Rational tuning of the thiolate donor in model complexes of superoxide reductase: Direct evidence for a trans influence in FeIII-OOR complexes

Article abstract of DOI:10.1021/ja8031828

Iron peroxide species have been identified as important intermediates in a number of nonheme iron as well as heme-containing enzymes, yet there are only a few examples of such species either synthetic or biological that have been well characterized. We describe the synthesis and structural characterization of a new series of five-coordinate (N₄S(thiolate))Fe^II complexes that react with tert-butyl hydroperoxide (^tBuOOH) or cumenyl hydroperoxide (CmOOH) to give metastable alkylperoxo-iron(III) species (N₄S(thiolate)Fe^III-OOR) at low temperature. These complexes were designed specifically to mimic the nonheme iron active site of superoxide reductase, which contains a five-coordinate iron(II) center bound by one Cys and four His residues in the active form of the protein. The structures of the Fe^II complexes are analyzed by X-ray crystallography, and their electrochemical properties are assessed by cyclic voltammetry. For the Fe^III-OOR species, low-temperature UV-vis spectra reveal intense peaks between 500-550 nm that are typical of peroxide to iron(III) ligand-to-metal charge-transfer (LMCT) transitions, and EPR spectroscopy shows that these alkylperoxo species are all low-spin iron(III) complexes. Identification of the vibrational modes of the Fe^III-OOR unit comes from resonance Raman (RR) spectroscopy, which shows ν(Fe-O) modes between 600-635 cm^-1 and ν(O-O) bands near 800 cm^-1. These Fe-O stretching frequencies are significantly lower than those found in other low-spin Fe^III-OOR complexes. Trends in the data conclusively show that this weakening of the Fe-O bond arises from a trans influence of the thiolate donor, and density functional theory (DFT) calculations support these findings. These results suggest a role for the cysteine ligand in SOR, and are discussed in light of the recent assessments of the function of the cysteine ligand in this enzyme.

Full text of DOI:10.1021/ja8031828

Published on Web 10/07/2008
Rational Tuning of the Thiolate Donor in Model Complexes of
Superoxide Reductase: Direct Evidence for a trans Influence
in FeIII-OOR Complexes
Frances Namuswe,† Gary D. Kasper,† Amy A. Narducci Sarjeant,†
Takahiro Hayashi,‡ Courtney M. Krest,§ Michael T. Green,§
Pierre Moe¨nne-Loccoz,*,‡ and David P. Goldberg*,†
Department of Chemistry, Johns Hopkins UniVersity, Baltimore, Maryland 21218, Department of
EnVironmental and Biomolecular Systems, OGI School of Science and Engineering, Oregon
Health and Science UniVersity, BeaVerton, Oregon 97006, and Department of Chemistry,
PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802
Received April 29, 2008; E-mail: ploccoz@ebs.ogi.edu; dpg@jhu.edu
Abstract: Iron peroxide species have been identified as important intermediates in a number of nonheme
iron as well as heme-containing enzymes, yet there are only a few examples of such species either synthetic
or biological that have been well characterized. We describe the synthesis and structural characterization
of a new series of five-coordinate (N4S(thiolate))FeII complexes that react with tert-butyl hydroperoxide
(tBuOOH) or cumenyl hydroperoxide (CmOOH) to give metastable alkylperoxo-iron(III) species
(N4S(thiolate)FeIII-OOR) at low temperature. These complexes were designed specifically to mimic the
nonheme iron active site of superoxide reductase, which contains a five-coordinate iron(II) center bound
by one Cys and four His residues in the active form of the protein. The structures of the FeII complexes are
analyzed by X-ray crystallography, and their electrochemical properties are assessed by cyclic voltammetry.
For the FeIII-OOR species, low-temperature UV-vis spectra reveal intense peaks between 500-550 nm
that are typical of peroxide to iron(III) ligand-to-metal charge-transfer (LMCT) transitions, and EPR
spectroscopy shows that these alkylperoxo species are all low-spin iron(III) complexes. Identification of
the vibrational modes of the FeIII-OOR unit comes from resonance Raman (RR) spectroscopy, which
shows ν(Fe-O) modes between 600-635 cm-1 and ν(O-O) bands near 800 cm-1. These Fe-O stretching
frequencies are significantly lower than those found in other low-spin FeIII-OOR complexes. Trends in the
data conclusively show that this weakening of the Fe-O bond arises from a trans influence of the thiolate
donor, and density functional theory (DFT) calculations support these findings. These results suggest a
role for the cysteine ligand in SOR, and are discussed in light of the recent assessments of the function of
the cysteine ligand in this enzyme.
extradiol,10-13and Rieske dioxygenases,10,11 and superoxide
reductases.14-19 The latter enzymes represent a new class of
Introduction
Iron peroxide species are proposed to play important roles
in the function of many mononuclear iron centers in biology,
including heme systems such as cytochrome P4501-6 and heme
oxygenase,7 and nonheme iron centers such as bleomycin,8-10
iron-containing proteins that is found in anaerobic and mi-
croaerophilic prokaryotes, and catalyzes the reduction of su-
peroxide to hydrogen peroxide through a reductive pathway,
-
O2 + e- + 2H+ f H2O2. SOR is thought to serve as an
† Johns Hopkins University.
(9) Stubbe, J.; Kozarich, J. W. Chem. ReV. 1987, 87, 1107–1136.
(10) Que, L., Jr.; Ho, R. Y. N. Chem. ReV. 1996, 96, 2607–2624.
(11) Kovaleva, E. G.; Neibergall, M. B.; Chakrabarty, S.; Lipscomb, J. D.
Acc. Chem. Res. 2007, 40, 475–483.
‡ Oregon Health and Science University.
§ Pennsylvania State University.
(1) Sono, M.; Roach, M. P.; Coulter, E. D.; Dawson, J. H. Chem. ReV.
1996, 96, 2841–2887.
(12) Kovaleva, E. G.; Lipscomb, J. D. Science 2007, 316, 453–457.
(13) Costas, M.; Mehn, M. P.; Jensen, M. P.; Que, L., Jr. Chem. ReV. 2004,
104, 939–986.
(2) Sligar, S. G.; Makris, T. M.; Denisov, I. G. Biochem. Biophys. Res.
Commun. 2005, 338, 346–354.
(3) Shaik, S.; Kumar, D.; de Visser, S. P.; Altun, A.; Thiel, W. Chem.
ReV. 2005, 105, 2279–2328.
(14) Emerson, J. P.; Coulter, E. D.; Cabelli, D. E.; Phillips, R. S.; Kurtz,
D. M., Jr. Biochemistry 2002, 41, 4348–4357.
(4) Meunier, B.; de Visser, S. P.; Shaik, S. Chem. ReV. 2004, 104, 3947–
3980.
(15) Katona, G.; Carpentier, P.; Nivie`re, V.; Amara, P.; Adam, V.; Ohana,
J.; Tsanov, N.; Bourgeois, D. Science 2007, 316, 449–453.
(16) Kurtz, D. M., Jr. Acc. Chem. Res. 2004, 37, 902–908.
(17) Lombard, M.; Houe´e-Levin, C.; Touati, D.; Fontecave, M.; Nivie`re,
V. Biochemistry 2001, 40, 5032–5040.
(5) Loew, G. H.; Harris, D. L. Chem. ReV. 2000, 100, 407–419.
(6) Denisov, I. G.; Makris, T. M.; Sligar, S. G.; Schlichting, I. Chem.
ReV. 2005, 105, 2253–2277.
(7) Unno, M.; Matsui, T.; Ikeda-Saito, M. Nat. Prod. Rep. 2007, 24, 553–
570.
(18) Mathe´, C.; Weill, C. O.; Mattioli, T. A.; Berthomieu, C.; Houe´e-Levin,
C.; Tremey, E.; Nivie`re, V. J. Biol. Chem. 2007, 282, 22207–22216.
(19) Nivie`re, V.; Lombard, M.; Fontecave, M.; Houe´e-Levin, C. Febs Lett.
2001, 497, 171–173.
(8) Stubbe, J.; Kozarich, J. W.; Wu, W.; Vanderwall, D. E. Acc. Chem.
Res. 1996, 29, 322–330.
9
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2008 American Chemical Society
J. AM. CHEM. SOC. 2008, 130, 14189–14200 14189

A R T I C L E S
Namuswe et al.
alternative to the classical superoxide dismutases (SODs),
allowing anoxic microorganisms to avoid the oxidative half-
reaction of SOD, which would produce deleterious dioxygen.
encouraging Fe-O bond cleavage and release of H2O2 as
opposed to the O-O cleavage pathway observed for other
enzymes such as cytochrome P450.16,33-35
-
The metal center in SOR that carries out the reduction of O2
In our earlier report we reasoned that the unexpected weakening
of the Fe-O bond in [FeIII([15]aneN4)(SC6H5)(OOR)]+ may arise
from a trans influence of the thiolate donor, provided that the
thiolate ligand was indeed coordinated in a trans configuration.
Immediately following this work a report from Kovacs and
Solomon36 described the low-temperature synthesis and charac-
terization of a high-spin FeIII-OOH model of SOR that also
exhibited a significantly weakened Fe-O bond, as evident from
the RR-detected Fe-O stretch for this complex (ν(Fe-O) ) 419
cm-1 versus 450-639 cm-1 for other high-spin iron(III) perox-
ides). In this study a thiolate-induced trans influence was also
invoked as the potential cause of the weak Fe-O bond. In contrast,
an earlier report by Que and Halfen33 showed that increasing the
electron-donating ability of a trans-oriented axial ligand (X) in a
series of six-coordinate, high-spin [FeIII(N4)(X)(OOR)]+ complexes
led to a decrease in the decay rate of these species. This trend is
opposite to that expected for a weakening of the Fe-O bond via
a trans influence of X, assuming that the decay involves Fe-O
bond cleavage. Interestingly, the RR spectroscopy on these high-
spin complexes displayed essentially no variance with the identity
of the axial ligand. We speculated that the synthesis of a series of
SOR models in which the electron-donating power of the thiolate
ligand was rationally varied would greatly aid in determining the
potential role of this ligand in influencing the properties of
Fe-OO(H or R) species.
