3
2
3
2
2
3
2
2
k/M's' 1.50 x 1.59 x 3.13 x 7.72 x 2.85 x 2.07 x 4.74 x 7.46 x 3.15x 4.12 x
Substrate 4-CH -Ph-CH OH 4-CH 0-Ph-CH OH Cyclobutanol CH 0 C H CHO PhCHO Et 0 THF CH COCH CH CN
2
10 10 10 10 10 10 10 10 10 10 10
3
2
2
3
2
3
2
2
3
2
3
2
5
2
2
4
3
4
3
3
4
10 10 10 10 10 10 10 10 10 10° 4
3
2
4
4
3
3
1
4
"Adapted from J. Am. Chem. Soc. 2004, 126, 13757-13764. American Chemical Society.
Copyright 2004
Table II. Kinetics Data for Reactions of Aqueous Ferryl(IV) with Organic Substrates in Individual Hydrogen Atom Transfer (le) and Hydride Transfer (2e) Pathways" Substrate CH OH CH 0 C H OH (CH ) CHOH cyclobutanol THF CH COCH CH CN
K, /M's' 5.3 x 10 4.0 x 10 2.3 x 10 1.7 x 10 2.4 x 10 7.5 x 10 3.2 x 10' 4.1 x 10° e
2
3
2
2
2
3
3
5
3
2
3
3
3
3
3
a
h
Adapted from J. Am. Chem. Soc. Chemical Society. Below detection limit
2004,
k /M's' 6.3 x 10 1.0 x 10 1.8 x 10 3.3 x 10 4.9 x 10 2.0 x 10 b b 2e
2
3
3
3
4
3
J 26, 13757-13764. Copyright 2004 American
In Ferrates; Sharma, V.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.
173 2+
2+
Two-electron reduction of F e 0 generates Fe which can be reoxidized with excess ozone to F e 0 to make the reaction catalytic. Obviously, the catalysis will be of limited importance if F e 0 is lost irreversibly in a parallel, 1-electron process, as is the case for all the reactions in Table II. More successful catalytic oxidations were carried out in oxidations taking place by oxygen atom transfer to sulfoxides as described below. Figure 3 shows kinetic traces for the loss of methyl para-tolyl sulfoxide in a reaction with ozone in the presence and absence of 5 micromolar Fe (36). The interpretation of the large catalytic effect of Fe appears straightforward aq
aq
2+
aq
2+
aq
2+
aq
2+
Downloaded by UNIV OF ARIZONA on December 13, 2012 | http://pubs.acs.org Publication Date: July 25, 2008 | doi: 10.1021/bk-2008-0985.ch011
aq
0.80
0.10
' 2000 4000 Time / s
1
1
0
1
6000
Figure 3. Oxidation of 100 jiM methyl para-tolyl sulfoxide with 100 ptM ozone catalyzed by Fe * in 0.10 M aqueous HC10 . Kinetic traces at 240 nm at 0 juM Fe * (a), and 5 y.M Fe (b). Adaptedfrom Inorg. Chem. 2006,126, 1375713764. Copyright 2006 American Chemical Society. aq
4
2
2+
aq
aq
and suggests that the oxidation of the sulfoxide by F e 0 oxygen atom transfer, i.e. eq 1 followed by eq 8
2+
aq
Fe 0 aq
2+
+ CH S(0)Ar - Fe 3
2+ aq
takes place by
+ CH S(0) Ar 3
2
2+
(8)
Indeed, the amount of Fe at the end of the reaction matched exactly the calculated value obtained in simulations under the assumption that only aq
In Ferrates; Sharma, V.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.
174 reactions 1, 3 and 8 are involved, i.e. in the complete absence of a 1-e path in the Fe 0 /sulfoxide reaction. The kinetics of Fe 0 /sulfoxide reactions were determined by setting up a competition with excess Fe and determining the amounts of Fe at the end of the reaction, according to the following scheme. 2+
aq
2+
aq
2+
3+
Downloaded by UNIV OF ARIZONA on December 13, 2012 | http://pubs.acs.org Publication Date: July 25, 2008 | doi: 10.1021/bk-2008-0985.ch011
aq
aq
2Fe
3 +
2 +
Fe
a q
+ R S0
a q
2
2
As shown in Table III, the rate constants are similar for all the sulfoxides studied. Moreover, the kinetics are insensitive to the choice of the isotope, H or D, in the alkyl group, as expected for an oxygen atom transfer. The fast and "clean" oxygen transfer to the sulfoxides provided for an opportunity to determine the rate constant for the oxo oxygen exchange with solvent water. To this goal, the reaction between Fe , 0 , and dimethylsulfoxide (DMSO) was carried out in H 0 , and the isotopic composition of the product sulfone was determined by GCMS. Under the experimental conditions, all the H 0 , including that coordinated to Fe , contained the 0 label, and 0 was the only source of 0. The reaction scheme is shown below. 2+
aq
3
I8
2
2+
2
aq
18
16
3
•
OH
n
2
H 0/,,v . pI '. .^, \O \ ^Hn 2
e
22
2 +
OH
+# tast fast
2
n
2 +
H nH w0/,, / / / , y I ,„*, \n\O ^n
3
2 2
Wfc "* 0
1 H u C n
22
e
H 0^ | ^» 2
OH
2
^
C nH u
33
M
^H C
t
w
