Sulfur versus iron oxidation in an Iron-Thiolate model complex

Article abstract of DOI:10.1021/ja1045428

In the absence of base, the reaction of [Fe^II(TMCS)]PF ₆ (1, TMCS = 1-(2-mercaptoethyl)-4,8,11-trimethyl-1,4,8,11- tetraazacyclotetradecane) with peracid in methanol at -20 °C did not yield the oxoiron(IV) complex (2, [Fe^IV(O)(TMCS)]PF₆), as previously observed in the presence of strong base (KO^tBu). Instead, the addition of 1 equiv of peracid resulted in 50% consumption of 1. The addition of a second equivalent of peracid resulted in the complete consumption of 1 and the formation of a new species 3, as monitored by UV-vis, ESI-MS, and Moessbauer spectroscopies. ESI-MS showed 3 to be formulated as [Fe ^II(TMCS) + 2O]⁺, while EXAFS analysis suggested that 3 was an O-bound iron(II)-sulfinate complex (Fe-O = 1.95 A, Fe-S = 3.26 A). The addition of a third equivalent of peracid resulted in the formation of yet another compound, 4, which showed electronic absorption properties typical of an oxoiron(IV) species. Moessbauer spectroscopy confirmed 4 to be a novel iron(IV) compound, different from 2, and EXAFS (Fe=O = 1.64 A) and resonance Raman (v_Fe=O = 831 cm^-1) showed that indeed an oxoiron(IV) unit had been generated in 4. Furthermore, both infrared and Raman spectroscopy gave indications that 4 contains a metal-bound sulfinate moiety (v_s(SO₂) ≈ 1000 cm ^-1, v_as(SO₂) ≈ 1150 cm ^-1). Investigations into the reactivity of 1 and 2 toward H⁺ and oxygen atom transfer reagents have led to a mechanism for sulfur oxidation in which 2 could form even in the absence of base but is rapidly protonated to yield an oxoiron(IV) species with an uncoordinated thiol moiety that acts as both oxidant and substrate in the conversion of 2 to 3.

Full text of DOI:10.1021/ja1045428

Published on Web 11/11/2010
Sulfur versus Iron Oxidation in an Iron-Thiolate Model
Complex
Aidan R. McDonald,† Michael R. Bukowski,†,| Erik R. Farquhar,†
Timothy A. Jackson,†,# Kevin D. Koehntop,† Mi Sook Seo,‡ Raymond F. De Hont,§
Audria Stubna,§ Jason A. Halfen,⊥ Eckard Mu¨nck,*,§ Wonwoo Nam,*,‡ and
Lawrence Que, Jr.*,†
Department of Chemistry and Center for Metals in Biocatalysis, 207 Pleasant Street SE,
UniVersity of Minnesota, Minneapolis, Minnesota 55455, United States, Department of
Bioinspired Science, Department of Chemistry and Nano Science, and Center for Biomimetic
Systems, Ewha Womans UniVersity, Seoul 120-750, Korea, Department of Chemistry, Carnegie
Mellon UniVersity, Mellon Institute, 4400 Fifth AVenue, Pittsburgh, PennsylVania 15213, United States,
and Department of Chemistry, UniVersity of Wisconsin-Eau Claire, 443 Phillips Hall,
Eau Claire, Wisconsin 54702, United States
Received May 25, 2010; E-mail: larryque@umn.edu; wwnam@ewha.ac.kr; emunck@cmu.edu
Abstract: In the absence of base, the reaction of [FeII(TMCS)]PF6 (1, TMCS ) 1-(2-mercaptoethyl)-4,8,11-
trimethyl-1,4,8,11-tetraazacyclotetradecane) with peracid in methanol at -20 °C did not yield the oxoiron(IV)
complex (2, [FeIV(O)(TMCS)]PF6), as previously observed in the presence of strong base (KOtBu). Instead,
the addition of 1 equiv of peracid resulted in 50% consumption of 1. The addition of a second equivalent
of peracid resulted in the complete consumption of 1 and the formation of a new species 3, as monitored
by UV-vis, ESI-MS, and Mo¨ssbauer spectroscopies. ESI-MS showed 3 to be formulated as [FeII(TMCS)
+ 2O]+, while EXAFS analysis suggested that 3 was an O-bound iron(II)-sulfinate complex (Fe-O )
1.95 Å, Fe-S ) 3.26 Å). The addition of a third equivalent of peracid resulted in the formation of yet
another compound, 4, which showed electronic absorption properties typical of an oxoiron(IV) species.
Mo¨ssbauer spectroscopy confirmed 4 to be a novel iron(IV) compound, different from 2, and EXAFS (FedO
) 1.64 Å) and resonance Raman (νFedO ) 831 cm-1) showed that indeed an oxoiron(IV) unit had been
generated in 4. Furthermore, both infrared and Raman spectroscopy gave indications that 4 contains a
metal-bound sulfinate moiety (νs(SO2) ≈ 1000 cm -1, νas(SO2) ≈ 1150 cm -1). Investigations into the reactivity
of 1 and 2 toward H+ and oxygen atom transfer reagents have led to a mechanism for sulfur oxidation in
which 2 could form even in the absence of base but is rapidly protonated to yield an oxoiron(IV) species
with an uncoordinated thiol moiety that acts as both oxidant and substrate in the conversion of 2 to 3.
1. Introduction
with four equatorial His ligands and an axial Cys, where the
thiolate acts to stabilize the O-O bond of a peroxoiron(III)
intermediate and to weaken the Fe-O2 bond to facilitate
dissociation of H2O2.3 For both cysteine dioxygenase (CDO)
and isopenicillin N synthase (IPNS), the substrate thiolate ligates
cis to the O2 binding site and becomes primed for oxidation.
CDO catalyzes the conversion of cysteine to cysteinesulfinate.4
It is proposed that dioxygen reacts with the cysteinyl thiolate-
boundiron(II)centerinCDO,formingasuperoxoiron(III)-cysteinate
intermediate, which converts to a persulfenate intermediate and
ultimately an S-bound iron(II)-cysteinesulfinate.5 IPNS cata-
lyzes the oxidation of the thiolate-containing tripeptide substrate
to the isopenicillin product containing a thiazolidine ring in a
two-step process involving superoxoiron(III) and oxoiron(IV)
oxidants.6 With a substrate analogue, the thiol function can
Thiolate ligation plays a vital role in a wide variety of iron-
containing enzymes involved in the activation of dioxygen and
deactivation of reactive oxygen species (ROS). In cytochrome
P450, the highly donating axial Cys ligand acts to facilitate
heterolytic cleavage of an O-O bond in an FeIII-OOH precursor
to form Compound I [(L+•)FeIVdO]+ (L ) porphyrin).1,2 In
contrast, superoxide reductase (SOR) has a non-heme iron center
† University of Minnesota.
‡ Ewha Womans University.
§ Carnegie Mellon University.
⊥ University of Wisconsin-Eau Claire.
| Current address: Department of Chemistry, Penn State Altoona, Altoona,
PA 16601.
# Current address: Department of Chemistry, University of Kansas,
Lawrence, KS 66045.
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10.1021/ja1045428  2010 American Chemical Society

S vs Fe Oxidation in an Iron-Thiolate Model
A R T I C L E S
instead be oxidized to sulfenate.7 Lastly, iron(III) and cobalt(III)
nitrile hydratases (NHase) have active sites where two of the
three Cys ligands are oxidized to sulfenate and sulfinate that
are S-bound to the metal center.8 The maturation of the NHase
active site occurs post-translationally9 and presumably by metal-
assisted oxidation.
Scheme 1. Structures of 1, 2, and Some Synthetic Oxoiron(IV)
Complexes (TMC, TPA, and N4Py)
Recent biomimetic efforts have focused on modeling the
iron-thiolate chemistry of SOR and NHase.10 In efforts to
model putative peroxo intermediates of SOR, [FeIII(SR)(OOH-
(R))] complexes were characterized by several groups.11 In those
studies there was little mention of any oxidation byproducts
involving the thiolate sulfur. In efforts to mimic the active site
of NHase, thiolatoiron(III) or -cobalt(III) complexes were found
to be oxidized to derivatives with S-bound sulfenate and/or
sulfinate ligands as found in the enzymes. Artaud reported a
bis(thiolate)iron(III) complex that reacted with O2 to form an
S-bound bis(sulfinato)iron(III) complex.12 The same group
showed that selective sulfur oxidation of a dithiol ligand can
occur prior to metal (cobalt(III)) insertion, suggesting that metal-
mediated oxidation may not necessarily be involved in NHase.13
Kovacs found iron(III) and cobalt(III) di- and trithiolate
complexes that reacted readily with O2 to form a variety of O-
and S-bound metal(III)-sulfenate and -sulfinate species.14
Grapperhaus demonstrated that the spin-state of iron(III)-thiolate
complexes can modulate the reactivity of bound thiolate to
dioxygen.15 Goldberg has recently reported an iron(II)-thiolate
complex that was converted to an iron(II)-sulfonate complex
upon exposure to O2, demonstrating the first functional mimic
of CDO.16 The analogous zinc(II)-thiolate complex showed
no reactivity toward O2, implicating the iron site in sulfur
oxidation. Heme iron(III)-thiolate model complexes have, in
some cases, also been found to react with oxidants to yield axial
iron(III)-sulfonate complexes.17 Unfortunately, very little
insight into the mechanism of sulfur oxidation in all of these
model complexes was obtained. One mass spectrometry study
into the reaction of an iron(III)-thiolate and dioxygen identified
the intermediate species to be iron(III)-sulfenate, which over
time was converted to iron(III)-sulfinate.18
In the course of our work on synthetic high-valent iron
complexes, we investigated the reaction of [FeII(TMCS)]+ (1,
TMCS ) 1-(2-mercaptoethyl)-4,8,11-trimethyl-1,4,8,11-tetra-
azacyclotetradecane, Scheme 1) with 3-chloroperoxybenzoic
acid (m-CPBA). In the presence of strong base, this reaction
generates 2, an oxoiron(IV) complex with a thiolate ligand trans
to the oxo group that is the only synthetic complex thus far to
model the RSsFeIVdO unit associated with the active oxidants
of cytochrome P450 and chloroperoxidase.1,19 Complex 2
belongs to a growing family of oxoiron(IV) complexes sup-
ported by non-porphyrin ligands,20 including 1,4,8,11-tetram-
ethyl-1,4,8,11-tetraazacyclotetradecane (TMC, Scheme 1),21
tris(2-pyridylmethyl)amine (TPA, Scheme 1),22 and N,N-bis(2-
pyridylmethyl)-N-bis(2-pyridyl)methylamine (N4Py, Scheme
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A R T I C L E S
McDonald et al.