In this paper, we describe the modular synthesis of a series
of SOR model complexes of the type [FeII([15]aneN4)(SAr)]+,
wherein the thiolate ligand has been varied with regard to its
electron-donating ability. These complexes have allowed us to
systematically assess the influence of the thiolate donor on the
structural and physical properties, as well as the reactivity of
this family of SOR model complexes while holding the
remaining ligand set constant. The ability of these complexes
to form FeIII-OOR species is evaluated, and a mechanism of
formation is proposed. The UV-vis features, spin-states, and
vibrational signatures of the FeIII-OOR species are determined
by low-temperature electronic absorption, EPR, and RR spec-
troscopies. The influence of the thiolate donor on the Fe-O
and O-O bond strengths is assessed. DFT calculations were
performed in support of the spectroscopic data. These findings
are discussed in light of recent results on SOR in which the
influence of the Cys donor on the properties of an FeIII-OO(H)
intermediate has been investigated through site-directed mu-
tagenesis.18
is comprised of an FeII ion bound to the protein by four His
donors in a plane and a single axial cysteinate donor, completing
a square pyramidal geometry for the high-spin ferrous (reduced)
enzyme. High-resolution X-ray structures of SOR have also
shown that the Cys sulfur atom is within hydrogen-bonding
distance of two N-H amide groups (N-H---S bonds) from the
peptide backbone.20 The mechanism of O2- reduction has been
studied by several methods, and there is one intermediate,
characterized by an absorption band at ∼600 nm,16,17,19,21 that
has been consistently observed. This species has been formulated
as an iron(III)-(hydro)peroxo species. However, the structure,
spin state, and protonation state of this intermediate have not
been definitively determined, and the possible existence of other
iron peroxide-type intermediates along the catalytic pathway
remains under investigation. In addition, the thiolate ligand in
SOR is unique among nonheme iron enzymes, and its role in
-
the mechanism of O2 reduction has been a subject of much
interest.18,20,22,23
We previously reported the synthesis of the SOR model
-
complex [FeII([15]aneN4)(SC6H5)]+BF4 and showed that it
reacted with alkylhydroperoxides at low temperature in CH2Cl2
to give the metastable iron(III)-alkylperoxo species
[FeIII([15]aneN4)(SC6H5)(OOR)]+ (R ) cumenyl or tert-bu-
tyl).24 These complexes were characterized by UV-vis, EPR,
and resonance Raman (RR) spectroscopies and were determined
to be low-spin FeIII-OOR species with weak Fe-O bonds, i.e.,
with ν(Fe-O) ≈ 90 cm-1 lower than in other low-spin
FeIII-OOR species. Analysis of other low-spin and high-spin
FeIII-OO(H or R) species by RR spectroscopy has led to a trend
in which the low-spin complexes exhibit strong Fe-O and weak
O-O bonds, while the opposite trend is observed for the high-
spin species.25-32 Thus, the [FeIII([15]aneN4)(SC6H5)(OOR)]+
complexes were the first examples of low-spin FeIII-OOR
species to exhibit dramatically weakened Fe-O bonds. The
former trend was used as precedent for the suggestion that the
-
spin-state of the iron during O2 reduction may play a part in
(20) Adam, V.; Royant, A.; Nivie`re, V.; Molina-Heredia, F. P.; Bourgeois,
D. Structure 2004, 12, 1729–1740.
(21) Coulter, E. D.; Emerson, J. P.; Kurtz, D. M., Jr.; Cabelli, D. E. J. Am.
Chem. Soc. 2000, 122, 11555–11556.
(22) Dey, A.; Jenney, F. E.; Adams, M. W. W.; Johnson, M. K.; Hodgson,
K. O.; Hedman, B.; Solomon, E. I. J. Am. Chem. Soc. 2007, 129,
12418–12431.
(23) Kovacs, J. A.; Brines, L. M. Acc. Chem. Res. 2007, 40, 501–509.
(24) Krishnamurthy, D.; Kasper, G. D.; Namuswe, F.; Kerber, W. D.;
Sarjeant, A. A. N.; Moe¨nne-Loccoz, P.; Goldberg, D. P. J. Am. Chem.
Soc. 2006, 128, 14222–14223.
Experimental Section
General Procedures. All reactions were carried out under an
atmosphere of N2 or Ar using a drybox or standard Schlenk
techniques. 1,4,8,12-Tetraazacyclopentadecane ([15]aneN4) (98%)
was purchased from Strem Chemicals. All other reagents were
purchased from commercial vendors and used without further
purification unless noted otherwise. Diethyl ether and dichlo-
romethane were purified via a Pure-Solv Solvent Purification System
(25) Girerd, J. J.; Banse, F.; Simaan, A. J. Struct. Bonding (Berlin) 2000,
97, 145–177.
(26) Jensen, M. P.; Payeras, A. M. I.; Fiedler, A. T.; Costas, M.; Kaizer,
J.; Stubna, A.; Munck, E.; Que, L., Jr. Inorg. Chem. 2007, 46, 2398–
2408.
(27) Lehnert, N.; Fujisawa, K.; Solomon, E. I. Inorg. Chem. 2003, 42, 469–
481.
(28) Lehnert, N.; Ho, R. Y. N.; Que, L., Jr.; Solomon, E. I. J. Am. Chem.
Soc. 2001, 123, 12802–12816.
(29) Lehnert, N.; Ho, R. Y. N.; Que, L., Jr.; Solomon, E. I. J. Am. Chem.
Soc. 2001, 123, 8271–8290.
(33) Bukowski, M. R.; Halfen, H. L.; van den Berg, T. A.; Halfen, J. A.;
Que, L., Jr. Angew. Chem., Int. Ed. 2005, 44, 584–587.
(34) Clay, M. D.; Cosper, C. A.; Jenney, F. E.; Adams, M. W. W.; Johnson,
M. K. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 3796–3801.
(35) Kovacs, J. A. Chem. ReV. 2004, 104, 825–848.
(36) Kitagawa, T.; Dey, A.; Lugo-Mas, P.; Benedict, J. B.; Kaminsky, W.;
Solomon, E.; Kovacs, J. A. J. Am. Chem. Soc. 2006, 128, 14448–
14449.
(30) Me´nage, S.; Wilkinson, E. C.; Que, L., Jr.; Fontecave, M. Angew.
Chem., Int. Ed. 1995, 34, 203–205.
(31) Wada, A.; Ogo, S.; Watanabe, Y.; Mukai, M.; Kitagawa, T.; Jitsukawa,
K.; Masuda, H.; Einaga, H. Inorg. Chem. 1999, 38, 3592–3593.
(32) Zang, Y.; Kim, J.; Dong, Y. H.; Wilkinson, E. C.; Appelman, E. H.;
Que, L., Jr. J. Am. Chem. Soc. 1997, 119, 4197–4205.
9
14190 J. AM. CHEM. SOC. VOL. 130, NO. 43, 2008

Evidence for a trans Influence in FeIII-OOR Complexes
A R T I C L E S
from Innovative Technology, Inc. THF was distilled from sodium/
benzophenone. Methanol was distilled over CaH2. All solvents were
degassed by repeated cycles of freeze-pump-thaw and then stored
in a drybox. Sodium hydride (60% in mineral oil) was washed with
hexanes prior to use. Tert-butylhydroperoxide was purchased from
Aldrich as an ∼5.5 M solution in decane over molecular sieves.
yellow solution. A methanolic solution of NaSC6H4-p-OMe (0.227
g, 1.40 mmol; prepared from the addition of 4-methoxybenzenethiol
to NaH in THF) was added to the mixture and stirred overnight to
give a pale green-yellow solution. Methanol was removed under
vacuum to give a green solid, which was redissolved in CH2Cl2
and filtered through Celite to give a yellow-green solution. Vapor
diffusion of diethyl ether into this solution resulted in colorless
rods (which appear pale-green when grown together in clusters) of
3 after one week. Yield: 0.226 g (65%); Anal. Calcd for
C18H33N4OSFeBF4: C, 43.57; H, 6.70; N, 11.29. Found: C, 43.51;
H, 6.73; N, 11.31.
t
The concentration of BuOOH was measured via titration with
potassium iodide according to a published procedure.37 tBu18O18OH
t
was synthesized by the reaction of BuMgCl with 18O2 following
a published procedure.38 Cumene18O18OH was synthesized by the
reaction of cumene with 18O2 following a published procedure.39
18O2 was purchased from Icon Isotopes (99% atom purity).
Physical Methods. Electron paramagnetic resonance (EPR)
spectra were obtained on a Bruker EMX EPR spectrometer
controlled with a Bruker ER 041 X G microwave bridge at 15 K.
The EPR spectrometer was equipped with a continuous-flow liquid
He cryostat and an ITC503 temperature controller made by Oxford
Instruments, Inc. Low-temperature UV-visible spectra were re-
corded at -78 °C on a Cary 50 Bio spectrophotometer equipped
with a fiber-optic coupler (Varian) and a fiber optic dip probe
(Hellma 661.302-QX-UV, 2 mm path length) for low temperature,
using custom-made Schlenk tubes designed for the fiber-optic dip
probe. Room temperature UV-vis spectra were recorded on a
Hewlett-Packard 8453 diode array spectrophotometer. Resonance
Raman spectra were obtained on a custom McPherson 2061/207
spectrometer equipped with a Princeton Instrument liquid-N2 cooled
CCD detector (LN-1100OB). The 514-nm excitation from an Ar
laser (Innova 90, Coherent) was set at 50 mW, and continuous
spinning of the sample at 110 K was used to prevent adverse effect
from the laser illumination. A Semrock RazorEdge filter was used
to attenuate Rayleigh scattering. Sets of ∼15 min accumulations
were acquired at 4-cm-1 resolution. Frequency calibrations were
performed using aspirin and are accurate to ( 1 cm-1. Elemental
analyses were performed by Atlantic Microlab Inc., Norcross, GA.
Synthesis of [FeII([15]aneN4)(SC6H5)]BF4 (1). To a solution
of Fe(BF4)2 ·6H2O (0.395 g, 1.17 mmol) in MeOH (4 mL) was
added a solution of [15]aneN4 (0.250 g, 1.17 mmol) in MeOH (6
mL) dropwise, and stirred for 45 min resulting in a pale-yellow
solution. A methanolic solution of NaSC6H5 (0.185 g, 1.40 mmol)
(prepared from addition of benzenethiol to NaH in THF) was added
to the mixture and stirred overnight to give a pale-green solution.
Methanol was removed under vacuum to give a green solid, which
was redissolved in CH2Cl2 and filtered through Celite to give a
green solution. Vapor diffusion of diethyl ether into this solution
resulted in colorless prisms (which appear light-blue when grown
together in clusters) of 1 after a week. Yield: 0.336 g (65%); Anal.
