Non-heme Iron Complex

DOI: 10.1002/anie.200502656

Regiospecific Ligand Oxygenation in IronComplexes of a Carboxylate-Containing LigandMediated by a Proposed FeV–Oxo Species**

Anne Nielsen, Frank B. Larsen, Andrew D. Bond, andChristine J. McKenzie*

Metal-promoted oxidations of coordinated ligands in syn-thetic metal complexes mimic the reactions catalyzed bymetalloenzymes involved in oxidation/oxygenation of sub-strates using O2 or its activated derivatives as terminaloxidants. In the biomimetic non-heme genre, observedreactions include oxygen-atom insertion into aliphatic[1] andaromatic[2–4] C�H bonds, and oxidative N-dealkylation.[5]

Herein we communicate regiospecific oxygen-atom inser-tion into iron(iii) complexes of carboxylate-containing penta-dentate ligand systems based onN-carboxymethyl-N’-R-N,N’-bis(2-pyridylmethyl)-1,2-ethandiamine (R=methyl orbenzyl;[6] Scheme 1a) and propose involvement of an FeV-based oxidant. A single oxygen atom can be inserted intothese complexes in two ways, using H2O2 and tBuOOH asoxidants: a phenolato group is produced in the systemcontaining the benzyl substituent (Scheme 1b), and anunprecedented iron(iii) complex containing a coordinatedN-oxide group is produced in the related methyl-substitutedligand (Scheme 1c).

The monoanionic precursor ligands (L1)� and (L2)�

contain one carboxylate donor, resembling in this respect

[*] A. Nielsen, F. B. Larsen, Dr. A. D. Bond, Prof. C. J. McKenzieUniversity of Southern DenmarkDepartment of ChemistryCampusvej 55, 5230 Odense M (Denmark)Fax: (+45)6615-8780E-mail: chk@chem.sdu.dk

[**] This work was supported by a grant (2058-03-0036) from the DanishTechnical Research Council. We express our gratitude to the refereesfor helpful comments.

Supporting information for this article (syntheses and character-izations, full details of single-crystal X-ray analyses, list of structuresconsidered for definition of average N�O bond distance incoordinated N-oxides, ESI MS of 5-FeCl4·H2O and the N-oxygen-ation reaction mixture in H2O) is available on the WWW underhttp://www.angewandte.org or from the author.

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many non-heme iron enzymes involved in O2 metabolism. Inthese enzymes, the proteins furnish endogenous mononeg-ative amino-acid donor sets containing one aspartate orglutamate group along with predominately histidine donors.A common motif is the 2-his-1-carboxylate triad which occursin extradiol cleaving catechol dioxygenases, Rieske dioxyge-nases, a-ketoglutarate-dependent oxidases, pterin-dependenthydroxylases, and other non-heme iron oxidases.[7]

Reaction of L1H with iron(ii) or iron(iii) perchlorate inwater, methanol, or acetonitrile gives the brown unsupportedm-oxo-bridged diiron(iii) complex [(L1)FeOFe(L1)](ClO4)2 (1-(ClO4)2).

[8] When water, acetonitrile, methanol, or D2Osolutions of 1-(ClO4)2 are treated with four equivalents ofperchloric acid, evidence of hydrolysis and cleavage of theoxide bridge is furnished by the generation of mononucleariron(iii) cations [(L1)FeOH]+ (2a+, m/z 462.3, in water oracetonitrile), [(L1)FeOCH3]

+ (2b+, m/z 476.3, in methanol),and [(L1)FeOD]+ (2c+, m/z 463.3, in D2O). When 10–15 equivalents of H2O2 or tBuOOH per iron atom aresubsequently added, a dark blue color evolves within minutesto hours, depending on concentration and the requirementthat a final pH of 2 is reached. Above or below pH 2, bubblesare evolved, indicating catalase activity. The electrospray-ionization mass spectra (ESI MS) of all these blue solutionsshow that a new singly charged ion evolves at m/z 460.2, twomass units less than for 2a+. As the intense color of thesolution (l= 588 nm) is consistent with a phenolato–FeIII

ligand-to-metal charge transfer (LMCT), the most reasonableassignment for this peak is the mononuclear iron(iii) complex[(L1O)Fe]+ (3+) of the previously unknown ligand L1O(Scheme 1).[9]