1).23 The presence of the thiolate ligand trans to the oxo group
enhanced the H-atom abstraction reactivity of the FedO unit
but diminished its oxo-transfer ability.21c
filtered over Celite. Subsequent recrystallization from CH2Cl2/Et2O
(2×) yielded the desired product as a colorless crystalline material.
Yield 60%. Anal. Calcd for C26H42O4N4S2Fe: C, 52.52; H, 7.12;
N, 9.42. Found: C, 52.01; H, 7.24; N, 9.45. ESI-MS: calcd for
C26H42O4N4S2Fe, 594.20; found, 453.30 (loss of one sulfinate
[FeII(TMC)(O2SPh)]+). 1H NMR (acetone-d6, 300 MHz): δ ) -9.5,
-8.7, -6.7, -4.5, 1.1, 3.4, 5.5, 6.6, 9.4, 10.7, 12.9, 44, 57, 75,
109, 113, 271, 281.
Herein we show that, in the absence of base, 1 is rapidly
converted upon exposure to 2 equiv of peracid to an S-oxidized
species [FeII(TMCSO2)] (3), which can further be oxidized to
[FeIV(O)(TMCSO2)] (4) with the addition of a third equivalent
of peracid. Both new species have been extensively character-
ized, with the sulfinate functionality likely to be O-bound to
the iron center. Experiments probing the mechanism of sulfur
oxidation are presented and show that the oxoiron(IV) moiety
of 2 could be involved in the oxidation of thiolate to sulfinate.
Preparation of [ZnII(TMCS)]PF6 (7). Zinc complex 7 was
synthesized using the same procedure as reported for the synthesis
of iron complex 1.24 1-(2-Mercaptoethyl)-4,8,11-trimethyl-1,4,8,11-
tetraazacyclotetradecane (0.2 g, 0.66 mmol) and LiOH (0.016 g,
0.66 mmol) were mixed at room temperature in MeOH (3 mL)
for 1 h. Zn(OTf)2 (0.24 g, 0.66 mmol) was then added, and the
resulting mixture was stirred for 1 h at room temperature and
subsequently filtered over Celite. NaPF6 (0.44 g, 2.6 mmol) in
MeOH (3 mL) was then added, resulting in the precipitation of
a white crystalline material that was then collected by vacuum
filtration. Recrystallization from CH2Cl2/Et2O yielded the desired
product in 55% yield. Anal. Calcd for [ZnII(TMCS)CH3CN]PF6 ·
2CH2Cl2 (C19H40Cl4F6N5PSZn): C, 31.57; H, 5.58; N, 9.69. Found:
C, 31.26; H, 5.70; N, 9.08. ESI-MS: calcd for C15H33F6N4PSZn,
2. Experimental Section
Materials. All reagents and solvents were purchased from
commercial sources and used as received, unless otherwise stated.
All reactions with air-sensitive materials were conducted under an
inert atmosphere either using standard Schlenk techniques or in a
glovebox. Methanol was purified by refluxing over metallic
magnesium and subsequent distillation. 1-(2-Mercaptoethyl)-4,8,11-
trimethyl-1,4,8,11-tetraazacyclotetradecane (TMCSH) and its iron-
(II) complex (1, [FeII(TMCS)]PF6) were synthesized according to
literature procedures.24,25 57Fe-enriched 1 was synthesized using
the same procedure but with 57Fe(OTf)2(NCCH3)2 as the source of
Fe. 3-Chloroperoxybenzoic acid (77% Aldrich, m-CPBA) was
purified according to literature procedures (titrated iodometrically,
85% pure).26
Preparation of Intermediates 3 and 4. The preparation of
intermediates 3 and 4 was carried out under an inert gas atmosphere.
Intermediate 3 was prepared by the addition of 2 equiv of m-CPBA
to a stirring solution of 1 (1 mM, MeOH) at -20 °C. Intermediate
4 was prepared by addition of 3 equiv of m-CPBA to a stirring
solution of 1 (1 mM, MeOH) at -20 °C. Both intermediates 3 and
4 were stable at -20 °C, but intermediate 4 reacted further upon
warming to room temperature.
1
m/z 365.17 ([ZnII(TMCS)]+). H NMR (DMSO-d6, 300 MHz): δ
) 1.52 (t, 2H), 1.80 (m, 2H), 2.10-2.35 (m, 9H), 2.39 (s, 3H),
2.41 (s, 3H), 2.55- 3.0 (br m, 8H), 3.0 -3.3 (br m, 6H).
Reactivity Studies. Kinetic studies on the oxidative reactivity
of 4 were performed by adding varying amounts of either
triphenylphosphine (PPh3) or dihydroanthracene (DHA) to a solution
of 4 in MeOH/CH3CN (1:1). Rate constants (kobs) were determined
by pseudo-first-order fitting of the disappearance of the electronic
absorption band at 830 nm. The second-order rate constants (k2)
were determined by plotting kobs versus [substrate].21c Product
analysis for the oxidation of PPh3 was done using 31P NMR. Product
analysis for the oxidation of DHA was done using UV-vis
spectroscopy and GC-MS.
X-ray Crystal Structure Determination of 6. A crystal
(approximate dimensions 0.45 × 0.42 × 0.08 mm) was placed onto
the tip of a 0.1 mm diameter glass capillary and mounted on a
CCD area detector diffractometer for data collection at 173(2) K.27
A preliminary set of cell constants was calculated from reflections
harvested from three sets of 20 frames. These initial sets of frames
were oriented such that orthogonal wedges of reciprocal space were
surveyed. This produced initial orientation matrices determined from
104 reflections. The data collection was carried out using Mo KR
radiation (graphite monochromator) with a frame time of 20 s and
a detector distance of 4.9 cm. A randomly oriented region of
reciprocal space was surveyed to the extent of one sphere and to a
resolution of 0.80 Å. Four major sections of frames were collected
with 0.30° steps in ω at four different φ settings and a detector
position of -28° in 2θ. The intensity data were corrected for
absorption and decay (SADABS).28 Final cell constants were
calculated from 2899 strong reflections from the actual data collection
after integration (SAINT).29 Refer to Table 1 for additional crystal
and refinement information. The structure was solved using Bruker
SHELXTL and refined using Bruker SHELXTL.30 The space group
P212121 was determined on the basis of systematic absences and
intensity statistics. A direct-methods solution was calculated which
provided most non-hydrogen atoms from the E-map. Full-matrix
least-squares/difference Fourier cycles were performed which
located the remaining non-hydrogen atoms. All non-hydrogen atoms
were refined with anisotropic displacement parameters. All hydro-
gen atoms were placed in ideal positions and refined as riding atoms
Preparation of [FeII(TMC)(O2SPh)2] (6). The preparation of
6 was carried out under an inert gas atmosphere. First, 0.290 g
(2.29 mmol) of FeCl2 and 0.590 g (2.29 mmol) of 1,4,8,11-
tetramethyl-1,4,8,11-tetraazacyclotetradecane (TMC) were mixed
together in MeOH (5 mL) for 1 h. Next, 0.750 g (4.58 mmol) of
sodium benzenesulfinate was added, and the resulting solution was
stirred at room temperature for 2 days. MeOH was removed in
Vacuo, and the resulting oily solid was dissolved in CH2Cl2 and
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Chem. Soc. 2005, 127, 1675–1689.
(26) Armarego, W. L. F.; Perrin, D. D. Purification of Laboratory
Chemicals; Butterworth-Heinemann: Oxford Press, 1997.
(28) Blessing, R. Acta Crystallogr. 1995, A51, 33–38.
(29) SAINT+, V6.45; Bruker Analytical X-Ray Systems: Madison, WI,
2003.
(30) SHELXTL, V6.14; Bruker Analytical X-Ray Systems: Madison, WI,
2000.
