A high-valent iron-oxo corrolazine activates C-H bonds via hydrogen-atom transfer

Article abstract of DOI:10.1021/ja3018658

Oxidation of the Fe^III complex (TBP₈Cz)Fe ^III [TBP₈Cz = octakis(4-tert-butylphenyl)corrolazinate] with O-atom transfer oxidants under a variety of conditions gives the reactive high-valent Fe(O) complex (TBP₈Cz^+?)Fe ^IV(O) (2). The solution state structure of 2 was characterized by XAS [d(Fe-O) = 1.64 A]. This complex is competent to oxidize a range of C-H substrates. Product analyses and kinetic data show that these reactions occur via rate-determining hydrogen-atom transfer (HAT), with a linear correlation for log k versus BDE(C-H), and the following activation parameters for xanthene (Xn) substrate: ΔH^? = 12.7 ± 0.8 kcal mol ^-1, ΔS^? = -9 ± 3 cal K^-1 mol^-1, and KIE = 5.7. Rebound hydroxylation versus radical dimerization for Xn is favored by lowering the reaction temperature. These findings provide insights into the factors that control the intrinsic reactivity of Compound I heme analogues.

Full text of DOI:10.1021/ja3018658

Article
pubs.acs.org/JACS
A High-Valent Iron−Oxo Corrolazine Activates C−H Bonds via
Hydrogen-Atom Transfer
Kevin Cho,† Pannee Leeladee,† Amanda J. McGown,† Serena DeBeer,*,‡ and David P. Goldberg*,†
†Department of Chemistry, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, United States
‡Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, United States, and
Max-Planck-Institut fur Bioanorganische Chemie, Stiftstrasse 34-36, D-45470 Mulheim an der Ruhr, Germany
̈
̈
S
* Supporting Information
ABSTRACT: Oxidation of the FeIII complex (TBP8Cz)FeIII
[TBP8Cz = octakis(4-tert-butylphenyl)corrolazinate] with O-
atom transfer oxidants under a variety of conditions gives the
reactive high-valent Fe(O) complex (TBP8Cz+•)FeIV(O) (2).
The solution state structure of 2 was characterized by XAS
[d(Fe−O) = 1.64 Å]. This complex is competent to oxidize a
range of C−H substrates. Product analyses and kinetic data show
that these reactions occur via rate-determining hydrogen-atom
transfer (HAT), with a linear correlation for log k versus
BDE(C−H), and the following activation parameters for
xanthene (Xn) substrate: ΔH⧧ = 12.7 0.8 kcal mol−1, ΔS⧧ = −9 3 cal K−1 mol−1, and KIE = 5.7. Rebound hydroxylation
versus radical dimerization for Xn is favored by lowering the reaction temperature. These findings provide insights into the
factors that control the intrinsic reactivity of Compound I heme analogues.
catalysis is presumed to be a high-valent iron−oxo corrole,
although such a species was not characterized. Hayashi and co-
workers have been successful in reconstituting the apo forms of
horseradish peroxidase (HRP) and myoglobin (Mb) with an
iron corrole.5g The iron corrole bound in the proteins was
catalytically active toward the oxidation of 2-methoxyphenol
with hydrogen peroxide as the external oxidant. It is expected
that the iron corrole activates hydrogen peroxide to form an
HRP or Mb Cpd-I intermediate before reacting with the
substrate. However, there has been no characterization of the
reactive intermediate in the corrole-reconstituted metallopro-
teins. Our group has found that (TBP8Cz)FeIII is also a good
catalyst for oxygen-atom transfer and hydroxylation of
cyclohexene in organic solution, as well as in the form of
nanoparticles [(TBP8Cz)FeIII-NP] in an aqueous environment
with pentafluoroiodosobenzene (PFIB) as a sacrificial oxi-
dant.4d,h In the former case, there was spectroscopic evidence
that indicated a high-valent iron−oxo corrolazine,
(TBP8Cz+•)FeIV(O), is the active intermediate for the
oxidation of olefins. For the latter case involving nanoparticles,
the catalytic reactivity of (TBP8Cz)FeIII nanoparticles in an
aqueous environment was different compared to that of
(TBP8Cz)FeIII in an organic solution. The catalytic oxidation
of cyclohexene with PFIB and (TBP8Cz)FeIII-NP favored the
formation of the hydroxylation products over the epoxidation
product,4h as opposed to the product distribution for the
molecular, solution state study. The mechanism of catalytic
INTRODUCTION
■
High-valent iron−oxo complexes have been implicated as
reactive intermediates in a range of biological transformations
and in synthetic catalytic processes. In particular, FeIV−oxo
porphyrin π-cation radical species, designated as Compound I
(Cpd-I) in heme proteins (e.g., cytochrome P450), are
proposed to mediate many key oxidative processes.1 Synthetic
FeIV(O)(porph+•) complexes are difficult to generate and study
because of their instability,2 but much effort has gone into
preparing these and related high-valent metal−oxo complexes
because of the potential they offer in helping to define patterns
of reactivity and in testing mechanistic hypotheses.3 Our group
has been involved in developing ring-contracted porphyrins
known as corrolazines, which are related to the more widely
studied corroles. Corrolazines and corroles are designed to
stabilize high oxidation states. Although a number of high-
valent metal−oxo corrolazines4 and related corroles5 have been
described, only one example of a metastable mononuclear,
high-valent Fe(O) complex has been reported thus far,
prepared by our group with an octaaryl-substituted corrola-
zine.4d,6
A few reports have indicated that iron corroles and
corrolazines can serve as potent oxidation catalysts, which
may utilize one or more high-valent metal−oxo intermediates.
