A high-spin iron(IV)-oxo complex supported by a trigonal nonheme pyrrolide platform

Article abstract of DOI:10.1021/ja207048h

We report the generation and characterization of a new high-spin iron(IV)-oxo complex supported by a trigonal nonheme pyrrolide platform. Oxygen-atom transfer to [(tpa^Mes)Fe^II]^- (tpa^Ar = tris(5-arylpyrrol-2-ylmethyl)amine) in acetonitrile solution affords the Fe(III)-alkoxide product [(tpa^Mes2MesO)Fe ^III]^- resulting from intramolecular C-H oxidation with no observable ferryl intermediates. In contrast, treatment of the phenyl derivative [(tpa^Ph)Fe^II]^- with trimethylamine N-oxide in acetonitrile solution produces the iron(IV)-oxo complex [(tpa ^Ph)Fe^IV(O)]^- that has been characterized by a suite of techniques, including mass spectrometry as well as UV-vis, FTIR, Moessbauer, XAS, and parallel-mode EPR spectroscopies. Mass spectral, FTIR, and optical absorption studies provide signatures for the iron-oxo chromophore, and Moessbauer and XAS measurements establish the presence of an Fe(IV) center. Moreover, the Fe(IV)-oxo species gives parallel-mode EPR features indicative of a high-spin, S = 2 system. Preliminary reactivity studies show that the high-spin ferryl tpa^Ph complex is capable of mediating intermolecular C-H oxidation as well as oxygen-atom transfer chemistry.

Full text of DOI:10.1021/ja207048h

Article
pubs.acs.org/JACS
A High-Spin Iron(IV)−Oxo Complex Supported by a Trigonal
Nonheme Pyrrolide Platform
Julian P. Bigi,†,§ W. Hill Harman,†,§ Benedikt Lassalle-Kaiser,⊥ Damon M. Robles,∥ Troy A. Stich,∥
Junko Yano,⊥ R. David Britt,*,∥ and Christopher J. Chang*,†,‡,§
†Department of Chemistry and ‡The Howard Hughes Medical Institute, University of California Berkeley, Berkeley, California 94720,
United States
§Chemical Sciences Division and ⊥Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720,
United States
∥Department of Chemistry, University of Califoria, Davis, Davis, California 95616, United States
S
* Supporting Information
ABSTRACT: We report the generation and characterization of
a new high-spin iron(IV)−oxo complex supported by a trigonal
-
nonheme pyrrolide platform. Oxygen-atom transfer to [(tpaMes
)
FeII]− (tpaAr = tris(5-arylpyrrol-2-ylmethyl)amine) in acetoni-
trile solution affords the Fe(III)−alkoxide product
[(tpaMes2MesO)FeIII]− resulting from intramolecular C−H
oxidation with no observable ferryl intermediates. In contrast,
treatment of the phenyl derivative [(tpaPh)FeII]− with trimethylamine N-oxide in acetonitrile solution produces the iron(IV)−
oxo complex [(tpaPh)FeIV(O)]− that has been characterized by a suite of techniques, including mass spectrometry as well as UV−
vis, FTIR, Mossbauer, XAS, and parallel-mode EPR spectroscopies. Mass spectral, FTIR, and optical absorption studies provide
̈
signatures for the iron−oxo chromophore, and Mossbauer and XAS measurements establish the presence of an Fe(IV) center.
̈
Moreover, the Fe(IV)−oxo species gives parallel-mode EPR features indicative of a high-spin, S = 2 system. Preliminary reactivity
studies show that the high-spin ferryl tpaPh complex is capable of mediating intermolecular C−H oxidation as well as oxygen-
atom transfer chemistry.
spectrometry, UV−vis, FTIR, Mossbauer, XAS, and
̈
INTRODUCTION
■
parallel-mode EPR measurements provides a coherent
picture for this rare synthetic, S = 2 ferryl system, and
preliminary reactivity studies show that this species is capable
of intermolecular C−H oxidation and oxygen-atom transfer
chemistry.
Iron centers supported by heme and nonheme ligands are
potent oxidants in natural and synthetic systems.1 In particular,
nonheme iron(IV)−oxo intermediates are implicated in a diverse
array of important biological oxidation processes, including hy-
droxylation, desaturation, ring-closing, and halogenation reaction
pathways.2−7 Interestingly, whereas iron(IV)−oxo species pro-
posed as active oxidants in these aforementioned reactions
possess high-spin, S = 2 ground states, the vast majority of
synthetic iron(IV)−oxo complexes reside in intermediate-
spin, S = 1 ground states.8−30 Indeed, synthetic high-
spin iron(IV)−oxo complexes remain rare and are limited to
seminal contributions by Bakac, Que, and Borovik.31−34 As
such, the identification and characterization of new iron−oxo
species, particularly with high-spin ground states, is of
fundamental interest in elucidating underlying principles of
their reactivity.
