Iron(III) complex of a crown ether-porphyrin conjugate and reversible binding of superoxide to its iron(II) form

Article abstract of DOI:10.1021/ja064984p

The synthesis and characterization of the Fe(III) complex of a novel crown ether-porphyrin conjugate, 5²-N-(4-aza-18-crown-6)methyl-5 ⁴,10⁴,15⁴,20⁴-tetra-tert-butyl- 5⁶-methyl-5,10,15,20-tetraphenylporphyrin (H₂Porph), as well as the corresponding hydroxo, dimeric, Fe(II), and peroxo species are reported. The crystal structure of [Fe^III(Porph)Cl]·H ₃O⁺·FeCl₄^-·C ₆H₆·EtOH is also reported. [Fe^III(Porph) (DMSO)₂]⁺ and K[Fe^III(Porph)(O₂ ^2-)] are high-spin species (Moessbauer data: δ = 0.38 mm s^-1, ΔE_q = 0.83 mm s^-1 and δ = 0.41 mm s^-1, ΔE_q = 0.51 mm s^-1, respectively), whereas in a solution of reduced [Fe^III(Porph)-(DMSO) ₂]⁺ complex the low-spin [Fe^II(Porph)(DMSO) ₂] (δ = 0.44 mm s^-1, ΔE_q = 1.32 mm s^-1) and high-spin [Fe^II(Porph)(DMSO)] (δ = 1.27 mm s^-1 ΔE_q = 3.13 mm s^-1) iron(II) species are observed. The reaction of [Fe^III(Porph)(DMSO)₂] ⁺ with KO₂ in DMSO has been investigated. The first reaction step, involving reduction to [Fe^II(Porph)(DMSO) ₂], was not investigated in detail because of parallel formation of an Fe-(III)-hydroxo species. The kinetics and thermodynamics of the second reaction step, reversible binding of superoxide to the Fe(II) complex and formation of an Fe(III)-peroxo species, were studied in detail (by stopped-flow time-resolved UV/vis measurements in DMSO at 25°C), resulting in k _on = 36 500 ± 500 M^-1 s^-1, k _off = 0.21 ± 0.01 s^-1 (direct measurements using an acid as a superoxide scavenger), and K_O2- = (1.7 ± 0.2) × 10⁵ (superoxide binding constant kinetically obtained as k_on/k_off), (1.4 ± 0.1) × 10⁵, and (9.0 ± 0.1) × 10⁴ M^-1 (thermodynamically obtained in the absence and in the presence of 0.1 M NBu₄PF ₆, respectively). Temperature-dependent kinetic measurements for k_on (-40 to 25°C in 3:7 DMSO/CH₃CN mixture) yielded the activation parameters ΔH^? = 61.2 ± 0.9 kJ mol^-1 and ΔS^? = +48 ± 3 J K ^-1 mol^-1. The observed reversible binding of superoxide to the metal center and the obtained kinetic and thermodynamic parameters are unique. The finding that fine-tuning of the proton concentration can cause the Fe(III)-peroxo species to release O₂^- and form an Fe(II) species is of biological interest, since this process might occur under very specific physiological conditions.

Full text of DOI:10.1021/ja064984p

Published on Web 03/20/2007
Iron(III) Complex of a Crown Ether-Porphyrin Conjugate and
Reversible Binding of Superoxide to Its Iron(II) Form
Katharina Du¨rr,†,‡ Brendan P. Macpherson,† Ralf Warratz,§ Frank Hampel,‡
Felix Tuczek,§ Matthias Helmreich,‡ Norbert Jux,*,‡ and
Ivana Ivanovic´-Burmazovic´*,†
Contribution from the Institute of Inorganic Chemistry, UniVersity of Erlangen-Nu¨rnberg,
Egerlandstrasse 1, 91058 Erlangen, Germany, Institute of Organic Chemistry, UniVersity of
Erlangen-Nu¨rnberg, Henkestrasse 42, 91054 Erlangen, Germany, and Institute of Inorganic
Chemistry, Christian-Albrechts-UniVersity Kiel, Otto Hahn Platz 6/7, 24098 Kiel, Germany
Received July 13, 2006; E-mail: ivana.ivanovic@chemie.uni-erlangen.de; norbert.jux@chemie.uni-erlangen.de
Abstract: The synthesis and characterization of the Fe(III) complex of a novel crown ether-porphyrin
conjugate, 52-N-(4-aza-18-crown-6)methyl-54,104,154,204-tetra-tert-butyl-56-methyl-5,10,15,20-tetraphenylpor-
phyrin (H2Porph), as well as the corresponding hydroxo, dimeric, Fe(II), and peroxo species are reported.
The crystal structure of [FeIII(Porph)Cl]‚H3O+‚FeCl4-‚C6H6‚EtOH is also reported. [FeIII(Porph)(DMSO)2]+
and K[FeIII(Porph)(O22-)] are high-spin species (Mo¨ssbauer data: δ ) 0.38 mm s-1, ∆Eq ) 0.83 mm s-1
and δ ) 0.41 mm s-1, ∆Eq ) 0.51 mm s-1, respectively), whereas in a solution of reduced [FeIII(Porph)-
(DMSO)2]+ complex the low-spin [FeII(Porph)(DMSO)2] (δ ) 0.44 mm s-1, ∆Eq ) 1.32 mm s-1) and high-
spin [FeII(Porph)(DMSO)] (δ ) 1.27 mm s-1, ∆Eq ) 3.13 mm s-1) iron(II) species are observed. The reaction
of [FeIII(Porph)(DMSO)2]+ with KO2 in DMSO has been investigated. The first reaction step, involving
reduction to [FeII(Porph)(DMSO)2], was not investigated in detail because of parallel formation of an Fe-
(III)-hydroxo species. The kinetics and thermodynamics of the second reaction step, reversible binding of
superoxide to the Fe(II) complex and formation of an Fe(III)-peroxo species, were studied in detail (by
stopped-flow time-resolved UV/vis measurements in DMSO at 25 °C), resulting in kon ) 36 500 ( 500 M-1
s-1, koff ) 0.21 ( 0.01 s-1 (direct measurements using an acid as a superoxide scavenger), and KO
)
-
2
(1.7 ( 0.2) × 105 (superoxide binding constant kinetically obtained as kon/koff), (1.4 ( 0.1) × 105, and (9.0
( 0.1) × 104 M-1 (thermodynamically obtained in the absence and in the presence of 0.1 M NBu4PF6,
respectively). Temperature-dependent kinetic measurements for kon (-40 to 25 °C in 3:7 DMSO/CH3CN
mixture) yielded the activation parameters ∆Hq ) 61.2 ( 0.9 kJ mol-1 and ∆Sq ) +48 ( 3 J K-1 mol-1
.
The observed reversible binding of superoxide to the metal center and the obtained kinetic and
thermodynamic parameters are unique. The finding that fine-tuning of the proton concentration can cause
-
the Fe(III)-peroxo species to release O2 and form an Fe(II) species is of biological interest, since this
process might occur under very specific physiological conditions.
applications.6,7 In general, the biological relevance of the iron-
Introduction
porphyrins and superoxide radical anion makes the investigation
of their chemical reactions deservedly attractive.
-
The biochemistry of dioxygen and its reduced forms (O2
and O22-) is closely related to that of iron-porphyrin centers
in different hemoproteins. Iron(III)-superoxo and iron(III)-
peroxo species are involved, for example, in the transport of
dioxygen, the catalytic cycle of the monooxygenase enzyme
cytochrome P450, and heme-copper assemblies.1-5 Some iron-
porphyrin complexes possess superoxide dismutase (SOD)
activity and have been studied for their possible therapeutic
It is well established that, in aprotic coordinating solvents
(e.g., dimethyl sulfoxide (DMSO) or MeCN), 1 equiv of KO2
reduces Fe(III)-porphyrins to Fe(II), whereas an additional
equivalent of KO2 produces an Fe(III)-peroxo-porphyrin
species (Scheme 1).8,9
Scheme 1
[FeIII(porphyrin)]+ + O2- f [FeII(porphyrin)] + O2 (1)
† Institute of Inorganic Chemistry, University of Erlangen-Nu¨rnberg.
‡ Institute of Organic Chemistry, University of Erlangen-Nu¨rnberg.
§ Institute of Inorganic Chemistry, Christian-Albrechts-University Kiel.
(1) Momenteau, M.; Reed, C. A. Chem. ReV. 1994, 94, 659-598.
(2) Schlichting, I.; Berendzen, J.; Chu, K.; Stock, A. M.; Maves, S. A.; Benson,
D. E.; Sweet, R. M.; Ringe, D.; Petsko, G. A.; Sligar, S. G. Science 2000,
287, 1615-1622.
(3) Jensen, K. P.; Ryde, U. J. Biol. Chem. 2004, 279, 14561-14569.
(4) Wertz, D. L.; Valentine, J. S. Struct. Bonding (Berlin) 2000, 97, 37-60.
(5) Kim, E.; Chufan, E. E.; Kamaraj, K.; Karlin, K. D. Chem. ReV. 2004, 104,
1077-1133.
[FeII(porphyrin)] + O2- f [FeIII(porphyrin)(O22-)]- (2)
Fe(III)-peroxo-porphyrin complexes comprising ligands of
different electronic properties were extensively investigated by
Valentine et al.10,11 as synthetic analogues of intermediates that
might occur during enzymatic reactions. All these complexes
9
10.1021/ja064984p CCC: $37.00 © 2007 American Chemical Society
J. AM. CHEM. SOC. 2007, 129, 4217-4228
4217

A R T I C L E S
Scheme 2
Du¨rr et al.
of KO2 and the Fe(II) form of the complex, which leads to a
quite stable Fe(III)-peroxo-porphyrin species, K[FeIII(Porph)-
(O22-)]. To our knowledge, kinetic and thermodynamic param-
eters for such reactions have not been previously reported. Even
more interesting is the fact that the Fe(II) form of the studied
complex can reversibly bind superoxide. The crown ether
covalently attached to the porphyrin, as chelator of the elec-
trophilic potassium cation, seems to play a role in the stabiliza-
tion and reactivity of the peroxo complex studied. The release
of O2- by the Fe(III)-peroxo species to form an Fe(II) species
upon fine-tuning of the proton concentration is of biological
interest, since this process might occur under physiological
conditions.
are high-spin species. The same authors studied the effect of
electron-withdrawing groups on the stability and type of
reactivity of the corresponding Fe(III)-peroxo complexes and
showed that electron-withdrawing groups attached to the por-
phyrin macrocycle stabilize peroxo species and decrease nu-
cleophilic reactivity.10 Even the most stable Fe(III)-peroxo-
porphyrin complex, [FeIII(F20tpp)(O22-)]-, containing the very
electron-poor F20tpp ligand (F20tpp ) 5,10,15,20-tetrakis-
(pentafluorophenyl)porphyrin), is stable only under an inert
atmosphere for several weeks.10 Interestingly, the same authors
reported that addition of a potassium chelator to a solution of
Fe(III)-peroxo species increases its stability,9 suggesting the
existence of an interaction between the potassium ion and the
coordinated peroxo ligand.