The first study of catalysis by a high-valent iron corrole,
(tpfc)FeIVCl [tpfc = meso-tris(pentafluorophenyl)corrolate],
was reported by Gross and co-workers.5i In this study,
(tpfc)FeIVCl was shown to be active for catalytic hydroxylation
of ethyl benzene and epoxidation of styrene with iodosylben-
zene as the sacrificial oxidant. The reactive intermediate of this
Received: December 14, 2011
Published: April 10, 2012
© 2012 American Chemical Society
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Article
element array detector). XAS data were measured to k = 11 Å−1.
Samples were monitored for photoreduction throughout the course of
data collection. Only those scans, which showed no evidence of
photoreduction, were used in the final average.
oxidations with (TBP8Cz)FeIII-NP may also utilize the same
high-valent iron−oxo intermediate as the molecular catalyst,
but the nanoparticle architecture and/or the change in solvent
(aqueous versus organic) may be responsible for the observed
differences in reactivity.
The data were calibrated and averaged using EXAFSPAK.8 Pre-edge
subtraction and splining were conducted using PYSPLINE.9 A three-
region cubic spline of order 2, 3, 3 was used to model the smooth
background above the edge. Normalization of the data was achieved by
subtracting the spline and normalizing the postedge region to 1. The
resultant EXAFS was k3-weighted to enhance the impact of high-k
data.
Recently, we have been interested in exploiting the stability
of the high-valent iron−oxo corrolazine to examine C−H
activation by (TBP8Cz+•)FeIV(O), a reaction relevant to the
proposed mechanism for Cpd-I in heme oxygenases. Herein we
describe new, efficient methods of generating this complex for
spectroscopic and reactivity studies. These efforts have allowed
for the first structural characterization of an Fe(O) corrolazine
or corrole by X-ray absorption spectroscopy (XAS). We also
provide conclusive evidence for the first time that an Fe(O)
corrole-type complex is capable of C−H bond activation, and
hydrogen-atom abstraction followed by rebound (or a second
H-atom transfer) is the likely mechanism for these reactions,
mimicking the proposed reaction course for heme-based
oxidations.
Theoretical EXAFS signals c(k) were calculated using FEFF
(version 7.0)10 and fit to the data using EXAFSPAK.8 The
nonstructural parameter E0 was also allowed to vary but was restricted
to a common value for every component in a given fit. The structural
parameters that were varied during the refinements were the bond
distance (R) and the bond variance (s2). s2 is related to the Debye−
Waller factor, which is a measure of thermal vibration, and to the static
disorder of the absorbers and scatterers. Coordination numbers were
systematically varied in the course of the analysis, but they were not
allowed to vary within a given fit.
Formation of 2 at −78 °C for XAS Spectroscopy. In a one
dram vial, 1 was dissolved in 150 μL of toluene (13.3 mM) and cooled
to −78 °C. To this solution was added PFIB (50 μL, 0.4 M stock
solution in CH3OH, 10 equiv) to initiate the reaction. The reaction
mixture was kept at −78 °C for ∼1 min, upon which a color change
from green to dark brown was observed. The sample was then
transferred via a precooled pipet to a precooled XAS sample cell and
carefully frozen in liquid nitrogen.
