Lewis acid trapping of an elusive copper-tosylnitrene intermediate using scandium triflate

Article abstract of DOI:10.1021/ja306674h

High-valent copper-nitrene intermediates have long been proposed to play a role in copper-catalyzed aziridination and amination reactions. However, such intermediates have eluded detection for decades, preventing the unambiguous assignments of mechanisms.

Full text of DOI:10.1021/ja306674h

Communication
pubs.acs.org/JACS
Lewis Acid Trapping of an Elusive Copper−Tosylnitrene Intermediate
Using Scandium Triflate
Subrata Kundu,† Enrico Miceli,† Erik Farquhar,‡ Florian Felix Pfaff,† Uwe Kuhlmann,§ Peter Hildebrandt,§
Beatrice Braun,† Claudio Greco,† and Kallol Ray*,†
†Institut fur Chemie, Humboldt-Universitat zu Berlin, Brook-Taylor-Straße 2, D-12489 Berlin, Germany
̈
̈
‡Case Western Reserve University Center for Synchrotron Biosciences and Center for Proteomics and Bioinformatics, National
Synchrotron Light Source, Brookhaven National Laboratory, Upton, New York 11973, United States
§Institut fur Chemie, Technische Universitat Berlin, Sekr. PC14, Straße des 17 Juni 135, D-10623 Berlin, Germany
̈
̈
S
* Supporting Information
its detailed characterization by spectroscopy and theory, which
enabled an unambiguous assignment of its electronic structure.
Moreover, complex 3-Sc comprising the CuII−N•Ts core rapidly
abstracts H atoms from the strong C−H bonds of cyclohexane,
thus providing a key precedent for the possible involvement of
CuII−N•Ts in oxidation catalysis.
The reaction of the complex [Cu(L1)](BF4) (1-BF4) [L1 =
3,3′-iminobis(N,N-dimethylpropylamine)]7 with 2 equiv of [N-
(p-toluenesulfonyl)imino](2-tert-butylsulfonyl)phenyliodinane
(sPhINTs)8 at 25 °C resulted in the immediate formation of the
bluish-green complex 2 (Scheme 1). Greenish-blue crystals
ABSTRACT: High-valent copper−nitrene intermediates
have long been proposed to play a role in copper-catalyzed
aziridination and amination reactions. However, such
intermediates have eluded detection for decades, prevent-
ing the unambiguous assignments of mechanisms. More-
over, the electronic structure of the proposed copper−
nitrene intermediates has also been controversially
discussed in the literature. These mechanistic questions
and controversy have provided tremendous motivation to
probe the accessibility and reactivity of CuIII−NR/
CuIIN•R species. In this paper, we report a breakthrough
in this field that was achieved by trapping a transient
copper−tosylnitrene species, 3-Sc, in the presence of
scandium triflate. The sufficient stability of 3-Sc at −90 °C
enabled its characterization with optical, resonance Raman,
NMR, and X-ray absorption near-edge spectroscopies,
which helped to establish its electronic structure as
CuIIN•Ts (Ts = tosyl group) and not CuIIINTs. 3-Sc
can initiate tosylamination of cyclohexane, thereby
suggesting CuIIN•Ts cores as viable reactants in oxidation
catalysis.
a
Scheme 1. Formation and Reactivity of 2 and 3-Sc
erminal copper−nitrene units have been proposed as
Treactive intermediates in a number of copper-catalyzed
alkane amination and alkene aziridination reactions.1−4 Under-
standing the structures, properties, and reactivities of such
species is critical for obtaining mechanistic insights into oxidation
catalysis and developing new, selective catalytic reagents and
processes. Although examples of isolated nitrenes of late
transition metals such as Fe,5a Co,5b and Ni5c are known, direct
spectroscopic characterizations of copper−nitrene intermediates
have not been reported, leaving the mechanism ambiguous.
