Mechanistic insights into the C-H bond activation of hydrocarbons by chromium(IV) oxo and chromium(III) superoxo complexes

Article abstract of DOI:10.1021/ic402831f

The mechanism of the C-H bond activation of hydrocarbons by a nonheme chromium(IV) oxo complex bearing an N-methylated tetraazamacrocyclic cyclam (TMC) ligand, [Cr^IV(O)(TMC)(Cl)]⁺ (2), has been investigated experimentally and theoretically. In experimental studies, reaction rates of 2 with substrates having weak C-H bonds were found to depend on the concentration and bond dissociation energies of the substrates. A large kinetic isotope effect value of 60 was determined in the oxidation of dihydroanthracene (DHA) and deuterated DHA by 2. These results led us to propose that the C-H bond activation reaction occurs via a H-atom abstraction mechanism, in which H-atom abstraction of substrates by 2 is the rate-determining step. In addition, formation of a chromium(III) hydroxo complex, [Cr^III(OH)(TMC)(Cl)] ⁺ (3), was observed as a decomposed product of 2 in the C-H bond activation reaction. The Cr^IIIOH product was characterized unambiguously with various spectroscopic methods and X-ray crystallography. Density functional theory (DFT) calculations support the experimental observations that the C-H bond activation by 2 does not occur via the conventional H-atom-abstraction/oxygen-rebound mechanism and that 3 is the product formed in this C-H bond activation reaction. DFT calculations also propose that 2 may have some Cr^IIIO^?- character. The oxidizing power of 2 was then compared with that of a chromium(III) superoxo complex bearing the identical TMC ligand, [Cr^III(O ₂)(TMC)(Cl)]⁺ (1), in the C-H bond activation reaction. By performing reactions of 1 and 2 with substrates under identical conditions, we were able to demonstrate that the reactivity of 2 is slightly greater than that of 1. DFT calculations again support this experimental observation, showing that the rate-limiting barrier for the reaction with 2 is slightly lower than that of 1.

Full text of DOI:10.1021/ic402831f

Article
pubs.acs.org/IC
Mechanistic Insights into the C−H Bond Activation of Hydrocarbons
by Chromium(IV) Oxo and Chromium(III) Superoxo Complexes
†
,‡
†,‡
†
†
†
,§
Kyung-Bin Cho, Hyeona Kang, Jaeyoung Woo, Young Jun Park, Mi Sook Seo, Jaeheung Cho,*
,
†
and Wonwoo Nam*
†Department of Chemistry and Nano Science, Center for Biomimetic Systems, Ewha Womans University, Seoul 120-750, Korea
§Department of Emerging Materials Science, DGIST, Daegu 711-873, Korea
*
S Supporting Information
ABSTRACT: The mechanism of the C−H bond activation of hydro-
carbons by a nonheme chromium(IV) oxo complex bearing an N-
IV
methylated tetraazamacrocyclic cyclam (TMC) ligand, [Cr (O)(TMC)-
+
(
Cl)] (2), has been investigated experimentally and theoretically. In
experimental studies, reaction rates of 2 with substrates having weak C−H
bonds were found to depend on the concentration and bond dissociation
energies of the substrates. A large kinetic isotope effect value of 60 was
determined in the oxidation of dihydroanthracene (DHA) and deuterated
DHA by 2. These results led us to propose that the C−H bond activation
reaction occurs via a H-atom abstraction mechanism, in which H-atom
abstraction of substrates by 2 is the rate-determining step. In addition,
III
+
formation of a chromium(III) hydroxo complex, [Cr (OH)(TMC)(Cl)]
(
3), was observed as a decomposed product of 2 in the C−H bond
III
activation reaction. The Cr OH product was characterized unambiguously with various spectroscopic methods and X-ray
crystallography. Density functional theory (DFT) calculations support the experimental observations that the C−H bond
activation by 2 does not occur via the conventional H-atom-abstraction/oxygen-rebound mechanism and that 3 is the product
formed in this C−H bond activation reaction. DFT calculations also propose that 2 may have some Cr O character. The
oxidizing power of 2 was then compared with that of a chromium(III) superoxo complex bearing the identical TMC ligand,
III •−
III
+
[
Cr (O )(TMC)(Cl)] (1), in the C−H bond activation reaction. By performing reactions of 1 and 2 with substrates under
2
identical conditions, we were able to demonstrate that the reactivity of 2 is slightly greater than that of 1. DFT calculations again
support this experimental observation, showing that the rate-limiting barrier for the reaction with 2 is slightly lower than that of
1
.
