Ferrates Synthesis, Properties, and Applications in Water and

Chapter 12

Ferrate(VI) Oxidation of Nitrogenous Compounds

Downloaded by UNIV OF MISSOURI COLUMBIA on March 8, 2013 | http://pubs.acs.org Publication Date: July 25, 2008 | doi: 10.1021/bk-2008-0985.ch012

Michael D. Johnson, Brooks J. Hornstein and Jacob Wischnewsky Department of Chemistry and Biochemistry, New Mexico State University, LasCruces, NM 88003

The oxidation kinetics of a series of nitrogen containing compounds by ferrate(VI), Fe0 \ is described. Each of these reactions was studied at 25°C using spectrophotometric techniques. These included stopped-flow, rapid scanning spectrophotometry and convention Diode array spectrophotometry. Mechanistic schemes are proposed for each system studied along with potential intermediates when observed or required by kinetic data. 2

4

Introduction With the investigation of the oxidation reaction mechanisms of metalloproteins, the importance of iron in its high oxidation states has emerged (7). For example, in the functioning of catalases and peroxidases, the formation of an iron(IV) intermediate has been postulated as a key step in their enzymatic activity (2). In the reactions of cytP450, the generation of an iron(IV) or iron(V) heme complex is crucial in its catalytic cycle (5). It has been long recognized that in these enzymes, the iron exists in a porphyrin ring which imparts a unique stabilization of hypervalent iron along with n cation radicals. In contrast, recent studies of non-heme enzymes suggest that iron utilizes oxidation states greater than +3 in their catalytic cycles despite the absence of a porphyrin ring. Important representatives of these enzymes include the hydroxylase component of methane monooxygenase(MMO) (4), the R2 subunit of ribonucleotide reductase (RNR R2) (5), Rieske dioxygenase (6) and other less well defined enzymes like squalene epoxidase and tyrosine hydroxylase (7). In addition, the involvement of non-heme iron complexes in disease states such as tyrosinemia, phenylketouria and Refusm's disease has been proposed. To date, these non© 2008 American Chemical Society

In Ferrates; Sharma, V.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

177


178 heme iron systems possess either monomeric or dimeric Fe(II) cores that interact with molecular oxygen to generate species that carry out oxidations. Though model compounds are emerging that mimic these enzymes, difficulties in their preparation and data interpretation exist (1-8). In the past few years, the Que group has undertaken the development of iron(III) and iron(IV) complexes for the understanding of dinuclear non-heme iron enzymes (1,9). He has also characterized and developed the chemistry of iron(III)-TPA, and now analogous cyclam complexes (10). Recently he has reported on an important new iron(IV) complex as well. His approach has been to start with low oxidation state iron complexes and oxidize them to higher states. This work dovetails well with earlier work by Wieghardt and Eckardt on iron(V) nitrido complexes using cyclam (11). In order to provide a basis for the planning and interpretation of studies on related systems, both enzymes and other model systems, knowledge of the fundamental chemistry of iron in high oxidation states is urgently needed. For example, Valentine has characterized an olefin epoxidation catalyzed by an ironcyclam complex (12). It was suggested that the intermediate responsible for addition across the double bond was a "ferryl" species, (cyclam)Fe=O. Que et al. are now exploring this possibility further. Increased fundamental data on high oxidation state compounds will aid in such studies. Other iron oxidation catalysts have also been suggested to react in a similar fashion and it is interesting to note that the nature of the iron intermediates vary when other oxidants (e.g., iodosylbenzene vs. hydrogen peroxide) are used. These conclusions are based on the nature of products, product yields, Hammett correlations for reactivity, and intra- and intermolecular competitive epoxidation studies. Barton (13) and Sawyer (14) examined iron catalyzed peroxide oxidations and proposed several active metal intermediates. While limited evidence for these species exists, few high oxidation state non-heme iron complexes have been isolated. The elegant work of the Que group has provided the best characterized examples of Fe(IV). While his early work has focused more on the binuclear iron complexes, his Fe(TPA) systems provide an important entry into mononuclear iron(IV) chemistry. Despite his advances, new complexes need to be synthesized, characterized and their reactivity studied in order to understand the broader chemistry involved in this important class of reactions. Currently, the only "simple" high oxidation state complex of iron that is easily prepared are the ferrates, [Fe0 ] where iron is in the +6 oxidation state (15) In view of the paucity of information on such iron complexes and their importance in a host of reactions, we have studied the chemistry of the FeO ion to provide a new and unique entry into hypervalent iron chemistry. While ferrate(VI) is a strong oxidant, it appears, from preliminary studies, to be selective in its organic oxidations. We have demonstrated that reaction conditions, such as pH and order of addition can serve to alter the final products. 2-

