Modeling TauD- J: A high-spin nonheme oxoiron(IV) complex with high reactivity toward C-H bonds

Article abstract of DOI:10.1021/ja511757j

High-spin oxoiron(IV) species are often implicated in the mechanisms of nonheme iron oxygenases, their C-H bond cleaving properties being attributed to the quintet spin state. However, the few available synthetic S = 2 Fe^IV=O complexes supported by polydentate ligands do not cleave strong C-H bonds. Herein we report the characterization of a highly reactive S = 2 complex, [Fe^IV(O)(TQA)(NCMe)]²⁺ (2) (TQA = tris(2-quinolylmethyl)amine), which oxidizes both C-H and C=C bonds at -40 °C. The oxidation of cyclohexane by 2 occurs at a rate comparable to that of the oxidation of taurine by the TauD-J enzyme intermediate after adjustment for the different temperatures of measurement. Moreover, compared with other S = 2 complexes characterized to date, the spectroscopic properties of 2 most closely resemble those of TauD-J. Together these features make 2 the best electronic and functional model for TauD-J to date.

Full text of DOI:10.1021/ja511757j

Communication
pubs.acs.org/JACS
Modeling TauD‑J: A High-Spin Nonheme Oxoiron(IV) Complex with
High Reactivity toward C−H Bonds
Achintesh N. Biswas,† Mayank Puri,† Katlyn K. Meier,‡ Williamson N. Oloo,† Gregory T. Rohde,†
Emile L. Bominaar,*,‡ Eckard Munck,*,‡ and Lawrence Que, Jr.*,†
̈
†Department of Chemistry and Center for Metals in Biocatalysis, University of Minnesota, Minneapolis, Minnesota 55455, United
States
‡Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States
S
* Supporting Information
tertiary amine/N-heterocycle combinations used to date. Bakac
and co-workers have shown that the S = 2 [FeIV(O)(H2O)5]2+
ABSTRACT: High-spin oxoiron(IV) species are often
implicated in the mechanisms of nonheme iron oxy-
genases, their C−H bond cleaving properties being
attributed to the quintet spin state. However, the few
available synthetic S = 2 FeIVO complexes supported by
polydentate ligands do not cleave strong C−H bonds.
Herein we report the characterization of a highly reactive S
= 2 complex, [FeIV(O)(TQA)(NCMe)]2+ (2) (TQA =
tris(2-quinolylmethyl)amine), which oxidizes both C−H
and CC bonds at −40 °C. The oxidation of cyclohexane
by 2 occurs at a rate comparable to that of the oxidation of
taurine by the TauD-J enzyme intermediate after adjust-
ment for the different temperatures of measurement.
Moreover, compared with other S = 2 complexes
characterized to date, the spectroscopic properties of 2
most closely resemble those of TauD-J. Together these
features make 2 the best electronic and functional model
for TauD-J to date.
ion can be obtained in acidic aqueous solution and is in fact
highly reactive,8 but its short lifetime (t1/2 ∼ 20 s at 25 °C) has
made its detailed characterization challenging. We have thus
sought to identify a tripodal ligand with weaker-field donors that
would allow the trapping of a sterically less hindered S = 2 FeIV
O species that would exhibit higher reactivity. Herein we report
the synthesis and characterization of the S = 2 complex
[FeIV(O)(TQA)]2+ (2) (TQA = tris(2-quinolylmethyl)amine),
with weaker-field quinolines9 replacing the pyridines of the
popular TPA ligand. Complex 2 exhibits spectroscopic proper-
ties distinct from [FeIV(O)(TPA)(NCCH3)]2+10 and remarkably
higher reactivity toward alkanes and alkenes.