INTRODUCTION
that there is an alternative to the oxygen-rebound mechanism in
■
the C−H bond activation by nonheme iron(IV) oxo and
The C−H bond activation of hydrocarbons by metal-bound
active oxygen species, such as superoxo, peroxo, hydroperoxo,
and oxo, is one of the most important subjects in bioinorganic
8
manganese(IV) oxo complexes. That is, the dissociation of the
substrate radical (Scheme 1B, pathway c), which is formed by
H-atom abstraction by the nonheme iron(IV) oxo and
manganese(IV) oxo complexes (Scheme 1B, pathway a), is
more favorable than the oxygen-rebound process (Scheme 1B,
pathway b). Thus, the latter result casts doubt that the C−H
bond activation by high-valent metal oxo species occurs
invariably via the conventional H-atom-abstraction/oxygen-
rebound mechanism (Scheme 1).
1
,2
and oxidation chemistry. Among the metal-active oxygen
species, high-valent metal oxo intermediates have attracted
particular attention as reactive species in the C−H bond
activation reactions by enzymes and synthetic catalysts. For
instance, high-valent iron(IV) oxo intermediates in heme and
nonheme iron enzymes and their models have been intensively
investigated to elucidate mechanism(s) of the C−H bond
2
−4
Much attention has recently been focused on the reactivities
of metal superoxo species because the intermediates are
postulated to be involved as reactive species in the C−H
bond activation reactions by copper enzymes (e.g., peptidyl-
glycine-α-amidating monooxygenase and dopamine β-mono-
activation by the iron oxo oxidants.
In heme enzymes such
as cytochrome P450 and iron porphyrin models, the C−H
bond activation by iron(IV) oxo porphyrin π-cation radicals,
•+
IV
(
Porp )Fe O (Cpd I), occurs via a rate-determining H-
atom abstraction step (Scheme 1A, pathway a), followed by an
9
IV
oxygenase) and nonheme iron enzymes (e.g., isopenicillin N
oxygen-rebound step between the resulting (Porp)Fe OH and
substrate radical species (Scheme 1A, pathway b). This
5
synthase, myo-inositol oxygenase, and 2-hydroxyethyl phospho-
oxygen-rebound mechanism has been supported strongly in
both experiments and theoretical calculations. In nonheme
iron and manganese models, it has been shown very recently
6
,7
Received: November 12, 2013
©
XXXX American Chemical Society
A
dx.doi.org/10.1021/ic402831f | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Scheme 1. Proposed Mechanisms of C−H Bond Activation
of hydrocarbons and the O-atom transfer (OAT) to substrates
16
(
e.g., the oxidation of phosphine and sulfides). In the latter
IV
reaction, a high-valent chromium(IV) oxo complex, [Cr (O)-
TMC)(Cl)]+ (2; Chart 1), was successfully isolated as a
product and characterized with various spectroscopic methods
(
16b
and X-ray crystallography.
As part of our ongoing efforts to understand the mechanism
of the C−H bond activation by nonheme metal oxo complexes,
we now report for the first time a combined experimental and
computational study on the mechanism of the C−H bond
activation of hydrocarbons by a nonheme chromium(IV) oxo
complex and the direct reactivity comparison of the chromium-
(
III) superoxo and chromium(IV) oxo complexes bearing a
common supporting ligand in C−H bond activation reactions
carried out under identical conditions.