Calcd for C17H31N4SFeBF4: C, 43.80; H, 6.71; N, 12.01. Found:
C, 43.34; H, 6.85; N, 12.08.
Synthesis of [FeII([15]aneN4)(SC6F4-p-SC6F5]BF4 (4). To a
solution of Fe(BF4)2 ·6H2O (0.284 g, 0.84 mmol) in MeOH (3 mL)
was added a solution of [15]aneN4 (0.180 g, 0.84 mmol) in MeOH
(4 mL) dropwise, and stirred for 45 min resulting in a pale-yellow
solution. A methanolic solution of NaSC6F4-p-SC6F5 (0.266 g, 0.66
mmol) (4 mL) (prepared in situ from addition of 2,3,4,5,6-
pentafluorobenzenethiol to NaH in THF) was added dropwise to
give a dark-yellow solution, which was stirred for 30 min. Methanol
was removed under vacuum to give a dark-yellow solid. The solid
was dissolved in CH2Cl2 and filtered over Celite to give a dark-
yellow solution. Vapor diffusion of diethyl ether into this solution
resulted in colorless thick plates (which appear yellow when grown
together in clusters) of 4 after 3 weeks. Anal. Calcd for
C23H26N4S2FeBF13: C, 37.52; H, 3.56; N, 7.61. Found: C, 37.33;
H, 3.49; N, 8.16.
Synthesis of [FeII([15]aneN4)(SC6H4-p-NO2)]BF4 (5). To a
solution of Fe(BF4)2 ·6H2O (0.239 g, 0.707 mmol) in MeOH (3
mL) was added a solution of [15]aneN4 (0.152 g, 0.707 mmol) in
MeOH (4 mL) dropwise, and stirred for 45 min resulting in a pale-
yellow solution. A red solution of NaSC6H5-p-NO2 (0.179 g, 1.01
mmol) in MeOH (4 mL) (prepared from the addition of 4-nitroben-
zenethiol to NaH in THF) was added dropwise to give a dark-red
solution with orange-yellow precipitate, which was stirred for 30
min. The solvent was removed under vacuum to give a dark-red
solid interspersed with small amounts of an orange solid. The solid
was dissolved in CH2Cl2 and filtered through Celite to give a dark-
red solution. Vapor diffusion of diethyl ether into this solution
resulted in dark-red prisms of 5 after 1 day. Yield: 0.240 g (66%);
UV-vis (CH2Cl2), λmax 370 nm (11463 M-1 cm-1), 490 nm (sh);
Anal. Calcd for C17H30N5SFeO2BF4: C, 39.94; H, 5.92; N, 13.70.
Found: C, 39.98; H, 5.96; N, 13.48.
X-ray Crystallography. All X-ray diffraction data were collected
on an Oxford Diffraction Xcalibur3 diffractometer, equipped with
a Mo KR sealed-tube source and a Sapphire CCD detector. Single
crystals of each compound were mounted in oil under a nitrogen
cold stream at 110(2) K. Data collection parameters are provided
in Supporting Information as CIF files. Selected crystallographic
details are provided in Table 1. Data were collected, integrated,
scaled, and corrected for absorption using CrysAlis PRO software.40
Structures were solved and refined using the SHELXTL suite of
software.41
Synthesis of [FeII([15]aneN4)(SC6H4-p-Cl)]BF4 (2). To a solu-
tion of Fe(BF4)2 ·6H2O (0.378 g, 1.12 mmol) in MeOH (3 mL)
was added a solution of [15]aneN4 (0.267 g, 1.25 mmol) in MeOH
(4 mL) dropwise, and stirred for 45 min resulting in a pale-yellow
solution. A methanolic solution of NaSC6H4-p-Cl (0.236 g, 1.39
mmol) (prepared from addition of 4-chlorobenzenethiol to NaH in
THF) was added to the mixture and stirred for 30 min to give a
pale-yellow solution. Methanol was removed under vacuum to give
a yellow-brown solid, which was redissolved in CH2Cl2 and filtered
through Celite to give a forest green solution. Vapor diffusion of
diethyl ether into this solution resulted in colorless prisms (which
appear pale-green when grown together in clusters) of 2 after 2
days. Yield: 0.362 g (65%); Anal. Calcd for C17H30N4SFeClBF4:
C, 40.79; H, 6.04; N, 11.19. Found: C, 40.79; H, 5.66; N, 11.33.
Synthesis of [FeII([15]aneN4)(SC6H4-p-OMe)]BF4 (3). To a
solution of Fe(BF4)2 ·6H2O (0.236 g, 0.700 mmol) in MeOH (4
mL) was added a solution of [15]aneN4 (0.150 g, 0.700 mmol) in
MeOH (6 mL) dropwise and stirred for 45 min resulting in a pale-
Disorder in structures comprising the [15]aneN4 ligand is well
documented.42-55 The structures presented here were refined in
noncentrosymmetric space groups in order to better model the
disorder in the macrocycle. Elongated thermal parameters for the
(40) CrysAlis PRO; Oxford Diffraction Ltd.: Wroclaw, Poland, 2006.
(41) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112–122.
(42) Ballester, L.; Gutierrez, A.; Perpinan, M. F.; Sanchez, A. E.; Azcondo,
M. T.; Gonzalez, M. J. Inorg. Chim. Acta 2004, 357, 1054–1062.
(43) Blake, A. J.; Li, W. S.; Lippolis, V.; Schroder, M. Acta Crystallogr.
1998, C54, 299–302.
(44) Chen, L. F.; Cotton, F. A. J. Mol. Struct. 1998, 470, 161–166.
(45) Clegg, W. Acta Crystallogr. 1986, C42, 1463–1464.
(46) Clegg, W.; Leupin, P.; Richens, D. T.; Sykes, A. G.; Raper, E. S.
Acta Crystallogr. 1985, C41, 530–532.
(47) Ito, T.; Kato, M.; Ito, H. Bull. Chem. Soc. Jpn. 1984, 57, 2634–2640.
(48) Ito, T.; Kato, M.; Ito, H. Bull. Chem. Soc. Jpn. 1984, 57, 2641–2649.
(49) Jacobsen, C. J. H.; Hyldtoft, J.; Larsen, S.; Pedersen, E. Inorg. Chem.
1994, 33, 840–842.
(37) Kokatnur, V. R.; Jelling, M. J. Am. Chem. Soc. 1941, 63, 1432–1433.
(38) Walling, C.; Buckler, S. A. J. Am. Chem. Soc. 1955, 77, 6032–6038.
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9
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A R T I C L E S
Namuswe et al.
Table 1. Crystallographic Information for Complexes 1-5
parameter
1
2
3
4
5
formula
Mr
C17H31BF4FeN4S
466.18
C17H30BClF4FeN4S
500.62
C18H33BF4FeN4OS
496.20
C23H26BF13FeN4S2
736.26
C17H30BF4FeN5O2S
511.18
cryst syst
space group
a (Å)
b (Å)
c (Å)
R (deg)
ꢀ (deg)
γ (deg)
V, Å3
monoclinic
P21/c
12.4027(7)
10.1280(6)
16.9164(11)
90
92.727(5)
90
2122.5(2)
4
orthorhombic
Pna21
16.1010(4)
13.7952(4)
9.9077(3)
90
orthorhombic
Pna21
15.7595(4)
14.3395(3)
10.0194(3)
90
monoclinic
Cc
8.8233(2)
22.7707(6)
15.2423(4)
90
103.444(3)
90
2978.46(13)
4
orthorhombic
Pna21
16.0267(3)
13.6455(3)
10.1189(2)
90
90
90
90
90
90
90
2200.66(11)
4
2264.22(10)
4
2212.93(8)
4
Z
Cryst color
T, K
colorless
110
colorless
110
colorless
110
colorless
110
dark red
110
R1 [I > 2σ(I)]
0.0707
1.253
0.0719
1.071
0.0947
1.076
0.0401
1.047
0.0831
1.080
GOF on F2
Scheme 1
macrocycle atoms suggest that these atoms are significantly
disordered. Attempts to further model this disorder did not improve
the refinement. In structures 1, 2, 3, and 5, the C-C and C-N
bonds were restrained using values given by Blake, et al.43 The
hydrogen atoms were refined in calculated positions, using ap-
propriate riding models. The atoms in the disordered C6H4-p-OMe
and C6H4-p-NO2 moieties of 3 and 5, respectively, were refined
isotropically. Complex 4, which is rather well-ordered was refined
with no distance restraints, and all non-hydrogen atoms were refined
anisotropically. The N-H hydrogen atoms were located from the
residual electron density maps and freely refined, while all C-H
hydrogen atoms were refined in calculated positions. For further
details on all structures, see CIF files in Supporting Information.
Electrochemistry. Cyclic voltammograms were measured with
an EG&G Princeton Applied Research potentiostat/galvanostat
model 263A at scan rates of 0.025-0.5 V/s. A three-electrode
configuration made up of a platinum working electrode, a Ag/AgCl
reference electrode (3.5 M KCl), and a platinum wire auxiliary
electrode was employed. Measurements were performed with 0.1
mM analyte in CH2Cl2 at ambient temperatures under argon using
0.10 M tetra-n-butylammonium hexafluorophosphate (recrystallized
three times from ethanol/water, dried over high vacuum for 3 days,
and stored in the dry box) as the supporting electrolyte. The
ferrocenium/ferrocene couple (FeCp2+/0) at 0.45 V was used as an
external reference.
optic dip probe with a path length of 2 mm was inserted into the
flask under argon, and the solution was cooled to -78 °C. After
recording the UV-vis spectrum of the FeII starting material, 3 equiv
t
of BuOOH (stock solution in n-decane) was diluted into CH2Cl2
and added to the FeII complex, and a new spectrum was im-
mediately recorded. The colorless solution instantly turned deep
red except in the case of 5 where the light-orange starting material
gradually turned fuchsia. The formation and decay of the new
species were followed by UV-vis spectroscopy.
Computational Methods. Frequency calculations were per-
formed at optimized geometries. Initial coordinates were taken from
the X-ray crystal structures of the iron(II) complexes 1-5.