9
17120 J. AM. CHEM. SOC. VOL. 132, NO. 48, 2010

S vs Fe Oxidation in an Iron-Thiolate Model
A R T I C L E S
Table 1. Crystal Data and Structure Refinement for 6
or a 30-element (SSRL) solid-state germanium detector array
(Canberra). An iron foil spectrum was recorded concomitantly for
internal energy calibration, and the first inflection point of the
K-edge was assigned to 7112.0 eV.
empirical formula
formula weight
temperature
C26H42FeN4O4S2
594.61
173(2) K
XAS Data Analysis. Data reduction, averaging, and normaliza-
tion were performed using the program EXAFSPAK.31 Following
calibration and averaging of the data, background absorption was
removed by fitting a Gaussian function to the pre-edge region and
then subtracting this function from the entire spectrum. A three-
segment spline with fourth-order components was then fit to the
EXAFS region of the spectrum in order to extract ꢀ(k). Analysis
of the pre-edge features was carried out with the program SSExafs32
using a previously described protocol.33 Theoretical phase and
amplitude parameters for a given absorber-scatterer pair were
calculated using FEFF 8.4034 at the single-scattering level of theory
and were utilized by the opt program of the EXAFSPAK package
during curve-fitting. Parameters for 1 were calculated using the
coordinates of the crystal structure reported.25 Parameters for 3 were
calculated for a modification of the starting complex structure in
which an O-bound sulfinate was introduced, such that the Fe · · ·S
distance was increased to 3.3 Å from the 2.3 Å distance in 1 and
an O-atom bound to the sulfur was placed at a distance of 2.0 Å
from the Fe center. The Fe-O-S bond angle was ∼130°. These
parameters are consistent with other iron(II)-sulfinate moieties that
have been described in the literature.41,42 Parameters for 4 were
calculated for a modification of the DFT structure published for 2
in which an O-bound sulfinate was introduced, such that the Fe · · ·S
distance was increased to 3.3 Å and an O-atom bound to the sulfur
was placed at a distance of 2.0 Å from the Fe center. The Fe-O-S
bond angle was ∼130°. The FedO bond length was adjusted to
1.64 Å. A path for an Fe-S interaction at 2.3 Å (as seen in the
crystal structure of 1 and the EXAFS analysis of 2) was also
considered in order to test a wider range of possible Fe · · ·S
distances. Finally, parameters for 6 were calculated using the
coordinates of the crystal structure reported in this work (Vide infra).
Structures of the models for 3, 4, and 6 are shown in Figure S1 of
the Supporting Information. In all analyses, the coordination number
of a given shell was a fixed parameter and was varied iteratively,
while bond lengths (r) and Debye-Waller factors (σ2) were allowed
to freely float. The amplitude reduction factor S0 was fixed at 0.9,
while the edge shift parameter E0 was allowed to float as a single
value for all shells [thus, in any given fit, the number of floating
parameters ) (2 × num shells) + 1]. The goodness-of-fit F (or
crystal system
space group
unit cell dimensions
orthorhombic
P212121
a ) 8.8119(14) Å
b ) 12.913(2) Å
c ) 24.946(4) Å
2838.6(8) Å3
R ) 90°
) 90°
γ ) 90°
volume
Z
4
density (calculated)
crystal color, morphology
crystal size
1.391 mg/m3
colorless, plate
0.45 × 0.42 × 0.08 mm3
1.068
R1 ) 0.0382, wR2 ) 0.0765
R1 ) 0.0487, wR2 ) 0.0798
Goodness-of-fit on F2
final R indices [I > 2σ(I)]
R indices (all data)
absolute structure parameter -0.026(15)
with relative isotropic displacement parameters. The final full matrix
least-squares refinement converged to R1 ) 0.0382 and wR2 )
0.0798 (F2, all data).
Physical Methods. Electronic absorption spectra were recorded
on a Hewlett-Packard (Agilent) 8453 diode array spectrophotometer
(190-1100-nm range) in quartz cuvettes cooled using a liquid
nitrogen cooled cryostat from Unisoku Scientific Instruments
(Osaka, Japan). Mo¨ssbauer spectra were recorded with two
spectrometers, using Janis Research Super-Varitemp dewars that
allowed studies in applied magnetic fields up to 8.0 T in a
temperature range of 1.5-200 K. Mo¨ssbauer spectral simulations
were performed using the WMOSS software package (WEB
Research, Edina, MN). Isomer shifts are quoted relative to Fe metal
at 298 K. High-resolution electrospray mass spectrometry was
performed using a Bruker Bio-TOF II spectrometer. Metastable
species were infused directly into the instrument using a pre-chilled
gas-tight syringe. Fourier transform infrared (FT-IR) transmission
spectra were recorded at room temperature on a solid sample, on
a Thermo-Nicolet Avatar 370 FT-IR. Resonance Raman (rR) spectra
were collected on an ACTON AM-506M3 monochromator with a
Princeton LN/CCD data collection system using a Spectra-Physics
Model 2060 krypton laser. All measurements were carried out on
frozen solutions at 77 K with ∼10 mW power at the samples.
Samples were frozen onto a gold-plated copper coldfinger in thermal
contact with a Dewar flask containing liquid nitrogen. Raman shifts
were calibrated with indene (accuracy (1 cm-1).
F-factor) was defined as [∑k6(ꢀexptl - ꢀcalc)2/∑k6ꢀexptl
]
.
2
1/2
XAS Data Collection. X-ray absorption spectroscopic data for
3 were collected on beamline X9B of the National Synchrotron
Light Source (NSLS) at Brookhaven National Laboratory, with
storage ring conditions of 2.8 GeV and 100-300 mA, while data
for 1, 4, and 6 were collected on beamline 7-3 of the Stanford
Synchrotron Radiation Lightsource (SSRL) of SLAC National
Accelerator Laboratory with storage ring conditions of 3.0 GeV
and 80-100 mA. At NSLS, Fe K-edge XAS data were collected
for a frozen solution of 3 (in MeOH, six complete scans) maintained
at ca. 15 K over an energy range of 6.9-8.0 keV using a Si(111)
double-crystal monochromator for energy selection and a Displex
closed-cycle cryostat for temperature control. A bent focusing mirror
was used for harmonic rejection. The sample of 3 studied by XAS
contained 14 mM Fe; Mo¨ssbauer analysis indicated a yield for 3
of 75%, with the balance of the sample consisting of 4 (11%) and
an iron(III) species (14%). At SSRL, Fe K-edge XAS data were
collected for frozen solutions of 1 (in MeCN, 40 mM [Fe]T, four
complete scans), 4 (in MeCN, 8.0 mM [Fe]T, 80% iron(IV) by UV/
vis, 14 complete scans), and 6 (in THF, 30 mM [Fe]T, 4 complete
scans) maintained at a temperature of ca. 10 K over an energy range
of 6.9-8.0 keV using a Si(220) double-crystal monochromator for
energy selection and an Oxford Instruments CF1208 continuous-
flow liquid helium cryostat for temperature control. Harmonic
rejection was achieved by a 9 keV cutoff filter. Data were obtained
as fluorescence excitation spectra with either a 13-element (NSLS)
3. Results and Discussion
3.1. Identifying Intermediates: Defining the Reaction Land-
scape. 3.1.1. Electronic Absorption Spectroscopy. The iron(II)
complex of the macrocyclic ligand TMCS (1, [FeII(TMCS)-
(PF6)], Scheme 1) dissolved in anhydrous, anaerobic methanol
(MeOH) exhibits an electronic absorption spectrum with an
intense UV feature at λmax ) 320 nm (ε ) 1500 M-1cm-1),
previously assigned to a thiolate-to-iron(II) charge-transfer (CT)
transition.25 The addition of 1 equiV of 3-chloroperoxybenzoic
acid (m-CPBA) to 1 at -20 °C showed the rapid (1 min)
consumption (∼50%, see Figures 1 and 2) of the λmax ) 320
nm band, with no features appearing in the visible or near-IR
regions of the absorption spectrum. The addition of a second
(31) George, G. N. EXAFSPAK; Stanford Synchrotron Radiation Light-
source, SLAC National Accelerator Laboratory: Stanford, CA, 2000.
(32) Scarrow, R. C.; Brennan, B. A.; Cummings, J. G.; Jin, H.; Duong,
D. J.; Kindt, J. T.; Nelson, M. J. Biochemistry 1996, 35, 10078–10088.
(33) Rohde, J.-U.; Torelli, S.; Shan, X.; Lim, M. H.; Klinker, E. J.; Kaizer,
J.; Chen, K.; Nam, W.; Que, L., Jr. J. Am. Chem. Soc. 2004, 126,
16750–16761.
(34) Ankudinov, A. L.; Ravel, B.; Rehr, J. J.; Conradson, S. D. Phys. ReV.
B 1998, 58, 7565.
9
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A R T I C L E S
McDonald et al.
Scheme 2. Reaction of 1 with m-CPBA
Figure 1. UV-vis spectral changes upon titration of 1 with m-CPBA in
MeOH. Red solid line, 0 equiv; red dashed line, 1 equiv; green dashed
line, 2 equiv to form 3; blue solid line, 3 equiv to form 4.
is added to the reaction mixture. Scheme 2 depicts the proposed
stepwise conversion of iron(II)-thiolate 2 to iron(II)-sulfinate
3 and subsequently oxoiron(IV)-sulfinate 4, as well as the
conversion of 1 to 2 in the presence of base.
3.1.2. Mo¨ssbauer Spectroscopy. Mo¨ssbauer spectroscopy
gave insights into the iron oxidation states of intermediates 3
and 4. Complex 1 displayed Mo¨ssbauer parameters typical of a
high-spin iron(II) complex (δ ) 0.90 mm/s, ∆EQ ) 3.06 mm/
s, Figure 3A, Table 2).37 The Mo¨ssbauer spectrum observed
when 1 equiv of m-CPBA was added to 1 showed the formation
of a new high-spin iron(II) species (3, 50%, δ ) 1.06 mm/s,
∆EQ ) 3.77 mm/s), along with unreacted 1 (50%). The addition
of a second equivalent of oxidant showed formation of 3 in
92% yield, with 8% of the iron present belonging to an iron(IV)
species (5, δ ) 0.16 mm/s, ∆EQ ) 1.15 mm/s, Figure 3C). We
assign 5 to be a transient precursor to 4, which is discussed in
greater detail in section 3.2. The conversion of 1 to 3 is thus
clean and rapid and indicates that indeed the reaction between
1 and 2.0 equiv peracid is likely purely a ligand modification
reaction. The change in isomer shift on going from 1 to 3 and
the increase in quadrupole splitting (Table 2) are both consistent
with the replacement of the thiolate in 1 with a much less basic
axial ligand, putatively a sulfinate.