Synthesis of 9,9′-Bixanthene. A solution of 1 (0.53 mM) and
xanthene (0.53 M) was dissolved in 20 mL of a 1:1 (v/v) CH2Cl2/
CH3OH mixture and stirred at room temperature. To this solution was
added mCPBA in 20 mg increments up to a total of 80 mg. A transient
brown species was observed after each addition. A final addition of 100
mg of mCPBA was then made, causing the green color of 1 to fade to
yellow. The solvent was removed in vacuo. The crude reaction mixture
was purified by column chromatography [silica gel, 2:1 (v/v) hexane/
CH2Cl2 mixture], yielding 9,9′-bixanthyl as a white solid (11.8 mg,
3%): 1H NMR (CD2Cl2) δ 7.21 (t, J = 7.60 Hz, 4H), 6.95 (t, J = 7.36
Hz, 4H), 6.83 (d, J = 8.16 Hz, 4H), 6.70 (d, J = 7.52 Hz, 4H), 4.26 (s,
2H).11
EXPERIMENTAL DETAILS
■
Materials. All reactions were performed using dry solvents and
standard Schlenk techniques. The iron(III) corrolazine (TBP8Cz)FeIII
(1) and pentafluoroiodosylbenzene were synthesized according to
previously published methods.4d Dichloromethane, diethyl ether,
toluene, and methanol were purified via a Pure-Solv solvent
purification system from Innovative Technologies, Inc. meta-
Chloroperbenzoic acid (mCPBA) was purchased from Sigma-Aldrich,
washed with phosphate buffer, and then recrystallized from a CH2Cl2/
diethyl ether mixture. Xanthene, 9,10-dihydroanthracene (DHA),
triphenylmethane (TPM), and ferrocene (Cp2Fe) were purchased
from commercial sources and recrystallized from ethanol. Decamethyl
ferrocene (Cp*2Fe) was purchased from Sigma-Aldrich and recrystal-
lized from cyclohexane. 9,10-d2-Xanthene was synthesized according to
a literature procedure.3b 9,9′-Bixanthene was synthesized as described
below. 9-Ethyl-9,10-dihydro-10-methylacridine (AcrHEt) was a gift
from S. Fukuzumi.7 1,4-Cyclohexadiene (CHD), xanthydrol, eicosane,
and anthracene were purchased from commercial sources and used as
received.
General Instrumentation. UV−vis spectroscopy was performed
on a Varian Cary 50 Bio spectrophotometer, and low-temperature data
were collected by using a fiber optic coupler and a Hellma 2 mm
quartz immersion probe. Electron paramagnetic resonance (EPR)
spectra were recorded on a Bruker EMX EPR spectrometer controlled
with a Bruker ER 041 X G microwave bridge at 15 K and equipped
with a continuous-flow liquid helium cryostat (ESR900) coupled to a
TC503 temperature controller made by Oxford Instruments, Inc. Gas
chromatography (GC) was performed on an Agilent 6890N gas
chromatograph fitted with an HP-5 (5% phenyl)-methylpolysiloxane
capillary column (30 m × 0.32 mm × 0.25 mm) and equipped with a
flame-ionization detector. GC-FID response factors for xanthydrol,
xanthone, bixanthene, and anthracene were prepared versus eicosane
as the internal standard. Temperatures for kinetics were measured by a
VWR two-channel thermometer with offsets equipped with both a
beaded and stainless steel probe. 1H NMR spectra were recorded on a
Bruker AMX-400 spectrometer (400 MHz). Chemical shifts were
referenced to the residual solvent peaks.
Formation of (TBP8Cz+•)FeIV(O) (2) with mCPBA at −78 °C. In
a typical experiment, a solution of 1 was prepared under Ar in 5.0 mL
of a 1:1 (v/v) CH2Cl2/CH3OH mixture (80 μM) in a custom-made
Schlenk flask fitted with a quartz immersion probe and attached to a
UV−vis spectrophotometer via a fiber optics coupler. The solution was
cooled to −78 °C, and mCPBA (100 μL, 4 mM stock solution in a 1:1
CH2Cl2/MeOH mixture, 1 equiv) was added to the reaction mixture.
The reaction mixture changed color from green to brown after the
addition of oxidant. The temperature was maintained at −78 °C, and
the reaction showed isosbestic conversion from 1 (λmax = 440, 611, and
747 nm) to 2 (λmax = 396, 730, and 843 nm) with isosbestic points at
394 and 770 nm determined by UV−vis spectroscopy. The reaction
was monitored until complete formation of 2 [λmax = 396 nm (ε = 3.8
× 104), 730 nm (ε = 3.9 × 103), and 843 nm (ε = 1.6 × 103)] was
observed.
Reaction of 2 with Cp2Fe and Cp*2Fe. Formation of 2 at −78
°C was conducted as described above. The solution containing 2 (0.08
mM) was titrated with Cp2Fe or Cp*2Fe (6.4 mM stock solution in a
1:1 CH2Cl2/MeOH mixture). Cp2Fe or Cp*2Fe was added in 0.4
equiv increments in 25 μL of CH2Cl2 for each addition up to a total of
4 equiv. After each addition, an immediate change was observed by
UV−vis spectroscopy up to 2 equiv, at which point no further change
in the spectrum was noted. A color change from brown to green was
observed as 2 was reduced to 1.
X-ray Absorption Spectroscopy. XAS data were recorded at the
Stanford Synchrotron Radiation Laboratory (SSRL) on focused
beamline 9-3, under ring conditions of 3 GeV and 60−100 mA. A
Si(220) double-crystal monochromator was used for energy selection,
and a Rh-coated mirror (set to an energy cutoff of 10 keV) was used
for harmonic rejection. Internal energy calibration was performed by
assigning the first inflection point of the Fe foil spectrum to 7111.2 eV.