To meet this goal, we have initiated a program aimed at
studying basic aspects of electronic structure, magnetism, and
reactivity at synthetic iron centers,35−38 with recent efforts focusing
on hybrid ligand architectures that combine features of both heme
and nonheme systems.35,37−43 We now report the generation,
spectroscopic characterization, and reactivity of a new high-
spin iron(IV)−oxo complex supported by a three-fold
symmetric pyrrolide platform. A combination of mass
RESULTS AND DISCUSSION
■
Oxidation of [(tpaMes)Fe]− Provides the Correspond-
ing FeIII−Alkoxide Complex. Previous work from our labo-
ratory described N2O activation and intra- and intermolecular
C−H oxidation reactivity with iron complexes supported by
three-fold symmetic tpaAr ligands (tpaAr = tris(5-arylpyrrol-2-
ylmethyl)amine) and suggested the involvement of a putative
iron(IV)−oxo species.35 We sought to identify and characterize
discrete ferryl intermediates in this platform, and initial experiments
centered on oxygen-atom transfer reactions to the mesityl
derivative [(tpaMes)FeII]− (1). In contrast to the intermolecular
oxidation reactivity observed in ethereal solvents with
relatively weak C−H bonds (THF, DME, etc.) to give the
iron(III)−hydroxide complex [(tpaMes)FeIII(OH)]− (2) after
oxygen transfer and subsequent hydrogen-atom abstraction
Received: July 27, 2011
Published: January 4, 2012
© 2012 American Chemical Society
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Scheme 1. Oxygen-Atom Transfer Reactivity and Formation of an Iron(IV)−Oxo Species Supported by Nonheme tpaR Ligands
chemistry,35 treatment of 1 with trimethylamine N-oxide in
the more oxidatively robust solvent acetonitrile exclusively
affords the iron(III)−alkoxide product [(tpaMes2MesO)FeIII]−
(3) resulting from intramolecular benzylic C−H oxidation of
one of the pendant mesityl arms (Scheme 1 and Figure S1
of Supporting Information [SI]). We identified the iron-
(III)−alkoxide product by mass spectrometry and the
appearance of a new species 5 with a strong absorption band
at 400 nm and a weaker absorption feature centered around
900 nm; the latter peak is reminiscent of the near-IR absorp-
tions that are characteristic of synthetic iron(IV)−oxo com-
plexes (Figure 1).21 Importantly, the conversion of 4 to 5 proceeds
Mossbauer spectrum of a frozen solution of the reaction
̈
mixture at 100 K displayed a single iron(III) product with
δ = 0.32 mm/s and ΔEq = 0.88 mm/s (Figure S2, SI). No
intermediates were observed in the conversion from 1 to 3 at
1
a variety of different temperatures. Moreover, H NMR and
LC/MS analysis of the free tpa ligand isolated after
demetalation confirmed ligand oxidation and desymmetriza-
tion (see SI and Figure S3).
Oxidation of [(tpaMes)FeOH]− Also Provides the FeIII−
Alkoxide Product. Inspired by elegant work from Borovik
showing that a hydrogen-bond-stabilized iron(IV)−
oxo species could be obtained by oxidation of an
iron(III)−hydroxide precursor,34 we attempted to treat
our iron(III)−hydroxide complex 2 with outer-sphere
oxidants, as the cyclic voltammogram of this species shows
an irreversible electrochemical oxidation process at reasonable
potentials (Figure S4 [SI]). However, even at temperatures as
low as −60 °C, reactions of 2 with oxidants like ferrocenium
hexafluorophosphate produced the same iron(III)−alkoxide
3 directly with no observable Fe(IV) intermediates (Figure S5
[SI], Scheme 1).