The reaction of TQA and FeII(OTf)2(CH3CN)2 in CH3CN
affords [FeII(TQA)(OTf)2] (1). Its crystal structure shows a six-
coordinate high-spin iron(II) center (Figure 1 left and Tables S1
and S2). Treatment of 1 with 2-(tBuSO2)C6H4IO (ArIO)11 at
−40 °C leads to the formation of a highly reactive intermediate 2
that has a half-life of 15 min and exhibits a near-UV shoulder at
400 nm and weak bands at 650 nm (εM = 300 M−1 cm−1) and 900
igh-spin oxoiron(IV) species have been trapped and
H
characterized as key intermediates in the dioxygen
activation mechanisms of mononuclear nonheme iron enzymes1
by the seminal efforts of Bollinger and Krebs.2 A related species is
proposed to be the oxidant formed in the reactions of N2O with
coordinately unsaturated iron(II) centers in carboxylate-based
metal−organic frameworks that catalyze ethane hydroxylation.3
The quintet (S = 2) species is predicted by density functional
theory (DFT) calculations to be more reactive in H atom transfer
(HAT) than the corresponding triplet (S = 1) species.4 In
particular, Shaik has emphasized the role of exchange to stabilize
the high-spin FeIII−OH transition state in HAT by high-spin
FeIV(O) complexes.4e
These developments have fueled a significant synthetic effort
that has led to the characterization of over 60 oxoiron(IV)
complexes in the past decade.5 However, most of these
complexes have triplet ground states, and only a handful possess
a quintet ground state. One successful strategy to obtain S = 2
FeIVO complexes has been to employ sterically bulky tripodal
ligands that enforce a trigonal-bipyramidal geometry on the
iron(IV) center,6,7 but these complexes exhibit rather sluggish
intermolecular reactivity, perhaps because of steric constraints.
An alternative strategy to obtain S = 2 FeIVO complexes is to
use a weaker-field ligand environment than that provided by the
Figure 1. (left) ORTEP plot for [FeII(TQA)(OTf)2] (1) with 50%
probability ellipsoids and H atoms omitted for clarity. Selected bond
lengths (Å): Fe(1)−N(1), 2.2155(17); Fe(1)−N(2), 2.2308(17);
Fe(1)−N(3), 2.1954(17); Fe(1)−N(4), 2.1748(17); Fe(1)−O(1),
2.0450(14); Fe(1)−O(4), 2.1965(15). (right) UV−vis spectrum of 2
from the reaction of 0.4 mM 1 with 2 equiv of ArIO in CH3CN at −40
°C. Inset: resonance Raman spectra (λex = 514.5 nm, 60 mW power) of
[16O]2 (top) and [18O]2 (bottom) in frozen CH3CN solution. (Off-
resonance excitation was used to minimize the effects of photo-
reduction.6a).
Received: November 15, 2014
© XXXX American Chemical Society
A
DOI: 10.1021/ja511757j
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
a
Table 1. Properties of Synthetic [FeIV(O)L] Complexes
L
TQA
TPA
724
1
Me3NTB
380, 770
1
TMG3tren
400, 825, 866
2
(H2O)5
TauD-J
∼318
2
λmax (nm)
400(sh), 650, 900
S
2
2
D (cm−1
)
17(1)
28
28
5.0
9.7
10.5
ΔEQ (mm/s)
Ax,y/gnβn (T)
Az/gnβn (T)
δ (mm/s)
ref
−1.05(2)
0.92
−23.5
−5.0
0.01
10
1.53
−19.0
0
−0.29
−15.5
−28.0
0.09
−0.33
−20.3
−
−0.90
−18.4, −17.6
−31.0
0.30
−18.2(4), −16.6(4)
b
−31.7
0.24(2)
0.02
12
0.38
this work
6a
8
2c
a
Abbreviations used: Me3NTB = tris(1-methylbenzimidazol-2-ylmethyl)amine, TMG3tren =1,1,1-tris{2-[N2-(1,1,3,3-tetramethylguanidino)]ethyl}-
b
amine. Az/gnβn(T) could not be determined experimentally for 2 and was estimated from calculations described below.
nm (εM = 75 M−1 cm−1) (Figure 1 right). Its properties
suggesting the formation of an FeIVO complex are compared
with those of others in Table 1. A resonance-enhanced Raman
vibration is found at 838 cm−1, which shifts to 803 cm−1 upon
18O-labeling of 2, as expected for the ν(FeO) mode.