All samples were maintained at 10 K during data collection using an
Oxford Instruments CF1208 continuous-flow liquid helium cryostat.
Data were measured in fluorescence mode (using a Canberra Ge 30-
Reaction of 2 and DHA for Product Analysis. In a typical
reaction, a solution of 1 (16 μM) and DHA (16 mM) was dissolved
under Ar in 90 mL of a 1:1 (v/v) CH2Cl2/MeOH mixture. To this
solution was added mCPBA (200 μL, 14 mM stock solution in a 1:1
CH2Cl2/MeOH mixture, 1 equiv). The reaction mixture instantly
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turned brown and then slowly returned to a green color. After ∼30
min, the solvent was removed under vacuum to dryness, and the solid
was redissolved in 500 μL of CH2Cl2 with eicosane added as an
internal standard. The reaction mixture was directly injected onto the
gas chromatograph for analysis. The product, anthracene, was
identified by GC in comparison with an authentic sample and
quantitated by integration and comparison with an eicosane calibration
curve. The yield was obtained from an average of three runs.
vis spectroscopy until no further change in the spectrum was observed.
Excess xanthene dissolved in CH2Cl2 (0.1−1.2 mM) was added to the
stirring solution. The change in absorbance at 440 nm was observed
over time by UV−vis spectroscopy. The pseudo-first-order rate
constants, kobs, and second-order rate constants at each temperature
were obtained as described above. Activation parameters were
determined from the linear fit of ln(k/T) versus 1/T and the Eyring
equation (eq 4).14
Reaction of 2 and Xanthene for Product Analysis. A solution
of 1 (1.2 mM) and xanthene (60 mM) was prepared under Ar in a 1:1
(v/v) CH2Cl2/MeOH mixture (500 μL) at room temperature or −25
°C. To this solution was added mCPBA (100 μL, 6 mM stock solution
in a 1:1 CH2Cl2/MeOH mixture, 1 equiv). The reaction mixture
instantly turned brown, followed by a rapid return to a green color. A
solution of eicosane was added as an internal standard, and the
reaction mixture was directly injected onto the gas chromatograph for
analysis. The products, xanthydrol and 9,9′-bixanthyl, were identified
by GC in comparison to authentic samples and quantitated by
integration and comparison with eicosane calibration curves. Yields
were obtained from an average of two runs.
⧧
k
T
ΔH
R
1
T
ΔS⧧
R
⎛
= −⎜
⎝
⎞
⎟
⎠
⎛
⎜
⎞
⎟
kb
h
⎛
⎜
⎞
⎟
ln
+
+ ln
⎝
⎠
⎝
⎠
(4)
RESULTS AND DISCUSSION
■
In a previous report, we showed that the quantitative oxidation
of the FeIII complex 1 (λmax = 440, 611, and 747 nm) shown in
Scheme 1 could be accomplished by the addition of 1 equiv of
Scheme 1
Kinetics. In a typical experiment, a mixture of 1 (16 μM) and C−H
substrate (AcrHEt, xanthene, 9,10-d2-xanthene, DHA, CHD, or TPM
from 0.12 to 240 mM) was dissolved in 5 mL of a 1:1 (v/v) CH2Cl2/
CH3OH solution in a custom quartz cuvette fitted with a 100 mL
round-bottom Schlenk flask. To this solution was added mCPBA (100
μL, 0.8 mM stock solution in a 1:1 CH2Cl2/MeOH mixture, 1 equiv),
and the solution was mixed by hand for <5 s. A color change occurred
immediately after the addition of oxidant from green to brown
associated with the conversion of 1 (λmax = 440, 611, and 747 nm) to 2
(λmax = 397, 730, and 843 nm). Immediately after the solution had
been mixed, the reaction was monitored by UV−vis spectroscopy until
no further change in the spectrum was observed. Each substrate
reaction resulted in a color change from brown 2 to the green color of
pentafluoroiodosylbenzene (PFIB) in a CH2Cl2/MeOH
mixture at −78 °C, giving isosbestic conversion to a metastable
brown product (λmax = 396, 730, and 843 nm).4d The low-
temperature UV−vis and EPR data, along with redox titrations
and oxygen-atom transfer reactivity, provided evidence of the
assignment of 2 as the high-valent iron−oxo complex
[(TBP8Cz+•)FeIV(O)], a corrolazine analogue of Cpd-I.
However, no direct structural information about 2 was
obtained. We sought to characterize 2 by X-ray absorption
spectroscopy (XAS) to obtain information about the solution
state structure and iron oxidation state of samples of 2
generated at low temperatures.
starting FeIII complex 1. The pseudo-first-order rate constants, kobs
,
were obtained from nonlinear fitting of the growth in absorbance at
440 nm versus time (t) by eq 1, where Absf is the final absorbance and
Abs0 is the initial absorbance.
obst)
Abst = Absf − (Absf − Abs0)e(−k
(1)
The reaction was repeated for all substrates over a range of at least
three different concentrations. The plot of kobs versus substrate
concentration was found to be linear for each substrate. Second-order
rate constants (k) were obtained from the slope of the best-fit lines.