Figure 1. UV/vis spectra monitoring the oxidation of a 0.40 mM
solution of 4 in acetonitrile with trimethylamine-N-oxide at −40 °C
to produce iron(IV)−oxo complex 5. (Inset) Growth of near-IR
band for an analogous experiment run at a higher concentration of 4
(8.0 mM).
with clean isosbestic behavior. The spectrum of 5 remains
unchanged for hours when kept at −40 °C and at concen-
trations near 0.5 mM. However, more concentrated solutions of
5 (∼10 mM) decompose with a half-life of approximately one
hour, which to date has precluded isolation of solid samples of
5. Nevertheless, identification of 5 as an iron(IV)−oxo complex
comes from electrospray mass spectrometry on a cold ace-
tonitrile solution of this species, where the most prominent
signal in the spectrum has an m/z value of 551.151 that is
consistent with the value expected for the iron(IV)−oxo com-
plex [(tpaPh)Fe(O)]− (predicted m/z = 551.153) (Figures 2
and S6 [SI]). Moreover, within seconds, the most prominent
Oxidation of [(tpaPh)Fe]− Yields the Corresponding
FeIV−Oxo Complex. Undeterred, we turned our attention to
the related phenyl derivative [(tpaPh)Fe]− (4) to pursue high-
valent ferryl species. We reasoned that the aryl C−H bonds on
the pendant phenyl groups of this ligand would be com-
paratively less reactive than their benzylic C−H counterparts
on tpaMes and thus enhance the stability of potential high-valent
iron species prone to decomposition by C−H functionaliza-
tion.44 Indeed, upon oxidation of 4 with trimethylamine N-
oxide in acetonitrile solution at −40 °C, we observe the
signal shifts to an m/z value of 550.147, corresponding to the
-
previously reported iron(III)-phenoxide complex [(tpaPh2PhO
)
FeIII]− (6) (predicted m/z = 550.146) produced from
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Figure 4. Mossbauer spectrum of a 85 mM frozen solution of 5 in
̈
acetonitrile acquired at 4.2 K. A least-squares fit to the data (solid
black line) provided the following parameters: δ = 0.09 mm/s, ΔEq =
0.51 mm/s.
and is in close agreement with other ferryl species in the
literature.21,31,32,34
XAS/EXAFS. The Fe K-edge X-ray absorption spectrum of a
frozen acetonitrile solution of 5 shows an edge energy of 7122.0
eV (at F/I0 = 0.5), which represents a shift of 3.2 eV as
compared to 4 and is consistent with an oxidation of Fe(II) to
Fe(IV) (Figure 5 top). The pre-edge region in 4 possesses two
Figure 2. The electrospray ionization mass spectrum of 5, showing the
experimentally observed (top) and theoretical (bottom) isotopic
distribution pattern for [(tpaPh)FeO]−.
intramolecular aryl C−H oxidation (Figures S7 and S8 [SI],
Scheme 1).
Spectroscopic Characterization of the FeIV−Oxo
Complex: Identifying the Ferryl Chromophore and
Fe(IV) Oxidation State. With the aforementioned result in
hand, we proceeded to characterize this new iron(IV)−oxo
complex using a variety of spectroscopic methods. Our findings
from FTIR, Mossbauer, and XAS/EXAFS experiments are
̈
summarized as follows.
FTIR Spectroscopy. First, in situ react-IR spectroscopy of 5
generated from reaction of 4 with trimethylamine N-oxide in
acetonitrile solution at −35 °C showed a new band at 850 cm−1,
which shifted to 814 cm−1 upon 18O labeling (Figure 3).
Figure 5. (Top) Fe K-edge XANES and pre-edge (inset) of 4 (dotted
trace) and 5 (solid trace). (Bottom) EXAFS Fourier transform and k3-
weighted oscillations (inset) of 5 (solid trace) and the best fit (dotted
trace).
features at 7110.9 and 7112.5, with areas of 19.7 and 2.7 units,
respectively (Figure S9 and Table S1,SI). Such a pre-edge splitt-
ing has been observed previously in the case of iron complexes
in C3v geometries46 and is explained by the noncentrosym-
metric nature of the coordination environment as observed
in 4. Upon oxidation to 5, the pre-edge region shows a strong
main feature at 7112.3 eV, which is best fit with two com-
ponents at 7112.3 and 7114.0 eV, with areas of 28.0 and 4.3
units, respectively (Figure S10 and Table S1, SI). The presence
of two peaks in this pre-edge is in agreement with the observa-
tions made by England et al. on a S = 2 Fe(IV)−oxo complex32
Figure 3. IR spectrum of 5 generated in situ by trimethylamine-N-
oxide oxidation of 4 in acetonitrile at −35 °C (16O: solid line; 18O:
dotted line).
We assign this band to an FeO vibration on the basis of
agreement with the shift expected for a harmonic Fe−O
oscillator model (38 cm−1) and with the values reported for the
related TMG3tren and H3buea ferryl systems.32,34,45
̈
Mossbauer Spectroscopy. The Mossbauer spectrum of a
̈
frozen solution of 5 at 4.2 K showed a single iron-containing
product with parameters consistent with iron(IV) with δ = 0.09
mm/s and ΔEq = 0.51 mm/s (Figure 4). In particular, the near
zero isomer shift is characteristic of an Fe(IV) oxidation state
2
and is theoretically explained by a splitting of the α and β dz
orbitals.47
The first shell of the EXAFS Fourier transform of 4 was fit with
four scatterers at 2.03 Å from the iron(II) center (Table S2, SI),
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corresponding to an average of the equatorial and axial N ligands.