4

2=

4


179 Experimental The chemicals used in these studies were reagent grade or higher purity. Potassium ferrate(VI) was synthesized using the method of Schreyer and Ockerman and recrystallized until a >95% purity was achieved. Purities were determined using spectrophotometric determination at 505nm where ferrate has an extinction coefficient of 1175M"cm" . Reaction rates were monitored using UV-vis detection. Depending on the rates of reaction different instruments were used to carry out the kinetic studies. For reactions occurring in less than 1 minute, rapid scanning spectrophotometry was used as coupled with a stopped-flow as found in the OLIS-RSM1000 system. For slower reactions, an HP8452A Diode Array spectrophotometer was used. In both cases, reaction rate constants were determined using OLIS kinetic packages.


l

1

Results and Discussion Reactions with Hydrazines (N H or CH N H ) (16) 2

4

3

2

3

We studied the ferrate(VI) oxidation of hydrazines and measured the reaction kinetics in aqueous media. Hydrazine, monomethylhydrazine (MMH) and phenylhydrazine (PH) each produced molecular nitrogen and the latter two produced methanol and phenol respectively. The ferrate was consistently reduced to iron(II). In each case, the following "simple" rate law was observed. +

2

Rate = (k + k [H ]) [Fe0 '][hydrazine] 0

H

4

Protonation was assumed to occur on the ferrate center since protonated hydrazine is typically oxidized at a slower rate than its deprotonated form. Checks for a radical mechanism using acrylonitrile showed no evidence for this one-electron pathway for any of the reactions. Support for a two electron pathway for N H is supported since unsaturated carboxylic acids were saturated when present in the oxidation mixture. This was assumed to occur via formation of diazine (N H ) which is well known to react with double bonds. 2

2

4

2

Table 1. Rate Constants of Ferrate(VI) oxidation of hydrazines. Conditions: T = 25°C, I = 1.0M (NaCI0 ). 4

Compound N H CH N H 2

3

3

4

2

1

ko (M-'s' ) 5.0 x 10 4.4 x 10 3

3

MM-'s") 3.8 x 10 6.2 x 10 5

5


180 Therefore, a series of two electron transfers, presumably inner-sphere, are proposed to account for these observations, Scheme I. HFeCV + N H — N H + Fe(IV) HFeCV + N H - * N H + Fe(IV) Fe(IV) + N H -> N H + Fe(II) N H + (H)Fe0 ' - -> N + Fe(IV) 2

4

2

2

2

4

2

2

2

4

2

(

2

2

kO kH rapid rapid

2

2)

4

2


Methanol and phenol are produced by oxidation followed by rapid hydrolysis of their corresponding diazene intermediates. +

RN H + HFe0 " — R N + Fe(IV) R N + H 0 - > ROH + N + H 2

4

2

+

+

2

2

2

Unfortunately, no other information may be obtained about the existence of the proposed intermediate.