RESULTS AND DISCUSSION
■
Experimental Studies on the C−H Bond Activation by
Chromium(IV) Oxo and Chromium(III) Superoxo Com-
plexes. The chromium(IV) oxo complex 2 was prepared by
reacting its corresponding chromium(III) superoxo complex, 1,
10
nate dioxygenase). In biomimetic studies, it has been shown
that synthetic copper(II) superoxo complexes are capable of
performing C−H bond functionalization of a supporting ligand
and weak O−H and N−H bond activation of external
16b
with 1 equiv of PPh3.
The intermediate 2 was stable enough
to be used in reactivity studies under stoichiometric conditions
17
1
1
(e.g., t1/2 of ∼40 min at −10 °C). Upon the addition of
substrates. In nonheme iron models, although iron(III)
superoxo intermediates have been proposed as active oxidants
cyclohexadiene (CHD) to the solution of 2 in CH CN at −10
3
°
C, the characteristic UV−vis absorption bands of 2
disappeared with a pseudo-first-order decay profile (Figure
a), and product analysis of the reaction solution revealed that
in the course of O activation, no direct evidence for the
2
iron(III) superoxo species participating in the C−H bond
1
activation of hydrocarbons has been obtained yet, probably
12
benzene (43 ± 2% based on 2) was produced as a sole product
in the oxidation of CHD (see the Experimental Section). Clean
isosbestic points were observed at 527, 574, 615, and 755 nm in
this reaction (Figure 1a). A pseudo-first-order fitting of the
because of the nature of its instability and short lifetime. The
lack of direct evidence for the heme and nonheme iron(III)
superoxo species in experiments was complemented by density
functional theory (DFT) calculations, in which the nonheme
iron(III) superoxo species is shown to be an electrophilic
oxidant in H-atom-abstraction reactions, whereas the heme
iron(III) superoxo species is a sluggish oxidant in oxidation
1
3−15
reactions.
In addition, a comparison of the oxidizing power
of heme and nonheme iron(IV) oxo and iron(III) superoxo
intermediates in the C−H bond activation reactions has been
another topic that could be addressed in the computational
1
3
studies.
Very recently, a chromium(III) superoxo complex bearing an
III
N-methylated tetraazamacrocyclic ligand, [Cr (O )(TMC)-
(
2
+
Cl)] (1, where TMC = 1,4,8,11-tetramethyl-1,4,8,11-
tetraazacyclotetradecane; Chart 1), was successfully synthesized
Chart 1
1
6a
by reacting a chromium(II) precursor with O2.
The
intermediate 1 was thermally stable so that we were able to
isolate and use it directly in spectroscopic and structural
characterization as well as in reactivity studies. For example, it
has been shown that 1 is capable of conducting electrophilic
oxidative reactions, such as the activation of weak C−H bonds
Figure 1. (a) UV−vis spectral changes of 2 (2 mM) upon the addition
of CHD (20 equiv to 2, 40 mM) in CH CN at −10 °C. Inset: time
3
course of the decay of 2 monitored at 603 nm. (b) Plot of k against 1/
2
T to determine the activation parameters for the reactions of CHD
with 1 (blue ■) and 2 (red ●).
B
dx.doi.org/10.1021/ic402831f | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
kinetic data gave a kobs value of 3.8 × 10− s−1. The rate
3
2 (see the data with blue and orange colors in Figure 2a). Such
a large KIE value suggests a tunneling behavior in the H-atom-
abstraction reaction, as is frequently observed in H-atom-
abstraction reactions by iron(IV) oxo intermediates of
constant increased linearly with an increase of the CHD
concentration, affording a second-order rate constant (k ) of
2
2
−1 −1
9
.6 × 10−
M
s
at −10 °C (Figure 2a). The reaction rate
19
nonheme iron enzymes and models. It is worth noting that
1
also gave a high KIE value (∼50) in the C−H bond activation
16a
of DHA.