The EXAFS of 5 could be fit with similar parameters, using four
paths at 1.99 Å. However, the fit quality was significantly im-
proved (as seen from the decrease in R factor and reduced χ2
values) by adding a Fe−O path at 1.62 Å (Figure 5 bottom and
Table S1, SI). The contribution of this Fe−O vector is not seen
as a separated peak in the Fourier transform due to its interac-
tion with the four scatterers at 1.99 Å. The 1.62 Å scatterer
observed in 5 is attributed to the oxygen of a newly formed
Fe−O bond, the distance being in line with that of previously
reported S = 2 Fe(IV)−oxo compounds.32−34
Parallel-Mode EPR Spectroscopy Reveals a High-Spin
S = 2 FeIV−Oxo Complex. With information on the iron−
oxo chromophore and iron(IV) oxidation state in hand, we
proceeded to examine the magnetic properties of this new ferryl
species by EPR. Parallel-mode X-band EPR spectra of 5
acquired below 8 K possess almost no intensity (Figure 6).
Figure 7. Parallel-mode X-band (9.38 GHz) EPR spectrum of 5
(black) and simulation (red) in 1:1 acetonitrile/toluene. Spectrometer
conditions: temperature 40 K, incident power 2.02 mW, modulation
amplitude 0.8 mT. Simulation parameters: D = +4.32 cm−1, E/D =
0.098, σE/D = 0.02, and a line broadening of 8 mT.
behavior reflects a distribution in the precise electronic-structure
description of 5 that likely results from increased disorder of the
phenyl groups on the tpa ligand relative to the urealato groups of
[H3buea]3‑ which can hydrogen bond to the ferryl-oxo.
To further confirm the electronic structure, spectra were
calculated for an intermediate, S = 1, spin system. However, no
single parameter set was found that correctly modeled both the
line shape and field position of the observed resonance signal.
Furthermore, to the best of our knowledge, there are no examples
in the literature of 3d4 complexes with trigonal bipyramidal ligand
field symmetry that have been characterized as S = 1 spin systems.
A dramatic distortion in the trigonal ligand field would be required
to produce this state, one that is not supported by the EXAFS data
(vide supra). With the S = 1 formulation of 5 ruled out on this
basis, and in concert with the combined data from a suite of
other spectroscopic measurements we assign 5 as being a high-
spin, S = 2 iron(IV)−oxo complex.
Preliminary Reactivity Studies Establish that the tpa-
Supported Ferryl Can Participate in C−H Abstraction
and Oxygen-Atom Transfer Chemistry. Finally, prelimi-
nary reactivity studies of high-spin ferryl 5 are consistent with
the presence of a reactive yet sterically shielded iron−oxo unit,
which is expected for this complex bearing a bulky ancillary
ligand. For example, 5 fails to react with oxygen-atom acceptors
ranging from cyclooctene to thioanisole to triphenylphosphine.
In contrast, 5 reacts rapidly upon mixing with the smaller
phosphine PMe2Ph to produce the phosphine oxide in 89%
yield (Figure S14, SI). In addition, a 3.0 mM acetonitrile
solution of 5 reacts rapidly with 1,4-cyclohexadiene (CHD) to
produce benzene (90% yield) with a second-order rate constant
of 1.4 M−1 s−1 at −30 °C, showing that ferryl 5 is capable of
intermolecular hydrogen-atom transfer (HAT) chemistry
(Figures 8 and S15 [SI]). The steric demands on 5 preclude
Figure 6. Temperature dependence of parallel-mode EPR spectrum
of 5 from 3 to 90 K (top). Relative intensity of g = 8.50 feature versus 1/T
for experimental (○) and calculated (+) spectra (bottom). Best fit analysis
provides a value for the axial zero-field splitting paramater, D, of 4.3 cm−1.
As the temperature increased, however, a broad derivative-shaped
feature centered at g = 8.5 grows in, which is distinct from
spectral features corresponding to either 4 or 6 (cf. Figures S12,
S13 [SI] and 6 and 7). Such behavior is consistent with 5 being
a S = 2 system with a positive axial zero-field splitting con-
stant D. The temperature-dependence of the intensity of this
derivative feature is best fit using a value of D = +4.3 cm−1.