Reactions with Hydroxylamines (RNHOH) (77) Hydroxylamines react rapidly with ferrate to produce a variety of oxidation products and iron(III) as shown in the following reactions. +

2

2 NH OH + 4 H + Fe0 " — N 0 + Fe(II) + 5 H 0 2 CH3NHOH + 4 H + Fe0 - — 2 CH NO + Fe(II) + 4 H 0 2 PhNHOH + 4 H + Fe0 " — 2 PhNO + Fe(II) + 4 H 0 2 CH ONH + 4 H + Fe0 -> 2 CH OH + N 0 + Fe(II) + 3 H 0 2

4

2

+

2

2

4

+

3

2

2

4

2

+

3

2_

2

4

3

2

2

A general rate law maybe written for each of these reactions as follows, +

2

Rate = (ko + k )[H ])[Fe0 '][hyroxylamine] H

4

where ko and k represent deprotonated and protonated pathways for the oxidation process. These values are shown in Table 2. H

Table 2. Rate Constants of Ferrate(VI) oxidation of hydroxylamines Conditions: T = 25°C, I = 1.0M (NaCI0 ). 4

Compound NH OH CH3NHOH CH ONH 2

3

2

1

k (M-'s" ) 4.8 x 10 3.5 x 10 1.9 0

3

3

kHCM-V) 3.3 x 10 4

1.6 x l O 110


5

181 For the hydroxylamine or N-substituted hydroxylamines a general reaction mechanism may be written as follows. (R = H, C H or Ph) 3

2

+

HFe0 ' *-* Fe0 " + H Fe0 " + RNHOH -> RNO + Fe(IV) HFeCV + RNHOH — N H + Fe(IV) + H Fe(IV) + RHNOH — RNO + Fe(II) 4

4

4


2

2

A similar reaction scheme may be written for NH OCH 2

2

NH OCH + Fe0 " -> NOR + Fe(IV) NH OCH + HFe0 " — NOR + Fe(IV) + H NH OCH + Fe(IV)- — NOR + Fe(Il) H 0 + 2 NOR -> N 0 + 2ROH + Fe(ll) 2

2

3

2

3

2

3

4

4

2

K ko k rapid e q

2

+

H

3

k k rapid rapid 0

+

H

The latter reaction is similar to that for the aqueous decomposition of NOH radicals into nitrous oxide and water. Although no reaction intermediate was observed in any of the system, we propose one where the hydroxylamine is either O-bonded, N-bonded or side-on bonded to the ferrate ion, presumably via expansion of the coordination environment around the iron center. This is "required" since these reactions occur faster than oxygen exchange on the iron(VI) center. Unfortunately, no information regarding the exact coordination environment is known.

Reactions with Anilines (18) Preliminary investigation of the ferrate oxidation of aniline and substituted anilines in aqueous media showed that the ferrate(VI) oxidation proceeds smoothly to produce a single product. A unique feature of the lower pH process is that the product possesses only a cis-conformation, and represents the only chemical process to form cis-azo complexes. With excess aniline at pH 9, ferrate(VI) rapidly (seconds) disappears to form an intermediate that subsequently disappears by a slower reaction (secsmins) with the committant appearance of azobenzene. This is in contrast to the observations of Eyring et al. In their study, they only monitored the reaction at 505nm. At this wavelength, it is easy to miss the rapid formation of the intermediate and only observe the loss of intermediate. They also did not explore the possible formation of cis-azobenzene. 19


182 Aniline Aniline Fe0 " —• imidoiron(VI) intermediate —• cis-azobenzene + Fe(II) m illiseconds-seconds seconds-m inutes 2

4


Although the intermediate has not been characterized, a likely prospect is an imidoferrate(VI) species formed by the simple substitution of an oxide (hydroxide) on iron(VI). The presence of an isosbestic point at 480nm shows that this reaction is a clean, single step process with a rate law first order with respect to ferrate(VI), aniline and hydrogen ions.