Electrospray ionization mass spectrometry (ESI-MS) analysis
of 2 supports its formulation as [FeIV(O)(TQA)]2+, with a peak
observed at m/z 256.1. However, the isotope distribution pattern
suggests contamination with [FeIII(OH)(TQA)]2+ (m/z 256.6)
due to the instability of 2 (Figure S1 in the SI). Indeed, the
dominant feature in its ESI-MS spectrum is a peak at m/z 662
that corresponds to [FeIII(OH)(TQA)(O3SCF3)]+. Also present
is a peak at m/z 676 that corresponds to {[FeIII(OH)(TQA)-
(OTf)] + O − 2H}+, suggesting the oxidation of one TQA CH2
group to a carbonyl.
in Figure S3) these contaminants were subtracted from the raw
data to yield the spectra in Figures 2B,C and S2. The isomer shift
of 2 is quite positive compared with values for other oxoiron(IV)
complexes supported by polypyridine ligands,12,13 suggesting
that this species is a high-spin iron(IV) complex. This spin-state
assignment is strongly supported by analysis of the magnetic
hyperfine structure. The spectra of 2 exhibit increased magnetic
hyperfine splittings with increasing applied field. This increase
follows a “magnetization” curve with a shape determined by the
magnitude of the zero-field splitting parameter D. From spectral
simulations using an S = 2 spin Hamiltonian (red lines in Figures
2 and S2) we obtained the parameters listed in Table 1.
Simulating the spectra for S = 1 would require Ax,y/gnβn to be
(−33, 32) T, which is unreasonably large compared with the
typical Ax,y/gnβn = (−21, 21) T,14 ruling out the S = 1 assignment.
Table 1 shows that the magnetic hyperfine parameters previously
reported for TauD-J are best reproduced by 2, making it the best
spectroscopic model for TauD-J to date.
Figure 2 shows 4.2 K Mossbauer spectra of 2 recorded at
̈
various applied fields; additional spectra are shown in Figure S2.
The change in spin state in going from [FeIV(O)(TPA)-
(NCMe)]2+ (S = 1) to 2 (S = 2) can be rationalized by an earlier
study on Fe(TPA) complexes.9a Lower spin states could be
attained for FeII and FeIII complexes of the parent TPA ligand but
became less likely as the number of pyridine α-substituents
increased because of steric interactions involving the α-C−H
bonds, which prevent formation of the shorter Fe−Npy bonds
required by a lower-spin iron center.9a For the oxoiron(IV)
series, S = 1 complexes were obtained for TPA and derivatives
with one α-substituent.9b However, with all three pyridine
donors having α-substituents, TQA is not able to support an S =
1 FeIVO center and instead gives rise to an S = 2 FeIVO
complex. The principle for this design strategy is supported by a
computational comparison of several structures related to TQA
(see pp S14−S16 in SI for additional details).