The kinetic isotope effect (KIE) of xanthene was obtained from the
second-order rate constants of xanthene (kH) and 9,10-d2-xanthene
(kD) as shown in eq 2.
XAS Studies. The generation of 2 in a CH2Cl2/MeOH
mixture was not practical for XAS because of the strong
absorption background from the chlorinated solvent. Oxidation
of 1 with PFIB in a toluene/MeOH mixture at −78 °C to
afford 2 (Scheme 1) proceeded with the same isosbestic
conversion (isosbestic points at 394, 500, and 770 nm) and
gave the same EPR spectrum that was seen previously in a
CH2Cl2/MeOH mixture (Figure 1).4d The X-ray absorption
spectrum in the near-edge region for 2 in a toluene/MeOH
mixture is shown in Figure 2. The energy of the rising Fe K-
edge is dependent upon both oxidation state and structure. The
edge energy of 7124 eV for 2 is characteristic of reported low-
spin (S = 1) FeIV complexes.15−17 As seen in Figure 2, 2 is
shifted by ∼1 eV to a higher energy in comparison to that of
the FeIII starting material. In addition, an intense pre-edge
feature (∼7113 eV) is seen for complex 2 that is significantly
more intense than that seen for complex 1. This peak is
kH
kD
KIExanthene
=
(2)
BDE(C−H) values for xanthene, CHD, DHA, and TPM were
obtained from ref 12. The BDE(C−H) value for AcrHEt was obtained
from eq 3, where ΔEAcrHEt−AcrEt (393.1 kcal mol−1) and ΔEAcrH −AcrH
•
•
2
(392.3 kcal mol−1) were calculated from DFT with a B3LYP/6-
31+(d,p) basis set and the experimental value for BDE(C−H) of
AcrH2 (73.7 kcal mol−1) was obtained from ref 13.
•
•
BDEAcrHEt − BDE
= ΔEAcrHEt−AcrEt − ΔE
AcrH2
AcrH2−AcrH
(3)
Temperature Dependence of the Rate of the Reaction of 2
and Xanthene (−35 to 22 °C). In a typical experiment,
(TBP8Cz)FeIII was dissolved in 5 mL of a 1:1 (v/v) CH2Cl2/
CH3OH solution (80 μM) in a custom-made Schlenk flask fitted with
a quartz immersion probe and attached to a UV−vis spectropho-
tometer via a fiber optics coupler. The solution was cooled to −35 °C,
and mCPBA (100 μL, 4 mM stock solution in a CH2Cl2/MeOH
mixture, 1 equiv) was added to the mixture. The reaction mixture was
magnetically stirred, and the reaction temperature was maintained
within 0.5 °C. The isosbestic conversion of 1 (λmax = 440, 611, and
747 nm) to 2 (λmax = 385, 730, and 843 nm) was monitored by UV−
2
attributed primarily to a 1s → (M3dz ) transition, which gains
intensity because of the increased level of metal 4p mixing
induced by M−L overlap.18 The increase in the intensity of this
pre-edge feature is a hallmark of terminal, MX multiply
bonded complexes (X = O or N) and provides strong evidence
of the presence of the terminal oxo ligand in 2.4g Best fits to the
EXAFS data (Figure 2 and Table 1) for 2 are obtained with
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0.65). Importantly, no similar short vector can be fit to ferric
precursor 1 (Table 1). The EXAFS data thus confirm the
presence of a multiply bonded terminal oxo group in 2, with a
d(Fe−O) of 1.64 Å that is in excellent agreement with Fe−O
distances reported for FeIV(O) porphyrins15 and nonheme
FeIV(O) complexes.17 Thus, the XAS data fully support the
assignment of 2 as a Cpd-I analogue.
Generation of 2 with mCPBA. With the assignment of 2
as an iron−oxo complex confirmed by XAS, we sought to
examine its reactivity toward C−H bond cleavage. The
generation of 2 with PFIB at −78 °C was useful for
spectroscopic characterization but was prohibitively slow
(12−14 h for completion), preventing the practical examination
of reactions of 2. We therefore sought a method that would
result in the more rapid formation of 2 and examined the
stronger oxidant mCPBA for this purpose. Addition of 1.0
equiv of mCPBA to 1 at −78 °C (Scheme 2) led to the
generation of 2 in 1 min as seen by UV−vis spectroscopy
(Figure 3a). The final spectrum (red) in Figure 3a matches that
Scheme 2
Figure 1. (a) UV−vis spectra of the reaction of 1 (blue line) with 10
equiv of PFIB to give 2 (red line) in a toluene/MeOH mixture (3:1, v/
v) at −78 °C. (b) X-Band EPR spectrum at 15 K of 1 and PFIB in a
toluene/MeOH mixture (3:1, v/v). Experimental conditions:
frequency, 9.48 GHz; microwave power, 2.01 mW; modulation
frequency, 100 kHz; modulation amplitude, 10.0 G.