It has been postulated, on the basis of the crystal structure of
[MnIII(tpp)(O22-)]-, that the peroxide ligand in an Fe(III)-
peroxo-porphyrin species is coordinated in a side-on bidentate
manner.12 However, it seems that the end-on form of these
complexes leads to nucleophilic attack and that axial trans
coordination of DMSO results in the opening of the triangular
peroxo chelate ring, which increases the nucleophilicity of Fe-
(III)-peroxo species.13a Coordination of the axial ligand as a
switch for chelate ring-opening of the peroxo ligand and a
promoter of the nucleophilic reactivity was proposed to play a
role in reactions of peroxo complexes derived from group VIII
transition metals, as well as in enzymatic reactions.13a Interest-
ingly, it has been demonstrated that side-on peroxo non-heme
Fe(III) and Mn(III) species exhibit nucleophilic but not elec-
trophilic reactivity.13bc
Experimental Section
Materials. Reagents and solvents were obtained from commercial
sources and were of reagent quality unless otherwise stated. DMSO
and acetonitrile were purchased as extra-dry solvents. All chemicals
were used as received without further purification. The preparation of
the zinc complex of H2Porph is described elsewhere.14 The ligand H2-
ttbpp (54,104,154,204-tetra-tert-butyl-5,10,15,20-tetraphenylporphyrin)
was obtained as a byproduct in the synthesis of a precursor of 1. K18O2
was prepared according to the published method.15 18O2 was obtained
from Eurisotop, France. Fe(tpp) and Fe(oep) (tpp ) meso-tetraphe-
nylporphinato, oep ) octaethylporphinato) were prepared according
to an established literature method by metalation of the free ligands.16
57FeCl2 was synthesized using 57Fe powder (enrichment 95.95%,
Medical Isotopes). 57FeCl2 dihydrate was synthesized according to a
literature method.17 The light-blue dihydrate was dried in a drying pistol
at 220 °C until a light brown color was obtained.
H2Porph. The bromomethylporphyrin precursor 118 (950 mg, 1
mmol), monoaza-18-crown-6 (290 mg, 1.1 mmol), and NaHCO3 (92
mg, 1.1 mmol) were dissolved in dry toluene (25 mL) and refluxed for
24 h. The solvent was removed in vacuo, and the residue was purified
by column chromatography (silica gel, eluent ethyl acetate/methanol
9:1). Yield: 914 mg (81% based on porphyrin precursor) of a dark
1
violet product with metallic luster. H NMR (400 MHz, CDCl3, 25
3
°C): δ [ppm] ) 8.91 (s, 4 H, â-pyrrole), 8.87 (d, 2 H, J ) 4.7 Hz,
3
3
â-pyrrole), 8.67 (d, 2 H, J ) 4.7 Hz, â-pyrrole), 8.22 (d, 2 H, J )
3
8.1 Hz, o-aryl-H), 8.17 (d, 4 H, J ) 7.7 Hz, o-aryl-H), 7.87 (d, 1 H,
4
4J ) 1.7 Hz, aryl-H), 7.79 (m, 6 H, m-Ar-H), 7.53 (d, 1 H, J ) 1.7
Hz, aryl-H), 3.19 (s, 2 H, aryl-methylene-H), 3.16 (m, 4 H, methylene-
H), 3.14 (m, methylene-H), 2.98 (m, 4 H, methylene-H), 2.92 (m, 4 H,
methylene-H), 2.69 (t, 4 H, J ) 6.1 Hz, methylene-H), 2.26 (t, 4 H,
In this paper, we present the synthesis and characterization
of the Fe(III) complex of a novel crown ether-porphyrin
conjugate (H2Porph) as ligand (Scheme 2, H2Porph ) 52-N-(4-
aza-18-crown-6)methyl-54,104,154,204-tetra-tert-butyl-56-methyl-
5,10,15,20-tetraphenylporphyrin)14 and its reaction with KO2.
We also studied the kinetics and thermodynamics of the reaction
3
3J ) 6.1 Hz, methylene-H), 1.97 (s, 3 H, methyl-H), 1.64 (s, 27 H,
aryl-tert-butyl-H), 1.63 (s, 9 H, aryl-tert-butyl-H), -2.58 (s, 2 H, NH).
13C NMR (100 MHz, CDCl3, 25 °C): δ [ppm] ) 150.9, 150.4 (3 C,
p-aryl-Cq), ca. 147-145 (4 C, R-pyrrolic-C), 140.9, 139.3, 139.1, 138.9,
138.0, 134.5, ca. 131-130 (4 C, â-pyrrolic-C), 124.6, 123.6, 123.6,
123.12, 120.2, 119.8, 117.5, 70.2, 70.0, 69.7, 69.4, 59.1, 53.5, 34.9,
31.8, 31.7, 21.9. FAB-MS: m/z ) 1129 (M+), 865 (M+ - monoaza-
18-crown-6). UV/vis (CH2Cl2): λmax [nm] (ꢀ [L mol-1 cm-1]) ) 421
(335 000), 517 (13 500), 552 (6800), 590 (4100), 646 (4100). IR
(KBr): ν˜ [cm-1] ) 3316, 2957, 2902, 2865, 1474, 1362, 1350, 1108,
967, 801. Elemental analysis for C74H89N5O5‚2H2O calculated: C,
76.32; H, 8.05; N, 6.01. Found: C. 76.71; H, 7.72; N, 6.05.
(6) Batinic-Haberle, I.; Spasojevic, I.; Hambright, P.; Benov, L.; Crumbliss,
A. L.; Fridovich, I. Inorg. Chem. 1999, 38, 4011-4022.
(7) Kasugai, N.; Murase, T.; Ohse, T.; Nagaoka, S.; Kawakami, H.; Kubota,
S. J. Inorg. Biochem. 2002, 91, 349-355.
(8) McCandlish, E.; Miksztal, A. R.; Nappa, M.; Sprenger, A. Q.; Valentine,
J. S.; Stong, J. D.; Spiro, T. G. J. Am. Chem. Soc. 1980, 102, 4268-4271.
(9) Burstyn, J. N.; Roe, J. A.; Miksztal, A. R.; Shaevitz, B. A.; Lang, G.;
Valentine, J. S. J. Am. Chem. Soc. 1988, 110, 1382-1388.
(10) Selke, M.; Sisemore, M. F.; Valentine, J. S. J. Am. Chem. Soc. 1996, 118,
2008-2012.
[FeIII(Porph)Cl]. H2Porph (300 mg, 0.266 mmol) was dissolved in
chloroform (30 mL). A solution of anhydrous FeCl2 (50 mg, 0.391
mmol) in ethanol (30 mL) was added. Two drops of the base 2,6-
(11) Sisemore, M. F.; Selke, M.; Burstyn, J. N.; Valentine, J. S. Inorg. Chem.
1997, 36, 979-984.
(12) VanAtta, R. B.; Strouse, C. E.; Hanson, L. K.; Valentine, J. S. J. Am. Chem.
Soc. 1987, 109, 1425-1434.
(13) (a) Selke, M.; Valentine, J. S. J. Am. Chem. Soc. 1998, 120, 2652-2653.
(b) Park, M. J.; Lee, J.; Suh, Y.; Kim, J.; Nam, W. J. Am. Chem. Soc.
2006, 128, 2630-2634. (c) Seo, M. S.; Kim, J. Y.; Annaraj, J.; Kim, Y.;
Lee, Y.-M.; Kim, S.-J.; Kim, J.; Nam, W. Angew. Chem., Int. Ed. 2007,
46, 377-380.
(15) Rosenthal, I. J. Labelled Compd. Radiopharm. 1976, 12, 317-318.
(16) Borovkov, V. V.; Lintuluoto, J. M.; Inoue, Y. Synlett 1999, 61-63.
(17) Janiak, C.; Dorn, T.; Paulsen, H.; Wrackmeyer, B. Z. Anorg. Allg. Chem.
2001, 627, 1663.
(18) Huyen, N. H.; Jannsen, U.; Mansour, H.; Jux, N. J. Porphyrins Phthalo-
cyanines 2004, 8, 1356-1365.
(14) Helmreich, M. Ph.D. thesis, University of Erlangen-Nu¨rnberg, 2005.
9
4218 J. AM. CHEM. SOC. VOL. 129, NO. 14, 2007

Iron(III) Complex of a Crown Ether−Porphyrin Conjugate
A R T I C L E S
lutidine were added, and the mixture was refluxed for 24 h open to the
atmosphere. The reaction was monitored by thin-layer chromatography
on alumina plates with CH2Cl2/methanol 9:1 as eluent. The reaction
was completed when the typical fluorescence of the ligand had
completely vanished. The reaction mixture was filtered to remove excess
iron salts, and the solvent was evaporated. The product was purified
by recrystallization from a mixture of pentane and dichloromethane to
give a microcrystalline purple-black substance in 80% yield. 1H NMR
(400 MHz, CDCl3, 25 °C): δ [ppm] ) 84.8, 79.2, 77.9 (bs, 8 H,
â-pyrrole), 17.9 (s, 1 H, m-aryl-H), 16.1 (s, 1 H, m-aryl-H), 13.9 (s, 3
H, m-aryl-H), 12.7 (s, 3 H, m-aryl-H), 2.61 (s, 36 H, aryl-tert-butyl-
H). FAB-MS: m/z ) 1217 (M+), 1182 (M+ - Cl). UV/vis (CH2Cl2):
δ [ppm] ) 80.46. UV/vis (DMSO): λmax [nm] (ꢀ [L mol-1 cm-1]) )
423 sh (76 600), 429 (80 000), 518 (6670), 563 (4550), 583 (4180),
645 (2630), 710 (560).