The right panel of Figure 2 shows the DFT-optimized
geometry of 2, formulated as [FeIV(O)(TQA)(NCMe)]2+,
obtained using Gaussian 09 (Table S3). The calculations yielded
ΔEQ = −1.14 mm/s, η = 0.34 (0 ≤ ηexp < 0.3), and δ = 0.24 mm
s−1, in excellent agreement with the experimental data.15 The
validity of two alternative formulations for 2 was also assessed by
DFT. Five-coordinate [FeIV(O)(TQA)]2+ was ruled out, as it
yielded a δ of 0.15 mm s−1 that was inconsistent with experiment,
but [FeIV(O)(TQA)(OTf)]+ gave rise to the same isomer shift as
predicted for [FeIV(O)(TQA)(NCMe)]2+ and observed exper-
imentally (Table S3). To distinguish between the two six-
coordinate formulations, a 19F NMR study of 2 was performed at
−40 °C, which revealed only one peak at −80 ppm, the same
chemical shift as for free triflate (Figures S8−S13). This peak
represents all of the triflate in the sample and has a line width 5-
fold broader than that found for free triflate. When these
Figure 2. (left) Mossbauer spectra recorded at 4.2 K in parallel applied
̈
magnetic fields as indicated. The blue line in (A) outlines a diiron(III)
contaminant (12% of Fe). We have subtracted the spectra of this
contaminant from the data in (B) and (C). A second contaminant, a
high-spin Fe(III) species (18%), has been removed in (B) and (C);
details are given in the SI. For the spectral simulations for 2 (red lines),
̂
we employed the software WMOSS for the S = 2 spin Hamiltonian H =
2
2
2
̂
̂
̂
̂
̂
̂
̂
D[Sz − 2 + (E/D)(Sx 2 − Sy2)] + 2βS·B + S·A·I − gnβnB·I + (eQVzz/
2
̂
̂
̂
12)[3Iz − 15/4 + η(Ix − Iy )] using the parameters listed in Table 1
and n = 0. (right) Geometry-optimized structure of 2 in the gas phase
with Fe−ligand distances shown.
The zero-field spectrum exhibits a quadrupole doublet (∼70% of
Fe) with splitting ΔEQ = −1.05 mm/s and isomer shift δ = 0.24
mm/s. The remainder of the Fe in the sample belongs to high-
spin Fe(III) decay products, 18% representing mononuclear
Fe(III) that presumably corresponds to the [FeIII(OH)(L)]2+
species observed in the ESI-MS spectrum (Figure S1) and 12%
from a diferric complex (ΔEQ = +1.90 mm/s, δ = 0.45 mm/s;
blue line in Figure 2A). By means of spectral simulations (shown
B
DOI: 10.1021/ja511757j
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Journal of the American Chemical Society
Communication
observations are judged against the data accumulated in Table
S6, we surmise that only a small fraction of the triflate binds to the
FeIV(O) unit and is in rapid equilibrium with unbound triflate,
making [FeIV(O)(TQA)(OTf)]+ a minor component of 2 in
MeCN solution. Thus, taking the 19F NMR and DFT results
together, we conclude that 2 is best formulated as [FeIV(O)-
(TQA)(NCMe)]2+.
more reactive in cleaving C−H bonds than other well-
characterized S = 2 FeIV(O) complexes reported to date.6,7 In
fact, 2 exhibits the highest cyclohexane oxidation rate found to
date for this class of complexes (k2 = 0.37 M−1 s−1 at −40 °C).
Notably, the closely related S = 1 [FeIV(O)(TPA)(NCMe)]2+
complex does not react at all with cyclohexane at −40 °C. Even
without correction for the temperature difference, the cyclo-
hexane oxidation rate found for 2 at −40 °C is 3−4 orders of
magnitude higher than the k2 values at 25 °C for S = 1
[FeIV(O)(N4Py)]2+ and [FeIV(O)(BnTPEN)]2+ (N4Py = bis(2-
pyridylmethyl)(bis(2-pyridyl)methyl)amine); BnTPEN = N-
benzyl-N,N′,N′-tris(2-pyridylmethyl)-1,2-diaminoethane)13b
and an order of magnitude higher than that for a recently
reported FeV(O) complex.16 However, the S = 1 complex
[FeIV(O)(Me3NTB)]2+ (3) has reactivity comparable to that of
2, with k2(cyclohexane) = 0.23 M−1 s−1 at −40 °C.12 Indeed, the
HAT data points for 3 (blue triangles) reported by Nam and
Shaik12 fall on the same trend line as those for 2 (red squares) in
Figure 3. This surprising resemblance suggests that FeIV(O)
complexes, despite having different ground spin states, can
achieve comparably high HAT reactivities. On the basis of DFT
calculations, Nam and Shaik rationalized the high HAT reactivity
of 3 by invoking a highly reactive close-lying S = 2 excited state.12
More examples are needed to clarify the basis for the similar
reactivities of 2 and 3 (see p S16 in the SI).