Figure 2. Comparison of the Fe K-edge XAS data for 1 and 2 (left).
Fourier transforms of the Fe K-edge EXAFS data (solid lines) and the
corresponding fits (dashed lines) for 1 (black) and 2 (red) (right).
Table 1. EXAFS Fit Results for 1 and 2
1
2
R (Å)
σ2 (Å2)
R (Å)
σ2 (Å2)
4 Fe−N
1 Fe−O
4 Fe−C
16 Fe−C
16 Fe−C
ΔE0
1.85
−
2.88
3.15
4.24
−15.4
0.35
0.0011
−
0.0073
0.0019
0.0107
1.84
1.64
0.0026
0.0010
0.0049
0.0115
0.0163
Figure 3. (a) UV−vis spectral changes for the reaction of 1 and
mCPBA (1 equiv) at −78 °C to give 2. (b) Changes in absorbance at
440 nm for 1 as a function of the ratio of reductant to 2.
2.87
3.12
4.15
−15.1
0.21
a
error
seen for the product of oxidation of 1 with PFIB. Care must be
taken to employ a stoichiometric amount of mCPBA, because
the addition of even a small excess of this oxidant leads to
significant decomposition of the iron corrolazine. EPR
spectroscopy of 2 generated via mCPBA oxidation revealed a
spectrum similar to that seen for 2 from PFIB, although a
a
The error is given by ∑[(χobsd − χcalcd)2k6]/∑(χobsd2k6).
four N donors at 1.84 Å and one O donor at a short distance of
1.64 Å (Table 1). Without the inclusion of the short Fe−O
vector, the fit error value increases significantly (from 0.21 to
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radical impurity appears to overlap with the signal in the g = 2
region (Figure S2 of the Supporting Information).
(Scheme 3). Although these products were consistent with
direct oxidation of xanthene by 2 under strict anaerobic
conditions, the combined yield based on total oxidizing
equivalents for 2 was modest (44%). We speculated that the
modest yield was due to the competing background decay of 2
in this case. To enhance the stability of 2, xanthene oxidation
was examined at a lower temperature (−25 °C). The combined
yield of xanthydrol and 9,9′-bixanthene was significantly
improved, to 81% at −25 °C. Interestingly, there was also a
dramatic change in product distribution, with 75% xanthydrol
and 6% 9,9′-bixanthene formed at −25 °C.
Kinetics. To gain insight into the mechanism of C−H bond
oxidation by 2, we performed several kinetic experiments. A
series of C−H substrates with a range of BDE(C−H) values
were reacted with 2 under pseudo-first-order conditions, and
rates of reaction were measured by UV−vis spectroscopy. The
representative time-resolved UV−vis spectra for the reaction
with cyclohexadiene are shown in Figure 4, together with the
Complex 2 is stable at −78 °C but shows rapid reduction
back to 1 upon addition of the one-electron reductant Cp2Fe or
Cp*2Fe. Spectral titration with the ferrocene derivatives (Figure
3b) led to the smooth regeneration of the FeIII complex (Figure
S2 of the Supporting Information). The titration shows that 2
equiv of reductant is required for the quantitative conversion of
2 to 1, confirming that 2 behaves as a 2e− oxidant and is two
oxidation levels above 1, as expected for an FeIV(O) π-cation
radical complex. Isosbestic conversion was observed for the
reactions with the ferrocene derivatives, indicating that the one-
electron-reduced intermediate [(TBP8Cz)FeIV(O)]− does not
build up to any extent. These results imply a mechanism in
which the first reduction step is rate-determining. Both
reduction steps are also likely to be accompanied by
protonation at the terminal oxo position. This mechanism is
the same as that described for the reduction of (TBP8Cz)-
MnV(O) with ferrocene derivatives.4i
Notably, we found that mCPBA could also be used at room
temperature (22 °C) to generate 2. Reactions at room
temperature with a stoichiometric amount of PFIB as the
oxidant led to insufficient buildup of 2 because of rapid decay
back to 1. When excess PFIB was employed to increase the
oxidation rate, significant decomposition (bleaching) of 1 was
noted by UV−vis spectroscopy. However, oxidation of 1 by
mCPBA was sufficiently fast at 22 °C to give a reasonable
buildup of 2 prior to the decay and partial return of 1 (Figure
S3 of the Supporting Information). These results suggested that
2 could be generated in situ at room temperature in the
presence of potential substrates.