[FeII(Porph)]. The reduced form of [FeIII(Porph)Cl] could be
obtained by chemical reduction or bulk electrolysis. Chemical reduction
in dry DMSO was achieved by using sodium dithionite (saturated
solution), potassium superoxide (KO2, see below), cobaltocene, or
nickelocene (1:5 Fe(III):reductant molar ratio) as reductant. In the case
of superoxide, a 10-5 M solution of [FeIII(Porph)Cl] in dry DMSO was
prepared, and a solution containing a 10-fold excess of KO2 in dry
DMSO was added. UV/vis (DMSO): λmax [nm] (ꢀ [L mol-1 cm-1]) )
412 sh (35 500), 430 (230 000), 531 (9670), 561 sh (5110), 585 sh
(3550), 650 sh (2220), 700 sh (1400). 1H NMR (reduction with
cobaltocene, 300 MHz, DMSO-d6, 25 °C): δ [ppm] ) 10.37, 7.77,
7.53, 7.28, 6.51, 6.13, 6.036.19, 3.07, 2.96, 2.88, 2.25, 1.99, 1.70, 1.30.
K[FeIII(Porph)(O22-)]. KO2 was suspended in a solution of [FeIII-
(Porph)Cl] in dry DMSO. The solution was stirred until the color
changed to green and was filtered under argon atmosphere. UV/vis
(DMSO): λmax [nm] (ꢀ [L mol-1 cm-1]) ) 429 sh (39 500), 440
(197 000), 550 sh (8320), 570 (11 280), 597 sh (6230), 615 (5500),
628 (5120).
λ
max [nm] (ꢀ [L mol-1 cm-1]) ) 384 (31 300), 420 (64 700), 512 (7740),
579 (2900), 701 (1860). IR (KBr): ν˜ [cm-1] ) 3450, 2960, 2903, 2868,
1462, 1396, 1363, 1333, 1296, 1268, 1202, 1110, 1071, 999, 806, 723.
Elemental analysis for C74H87ClFeN5O5‚3H2O calculated: C, 69.88;
H, 7.37; N, 5.51. Found: C, 69.50; H, 7.56; N, 5.36.
[57FeIII(Porph)Cl]. The synthesis was carried out as for that of [FeIII-
(Porph)Cl], except that anhydrous 57FeCl2 was used instead of FeCl2.
The spectroscopic data (UV/vis, IR) were identical to those of the
unlabeled compound.
[(FeIII(Porph))2O]. The bis-µ-oxo dimer was prepared by shaking
a solution of [FeIII(Porph)Cl] (50.0 mg, 0.041 mmol) in CH2Cl2 (20
mL) with a 2 M solution of aqueous NaOH (20 mL). The organic layer
was separated and dried over anhydrous MgSO4, and the solvent was
Equipment. Elemental analysis was performed on a HERAEUS
CHN-Mikroautomat. A Hewlett-Packard 8452A spectrophotometer was
used for UV/visible spectrophotometric measurements. NMR spectra
in CDCl3 and DMSO-d6 were measured on a JEOL GX 400 NMR
instrument or a Bruker Avance 300. All spectra were recorded using 5
mm o.d. NMR tubes, and chemical shifts were reported as δ (ppm)
values calibrated to natural abundance deuterium solvent peaks (ppm).
The IR spectra were recorded on a Mattson FT IR 60 AR instrument
using liquid samples as films between KBr and NaCl plates, respec-
tively. The spectrum of DMSO was used as reference. Porphyrin
solutions were prepared by dissolving the solid sample in dry DMSO
to give a 0.5 mM solution. The samples of the peroxo species were
prepared by suspending solid KO2 in the complex solution until the
color change occurred, indicating formation of the peroxo species. After
that, the mixture was filtered using a syringe filter, and the UV/vis
spectrum was recorded to confirm the presence of the peroxo species.
The neat KO2 solutions were prepared by suspending solid KO2 in dry
DMSO and filtered through a syringe filter.
Cyclic voltammetric measurements were carried out using an Autolab
instrument with a PGSTAT 30 potentiostat. A conventional three-
electrode arrangement was employed, consisting of a gold working disc
electrode (Metrohm, geometric area 0.07 cm2), a platinum wire
(Metrohm) as the auxiliary electrode, and Ag wire as a pseudo reference
electrode. All measurements were done in DMSO in the presence of
0.1 M tetraethylammonium hexafluorophosphate as supporting elec-
trolyte. The Fc+/Fc (Fc ) ferrocene) couple was used to calibrate the
redox potentials, which are reported in volts vs SCE (E1/2(Fc+/Fc) )
0.43 V vs SCE).19 All solutions without superoxide were thoroughly
degassed with nitrogen prior to being used, and during the measure-
ments a nitrogen atmosphere was maintained. Measurements with
superoxide were carried out by saturating the solution with dry air.
Sample concentration was 1.0 mM. All experiments were performed
at room temperature.
1
evaporated. A brown solid was obtained in 94% yield. H NMR (400
MHz, CDCl3, 25 °C): δ [ppm] ) 16.0, 14.3, 13.8, 12.6 (bs, 16 H,
â-pyrrole), 8.30, 8.08, 7.78, 7.65, 7.53, 7.34, 7.32 (m, 28 H, o-aryl-H,
m-aryl-H), 3.81, 3.74, 3.68, 3.63,3.61, 3.60, 3.55 (m, methylene-H of
crown ether), 1.28 (s, 18 H, aryl-tert-butyl-H), 1.22 (s, 54 H, aryl-tert-
butyl-H), 1.73-0.756 (m, aliphatic groups). 13C NMR (100 MHz,
CDCl3, room temperature): δ [ppm] ) 150.4, 123.3, 70.9, 70.8, 70.6,
70.4, 60.9, 52.8, 37.9, 34.4, 33.5, 31.8, 31.5, 29.4, 22.4. FAB-MS: m/z
) 2380 (M+). UV/vis (CH2Cl2): λmax [nm] (ꢀ [L mol-1 cm-1]) ) 414
(151 000), 514 (10 100), 574 (9930), 615 (5640). IR (KBr): ν˜ [cm-1
]
) 2961, 2925, 2866, 1731, 1461, 1394, 1362, 1335, 1262, 1201, 1110,
1026, 999, 863, 720.
K[FeIII(Porph)(CN)2]. [FeIII(Porph)Cl] (50.0 mg, 0.041 mmol) was
dissolved in CH2Cl2 (20 mL), and KCN (200 mg, 2.90 mmol) was
added as a solid. The mixture was stirred for 24 h. The solution was
filtered and the solvent evaporated. A green solid was obtained in 90%
1
yield. H NMR (400 MHz, CDCl3, 25 °C): δ [ppm] ) 9.78, 8.70,
8.53, 8.42 (8 H, m-aryl-H), 3.65, 3.64, 3.62, 3.61, 3.51, 3.40 (m-aryl-
H, methylene-H), 3.19 (s, 2 H, aryl-methylene-H), 2.80 (s, 3 H, methyl-
H), 2.49, 1.60 (methylene-H), 1.47, 1.35 (aryl-tert-butyl-H), 1.33, 1.25,
1.16, 1.14, 1.13 (methylene-H), -3.13, -3.32, -4.16, -4.39 (4s, 8 H,
â-pyrrole). 13C NMR (100 MHz, CDCl3, room temperature): δ [ppm]
) 156.6, 155.4, 150.6 (4 C, p-aryl-Cq), 127.8 (8 C, o-aryl-CH), 123.8
(8 C, m-aryl-CH), 85.4, 85.1, 84.0, 80.6 (8 C, â-pyrrole), 75.2, 75.1,
74.9, 73.3, 72.8, 70.9, 70.8, 70.7, 70.5 (OCH2), 52.5 (aryl-CH2), 33.6,
33.2 (8 C, tert-butyl-Cq), 32.0, 31.6 (24 C, tert-butyl-CH3), 16.7 (6 C,
CH3). FAB-MS: m/z ) 1272 (M+). UV/vis (CH2Cl2): λmax [nm] (ꢀ [L
mol-1 cm-1]) ) 333 (17 300), 433 (76 300), 539 (2530), 582 (2660),
678 (3110). IR (KBr): ν˜ [cm-1] ) 2961, 2902, 2866, 2115, 1794, 1529,
1505, 1461, 1394, 1351, 1300, 1265, 1201, 1110, 1011, 951, 812, 792,
712.
[FeIII(Porph)(DMSO)2]+. A solution of [FeIII(Porph)Cl] in DMSO
yielded the bis-solvent complex of the iron-porphyrin. 1H NMR (300
MHz, DMSO-d6, room temperature): δ [ppm] ) 71.46, 70.74, 11.33,
9.67, 8.27, 7.52, 7.01, 5.08, 4.97, 4.24, 2.59, 2.49, 2.39, 2.06, 1.89,
1.57, 1.22, 1.13. UV/vis (DMSO): λmax [nm] (ꢀ [L mol-1 cm-1]) )
399 sh (69 660), 420 (80 800), 531 (9870), 588 sh (1010), 650 (1850),
695 (2520).
Electrochemical reduction was performed under nitrogen at a Pt
gauze working electrode with a Ag wire pseudo reference electrode
and a Pt mesh auxiliary electrode, separated from the working electrode
compartment by a glass frit. The electrolysis was terminated when the
potential exceeded 0.5 V more negative than that of the FeIII/II couple.
Mo1ssbauer Measurements. 57Fe Mo¨ssbauer spectroscopy was done
in frozen DMSO solution on a Mo¨ssbauer spectrometer operating in
standard transmission geometry with an MR260A drive system and an
MVT-1000 transducer (both from Wissenschaftliche Elektronik GmbH,
Starnberg) with a 25 mCi 57Co(Rh) source in symmetric triangle velocity
[FeIII(Porph)OH]. Addition of hydroxide ions as water or NaOH
to a solution of [FeIIIPorph(DMSO)2]+ yielded the hydroxo complex
[FeIII(Porph)OH]. 1H NMR (300 MHz, DMSO-d6, room temperature):
(19) Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877-910.