The field position of this resonance and the asymmetry of the
corresponding line shape are diagnostic of the central value and
distribution of the rhombic zero-field splitting term E. All EPR
spectral features and their corresponding temperature dependence
are well-simulated using a S = 2 spin system with g = 2,D =
+4.3 cm−1, E/D = 0.098 and σE/D = 0.02 (Figures 6 and 7). This
value of D is very similar to that found for other high-spin Fe(IV)
oxo species with trigonal bipyramidal ligand sets (D = 4.0 cm−1 for
H3buea;34 5.0 cm−1 for TMG3tren;32 and 10.0 cm−1 TauD-J).48
The spectral fit requires a rather large line width (8 mT) and
distribution in E (σE/D, one standard deviation in the E/D ratio)
compared to those determined for Fe(IV)O[H3buea]−.34 This
Figure 8. Plot of kobs/2 vs [CHD] for the reaction of 5 with CHD at
−30 °C. The second-order rate constant is obtained from the slope to
be 1.4 M−1 s−1.
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spectrometry experiments were performed on thawing acetonitrile
solutions (vide infra). ESI MS ([5-Na]−): m/z calcd for C33H27FeN4O
551.153, found at 551.151 (Figure 2).
reactivity with other hydrogen-atom donors such as 9,10-
dihydroanthracene (DHA) or xanthene despite their weaker
C−H bond strengths compared to those of CHD. As such, current
efforts are aimed at reducing the steric crowding at the iron−oxo
unit with the hope of increasing reactivity with external substrates.
General Physical Methods. Mass spectrometry measurements
were performed on either an LTQ Orbitrap (Thermo Scientific,
West Palm Beach, FL) or Waters Q-TOF Premier (Milford, MA) spec-
trometer at the QB3/Chemistry Mass Spectrometry Facility at UC
Berkeley. Elemental analyses were performed on a Perkin-Elmer 2400
Series II combustion analyzer (Waltham, MA) in the Microanalytical
Laboratory in the College of Chemistry, University of California,
Berkeley, California. Cyclic voltammetry experiments were conducted
on a BASi Epsilon potentiostat (West Lafayette, IN) using a glassy
carbon disk working electrode, a platinum wire auxiliary, and a plati-
num wire as a floating reference. Potentials were referenced using
either ferrocene or cobaltocene as internal standards. In situ IR spectra
were measured on a Mettler-Toledo ReactIR iC10 instrument (Columbus,
OH). 1H NMR spectra were recorded on Bruker spectrometers
operating at 300 or 400 MHz as noted. Chemical shifts are reported in
ppm and referenced to residual protiated solvent, and coupling
constants are reported in hertz. UV−vis spectra were acquired on
a Varian Cary 50 BIO UV−visible spectrophotometer (Agilent
Technologies, Santa Clara, CA) with a Unisoko cryostat attachment
for temperature control (Unisoku Co, Osaka, Japan).
CONCLUDING REMARKS
■
To close, we have presented the generation of a new high-spin
iron(IV)−oxo complex, its characterization by a suite of spec-
troscopic methods, and its ability to participate in intra- and
intermolecular C−H oxidation reactions as well as oxygen-atom
transfer to a sterically undemanding phosphine. In particular,
Mossbauer and XAS measurements establish the presence of an
̈
Fe(IV) center for [(tpaPh)Fe(O)]−, FTIR and UV−vis studies
identify the Fe−oxo chromophore, and parallel-mode EPR provides
signatures indicative of a high-spin, S = 2 system. Taken together,
this system adds to the small but growing number of high-spin
ferryl complexes in nonheme environments. Ongoing work is
geared toward further reactivity studies of high-spin ferryl species in
this general manifold, and related group-transfer processes on iron
and other first-row transition metals are also under investigation.