The best conditions found for the oxidation were thoseusing H2O2 in methanol. Under these conditions 3+ as theperchlorate salt was isolated in good yield. At least 45%conversion is possible, as determined on the basis of UV/Visspectroscopy (e 1900m�1 cm�1). The ESI MS of the motherliquors show that the cations 12+ and 2a+ or 2b+ remain insolution, and further 3+ can be generated by addition of more

oxidant. It is difficult to state precisely the number ofequivalents of H2O2 required to optimize the reaction sincethe oxidant is also consumed by competing catalase activity.However, it is notable that the ESI MS does not identify ionsthat might arise from alternative oxidation reactions, inparticular N-dealkylation.

X-ray analysis of 3-ClO4·0.5MeOH·0.5H2O[10] (Figure 1)

shows that the FeIII atom in the cation 3+ exhibits distorted

octahedral coordination geometry, with the O atoms of thecarboxylato and phenylato groups lying cis to each other. TheFe–Narom. and Fe–Naliph. bond lengths are consistent with thoseobserved in comparable FeIII complexes and display theexpected distortion on account of the trans influence of O.

Speculating that removal of the internal aromatic sub-strate may permit trapping of a reactive metal-based oxide orperoxide intermediate, we set out to examine the reactivity ofiron complexes of the analogous methylated ligand (L2)�

(Scheme 1). Using this system we observed a second,unprecedented type of oxygen-atom insertion. Since an oxo-bridged analogue to 12+ had not been adequately character-ized for the methylated ligand (L2)� , we were not able to carryout the series of reactions exactly analogous to those depictedin Scheme 2. Instead, we examined the reactions of theiron(iii) complex of the methyl ester derivative, [FeCl2-(MeL2)]ClO4 (4-ClO4), which is prepared from reaction ofequimolar amounts of FeCl3 and L2H in methanol withapproximately 10 equivalents of HCl(aq).

[11] Treatment of asuspension of 4-ClO4 in dichloromethane with tBuOOH (indecane) gave a yellow solution, from which crystals of[FeCl2(HL2O)][FeCl4]·H2O, 5-FeCl4·H2O were isolated aftera few days.

The single-crystal X-ray structure of 5-FeCl4·H2O[12]

(Figure 2) shows that the ligand in the cation has undergonetwo transformations: the methyl ester group of 4+ has beenhydrolyzed, and, more significantly, one tertiary amine group

Scheme 1. a) Protonated benzyl and methyl N-substituted pentaden-tate N4O ligands, (L1)� and (L2)� , together with a methylated deriva-tive, MeL2. b) Hexadentate phenolate-containing ligand, (L1O)2�, pre-pared by the Fe-promoted oxygenation of (L1)� . c) N-oxide-containingpentadentate ligand (L2O)� , prepared by oxygenation of (L2)� .

Figure 1. [(L1O)Fe]+ ion in the crystal structure 3-ClO4·0.5MeOH·0.5H2O, showing displacement ellipsoids at 50%probability. H atoms are omitted. Selected bond lengths [E]: Fe1�O11.864(4), Fe1�O2 1.951(4), Fe1�N1 2.161(4), Fe1�N2 2.218(4), Fe1�N3 2.188(5), Fe1�N4 2.107(5), C1�O1 1.344(6).

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has been oxygenated (Scheme 3). The methylpyridine armattached to the resultant N-oxide nitrogen atom is de-coordinated from the Fe atom and is protonated in the solidstate (forming a strong hydrogen bond to the uncoordinatedO atom of the carboxy group in an adjacent complex). The N–O bond length of 1.402(8) E is that expected for a coordi-nated N-oxide, R3NO (R¼6 H); the mean value from 29 suchstructures in the Cambridge Structural Database[13] is1.404(19) E.