On the basis of the observations of the linear
correlation between the reaction rates and BDEs of the
substrates and the large KIE value, we propose that the C−H
bond activation by 2 occurs via a H-atom-abstraction
mechanism and that the H-atom abstraction is the rate-
determining step, as observed in other metal oxo reac-
8,19,20
tions.
By analysis of the reaction solution of 2 and CHD with cold
spray ionization time-of-flight mass spectrometry (CSI-TOF
MS), we found that a chromium(III) hydroxo complex, 3, was
formed as a decomposed product of 2 in the C−H bond
activation reaction (Figure 3a). We also found an O atom in the
Figure 2. Reaction of 2 with external substrates in CH CN at −10 °C.
3
(
a) Plots of k of 2 against the concentration of xanthene (red ●, k
obs
2
−1
−1 −1
−1
−1 −1
=
4
×
8.6 × 10
M
s ), DHA (blue ▲, k = 2.1 × 10
M
s ), DHA-
2
−3
−1 −1
d (orange ▼, k = 3.5 × 10
M
s ), and CHD (black ■, k = 9.6
2
2
−2
−1 −1
10
M
s ) to determine second-order rate constants. A plot of
−
2
k
M
of 1 against the concentration of CHD (green ◆, k = 2.7 × 10
obs
2
−
1 −1
s ) under the same reaction condition has also been added. (b)
Plot of log keff of 2 against C−H BDEs of xanthene, DHA, and CHD,
showing a correlation of the reaction rates with the BDEs of the
substrates. The k values are adjusted for reaction stoichiometry to
2
yield keff based on the number of equivalent target C−H bonds of the
substrates.
Figure 3. (a) CSI-TOF MS showing the formation of 3 in the reaction
was dependent on the reaction temperature, and a linear Eyring
of 2 and CHD in CH CN at −10 °C. Insets: observed isotope
3
plot was obtained in the range of 243−273 K with activation
III 16
+
distribution patterns for [Cr ( OH)(TMC)(Cl)] at m/z 360.2 and
Cr ( OH)(TMC)(Cl)] at m/z 362.2. (b) ORTEP plot of 3 with a
⧧
−1
⧧
−1
parameters of ΔH = 3.5 kcal mol and ΔS = −49.5 cal mol
III 18
+
[
−1
K
(Figure 1b, red line).
3
0% probability thermal ellipsoid. All H atoms except H1 are omitted
In order to gain mechanistic insight into the C−H bond
for clarity. The hydroxyl proton was found in the Fourier difference
map. Selected bond lengths (Å) and angles (deg): Cr−O1 1.8630(12),
Cr−N1 2.1316(13), Cr−N2 2.1607 (13), Cr−N3 2.1746(14), Cr−N4
2.1477(13), Cr−Cl2 2.3731(4); O1−Cr−Cl2 173.50(4).
activation by 2, we carried out the reaction of 2 with other
substrates having weak C−H bond dissociation energies
−
1
(
BDEs), such as xanthene (75.5 kcal mol ) and 9,10-
−1
dihydroanthracene (DHA; 77 kcal mol ) along with CHD
78 kcal mol ), from which second-order rate constants were
−1
18
III
(
OH group of the Cr OH product derived from the oxo group
18
determined [Figure 2a; Supporting Information (SI), Table
S1]. Product analysis of the reaction solutions revealed that
xanthone (45 ± 5% based on 2) and anthracene (42 ± 2%
based on 2) were produced in the oxidation of xanthene and
DHA, respectively. The rate constants decreased with an
increase of the C−H BDEs of the substrates, giving a linear
correlation with the C−H BDE values of the substrates (Figure
in 2 by carrying out an O isotope labeling experiment with
IV 18
+
18
[Cr ( O)(TMC)(Cl)] (2- O; Figure 3a, upper inset). The
X-ray crystal structure of 3-Cl·CH CN·H O revealed that the
3
2
hydroxide ligand coordinates to the chromium center, affording
a distorted octahedral geometry with a Cr−O bond distance of
1.8630(12) Å (Figure 3b; SI, Tables S2 and S3). On the basis
of the structural and spectroscopic characterization of 3, we
III
2b). We also observed a large deuterium kinetic isotope effect
were able to conclude unambiguously that a Cr OH complex
(
KIE) value of 60(6) in the oxidation of DHA and DHA-d by
is the product formed in the C−H bond activation of
4
C
dx.doi.org/10.1021/ic402831f | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
2
1
hydrocarbons by a nonheme chromium(IV) oxo complex.