Calculations were performed with Gaussian 03 at the B3LYP/6-
311G level of theory.56 The potential energy distribution was
obtained by transforming the calculated force constants and normal
modes from Cartesian coordinates to an internal coordinate sys-
tem.57
Generation
of
[FeIII([15]aneN4)(SAr)(OOR)]+
for
Resonance Raman and EPR Spectroscopy. To a Wilmad WG-5
M economy NMR tube or Wilmad Quartz EPR tube containing a
known concentration of [FeII([15]aneN4)(SAr)]BF4 in CH2Cl2 (500
Generation of [([15]aneN4)FeIII(SAr)(OOR)]+ for UV-vis
Spectroscopy. In a typical reaction, a custom-made Schlenk flask
was loaded with a known amount of [FeII([15]aneN4)(SAr)]BF4
and dissolved in an appropriate volume of CH2Cl2 to give a final
concentration of the FeII complex between 0.3 and 3 mM. A fiber-
t
µL) cooled to -78 °C, was added 3 equiv of ROOH (R ) Bu or
cumenyl) in CH2Cl2. An instant color change from colorless to deep
red (light-orange to fuchsia in the case of 5) was observed,
indicating the formation of the FeIII-OOR species. The reaction
mixture was then bubbled with Ar or N2 gas in the EPR or NMR
tube to obtain a homogeneous solution. Bubbling was kept to a
minimum in order to minimize warming of the solution. The
solution was then frozen in liquid nitrogen and stored at 77 K until
either RR or EPR measurements were made.
(50) Kato, M.; Ito, T. Inorg. Chem. 1985, 24, 509–514.
(51) Mak, T. C. W.; Che, C. M.; Wong, K. Y. J. Chem. Soc., Chem.
Commun. 1985, 986–988.
(52) Notni, J.; Gorls, H.; Anders, E. Eur. J. Inorg. Chem. 2006, 144, 1444–
1455.
(53) Panneerselvam, K.; Lu, T. H.; Chi, T. Y.; Pariya, C.; Liao, F. L.;
Chung, C. S. Acta Crystallogr. 1998, C54, 712–714.
(54) Pleus, R. J.; Saak, W.; Pohl, S. Z. Anorg. Allg. Chem. 2001, 627,
250–253.
(55) Shi, S.; Espenson, J. H.; Bakac, A. J. Am. Chem. Soc. 1990, 112,
1841–1846.
(56) Frisch, M. J.; et al. Gaussian 03, ReVision C.02 2004.
(57) Green, M. T. J. Am. Chem. Soc. 2006, 128, 1902–1906.
Results and Discussion
Synthesis. The iron(II) complexes 1- 5 that form the basis
of this study were prepared according to Scheme 1. The
synthesis of complex 1 was communicated previously.24 Reac-
tion of Fe(BF4)2 with the tetraamine donor [15]aneN4 in MeOH
results in the insertion of FeII into the [15]aneN4 cavity. A dark-
9
14192 J. AM. CHEM. SOC. VOL. 130, NO. 43, 2008

Evidence for a trans Influence in FeIII-OOR Complexes
A R T I C L E S
green color forms immediately upon addition of [15]aneN4 to
colorless FeII(BF4)2 in MeOH, which gradually fades to a pale-
yellow over the course of 45 min. The sodium salt of the
appropriate arylthiolate ligand is then added, leading to the
production of complexes 1-5. Our initial synthesis involved
changing the reaction solvent to CH2Cl2 before adding the
thiolate ligand, but we have seen improved yields of the FeII
complexes when adding ArS-Na+ in MeOH and then removing
solvent. Recrystallization of 1- 5 from CH2Cl2/Et2O provided
crystals for X-ray diffraction, and was the general method used
for preparing pure 1-5 for all of the spectroscopic and reactivity
studies described herein. Complexes 1 and 2 recrystallize as
colorless prisms, complex 3 recrystallizes as colorless rods,
while complex 4 recrystallizes as colorless thick plates. The
sodium salt of -SC6H4-p-NO2 has an intense red color from an
intraligand π-π* transition, which causes crystalline complex
5 to appear as dark-red prisms.
The synthesis of these complexes is sensitive to the ratio of
arylthiolate ligand versus iron(II). A slight excess of the thiolate
ligand (1.25-2.0 equiv) compared to FeII is necessary to obtain
these complexes in good yield and avoid other side-products.
For example, when lower ratios of ArS- are employed in the
syntheses of 1 or 2, a sideproduct appears as white needle-
shaped crystals, easily discernible from crystalline 1 or 2 by
visual inspection. The use of too large an excess of thiolate
donor (3 equiv) in the case of the p-NO2 complex 5 led to the
formation of [FeII([15]aneN4)(SC6H4-p-NO2)][SC6H4-p-NO2],
-
Figure 1. ORTEP diagrams of the complex cations of 2, 3, 4, and 5
showing 30% probability ellipsoids. Hydrogen atoms were omitted for clarity
except for the N-H groups. The carbon atoms for the macrocycle in 2, 3,
and 5 as well as the phenyl substituents of 3 and 5 are drawn as isotropic
spheres for clarity.
where the BF4 counterion has been replaced by a noncoordi-
nating -SC6H4-p-NO2 ligand.58 For the synthesis of polyflu-
orinated derivative 4, the target complex was
[FeII([15]aneN4)(SC6F5)]+, but nucleophilic displacement of the
p-F substituent during the initial deprotonation of C6F5SH with
NaH led to the thioether-linked thiolate ligand shown in Scheme
1.
and angles for each complex are given in Table 2. The structure
of 1 has been reported previously,24 but is included in Tables 1
and 2 for comparison. Each cationic iron(II) complex is five-
coordinate, with four nitrogen donors from the [15]aneN4 ligand
and one sulfur donor from the ArS- ligand completing the
coordination sphere. The conformation for [15]aneN4 is the same
in each structure, with one N-H group (N(2)-H) on the same
side of the macrocyclic plane as the thiolate ligand, while the
other three N-H groups lie on the opposite side. The Fe-S
bond distances for 1-5 range from 2.3197(12)-2.3426(8) Å,
showing little variance across the series and falling in the typical
range for high-spin FeII-SR complexes.63-66 For complexes
1-5, the Fe-N distances range between 2.075(7)-2.273(3) Å
and are similar to other high-spin Fe(II) complexes, including
the related N4S(thiolate)FeII complex [FeII(Me4cyclam)(SC6H4-
p-OMe)]+ (Fe-N ) 2.183(6)-2.279(6) Å).60 For complexes
1-4 the Fe-N(3) distance (1: 2.138(3); 2: 2.075(5) Å; 3:
2.095(8) Å; 4: 2.088(3) Å) is slightly shorter than the other
three Fe-N bonds. These short distances may be a consequence
of steric constraints imposed by the [15]aneN4 ligand. The
longest Fe-N distance in 1-4 is observed for N(2), which is
also the only N-H group on the same side of the macrocycle
as the sulfur donor. The p-nitro-benzenethiolate complex 5
The modular synthetic approach taken in this study purpose-
fully avoided covalently linking the thiolate donor to the N4
ligand in order to allow for systematic variation of the electron-
donating/releasing properties of the thiolate ligand. We were
pleased to find that an extensive series of mononuclear, five-
coordinate iron(II) complexes could be assembled in this
manner, provided that the thiolate-to-iron ratios are carefully
controlled. Other iron(II) complexes with N4S(thiolate) donor
sets include [FeII(L8py2)(SAr or SC6H11)]BF4 (L8py2 ) 1,5-
Bis(pyridin-2-ylmethyl)-1,5-diazocane), which were prepared as
models of SOR by a similar self-assembly approach,59,60 and
the covalently linked, thiolate-ligated complexes [FeII(cyclam-
PrS)]BPh4 and [FeII(SMe2N4(tren))]PF6 (tren ) tris(2-amino-
ethyl)amine) from Kovacs and co-workers.36,61,62 However, this
study is the first report, to our knowledge, that allows for a
direct assessment of the electronic influence of the thiolate donor
at parity of ligand environment for a series of N4S(thiolate)-iron
complexes.
X-ray Structural Studies. The X-ray crystal structures of 2-5
are presented as ORTEP diagrams in Figure 1. Structural
parameters are shown in Table 1, and selected bond lengths
(58) Namuswe, F., Sarjeant, A. A. N.; Goldberg, D. P. Unpublished results.
(59) Halfen, J. A.; Moore, H. L.; Fox, D. C. Inorg. Chem. 2002, 41, 3935–
3943.
(63) The high-spin configuration for 1-3 has been confirmed by SQUID
measurements, giving ꢁMT ) 3.61-3.70 cm3 mol-1 K at 298 K
-1
(ꢁMT(theor.) ) 3.63 cm3 mol
K with g ) 2.2 for HS FeII).
(60) Fiedler, A. T.; Halfen, H. L.; Halfen, J. A.; Brunold, T. C. J. Am.
Chem. Soc. 2005, 127, 1675–1689.
(64) Krishnamurthy, D.; Sarjeant, A.; Goldberg, D. P.; Caneschi, A.; Totti,
F.; Zakharov, L. N.; Rheingold, A. L. Chem. Eur. J. 2005, 11, 7328–
7341.
(61) Shearer, J.; Scarrow, R. C.; Kovacs, J. A. J. Am. Chem. Soc. 2002,
124, 11709–11717.
(65) Mukherjee, R. N.; Abrahamson, A. J.; Patterson, G. S.; Stack, T. D. P.;
Holm, R. H. Inorg. Chem. 1988, 27, 2137–2144.
(62) Shearer, J.; Nehring, J.; Lovell, S.; Kaminsky, W.; Kovacs, J. A. Inorg.
Chem. 2001, 40, 5483–5484.
(66) Zang, Y.; Que, L., Jr. Inorg. Chem. 1995, 34, 1030–1035.
9
J. AM. CHEM. SOC. VOL. 130, NO. 43, 2008 14193

A R T I C L E S
Namuswe et al.