Mass Spectrometric Detection of 5. The thermally unstable
iron(IV)−oxo complex 5 was analyzed using electrospray ionization in
negative V mode on a Waters Q-TOF Premier spectrometer. Samples
of 5 were generated at −35 °C as ca. 1 mM acetonitrile solutions,
frozen in liquid nitrogen, and introduced into the mass spectrometer
as a thawing solution using a syringe pump. ESI source parameters
were adjusted as follows: capillary voltage, 0.8 kV; sampling cone voltage,
19 V; source temperature, 60 °C; desolvation temperature, 40 °C.
EXPERIMENTAL SECTION
■
General Synthetic Details. Unless otherwise noted, all synthetic
manipulations were performed under an inert atmosphere of
dinitrogen in a Vacuum Atmospheres glovebox or on a vacuum line
using standard Schlenk technique. Solvents were dried on a Vacuum
Atmospheres Solvent purification system and stored over 3 Å molec-
ular sieves. Acetonitrile was further dried by filtering through a plug of
basic alumina immediately prior to use. Molecular sieves, alumina, and
Celite were activated by heating at 200 °C under dynamic vacuum for
at least 24 h. Potassium hydride was purchased as a suspension in
mineral oil, washed with pentane and used as a dry solid in the
glovebox. 1,4-Cyclohexadiene was degassed with three freeze−pump−
thaw cycles and dried over 3 Å molecular sieves. 18O-labeled trimethyl-
̈
Mossbauer Spectroscopy. Mossbauer spectra were recorded in
̈
constant acceleration mode as frozen acetonitrile solutions (∼80−100 mM)
between room temperature and 4.2 K in a Janis Research Co. cryostat
(Willmington, MA) and analyzed using the WMOSS software package
(See Co, Medina, MN). Isomer shifts are reported relative to α-iron
(27 μm foil) at room temperature.
X-ray Absorption Spectroscopy. X-ray data were collected at
the Stanford Synchrotron Radiation Lightsource (SSRL) on beamline
7-3 at an electron energy of 3.0 GeV with an average current of 300 mA.
The radiation was monochromatized by a Si(220) double-crystal
monochromator. The intensity of the incident X-ray was monitored by
an N2-filled ion chamber (I0) in front of the sample. Solution samples
were placed in 60-μL plastic sample holders, frozen with liquid
nitrogen and collected at 20 K using a helium-cooled cryostat. Data
were collected as fluorescence excitation spectra with a Ge 30 element
detector (Canberra). Energy was calibrated by the first peak maximum
of the first derivative of an iron foil (7111.20 eV). The standard was
placed between two N2-filled ionization chambers (I1 and I2) after the
sample. The integrity of the sample upon measurement was assured by
the XANES energy shift, and we did not observe any edge shift during
the several scans under the current experimental condition.
XAS/EXAFS Data Reduction and Analysis. Pre-edge peaks of 4
and 5 were fit using the EDG_FIT program from the EXAFSPAK
suite. (Drs. Graham George and Ingrid Pickering, SSRL) The param-
eters obtained from the fits are gathered in Table S1 (SI).
Data reduction of the EXAFS spectra was performed using EXAFSPAK.
Pre-edge and post-edge backgrounds were subtracted from the XAS
spectra, and the results were normalized with respect to edge height.
Background removal in k-space was achieved through a five-domain
cubic spline. Curve fitting was performed with Artemis and IFEFFIT
software using ab initio-calculated phases and amplitudes from the
program FEFF 8.2.51,52 These ab initio phases and amplitudes were
used in the EXAFS equation:
amine-N-oxide was prepared according to a literature procedure49
using 18O-labeled hydrogen peroxide (90%
O
purchased from
18
2)
Cambridge Isotope Laboratories (Andover, MA). All other reagents
were purchased from Sigma-Aldrich and used as received. All glassware
was dried by storage in an oven at 170 °C for at least 12 h before use.
The iron complexes [(tpaMes)Fe][K(dme)2] (1), [(tpaMes)FeOH]
[K(dme)] (2) and [(tpaPh)Fe][Na(thf)] (4) were prepared according
to literature procedures.35
Synthesis of [(tpa2MesMesO)Fe]K (3). An acetonitrile solution of
trimethylamine-N-oxide (0.018 g, 0.24 mmol) was added to an ace-
tonitrile solution of [(tpaMes)Fe][K(dme)2] (1) (0.047 g, 0.054 mmol)
and stirred for 3 h. Over the course of 30 min the colorless solution
turned dark brown. The solvent was then removed in vacuo, and the
resulting brown solid residue was recrystallized by layering a con-
centrated THF solution of the product under ether to provide clumps
of brown crystals of 3. ESI MS ([3-K]−): m/z calcd for C42H44FeN4O
676.2870, found at 676.2849. The oxidized ligand was separated
from the metal and isolated by filtering an acetonitrile solution of 3
through a plug of silica and removing the solvent in vacuo. LC/MS
([H3tpa2MesMesO]−): m/z calcd for C42H47N4O 623.38, found at 623.4.
1H NMR spectrum (300 MHz, CD3CN): δ 9.66 (s, 1H), 9.37 (s, 2H),
7.06 (s, 1H), 6.98 (s, 1H), 6.86 (s, 4H), 6.05 (s, 3H), 5.90 (s, 1H),
5.79 (s, 2H), 4.23 (s, 2H), 3.54 (s, 6H), 2.28 (s, 3H), 2.23 (s, 6H),
2.10 (s, 3H), 2.01 (s, 12H) (Figure S3, SI).