III
III
We have shown previously that Fe OH and Mn OH are the
products formed in the C−H bond activation of hydrocarbons
by nonheme iron(IV) oxo and manganese(IV) oxo complexes,
8
respectively. On the basis of the present results, we propose
IV
that the C−H bond activation by the Cr O complex does not
occur via an oxygen-rebound mechanism (Scheme 1B, pathway
b). Instead, dissociation of the substrate radical is more
favorable than the oxygen-rebound process (Scheme 1B,
III
22
pathway c), giving the Cr OH complex product. A similar
mechanism was proposed in the C−H bond activation of
hydrocarbons by nonheme iron(IV) oxo and manganese(IV)
8
oxo complexes.
We then compared the reactivities of the chromium(III)
superoxo (1) and chromium(IV) oxo (2) complexes in the C−
H bond activation of hydrocarbons under identical reaction
Figure 4. TS structures of (a) 1 and (b) 2 performing C−H
abstraction reactions in the S = 1 spin state. Selected bond lengths are
given in angstroms.
conditions (e.g., in CH CN and at −10 °C). In these reactions,
3
2
showed a slightly greater reactivity, compared to that of 1 (SI,
23
Table S1). For example, in the CHD oxidation, 2 (k = 9.6 ×
2
−2
2
−1 −1
1
1
0
0
M
M
s ) was 3.6 times more reactive than 1 (k = 2.7 ×
2
−
−1 −1
s ; Figure 2a, black and green lines; also compare
the data in the SI, Table S1). A linear Eyring plot, obtained in
the range of 243−273 K for the reaction of 1 and CHD,
⧧
−1
afforded activation parameters of ΔH = 7.1 kcal mol and
⧧
−1 −1
ΔS = −38.2 cal mol
K
(Figure 1b). The ΔΔG value of 0.6
−1
kcal mol , which was calculated from the activation parameters
of 1 and 2, supports the greater reactivity of 2 in the C−H
bond activation reaction (see the SI, Table S4; vide infra). As
observed in the reaction of 2, 3 was the product formed in the
16a
C−H bond activation of hydrocarbons by 1. We propose
that the formation of 3 occurs via the formation of an
intermediate 2 in the C−H bond activation by 1. Because the
reactivity of 2 is greater than that of 1, 2 is not detected in this
reaction. Finally, it is worth noting that a chromium(V) oxo
complex bearing the same TMC ligand showed a much lower
reactivity than both 1 and 2 in the C−H bond activation
reactions, although the axial ligand of the chromium(V) oxo
complex was different from that of 1 and 2 [e.g., methoxide for
2
Figure 5. Five valence orbitals d , d , d , σ* , and σ* from the
(
xy
xz
yz
z
xy
II
Cl)Cr (TMC) moiety (left) and the two O orbitals π* and π*
2 y z
III +
2
(
right). In the [Cr (OOH)(TMC)(Cl)] complex, the σ* and π*z
z
orbitals (in gray) mix to form two new σ-type bonding (σ ) and
antibonding (σ* ) orbitals, where the former one becomes doubly
occupied. The d and π* orbitals also mix in a similar fashion, but this
mixing is weak and is therefore best described as an antiferromagnetic
coupling rather than an orbital mixing and pairing.
xz
xz
yz
y
24
chromium(V) oxo vs chloride for 1 and 2].