Table 2. Relevant Bond Distances (Å) and Angles (deg) for Complexes 1-5
cmpd
1
2
3
4
5
Fe -N1
Fe-N2
Fe-N3
Fe-N4
Fe-S
2.164(3)
2.273(3)
2.138(3)
2.206(3)
2.3316(11)
2.206(7)
2.251(8)
2.075(7)
2.160(7)
2.3197(12)
2.223(8)
2.161(3)
2.250((3)
2.088(3)
2.229(3)
2.3426(8)
2.132(10)
2.248(8)
2.095(8)
2.188(7)
2.3240(17)
2.097(11)
2.137(16)
2.069(9)
2.3305(15)
N1-Fe-N2
N1-Fe-N3
N1-Fe-N4
N2-Fe-N3
N2-Fe-N4
N3-Fe-N4
S-Fe-N1
79.45(12)
133.56(13)
85.98(12)
94.49(12)
163.08(13)
89.30(12)
115.33(10)
86.10(9)
110.03(10)
108.10(10)
116.90(12)
97.6(3)
77.7(3)
133.4(3)
85.8(3)
96.0(3)
162.0(3)
90.1(3)
77.8(3)
132.5(3)
84.9(3)
95.5(3)
161.0(3)
90.4(3)
110.8(2)
97.3(2)
79.86(11)
132.06(12)
84.60(10)
95.78(12)
162.26(11)
88.28(12)
107.85(8)
100.04(9)
119.83(9)
92.79(8)
72.3(6)
137.4(4)
86.4(6)
95.2(5)
153.2(8)
89.4(6)
107.0(3)
104.2(7)
109.4(2)
96.6(2)
S-Fe-N2
S-Fe-N3
117.2(2)
95.59(17)
111.34(15)
-90.6(5)
94.0(5)
116.7(2)
96.06(18)
115.5(3)
97.5(2)
S-Fe-N4
Fe-S-C
111.3(4),a 106.0(5)b
-80.8(10),a -105.9(8)b
102.2(12),a 82.7(13)b
112.29(10)
-132.8(2)
52.0(3)
109.7(6),a 110.3(5)b
90.3(11),a 89.7(10)b
-92.9(14),a -92.7(11)b
Fe-S-C(12)-C(13)
Fe-S-C(12)-C(17)
-87.5(3)
a Major component. b Minor component.
exhibits Fe-N bond distances that are on the lower end of the
range seen for 1-5. These shorter distances may be a
consequence of the weaker donation from the NO2-substituted
phenylthiolate ligand, which results in a more electropositive
iron(II) center as evidenced by electrochemistry (vide infra).
In addition, the NO2 complex 5 exhibits a geometry closer to
square pyramidal (sp) as determined by its τ value67 of 0.27 (τ
) 0.0 for idealized sp and 1.0 for idealized tbp) compared to τ
≈ 0.5 for 1-4, with its thiolate ligand occupying the axial
position. A combination of the electron-poor nature of 5 along
with this subtle geometry change may account for the shorter
Fe-N distances in 5.
The precise orientation of the thiolate donor in both
metal-thiolate models and metalloproteins has been suggested
to be important for controlling the sulfur-metal bonding
interactions and related properties, including reduction poten-
tials.60,68,69 The orientation of the arylthiolate ligands in 1-5
Figure 2. Cyclic voltammograms for [FeII([15]aneN4)(SC6H4-p-X)]BF4 (X
) H (1), Cl (2), OMe (3), NO2 (5)) and [FeII([15]aneN4)(SC6F4-p-
SC6F5)]BF4 (4), measured in CH2Cl2 using a Ag/AgCl reference electrode
(3.5 M KCl).
can be defined by the Fe-S-Caryl angle and Fe-S-Caryl-Caryl
′
dihedral angle. The Fe-S-Caryl angles for 1-5 are similar, lying
between 109.7(6)-116.90(12)°. The Fe-S-Caryl-Caryl′ dihedral
angles are also similar for 1-3 and 5, varying between
90.3(11)-105.9(8)°, while for the fluorinated complex 4 this
angle is 132.8(2)°. Interestingly, the packing diagram of complex
4 shows a π-stacking interaction in the solid state between the
C6F4 ring of one molecule and C6F5 ring of a neighboring
molecule (X, 1 - Y, -1/2 + Z), characterized by the following
geometric parameters: Ct-Ct ) 3.586 Å (Ct ) centroid of the
ring), R ) 9.30° (R ) dihedral angle between the stacking
planes) and D ) 3.296 Å and 3.403 Å (D ) perpendicular
distances from one ring to the plane of the other ring). This
the coordination sphere of the iron(II) center in the active site.
The FeII-S bond distances in 1-5, which fall in the normal
range, are somewhat shorter than that found in the enzyme (X-
ray structure:70 2.40-2.44 Å; EXAFS:71 2.37 Å). It is not clear
at present why SOR exhibits an elongated iron-sulfur bond.
In addition, complexes 1-5 show geometries between that of
square pyramidal and trigonal bipyramidal, whereas the protein
exhibits a more purely square pyramidal geometry at the iron(II)
center.
Electrochemistry. Complexes 1- 5 were characterized by
cyclic voltammetry, as shown in Figure 2. Each complex
exhibits a single irreversible oxidation whose position varies
with the identity of the para substituent on the arylthiolate
ligand. We assign these anodic peaks (Epa) for 1-5 to the FeIII/II
couples of these complexes, and they are listed in Table 3
together with other relevant FeIII/II redox potentials for related
complexes. Although caution must be used when analyzing
irreversible waves, the trends that emerge from comparing the
π-stacking interaction may be perturbing the Fe-S-Caryl-Caryl
′
dihedral angle.
Comparison to the Structure of the Reduced Enzyme,
SORred. All of the model complexes 1- 5 are five-coordinate
iron(II) complexes with one open site on the metal, as found in
SORred. The N4S(thiolate) donor set of 1-5 is a good mimic of
(67) Addison, A. W.; Rao, T. N.; Reedjik, J.; van Rijn, J.; Verschoor, G. C.
J. Chem. Soc., Dalton Trans. 1984, 1349–1456.
(70) Yeh, A. P.; Hu, Y. L.; Jenney, F. E., Jr.; Adams, M. W. W.; Rees,
D. C. Biochemistry 2000, 39, 2499–2508.
(68) McNaughton, R. L.; Tipton, A. A.; Rubie, N. D.; Conry, R. R.; Kirk,
M. L. Inorg. Chem. 2000, 39, 5697–5706.
(71) Clay, M. D.; Jenney, F. E., Jr.; Hagedoorn, P. L.; George, G. N.;
Adams, M. W. W.; Johnson, M. K. J. Am. Chem. Soc. 2002, 124,
788–805.
(69) Solomon, E. I.; Szilagyi, R. K.; George, S. D.; Basumallick, L. Chem.
ReV. 2004, 104, 419–458.
9
14194 J. AM. CHEM. SOC. VOL. 130, NO. 43, 2008

Evidence for a trans Influence in FeIII-OOR Complexes
A R T I C L E S
Table 3. Electrochemical Data for (N4S(thiolate))FeII Complexes
complex
E (mV) vs SHE (Epa (mV) vs Ag/AgCl)
solvent
ref
[FeII([15]aneN4)(SC6H5)]BF4 (1)
[FeII([15]aneN4)(SC6H4-p-Cl)]BF4 (2)
[FeII([15]aneN4)(SC6H4-p-OMe)]BF4 (3)
[FeII([15]aneN4)(SC6F4-p-SC6F5]BF4 (4)
[FeII([15]aneN4)(SC6H4-p-NO2)]BF4 (5)
FeII[(Me4cyclam)(SC6H4-p-OMe)]OTf
[FeII(L8py2)(SC6H4-p-CH3)]BF4
706a, b (501)
786a, b (581)
651a, b (446)
1018a,b (813)
853a, b (648)
812a, c
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH3CN
CH3CN
CH3CN
this work
this work
this work
this work
this work
60
857c
59
36
[FeII(cyclam-PrS)]BPh4
462c
a Anodic potentials for the irreversible oxidation wave. b E versus SHE values were obtained by adding 205 mV to the recorded values versus Ag/
AgCl. c E versus SHE values were obtained by adding 242 mV to the reported literature values versus SCE.
Scheme 2
different values in Table 3 deserve attention. There is a clear
increase in the FeIII/II potential for 1-5 as the para substituent
changes from electron-donating to electron-withdrawing, dem-
onstrating that the thiolate donor has a dramatic influence over
the FeIII/II couple. It is not easy to predict a priori where
fluorinated complex 4 should appear in the series, but the CV
data show that -SC6F4-p-SC6F5 has the weakest electron-
donating ability, exhibiting an FeIII/II couple that is 165 mV
more positive than the NO2-substituted complex 5. The p-OMe
complex 3 reveals a significantly less positive oxidation potential
(∆E ) 161 mV) than the related cyclam complex (FeII(Me4[14]-
aneN4)(SC6H4-p-OMe)]OTf.60 This complex contains the
same arylthiolate ligand as 3 but includes the smaller 14-
membered macrocyclic N4 donor and has N-methylated
groups instead of secondary N-H groups. The change in ring
size is unlikely to account for the observed difference in
oxidation potentials, given that the potentials for the cobalt
complexes [Co[15-14]aneN4)Cl2]+ are in the opposite order
([15]aneN4 stabilizes CoII relative to [14]aneN4).72 The
presence of the methyl groups on the amine donors might
be expected to cause the opposite shift as well. However,
experimental evidence and density functional theory (DFT)
calculations for [(cyclamacetate)FeF]PF6 and (trimethyl-
cyclamacetate)FeF]PF6 show that the N-methylated ligand
is in fact a weaker donor for both steric and electronic
reasons.73 Thus, N-methylation may be responsible for mak-
ing it significantly more difficult to oxidize (Me4[14]aneN4)-
FeII(SC6H4-p-OMe)]OTf compared to complex 3.
value,77,78 but computational models of the SOR active site show
a surprising opposite effect in which the incorporation of
N-H---S bonds causes a significant lowering of the FeIII/II redox
potential.22 Despite the discrepancies between the models and
the protein, the data for 1-5 provide a clear trend which shows
that the nature of the thiolate donor has a strong influence over
the oxidation potential, as proposed for SOR.16,22
Reaction of FeII Complexes with ROOH. In an earlier report
we showed that 1 reacts at low temperature (-78 °C) with the
t
alkylhydroperoxides BuOOH and cumenylOOH to give the
corresponding dark-red, alkylperoxo-iron(III) species. Char-
acterization of these low-temperature, metastable FeIII-OOR
intermediates by UV-vis, EPR, and RR spectroscopies showed
1
that they are low-spin (S ) /2) FeIII-OOR complexes with
weakened Fe-O bonds. In this study we have significantly
expanded this class of FeIII-OOR species via the reaction of
tBuOOH and cumenylOOH with complexes 2-5 to give low-
temperature metastable FeIII-OOR species.