Examples of such species have been characterized only through
theoretical calculations.6 The nature of their ground state is also
ambiguous. The singlet state (with a bent nitrene coordination
mode) and the triplet state (with linear nitrene coordination)
were theoretically calculated to be very close in energy, and the
predicted ground state was found to depend strongly on the
computational method used.6 We report herein the Lewis acid
trapping of an elusive copper−tosylnitrene (CuII−N•Ts) species,
3-Sc, in the presence of Sc(OTf)3 in near-quantitative yields and
a
Color code: Sc, purple; N, blue; S, yellow; O, red; Cu, orange; C,
gray.
suitable for X-ray diffraction [Table S1 in the Supporting
Information (SI)] were obtained by layering of hexane onto a
dichloromethane solution of 2. The molecular structure of 2
determined by X-ray crystallography shows a four-coordinate
copper−tosylamide complex cation (Scheme 1) with the
geometry of copper being best described as distorted tetrahedral.
The Cu−Namide distance of 1.934(2) Å is comparable to that
reported for other CuII−sulfonamide complexes [1.921(2)−
1.954(7) Å].9 The absorption spectrum of 2 displays a broad,
low-intensity band at λmax [εmax] = 660 nm [250 M−1 cm−1] and
extending from 550 to 1050 nm, which is assigned to the low-
Received: July 9, 2012
Published: August 28, 2012
© 2012 American Chemical Society
14710
dx.doi.org/10.1021/ja306674h | J. Am. Chem. Soc. 2012, 134, 14710−14713

Journal of the American Chemical Society
Communication
energy d−d transitions arising from an S = 1/2 d9 CuII center in a
pseudotetrahedral geometry (Figure 1 top; dash-dotted line).
[580 M−1 cm−1] (Figure 1 top; dashed line) was obtained when 1
equiv of Sc(OTf)3 was used in the reaction.12 Interestingly, the
530 nm absorption feature of the new intermediate resembles
that reported by Tolman and co-workers13 for a CuIII−OH
complex and by King et al.14 for the corresponding aryl−CuIII−X
complexes (X = Cl, Br, I). Generation of the purple intermediate,
designated as 3-Sc, was found to be complete immediately upon
addition of the oxidant sPhINTs to a CH2Cl2 solution of 1-BF4
and Sc(OTf)3 at −90 °C, after which it slowly decayed to a CuI
species15 with a half-life of 1600 s (Figure S2). In contrast to the
doublet (S = 1/2) ground state of 2, 3-Sc is diamagnetic (S = 0), as
demonstrated by the 1H NMR resonances spread over a chemical
shift range of 0 to +10 ppm (Figure S3).
3-Sc underwent a two-electron reduction process with a one-
electron reductant such as ferrocene (Fc) at −90 °C with the
corresponding formation of a CuI species15 and ferrocenium
cations (Fc+; 180% yield) (Figure 2); this confirmed that 3-Sc is
Figure 1. Top: Absorption spectra of 1-BF4 (solid line), 2 (dash-dotted
line), and 3-Sc (dashed line) in CH2Cl2 at −90 °C. Inset: rR spectra of 3-
Sc-14N (black), 3-Sc-15N (red) and the 3-Sc decay product at room
temperature (gray) upon excitation at 514 nm. Bands originating from
the solvent are marked with asterisks. Bottom: Normalized XANES
spectra of 1-BF4 (dotted line), 2 (solid line), and 3-Sc (dashed line).
The inset depicts an expansion of the pre-edge region for 2 and 3-Sc.
Figure 2. Changes in the absorption spectra associated with the reaction
of 3-Sc (0.34 mM) with ferrocene (30 equiv) at −90 °C. Inset:
Absorption spectra of 2 (dashed line) (0.36 mM) and the product of its
reaction with ferrocene (30 equiv) in the presence of 1 equiv of
Sc(OTf)3 (solid line) at −90 °C. The yield of Fc+ was determined on the
basis of the known extinction coefficient of its 620 nm band in CH2Cl2 at
−90 °C (ε = 505 M−1 cm−1).