Theoretical Calculations on the C−H Bond Activation
by Chromium(IV) Oxo and Chromium(III) Superoxo
Complexes. The reactivities of 1 and 2 in the C−H activation
reaction were investigated with DFT as well. CHD was used as
a substrate in both cases. The calculated transition-state (TS)
structures of 1 and 2 in the initial C−H bond activation
reactions are shown in Figure 4. Spin contamination issues have
been reviewed for both 1 and 2, which are presented in the SI.
The optimized geometry of 1 without the substrate shows a
root-mean-square deviation (RMSD) of 0.28 Å compared to
the crystal structure, indicating a fair agreement with experi-
ments. The ground state of 1 was calculated to be S = 1, which
π* and π* orbitals of the triplet oxygen are each singly
z
y
occupied by β electrons. Upon complexation, the orbitals from
2
both sides mix with each other (d with π* and d with π*z).
yz
y
z
We find that d with π* mixing is weak in the S = 1 state of 1.
A complete pairing of the α electron in d with the β electron
yz
y
yz
in π* is not seen because the superoxo moiety has a spin-
y
density distribution of −0.97 (SI, Table S9). Therefore, this
“pairing” is more correctly described as an antiferromagnetic
pairing rather than orbital mixing. However, this antiferromag-
netic coupling is not negligible, as is shown by the energy of its
−1
ferromagnetic counterpart (S = 2), which is 5.3 kcal mol
higher in energy. In contrast, the dz and π* mixing creates two
2
z
25
is also in agreement with experiments. The orbitals involved
in this structure are reminiscent of what has been described
new orbitals (σ and σ* in Figure 5), where electron pairing
xz xz
occurs in the bonding σ orbital. This bonding orbital has 49%
xz
13c
earlier for iron(III) superoxo species.
Figure 5 shows a
contribution from the O moiety and 32% from the Cr d
2
schematic diagram of the valence orbitals involved and their
occupations, where the z axis denotes the Cr−O bond direction
and the x axis denotes the projected direction of the O−O
bond that is perpendicular to the z axis. The simplest way to
orbital, with the rest coming from the TMC and Cl ligands.
Therefore, 1 is from a strict electron spatial distribution point
II
of view more correctly described as a Cr O moiety, which
2
happens to have an unpaired electron count that matches that
of the chromium(III) superoxo species. Mulliken spin-density
distribution indeed shows about 3.3 in α spin on Cr (SI, Table
analyze the orbitals is to consider the structure as a binding
II
between S = 2 of Cr and S = 1 of O . The five valence orbitals
2
III
2
(
here denoted as d , d , d , σ* , and σ* in 1) are usually the
S9), seemingly implicating a high-spin Cr antiferromagneti-
xy xz yz
z
xy
II
2
ones considered active. In Cr , d , d , d , and σ* are singly
cally coupled to a superoxo species. Hence, the exact oxidation
state of the metal depends on the point of view. We follow in
xy xz
yz
z
occupied, whereas σ*xy is unoccupied. From the O side, the
2
D
dx.doi.org/10.1021/ic402831f | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
this paper the latter convention because of its relevance to
spectroscopical assignments.
(Figure 6, gray arrows). If the mixing is complete, each of these
orbitals will have 50% contribution from both Cr and O (Figure
6, blue). In this case, the unpaired electron count gives that the
expected spin-density distribution would be 1.5 on Cr and 0.5
on O. Our calculated spin-density values fall between these
The C−H bond activation by 1 was investigated in three spin
states (S = 0, 1, and 2). The S = 0 state was found to have high
energies throughout the reaction and was ruled out (see the SI,
Table S7). The calculated electronic energy barrier for the
III •−
IV
extreme points (pure Cr O and pure Cr O). Hence, the
−1
IV
reaction for S = 1 was 17.5 kcal mol . The S = 2 state, while
calculations seem to suggest that the supposedly Cr O species,
−1
III •−
not ruled out, has a higher TS at 18.7 kcal mol . Electronic
energy barriers in this range are expected to be close to the
in fact, has some Cr O character mixed into it. Extensive
investigation into whether these are calculation artifacts (so-
called spin contamination) has been detailed in the SI. The
26
experimental free-energy values, and indeed the experimen-
−1
III •−
tally measured activation barrier is 17.2 kcal mol at −10 °C
result of this investigation is that this inclusion of some Cr O
(
SI, Table S4). This reaction is slightly exergonic (−2.7 kcal
character is more consistent with the crystal structure than pure
−1
III
+
IV
IV
mol ), forming a [Cr (OOH)(TMC)(Cl)] species that can
react further to form intermediate 2. At this intermediate stage,
the electron abstracted from the substrate resides in the π*y
orbital. The orbital occupation of the Cr side remains the same,
implicating an S = / chromium(III) hydroperoxo character.