9
J. AM. CHEM. SOC. VOL. 129, NO. 14, 2007 4219

A R T I C L E S
Du¨rr et al.
mode. The temperature was controlled by a type ITC502 temperature
controller (Oxford Instruments) inside a continuous-flow cryostat, type
CF 506 (Oxford Instruments). Data were collected with a CMCA-550
data acquisition card (Wissenschaftliche Elektronik GmbH, Starnberg).
Calibration was carried out by use of the inner four lines of the six-
line pattern of R-iron at room temperature. Spectra were fitted by
Lorentzians using the software “EFFI” (Hartmut Spiering, Mainz). The
[57FeIII(Porph)Cl] complex (7.00 mg) was dissolved in dry DMSO (0.4
mL) under an atmosphere of argon in a drybox. To obtain the Fe(II)
and Fe(III)-peroxo species, an excess of sodium dithionite or potassium
superoxide, respectively, was added to the [57FeIII(Porph)Cl] solution
and the resulting slurry was stirred for 15 min. This solution was filtered
through a syringe filter, put into a Kel-F polychlorotrifluoroethylene
(PCTFE) container, and sealed with a plug. The samples were
immediately frozen in liquid nitrogen upon removal from the drybox.
Structural Determination of [FeIII(Porph)Cl]‚H3O+‚FeCl4-‚C6H6‚
EtOH. Purple needle-shaped crystals suitable for X-ray diffraction were
obtained by slow diffusion of pentane into a benzene solution of the
raw material obtained directly from the reaction mixture after its
filtration and solvent evaporation. [FeIII(Porph)Cl] cocrystallized with
crown ether coordinated water, ethanol, benzene, and one unit of
tetrachloroiron(III) acid. The ethanol molecule in the structure showed
disorder due to an inversion center (50:50 occupation). An Enraf-Nonius
KappaCCD area detector was used for data collection at 173 K. Cell
parameters were obtained from 10 frames using a 10° scan and refined
with 17 871 reflections. The structure was solved by using direct
methods. The complex crystallizes in the space group P1h with two
formula units per unit cell. Calculations were carried out with the
SHELX software,20 and the graphics were generated with ORTEP-3.21
Further details can be found in Table S2 in the Supporting Information.
Preparation of Superoxide Solution. The moisture-sensitive KO2
solution was prepared and handled either under a nitrogen or an argon
atmosphere by using standard Schlenck techniques or in an MBraun
glovebox under an argon atmosphere. A slurry of KO2 (200 mg, 2.82
mmol) in a 0.1 M solution of NBu4PF6 in DMSO (20 mL) was
vigorously stirred for 30 min. The mixture was filtered through a syringe
filter and the filtrate kept in the dark. Such a stock solution was stable
(not more than 5% decrease in concentration) for about 5 h. Solutions
of desired superoxide concentrations were prepared by diluting this
stock solution with a DMSO solution of the electrolyte immediately
before use, and the concentration was determined spectrophotometrically
before and after measurement. Whenever the concentration of super-
oxide decreased during the course of an experiment by more than 5%,
the results were discarded.
stopped-flow module combined with a Huber CC90 cryostat and
equipped with a J&M TIDAS high-speed diode array spectrometer with
combined deuterium and tungsten lamps (200-1015 nm wavelength
range). Isolast O-rings were used for all sealing purposes to enable
measurements in DMSO, and solutions were delivered from 10 mL
gastight Hamilton syringes. The syringes are controlled by separate
drives, allowing for variation of the ratio of mixing volumes used in
the kinetic runs. Data were analyzed by using the integrated Bio-Kine
software, version 4.23, and also the Specfit/32 program. At least five
kinetic runs were recorded under all conditions, and the reported rate
constants represent the mean values. The stopped-flow instrument was
thermostated to the desired temperature (0.1 °C. Experiments at 25
°C were performed in DMSO solution, whereas for the low-temperature
measurements a solution of 30% DMSO in CH3CN was used. During
all kinetic measurements, the ionic strength was kept constant at 0.1
M NBu4PF6. The reaction was followed in excess of KO2, and the
applied complex concentration was in the range of 5 × 10-5-5 × 10-6
M. Concentration-dependent measurements were performed by mixing
different concentrations of KO2 with the same complex concentration
in a 1:1 volume ratio and also by using the same KO2 solution ([KO2]
) 2 and 0.2 mM, corresponding to 50% and 5% of the saturated KO2
solution in DMSO, respectively) and applying the variable-mixing-
volume-ratio option. For temperature dependence measurements in the
DMSO/CH3CN mixture within the -40 to 0 °C temperature range, a
1 mM solution of KO2 was mixed with a 1 × 10-5 M solution of the
complex in a 1:1 volume ratio. Because of the decreased stability of
KO2 in the DMSO/CH3CN solvent mixture, a fresh KO2 solution was
prepared every 0.5 h, so that the concentration decrease was not greater
than 5%. Values of ∆Hq and ∆Sq were calculated from the slope and
intercept, respectively, of the plot of ln(k/T) versus 1/T (the second-
order rate constant k was defined as kobs/[O2-]).
Equilibrium Measurement. A spectrophotometric study of the
binding of superoxide to [FeII(Porph)(DMSO)2] was performed in a
1 cm path length cuvette with a 25 mL reservoir under nitrogen. The
5 × 10-6 M solution (20 mL) of [FeII(Porph)(DMSO)2] complex in
0.1 M NBu4PF6 was purged with dry nitrogen for 20 min, and then
aliquots of spectrophotometrically verified saturated KO2 solution were
added from a Hamilton microsyringe. The titration spectra were
analyzed with Specfit/32 using the M/L/H+ model, treating all
components as colored species, and taking into consideration the dilution
effects. The same procedure was repeated by using a 1 × 10-5
M
solution of [FeII(Porph)(DMSO)2] and DMSO-KO2 solutions without
electrolyte.
Results and Discussion
The superoxide solution in the DMSO/CH3CN mixture was prepared
according to the following procedure. Solid KO2 (1 mg, 0.014 mmol)
and cis-dicyclohexano-18-crown-6 (52.1 mg, 0.140 mmol) were
weighed directly into the electrolyte solution (14 mL of 0.1 M (NBu4)-
PF6 in a 30:70 mixture of DMSO/CH3CN) and stirred until completely
dissolved. This solution was also kept in the dark; however, within
approximately 30 min the concentration decreased by 5%.
Concentration of Superoxide Solution. The concentration of
saturated KO2 in 0.1 M NBu4PF6 in DMSO was determined to be 4.0
( 0.1 mM by optical absorbance measurements (0.1 mm pathlength
cell)22 as well as by ferricytochrome c titration.23 The same methods
were used to determine the concentration of saturated KO2 in DMSO
without electrolyte, which was found to be 2.5 ( 0.2 mM.
Synthesis and Structure. The novel porphyrin-crown ether
conjugate H2Porph used in this work14 is a tetraphenylporphyrin
with tert-butyl groups at the para position of the porphyrin which
enhance the solubility in organic solvents. H2Porph is easily
accessible via a nucleophilic substitution reaction of the
bromomethylated porphyrin precursor 118 (Scheme 2) in the
presence of sodium bicarbonate as acid scavenger. Aza-18-
crown-6 is attached through its amine group to a methyl group
at the ortho position of one aryl ring. This close proximity of
the crown ether gives the metalated form of the ligand its unique
properties. The metalloporphyrin can function as a ditopic
receptor by coordinating a potassium cation, which can then
interact with a diatomic anionic ligand bound to the metal center.
Such an interaction has already been confirmed by X-ray
structure analysis of the corresponding zinc(II) and cobalt(III)
complexes with one and two coordinated cyanide anions,
respectively.14 In both cases, a potassium ion is coordinated in
the crown ether, which is positioned directly over the porphyrin
ring. A similar behavior is expected for the analogous iron-
(III)-porphyrin complex reported herein.
Kinetic Measurements. Kinetic data were obtained by recording
time-resolved UV/vis spectra using a modified µSFM-20 Bio-Logic
(20) Sheldrick, G. M. SHELXS-97 and SHELXL-97, programs for crystal
structure solution and structure refinement; Universita¨t Go¨ttingen: Go¨t-
tingen Germany, 1997.
(21) ORTEP-3 for Windows 1.06: Farrugia, L. J. J. Appl. Crystallogr. 1997,
30, 565.
(22) Sawyer, D. T.; Calderwood, T. S.; Yamaguchi, K.; Angelis, C. T. Inorg.
Chem. 1983, 22, 2577-2583 and references therein.
(23) Arudi, R. L.; Allen, A. O.; Bielski, B. H. J. FEBS Lett. 1981, 135, 265-
267.
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Iron(III) Complex of a Crown Ether−Porphyrin Conjugate
A R T I C L E S
probably engendered by electrostatic interaction between the
anionic chloro ligand and the cationic crown ether (the Cl1-
N101 distance is 4.44 Å). Further structural data are given as
Supporting Information (Tables S2 and S3).
The bis(cyano)iron(III) complex of H2Porph was also syn-
thesized to study the influence of the potassium ion on the spin
state of the iron center. Synthesis was performed by stirring
solid KCN with [FeIII(Porph)Cl] in dichloromethane, which takes
up the KCNsmost likely with the crown ether acting as a phase-
transfer catalyst. The bis(cyano) complex K[FeIIIPorph(CN)2]
was first examined by proton NMR spectroscopy. The spectrum
shows an iron(III)-porphyrin low-spin complex (S ) 1/2) with
four resonances for the pyrrolic protons between -3 and -5
ppm. This high-field shift is not very strong relative to those of
similar low-spin iron(III)-porphyrins24 that have pyrrole proton
resonances around -16 ppm. Thus, the complex exists in the
(dxz,dyz)4(dxy)1 spin state rather than the more usual (dxy)2(dxz,dyz)3
spin state. The (dxz,dyz)4(dxy)1 spin state is known25 to occur in
the presence of solvents that can form hydrogen bonds to the
cyanide ligands, thus increasing their π-acceptor ability, which
leads to back-bonding from the iron center to the cyanide ligands
and not to the porphyrin ring. In our case, this effect is too
strong to be caused by the deuterochloroform solvent, which
can only form weak hydrogen bonds to the cyanide ligands.