Figure 2. Plot of titration of oxidant (m-CPBA) with 1 (0.85 mM) against
intensity of absorption features assigned to 1 (O, λmax ) 320 nm, left) and
4 (9, λmax ) 830 nm, right).
equivalent of m-CPBA resulted in the complete consumption
of the λmax ) 320 nm feature (1 min) and the appearance of a
new shoulder at λmax ) 280 nm (ε ) 3000 M-1 cm-1) that is
associated with the oxidation product 3. From Figure 2, the
stoichiometry of the conversion of 1 to 3 can be deduced. The
addition of 0.5, 1, and 1.5 equiv of m-CPBA showed a linear
decrease of the λmax ) 320 nm feature, suggesting rapid and
quantitative conversion of 1 to 3. After addition of 1.5 equiv of
m-CPBA, new features in the UV region appear, and hence the
trace of the λmax ) 320 nm feature shows an increase in Figure
2. The new feature arises from an oxoiron(IV) complex
discussed below, but evidence for the formation of this high-
valent iron species was not observed prior to the addition of
1.5 equiv or more of peracid. We surmise that the reaction of
1 with 2 equiv of m-CPBA yields another low-valent iron
complex. The thiolate sulfur is the most likely site of oxidation
apart from the iron(II) center in 1, and the stoichiometry would
suggest that the thiolate is converted to a sulfinate s a stable
moiety at low temperatures.
Complex 4, observed upon addition of a third equivalent of
peracid, exhibited Mo¨ssbauer parameters typical of an iron(IV)
species (δ ) 0.19 mm/s, ∆EQ ) 1.28 mm/s), confirming the
UV-vis diagnosis (Figure 3D). Furthermore, the yield of 4 was
The addition of a third equivalent of m-CPBA to the solution
resulted in the gradual formation of oxoiron(IV) complex 4 over
30 min, with spectral features at λmax ) 330 nm (ε ) 9000
M-1 cm-1), 830 nm (ε ) 170 M-1cm-1), and 990 nm (ε ) 170
M-1cm-1). Near-IR transitions are found that are characteristic
of d-d transitions in S ) 1 [FeIV(O)(L)] complexes.35 The
observed “double-humped” near-IR features resemble those of
[FeIV(O)(TMC)(X)] complexes, where X is a weakly basic anion
like -O2CCF3.21d Both species 3 and 4 were stable at -20 °C,36
but intermediate 4 decomposed over a period of hours when
warmed to room temperature. Apparently iron oxidation is
preferential to sulfur oxidation in the reaction of 3 with peracid,
suggesting that the low-valent iron sulfinate 3 is converted to
an oxoiron(IV) sulfinate, 4, when the third equivalent of peracid
Figure 3. The 4.2-K Mo¨ssbauer spectra of 1, 3, and 4 recorded in zero
field (1.5 mM Fe, MeOH solvent). Spectra are shown as hash-marks, while
spectral simulations are shown as lines: (A) 1; (B) 1 + 1 equiv of m-CPBA
(50% 1, 50% 3); (C) 1 + 2 equiv of m-CPBA (92% 3, 8% 5); (D) 1 + 3
equiv of m-CPBA (86% 4, 14% 3).
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17122 J. AM. CHEM. SOC. VOL. 132, NO. 48, 2010

S vs Fe Oxidation in an Iron-Thiolate Model
A R T I C L E S
Table 2. Physical Properties of 1-6 and Related [FeIV(O)(TMC)(X)] Complexes
a
λmax (nm) (ε (M-1cm-1))
δa (mm/s)
∆EQ (mm/s)
νFedO (cm-1
)
pre-edge area
edge energy (eV)
1
320 (1500)
0.90
0.18
1.01
0.19
0.16
1.13
3.06
0.21
3.77
1.28
1.15
3.92
13.6, 3.6
20.0
13.4
31.0
n.d.
7122.4
7125.1
7123.0
7125.6
n.d.
2b
460 (1300), 570 (1200), 860 (200), 990 (150)
280 (3000)
830 (170), 990 (170)
n.d.
3
4
5
6
831
n.d.
830 (∼150), 1000 (∼150)
230 (2500), 270 (1700)
8.3, 1.5
7123.1
[FeIV(O)(TMC)(X)]c
X ) -CN
858 (250)
836 (250)
824 (400)
0.15
0.20
0.17
0.25
1.39
1.23
823
854
839
21.0
30.9
26.2
7124.8
7124.0
7124.5
X ) -O2CCF3
X ) NCCH3
a Estimated uncertainties for Mo¨ssbauer parameters are as follows: δ, (0.01 mm/s; ∆EQ, (0.05 mm/s. b Data from ref 19. c Data from ref 21d. n.d.
) not determined.
Figure 5. Resonance Raman spectrum of 4 obtained from an 8 mM CH3CN
frozen solution of 4 using 406.7 nm excitation (10 mW power, 32 scans of
16 s each). S ) solvent peak.
to 4 were observed after the addition of 2 equiv of oxidant (m/z
) 405.2, [FeIV(TMCS)(O)3]+).38 These results, together with
Figure 4. ESI-MS spectra following the conversion of 1 (m/z ) 357.1) to
the Mo¨ssbauer data, further corroborate the proposed formation
3 (389.2) and 4 (405.2): (A) 1; (B) 1 + 1 equiv of m-CPBA; (C) 1 + 2
of a sulfinate, demonstrating the incorporation of two oxygen
equiv of m-CPBA; (D) 1 + 3 equiv of m-CPBA.
atoms into 1 to generate 3, an iron(II)-sulfinate. As can be
seen in Figure 4, no evidence for a species incorporating one
found to be almost quantitative (86% FeIV). The observed isomer
oxygen atom was observed (2, m/z ) 373.1) in the conversion
shift (δ) compares favorably with results observed for analogous
of 1 to 3. This would suggest that the conversion from 1 to 3
[FeIV(O)(TMC)(X)] complexes, which generally show values
is rapid and clean, with any putative sulfenate intermediate very
in the range 0.15-0.20 mm/s.21d More importantly, the large
short-lived.
quadrupole splitting (∆EQ) gives a good indication as to the
electronic nature of the axial ligand, as the value of ∆EQ for 4
The addition of the third equivalent of oxidant resulted
in the appearance of a dominant peak at m/z ) 405.2
is comparable to those found for axial NCCH3 (∆EQ ) 1.24
([FeIV(TMCS)(O)3]+), which we have assigned to be 4, and
mm/s) and -O2CCF3 (∆EQ ) 1.39 mm/s) ligands, but much
much less intense features at 357.1 (1) and 389.2 (3). Combining
this observation with the spectral features observed upon
addition of a third equivalent of oxidant to 1, we conclude that
larger than those associated with more basic axial ligands (e.g.,
-SR, ∆EQ ) -0.22 mm/s; -CN, ∆EQ ) 0.25 mm/s). This
difference reflects low electronic symmetry around the iron
4 is most likely an oxoiron(IV) sulfinate complex. There was
center, consistent with the presence of a weakly basic axial
ligand.
3.1.3. Mass Spectrometry. Low-temperature electrospray
ionization mass spectrometry (ESI-MS) provides evidence for
some evidence for [FeIV(TMCS)(O)4]+ in the mass analysis of
4, but the relative intensity of the peak was small.38
3.1.4. Vibrational Spectroscopy. Methods to probe vibrational
modes proved useful in providing additional insight into the
the elemental compositions of intermediates 3 and 4 (Figure
location of the three oxygen atoms incorporated into 1 to make
4). The addition of 1 equiv of m-CPBA to 1 ([FeII(TMCS)]+,
4. Resonance Raman (rR) evidence for an oxoiron(IV) unit using
m/z ) 357.1) showed the formation of a peak at m/z ) 389.2
406.7-nm excitation revealed an enhanced band at 831 cm-1
of intensity comparable to that of residual 1, indicating the
typical of an FedO stretch (Figure 5). The observed stretching
incorporation of two oxygen atoms for 3 ([FeII(TMCS)(O)2]+).
frequency, together with the observed Mo¨ssbauer quadrupole
Addition of a second equivalent of m-CPBA showed a signifi-
cant decrease in intensity of the peak corresponding to 1 relative
to that of the m/z ) 389.2 mass peak (3). Peaks corresponding
(37) Mu¨nck, E. In Physical Methods in Bioinorganic Chemistry; Que, L.,
Jr., Ed.; University Science Books; Sausalito, CA, 2000; Chap. 6, pp
287-320.
(35) Decker, A.; Rohde, J.-U.; Que, L., Jr.; Solomon, E. I. J. Am. Chem.
Soc. 2004, 126, 5378–5379.
(38) Peaks corresponding to 4 are of comparable intensity to those of 3 in
Figure 4C, despite evidence from Mo¨ssbauer spectroscopy that 3 is
the dominant product (92%). We also saw evidence that the sulfinate
species 3 and 4 were being further oxidized in the ionization chamber
to sulfonate, thus producing signals corresponding to both iron(II)
sulfonate and oxoiron(IV)-sulfonate.
(36) ESI-MS analysis of the reaction solution once 4 had decayed showed
peaks that correspond to [Fe(III)(OMe)(TMCSO3)]+. This species
decays further when left to stand at room temperature to iron salts
and [2H+ + TMCSO3] according to ESI-MS.