The IR spectrum of 5-FeCl4·H2O is consistent with thecrystal structure: absorbances at 2962 and 2932 cm�1 areassigned to the protonated pyridine group and the shift ofn(C=O) from 1743 cm�1 in 4-ClO4 to 1666 cm�1 in 5-FeCl4·H2O shows conversion of the ester to a monodentatecarboxylate.[14] The ESI MS (THF or CH3CN) also supportsthe crystallographic results: the cation assignable to 5+ (m/z456.2) is observed along with a much more intense ion at m/z

420.3 (6+). This latter ion results from facile loss of the massequivalent of HCl from 5+. Two alternative structural assign-ments for m/z 420.3 are [FeIII(L2O)Cl]+ (6a+) or[FeVO(L2)Cl]+ (6b+; Scheme 3).

The lack of color change as a visual aid in the N-oxygenationmakes the reaction considerably more difficult tofollow compared to the aromatic oxygenation. However, ESIMS suggests that the N-oxygenation also occurs in aqueoussolution. In water, the ester group of 4-ClO4 is readilyhydrolyzed and the oxo-bridged analogue of 12+,[(L2)FeIIIOFeIII (L2)]2+ (72+) at m/z 377.3, is the dominantspecies along with a peak for [(L2)FeIIIOH]+ (8+) at m/z386.3.[15] Addition of 50 equivalents of tBuOOH causes noimmediate significant change to the spectrum of the solution,but a new singly charged cation is observed at m/z 402.2 afterstanding for 24 h.[16] Possible structural assignments for thiscation include the FeIII–(N-oxide) species [FeIII(L2O)OH]+

(9a+), the FeV–oxo species [FeVO(L2)OH]+ (9b+), and theFeIII–hydroperoxo species [FeIII(L2)OOH]+ (9c+), allexpected at m/z 402.2. Cations 9a+ and 9b+ are isoelectronicwith 6a+ and 6b+ (Scheme 3). An analogue of the cation 9c+

has been identified previously as a transient species for theneutral N5 Rtpen ligand system.[17] In the present case,however, cation 9c+ seems an unlikely assignment sincetBuOOH, and not H2O2, was used as the oxidant in thisparticular reaction, and since the species is unlikely to bestable in solution after standing under ambient conditions fora day. Furthermore the UV/Vis spectrum of the reactionmixture also lacks the characteristic CT band around 530 nmindicative of a FeIII–OOH species. The emergence of a peak atm/z 385.3 on collision-induced dissociation of the m/z 402.2

Scheme 2. a) 4 equiv HClO4 in H2O or D2O or MeOH; b) 4 equivHClO4, 20–30 equiv H2O2 or tBuOOH in H2O or MeOH or CH3CN.

Figure 2. [FeCl2(HL2O)]+ ion in the crystal structure 5-FeCl4·H2O, show-

ing displacement ellipsoids at 50% probability for non-H atoms. Hatoms are omitted (atom N4 is protonated). Selected bond lengths[E]: Fe1�O1 1.993(5), Fe1�O2 2.001(5), Fe1�N1 2.257(7), Fe1�N32.163(7), Fe1�Cl1 2.302(2), Fe1�Cl2 2.299(3), N2�O1 1.402(8).

Scheme 3. N-oxygenation reaction. Complexes 4+ and 5+ areisolated as their ClO4

� and FeCl4� salts, respectively. Ions 6a+ and 6b+

are alternative assignments for the major ion in the ESI MS of5-FeCl4·H2O.

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species (an MS/MS experiment) shows that the mass equiv-alent of HOC can be fragmented from the cation. This peak isassignable to either [(L2O)FeII]+ or [(L2)FeIVO]+ (bothexpected at m/z 385.2), which would arise by HOC loss from9a+ and 9b+, respectively. On the basis of the structure of 5+,we favor the assignment of m/z 402.2 to 9a+.