The X-band electron paramagnetic resonance (EPR) spectrum
of the sample frozen to −263 °C after mixing of 1-BF4 and
sPhINTs at 25 °C (Figure S1 in the SI) shows a slightly rhombic
signal with g values (gx = 2.09, gy = 2.07, gz = 2.27) and hyperfine
constants (Ax = 54 × 10−4 cm−1, Ay = 15 × 10−4 cm−1, Az = 128 ×
two oxidation levels above 1-BF4 and that its initial yield was at
least 90%. The two-electron reduction of 3-Sc in the presence of
Fc and also its decay to CuI at elevated temperatures (above
−90 °C) are in contrast to the spontaneous one-electron
reduction of transient 3 to 2 in the absence of Sc3+ ion. This
presumably results from much stronger binding of the Sc3+ ion to
the one-electron-reduced form of 3 due to the increased electron
density on the tosylimido group. This would facilitate further
reduction to a CuI complex, accompanied by removal of the
imido group with protons as tosylamide. Consistent with this
explanation, an independently generated solution of 2 in CH2Cl2
was found to be inefficient in oxidizing Fc at −90 °C in the
absence of Sc3+; in the presence of Sc3+, however, 2 was reduced
to a CuI species with the corresponding formation of Fc+ in 90%
yield (Figure 2 inset). A similar effect of Sc3+ ion was
demonstrated previously by Fukuzumi et al.16 in the reduction
of the complex cation [FeIV(O)(TMC)]2+ (TMC = 1,4,8,11-
tetramethyl-1,4,8,11-tetraazacyclotetradecane) in the presence
of Fc.
10−4 cm−1) consistent with the dx −y ground state of the CuII
center.10 Spin quantitation based on the EPR signal indicated
that at least 95% of the centers contained monomeric CuII
species.
2
2
s
The conversion of 1-BF4 to 2 in the presence of PhINTs
presumably involves the initial formation of a transient Cu−
nitrene intermediate, 3, followed by rapid H-atom abstraction
from the solvent and/or adventitious water (Scheme 1). We
therefore sought to trap the elusive CuIII−NTs/CuII−N•Ts
intermediate 3 before its conversion to 2 and demonstrate its
reactivity in a number of group transfer reactions. Monitoring of
s
the reaction of 1-BF4 with PhINTs in CH2Cl2 by UV−vis
spectroscopy over a range of temperatures from room temper-
ature to −90 °C, however, did not lead to the accumulation or
observation of any intermediate species competent for alkane or
alkene substrate oxidation. A near-quantitative yield of the CuII
species was obtained even at temperatures as low as −90 °C. We
then tried to trap 3 in the presence of a Lewis acid, a strategy that
The resonance Raman (rR) spectrum of 3-Sc using 514 nm
laser excitation in resonance with the 530 nm transition (inset in
Figure 1 top) displays three bands at 570, 660, and 887 cm−1 that
can be attributed to the CuIII−NTs/CuII−N•Ts core on the basis
of 15N isotope labeling. The band at 887 cm−1 showed the largest
3
we successfully used previously to stabilize an elusive S = /2
oxocobalt(IV) complex.11 Indeed, a metastable intermediate
with an intense purple color exhibiting absorption maxima
centered at λmax [εmax] = 530 nm [3500 M−1 cm−1] and 750 nm
14711
dx.doi.org/10.1021/ja306674h | J. Am. Chem. Soc. 2012, 134, 14710−14713

Journal of the American Chemical Society
Communication
underestimation of the calculated 15N/14N shifts by 1−8 cm−1
implies that DFT predicts a slightly different mode composition.
The oxidative reactivity of 3-Sc was examined for the oxidation
of hydrocarbons with C−H bond dissociation energies20 (BDEs)
of 67.9−99.5 kcal mol−1. Product analysis of the reaction
solutions revealed the formation of the amination or
dehydrogenation products in yields of 85−35% (Scheme 1 and
Table S5); the yields decreased with increasing C−H bond
strength. The kinetics of the reaction could be followed by UV-
vis spectroscopy (Figures S7−S9 ) at −90 °C when 1-benzyl-1,4-
dihydronicotinamide (BNAH), xanthene, dihydroanthracene
(DHA), 1,4-cyclohexadiene (CHD), triphenylmethane, fluo-
rene, and toluene were used as substrates.