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J. AM. CHEM. SOC. VOL. 132, NO. 48, 2010 17123

A R T I C L E S
McDonald et al.
Table 3. Selected Fe-O/S/N Distances (Å) in Species 1-4 and 6
Fe-O
Fe-S
FedO
ave. Fe-N
1a b
2.297(3)a
2.20a
2.19b
2.09
,
2.29b
2b
3b
4b
2.33
3.27
1.70
1.64
1.95
2.19
2.05
6a b
,
1.996(2)a
1.95b
3.1749(9)a
3.28b
2.22a
2.18b
a From X-ray crystallography. b From EXAFS analysis.
Figure 6. ORTEP plot of 6 (50% probability level). Hydrogen atoms and
the O2SPh counteranion are omitted for clarity. Selected interatomic
distances (Å): Fe-O1, 1.996(2); Fe-S1, 3.1749(9).
splitting and X-ray absorption pre-edge area (Table 2), suggests
the presence of a weak axial donor ligand comparable to CH3CN
in basicity, as correlated for a series of [FeIV(O)(TMC)(X)]
complexes.39 Unfortunately, we have been unsuccessful in
incorporating 18O into 4 by H218O exchange.40 Furthermore, we
were unable to obtain good rR spectra in attempts to obtain
18O-labeled 4 with H218O2.
Figure 7. Comparison of the Fe K-edge X-ray absorption edge and pre-
edge (inset) features of species 1 (black), 3 (red), 4 (green), and 6 (blue).
Interestingly, bands at 1003 and 1150 cm-1 were also
observed in the rR spectrum of 4, but these features were found
not to be resonance enhanced (Figure 5). Such vibrations are
characteristic of metal-bound sulfinate S-O vibrations (νs(SO2)
≈ 950-1050 cm-1, νas(SO2) ≈ 1100-1200 cm-1).41 Similar
features were observed at 1003 and 1140 cm-1 in the IR
spectrum of 3 (Supporting Information, Figure S7), demonstrat-
ing that a sulfinate moiety is also present in 3. Given the good
agreement between the vibrational data obtained for 3 and 4,
we conclude that the sulfur atom is the lone oxidation site when
2 equiv of peracid is reacted with 1, generating initially
iron(II)-sulfinate 3; the third equivalent of peracid then yields
oxoiron(IV)-sulfinate 4.
and 4 (Figure S8, Supporting Information). Furthermore, the
Mo¨ssbauer parameters for 6 correlate well with those obtained
for 3 (∆EQ ) 3.92 mm/s, δ ) 1.13 mm/s, see Table 2 and
Figure S9, Supporting Information).
3.1.6. X-ray Absorption Spectroscopy. XANES analysis
showed that 1, 3, and 6 possess relatively similar first inflection
points for the Fe K-edge of 7122.4, 7123.0, and 7123.1 eV,
respectively (Table 2), all of which are consistent with an iron(II)
oxidation state. The slightly higher edge energies for 3 and 6
may reflect a slightly higher effective nuclear charge on the
iron center relative to 1, as would be expected when comparing
a highly basic thiolate (1) to a relative weak sulfinate (3 and 6)
axial ligand. In contrast, the Fe K-edge of 4 is considerably
blue-shifted, with a first inflection point of 7125.7 eV, consistent
with two-electron oxidation of the iron center from iron(II) in
1 and 3 to iron(IV) in 4. The edge energy of 4 is similar to the
value of 7125.1 eV previously reported for 219 and is consistent
with the values reported previously for other oxoiron(IV)
complexes with the TMC ligand.21d
The pre-edge feature of 1 shows clear evidence for two peaks
with peak maxima at 7112.5 and 7114.4 eV, having respective
areas of 13.6 and 3.6 units (Figure 7). Complex 6 displays pre-
edge features similar to those of 1, with two peaks observed at
7112.4 and 7114.3 eV, although they are of lower intensity than
those of 1. Earlier XANES studies of five-coordinate square
pyramidal high-spin iron(II) complexes of the form [FeII(TMC)-
(X)] (where X ) -Cl, -Br, NCCH3, or -N3) indicated that these
complexes exhibited two pre-edge peaks split by 1.8-2.0 eV,
consistent with our observations for 1 and 6.43 On the other
hand, the pre-edge feature of 3 can be fit to a single Gaussian
peak at 7112.6 eV with a normalized pre-edge area of ca. 13.4
units.44 Complex 4 exhibits pre-edge peak properties similar to
those of other non-heme oxoiron(IV) complexes, with a single
3.1.5. Characterization of [FeII(TMC)(O2SPh)](O2SPh) (6).
In order to gain a greater understanding of the structural
nature of intermediates 3 and 4, we synthesized a non-
appended iron-sulfinate complex. The reaction of TMC
with FeCl2 with subsequent addition of sodium benzene-
sulfinate yielded quantitatively the iron(II)-sulfinate complex
[FeII(TMC)(O2SPh)](O2SPh) (6), which acts as an ideal struc-
tural mimic of 3. Crystals of 6 suitable for X-ray diffraction
analysis were attained from CH2Cl2/Et2O. The structure obtained
contains one benzenesulfinate bound apically through an oxygen
atom to the iron(II) center (Figure 6). The observed bond
distances in 6 correlate well with other iron(II)-sulfinate
species, showing Fe-O and Fe-S distances comparable to those
reported in the literature (Fe-O ) 1.9-2.0 Å, Fe-S ) 3.1-3.3
Å, Table 3).42 FTIR showed peaks at 1001 and 1152 cm-1 for
6 that compare favorably with those obtained for complexes 3
(39) See Figure 11 in ref 21d. Insertion of a point for 4 (pre-edge area )
31 au, νFedO ) 831 cm-1, ∆EQ ) 1.32 mm/s-1) shows good agreement
with the plot/trend observed, supporting the assignment of a weak
axial donor in 4.
(40) Seo, M. S.; In, J-H,; Kim, S. O.; Oh, N. Y.; Hong, J.; Kim, J.; Que,
L., Jr.; Nam, W. Angew. Chem., Int. Ed. 2004, 43, 2417–2420.
(41) Vitzthum, G.; Lindner, E. Angew. Chem., Int. Ed. 1971, 10, 315–
326.
(43) Westre, T. E.; Kennepohl, P.; DeWitt, J. G.; Hedman, B.; Hodgson,
K. O.; Solomon, E. I. J. Am. Chem. Soc. 1997, 119, 6297–6314.
(44) The lack of a second peak in 3 can be largely attributed to the lower
resolution of the Si(111) crystal monochromator at NSLS X9B
compared with the Si(220) crystal monochromator used at SSRL 7-3.
Similar observations have been made in XAS studies of MMOH.
(42) (a) Cocolios, P.; Lagrange, G.; Guilard, R.; Oumous, H.; Lecomte, C.
J. Chem. Soc., Dalton Trans. 1984, 567–574. (b) Heinrich, L.; Li, Y.;
Vaissermann, J.; Chottard, G.; Chottard, J.-C. Angew. Chem., Int. Ed.
1999, 38, 3526–3528. (c) Noveron, J. C.; Olmstead, M. M.; Mascharak,
P. K. J. Am. Chem. Soc. 2001, 123, 3247–3259.
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S vs Fe Oxidation in an Iron-Thiolate Model
A R T I C L E S
2.29 Å, parameters consistent with the structure derived from
X-ray crystallography.25 The outer-shell features require multiple
Fe· · ·C shells at 2.99, 3.16, and 3.54 Å in order to obtain an
accurate fit (Table 4, fit 1-5), all of which can be accounted for
in the crystal structure of 1. The improvement in F over a fit
with a single Fe · · ·C shell at 3.07 Å indicates that this
convoluted fit is justified, despite the additional complexity
imparted by the multiple shells of carbon scatterers.
The EXAFS analysis of 3 indicates that there are structural
differences between 3 and its precursor 1, as might be expected
from their significantly different k3ꢀ(k) EXAFS and r′-space
spectra. The first coordination sphere can be accounted for with
two shells of three Fe-N scatterers at 2.19 Å and a single Fe-O
scatterer at 1.95 Å, in line with the structure proposed for 3 in
Scheme 2. Although the number of scatterers in the 2.19-Å shell
is underestimated in our best fit (Table 4, fit 3-6), our fit results
are acceptable, given the 30% uncertainty typically associated
with coordination number determinations by EXAFS.45 The
EXAFS features of 3 beyond the first coordination sphere are
best simulated by a shell of four Fe · · ·C at 2.97 Å and a single
Fe· · ·S interaction at 3.27 Å (Table 4, fit 3-6). Importantly, it
was not possible to introduce the Fe-S distance of 2.3 Å seen
in 1 and obtain meaningful fits. Given the short Fe-O and the
long Fe · · ·S distances, we assign the O and S shells respectively
to oxygen and sulfur atoms arising from an O-bound sulfinate
moiety. This assignment is further corroborated by the results
obtained for 6, where a reliable analysis was obtained with one
Fe-O at 1.95 Å, four Fe-N/O at 2.18 Å, four Fe · · ·C at 2.99
Å, and one Fe · · ·S at 3.28 Å (Table 4, fit 5-4). We also note
that modeling the outer-shell features of 6 with two shells of
Fe· · ·C scatterers afforded a considerably poorer fit to the data
(Table 4, fit 5-5), lending further support to our contention that
the feature at r′ ) 2.9 Å is associated with a sulfur scatterer.
There is a discrepancy that we are unable to rationalize in the
Fe-O and Fe · · ·S bond distances obtained for 6 using EXAFS
relative to those from the crystal structure of 6 (Table 3).