Cr O. Also, this is reminiscent of calculations on any Fe O
species (supposedly 3/0 in spin density on Fe/O), which is
widely known to give spin-density distribution more consistent
III •−
26
with Fe O (i.e., 2/1). However, the exact nature of this
compound is immaterial to our conclusions below even though
it remains to be conclusively experimentally determined. The
orbital discussion below is therefore based on the simplified
3
2
Including the substrate spin, the S = 1 structure differs from the
S = 2 state only by the spin direction on the now-formed
substrate radical. This is also seen by the degenerate energies of
IV
Cr O view, where the orbital mixings are complete.
−1
these states (−2.6 kcal mol for both).
Upon C−H bond activation by 2, one α electron is
For 2, the calculated ground-state spin is also S = 1 (see the
transferred from the substrate to the empty π* orbital
xz
16b
III
SI, Table S6), in agreement with experiments. Comparing
the calculated structure to the crystal one, the obtained RMSD
is 0.26 Å. Interestingly, Mulliken spin-density distribution
shows spins on Cr and O of 2.71 and −0.46, respectively (SI,
Table S10). We can explain this spin polarization by orbital
mixing here as well. The starting point is to consider the CrO
together with the proton. The Cr OH moiety is most stable at
the S = / state, coupled to a β spin on the substrate (in total S
3
2
= 1). Also, like in the case of 1, the reactivity on the S = 0 state
1
III
(leading to S = / Cr OH coupled to a β-spin substrate) is
2
ruled out because of the high energies (see the SI, Table S8).
The initial C−H bond activation on the S = 1 state occurs over
III
•−
−1
complex as a mix of Cr and O . Figure 6 (red) shows that, in
a barrier of 15.8 kcal mol , close to the experimental value of
−
1
1
6.6 kcal mol . Thus, 2 is calculated to have a lower barrier
than 1 (Figure 7), and the calculations are in agreement with
Figure 7. Comparison of the rate-limiting barriers in the C−H
activation reactions of CHD by 1 (red) versus 2 (green).
III •−
IV
Figure 6. Valence orbitals of 2 such as Cr O (red) or Cr O (blue)
2
for the S = 1 state. The d orbital of Cr can mix with the p orbital of
O to form the bonding and antibonding pair of orbitals σz and σ*z
z
z
2
2
(
gray). Similarly, d mixes with p to form π /π* and d mixes with
experiments showing a greater reactivity for 2 (vide supra). The
reaction ends in a stable intermediate 3 (RMSD = 0.18 Å to the
crystal structure, with the only significant difference being the
O−H direction).
xz
x
xz
xz
yz
p to form π /π* . Depending on how well these orbitals from Cr and
O mix with each other, the CrO compound can be characterized as
anything between pure Cr O (no mixing) to Cr O (evenly mixed).