15N/14N downshift of 24 cm−1, whereas the bands at 570 and 660
cm−1 showed a much weaker sensitivity to 15N labeling with
downshifts of 2 and 4 cm−1, respectively. The rR spectra thus
established the 530 nm band in 3-Sc as having predominantly
nitrene-to-Cu charge transfer character on the basis of the
selective enhancement of modes involving Cu−NTs coordi-
nates.
We then turned to X-ray absorption spectroscopy (XAS) to
probe the Cu oxidation state in 3-Sc directly. As expected,
complexes 1-BF4 and 2 exhibited X-ray absorption near-edge
structure (XANES) features characteristic of CuI and CuII,
respectively (Figure 1 bottom).17 The edge features for 3-Sc
were altered relative to 2, with a hypsochromic shift of ca. 1 eV in
the energies of the two inflection points along the rising edge
(Figure S4). However, while the intensity of the pre-edge feature
weakened significantly in 3-Sc, its position at ca. 8978 eV was
unchanged from that of 2 (Figure S5). Edge energies are known
to be an ambiguous predictor for identifying whether a given
complex is CuII or CuIII, reflecting the fact that the Cu edge
includes 1s-to-4p transitions that are strongly influenced by
donor ligands and geometric structure. Instead, the energy of the
pre-edge feature associated with a 1s-to-3d transition provides
the most reliable basis for identifying CuIII complexes, as this
feature typically is upshifted by 2 eV relative to those of CuII
congeners.13,18 The absence of a hypsochromic shift in the pre-
edge energy strongly suggests that 3-Sc contains a CuII center
bound to a nitrene radical, consistent with a CuII−N•Ts
formulation. The present study therefore adds to the very few
extant reports where direct evidence of metal-bound nitrene
radicals has been obtained.5a,b,19
Unfortunately, the strongly absorbing CH2Cl2 solvent
prevented us from obtaining data of sufficient quality for an
extended X-ray absorption fine structure (EXAFS) analysis of 3-
Sc. Therefore, density functional theory (DFT) calculations were
performed to gain insights into the molecular structure of 3-Sc.
All of the calculations were done at the BP86/TZVP level in
acetone (ε = 21), which successfully reproduced the
experimentally observed geometry and, in particular, the trends
in the variation of the metal−nitrogen distances in 2 (Tables S2
and S3). The overestimation of the metal−nitrogen distances in
the calculated structure by 0.05−0.08 Å is typical of DFT
functionals. The optimized structure of 3-Sc in the exper-
imentally observed singlet state (Scheme 1 and Table S4) reveals
a bent copper−nitrene unit with a κ2-N,O binding mode of the
NSO2R tosylnitrene ligand, consistent with the results of a
previous calculation.6a The Cu−N(Sc)Ts distance in 3-Sc was
calculated to be 0.045 Å shorter than the Cu−NHTs distance in
2 (Table S2). Three Raman-active bands involving the Cu−
N(Sc)Ts core were calculated, and their positions and 15N/14N
isotopic shifts were in reasonable agreement with the
experimental values, thereby validating the proposed structure
of 3-Sc (Figure S6; also see the animation of the vibrational
motions provided in the SI). On the basis of the calculations, the
experimentally observed rR band at 887 cm−1 (15N/14N shift of
24 cm−1) is assigned to a symmetric bending of the N−Cu−S
angle (Figure S6a); the calculated frequency was 882 cm−1 with a
shift of 16 cm−1. The remaining 15N-sensitive rR bands at 660
cm−1 (15N/14N shift of 4 cm−1) and 570 cm−1 (15N/14N shift of 2
cm−1), on the other hand, correspond to Cu−N stretching
modes that are strongly coupled to the vibrations of the tosyl and
scandium moieties; the calculated frequencies were 654 cm−1
with an 15N/14N shift of 3 cm−1 (Figure S6b) and 589 cm−1 with
an 15N/14N shift of 1 cm−1 (Figure S6c), respectively. The slight
The rates of the reactions were found to be linearly correlated
with the BDEs of the substrates (Figure 3), thereby revealing a
Figure 3. Plot of log k′ for reactions with 3-Sc at −90 °C against the
BDEs for different C−H substrates. k′ was determined by dividing the
second-order rate constant (k2) by the number of equivalent H atoms in
the substrate. Inset: Plot of pseudo-first-order rate constants (kobs
)
against different concentrations of DHA and d4-DHA. The point at zero
substrate concentration corresponds to the pseudo-first-order rate
constant for the self-decay of 3-Sc at −90 °C.