Generation of [(tpaPh)FeO]Na (5). An acetonitrile solution of
trimethylamine-N-oxide (4 equiv) was added to an acetonitrile solu-
tion of [(tpaPh)Fe][Na(thf)] (4) at −40 °C to produce a dark-red/
brown solution which could be monitored by UV/vis for the charac-
teristic near-IR band near 900 nm (∼1−10 mM). The solutions were
N
2 2
−2σ k
j
2
0
j
χ(k) = S
f
(π, k, R ) e
sin(2kR + φ (k))
j
ij
∑
j
eff
2
j
kR
j
j
immediately frozen at 77K for Mossbauer and XAS experiments
̈
(∼70−100 mM), and EPR samples were prepared similarly using a 1:1
The neighboring atoms to the central atom(s) are divided into j
shells, with all atoms with the same atomic number and distance from
mixture of toluene and acetonitrile as solvent (∼1−10 mM). Mass
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the central atom grouped into a single shell. Within each shell, the
coordination number Nj denotes the number of neighboring atoms
to give the total spectral intensity. This is then compared to the ex-
perimentally determined spectral intensity. The resonant field position
of the derivative feature allows for the determination of the central
value in the distribution of E/D. All spectral features and their corres-
ponding temperature dependence are well simulated using a S = 2 spin
system with g = 2, D = +4.3( 0.1) cm−1, E/D = 0.098 and σE/D = 0.02
The 40 K perpendicular-mode spectrum of this sample, shows an
intense derivative-shaped feature at g = 4.3 that is assigned to the
Fe(III)-phenoxy product (Figure S13, SI). A small portion of this
signal bleeds through to the parallel-mode spectrum due to small
misalignments of the B0 relative to B1. Notably, the parallel-mode EPR
signals described above are distinct from those of the Fe(II) starting
complex (S = 2), which shows a strong negative feature at g = 9.50
(Figure S12, SI).
Phosphine-Oxide Yield from the Reaction of 5 with
Me2PPh. The reaction of 5 with an excess of Me2PPh was performed
in an NMR tube and the yield of Me2P(O)Ph calculated by integration
of the 31P NMR resonances relative to PPh3 as an internal standard.
See Figure S14 in SI for the spectrum.
Benzene Yield from the Reaction of 5 with 1,4-Cyclo-
hexadiene (CHD). The reaction of 5 with an excess of CHD was
performed in an NMR tube and the yield of benzene calculated by
integration of the 1H NMR resonances relative to 1,2,4,5-tetra-
methylbenzene as an internal standard. See Figure S15 in SI for the
spectrum.
in shell j at a distance of Rj from the central atom. feff (π,k,Rj) is the
j
ab initio amplitude function for shell j, and the Debye−Waller term
2
j
2
e−2σ k accounts for damping due to static and thermal disorder in
absorber−backscatterer distances. The mean free path term e−2R /λ (k)
reflects losses due to inelastic scattering, where λj(k) is the electron
mean free path. The oscillations in the EXAFS spectrum are reflected
in the sinusoidal term, sin(2kRj + fij(k)) where fij(k) is the ab initio
phase function for shell j. S02 is an amplitude reduction factor due to
shake-up/shake-off processes at the central atom(s). The EXAFS
equation was used to fit the experimental data using N, R, and the
EXAFS Debye−Waller factor (σ2) as variable parameters. For the
energy (eV) to wave vector (k, Å−1) axis conversion, E0 was defined as
7120 eV and the S02 value was fixed to 0.78. EXAFS curve-fitting
procedures and the estimation of the uncertainty in the parameters
from the fits are described in detail as follows.
j
j
EXAFS Curve Fitting Procedure. The best fit parameters for the
Fe EXAFS data are summarized in Table S2, SI. As a goodness-of-fit
index, we used the R-factor (the absolute difference between theory
and data), which is defined as the sum of the squares of the differences
between each experimental point and the fit normalized to the sum of
the squares of the experimental points.53 In all fits unless specified, N
values were set, while R and σ2 values were left floating.