0 o c CD n i—

o tfi n

as

X, nm Figure 1. Formation offerrate(Vl) - aniline intermediate. Conditions: T = 25°C,pH9, [Fe(VI)]~5xlO- M, [aniline] = I0~ M, I = l.OM(NaCIO ). 5

3

4

+

The kinetic data show a good Hammett correlation with a to give a p equal to a -2 for the formation reaction. This indicates build-up of a high positive charge at the reaction site. The second step rate law is identical to the first, but first order in intermediate, and well resolved in time, i.e. over ten times slower. See Figure 2. As observed for the first step, the Hammett correlated well with a to give a p equal to a -1.9. We believe this is the addition of a second aniline to the iron center, followed by a series of rapid intramolecular electron transfer steps involving N-N bond formation, to eventually produce c/s-azobenzene. The iron center apparently acts as a template in this two-step process, see Figure 3. +


183

o c CO

J3 i_

o Downloaded by UNIV OF MISSOURI COLUMBIA on March 8, 2013 | http://pubs.acs.org Publication Date: July 25, 2008 | doi: 10.1021/bk-2008-0985.ch012

.O CO

wavelength, nm

Figure 2. Loss of aniline-ferrate(VI) intermediate. Conditions: T = 25°C, pH 9, [intermediate] ~ 5xlff M, [aniline] = W M, I=1.0M (NaCl0 ). 5

3

4

Figure 3. Proposed template reaction for formation of cis-azobenzene.



184 Several other reactions with substituted anilines were examined in attempts to block either the coupling reaction or intramolecular electron transfer steps to form azo products. Substitution at the para position gave reaction schemes identical to those shown above and azobenzenes were observed. However, when both the ortho and para positions were substituted with methyl groups, we were able to isolate a hydrazine final product. We believe this indicates a change in reaction processes. Unlike the para substituted aniline, these reactions show positive tests for production of radicals. Also, iron(III) instead of iron(II) was produced. This indicates that the reaction is now proceeding via a radical intermediate, similar to the process suggested by Eyring. Support for this change in mechanism is found by placement of methyl groups on the aniline nitrogen. For example, N, N-dimethylaniline produced the know anilinium type radical, see Figure 4.

CO

€ o

10

Wavelength, nm Figure 4. Reaction with N, N-dimethylaniline. Conditions: T = 25°C, pH 9, [intermediate] - 5x10 M, [N, N-dimethylaniline] = 2xW' M, / = l.OM (NaClQ ). 5

3

4

It should be noted that the observed spectrum is very different from that observed with "normal" aniline. Also, the anilinium radical has been observed by Wheeler and Nelson (19) to occur at 420nm, vastly different than our observations at ~485nm.


185

Conclusions The ferrate(VI) oxidationreactions of nitrogen containing compounds have a vast chemistry that remains to be explored. Preliminary observations may be summarized as follows: 1. 2.


3. 4. 5.

Simple hyrazines and hydroxylamines proceed via 2-electron steps. Aniline and para-substituted anilines proceed via 2-electron steps and observation of intermediates is possible. Azobenzene and its para-substituted analogs are all produced as the cisisomers. Ortho-substituted anlines proceed via 1-electron steps and the final products are hydrazines. Substitution on the nitrogen in aniline causes all the reactions to proceed via radical mechanisms.

Acknowledgements The authors acknowledge WERC and the ACS/PRF funding agencies for their generous support of this work.

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b)

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Kovacs, J. "Synthetic Analogues of Cysteinate-Ligated Non-Heme Iron and Non-Corrinoid Cobalt Enzymes." Chem. Rev. 2004, 104, 825. Kovacs, J. "Biochemistry: how iron activates O ." Science 2003, 299, 1035. 2

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Rev. 2002, 102, 1201. 4.

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Lippard, S.J.; Berg, J.M. "Principles of Bioinorganic Chemistry" University Science Books, NY 1994.

6.

Feig, Andrew L.; Becker, Michael; Schindler, Siegfried; van Eldik, Rudi; Lippard, Stephen J.. "Mechanistic Studies of the Formation and Decay of Diiron(III) Peroxo Complexes in the Reaction of Diiron(II) Precursors with Dioxygen." Inorg. Chem. 1996, 35, 2590.

7.

Ortiz de Montellano, P.R. "Cytochrome P450: Structure, Mechanism, and Biochemistry" Plenum, NY 1985.

8.