The DFT-calculated spin-dipolar contribution to the 57Fe
magnetic hyperfine tensor is Ax,y,z(s‑d)/gnβn = (+3.0, +6.5, −9.5)
T. We estimated Aiso = (Ax + Ay +Az)/3 from the experimental
values Ax/gnβn= −18.2 T, Ay/gnβn= −16.6 T by using the
calculated spin-dipolar values along x and y, obtaining for the
contact term Ac/gnβn ∼ Aiso/gnβn = −22.2 T (as shown in the SI,
the g values of 2 are estimated as gx = gy ≈ 1.98 and gz = 2.00, and
therefore, the orbital contributions to A are expected to be quite
small). We then obtained Az/gnβn = −31.7 T from Aiso + Az(s‑d)
.
The entries in Table 1 reveal that the parameters of 2 are quite
close to those reported for TauD-J. As shown on p S13 in the SI,
the predominant contribution to the D value of 2 arises from
spin−orbit coupling between the S = 2 ground state and a low-
lying (∼2700 cm−1) S = 1 excited state. This is also the case for
TauD-J, and its smaller D value suggests that its S = 1 state has a
higher vertical excitation energy.2a
We also evaluated the reactivity of 2 in the oxidation of
hydrocarbons (Table S7). Oxidation of cyclohexane and
cyclooctane by 2 under nitrogen afforded cyclohexanone and
cyclooctanone in respective yields of 35% and 40%. As these
products represent four-electron oxidations, their yields
correspond essentially to quantitative conversion of the oxidizing
equivalents from 2 present in the solution.
Second-order rate constants were measured for the reaction of
2 with substrates having different C−H bond strengths (Table
S7). Figure 3 shows a plot of log k′2 (k2′ = k2/number of equivalent
Complex 2 also epoxidizes olefins (Table S7). Although
commonly associated with oxoiron(IV) porphyrin cation radical
complexes,17 olefin epoxidation has only been reported for three
nonheme oxoiron(IV) complexes, in reactions limited to only
one or two types of olefins.18 Banse reported the epoxidation of
cyclooctene and cis-stilbene in <20% yield by an [FeIV(O)(N5)]
complex.18a Rybak-Akimova found that a complex supported by
a pyridine-containing tetraaza macrocycle afforded an 80% yield
of cyclooctene oxide with k2 = 0.45 M−1 s−1 at 0 °C,18b while Nam
obtained a 60% yield of styrene oxide from [FeIV(O)(N3S2)]2+
with k2 = 0.03 M−1 s−1 at 25 °C.18c Although we initially reported
the epoxidation of cis-cyclooctene by [FeIV(O)(TPA)-
(NCMe)]2+,10 subsequent studies with rigorous exclusion of
O2 showed no epoxide formation from cyclooctene and
cyclohexene.19 In contrast, 2 reacted with cis-cyclooctene at
−40 °C to form epoxide in 80% yield relative to the amount of 2
present in solution. The k2 value for cyclooctene epoxidation was
determined to be 3.3 M−1 s−1 (Table S7), a value that is orders of
magnitude larger than those for the S = 1 oxoiron(IV) complexes
mentioned above (given the higher temperatures at which the
latter results were obtained). These comparisons suggest a
significantly higher reactivity of the S = 2 complex 2 toward C
C bonds than found for its S = 1 nonheme counterparts.
However, because of the paucity of rate data for such reactions,
additional examples are needed to draw firm conclusions on the
role of spin state in determining the rates of olefin epoxidation.