The FeIII/FeII redox potential for SORs isolated from different
organisms have been reported to fall between 200-365
mV.71,74-76 Complexes 1-5 and the other model complexes
in Table 3 are considerably more difficult to oxidize than the
enzyme. The difference between the enzyme potentials and the
models in Table 3 may arise from several environmental factors,
such as solvent (organic solvents for the models versus aqueous
conditions for the protein) and the presence of a Glu residue in
the enzyme that coordinates to the iron(III) form of the protein.
There are also N-H---S bonds in the protein that would be
expected to drive the redox potential toward a more positive
UV-Vis Spectroscopy. Complexes 1- 5 react with tBuOOH
at -78 °C in CH2Cl2 to give the low-temperature adducts 1a-5a
(Scheme 2). Low-temperature UV-vis spectra for these species
are shown in Figure 3. The iron(II) complexes 1-4 are rapidly
converted from colorless or light-yellow to dark-red upon
t
introduction of BuOOH at -78 °C in CH2Cl2. The p-NO2
complex 5 is red in color because of an intraligand absorption
band from the thiolate donor, and the solution of 5 changes to
fuchsia upon reaction with tBuOOH at -78 °C. Each complex
exhibits an intense band (ꢀ ) 1800-3100 M-1 cm-1) between
508 and 526 nm (Figure 3), for which the peak maxima and
molar absorptivity are characteristic of alkylperoxo-to-iron(III)
LMCT bands.26,29,30,79 This peak shifts to higher energy in the
order 1a (p-H) < 2a (p-Cl) < 5a (p-NO2) < 3a (p-OMe) < 4a
(SC6F4-p-SC6F5). This ordering follows an increase in the
(72) Hung, Y.; Martin, L. Y.; Jackels, S. C.; Tait, A. M.; Busch, D. H.
J. Am. Chem. Soc. 1977, 99, 4029–4039.
(73) Berry, J. F.; Bill, E.; Garcia-Serres, R.; Neese, F.; Weyhermuller, T.;
Wieghardt, K. Inorg. Chem. 2006, 45, 2027–2037.
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Litwiller, R.; Benson, L. M.; Moura, I.; Moura, J. J. G.; Radolf, J. D.;
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D. E. J. Biol. Inorg. Chem. 2007, 12, 248–256.
(78) Dey, A.; Okamura, T.; Ueyama, N.; Hedman, B.; Hodgson, K. O.;
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A R T I C L E S
Namuswe et al.
Figure 4. UV-vis spectra of [FeII([15]aneN4)(SC6H4-p-NO2)]BF4 (5) (blue
circles), [FeIII([15]aneN4)(SC6H4-p-NO2)(OOtBu)]+ (5a) (red triangles), and
the decomposition product (green squares).
broad band at 370 nm (blue circles). The sodium salt of the
free thiolate ligand (NaSC6H4-p-NO2) is also dark red in color
and exhibits a peak at 422 nm in CH2Cl2:MeOH (99:1), which
has been assigned as an intraligand transition81,82 (data not
shown). Thus coordination of -SC6H4-p-NO2 to FeII induces a
52 nm blue-shift of this intraligand band (blue circles), and this
transition is clearly sensitive to the local environment of the
sulfur atom as has been shown previously.82 Upon reaction of
t
5 with BuOOH to give the FeIII-OOtBu species 5a, the
transition associated with the p-nitro-benzenethiolate moves to
352 nm (red triangles), concomitant with the appearance of the
alkylperoxo LMCT band at 522 nm. The final spectrum exhibits
a band at 327 nm that matches that of the disulfide p-NO2C6H4S-
SC6H4-p-NO2 measured independently. These data indicate that
the thiolate donor remains coordinated to the iron center in 5a,
giving rise to the unique band at 352 nm, which converts to
disulfide upon decomposition of 5a. Attempts to isolate the
iron(III) product(s) after warming the reaction to room temper-
ature have been unsuccessful, probably due to the decomposition
of the complexes via disulfide formation.
Proposed Mechanism of Formation of FeIII-OOR. A possible
mechanism of formation of 1a- 5a, 1b, 2b, 4b and 5b, involves
initial oxidation of the iron(II) complex by ROOH to give an
FeIII-OH complex, which then reacts with ROOH by displace-
ment of OH- to give the FeIII-OOR species and H2O. This
mechanism has been proposed by Que and co-workers
for the formation of similar FeIII-OOR species, and in one
case the identification of an FeIII-OH complex [(6-Me3-
TPA)FeIII(O2CPh)(OH)]+ (6-Me3-TPA ) tris(6-methyl-2-py-
ridylmethyl)amine) was obtained via ESI-MS after addition of
Figure 3. UV-vis spectra of [FeIII([15]aneN4)(SAr)(OOtBu)]+ complexes
1a-5a in CH2Cl2 at -78 °C.
electron-withdrawing power of the para substituent and the
related FeIII/II oxidation potential for each complex, with the
exception of the p-OMe complex 3a. Interestingly, the shift of
the peak to higher energy for complexes 1a, 2a, 4a, and 5a
with an increase in the FeIII/II couple is opposite to that observed
foraseriesofnonthiolateligated,polypyridyliron(III)-hydroperoxo
species.80 The differences in the low-temperature UV-vis
spectra for 1a-5a provide strong evidence that the thiolate
ligands remain coordinated upon oxidation to the FeIII-OOR
species and exert a significant influence over the spectroscopic
features of these species.
The UV-vis data for the p-NO2 complex provides more
information regarding the fate of the thiolate donor because of
the intraligand band associated with -SC6H4-p-NO2. The
UV-vis spectra of [FeII([15]aneN4)(SC6H4-p-NO2)](BF4) 5, the
corresponding FeIII-OOtBu species 5a, and the product obtained
after the decay of 5a at -78 °C are shown in Figure 4. The
spectrum for the dark-red iron(II) complex 5 exhibits an intense,
0.5 equivalents of tBuOOH to [(6-Me3-TPA)FeII(O2CPh)] + 83
.
Subsequent formation of the FeIII-OOR species [(6-Me3-
TPA)FeII(O2CPh)]+(OOtBu)]+ was observed upon addition of
t
excess (5 equiv) BuOOH.
According to the former mechanism, the stoichiometry of the
first step should account for 0.5 equiv ROOH reacting with 1.0
equiv FeII to give the FeIII-OH species, and 1.0 equiv ROOH
displacing the putative OH- ligand, resulting in a final stoichi-
ometry of 1.5 ROOH:1.0 FeII in the production of the alkylp-
eroxo complex. We thus quantitated the stoichiometry of the
t
reaction between 1 and BuOOH to gain insight into the
proposed mechanism. Successive addition of tBuOOH (0.2 equiv
t
per addition, 4.0 M stock solution of BuOOH in decane) to 1
(81) Abu-Eittah, R. H.; Hilal, R. H. Appl. Spectrosc. 1972, 26, 270–277.
(82) Thompson, J. S.; Marks, T. J.; Ibers, J. A. J. Am. Chem. Soc. 1979,
101, 4180–4192.
(80) Roelfes, G.; Vrajmasu, V.; Chen, K.; Ho, R. Y. N.; Rohde, J. U.;
Zondervan, C.; la Crois, R. M.; Schudde, E. P.; Lutz, M.; Spek, A. L.;
Hage, R.; Feringa, B. L.; Mu¨nck, E.; Que, L., Jr. Inorg. Chem. 2003,
42, 2639–2653.
(83) Kim, J.; Zang, Y.; Costas, M.; Harrison, R. G.; Wilkinson, E. C.; Que,
L., Jr. J. Biol. Inorg. Chem 2001, 6, 275–284.
9
14196 J. AM. CHEM. SOC. VOL. 130, NO. 43, 2008

Evidence for a trans Influence in FeIII-OOR Complexes
A R T I C L E S
are resonance-enhanced with laser excitation coinciding with
the alkylperoxo-to-iron(III) LMCT band. In all compounds
studied here, the RR spectra obtained with a 514-nm excitation
show no evidence of resonance enhanced ν(Fe-S) modes
associated with the intermediate species (data not shown). The
lack of enhancement of ν(Fe-S) modes suggests that the LMCT
transitions have no sulfur-to-iron(III) LMCT character. The
ν(O-O) falls in the expected region for a low-spin FeIII-OOR
(R ) alkyl) species, but the ν(Fe-O) is lower than usual (Table
4). Confirmation of these assignments comes from the observed
isotopic shifts for 1a-18O18OtBu (ν(Fe-18O) ) 584 cm-1
;
ν(18O-18O) ) 757 cm-1).24 The 28- and 46-cm-1 downshifts
observed for the ∆ν(Fe-16/18O) and ∆ν(16/18O-16/18O) modes,
respectively, are in perfect agreement with predicted values for
isolated diatomic vibrations. Equivalent 18O-shifts are observed
for the ν(Fe-O) modes of 2a, 4a, and 5a (Figure S2, Supporting
Information). Overlapping solvent bands and a Fermi splitting
of the ν(18O-18O) modes makes it difficult to quantify the extent
of the 18O-shifts on the ν(O-O), but the spectral changes
observed are sufficient to confirm the assignment of the ν(O-O)
mode.
Figure 5. X-band, 9 GHz, EPR spectrum of [FeIII([15]aneN4)(SC6H4-p-
Cl)(OOtBu)]+ (2a) in CH2Cl2 at 15 K. Experimental conditions: Frequency,
9.475 GHz; microwave power, 2.012 mW; modulation frequency, 100.00
kHz; modulation amplitude, 10.00 G; receiver gain, 5.02 × 103.
Based on the RR data for 1a, we previously noted that the
Fe-O bond is surprisingly weak compared to other low-spin
FeIII-OOR complexes where R is an alkyl group.24 Indeed,
other low-spin FeIII-OOR complexes exhibit relatively strong
Fe-O bonds and weak O-O bonds,26,29,30,79 as opposed to their
high-spin analogues which give rise to weak Fe-O and strong
O-O bonds.28,31,33 The vibrational frequencies observed previ-
ously for other low-spin FeIII-OOtBu complexes and those
observed for 1a-5a are given in Table 4. An earlier study of
the low-spin complex [FeIII(TPA)(OHn)(OOtBu)]n+ (entry 6,
Table 4) showed evidence of vibrational mixing within the Fe-
OOtBu unit (exptl: ∆16/18O ) 24 cm-1; theor. diatomic approx.:
Figure 6. Resonance Raman spectra of [FeIII([15]aneN4)(SAr)(OOtBu)]+
complexes 1a-5a in CH2Cl2 at 110 K obtained with 514-nm excitation.
Intense Raman bands from the solvent are observed between 700 and 750
cm-1. The 849-cm-1 band in 5a is assigned to the NO2 scissoring mode.