In general, complexes with aliphatic N-oxides are rare.[18]

Examples are known for the early transition metals, but thereare no previous examples for iron. Interestingly, both thealiphatic nitrogen atoms of ethylenediaminetetraacetate(edta4�) (to which (L1)� and (L2)� are related by thecommon feature of a diaminoethane backbone) can beoxygenated in the presence of niobic acid[19] or tetraperoxo-tantalate[20] and excess H2O2 to give [M(O2)2(edtaO2)]

3� (M=

Nb, Ta). In the context of these observations, it is clear thatthe (L2)� system displays impressive selectivity: only one ofthe aliphatic nitrogen atoms is oxygenated, and neither of thearomatic nitrogen atoms is affected.

The mechanism of the oxygenation reactions conceivablyinvolves O�O homolysis of a [LFeIIIOOR]+ species (R=H ortBu) to give the oxygenating species [LFeIVO]+ and ROC, orO�O heterolysis to give [LFeVO]2+ and RO� . That theoxygenation is not effected by HOC or tBuOC can be justifiedby 1) the regioselectivity, 2) the high yield, 3) the fact thattBuOOH can be used as terminal oxidant,[21] and 4) the factthat no other oxidation products are detected. Furthermore,Haber–Weiss chemistry would imply involvement of an FeII

species and our experience here with (L1)� and (L2)� indicatesthat FeII complexes are not readily accessible for these ligandsin condensed phases. This contrasts with the behavior of theneutral Rtpen ligands, for which mononuclear FeII species aretypically isolated in air.[17] If the proposed FeIV species acts asthe O-atom donor, Equations (1) and (2) apply. If theproposed FeV species acts as the O-atom donor, Equations (3)and (4) apply. In each case, for simplicity, only the relevantparts of (L1)� and (L2)� are depicted as the substrate:

½FeIV¼O�þ þC6H5R ! ½FeIII�O�C6H4R�þ þ HC ð1Þ

½FeIV¼O�þ þNR3 ! ½FeIII�O�NR3�2þ þ e� ð2Þ

½FeV¼O�2þ þC6H5R ! ½FeIII�O�C6H4R�þ þ Hþ ð3Þ

½FeV¼O�2þ þNR3 ! ½FeIII�O�NR3�2þ ð4Þ

Of the two possibilities, Equations (3) and (4) are mostconsistent with our results. Proposal of an FeV–oxo species issupported by following observations: 1) An upward shift ofone oxidation state is observed for mononuclear iron oxidesof (L1)� and (L2)� relative to the related neutral aminopyridylsupported systems (such as Rtpen), for which the catalyticallycompetent [FeIVO] species have been proposed.[3] 2) Theintramolecular two-electron, one-atom, oxygen transfer rep-resented by Equation (4) resembles closely the mechanismsof the molybdenum oxotransferases and their models.[22]

These systems have been shown to transfer an oxygen atomfrom MoVI=O to an amine in two-electron reactions, giving aMoIV species and an N-oxide, and vice versa. By analogy, itcan be noted that minimal bond rearrangement is needed to

interconvert the FeV–oxo species 6b+ and theN-oxide species6a+. 3) Although we have so far avoided the use of buffers, weobserve that it is essential to maintain a pH of 2 for bothoxygenations in water. The acidic conditions would assist O�O heterolysis for [LFeIIIOOR]+ rather than homolysis, sincewater or tBuOH would be released as leaving groups. 4) N-oxygenation has been observed previously on reaction of[FeIIItmp(Cl)] (tmp= tetramesitylporphyrin) with m-chloro-peroxybenzoic acid in toluene at 0 8C under dilute conditions(< 10�4

m).[23] In that system, an intermediate [FeIV(tmpC+)]species was detected in CH2Cl2 solution, again indicating two-electron chemistry. 5) The results obtained using ligands (L1)�

and (L2)� are in clear contrast to the neutral Rtpen systems, inwhich transient FeIII peroxides have been identified in thecondensed phase,[17,24] but ligand oxygenation has not beenobserved.