Finally, the k3ꢀ(k) EXAFS data for 4 are best fit with the
following set of parameters: 0.8 FedO at 1.64 Å, five Fe-N at
2.06 Å, and four Fe · · ·C at 2.96 Å (Table 4, fit 4-5). The FedO
distance of 1.64 Å is typical of values found for other
[FeIV(O)(TMC)(X)] complexes (the coordination number for the
1.64-Å shell was fixed at 0.8 to account for the observation
that the EXAFS sample was found by UV/vis spectroscopy to
contain 80% 4).21d The 2.06-Å shell consists mainly of the TMC
nitrogen donors, and the observed contraction in distance by
more than 0.1 Å in 4 relative to those in 1, 3, and 6 is consistent
with the increase in oxidation state from iron(II) to iron(IV).
Splitting the 2.06-Å shell to include an Fe-O scatterer at a
shorter distance that could be associated with an oxygen-bound
sulfinate afforded generally poorer fits as defined by the
magnitude of F, and inclusion of this shell could therefore not
be justified (Table S4, Supporting Information). This is the case
as well in the EXAFS analyses of other [FeIV(O)(TMC)(X)]
complexes except for 2.21d Likewise, the available EXAFS data
for 4 do not provide evidence for an Fe · · ·S interaction at any
chemically reasonable distance. Indeed, even the addition of
an Fe · · ·C shell at 2.96 Å to the fit leads to very modest
improvements in fit quality, suggesting that scatterers other than
the primary Fe-N and FedO shells contribute only minimally
to the observed EXAFS. Consequently, EXAFS does provide
Figure 8. Fourier transforms of the Fe K-edge EXAFS data (k3ꢀ(k)) and
unfiltered EXAFS spectra (k3ꢀ(k), inset) obtained for 1, 3, 4, and 6.
Experimental data are shown with dotted lines, while fits are shown with
solid lines. Fourier transformation ranges k (Å-1) are as follows: 1, 2-15;
3, 2-14; 4, 2-14.95; and 6, 2-14.3. Fit parameters are shown in bold
italics in Table 4.
peak centered at 7114.1 eV having a weighted peak area of
31.0 units. In contrast, 2 exhibits a smaller pre-edge area of
20.0 units, likely as a result of the highly basic axial thiolate
donor. We may therefore conclude that XANES analysis
supports our earlier conclusions that 4 contains an oxoiron(IV)
unit with a greater distortion of electronic symmetry relative to
that of 2 as a result of a weaker axial donor.
We next turned to EXAFS analysis to obtain metrical details
of the iron coordination environment in each of these complexes.
The unfiltered k3ꢀ(k) EXAFS data and the corresponding Fourier
transforms for 1, 3, 4, and 6 are presented in Figure 8, along
with representative best fits. Complex 1 exhibits a set of EXAFS
oscillations with a largely unchanged amplitude to k ) 15 Å-1
,
and a relatively simple r′-space spectrum containing an intense
primary peak at r′ ≈ 1.8 Å and relatively weak peaks at higher
r′. Both 3 and 6 exhibit k3ꢀ(k) EXAFS spectra that are similar
to one another and very different from those of 1. Most notably,
the amplitudes of the EXAFS oscillations for 3 and 6 are
considerably smaller, and the r′-space spectra exhibit a clear
set of distinct peaks, relative to observations for 1. The high
similarity between 3 and 6 adds weight to our hypothesis that
3 contains an O-bound Fe-sulfinate moiety. Finally, 4 exhibits
k3ꢀ(k) EXAFS in which the EXAFS oscillations are considerably
attenuated for k > 10 Å-1, as well as a relatively simple r′-
space spectrum similar to those of other [FeIV(O)(TMC)(X)]
species in which the primary peak has shifted to r′ ≈ 1.6 Å.21d
The latter observation is consistent with a shortening of bond
lengths in the first coordination sphere upon oxidation of the
iron(II) center to iron(IV).
Table 4 lists a limited set of EXAFS fits for each of 1, 3, 4,
and 6, with the best fits for each species given in bold italics
(see Tables S2-S5, Figures S3-S6, and accompanying text in
the Supporting Information for full details of the EXAFS
analysis). The primary coordination sphere of 1 can be modeled
with four N/O scatterers at 2.19 Å and one sulfur scatterer at
(45) (a) Penner-Hahn, J. E. Coord. Chem. ReV. 1999, 190-192, 1101–
1123. (b) Yano, J.; Yachandra, V. K. Photosynth. Res. 2009, 102,
241–254.
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J. AM. CHEM. SOC. VOL. 132, NO. 48, 2010 17125

A R T I C L E S
McDonald et al.
Table 4. Selected EXAFS Fitting Results for Species 1, 3, 4, and 6a
Fe-N/O
Fe-O/N
Fe· · · S
Fe· · ·C
species
fit
n
r
σ2
n
r
σ2
n
r
σ2
n
r
σ2
Fb
1
1
2
3
4
5
6
4
4
2.18
2.18
2.19
2.19
1.6
2.6
2.2
2.1
0.506
0.558
0.436
0.389
1
1
2.29
2.29
2.6
2.7
5
3.07
9.7
4
4
6
2.99
3.16
3.54
2.3
1.0
8.1
5
4
2.19
2.2
1
2.29
2.2
0.312
3
1
2
3
4
5
6
4
5
3
4
3
3
2.21
2.22
2.19
2.19
2.19
2.18
11.8
24.5
3.1
5.8
3.3
0.754
0.770
0.582
0.605
0.507
0.378
1
1
1
1
1.97
1.95
1.96
1.95
1.1
0.7
1.2
0.8
1
1
3.27
3.27
5.5
2.9
3.5
4
4
2.97
2.96
4.6
4
6
1
2
3
4
5
4
5
5
5
5
2.07
2.07
2.06
2.06
2.06
3.8
5.1
5.3
5.2
5.2
0.529
0.531
0.443
0.430
0.411
1
1.64
1.64
1.64
4.2
2.7
2.7
0.8c
0.8c
9.3
4.3
1
2
3
4
5
5
4
4
4
4
2.20
2.20
2.20
2.18
2.18
10.4
4.2
4.4
4.5
4.7
0.717
0.593
0.492
0.402
1
1
1
1
1.96
1.96
1.95
1.95
2.3
1.9
1.3
1.7
1
1
3.29
3.28
1.4
0.7
4
4.4
2.99
2.98 3.16
1.6 1.4
0.467
a Fourier transform ranges: 1, k ) 2.0-15 Å-1 (resolution ) 0.12 Å); 3, k ) 2.0-14 Å-1 (resolution ) 0.13 Å); 4, k ) 2.0-14.95 Å-1 (resolution )
0.12 Å); 6, k ) 2.0-14.3 Å-1 (resolution ) 0.13 Å). r is in units of Å; σ2 is in units of 10-3 Å2. All fits are to unfiltered data. b Goodness-of-fit
] .
c The value of n for the 1.64-Å shell was fixed at 0.8 for fits 4-4 and 4-5 in view of the fact
2
1/2
parameter F, defined as [∑k6(ꢀexptl - ꢀcalc)2/∑k6ꢀexptl
that the sample was found to contain 80% 4 by UV/vis spectroscopy.
Table 5. Reactivity toward PPh3 and DHAa
rate found for 4 is a factor of 20 faster than that of [FeIV(O)-
(TMC)(NCCH3)]2+ and comparable to that of 2 is not consistent
with this conclusion. This suggests that our understanding of
what factors control the reactivity of an FeIVdO unit in H-atom
abstraction requires further refinement. It is also possible that
the presence of protons from m-CPBA may affect the rate of
H-atom abstraction. Given this complexity, the DHA oxidation
rate may not be reliable as an indicator of the axial-ligand donor
in an [FeIV(O)(TMC)(X)] complex.
k2 (M-1 s-1
)
PPh3
DHA
2b
0.016b
7.5b
4
12.2 ( 0.3c
5.5 ( 0.3c
3.5 ( 0.1d
[FeIV(O)(TMC)(NCCH3)]2+
0.20b
a Carried out at 0 °C in 1:1 MeOH/CH3CN. b From ref 21c. c OPPh3
was obtained in quantitative yield relative to FeIVdO. d Anthracene was
obtained in 40% yield relative to FeIVdO.
3.1.8. Summary. Our accumulated observations lead us to
conclude that the addition of up to 2 equiv of peracid to 1 results
in the oxidation of the bound thiolate to a sulfinate in 3, without
evidence for an intervening sulfenate species. The addition of
another (third) equivalent of peracid (Scheme 2) results in the
conversion of the formed O-bound iron(II)-sulfinate complex
to the corresponding oxoiron(IV)-sulfinate (4, νFedO ) 831
cm-1, Fe-O ) 1.64 Å) species with properties similar to those
of a family of [FeIV(O)(TMC)(X)] complexes reported
previously.21d The combined spectroscopic evidence indicates
the presence of a weak-field axial donor in 4, most likely the
sulfinate moiety identified by vibrational spectroscopy (νs(SO2)
) 1003 cm-1, νas(SO2) ) 1150 cm-1).
3.2. Elucidating the Mechanism of Sulfur Oxidation. In
metal-thiolate complexes, sulfur oxidation may occur either
as a result of direct reaction between the bound thiolate and
oxidant or as a result of metal-mediated oxidation. Darensbourg
and co-workers have carried out a number of investigations into
sulfur oxidation in nickel(II)-dithiolate complexes,47 in which
both sulfur-bound sulfenate and sulfinates were generated,
depending on the oxidant used.48 Sulfur oxidation was proposed
to result from nucleophilic attack by the bound thiolate on an
electrophilic oxygen atom in dioxygen or H2O2. No eVidence
for metal-based oxidants was found. In attempting to mimic
direct structural evidence for the FedO unit in 4, corroborating
results from vibrational spectroscopy (Vide supra), but not for
a sulfur-derived species bound to the iron(IV) center trans to
the oxo moiety.