rate-determining H-atom abstraction process. Furthermore, a
deuterium kinetic isotope effect (KIE) of 5.1 was obtained when
d4-DHA was used as a substrate at −90 °C (Figure 3 inset). The
reaction of 3-Sc with cyclohexane was found to be too slow
relative to its self-decay at −90 °C; thus, the kinetics could not be
monitored. However, when a solution of 3-Sc was allowed to
warm to 25 °C in the presence of 20 equiv of cyclohexane, a 35%
yield of the aminated products was obtained. Additionally, 3-Sc
could also initiate aziridination of cyclohexene and nitrene
transfer to triphenylphosphine (Figure S10) at −90 °C with the
respective formation of cyclohexene-N-tosylaziridine (60%
yield) and N-(p-toluenesulfonyl)iminotriphenylphosphorane
(85% yield).
The reactivity of 2 was also investigated and compared with
that of 3-Sc. In contrast to the strong oxidizing capabilities of 3-
Sc, complex 2 was found to be a sluggish oxidant that reacted
only with strong reductants such as triphenylphosphine at 25 °C
to form N-(p-toluenesulfonyl)iminotriphenylphosphorane (75%
yield; Table S5); the reaction was orders of magnitude slower
than that with 3-Sc (Figure S11). Additionally, 2 did not react
even at 25 °C with the weak C−H bonds of DHA or CHD in the
presence or absence of scandium.
In summary, we have demonstrated the Lewis acid trapping of
an elusive copper−tosylnitrene complex, 3-Sc, and shown it to be
diamagnetic and two oxidation levels above the CuI complex 1-
BF4. The stabilization of 3-Sc can be attributed to the binding of
14712
dx.doi.org/10.1021/ja306674h | J. Am. Chem. Soc. 2012, 134, 14710−14713

Journal of the American Chemical Society
Communication
Sc3+ to the [CuNTs]+ core, which may help reduce the strong
electron repulsion between the electron-rich nitrene and copper
centers, presumably by lowering the electron density at the
nitrene nitrogen. In other words, the binding of Sc3+ may help to
lessen the H-atom abstraction ability of 3, thereby preventing its
spontaneous decay to the copper(II)−amide species 2 (Scheme
1). Consistent with the above explanation, the use of weaker
Lewis acids such as Y3+, Zn2+, and Ca2+ instead of Sc3+ resulted in
a significantly lower yield of the copper−nitrene intermediate
and increased formation of 2.12 Sufficient stability of 3-Sc at
−90 °C enabled its characterization using a variety of
spectroscopic methods, which helped to establish its electronic
structure as CuII−N•(Sc)Ts with a copper-bound nitrene radical.
Additionally, the vibrational modes of the CuII−N•(Sc)Ts core
have been established through rR and DFT studies, which should
help in their elucidation under catalytic turnover conditions.
Finally, the present report of the stabilization of the CuII−N•Ts
core may validate the existence of the elusive isoelectronic CuIII−
O/CuII−O• units, which have been proposed as reactive
intermediates in a number of chemical and biological oxidation
reactions.21 In particular, the ability of 3-Sc to attack the strong
C−H bonds of cyclohexane is extraordinary in the context of the
known oxidizing capabilities of copper−dioxygen species.22 With
the assumption that the reactivity of the CuII−N•Ts core can be
extended to the CuIII−O/CuII−O• core, the present study
therefore suggests the possible involvement of CuIII−O/CuII−
O• active species in the catalytic cycle of mononuclear copper
monooxygenases.