The most interesting feature in our system is thebiomimetic carboxylate donor and how this might tune theredox properties of iron complexes and their ability to formthe iron centered oxidant/oxygen atom donor involved inthese two regiospecific reactions. The pentadentate ligandsystem contains an inherent flexibility for alternative de-coordination of either a pyridyl or carboxylate group duringreactions, which might be useful for supporting variousoxidation states and geometries.[25] We have not investigatedwhether solvent oxidation is occurring in parallel with ligandoxygenations. This, and the possibility that external substratesmight be oxygenated, particularly in catalytic reactions, isclearly an area of interest for further work. As a final remark,we note that ESI MS is often a principal means of character-ization for reactive biomimetic oxide and peroxide iron,manganese, and copper species. If our interpretation of the N-oxygenation reaction had been based solely on ESI MS,without knowledge from the X-ray structure analysis, thetempting but incorrect assignment of [FeVO(L2)Cl]+ (6b+)might have been made for the ion 6+, without consideration ofthe unprecedented N-oxygenated [FeIII(L2O)Cl]+ (6a+). Like-wise, an FeIII–hydroperoxo species (9c+) might have beenassigned rather than 9a+ or 9b+.

Received: July 28, 2005Revised: December 16, 2005

.Keywords: hydroxylation · iron · N ligands · N oxides · peroxides

[1] V. Mahadevan, Z. Hou, A. P. Cole, D. E. Root, T. K. Lal, E. I.Solomon, T. D. P. Stack, J. Am. Chem. Soc. 1997, 119, 11996 –11997; S. Hikichi, H. Komatsuzaki, N. Kitajima, M. Akita, M.Mukai, T. Kitagawa, Y. Moro-oka, Inorg. Chem. 1997, 36, 266 –267; S. Hikichi, H. Komatsuzaki, M. Akita, Y. Moro-oka, J. Am.Chem. Soc. 1998, 120, 4699 – 4710.

[2] S. MKnage, J. B. Galey, G. Hussler, M. SeitK, M. Fontecave,Angew. Chem. 1996, 108, 2535 – 2537; Angew. Chem. Int. Ed.Engl. 1996, 35, 2353 – 2355; S. MKnage, J. B. Galey, J. Dumats, G.Hussler, M. SeitK, I. G. Luneau, G. Chottard, M. Fontecave, J.Am. Chem. Soc. 1998, 120, 13370 – 13382; F. Avenier, L. Dubois,J.-M. Latour, New J. Chem. 2004, 28, 782 – 784.

[3] M. P. Jensen, S. J. Lange, M. P. Mehn, E. L. Que, L. Que, Jr., J.Am. Chem. Soc. 2003, 125, 2113 – 2128; J.-U Rohde, S. Torelli, X.

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Shan, M. H. Lim, E. J. Klinker, J. Kaizer, K. Chen, W. Nam, L.Que, Jr., J. Am. Chem. Soc. 2004, 126, 16750 – 16761.

[4] H. Furutachi, M. Murayama, A. Shiohara, S. Yamazaki, S.Fujinami, A. Uehara,M. Suzuki, S. Ogo, Y.Watanabe, Y.Maeda,Chem. Commun. 2003, 1900 – 1901.

[5] S. Mahapatra. J. A. Halfen, E. C. Wilkinson, G. Pan, X. Wang,V. G. Young, Jr., C. J. Cramer, L. Que, Jr., W. B. Tolman, J. Am.Chem. Soc. 1996, 118, 11555 – 11574; S. Mahapatra, V. G.Young, Jr., S. Kaderli, A. D. ZuberbNhler, W. B. Tolman,Angew. Chem. 1997, 109, 125 – 127; Angew. Chem. Int. Ed.Engl. 1997, 36, 130 – 133; Y. Mekmouche, S. MKnage, C. Toia-Duboc, M. Fontecave, J. B. Galey, C. Lebrun, J. PKcaut, Angew.Chem. 2001, 113, 975 – 978;Angew. Chem. Int. Ed. 2001, 40, 949 –952.