3.1.7. Reactivity Studies with 4. The reactivity of 4 toward
triphenylphosphine (PPh3) and 9,10-dihydroanthracene (DHA)
was investigated for comparison with 2 and [FeIV(O)(TMC)-
(NCCH3)]2+ (Table 5). Oxo transfer to triphenylphosphine was
found to be quantitative, while anthracene was obtained in 40%
yield, corresponding to the consumption of 80% of 4. The
obtained second-order rate constant (k2) values show that 4 is
comparable to [FeIV(O)(TMC)(NCCH3)]2+ in oxo-transfer re-
activity and to 2 in H-atom abstraction reactivity.21c Interpreta-
tion of these results within the context of the trends observed
for a series of four [FeIV(O)(TMC)(X)] complexes46 is not
straightforward. Consistent with the spectroscopic data obtained
for 4 described above, the PPh3 oxidation rates suggest that 4
has an axial ligand with a basicity that affords an FedO unit
of electrophilicity comparable to that of [FeIV(O)(TMC)-
(NCCH3)]2+. However, the observation that the DHA oxidation
(46) Hirao, H.; Que, L., Jr.; Nam, W.; Shaik, S. Chem.sEur. J. 2008, 14,
1740–1756.
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S vs Fe Oxidation in an Iron-Thiolate Model
A R T I C L E S
CDO, Goldberg presented evidence for metal-mediated oxy-
genation by O2 of an iron(II)-thiolate complex, yielding an
iron(II)-sulfonate complex.16 In studies on iron and cobalt
NHase mimics, a number of metal-sulfenate, -sulfinate, and
-sulfonate complexes have been synthesized by reaction
between the metal(III)-thiolate complex and various oxidants
(e.g., dioxygen, H2O2, oxiranes).12-18 Grapperhaus observed
1
that, in the reaction between an S ) /2 iron(III)-dithiolate
complex and dioxygen, an iron(III)-disulfonate was formed.15
Figure 9. ESI-MS spectra of 7 (A) and after reaction with 2 equiv of
m-CPBA (B).
5
Conversely, the S ) /2 analogue yielded disulfide and non-
ligated iron products. In one mass spectrometry study, the
reaction of an iron(III)-thiolate complex and dioxygen resulted
in the initial formation of an iron(III)-sulfenate, which over
time was converted to iron(III)-sulfinate.49 The reaction
between a heme iron(III)-thiolate model compound and 1 equiv
of m-CPBA yielded an oxoiron(IV) Compound I mimic.17c The
same reaction carried out with excess m-CPBA yielded the
S-oxidized iron(III)-sulfonate product. Unfortunately, not much
insight was reported as to the mechanism of sulfur oxidation in
any of these iron and cobalt model complexes. Given the dearth
of mechanistic information, we have further investigated the
conversion of 1 to 3 to elucidate whether S-oxidation was purely
an organic transformation or was metal-mediated.
To gain a better understanding of the susceptibility of the
thiolate moiety in 1 and 2 to S-oxidation by nucleophilic attack
of the sulfur on an electrophilic oxygen atom, we investigated
their reactivity toward oxo-atom transfer reagents other than
m-CPBA. As previously reported,19 addition of 6 equiv of H2O2
with no added acid or base to a methanolic solution of 1 at
-40 °C resulted in the formation of 2 in 45% yield in
approximately 10 min, as determined by electronic absorption
and Mo¨ssbauer spectroscopies. Mo¨ssbauer analysis showed that
neither 3 nor 4 was detected in significant amounts in these
samples. In contrast, the same reaction carried out in the
presence of excess benzoic acid (10 equiv PhCO2H, chosen to
mimic m-CBA) resulted in the formation of 4 (instead of 2)
over a period of 2 h (60% yield), as evidenced by the formation
of near-IR electronic absorption features (λmax ) 830 and 990
nm) and the observation of a mass peak (m/z ) 405.2) consistent
with 4. The observed differences in reactivity, in the absence
and in the presence of acid, show that the availability of H+
controls whether 2 or 4 forms upon exposure of 1 to H2O2. This
observation parallels observations made with m-CPBA in the
presence and in the absence of base.
equiv of PhCO2H showed evidence for the formation of 3 and
iron(III)-thiolate byproducts. Furthermore, the self-decay of 2
yielded the same iron(III)-thiolate species, and not the sulfur
oxidized product 3.19 It must also be noted that neither 1 nor 2
reacted with dioxygen. These results demonstrate that the
thiolate ligands of 1 and 2 are unlikely to undergo S-oxidation
by nucleophilic sulfur attack on an electrophilic oxygen atom,
unless an excess of H+ is present.
Further insight into the reaction chemistry between a
metal(II)-TMCS complex and m-CPBA was obtained with the
redox-inactive complex [ZnII(TMCS)]PF6 (7). Exposure of a 1
mM methanolic solution of 7 to 2 equiv of m-CPBA at -20 °C
(mimicking the conversion of 1 to 3) resulted in the formation
of sulfinate complex [ZnII(TMCSO2)]+ within 60 s. ESI-MS
showed the incorporation of two oxygen atoms (Figure 9), while
FTIR showed peaks at 1001 and 1145 cm-1 typical of metal-
bound sulfinate (Supporting Information, Figure S10). The FTIR
results compare favorably with those obtained by IR and rR
for 3 and 4, respectively. Importantly, no S-oxidized products
were observed in the presence of excess base (KOtBu). The
conclusion can therefore be drawn that direct S-oxidation by
m-CPBA can occur in the conversion of 1 to 3.
The reaction of 1 with peracid in the absence of base results
in preferential sulfur oxidation over iron oxidation (Scheme 2),
while excess strong base (KOtBu) is required for the generation
of 2. The strong base may act in one of two ways: (a) by
deprotonation of m-CPBA to enhance binding of its conjugate
base to the vacant site on the iron(II) center in 1 or (b) by
neutralizing H+ formed in the reaction that may promote sulfur
oxidation. That base is not required for the formation of 2 from
less acidic H2O2 suggests that deprotonation of H2O2 is not
necessary, showing that factor (a) does not play a significant
role in directing the formation of 2. To gain a greater
understanding of the mechanism of sulfur oxidation, we
investigated the effect of adding H+ to both 1 and 2.
The introduction of 10 equiv of PhCO2H (pKa ) 4.2) or
trifluoroacetic acid (TFA, pKa ) 0.5) to 1 in methanol at -20
°C did not affect the UV-vis spectrum of 1. The intensity of
the λmax ) 320 nm feature belonging to 1 was maintained over
a period of 20 min in the presence of benzoic acid, and no new
absorption features were observed. On the other hand, addition
of 6 equiv of H+ (to neutralize the 6 equiv of KOtBu present;
PhCO2H, TFA, and pyridinium triflate are all suitable proton
sources) to a solution of 2 at -20 °C resulted in the rapid
decomposition of 2, as indicated by the disappearance of the
CT bands in the visible region characteristic of 2 (λmax ) 460
and 570 nm) and a decrease in intensity of the near-IR features
(Figure 10). Mo¨ssbauer spectroscopy showed the complete loss
of 2 and the appearance of two components: 3 (47%, δ ) 1.01
mm/s, ∆EQ ) 3.77 mm/s) and an iron(IV) species (5, 38%, δ
) 0.16 mm/s, ∆EQ ) 1.15 mm/s, Figure 11). For 5, the isomer
shift is indicative of an [FeIV(O)(TMC)(X)] center,21d and its
We investigated the susceptibility of 1 to oxidation by
iodosylbenzene and oxoiron(IV) complexes to get a greater
understanding of the nucleophilicity of the bound thiolate ligand.
When 1 was exposed at -20 °C to either iodosylbenzene,
[FeIV(O)(TMC)(NCCH3)]2+, [FeIV(O)(N4Py)]2+, or 2, no sulfur
oxidation products were observed by ESI-MS. However, the
reaction between 1 and iodosylbenzene in the presence of 10
(47) (a) Grapperhaus, C. A.; Darensbourg, M. Y. Acc. Chem. Res. 1998,
31, 451–459. (b) Kaasjager, V. E.; Bouwman, E.; Gorter, S.; Reedijk,
J.; Grapperhaus, C. A.; Reibenspies, J. H.; Smee, J. J.; Darensbourg,
M. Y.; Derecskei-Kovacs, A.; Thomson, L. M. Inorg. Chem. 2002,
41, 1837–1844. (c) Chohan, B. S.; Maroney, M. J. Inorg. Chem. 2006,
45, 1906–1908. (d) Maroney, M. J.; Choudhury, S. B.; Bryngelson,
P. A.; Mirza, S. A.; Sherrod, M. J. Inorg. Chem. 1996, 35, 1073–
1076. (e) Liu, T.; Li, B.; Singleton, M. L.; Hall, M. B.; Darensbourg,
M. Y. J. Am. Chem. Soc. 2009, 131, 8296–8307.
(48) Buonomo, R. M.; Font, I.; Maquire, M. J.; Reibenspies, J. H.;
Tuntulani, T.; Darensbourg, M. Y. J. Am. Chem. Soc. 1995, 117, 963–
973.
(49) Noveron, J. C.; Olmstead, M. M.; Mascharak, P. K. J. Am. Chem.
Soc. 2001, 123, 3247–3259.
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J. AM. CHEM. SOC. VOL. 132, NO. 48, 2010 17127

A R T I C L E S
McDonald et al.