(b) Fructos, M. R.; Trofimenko, S.; Diaz-Requejo, M. M.; Perez, P. J. J.
Am. Chem. Soc. 2006, 128, 11784.
(4) (a) Badiei, Y. M.; Dinescu, A.; Dai, X.; Palomino, R. M.;
Heinemann, F. W.; Cundari, T. R.; Warren, T. H. Angew. Chem., Int. Ed.
2008, 47, 9961. (b) Li, Z.; Quan, R. W.; Jacobsen, E. N. J. Am. Chem. Soc.
1995, 117, 5889. (c) Diaz-Requejo, M. M.; Perez, P. J.; Brookhart, M.;
Templeton, J. L. Organometallics 1997, 16, 4399.
(5) (a) King, E. R.; Hennesy, E. T.; Betley, T. A. J. Am. Chem. Soc. 2011,
133, 4917. (b) Lyaskovskyy, V.; Suarez, A. I. O.; Lu, H.; Jiang, H.; Zhang,
X. P.; de Bruin, B. J. Am. Chem. Soc. 2011, 133, 12264. (c) Waterman, R.;
Hillhouse, G. L. J. Am. Chem. Soc. 2008, 130, 12628.
(6) (a) Brandt, P.; Sodergren, M. J.; Andersson, P. G.; Norrby, P.-O. J.
̈
Am. Chem. Soc. 2000, 122, 8013. (b) Gillespie, K. M.; Crust, E. J.; Deeth,
R. J.; Scott, P. Chem. Commun. 2001, 785. (c) Tekarli, S. M.; Williams, T.
G.; Cundari, T. R. J. Chem. Theory Comput. 2009, 5, 2959. (d) Cundari,
T. R.; Dinescu, A.; Kazi, A. B. Inorg. Chem. 2008, 47, 10067.
(7) Liang, H.-C.; Zhang, C. X.; Henson, M. J.; Sommer, R. D.; Hatwell,
K. R.; Kaderli, S.; Zuberbuhler, A. D.; Rheingold, A. L.; Solomon, E. I.;
̈
Karlin, K. D. J. Am. Chem. Soc. 2002, 124, 4170.
(8) Macikenas, D.; Skrzypczak-Jankun, E.; Protasiewicz, J. D. J. Am.
Chem. Soc. 1999, 121, 7164.
(9) (a) Gonzalez-Alvarez, M.; Alzuet, G.; Borras, J.; Agudo, L. C.;
Garcia-Granda, S.; Bernardo, J. M. M. J. Inorg. Biochem. 2004, 98, 189.
(b) Macias, B.; Villa, M. V.; Gomez, B.; Borras, J.; Alzuet, G.; Gonzalez-
Alvarez, M.; Castineiras, A. J. Inorg. Biochem. 2007, 101, 444.
(10) Que, L. Jr. Physical Methods in Bioinorganic Chemistry: Spectroscopy
and Magnetism; University Science Books: Sausalito, CA, 2000.
(11) Pfaff, F. F.; Kundu, S.; Risch, M.; Pandian, S.; Heims, F.;
Pryjomska-Ray, I.; Haack, P.; Metzinger, R.; Bill, E.; Dau, H.; Comba, P.;
Ray, K. Angew. Chem., Int. Ed. 2011, 50, 1711.
(12) Other metal ions could also be used to generate the formal CuIII
purple intermediate; however, the yields (Figure S12 and Table S6)
decreased in the order Sc3+ (90%) > Y3+ (54%) > Zn2+ (28%) > Ca2+
(0%), which follows the order of decreasing Lewis acidity of the metal
ions. See: Fukuzumi, S.; Ohkubo, K. J. Am. Chem. Soc. 2002, 124, 10270.
(13) Donoghue, P. J.; Tehranchi, J.; Cramer, C. J.; Sarangi, R.;
Solomon, E. I.; Tolman, W. B. J. Am. Chem. Soc. 2011, 133, 17602.