Our conclusion is that this strong increase in π-acceptor ability
at the cyanide ligands can only come from interaction with the
positively charged potassium ion coordinated by the crown ether,
which pulls electron density away from the ligands. This was
confirmed by a proton NMR experiment with Bu4N+CN-
instead of KCN. Bu4N+CN- was mixed with [FeIII(Porph)Cl]
in CDCl3 in the NMR tube, and the spectrum was recorded. In
this case, the shift to higher field by the pyrrolic protons is
stronger (-8 to -10 ppm) than that observed with our bis-
(cyano) complex. We interpret this to mean that the bulky
Bu4N+ cation cannot be coordinated by the crown ether, and,
therefore, only interaction between solvent and cyanide occurs.
These results show that our iron(III)-porphyrin acts as a ditopic
receptor for diatomic anions as it has been demonstrated in the
crystal structures of the corresponding zinc and cobalt com-
plexes.14 A strong electrostatic interaction, like the one between
chelated K+ and CN-, is also to be expected between chelated
K+ and the peroxide anion (see below) and may be one of the
reasons for the enhanced stability of the peroxo species.
We also performed proton NMR experiments in DMSO-d6
on [FeIII(Porph)Cl] because our further experiments were carried
out in this solvent. [FeIII(Porph)Cl] shows the behavior typical
of an iron(III)-porphyrin in DMSO, having lost the chloro
ligand and coordinated two DMSO molecules upon solva-
tion.26,27 This can be observed by the shift of the resonances of
the pyrrolic protons from around 82 ppm in CDCl3 to around
72 ppm in DMSO.27,28 Trace amounts of the hydroxo complex
were also observed in the NMR spectrum of the Fe(III) complex,
with a broad resonance at 80 ppm.
Figure 1. ORTEP representation of [FeIII(Porph)Cl]‚H3O+: (a) side view
and (b) top view.
The metalloporphyrin [FeIII(Porph)Cl] was prepared according
to an established procedure16 by refluxing a chloroform solution
of the ligand with an ethanolic solution of ferrous chloride in
the presence of 2,6-lutidine. When the material was crystallized
from dichloromethane/pentane mixtures, analytically pure high-
spin chloro iron(III)-porphyrin was obtained. The resonances
of the â-pyrrolic protons of the porphyrin were found in the
typical region of around 77-80 ppm. If pentane was allowed
to diffuse slowly into a solution of freshly prepared material in
benzene, crystals suitable for X-ray structural analysis grew.
Interestingly, [FeIII(Porph)Cl] cocrystallized with a crown-ether-
coordinated water molecule and one unit of tetrachloroiron(III)
acid, its proton most likely bound to the nitrogen atom of the
crown ether (Figure 1). Also, one molecule each of ethanol and
benzene are found in the lattice. Although structural resolution
was not formidable (R factor ) 7.5%), the data obtained show
a typical high-spin five-coordinate Fe(III) center with the chloro
ligand pointing to the coordinated water molecule in the crown
ether. The metal center is displaced by 0.47 Å from the mean
porphyrin plane toward the crown ether. A water molecule
bound within the crown ether interacts with the protonated
nitrogen atom of the crown ether. The overall topology is most
UV/Vis Spectra. The property of the [FeIII(Porph)Cl] com-
plex to act as a ditopic receptor toward diatomic anions is very
attractive for studying its reaction with superoxide. The reaction
(24) La Mar, G. N.; Walker, F. A. J. Am. Chem. Soc. 1973, 95, 1782-1790.
(25) Ikezaki, A.; Nakamura, M. Inorg. Chem. 2002, 41, 2761.
(26) Kadish, K. M.; Bottomley, L. A.; Beroiz, D. Inorg. Chem. 1978, 17, 1124.
(27) Shin, K.; Kramer, S. K.; Goff, H. M. Inorg. Chem. 1987, 26, 4103-4106.
(28) Zobrist, M.; La Mar, G. N. J. Am. Chem. Soc. 1978, 9, 61-79.
9
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A R T I C L E S
Du¨rr et al.
Figure 2. UV/vis absorption spectra of (a) [FeIIIPorph(DMSO)2]+, (b) [FeIII(Porph)OH], (c) [(FeIII(Porph))2O], (d) [FeII(Porph)(DMSO)2], and (e)
K[FeIII(Porph)(O22-)].
Scheme 3
of Fe(III)-porphyrins with superoxide is convenient to follow
in DMSO as aprotic solvent, in which the solubility and stability
of KO2 (a common source of O2-) are satisfactory. It is also
known that the use of potassium chelators will enhance the
solubility of KO2. Moreover, the presence of potassium chelators
can influence the properties of the product species that result
from the reaction between Fe(III)-porphyrin complexes and
superoxide.9
Dissolution of [FeIII(Porph)Cl] in DMSO produces an [FeIII-
(Porph)(DMSO)2]+ complex with a characteristic Soret band
at 420 nm and a shoulder at 399 nm (Figure 2a). In wet DMSO
2- -
spectrum of the peroxo [FeIII(tpp)O2
] complex obtained in
or upon addition of NaOH, a hydroxo complex is formed with
a Soret band at 429 nm (Figure 2b); a dimer does not form in
DMSO. The UV/vis spectrum of separately synthesized [(FeIII-
(Porph))2O] (see the Experimental Section) in DMSO shows a
Soret band at 414 nm with a shoulder at 429 nm (Figure 2c).
By adding acid (trifluoromethanesulfonic acid, HOTf) to the
solution of the hydroxo or dimer species, the spectrum of [FeIII-
(Porph)(DMSO)2]+ was obtained.
The reaction of [FeIII(Porph)(DMSO)2]+ and KO2 was
investigated using a tandem cuvette. When a 10-fold excess of
superoxide is mixed with the [FeIII(Porph)(DMSO)2]+ complex,
reduction of the Fe(III) center is observed, as indicated by the
shift of the Soret band maximum from 420 to 430 nm8 (step I
in Scheme 3). Reduction of the iron center by superoxide is
confirmed by comparison of the resulting spectrum (Figure 2d)
to the one obtained by electrochemical (see Experimental
Section) or chemical (by sodium dithionite) reduction of [FeIII-
(Porph)(DMSO)2]+ (Figure 3). When a larger excess of super-
oxide (g20-fold) is added to the solution of [FeIII(Porph)-
(DMSO)2]+, a peroxo complex, [FeIII(Porph)(O22-)]-, is formed
(step II in Scheme 3), accompanied by a shift in the Soret band
maximum to 440 nm. The spectrum of the obtained peroxo
species (Figure 2e) is strikingly similar to the visible absorption
the presence of a potassium chelating agent.29 The low-energy
Soret band (440 nm) and the splitting of the two Q bands are
characteristic for an iron(III)-porphyrin with a peroxo ligand.8
Reversible Binding of Superoxide. In the presence of excess
superoxide in the solution, the green [FeIII(Porph)(O22-)]-
complex (λmax ) 440 nm) is quite stable, and there is no need
to handle it under an inert atmosphere. The peroxo complex is
indefinitely stable in a frozen DMSO solution, even upon
exposure to air. In a closed system under air, it is stable at room
temperature for a week. When the solution is opened to the
atmosphere, slow transformation (within a few hours) to the
[FeII(Porph)] species is observed, as monitored by UV/vis
spectroscopy. This reaction was found to be caused by absorp-
tion of moisture from the atmosphere, as no spectral changes
are observed when the peroxo complex is purged with dry
oxygen. The rate of decay of [FeIII(Porph)(O22-)]- is accelerated
by passing wet air through the solution, which results in the
formation of the brown [FeII(Porph)] complex, as monitored
by UV/vis spectroscopy. Further addition of superoxide to the
brown [FeII(Porph)] complex results in re-formation of the green
peroxo complex. Dilution of a solution of the peroxo complex
(29) See Figure 1 (top spectrum) in ref 8 and Figure 6B in ref 9.
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4222 J. AM. CHEM. SOC. VOL. 129, NO. 14, 2007

Iron(III) Complex of a Crown Ether−Porphyrin Conjugate
A R T I C L E S
Figure 5. UV/visible absorption spectra of (a) [FeIII(Porph)(DMSO)2]+,
(b) K[FeIII(Porph)(O22-)], and (c) K[FeIII(Porph)(O22-)] + 0.1 M HOTf.
Figure 3. UV/vis spectra recorded in a tandem cuvette of the solution of
5 × 10-6 M [FeIIIPorph(DMSO)2]+ and 5 × 10-5 M KO2 in DMSO at
room temperature, (a) before (λmax ) 420 nm) and (b) after (λmax ) 430
nm) mixing. Inset: UV/vis spectra of 5 × 10-6 M [FeIIIPorph(DMSO)2]+,
(a) before (λmax ) 420 nm) and (b) after (λmax ) 430 nm) reduction by
dithionite.
Figure 6. Spectral changes upon mixing 10-3 M MCPBA with a solution
of 10-1 M TBPH and 1.3 × 10-5 M [FeIII(Porph)Cl] in KCl-saturated
DMSO/CH3CN (5% DMSO) solution at room temperature, before (black
solid line) and after mixing (red dotted line).
Figure 4. Time-resolved spectra for the reaction of K[FeIII(Porph)(O22-)]
and HOTf at 25 °C in DMSO; [K[FeIII(Porph)(O22-)]] ) 1 × 10-5 M,
[HOTf] ) 5 × 10-5 M.
separate experiment, we generated an iron-oxo species in the
presence of K+ (KCl), DMSO (30% DMSO in CH3CN), and
TBPH by using m-chloroperbenzoic acid (MCPBA) (Figure S1,
Supporting Information).30 As expected, formation of the high-
valent iron-oxo species in the solution caused increases in the
absorbance at 380, 400, and 630 nm (Figure 6). As MCPBA is
not very stable in DMSO, decreasing the amount of DMSO in
solution resulted in even more pronounced spectral changes
(Figure 6). However, upon addition of TBPH to the solution of
[FeIII(Porph)(O22-)]-, no evidence for a high-valent iron-oxo
species was observed, but [FeII(Porph)] was produced instead
(Figure S2, Supporting Information). (The same was observed
when triflic acid and an excess of TBPH were added to the
solution of [FeIII(Porph)(O22-)]-.) In a subsequent study, TBPH
was also found to react with superoxide; comparison of the
spectral changes with those upon reaction with NaOH indicates
that TBPH is acting as an acid. Phenols are known to act as
results in partial formation of the [FeII(Porph)] complex, whereas
further addition of superoxide to the diluted solution again
restores the peroxo complex. All these observations suggest that
the binding of superoxide to the Fe(II) complex is reversible
and that the decrease in concentration of O2-, caused by its
decomposition in the presence of protons, shifts the equilibrium
from the Fe(III)-peroxo to the Fe(II) complex. Addition of a
moderate excess of triflic acid to the peroxo complex causes
partial formation of the [FeII(Porph)] complex (Figure 4) by
facilitating the decay of superoxide in the solution. When a large
excess of acid is added, the [FeIII(Porph)(DMSO)2]+ complex
is formed (Figure 5). Scheme 3 summarizes the observed
reactions.