∆
16/18O ) 31 cm-1), and a normal coordinate analysis coupled
with DFT calculations was used to derive a force constant
of 3.53 mdyn/Å for the Fe-O bond in [FeIII(TPA)(OHn)-
(OOtBu)]n+ 29
This force constant together with those of the
.
other complexes in Table 4 are significantly larger than those
for 1a-5a. The 1a to 5a series presents weaker Fe-O bonds
than other iron(III)-alkylperoxo complexes lacking the thiolate
ligand and implicates the unique thiolate donor in weakening
this bond.
at -78 °C revealed maximal formation of 1a (λ ) 526 nm)
between 1.5 and 2.0 equiv of tBuOOH (Figure S1). The overall
t
stoichiometry of BuOOH to FeII matches well with that
predicted by the former mechanism.
The influence of the sulfur donor can be seen in the
correlation between the energy of the Fe-O vibrations for
1a-5a and the identity of the thiolate ligand. Specifically, the
ν(Fe-O) shifts to higher energy as the electron-withdrawing
character of the aryl group increases. In contrast, the ν(O-O)
is unaffected by the thiolate donor, and the kO-O is nearly
constant in the 1a-5a series and is similar to those of previously
reported low-spin complexes lacking a thiolate ligand (Table
4). These RR data show that the electron-donating ability of
the arylthiolate ligand controls the strength of the Fe-O bond
without affecting the O-O bond.
A similar dependence on the electron-donating ability of the
arylthiolate ligand is observed for the FeIII-OOCm (Cm )
C(CH3)2C6H5) species. The RR spectra of these complexes show
ν(Fe-O) modes between 622 and 631 cm-1 (Figure 7) which
are ∼10 cm-1 higher than in the Fe-OOtBu complexes. As
with the Fe-OOtBu complexes, the FeIII-18O18OCm complexes
show RR spectra that confirm the ν(Fe-O) assignment, but the
extent of the isotope shifts come short of the expected values
for isolated diatomic vibrations and suggest that mode mixing
occurs with the cumenyl derivatives (Figure S3). The frequency
EPR Spectroscopy. An X-band EPR spectrum collected at
15 K of the FeIII-OOtBu species 2a is shown in Figure 5, and
is representative of the series 1a- 5a, which all show very
similar EPR spectra. This spectrum is indicative of a low-spin
(S ) 1/2) FeIII complex with axial symmetry, with g values of
2.19 and 1.97. The small signal at g ) 4.3 is assigned to a
high-spin iron(III) impurity. It was shown previously24 that the
low-spin iron(III) EPR signal for 1a decays concomitantly with
the red chromophore at -78 °C, while the high-spin iron(III)
signal remains unchanged. This behavior was confirmed for 2a,
where a similar correlation for the decay of the EPR signal and
red chromophore has been observed. These observations rule
out the g ) 4.3 signal as being associated with the FeIII-OOR
chromophore.
Analysis of Fe-O and O-O Bond Strengths from RR
Spectroscopy. Definitive evidence for the identification of the
low-temperature chromophores 1a-5a as FeIII-OOtBu species
comes from RR spectroscopy (Figure 6). The dominant band
in the 600-650 cm-1 range is assigned to the Fe-O stretching
vibration of the Fe-OOtBu unit, while the band near 800 cm-1
is assigned to the O-O stretching vibration. These vibrations
9
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A R T I C L E S
Namuswe et al.
Table 4. Vibrational Data Obtained from Resonance Raman Spectroscopy for Low-Spin FeIII-OOtBu Complexes
low-spin FeIII-OOtBu complex
ν (cm-1) (Fe-O)
ν (cm-1) (O-O)
k(Fe-O)a (mdyn/Å)
k(O-O)a (mdyn/ Å)
ref
[FeIII([15]aneN4)(SC6H5)(OOtBu)]+ (1a)
[FeIII([15]aneN4)(SC6H4-p-Cl)(OOtBu)]+ (2a)
[FeIII([15]aneN4)(SC6H4-p-OMe)(OOtBu)]+ (3a)
[FeIII([15]aneN4)(SC6H4-p-NO2)(OOtBu)]+ (5a)
[FeIII([15]aneN4)(SC6F4-p-SC6F5)(OOtBu)]+ (4a)
[FeIII(TPA)(OHn)(OOtBu)]n+,c
611
612
608
615
623
696
693
691
678
685
680
803
803
802
800
799
796
788
789
808
793
789
2.74
2.75
2.71
2.77
2.85
3.53d
3.52
3.50
3.37
3.44
3.39
3.03
3.03
3.03
3.02
3.01
2.92d
2.93
2.93
3.08
2.96
2.93
b
b
b
b
b
29
26
26
30
79
79
[FeIII(TPA)(OOtBu)(acetone)]+
[FeIII(5-Me3-TPA)(OOtBu)(acetone)]+
[FeIII(bpy)2(OOtBu)(BzOH)]2+
[FeIII(ꢀ-BPMCN)(OOtBu)(X)]2+,e
[FeIII(ꢀ-BPMCN)(OOtBu)(X)]2+,f
a Force constants calculated for a diatomic oscillator using ν(cm-1) ) 1302.83[k(mdyn/Å)/µ]1/2, where µ is the diatomic reduced mass, unless
otherwise noted. b This work. c Recently reported as [FeIII(TPA)(OOtBu)(NCMe]n+ in the same solvent (CH3CN).26 d Obtained from DFT calculations
using a combined NCA/DFT approach. e X ) NCMe or H2O. f X) OTf or ROOH.
the Fe-O stretch strengthens upon decreasing the electron-
donating ability of the thiolate donor, whereas the O-O stretch
shows less variation: SAr (ν(Fe-O), ν(O-O)) ) SC6H4-p-OMe
(586, 776 cm-1); SC6H5 (584, 774 cm-1); SC6H4-p-Cl (595,
781 cm-1); SC6H4-p-NO2 (599, 775 cm-1); SC6F4-p-SC6F5
(600, 776 cm-1). The calculated Fe-O distances also vary with
the arylthiolate substituents, becoming shorter with decreasing
electron-donating character, while the O-O distance shows no
such correlation: SAr (d(Fe-O), d(O-O)) ) SC6H4-p-OMe
(1.875, 1.532 Å); SC6H5 (1.873, 1.532 Å); SC6H4-p-Cl (1.870,
1.532 Å); SC6H4-p-NO2 (1.864, 1.531 Å); SC6F4-p-SC6F5
(1.858, 1.530 Å). These findings are in agreement with the trends
observed experimentally for the Fe-O and O-O vibrations with
changes in the identity of the thiolate ligand, and support the
idea that the thiolate donor modulates the strength of the Fe-O
bond in the alkylperoxo species.
The [15]aneN4 ligand, as well as related macrocycles such
as [14]aneN4 (cyclam), are flexible and potentially allow for
both cis and trans geometries in complexes of the type
Figure 7. Resonance Raman spectra of [FeIII([15]aneN4)(SAr)(OOCm)]+
complexes 1b, 2b, 4b, and 5b (same experimental conditions as in Figure
6).
[M(macrocycle)L2], depending on the size of the metal ion and
nature of the coligands L. Although there are no structurally
well-defined iron(III) complexes of the [15]aneN4 ligand,85 there
up-shifts observed with the ν(Fe-O) may be due to the different
steric requirements of the cumenyl group versus the tert-butyl
group and conformational changes in the Fe-OOR moiety.
Despite these changes, and as seen earlier with the tBu series,
the Fe-O stretching frequencies increase with increasing
electron-withdrawing character from the para-substituent on the
arylthiolate ligand. Once again, these results support a trans
effect of the thiolate donor, where both σ- and π-bonding
interactions between the iron(III) ion and the alkylperoxo ligand
are weakened by the thiolate donor competing for the same
metal d orbitals.
Theoretical calculations on the alkylperoxo species were
carried out to obtain further insights into the trends observed
in the vibrational data. Density functional theory calculations
(B3LYP/6-311G)56 were performed on a model of the alkyl-
peroxo intermediates in which the tBu or Cm group was replaced
with a methyl group, [(FeIII([15]aneN4)(OOMe)(SAr)]+. The
geometries were optimized beginning from the X-ray structural
coordinates of the corresponding iron(II) starting materials, and
both high-spin and low-spin iron(III) configurations (S ) 5/2 or
1/2) were evaluated. The low-spin species were found to be the
lowest in energy for all five thiolate ligands.84 The calculated
vibrational frequencies for the low-spin complexes reveal that
are
a
number of other six-coordinate MIII complexes
([MIII([15]aneN4)L2] (M ) Co, Cr, Rh; L ) monodentate
ligand)). 46,72,86-88 All of these complexes exist exclusively in
the trans geometry, as opposed to smaller macrocycles such as
cyclam, which exhibit both cis- and trans-isomers for complexes
of the type [MIII(cyclam)L2]. Molecular mechanics calculations
from Busch and Hay indicated that the trans geometry is
energetically preferred for the [15]aneN4 ligand, providing a
proper “fit” around the metal center.72,86 Thus we conclude that
the thiolate ligands in the Fe-OOR complexes are coordinated
trans to the OOR ligand and are weakening the Fe-O
interaction through a trans effect.
Our spectroscopic study of these Fe-OOR complexes shows
that low-spin iron(III) centers do not necessarily give rise to
strong Fe-O bonds in FeIII-OOR species. Interestingly, a
weakened Fe-O bond was also recently observed by Kovacs
and co-workers for the high-spin complex [FeIII(cyclam-
PrS)(OOH)]+, and was attributed to a trans influence of the
thiolate donor.36 These observations contrast with a series of
high-spin FeIII-OOR complexes [(L8py2)(X)FeIII(OOR)]+
(85) Hay, R. W.; Fraser, I. Polyhedron 1997, 16, 2223–2227.
(86) Hay, R. W.; Tarafder, M. T. H. J. Chem. Soc., Dalton Trans. 1991,
823–827.
(84) It should be noted that there are difficulties in determining the relative
energies of spin states for open-shell metal ions by DFT calculations.
(87) Islam, M. S.; Uddin, M. M. Polyhedron 1993, 12, 423–426.
(88) Bhattacharya, P. K. J. Chem. Soc., Dalton Trans. 1980, 810–812.