The reactions of 2 with other olefins were investigated. The
epoxidations of cis- and trans-2-heptene exhibited high degrees of
stereoretention (87% and >99%, respectively), indicating that if
substrate-based radical intermediates are formed, they must be
short-lived. Interestingly, 1-octene was oxidized by 2 with k2 = 5.3
Figure 3. Plot of log k′2 values for the reactions of various substrates with
2 (squares) and [FeIV(O)(Me3NTB)]2+ (3) (triangles) at −40 °C vs
their C−H bond dissociation energies (k2′ = k2/n, where n is the number
of equivalent substrate C−H bonds). See Table S7 for details.
target C−H bonds) as a function of bond dissociation energy
(DC−H), for which a linear correlation is seen, as previously found
for other high-valent oxoiron centers. In addition, 2 exhibits C−
H/D kinetic isotope effects (KIEs) of 28 and 25 for toluene and
cyclohexane oxidation, respectively, showing that C−H bond
cleavage is the rate-determining step (Figures S17 and S21). The
nonclassical KIE values are suggestive of hydrogen tunneling
effects.12,13b
M
−1 s−1; however, in this case both the epoxide and 1-octen-3-
one were obtained in respective yields of 35% and 15% relative to
1, representing 90% of the oxidizing equivalents available from 2.
On the other hand, cyclohexene oxidation by 2 afforded only
allylic oxidation products. These results show that 2 can mediate
the oxidation of C−H and CC bonds at comparable rates.
Indeed, the k2 values for cyclooctene and cyclooctane oxidation
Comparison of the HAT reactivities of 2 and other high-valent
nonheme oxoiron complexes (Table S8) shows that 2 is much
C
DOI: 10.1021/ja511757j
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Journal of the American Chemical Society
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Solomon, E. I. J. Am. Chem. Soc. 2007, 129, 15983. (c) Bernasconi, L.;
Louwerse, M. J.; Baerends, E. J. Eur. J. Inorg. Chem. 2007, 3023.
(d) Geng, C.; Ye, S.; Neese, F. Angew. Chem., Int. Ed. 2010, 49, 5717.
(e) Shaik, S.; Chen, H.; Janardanan, D. Nat. Chem. 2011, 3, 19.
(5) (a) Que, L., Jr. Acc. Chem. Res. 2007, 40, 493. (b) McDonald, A. R.;
Que, L., Jr. Coord. Chem. Rev. 2013, 257, 414. (c) Ray, K.; Pfaff, F. F.;
Wang, B.; Nam, W. J. Am. Chem. Soc. 2014, 136, 13942.
by 2 are identical, indicating that 2 attacks the cyclooctene CC
bond only 16-fold faster than the 95 kcal/mol C−H bond of
cyclooctane, which was confirmed by a competitive oxidation of
equimolar amounts of the two substrates, which afforded 35%
cyclooctene oxide and 15% cyclooctanone. These results
represent the first instance for which the rates of CC and
C−H bond attack by a nonheme FeIVO complex can be
compared directly, which emphasizes the uniqueness of 2 within
the nonheme oxoiron family.20
(6) (a) England, J.; Martinho, M.; Farquhar, E. R.; Frisch, J. R.;
Bominaar, E. L.; Munck, E.; Que, L., Jr. Angew. Chem., Int. Ed. 2009, 48,
̈
3622. (b) England, J.; Guo, Y.; Van Heuvelen, K. M.; Cranswick, M. A.;
In summary, we have generated the highly reactive high-spin
oxoiron(IV) complex 2. Introduction of α-substituents on all
three pyridines of the TPA ligand weakens the ligand field about
the FeIVO unit,9a and S = 1 [FeIV(O)(TPA)(NCMe)]2+
becomes S = 2 [FeIV(O)(TQA)(NCMe)]2+ (2). The FeIVO
center goes from being unreactive toward cyclohexane at −40 °C
to being the fastest in its class to date at oxidizing cyclohexane.
The rate of 0.37 s−1 at −40 °C for oxidation of 1 M cyclohexane
by 2 compares favorably with the rate of 13 s−1 for taurine
oxidation by TauD-J at 5 °C, after correction for the 45°
temperature difference.2b This observation and the strikingly
similar spectroscopic parameters of 2 and TauD-J (Table 1)
make 2 the best electronic and functional model for TauD-J to
date.
Rohde, G. T.; Bominaar, E. L.; Munck, E.; Que, L., Jr. J. Am. Chem. Soc.
2011, 133, 11880.