Rosenzweig, A.C.; Frederick, C.A.; Lippard, S.J.; Nordlund, P. "Crystal structure of a bacterial non-heme iron hydroxylase that catalyzes the biological oxidation of methane." Nature 1993, 366, 537. Fontecave M; Nordlund P; Eklund H; Reichard P "The redox centers of ribonucleotide reductase of Escherichia coli." Adv. Enzymol. 1992, 675, 147. Lipscomb J. D. "Biochemistry of the soluble methane monooxygenase." Ann. Rev. Microbiol. 1994, 45, 371.


187 9.

Dong, Y.; Fujii, H.; Hendrick, M.P.; Leising, R.A.; Pan, G.; Randall, C.R.; Wilkinson, E.; Zang, Y. Que, L.; Fox, B.G.; Kauffman, K.; Munck, E. "A High-Valent Nonheme Iron Intermediate. Structure and Properties of [Fe (µ-O) (5-Me-TPA) ](C10 ) ." J. Am. Chem. Soc. 1995, 117, 2778. Leising, R.A.; Brennan, B.A.; Que, L.; Fox, B.G.; Munck, E. "Models for non-heme iron oxygenases: a high-valent iron-oxo intermediate." J. Am. Chem. Soc 1991, 113, 3988. Leising, Randolph A.; Kojima, Takahiko; Que, Lawrence, Jr. "Alkane functionalization at nonheme iron centers: mechanistic insights." Act. Dioxygen Homogen. Catal Oxid., [Proc. Int. Symp.], 5th 1993, 321.


2

2

2

4

3

10. Kauppi, B.; Lee, K.; Carredano, E.; Parales, R.E.; Gibson, D.T.; Eklund, H.; Ramaswamy, S. "Purification and crystallization of the oxygenase component of naphthalene dioxygenase in native and selenomethioninederivatized forms." Structure 1998, 6, 571. 11. Que, L.; True, A.E. Progress in Inorganic Chemistry: Bioinorganic Chemistry: S.J. Lippard, ed., Wiley, NY 1990. Nam, W.; Valentine, J.S. "Zinc(II) complexes and aluminum(III) porphyrin complexes catalyze the epoxidation of olefins by iodosylbenzene." J. Am. Chem. Soc 1990, 112, 4987. 12. Holz, R.C.; Elgren, T.E.; Pearce, L.L.; Zhang, J.H.; O'Connor, C.J.; Que, L., "Spectroscopic and electrochemical properties of (µ-oxo)diiron(III) complexes related to diiron-oxo proteins. Structure of [Fe O(TPA) (MoO )](C1O ) ." Inorg. Chem. 1993, 32, 5844. Chen, K.; Que, L. "cis-Dihydroxylation of olefins by a non-heme iron catalyst: a functional model for Rieske dioxygenases." Angew. Chem. 1999, 38, 2227. 2

2

4

4

2

14. Zang, Y.; Kim, J.; Dong, Y.H.; Wilkinson, E.C.; Appleman, E.H.; Que, L. " Models for Nonheme Iron Intermediates: Structural Basis for Tuning the Spin States of Fe(TPA) Complexes." J. Am. Chem. Soc 1997, 119, 4197. 15. Meyer, K., E. Bill, B. Mienert, T. Weyhermuller and Wieghardt, K. "Photolysis of cis- and trans-[Fe (cyclam)(N ) ]+ Complexes: Spectroscopic Characterization of a Nitridoiron(V) Species." J. Am. Chem. Soc. 1999, 727, 4859. Grapperhaus, C. A., B. Mienert, E. B., Weyhermuller, T. and Wieghardt, K. "Mononuclear (Nitrido)iron(V) and (Oxo)iron(IV) Complexes via Photolysis of [(cyclam-acetato)FeIII(N )]+ and Ozonolysis of [(cyclam-acetato)Fe (O SCF )]+ in Water/Acetone Mixtures." Inorg. Chem. 2000, 39, 5306. III

3 2

3

HI

3

3

16. Johnson, M. D.; Hornstein, B. J. "Kinetics and Mechanism of the Ferrate Oxidation of Hydrazine and Monomethylhydrazine" Inorg. Chim. Acta 1994, 225, 145.


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Ferrates Synthesis, Properties, and Applications in Water and

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