To test for the presence of a high-valent iron-oxo species
as a decay product of the peroxo complex, the trap 2,4,6-tri-
(tert-butyl)phenol (TBPH) was utilized. Typically, upon oxida-
tion, TBPH forms an oxygen-centered radical, which results in
an increase of absorbance at 380, 400, and 630 nm.30 In a
(30) (a) Traylor, T. G.; Lee, W. A.; Stynes, D. V. J. Am. Chem. Soc. 1984, 106,
755-764. (b) Traylor, T. G.; Lee, W. A.; Stynes, D. V. Tetrahedron 1984,
40, 553-568.
9
J. AM. CHEM. SOC. VOL. 129, NO. 14, 2007 4223

A R T I C L E S
Scheme 4
Du¨rr et al.
Table 1. Mo¨ssbauer Parameters of the Studied Complexes in
Frozen DMSO at 80 K
δ
∆
EQ
Γli
(mm s-
Γre
(mm s-
1
1
1
1
compound
(mm s-
)
(mm s-
)
)
)
[FeIII(Porph)(DMSO)2]Cl
[FeII(Porph)(DMSO)2]
[FeII(Porph)(DMSO)]
K[FeIII(Porph)(O22-)]a
0.375
0.442
1.270
0.407
0.829
1.314
3.114
0.512
0.863
0.274
0.534
0.319
1.516
0.249
0.463
0.322
moderate acids to protonate superoxide ion in aprotic solvents
and induce its efficient disproportionation (K ) 1018).31 The
protonation of superoxide present in an equilibrium solution
causes its decay,31,32 shifting the equilibrium from [FeIII(Porph)-
(O22-)]- to [FeII(Porph)] (Scheme 3, Figure S2). This process
has been exploited in the kinetic studies of the unique reversible
binding of superoxide for direct determination of the rate
constant for dissociation of superoxide (back reaction of step
II in Scheme 3).
a A low-spin Fe(II) species present in the same solution (see text) has δ
) 0.358 mm s-1, ∆EQ ) 1.377 mm s-1, Γli ) 0.034 mm s-1, and Γre
0.369 mm s-1
)
.
the electrochemical behavior of the porphyrin ring itself, as zinc-
(II) is electrochemically inactive. In general, the redox behavior
of [FeIII(Porph)Cl] and [ZnII(Porph)] is in accordance with the
redox behavior of [FeIII(tpp)]+ and [ZnII(tpp)], respectively.34
The results of the electrochemical studies are summarized in
Scheme S1 and Figure S5 (Supporting Information).
The reaction between [FeII(Porph)] and O2 (the reverse of
the reaction between [FeIII(Porph)(DMSO)2]+ and O2-) was
investigated to check the reversibility of the first step in Scheme
3. [FeII(Porph)] (2.5 × 10-6 M), prepared by electrochemical
reduction of [FeIII(Porph)(DMSO)2]+, and O2-saturated DMSO
(1 atm, [O2] ) 2.2 mM)21 were mixed in equal proportions in
an Ar-flushed tandem cuvette. The resulting product (Figures
S3 and S4, Supporting Information) was found to be not [FeIII-
(Porph)(DMSO)2]+ but rather the hydroxo species, as indicated
by the absence of a Q-band at 530 nm, which is observed for
both [FeIII(Porph)(DMSO)2]+ and [FeII(Porph)]. There is also
the possibility that an O-bound adduct is formed, as the spectra
of [FeIII(Porph)(O2-)] and [FeIII(Porph)OH] are expected to be
similar, since both involve a negatively charged O ligand. A
similar spectrum was observed for an intermediate or side
product in the reaction of [FeIII(Porph)(DMSO)2]+ with KO2
(Figure S4), which we have shown to be the spectrum of the
hydroxo complex [FeIII(Porph)OH] (see below). The reaction
of [FeII(Porph)] and O2 did not follow a single-exponential
decay. It is possible that an [FeII(Porph)O2] adduct is produced,
which may decay further to the hydroxo complex.33 When a
solution of [FeII(Porph)] was purged at first with dried air (until
the band at 430 nm decreased in intensity and broadened) and
then with Ar, the spectrum of [FeII(Porph)] was recovered. Upon
subsequent bubbling of air for a longer period, the 430 nm peak
was observed to decrease further in intensity and increase again
upon bubbling with Ar. Recovery of the spectrum of [FeII-
(Porph)] was not complete in this case, which we attribute to
formation of the hydroxo complex. Scheme 4 summarizes the
observed reactions (see also Kinetics and Thermodynamics,
below).
The redox potentials corresponding to the Fe(III)/Fe(II)
couples of our complex are more positive than those of [FeIII-
(tpp)]+,34 suggesting higher stability of the complex in the Fe-
(II) oxidation state. This was observed when [FeII(Porph)] was
chemically or electrochemically generated, since it is stable
under air for a few hours, whereas [FeII(tpp)] oxidizes im-
mediately under air. The significant stability of [FeII(Porph)]
enabled further kinetic and thermodynamic investigations.
Mo1ssbauer Spectra. Mo¨ssbauer spectra of the 57Fe-enriched
[FeIII(Porph)Cl], [FeII(Porph)], and K[FeIII(Porph)(O22-)] species
in DMSO solution were measured at 80 K. The Mo¨ssbauer
spectrum of the starting complex, [FeIII(Porph)(DMSO)2]+
(obtained by dissolving 57Fe-enriched [FeIII(Porph)Cl] in DMSO),
exhibits an isomer shift (δ) of 0.38 mm s-1 and a quadrupole
splitting (∆Eq) of 0.83 mm s-1 (Table 1), which are character-
istic of high-spin porphyrinatoiron(III) complexes.35 The Mo¨ss-
bauer spectrum of the chemically reduced (with sodium
dithionite and also with cobaltocene), 57Fe-enriched [FeIII-
(Porph)]+ complex (Figure 7a) shows that there are two different
iron(II) species in solution, a low-spin (δ ) 0.44 mm s-1, ∆Eq
) 1.32 mm s-1) and a high-spin (δ ) 1.27 mm s-1, ∆Eq )
3.11 mm s-1) species (Table 1). Most probably, the low-spin
species is the six-coordinate bis-DMSO [FeII(Porph)(DMSO)2]
complex and the corresponding high-spin species is the five-
coordinate mono-DMSO [FeII(Porph)(DMSO)] complex.35a,36,37
To obtain the Mo¨ssbauer spectrum of [FeIII(Porph)(O22-)]-,
the 57Fe-enriched [FeIII(Porph)]+ complex was treated with a
large excess of KO2 in DMSO. The spectrum (Figure 7b) shows
the presence of both low-spin iron(II) species (δ ) 0.36 mm
s-1, ∆Eq ) 1.38 mm s-1) and high-spin [FeIII(Porph)(O22-)]-
(δ ) 0.41 mm s-1, ∆Eq ) 0.51 mm s-1) (Table 1) and suggests
an equilibrium between these two iron species and the reversible
nature of superoxide binding. The Mo¨ssbauer parameters of the
iron(II) species are typical for low-spin Fe(II) in hexacoordinate
porphyrin complexes36 and are in good agreement with those
of the low-spin iron(II) species obtained by chemical (by sodium
Electrochemistry. To examine the electrochemical properties
of [FeIII(Porph)Cl] and its reaction product with superoxide,
cyclic voltammetric experiments were carried out in nitrogen-
and oxygen-saturated DMSO solutions, respectively. By way
of comparison, the cyclic voltammograms of the corresponding
Zn(II) complex [ZnII(Porph)] were measured in order to check
(31) Chin, D.-H.; Chiericato, G., Jr.; Nanni, E. J., Jr.; Sawyer, D. T. J. Am.
Chem. Soc. 1982, 104, 1296-1299.
(32) The products of superoxide disproportionation, O2 and H2O2, were
qualitatively detected in both experiments with triflic acid and TBPH. For
the O2 detection, see: Karlin, K. D.; Cruse, R. W.; Gultneh, Y.; Farooq,
A.; Hayes, J. C.; Zubieta, J. J. Am. Chem. Soc. 1987, 109, 2668-2679.
For the H2O2 detection, a peroxide indicator paper suitable for the organic
solvents (QUANTOFIX-Peroxide 100) was used.
(34) Jones, S. E.; Srivatsa, G. S.; Sawyer, D. T. Inorg. Chem. 1983, 22, 3903-
3910.
(35) (a) Sams, J. R.; Tsin, T. B. Mo¨ssbauer Spectroscopy of Iron Porphyrins.
In The porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1979;
Vol. 4, pp 425-478. (b) Abu-Soud, H.; Silver, J. Inorg. Chim. Acta 1988,
52, 61-66.
(33) Ghiladi, R. A.; Kretzer, R. M.; Guzei, I.; Rheingold, A. L.; Neuhold, Y.-
M.; Hatwell, K. R.; Zuberbu¨hler, A. D.; Karlin, K. D. Inorg. Chem. 2001,
40, 5754-5767.
(36) Abu-Soud, H. M.; Silver, J. Inorg. Chim. Acta 1989, 161, 139-141.
(37) Andersson, L. A.; Mylrajan, M.; Loehr, T. M.; Sullivan, E. P.; Strauss, S.
H. New J. Chem. 1992, 16, 569-576.
9
4224 J. AM. CHEM. SOC. VOL. 129, NO. 14, 2007

Iron(III) Complex of a Crown Ether−Porphyrin Conjugate
A R T I C L E S
Figure 8. Time-resolved spectra for the reaction between [FeIII(Porph)Cl]
and KO2 at 25 °C in DMSO; [[FeIII(Porph)Cl]] ) 5 × 10-6 M, [KO2] )
2.5 × 10-5 M.