C−H Bond Activation. Treatment of 1 with 1 equiv of
mCPBA in the presence of excess 9,10-dihydroanthracene
(DHA, 1000 equiv) in a CH2Cl2/MeOH mixture at 22 °C led
to the rapid formation of 2 as seen by UV−vis spectroscopy,
followed by conversion of 2 back to 1 (73% recovery of 1 based
on absorbance at 440 nm). Product analysis of the reaction of 2
and DHA under Ar by GC-FID showed the production of
anthracene in good yield (74%), without any evidence of other
oxidation products (i.e., anthrone, anthraquinone, or anthra-
cene dimer) (Scheme 3). These data show that 2 is a
Scheme 3
Figure 4. (a) Time-resolved UV−vis spectra of the reaction of 2 (16
μM) with excess CHD (2.0 mM), in a CH2Cl2/MeOH mixture (1:1,
v/v, 5 mL) at 22 1 °C. (b) Change in absorbance at 440 nm vs time
corresponding to the growth of 1 for the data in panel a.
first-order curve for this reaction. Good first-order kinetics were
observed for each of the substrates up to five half-lives, yielding
kobs values that correlated linearly with substrate concentration
(Figure S4 of the Supporting Information). The derived
second-order rate constants were plotted versus BDE(C−H)
values for each substrate, and a linear relationship was found
with a slope of −0.42, as shown in Figure 5a. The slope in
Figure 5a also matches that seen for a porphyrin Cpd-I
analogue recently reported by Nam and Shaik,
[(tmp)+•FeIV(O)(p-X-PyO)]+ (slope of −0.43),3b,19 as well as
MnV(O) corrolazines (slope of −0.42 to −0.62).4e In addition,
a significant kinetic isotope effect (KIE = 5.7) for xanthene/
9,10-d2-xanthene was observed (Figure 5b). Taken together,
these data provide strong support for a mechanism that
competent oxidant for DHA, and the production of anthracene
is consistent with two consecutive hydrogen-atom abstractions,
as expected for the two oxidizing equivalents of 2. Control
reactions with mCPBA showed no anthracene product under
the same conditions.
Addition of xanthene, an alternative substrate with a
BDE(C−H) similar to that of DHA, also resulted in the
rapid conversion of 2 to 1 as determined by UV−vis
spectroscopy. Product analysis revealed xanthydrol (32%) and
9,9′-bixanthene (12%) as the major oxidation products
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influence on the kinetic parameters. These data provide some
of the first activation parameters for an FeIV(O) porphyrinoid
π-cation radical participating in C−H activation.20 Limited data
are available for other high-valent MO complexes and C−H
substrates. Our ΔH⧧ value falls in the range reported for
abstraction of a hydrogen from C−H substrates by Ru−oxo,
Mn−oxo, and nonheme Fe−oxo complexes.21 The negative
activation entropy observed for 2 is consistent with a
bimolecular, second-order process but is relatively high (more
positive) compared to those of the former complexes. This ΔS⧧
value suggests a less constrained transition state for 2 and
xanthene, keeping the overall barrier (ΔG⧧ = 15 2 kcal
298
mol−1) relatively low.
A proposed mechanism to account for all of the observations
regarding the reaction of 2 with C−H substrates is illustrated in
Scheme 4 for the reaction between 2 and xanthene. An initial
Scheme 4. Proposed Mechanism of C−H Bond Activation
Figure 5. (a) Dependence of the log k values for the reaction of 2 with
C−H substrates on the BDE values of the scissile C−H bond,12 where
k values are normalized per reactive C−H bond. (b) Second-order
plots for xanthene [green circles (experiment), solid line (fit)] and
9,10-d2-xanthene [blue diamonds (experiment), dashed line (fit)].
HAT step yields an FeIV(OH) intermediate along with the
xanthyl radical (Xn•). At this point, there are two possible
reaction pathways, one in which a rebound step occurs to give
the alcohol and the other in which cage escape of Xn• is
followed by dimerization to give the radical coupled product.
The isosbestic conversion of 2 to 1 upon reaction with the C−
H substrates indicates that the proposed FeIV(OH) inter-
mediate does not build up to any measurable extent over the
course of the reaction. This observation is consistent with
hydroxylation via a fast rebound step being the dominant
pathway, which prevents the buildup of FeIV(OH) and
minimizes cage escape of the xanthyl radical.22 In the
dimerization pathway, the FeIV(OH) intermediate either reacts
with another molecule of Xn to generate Xn• and FeIII(OH2) or
possibly decays through an unknown decomposition step.
Similarly, the oxidation of DHA likely goes through an initial
HAT step, but instead of rebound hydroxylation, the FeIV(OH)
intermediate can immediately abstract a second H atom from
the DHA radical, giving the dehydrogenated anthracene
product. If cage escape for DHA• occurs, it likely rapidly
loses the second H• to 2 or another FeIV(OH) intermediate,
because no evidence of dimerization of DHA• was obtained.