̈
(7) (a) Lacy, D. C.; Gupta, R.; Stone, K. L.; Greaves, J.; Ziller, J. W.;
Hendrich, M. P.; Borovik, A. S. J. Am. Chem. Soc. 2010, 132, 12188.
(b) Bigi, J. P.; Harman, W. H.; Lassalle-Kaiser, B.; Robles, D. M.; Stich,
T. A.; Yano, J.; Britt, R. D.; Chang, C. J. J. Am. Chem. Soc. 2012, 134,
1536.
(8) Pestovsky, O.; Stoian, S.; Bominaar, E. L.; Shan, X.; Munck, E.;
̈
Que, L., Jr.; Bakac, A. Angew. Chem., Int. Ed. 2005, 44, 6871.
(9) (a) Zang, Y.; Kim, J.; Dong, Y.; Wilkinson, E. C.; Appelman, E. H.;
Que, L., Jr. J. Am. Chem. Soc. 1997, 119, 4197. (b) Paine, T. K.; Costas,
M.; Kaizer, J.; Que, L., Jr. J. Biol. Inorg. Chem. 2006, 11, 272. (c) Wei, N.;
Murthy, N. N.; Chen, Q.; Zubieta, J.; Karlin, K. D. Inorg. Chem. 1994, 33,
1953.
(10) Lim, M. H.; Rohde, J.-U.; Stubna, A.; Bukowski, M. R.; Costas, M.;
Ho, R. Y. N.; Munck, E.; Nam, W.; Que, L., Jr. Proc. Natl. Acad. Sci. U.S.A.
̈
2003, 100, 3665.
ASSOCIATED CONTENT
* Supporting Information
Experimental details, additional ESI-MS and Mossbauer data,
■
(11) Macikenas, D.; Skrzypczak-Jankun, E.; Protasiewicz, J. D. J. Am.
Chem. Soc. 1999, 121, 7164.
(12) Seo, M. S.; Kim, N. H.; Cho, K.-B.; So, J. E.; Park, S. K.;
S
̈
DFT results and insights, kinetic data, and crystallographic data
for 1 (CIF). This material is available free of charge via the
Internet at http://pubs.acs.org.
Clem
́
ancey, M.; Garcia-Serres, R.; Latour, J.-M.; Shaik, S.; Nam, W.
Chem. Sci. 2011, 2, 1039.
(13) (a) Xue, G.; De Hont, R.; Munck, E.; Que, L., Jr. Nat. Chem. 2010,
̈
2, 400. (b) Kaizer, J.; Klinker, E. J.; Oh, N. Y.; Rohde, J.-U.; Song, W. J.;
Stubna, A.; Kim, J.; Munck, E.; Nam, W.; Que, L., Jr. J. Am. Chem. Soc.
2004, 126, 472.
(14) Jackson, T. A.; Rohde, J.-U.; Seo, M. S.; Sastri, C. V.; DeHont, R.;
̈
AUTHOR INFORMATION
Corresponding Authors
*larryque@umn.edu
*emunck@cmu.edu
*eb7g@andrew.cmu.edu
Notes
■
Stubna, A.; Ohta, T.; Kitagawa, T.; Munck, E.; Nam, W.; Que, L., Jr. J.
̈
Am. Chem. Soc. 2008, 130, 12394.
(15) The ΔEQ values were calculated using a conversion factor of
−1.43 mm s−1/AU instead of the conventional −1.7 mm s−1/AU. See
Figure 10 in: Chanda, A.; Shan, X.; Chakrabarti, M.; Ellis, W. C.;
Popescu, D. L.; Tiago de Oliveira, F.; Wang, D.; Que, L., Jr.; Collins, T.
The authors declare no competing financial interest.
J.; Munck, E.; Bominaar, E. L. Inorg. Chem. 2008, 47, 3669.
̈
ACKNOWLEDGMENTS
■
(16) Ghosh, M.; Singh, K. K.; Panda, C.; Weitz, A.; Hendrich, M. P.;
Collins, T. J.; Dhar, B. B.; Gupta, S. S. J. Am. Chem. Soc. 2014, 136, 9524.