0.56 mm s-1), with the quadrupole splitting in the expected
range (∆Eq ) 1.17 mm s-1).40 The iron(III)-peroxo species
with the Mo¨ssbauer parameters most similar to those of our
[FeIII(Porph)(O22-)]- is the [FeIII(oep)(O22-)]- complex.9 How-
ever, its isomer shift (δ ) 0.67 mm s-1) and quadrupole splitting
(∆Eq ) 0.62 mm s-1) are larger than in our case (Table 1). It
should be mentioned that the Mo¨ssbauer measurements of
[FeIII(oep)(O22-)]- were performed on solid samples, whereas
our measurements were performed on solutions in DMSO. This
could have an influence on the type of interactions the peroxo
ligand undergoes and consequently on its coordination mode.
Infrared Spectroscopy. As in the case of [FeIII(tpp)]+,8 we
could not identify the O-O stretch in the IR spectrum of
[FeIII(Porph)(O22-)]-, prepared by using either K16O2 or K18O2,
because a very strong band of [FeIII(Porph)(DMSO)2]+ is present
Figure 7. Mo¨ssbauer spectra of reduced 57Fe-enriched [FeIII(Porph)Cl] (a)
and K[FeIII(Porph)(O22-)] (b) complexes in frozen DMSO at 80 K.
dithionite and cobaltocene) reduction of the starting iron(III)
complex (Table 1). The absence of the high-spin five-coordinate
iron(II) species in the superoxide-saturated DMSO solution can
be easily understood in terms of its higher reactivity and the
presence of a vacant coordination site, resulting in its efficient
trapping by superoxide and DMSO. This also suggests that the
dissociation of DMSO from low-spin [FeII(Porph)(DMSO)2] and
formation of high-spin five-coordinate [FeII(Porph)(DMSO)] as
a reactive intermediate is the slowest reaction step in the
formation of the Fe(III)-peroxo complex (see below).
The Mo¨ssbauer parameters of our iron(III)-peroxo complex
are interesting. Although they are typical for high-spin iron-
(III)-porphyrin complexes, they are lower than those of known
iron(III)-peroxo complexes.35,38-40 Non-heme iron(III)-peroxo
complexes, all of which are considered to be side-on η2-O2
peroxo species, have a characteristic large isomer shift located
in the 0.62-0.65 mm s-1 range and a large quadrupole splitting
in the 0.72-1.37 mm s-1 range.38 The isomer shift of one of
the first such species in a biological system for which Mo¨ssbauer
parameters are available, the seven-coordinate side-on iron(III)-
peroxo species of superoxide reductase, is slightly lower (δ )
0.54 mm s-1), which was explained by the presence of the
coordinated thiolate ligand.39 Another example of an iron(III)-
peroxo species with an isomer shift below the characteristic
range is a µ-η2:η1 peroxo-bridged heme-copper complex (δ )
(Figure S6, Supporting Information) between 900 and 700 cm-1
,
which is the region expected for the O-O stretch vibration of
a coordinated peroxo ligand. We repeated the IR measurement
for [FeIII(tpp)]+, [FeIII(oep)]+, and the corresponding peroxo
species (prepared by using KO2/18-crown-6 according to the
reported procedure).8 Interestingly, we observed that the peak
at 806 cm-1 with a shoulder at 796 cm-1, which was ascribed
to the O-O vibration due to Fe-bound peroxide ligand, is also
present in the IR spectrum of KO2 with 18-crown-6 (Figure
S7, Supporting Information), suggesting that this band is due
to the ion pair of K+-crown ether and O2-. At the same time,
we could not observe any maximum in the IR spectrum of [FeIII-
(tpp)]+ around 803 cm-1 (Figure S8, Supporting Information).8
This suggests that the band of KO2/18-crown-6 and not of a
porphyrin might be the reason that, in the IR spectrum of
[Fe(tpp)(O22-)] (prepared by using KO2/18-crown-6), it is not
possible to observe the real O-O vibration of coordinated
peroxide.
Kinetics and Thermodynamics. Depending on the selected
superoxide concentration, two separate reaction steps (Scheme
3) can be studied by the stopped-flow technique. Using less
than a 10-fold excess of O2- in DMSO at 25 °C or in a mixture
of DMSO/CH3CN at sub-zero temperatures resulted in the same
pattern as was observed in the time-resolved spectra, as
presented in Figures 8 and S9 (Supporting Information),
respectively. The spectrum of the product species is very similar
(38) (a) Horner, O.; Jeandey, C.; Oddou, J.-L.; Bonville, P.; McKenzie, C. J.;
Latour, J.-M. Eur. J. Inorg. Chem. 2002, 3278-3283. (b) Simaan, A. J.;
Banse, F.; Girerd, J.-J.; Wieghardt, K.; Bill, E. Inorg. Chem. 2001, 40,
6538-6540.
(39) Horner, O.; Mouesca, J.-M.; Oddou, J.-L.; Jeandey, C.; Niviere, V.; Mattioli,
T. A.; Mathe, C.; Fontecave, M.; Maldivi, P.; Bonville, P.; Halfen, J. A.;
Latour, J.-M. Biochemistry 2004, 43, 8815-8825.
(40) Chishiro, T.; Shimazaki, Y.; Tani, F.; Tachi, Y.; Naruta, Y.; Karasawa, S.;
Hayami, S.; Maeda, Y. Angew. Chem., Int. Ed. 2003, 42, 2788-2791.
9
J. AM. CHEM. SOC. VOL. 129, NO. 14, 2007 4225

A R T I C L E S
Du¨rr et al.
Figure 10. Plots of kobs versus [O2-] for the second step of the reaction of
5 × 10-6 M complex and KO2 at 25 °C in DMSO. Experimental
conditions: 9, starting from the Fe(II) complex and using different mixing
volume ratios; O, starting from the Fe(II) complex and preparing a new
solution for each [O2-]; and 4, starting from the Fe(III) complex and
preparing a new solution for each [O2-].
Figure 9. Time-resolved spectra for the reaction between [FeII(Porph)]
and KO2 at 25 °C in DMSO; [[FeII(Porph)]] ) 5 × 10-6 M, [KO2] ) 5 ×
10-4 M.
to that of the Fe(III)-hydroxo complex, with the difference
being that the Soret band with a maximum at 430 nm is sharper.
The reaction of [FeIII(Porph)(DMSO)2]+ with saturated KOH
solution in DMSO led to an almost identical time-resolved
spectrum (Figure S10, Supporting Information). The only
difference is that the Soret band is broader, which is typical for
the hydroxo species. This can be explained by the fact that KO2
inevitably contains some KOH, which leads to the formation
of the Fe(III)-hydroxo complex in a side reaction (Scheme 4).
A change in superoxide concentration causes a slight change
in kobs, but determination of the second-order rate constant was
not attempted because of uncertainties in the KOH concentration.
To check whether the Fe(III)-hydroxo complex is able to
react with superoxide, we tested its reaction with a 10-fold
excess of O2-. The spectrum of the product obtained within 1
s confirms the formation of the Fe(II) complex (Figure S11,
Supporting Information, and Scheme 4); moreover, with a
Table 2. Kinetic and Thermodynamic Parameters for Binding of
Superoxide to FeII(Porph) at 25 °C (Second Reaction Step in
Scheme 3)
k
k
on (M-1 s-1
)
36 500 ( 500
off (s-1
)
0.21 ( 0.01
kon/koff ) KO a (M-1
)
(1.7 ( 0.2) × 105
(0.9 ( 0.1) × 105,c (1.4 ( 0.1) × 104 d
61.2 ( 0.9
-
2
KO b (M-1
)
-
2
∆Hq (kJ mol-1
∆Sq (J K-1 mol-1
)
)
48 ( 3
a Kinetically and b thermodynamically determined equilibrium constants,
c with and d without electrolyte.
each concentration of KO2. The results of all these measurements
are presented in Figure 10. Although these data were collected
by using different starting complexes, viz. Fe(III) and Fe(II)
species, respectively, and by applying different methods for
varying the superoxide concentration, a very good linear plot
of kobs vs [O2-] was obtained. This shows that (1) the kinetics
of the second step can be studied independently of the first step
in the presence of an excess of O2- and (2) a good control over
the superoxide concentration was achieved in the present study.
From the slope of the plot in Figure 10, the second-order rate
constant kon was determined to be 36 500 ( 500 M-1 s-1 (Table
2).
-
slightly higher concentration of O2 (achieved by using the
variable-mixing-volume-ratio option), the Fe(II) complex that
formed within the dead time of the instrument slowly reacted
further toward the peroxo species.
The second reaction step (Scheme 3), formation of the Fe-
(III)-peroxo species, can be monitored in two different ways.
When a larger than 10-fold excess of superoxide is applied to
the solution of [FeIII(Porph)(DMSO)2]+, the reduced [FeII-
(Porph)] complex is formed during the dead time of
the instrument, and the formation of the peroxo complex,
[FeIII(Porph)(O22-)]-, is observed. The second reaction step
(Scheme 3) can also be followed starting with electrochemically
generated [FeII(Porph)]. All the time-resolved spectra obtained
by these two methods were identical, and a typical example is
illustrated in Figure 9. Typically, the absorbance maximum at
430 nm decreases while a new band at 440 nm appears. The
clear isosbestic points indicate that only one reaction step occurs.
The time-resolved spectra can be fitted to a single-exponential
As a result of the large error in the intercept, an accurate
value for the rate constant of the back reaction from these
experiments could not be estimated. As reported above, addition
of a carefully controlled amount of acid, moderate TBPH or
strong HOTf, to the green solution of the Fe(III)-peroxo species
in DMSO causes its conversion to the Fe(II) complex. Therefore,
this reaction, in which acid is a trap for superoxide anions, can
be used for direct determination of the rate constant of the back
reaction, koff, in a stopped-flow experiment in which a 1 × 10-5
M solution of Fe(III)-peroxo-porphyrin was mixed with the
acid solution in DMSO at 25 °C (Figure 4 and Figure S2). It
should be mentioned that the high stability of the Fe(III)-peroxo
complex enables such an experiment. For the quantitative
determination of koff, HOTf was chosen since it is known that
strong acids react with superoxide in aprotic solvents extremely
rapidly (k′ > 1 × 107 M-1 s-1),31 which makes them very
function to give values for the observed rate constant, kobs
.