Scheme 3. Postulated Mechanism for the Conversion of 1 to 3 in
the Reaction between 1 and m-CPBA without Added Basea
a m-CPBA could react either at the iron site (pathway A) or at the sulfur
site (pathway B) of 1. Evidence for both pathways is provided.
Figure 10. UV-vis spectral changes observed in the reaction of 2 (0.5
mmol/L, blue solid line) with 6 equiv of PhCO2H in MeOH at -20 °C
over a 3-min period, yielding a mixture of 3 and 5 (yellow solid line).
presumably because of the trans influence of the oxo ligand.
Thus, the weaker than expected iron(IV)-thiolate interaction
allows the thiolate to be protonated by PhCO2H and become
displaced from the iron(IV) center by MeOH, resulting in the
species we assign as 5.
In the reaction between 2 and H+ to yield 3 and 5, the loss
of the thiolate ligand from 2 should make 5 a much better
oxygen atom transfer reagent than 2, which has been shown to
be very sluggish in such reactions, because the highly basic
thiolate has been replaced by a less basic ligand (Scheme 3).21c
At the same time, the hypothesized free thiol becomes a target
for S-oxidation. Because we have found no evidence for the
formation of an oxoiron(IV) complex with a sulfenate tail, such
a species must be oxidized more rapidly than 5 to be consistent
with our observations. The product of the second cycle of
oxidation would be 4, which in turn is converted to 3 upon
transfer of its oxo atom to another molecule of 5 (for complete
Scheme 3, see Scheme S11 in Supporting Information). Thus,
in the conversion of 1 to 3, a plausible reaction mechanism is
that m-CPBA reacts with the iron(II) center of 1 to generate 5,
which in turn transfers the oxygen atom to the thiol of a second
molecule of 5 (Scheme 3, pathway A).50 Indeed, a small amount
of 5 was observed to form in the conversion of 1 to 3 (Figures
1, 3C, and 4) (Mo¨ssbauer, 8% yield), thereby implicating 2 as
a likely intermediate. However, direct sulfur oxidation by
m-CPBA of 1, yielding 3, is also a plausible outcome, as
demonstrated by the reaction between 7 and m-CPBA (Scheme
3, pathway B).
Investigations into IPNS using substrate analogues have
shown that the thiol sulfur can be oxidized to a sulfenic acid,
presumably via the putative oxoiron(IV) oxidant formed in the
catalytic cycle.6 The mechanism of conversion of cysteine to
cysteinesulfinic acid by CDO is currently hypothesized to be
effected by either a low-valent iron superoxide moiety or a high-
valent iron-oxo moiety as a basis for the mechanism of sulfur
oxidation.5a Recent X-ray crystallographic work showed evi-
dence for an intermediate iron(II)-persulfenate species, leading
the authors to propose that the catalytic cycle involves no high-
valent iron intermediates.5d However, elsewhere oxoiron(IV)
species have been hypothesized as likely oxidizing moieties in
CDO.5a,c Little is known about the mechanism of S-oxidation
in the post-translational formation of sulfenate and sulfinate
Figure 11. The 4.2-K Mo¨ssbauer spectrum obtained after adding ∼6 equiv
of pyridinium triflate at -40 °C to 1.3 mM 2. 38% of the iron belongs to
complex 5 and 47% to 3; the remainder of the iron belongs to broad and
unresolved FeIII species.
quadrupole splitting is suggestive of an axial donor considerably
less basic than the thiolate ligand in 2 and somewhat more basic
than that of 4.21d We propose that this species is the conjugate
acid of 2, where the thiolate ligand is protonated and could be
displaced by the solvent MeOH or benzoate, formed as a result
of PhCO2H neutralization of KOtBu.
These results show that 2 can be converted to 3, implicating
a metal-based oxidant in sulfur oxidation. The yield of S-
oxidized product 3 (47%) accounts for ∼100% of O-atoms
derived from starting material 2. We believe that 5 (yield 38%)
retains an O-atom from 2, thus giving an overall oxygen atom
stoichiometry of 140%. We suspect that the extra oxygen atoms
are derived from excess peracid present in solution in the
Mo¨ssbauer experiment (1.2 equiv of m-CPBA was added to 1
to generate 2 in 75% yield). Alternatively, dioxygen may play
a role in converting the free thiol of 5 or a transient sulfenate
to sulfinate 3, thus accounting for the higher than expected
oxygen atom yield.
These results show that the iron(II)-thiolate bond in 1 is
stable to acidic conditions. On the other hand, the iron(IV)-
thiolate bond in 2 appears more susceptible to protonation by
PhCO2H, as indicated by the disappearance of the visible
features corresponding to thiolate-to-iron CT transitions (Figure
10). The greater susceptibility of the iron(IV)-thiolate bond to
protonation compared to the iron(II)-thiolate may seem coun-
terintuitive at first glance, because the iron(IV)-thiolate bond
may be expected to be the stronger of the two. However, the
oxo group trans to the thiolate ligand in 2 could exert a labilizing
influence. Indeed, the structural analysis of 125 and 219 shows
that the Fe-S bond increases marginally in length upon going
from 1 to 2 (2.30 vs 2.33 Å, respectively), despite the increase
in iron oxidation state from +2 to +4 as well as a change in
spin-state from S ) 2 to S ) 1. Both of these factors are
expected to shorten the Fe-S bond in 2 but apparently do not,
(50) In contrast, Kodanko has recently shown that the reaction between an
oxoiron(IV) complex (FeIV(O)(N4Py)]2+) and the thiol-containing
tripeptide glutathione yields thiolatoiron(III)-N4Py and disulfide. No
sulfenate, sulfinate, or sulfonate products were observed. Campanali,
A. A.; Kwiecien, T. D.; Hryhorczuk, L.; Kodanko, J. J. Inorg. Chem.
2010, 49, 4759–4761.
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17128 J. AM. CHEM. SOC. VOL. 132, NO. 48, 2010

S vs Fe Oxidation in an Iron-Thiolate Model
A R T I C L E S
Acknowledgment. This publication was made possible by
funding from the U.S. National Institutes of Health (grants GM-
33162 to L.Q. and EB-001475 to E.M., postdoctoral fellowship
GM-087895 to A.M., and predoctoral traineeship GM-08700 to
E.R.F.), NRF of Korea through CRI and WCU (R31-2008-000-
10010-1) programs (W.N.), and the U.S. National Science Founda-
tion (J.A.H., CHE-0615479). The authors thank Dr. Victor G.
Young, Jr., of the University of Minnesota X-Ray Crystallographic
Laboratory for the solution of the X-ray crystal structure of 6. XAS
data were collected on beamline X9B at the National Synchrotron
Radiation Lightsource (NSLS), Brookhaven National Laboratory,
and on beamline 7-3 at the Stanford Synchrotron Radiation Light
Source (SSRL). NSLS is supported by the U.S. Department of
Energy, Office of Science, Office of Basic Energy Sciences, under
Contract No. DE-AC02-98CH10886. NSLS Beamline X9B was
supported by the National Institute for Biomedical Imaging and
Bioengineering under P41-EB-001979. SSRL is a national user
facility operated by Stanford University on behalf of the U.S.
Department of Energy, Office of Basic Energy Sciences. The SSRL
Structural Molecular Biology Program is supported by the Depart-
ment of Energy, Office of Biological and Environmental Research,
and by the National Institutes of Health, National Center for
Research Resources, Biomedical Technology Program. We thank
Drs. Nebojsa Marinkovic (NSLS X9B), Allyson Aranda (SSRL
7-3), and Matthew Latimer (SSRL 7-3) for excellent technical
support of our XAS experiments.
functionalities on active-site cysteine residues in the maturation
of Fe-NHase.8,15 Furthermore, the role of metal-based oxidants
and in particular oxoiron(IV) species in this transformation has
not been investigated to date. Our observations compellingly
show that indeed under the correct conditions an oxoiron(IV)
moiety can reliably generate a sulfinate in high yield.
4. Conclusions
Drastically different reactivities between an iron(II)-thiolate
complex and peracid were observed depending on the avail-
ability of protons. Under highly basic conditions oxoiron(IV)-
thiolate species 2 is formed,19 but under slightly acidic condi-
tions (reported herein) iron(II)-sulfinate complex 3 is obtained
instead. We have investigated the structural and electronic
properties of the formed iron(II)-sulfinate, proving its existence,
and have also shown that it will readily convert to oxoiron(IV)-
sulfinate 4, whose properties have also been thoroughly
investigated. The apparent preferential sulfur versus iron oxida-
tion, depending on the availability of protons, is in fact quite
complex. Investigations into [ZnII(TMCS)]+ have demonstrated
that direct S-oxidation of a metal-bound thiolate can occur,
showing that the conversion of 1 to 3 does not necessarily
require a redox-active metal center. Conversely, we have
observed metal-oxidized intermediates ([FeIV(O)(TMCSH)]) in
the conversion of 1 to 3. Furthermore, we have demonstrated
that oxoiron(IV)-thiolate 2 will readily convert to 3 in the
presence of H+, suggesting that metal-mediated S-oxidation is
also a credible pathway in the conversion of 1 to 3. These results
demonstrate that oxoiron(IV) intermediates are plausible as
S-oxidants in the formation of oxygenated thiolate moieties
found in the chemistry of CDO, NHase, and IPNS.
Supporting Information Available: Iron K-edge XANES and
EXAFS data and fitting analyses for 1, 3, 4, and 6; FT-IR spectra
of 3, 6, and [ZnII(TMCSO2)]+; Mo¨ssbauer spectrum of 6;
extended Scheme 3. This material is available free of charge
via the Internet at http://pubs.acs.org.
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