This result is not surprising given the very weak C−H bond of
DHA• [BDE(C−H) = 43 kcal mol−1].21a The dramatic change
in product ratios with temperature for xanthene oxidation may
also be explained within the context of the mechanism in
Scheme 4. A large shift toward production of xanthydrol at −25
°C may be explained by suppression at low temperatures of the
bimolecular radical dimerization pathway versus the rebound
step in Scheme 4.
involves HAT between the substrate C−H bond and 2 as the
rate-determining step. A similar conclusion has been reached
previously regarding the mechanism of (TBP8Cz)MnV(O) and
O−H and C−H substrate oxidations.4e,f
The relative stability of 2 at moderate temperatures (−35 °C
to room temperature) gave us the ability to measure activation
parameters for C−H activation. The temperature dependence
of the rate constants for the reaction of 2 with xanthene was
examined from −35 to 22 °C (Figure S5 of the Supporting
Information), and an Eyring plot was constructed as shown in
Figure 6. The plot is nicely linear and yields a ΔH⧧ of 12.7
0.8 kcal mol−1 and a ΔS⧧ of −9 3 cal K−1 mol−1. A direct
comparison with 2 generated from PFIB at −25 °C showed no
significant change in the rate constant for oxidation of
xanthene, indicating that the identity of the oxidant has no
Comparison of 2 to Other Cpd-I Analogues. It is of
interest to compare the intrinsic reactivity of 2 with that of
porphyrin Cpd-I analogues. A recent study of
[(TMP)+•FeIV(O)(p-X-PyO)]+ reports rate constants for
Figure 6. Temperature dependence of the rate constants for xanthene
plotted as ln(k/T) vs 1/T.
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xanthene that are approximately 50−225 times larger than that
found for 2.3b The greater reactivity of the tmp complex may,
to a first approximation, be assigned to the differences in charge
between porphyrin and corrolazine, where the extra negative
charge provided by the corrolazine ligand reduces the
electrophilicity of 2 (a neutral complex) in comparison to
that of the cationic [(TMP)+•FeIV(O)]+. In support of this,
recent work from Groves on a tmp derivative bearing four
positively charged methylpyridinium meso substituents showed
that it reacts with a rate constant ∼105 times higher than that of
the parent tmp complex in xanthene oxidation.3a However,
differences in charge are only one aspect that may influence the
reactivity of high-valent metal−oxo porphyrinoid compounds.
The much greater reactivity for the former methylpyridinium
derivative may be due to the strong electron withdrawing
properties of the meso substituents and their consequential
effect on the redox potential of the central iron.2 Possible spin
state crossing phenomena may also be involved in enhancing
the reactivity.3a In fact, we have shown that the addition of
negatively charged axial ligands (X−) to neutral
[(TBP8Cz)MnV(O)]0 greatly increases its reactivity toward
C−H substrates.4e This effect was attributed to an increase in
the basicity (i.e., the pKa) of the metal−oxo unit, similar to the
proposed influence of the anionic thiolate donor trans to the
ferryl−oxo unit in Cyt-P450.23 These previous results, taken
together with the findings presented here, suggest that simple
differences in charge do not readily explain all of the reactivity
trends, and a number of other factors can influence the
reactivity of high-valent iron−oxo porphyrinoid complexes.24
The stabilization of high-valent oxidation states by trianionic
corroles has been attributed to the increase in the energy of the
d orbitals relative to other porphyrin derivatives. This increase
results in the energy of the metal d orbitals being raised above
the ligand HOMO, thus allowing for the stabilization of high
oxidation states and causing a consequent reduction in
reactivity.5m The corrolazine ligand may have a similar
influence on the d orbital manifold, helping to stabilize the
high-valent Fe−oxo intermediate and as a result lowering the
reaction rates for C−H activation.
ASSOCIATED CONTENT
■
S
* Supporting Information
Spectroscopic and kinetic data (Figures S1−S5). This material
is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
dpg@jhu.edu; serena.debeer@mpi-mail.mpg.de
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Science Foundation
(Grant CHE0909587 to D.P.G.) and a seed grant from the
Environment, Energy, Sustainability & Health Institute of The
Johns Hopkins University. S.D. thanks Cornell University, the
Alfred P. Sloan Foundation, and the Max Planck Society for
funding. Portions of this research were conducted at SSRL,
SLAC, a user facility operated by the U.S. Department of
Energy (DOE) and Stanford University. The SSRL SMB
Program is supported by the DOE, OBER, and the National
Institutes of Health. We are grateful to S. Fukuzumi for a gift of
9-ethyl-9,10-dihydro-10-methylacridine. P.L. is grateful for the
Queen Sirikit Scholarship (Thailand).
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CONCLUSIONS
■
In summary, we have generated a Cpd-I analogue with a
corrole-type framework and conducted the first structural
characterization of such a species by XAS. This complex is
competent to oxidize a range of C−H substrates, and a
concerted HAT mechanism for the key C−H activation step is
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