(17) (a) Meunier, B. Chem. Rev. 1992, 92, 1411. (b) Groves, J. T.;
Nemo, T. E. J. Am. Chem. Soc. 1983, 105, 5786.
(18) (a) Balland, V.; Charlot, M.-F.; Banse, F.; Girerd, J.-J.; Mattioli, T.
A.; Bill, E.; Bartoli, J.-F.; Battioni, P.; Mansuy, D. Eur. J. Inorg. Chem.
2004, 301. (b) Ye, W.; Ho, D. M.; Friedle, S.; Palluccio, T. D.; Rybak-
Akimova, E. V. Inorg. Chem. 2012, 51, 5006. (c) Annaraj, J.; Kim, S.; Seo,
M. S.; Lee, Y.-M.; Kim, Y.; Kim, S.-J.; Choi, Y. S.; Nam, W. Inorg. Chim.
Acta 2009, 362, 1031.
(19) (a) Mas-Balleste, R.; Que, L., Jr. J. Am. Chem. Soc. 2007, 129,
́
15964. (b) Oloo, W. N.; Feng, Y.; Iyer, S.; Parmelee, S.; Xue, G.; Que, L.,
Jr. New J. Chem. 2013, 37, 3411.
(20) Betley has reported a high-spin iron(III)−imido radical complex
that can attack olefin C−H and CC bonds, the only other nonheme
high-spin iron complex to exhibit such reactivity. See: Hennessy, E. T.;
Liu, R. Y.; Iovan, D. A.; Duncan, R. A.; Betley, T. A. Chem. Sci. 2014, 5,
1526. Having more such examples will be useful for comparison.
This work was supported by grants from the National Science
Foundation (CHE-1058248 and CHE-1361773 to L.Q. and
CHE-1305111 to E.M.). A.N.B. thanks the Indo-US Science &
Technology Forum (IUSSTF) for a postdoctoral fellowship, and
M.P. and G.T.R. thank the University of Minnesota for graduate
dissertation fellowships. We thank Mr. Ang Zhou for his
assistance with 19F NMR experiments.
REFERENCES
■
(1) (a) Costas, M.; Mehn, M. P.; Jensen, M. P.; Que, L., Jr. Chem. Rev.
2004, 104, 939. (b) Solomon, E. I.; Brunold, T. C.; Davis, M. I.;
Kemsley, J. N.; Lee, S.-K.; Lehnert, N.; Neese, F.; Skulan, A. J.; Yang, Y.-
S.; Zhou, J. Chem. Rev. 2000, 100, 235.
(2) (a) Krebs, C.; Fujimori, D. G.; Walsh, C. T.; Bollinger, J. M., Jr. Acc.
Chem. Res. 2007, 40, 484. (b) Price, J. C.; Barr, E. W.; Glass, T. E.; Krebs,
C.; Bollinger, J. M., Jr. J. Am. Chem. Soc. 2003, 125, 13008. (c) Sinnecker,
S.; Svensen, N.; Barr, E. W.; Ye, S.; Bollinger, J. M., Jr.; Neese, F.; Krebs,
C. J. Am. Chem. Soc. 2007, 129, 6168.
(3) Xiao, D. J.; Bloch, E. D.; Mason, J. A.; Queen, W. L.; Hudson, M. R.;
Planas, N.; Borycz, J.; Dzubak, A. L.; Verma, P.; Lee, K.; Bonino, F.;
Crocella, V.; Yano, J.; Bordiga, S.; Truhlar, D. G.; Gagliardi, L.; Brown,
̀
C. M.; Long, J. R. Nat. Chem. 2014, 6, 590.
(4) (a) Shaik, S.; Hirao, H.; Kumar, D. Acc. Chem. Res. 2007, 40, 532.
(b) Decker, A.; Rohde, J.-U.; Klinker, E. J.; Wong, S. D.; Que, L., Jr.;
D
DOI: 10.1021/ja511757j
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