Concentration-dependent measurements were performed start-
ing from the Fe(III) complex and also from the electrochemically
generated Fe(II) complex. Superoxide concentrations were
varied by using the same KO2 solution and variation of the
mixing volume ratios, and also by preparing new solutions for
-
efficient O2 scavengers, even at lower concentrations. The
9
4226 J. AM. CHEM. SOC. VOL. 129, NO. 14, 2007

Iron(III) Complex of a Crown Ether−Porphyrin Conjugate
A R T I C L E S
Figure 11. Changes in absorbance at 440 nm upon addition of O2- to a 5
× 10-6 M solution of [FeII(Porph)(DMSO)2] in DMSO at 25.0 °C (I ) 0.1
M (NBu4PF6)). The solid lines are a fit of the data obtained by global
analysis of the resulting spectra (inset) in SPECFIT.
Figure 12. Eyring plot for the second reaction step in the DMSO/CH3CN
mixture (30% DMSO; [[FeIII(Porph)Cl]] ) 5 × 10-6 M, [KO2] ) 1 mM).
Scheme 5
observed first-order rate constant for the reaction of [FeIII-
(Porph)(O22-)]- and HOTf does not depend on HOTf concen-
tration in the range of 5 × 10-5-5 × 10-4 M and was found
to be kobs ) koff ) 0.21 ( 0.01 s-1 (Table 2). From the obtained
second-order rate constant kon and the first-order rate constant
koff, the kinetically determined equilibrium constant for the
binding of superoxide is KO ) (1.7 ( 0.2) × 105 M-1 at 25
-
2
in pure DMSO and DMSO/CH3CN mixtures, suggests that in
both solutions the same reacting bis-DMSO-Fe(III) species
exists. Thus, the reactions and the corresponding rate constants
in pure DMSO and DMSO/CH3CN mixtures are fully compa-
rable.
°C (Table 2).
The reversible binding of O2- to [FeII(Porph)] and the relative
stability of the [FeIII(Porph)(O22-)]- complex allowed further
investigation of the binding thermodynamics. The equilibrium
constant for this reaction was determined by titrating an
electrochemically prepared solution of [FeII(Porph)] with O2
-
The activation parameters resulting from the temperature-
dependent measurements of kon were found to be ∆Hq ) 61.2
( 0.9 kJ mol-1 and ∆Sq ) +48 ( 3 J K-1 mol-1 (Table 2).
The relatively high value of the activation enthalpy and a
positive value of the activation entropy speak for a dissociative
mode of the second reaction step (Scheme 3), in which the redox
process is substitution controlled. Dissociation of coordinated
DMSO from the six-coordinate [FeII(Porph)(DMSO)2] complex
(a D mechanism, Scheme 5), resulting in the five-coordinate
intermediate, is suggested to be the rate-determining step. This
is also in agreement with the strong coordinating ability of
DMSO. In a limiting dissociative D mechanism according to
eq 3, a kinetic saturation behavior is expected, where k1 and
under a nitrogen atmosphere. Spectra were recorded about 30 s
after addition of the O2- aliquot to ensure that the reaction had
attained equilibrium. The experiment was performed twice with
some variation of the conditions (see Experimental Section),
and typical spectra along with the corresponding absorbance
changes at 430 and 440 nm are shown in Figure 11. A global
analysis of the spectra obtained in two independent experiments
(with and without electrolyte) resulted in equilibrium constants
for the reaction of (0.9 ( 0.1) × 105 and (1.4 ( 0.1) × 104
M-1 (Table 2), respectively, which is within the expected trend
for ion-pair formation constants as a function of ionic strength.
Thus, both thermodynamically and kinetically determined
equilibrium constants for superoxide binding are in good
agreement.
Starting from the [FeIII(Porph)(DMSO)2]+ complex and
applying high enough [O2-], the second reaction step (Scheme
3) was studied as a function of temperature (-40 to 0 °C) in a
DMSO/CH3CN mixture (30% DMSO with the lowest freezing
point about -50 °C). The kon () kobs/[O2-]) values obtained in
the DMSO/CH3CN mixture at low temperatures are in very good
agreement with the second-order rate constant obtained in
DMSO solution at 25 °C (Figure 12, Table 2, Table S1,
Supporting Information). This is indicated by a very good linear
correlation between ln(kon/T) and 1/T over a wide temperature
range (-40 to +25 °C) (Figure 12 and Table S1). Coordination
of DMSO to Fe(III)-porphyrin-peroxo complexes already
occurs in the presence of 20% DMSO in CH3CN, resulting in
a bis-axially coordinated species.13 This observation, together
with the comparative kinetic behavior of the studied reaction
-
k1k2[O2 ] + k-1k-2
kobs
)
(3)
-
k-1 + k2[O2 ]
k-1 are the forward and reverse rate constants for the dissociation
of DMSO, and k2 and k-2 are the forward and reverse rate
constants for the coordination of O2-. However, under the
selected experimental conditions, k2[O2-] is much smaller than
k-1, such that eq 3 can be simplified to kobs ) kon[O2-] + koff,
with the overall second-order rate constant for superoxide
binding kon ) k1k2/k-1 and the first-order rate constant for the
superoxide release koff ) k-2. Looking at eq 3, the linear
concentration dependence of kobs can easily be understood since
superoxide is present in a much lower concentration than DMSO
-
to scavenge the five-coordinate intermediate, such that k2[O2
]
in eq 3 is negligible in comparison to k-1. Thus, the activation
parameters obtained involve contributions from the effect of
9
J. AM. CHEM. SOC. VOL. 129, NO. 14, 2007 4227

A R T I C L E S
Scheme 6
Du¨rr et al.
Chart 1
temperature on the formation of the five-coordinate intermediate
(∆H(K1) and ∆S(K1)) and on the rate of superoxide binding, k2
(∆Hq(k2) and ∆Sq(k2)). In this case, there is a combination of
two opposite effects, which explains the rather moderate positive
value obtained for ∆Sq.
P450). Specifically, we have shown that, if the Fe(III)-peroxo
species is in equilibrium with superoxide, fine-tuning of the
proton concentration can lead to superoxide trapping and
formation of an Fe(II) species. Whether this type of reactivity
is a result of the unique structural feature of our Fe(III)-
peroxo-porphyrin (viz. the presence of the nearby K+-crown
ether moiety) or a more general feature that could not be
observed before because of the much lower stability of the
previously studied Fe(III)-peroxo-porphyrin complexes re-
mains to be seen. This also stresses the possible effect of the
electrostatic interactions between the coordinated peroxide and
the positively charged surrounding groups, as well as the effect
of the axially coordinated group, on the stability of the peroxo
species and molecular mechanism of O-O bond cleavage. In
any case, this type of reactivity of the Fe(III)-peroxo species
and reversible binding of superoxide could be of significant
biological importance since it could operate in a specific
biological environment as well.
Conclusions
We have synthesized and characterized the new Fe(III)-
porphyrin complex [FeIII(Porph)Cl], which carries a covalently
bound aza-crown ether in close proximity to the iron center.
The corresponding hydroxo, µ-oxo dimer, Fe(II), and Fe(III)-
peroxo species were also prepared. The quite high stability of
the Fe(II)-porphyrin and Fe(III)-peroxo-porphyrin species
enabled kinetic and thermodynamic measurements to be made
for the reaction with KO2 in DMSO. As mentioned above, in
contrast to the Fe(III)-peroxo-porphyrin species reported in
the literature,10 which are only stable under an inert atmosphere,
our peroxo species is quite stable in solution with an excess of
superoxide, even when exposed to air. The presence of nearby
K+ can account for this increased stability of K[FeIII(Porph)-
(O22-)].
The reactions studied are summarized in Scheme 6. The first
reaction step, reduction of the Fe(III) to the Fe(II)-porphyrin
complex by KO2, could not be studied in detail because of
interference from the formation of the Fe(III)-hydroxo species.
The second reaction step, binding of superoxide to the Fe(II)
species and formation of the Fe(III)-peroxo complex, could
be studied in detail. To our knowledge, this is the first time
that superoxide concentration and temperature-dependent kinetic
studies of reactions with superoxide have been performed by
stopped-flow UV/vis measurements, and as a result the second-
order rate constant and corresponding activation parameters
could be obtained. Moreover, we have observed for the first
time that the superoxide anion can bind reversibly to a metal
center. We have determined the rate constant for the reverse
reaction as well as measured kinetic and thermodynamic
superoxide binding constants.
It is not possible to conclude whether the peroxo ligand in
our [FeIII(Porph)(O22-)]- complex is coordinated in a side-on
or end-on fashion. However, with coordinated DMSO and the
electrophilic potassium cation in the crown ether lying above
the peroxo ligand, [FeIII(Porph)(O22-)]- may in a way represent
a model for the proposed13 nucleophilic attack of the end-on
peroxo form, with an axially coordinated solvent molecule, to
an electron-deficient substrate (see Chart 1).
Similar studies on the related Mn(III) complex of our crown
ether-porphyrin conjugate, as well as on the corresponding Fe-
(III) and Mn(III) complexes of the porphyrin ligand without a
crown ether moiety, are in progress.
Acknowledgment. The authors gratefully acknowledge fi-
nancial support from the Deutsche Forschungsgemeinschaft
through SFB 583 “Redox-active metal complexes” (I.I.-B. and
N.J.).
On the basis of the results obtained, we can conclude that
strongly coordinated DMSO controls the reaction mechanism
in the sense that its dissociation is the rate-determining step.
We have also shown that addition of acid (or proton sources in
general) to the Fe(III)-peroxo species does not necessarily lead
to dissociation of hydrogen peroxide and formation of the Fe-
(III) complex (as in the case of the SOD active enzymatic and
mimetic systems)6,7 or O-O bond cleavage and formation of a
high-valent oxo-iron species (as in the case of cytochrome
Supporting Information Available: UV/vis spectra of dif-
ferent species, time-resolved UV/vis spectra of different reaction
steps, IR spectra, and results of the electrochemical studies
(PDF); crystallographic data for {[FeIII(Porph)Cl]‚H3O+}FeCl4
-
(CIF; also deposited with the Cambridge Crystallographic Data
Centre as CCDC 612689). This material is available free of
charge via the Internet at http://pubs.acs.org.
JA064984P
9
4228 J. AM. CHEM. SOC. VOL. 129, NO. 14, 2007