[6] C. Baffert, M.-N. Collomb, A. Deronzier, S. Kjærgaard-Knud-sen, J.-M. Latour, K. H. Lund, C. J. McKenzie, M. Mortensen,L. P. Nielsen, N. Thorup, Dalton Trans. 2003, 1765 – 1772.

[7] L. Que, Jr., Nat. Struct. Biol. 2000, 7, 182 – 184; K. D. Koehntop,J. P. Emerson, L. Que, Jr., J. Biol. Inorg. Chem. 2005, 10, 87 – 93.

[8] 1-(ClO4)2 has been characterized in the solid state as twodifferent solvates, 1-(ClO4)2·CH3OH·2H2O and 1-(ClO4)2·0.5H2O; see Supporting Information.

[9] Treatment of 1-(ClO4)2 dissolved in methanol with 10 equiv-alents of ascorbic acid and exposure to dioxygen results in a lessintense absorption at 585 nm suggesting that the combination ofdioxygen and a reductant can also effect the oxygenation, albeitin a much lower yield.

[10] Crystal data for 3-(ClO4)·0.5CH3OH·0.5H2O. C23.5H27ClFeN4O8,Mr= 584.79, monoclinic, P21/m, Z= 4, a= 8.0335(2), b=36.5599(11), c= 8.8232(2) E, b= 107.693(2)8, V=

2468.83(11) E3, T= 180(2) K, 1calcd= 1.573 gcm�3, m(MoKa)=0.777 mm�1. Of 30287 data measured, 4750 were unique (Rint=

0.079). Final R1= 0.074 (3300 data with I> 2s(I)) and wR2=0.185 (all data).

[11] Crystal data for 4-(ClO4)·0.25CH3OH. C18.25H25Cl3FeN4O6.25,Mr= 562.62, triclinic, P1̄, Z= 4, a= 9.0165(8), b= 14.8220(16),c= 19.630(2) E, a= 69.953(3), b= 84.028(4), g= 80.974(4)8, V=

2430.3(4) E3, T= 180(2) K, 1calcd= 1.538 gcm�3, m(MoKa)=0.993 mm�1. Of 39082 data measured, 8150 were unique (Rint=

0.073). Final R1= 0.064 (4888 data with I> 2s(I)) and wR2=0.196 (all data).

[12] Crystal data for 5-(FeCl4)·H2O. C17H24Cl6Fe2N4O4, Mr= 672.80,monoclinic, P21, Z= 2, a= 8.5123(11), b= 9.9011(10), c=15.876(2) E, b= 102.141(5)8, V= 1308.2(3) E3, T= 180(2) K,1calcd= 1.708 gcm�3, m(MoKa)= 1.753 mm�1. The crystal wastwinned by two-fold rotation about c*. Of 21521 data measured,6516 were assigned exclusively to component 1, 6485 wereassigned exclusively to component 2, and 8520 were overlapped.The structure was refined against both components (HKLF-5 inSHELXL). Final R1= 0.065 (4760 data with I> 2s(I)) andwR2= 0.157 (all data). CCDC-276436 (3), -276438 (4), and-276439 (5) contain the supplementary crystallographic data forthis paper. These data can be obtained free of charge from TheCambridge Crystallographic Data Centre via www.ccdc.cam.a-c.uk/data_request/cif.

[13] F. H. Allen, Acta Crystallogr. Sect. B 2002, 58, 380 – 388. Cam-bridge Structural Database, November 2004 release plus 2updates (347763 structures in total). Details of the structuresused to define the average are included in the SupportingInformation.

[14] There are no signals that can be assigned unambiguously to acoordinated tertiary amineN-oxide. These would be expected tobe weak, appearing around 1270 and 860 cm�1, in this caseoverlapping with ligand bands.