We first carried out a fit for 4, using the parameters obtained from
the crystal structure. The EXAFS data are in good agreement with the
XRD values. The Fe−N shell at 2.04 Å reflects an average of three
equatorial (2.00 Å) and one axial (2.17 Å) N atoms. The Fe−C1 shell
at 2.91 Å reflects an average of the Fe to methylene carbon (2.92 Å)
and Fe to pyrrolide proximal carbon (2.85 Å) distances. The Fe−C2
shell at 3.20 Å reflects a Fe to pyrrolide distal carbon (3.29 Å) pathway
with multiple scattering. The Fe−C3 shell at 4.26 Å corresponds to the
distance from Fe to the pyrrolide carbon 4.23 Å away from the Fe
center. The 4.26 Å feature arising from the distal pyrrolide ring was
absent in the EXAFS spectrum of the S = 2 Fe(IV)−O compound
reported by England et al.32
For 5, the data was at first fit using the same parameters as 4
(Fit #1). A slight contraction of the Fe−N shell to 1.99 Å and the
Fe−C2 shell to 3.02 Å is observed, which is consistent with an oxida-
tion of the iron center. However, by introducing a short Fe−O shell
at 1.62 Å, the fit quality improved significantly, as is shown by the
decrease of the R-factor and reduced χ2 values (Fit #2). The Fe−O
vector does not appear as a separate peak in the Fourier transform due
to its interaction with the Fe−N shell.
Kinetics Studies of Reaction of 5 with 1,4-Cyclohexadiene
(CHD). A 3.0 mM acetonitrile solution of 4 was prepared in a cuvette
in an inert atmosphere glovebox and cooled to −30 °C in the UV/vis
spectrophotometer cryostat. An acetonitrile solution of trimethyl-
amine-N-oxide was then added via syringe (four equivalents in ca.
0.1 mL) and the formation of 5 was monitored at 900 nm. Once the
absorbance of 5 at 900 nm reached a steady value, a precooled (−30 °C)
acetonitrile solution of 10−100 equiv of CHD was added via syringe,
and the decay of 5 was monitored at 900 nm holding the temperature
at −30 °C. The resulting decay curves were fit to a single exponential,
allowing extraction of observed rate constants. The observed rate
constants were then divided by two to account for the stoichiometry of
the reaction and plotted against the concentration of CHD; the slope
of the resulting line is the second-order rate constant (Figure 8).
ASSOCIATED CONTENT
■
S
* Supporting Information
Supplementary figures. This material is available free of charge
via the Internet at http://pubs.acs.org.
EPR Spectroscopy. All continuous-wave parallel-mode (B0 || B1)
X-band (9.38 GHz) EPR spectroscopic characterization was
performed at the CalEPR facility at UC Davis using a Bruker ECS
106 spectrometer (Billerica, MA) equipped with an Oxford Instru-
ments ESR900 liquid helium cryostat and a Bruker TE102/TE012 dual-
mode cavity (ER4116DM). Temperature control was achieved using
an Oxford ITC 403 temperature controller. Spectra were collected
under slow passage and nonsaturating conditions. For all presented
data, a corresponding spectrum of the solvent (1:1 acetonitrile/toluene)
was subtracted to remove signals from baseline artifacts and paramagnetic
contaminants present in the cavity. All spectral simulations were carried
out using the EasySpin50 toolbox for MatLab R2010a (MathWorks, Inc.,
Natwick, MA).
Parallel-Mode EPR Experiments. At 3 K there is essentially no
spectral intensity. Signals grow in with increasing temperature that are
indicative of a high-spin (S = 2) species with a positive axial zero-field
splitting constant D (Figure 6). At 40 K, a broad derivative-shaped
feature is present at g = 8.5 and is assigned to the transition between
the levels of the mS = 2 doublet (Figure 7). A best-fit analysis of the
temperature dependence of the intensity of this derivative feature
allows for the determination of the magnitude of D. Predicted
intensities are computed in the following manner. The energy levels
for the system are calculated and then populated according to
Boltzmann statistics for a particular temperature. The predicted spectrum
is then computed as a function of these populations and doubly integrated
AUTHOR INFORMATION
Corresponding Author
chrischang@berkeley.edu; rdbritt@ucdavis.edu
■
ACKNOWLEDGMENTS
■
This research was supported by DOE/LBNL (403801) to
C.J.C., DOE (DE-AC02-05CH11231) to J.Y., and DOE (DE-
FG02-10ER16150) to R.D.B. C.J.C. is an investigator with the
Howard Hughes Medical Institute. J.P.B. thanks the National
Science Foundation for a graduate fellowship. D.M.R. thanks
the Alfred P. Sloan Foundation for support. We also thank
Rupal Gupta (Carnegie Mellon University) and Stefan Stoll
(University of California-Davis) for helpful discussions.
Synchrotron facilities were provided by the Stanford Synchro-
tron Radiation Lightsource (SSRL) operated by DOE OBES.
The SSRL Biomedical Technology program is supported by
NIH, the National Center for Research Resources, and the
DOE Office of Biological and Environmental Research.
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