(14) King, A. E.; Huffman, L. M.; Casitas, A.; Costas, M.; Ribas, X.;
Stahl, S. S. J. Am. Chem. Soc. 2010, 132, 12068.
ASSOCIATED CONTENT
* Supporting Information
■
S
Additional syntheses and characterization data (UV−vis, EPR,
NMR), kinetic data, experimental X-ray diffraction parameters
and crystal data, computational data, and a figure and an
animation showing the vibrational modes arising from the Cu−
NTs core. This material is available free of charge via the Internet
at http://pubs.acs.org.
(15) The formation of CuI was inferred from EPR studies (it was EPR-
silent) and UV−vis studies (no CuII d−d bands were observed).
(16) Fukuzumi, S.; Morimoto, Y.; Kotani, H.; Naumov, P.; Lee, Y.-M.;
Nam, W. Nat. Chem. 2010, 2, 756.
(17) Kau, L. S.; Spira-Solomon, D. J.; Penner-Hahn, J. E.; Hodgson, K.
O.; Solomon, E. I. J. Am. Chem. Soc. 1987, 109, 6433.
AUTHOR INFORMATION
Corresponding Author
kallol.ray@chemie.hu-berlin.de
■
(18) DuBois, J. L.; Mukherjee, P.; Stack, T. D. P.; Hedman, B.;
Solomon, E. I.; Hodgson, K. O. J. Am. Chem. Soc. 2000, 122, 5775.
Notes
The authors declare no competing financial interest.
(19) Lu, C. C.; George, S. D.; Weyhermuller, T.; Bill, E.; Bothe, E.;
̈
Wieghardt, K. Angew. Chem., Int. Ed. 2008, 47, 6384.
ACKNOWLEDGMENTS
(20) Luo, Y.-R. Comprehensive Handbook of Chemical Bond Energies;
CRC Press: Boca Raton, FL, 2007.
■
We gratefully acknowledge financial support of this work from
the Cluster of Excellence “Unifying Concepts in Catalysis” (EXC
314/1), Berlin. XAS data were obtained on Beamline X3B of the
National Synchrotron Light Source (NSLS, Brookhaven Na-
tional Laboratory, Upton, NY). Beamline X3B is operated by the
Case Western Reserve University Center for Synchrotron
Biosciences, supported by NIH Grant P30-EB-009998. NSLS
is supported by the U.S. Department of Energy, Office of Science,
Office of Basic Energy Sciences, under Contract DE-AC02-
98CH10886. We also thank Dr. E. Bill, Prof. F. Neese, and Prof.
(21) (a) Yoshizawa, K.; Kihara, N.; Kamachi, T.; Shiota, Y. Inorg. Chem.
2006, 45, 3034. (b) Crespo, A.; Marti, M. A.; Roitberg, A. E.; Amzel, L.
M.; Estrin, D. A. J. Am. Chem. Soc. 2006, 128, 12817. (c) Hong, S.;
Huber, S. M.; Gagliardi, L.; Cramer, C. C.; Tolman, W. B. J. Am. Chem.
Soc. 2007, 129, 14190.
(22) (a) Lewis, E. A.; Tolman, W. B. Chem. Rev. 2004, 104, 1047.
(b) Himes, R. A.; Karlin, K. D. Curr. Opin. Chem. Biol. 2009, 13, 119.
R. Stoßer for access to the EPR instruments and Prof. C. Limberg
̈
for access to the GC−MS instrument.
REFERENCES
(1) Kwart, H.; Kahn, A. A. J. Am. Chem. Soc. 1967, 89, 1950.
■
(2) Muller, P.; Fruit, C. Chem. Rev. 2003, 103, 2905.
̈
(3) (a) Diaz-Requejo, M. M.; Belderrain, T. R.; Nicasio, M. C.;
Trofimenko, S.; Perez, P. J. J. Am. Chem. Soc. 2003, 125, 12078.
14713
dx.doi.org/10.1021/ja306674h | J. Am. Chem. Soc. 2012, 134, 14710−14713