[15] Low intensity peaks are also present for the free ligands [L2H2]+

(m/z 314.9) and [MeL2H]+ (m/z 329.0). This was not the case forthe reactions starting with 1-(ClO4)2. Ligand release is presum-

ably promoted by the presence of chloride ions combined withthe low pH. This is consistent with the formation of the FeCl4

�

counteranion in 5-(FeCl4)·H2O.[16] Analysis of the analogous reaction with H2O2 in water is

hampered by a competing catalase reaction. We have observedthe m/z 402.2 peak in the ESI MS of this solution, albeit as aweaker signal.

[17] Rtpen=N-R-N,N’,N’-tris(2-pyridylmethyl)ethane-1,2-diamine,R=CH3CH2, CH3CH2CH2, HOCH2CH2, (CH3)2CH, C6H5, andC6H5CH2. O. Horner, C. Jeandey, J.-L. Oddou, P. Bonville, C. J.McKenzie, J.-M. Latour, Eur. J. Inorg. Chem. 2002, 3278 – 3283;A. Hazell, C. J. McKenzie, L. P. Nielsen, S. Schindler, M.Weitzer, J. Chem. Soc. Dalton Trans 2002, 310 – 317; K. B.Jensen, C. J. McKenzie, L. P. Nielsen, J. Z. Pedersen, H. Moli-na Svendsen,Chem. Commun. 1999, 1313 – 1314; I. Bernal, I.-M.Jensen, K. B. Jensen, C. J. McKenzie, H. Toftlund, J.-P. Tucha-gues, J. Chem. Soc. Dalton Trans. 1995, 3667 – 3675.

[18] W. R. Thiel, T. Priermeier, T. Bog, J. Chem. Soc. Chem.Commun. 1995, 1871 – 1872; R. F. Bezerra, D. M. Araujo Melo,G. Vicentini, K. Zinner, K. L. B. Zinner, J. Alloys Compd. 2002,344, 120 – 122; D. Bayot, B. Tinant, M. Devillers, Inorg. Chem.2004, 43, 5999 – 6005; M. Capo, J. Gonzalez, H. Adams, Eur. J.Inorg. Chem. 2004, 17, 3405 – 3408; A. J. Bailey, M. G. Bhowon,W. P. Griffith, A. G. F. Shoair, A. J. P. White, D. J. Williams, J.Chem. Soc. Dalton Trans. 1997, 3245 – 3250; R. B. Brown, M. M.Williamson, C. L. Hill, Inorg. Chem. 1987, 26, 1602 – 1608.

[19] D. Bayot, B. Tinant, B. Mathieu, J.-P. Declercq, M. Devillers,Eur. J. Inorg. Chem. 2003, 737 – 743.

[20] D. Bayot, B. Tinant, M. Devillers, Inorg. Chem. 2004, 43, 5999 –6005.

[21] Using the same argument as in Ref. [3]: aromatic hydrogenabstraction by tBuOC is strongly disfavored on thermodynamicgrounds.

[22] B. E. Schultz, S. F. Gheller, M. C. Muetterties, M. J. Scott, R. H.Holm, J. Am. Chem. Soc. 1993, 115, 2714 – 2722.

[23] J. T. Groves, Y. Watanabe, J. Am. Chem. Soc. 1988, 110, 8443 –8452. Changed Soret bands were the only spectroscopic signa-ture of the non-isolated oxidized complex. Subsequent demeta-lation and analysis of the isolated porphyrin by NMR spectros-copy showed that a pyrrole nitrogen atom was oxygenated in25% yield.

[24] We have not yet detected FeIII–peroxide species for L= (L1)�

and (L2)� analogue to those obtained for Rtpen, despiteattempts to generate them using similar reactions.[20] The failureto detect [FeIII(L)OOH]+ for (L1)� and (L2)� might imply thatthey are much more reactive than their Rtpen counterparts.

[25] A. K. Poulsen, A. Rompel, C. J. McKenzie, Angew. Chem. 2005,117, 7076 – 7080; Angew. Chem. Int. Ed. 2005, 44, 6916 – 6920.

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