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Evidence for a trans Influence in FeIII-OOR Complexes
A R T I C L E S
-
(L8py2 ) neutral N4 donor, X ) CH3-p-C6H4S-, C6H5CO2
,
Nivie`re and co-workers, who sought to determine the role of
the cysteine donor by examining a H2O2-derived intermediate
for a mutant form of SOR from D. baarsii in which the
iron-sulfur bond was perturbed.18 In this study Glu114, which
is found in proximity to the Fe-S bond, was replaced with Ala,
causing a disruption of the dipolar interaction between Glu114
and the Fe-S unit. In the oxidized form of the mutant (obtained
by treating the mutant with K2IrCl6), a downshift of the Fe3+-S
vibrational modes (291/313/311 cm-1 for E114A; 299/316/323
for wild-type) was observed, indicating a significant weakening
of the iron-sulfur bond. Reaction of the reduced form of the
mutant with a slight excess of H2O2 followed by rapid freezing
(<5 s) allowed for freeze-trapping of an iron-peroxo intermedi-
ate and subsequent examination by RR spectroscopy. The RR
spectrum showed intense ν(Fe-O) and ν(O-O) bands at 446
cm-1 and 851 cm-1. In comparison, RR data on H2O2-derived
intermediates from both wild-type and E47A-SOR revealed
lower Fe-O stretching frequencies (ν(Fe-O) ) 438 cm-1) but
the same O-O stretch (ν(O-O) ) 850-851 cm-1). Thus a
major conclusion from this work was that the E114A mutation
induces a weakening of the Fe-S bond, which in turn causes
a significant strengthening of the Fe-O bond in the (hydro)-
peroxo intermediate, but no change in the O-O bond.
The results presented herein are strikingly similar to the recent
findings for the E114A mutant of the enzyme, namely that the
weakening of the arylthiolate donor in [FeIII([15]aneN4)-
(SAr)(OOR)]+ induces a significant strengthening of the Fe-O
bond, while leaving the O-O bond unperturbed. Although these
FeIII-OOR model complexes are low-spin, while the
FeIII-OOH intermediate in E114A SOR is suggested to be high-
spin, our results provide strong evidence to support the
hypothesis that the trans thiolate donor in SOR significantly
modulates the strength of the Fe-O bond.
In addition to modulation of the Fe-O bond strength for
E114A SOR, the stability of the Fe-OO(H) intermediate was
also significantly increased with the weakening of the Fe-S
interaction, as demonstrated by increased yields of the inter-
mediate and slower decay kinetics.18 The kinetics of decay for
2a-5a are complicated and do not follow a simple first-order
rate law. Similarly, complex kinetic profiles were previously
observed for the decay of other low-spin FeIII-OOtBu com-
plexes.26 However, monitoring the disappearance of the LMCT
bands for 1a- 5a by low-temperature UV-vis reveal a
qualitative trend wherein the FeIII-OOR species decay more
slowly as the electron-donating character of ArS- is reduced.
For example, the fluorinated derivative 4a exhibits a remarkably
slow decay rate, showing almost no decomposition at -78 °C
for over 48 h, as opposed to 1.5 h for the complete decay of
the p-H complex 1a. As described in the Introduction (vide
supra), a study by Que and Halfen on the high-spin model
complexes (N4L)FeIII-OOR (L ) -OTf, -O2CC6H5, -SC6H4-
p-CH3)33 showed an increased stability with electron-donating
trans axial ligands (for kdecomposition, L ) -OTf > -O2CC6H5 >
-SC6H4-p-CH3) and no impact of the trans ligand on the
ν(Fe-O) in the RR spectra. Indeed, to our knowledge com-
pounds 1a-5a are the first series of model complexes that
provide direct evidence for such a trans influence in FeIII-OO(H
or R) species.
OTf-), where the N4 ligand enforces a trans X-Fe-OOR
arrangement, and where the Fe-O bond is not affected by the
nature of the trans ligand.33 The [FeIII(cyclam-PrS)(OOH)]+
complex is currently unique among high-spin FeIII-OOH
species in exhibiting the same trans influence on the Fe-O bond
as observed in 1a-5a, 1b, 2b, 4b, and 5b. It should be noted
that with the exception of [FeIII(cyclam-PrS)(OOH)]+, all trans-
[FeIII(cyclam)L2] complexes are found in low-spin configura-
tions. For example, even the very weak-field donor OTf- yields
low-spin iron(III) in [trans-(cyclam-acetato)FeIII(OTf)]+,89 whereas
cis-[FeIII(cyclam)L2] complexes are uniformly high-spin. Al-
though a cis configuration for [FeIII(cyclam-PrS)(OOH)]+ can
not be entirely ruled out based on the available data, as described
this complex provides another example of a thiolate-induced
trans influence for an Fe-OO(H or R) species.
Implications for the SOR Mechanism. Examination of the
reaction between native SOR and O2 generated by pulse
-
radiolysis by different research groups has led to the observation
of a transient intermediate characterized by a UV-vis band with
λmax ) 600 nm.14,17,19,21,90 Kurtz has recently reported the
generation of this same 600 nm intermediate by stopped-flow
mixing of SOR and KO2.91 This intermediate has been proposed
to be an FeIII-OO(H) species, but has thus far eluded
characterization by other spectroscopic methods. However,
reaction of SOR with H2O2 has allowed for the trapping and
characterization of an FeIII-peroxo species, initially formulated
as an FeIII-η2-O2
(“side-on” peroxo) species.92,93 High-
2-
resolution X-ray structural data has become available on a
mutant SOR (E114A) in which the crystals were first oxidized
to FeIII and then soaked in H2O2 prior to freezing.15 The
structural data revealed a terminal, “end-on” bound hydroper-
oxo-iron(III) species trapped in different configurations. Earlier
predictions from DFT calculations suggested that a side-on
peroxide bonding mode was not sterically feasible for the SOR
active site and favored a low-spin, end-on FeIII-OOH species.94
These calculations combined with the recent X-ray structural
analysis argue for assignment of the species generated in solution
between SOR and H2O2 as a terminally bound FeIII-OOH
species. New DFT calculations from Solomon and co-workers,22
which take into account the influence of the putative N-H---
SCys hydrogen bonds in SOR, also predicts an end-on, protonated
peroxide (FeIII-OOH) as the first detectable intermediate in the
catalytic mechanism, although these calculations favor a high-
spin FeIII ground-state as opposed to the low-spin FeIII-OOH
predicted in the earlier DFT study.94
Although the relationship of the H2O2-derived species to the
native catalytic pathway is an open question, analysis of the
structural and spectroscopic data of the H2O2-derived intermedi-
ate has provided valuable information regarding the inherent
properties of an Fe-(hydro)peroxo moiety in the SOR active
site. Most pertinent to the present study is recent work by
(89) Grapperhaus, C. A.; Mienert, B.; Bill, E.; Weyhermuller, T.; Wieghardt,
K. Inorg. Chem. 2000, 39, 5306–5317.
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2006, 45, 9266–9278.
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Summary and Perspective
We have prepared a series of iron(II) complexes as models
of the SOR active site. These complexes were designed to
examine the influence of the thiolate donor on the structural
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A R T I C L E S
Namuswe et al.
and physical properties as well as the reactivity of the metal
center at parity of ligand environment. The metrical parameters
as determined by X-ray crystallography do not vary significantly
with the change in thiolate donor. In contrast, the nature of the
arylthiolate donor has a significant impact on the FeIII/FeII
potential, in support of the suggestion that the cysteine ligand
in SOR plays a major role in controlling the iron redox
potential.16,22 The stabilization of FeIII-OOR species at low
temperature for all of the thiolate-ligated complexes was
demonstrated, and spectroscopic characterization of these alkyl-
peroxo complexes shows that they are low-spin iron(III) species
with weak Fe-O bonds. These findings provide an exception
to the hypothesis that low-spin FeIII-OO(H or R) species
necessarily exhibit strong Fe-O bonds in comparison to their
high-spin counterparts.
The electron-donating power of the sulfur donor in the
FeIII-OOR complexes described herein directly influences the
strength of the Fe-O bond through a trans influence from the
thiolate ligand. In contrast, the thiolate donor does not impact
the O-O bond strength. DFT calculations on a simplified model
for the FeIII-OOR complexes support these experimental
findings. A similar observation was recently described for the
E114A SOR enzyme, in which a H2O2-derived FeIII-OO(H)
species was trapped and characterized by RR spectroscopy.18
Thus, our results, combined with the results for E114A SOR
suggest that an important function of the Cys donor in SOR is
to weaken the Fe-O bond without weakening the O-O bond,
thereby favoring Fe-O over O-O bond cleavage. Recent DFT
calculations on SOR also support this conclusion.22
ligated iron proteins it has been proposed that N-H---S
hydrogen bonds assist in tuning the S-Fe bonding interaction
and the iron redox potentials,78 and similar proposals have been
made for SOR.20,22 We suggest that perhaps the presence of
the N-H---S bonds in SOR help to modulate the Fe-S
interaction in much the same way as addition of an electron-
withdrawing para substituent to the arylthiolate ligand of our
model system; they provide fine-tuning of the Cys ligand’s
ability to influence the Fe-O bond while leaving the O-O bond
unperturbed in an FeIII-OOH intermediate, ultimately favoring
protonation and formation of H2O2. Generation of FeIII-OO(H)
adducts via reactions between our model complexes and H2O2
-
and O2 is currently being examined.
Acknowledgment. D.P.G. is grateful to the National Institute
of General Medical Sciences at the NIH for funding this work
(GM62309). F.N. is thankful for financial support provided by a
Zeltmann Fellowship sponsored through Johns Hopkins University.
Prof. V. Szalai (University of Maryland Baltimore County) is
acknowledged for her help with EPR spectroscopy. We thank Dr.
Andrea Caneschi (University of Florence) for collecting SQUID
data.
Supporting Information Available: X-ray structure files for
2, 3, 4, and 5 (CIF). UV-vis spectra of the titration of
t
[FeII([15]aneN4)(SPh)(BF4)] with BuOOH; resonance Raman
spectra of [FeIII([15]aneN4)(SAr)(18O18OtBu)]+ and [FeIII([15]-
aneN4)(SAr)(18O18OCm)]+ complexes; complete ref 56. This
material is available free of charge via the Internet at http://
pubs.acs.org.
The sulfur atom of the Cys ligand in SOR is within weak
hydrogen bond contact of two nearby amide N-H groups as
determined by X-ray crystallography.20 For many other cysteine-
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14200 J. AM. CHEM. SOC. VOL. 130, NO. 43, 2008