y
yz
yz
III •−
IV
For the ensuing steps after this initial C−H bond activation
by 2, one can envision four scenarios in the cage (i.e.,
this configuration, there are three unpaired α electrons on the
Cr side (d , d , and d ) and one unpaired β electron on the
III
•
‡
8
[(L)M OH CR ] in Scheme 1B): (1) a rebound step where
xz
yz
xy
3
oxo side (p ). The expected spin-density distributions in this
3 returns its OH to the CHD radical to form a hydroxylated
product (Scheme 1B, pathway b), (2) further abstraction of a H
atom from the substrate radical by 3 to form water and benzene
(i.e., desaturation), (3) simply a dissociation of the substrate
radical to react with a second catalyst 2 (Scheme 1B, pathways
x
case on Cr and O are 3 and −1, respectively. Upon orbital
mixing, d mixes with p to form the bonding and antibonding
xz
x
orbital pair denoted as π and π* , respectively. Similarly, π ,
xz
xz
yz
2
z
2
z
π* , σ , and σ* are formed by mixing the other orbitals
yz
E
dx.doi.org/10.1021/ic402831f | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
c and d), or (4) a substrate spin flip to obtain an overall S = 2
than the oxygen-rebound process, as we have shown in both8
nonheme iron(IV) oxo and manganese(IV) oxo reactions.
These results are supported by DFT calculations, showing that
the energy barrier of the substrate radical dissociation is much
lower than those of the oxygen-rebound and desaturation
processes.
spin state that has a lower desaturation barrier. As noted earlier
III
8c
in the similar Mn OH case, a spin flip for an organic radical
27
is likely a relatively slow process compared to other options
available, especially dissociation (vide infra). Hence, we do not
consider the fourth option feasible for this reaction as whole.
The three remaining pathways are depicted in Figure 8. We
We have also shown that the reactivity patterns of the
nonheme chromium(III) superoxo and chromium(IV) oxo
complexes are similar in the C−H bond activation reactions.
For example, the H-atom abstractions by the chromium(III)
superoxo and chromium(IV) oxo complexes are the rate-
determining steps, and large KIE values were obtained in these
reactions. By comparing their reactivities under identical
reaction conditions, the reactivity of the chromium(IV) oxo
complex was found to be slightly greater than that of the
chromium(III) superoxo complex in the C−H bond activation
reactions. Theoretical calculations have shown that the energy
difference between the TSs for the C−H bond activation
reactions of CHD by the chromium(III) superoxo and
−1
chromium(IV) oxo complexes is 1.7 kcal mol , which is in
reasonable agreement with the experimental result (0.6 kcal
−1
mol ). In addition, the calculations show that the chromium-
Figure 8. Potential energy surface on the S = 1 surface of 2 performing
a C−H abstraction reaction (far end) to form the hydroxylated
intermediate 3 (center). The energy barriers for the rebound (left) and
desaturation (right) reactions are high, whereas the dissociation
reaction (near end) is thermoneutral.
III •−
(
IV) oxo species may have some Cr O character mixed,
although future verifications are needed through experiments
and accurate ab initio calculations. Finally, the present results
provide clues that may help us understand the role(s) of the
metal oxo and metal superoxo intermediates involved in C−H
bond activation reactions by nonheme iron and copper
9
,10
have shown earlier that, in C−H bond activation reactions by
enzymes.
IV
IV
both nonheme Fe O and Mn O species, the third option is
8
b,c
preferable, which can be experimentally verified by detecting
Mn /Fe products instead of Mn /Fe products, with a
substrate product yield of 50% relative to the catalyst. Our
calculated barrier values for the rebound (49.3 kcal mol
relative to the intermediate), desaturation (26.4 kcal mol ),
EXPERIMENTAL SECTION
■
III
III
II
II
Materials. All chemicals obtained from Aldrich Chemical Co. were
the best available purity and used without further purification unless
otherwise indicated. Solvents were dried according to published
−1
−1
2
9
18
18
procedures and distilled under argon prior to use.
enriched) was purchased from ICON Services Inc. (Summit, NJ). The
deuterated substrate, DHA-d , was prepared by taking DHA (0.5 g, 2.7
mmol) in DMSO-d (3 mL) along with NaH (0.2 g, 8.1 mmol) under
O (98% O-
2
−
1
and dissociation (0.03 kcal mol ) reactions confirm that the
substrate radical dissociation from the cage is a preferred
4
IV
pathway in the case of Cr O as well. The relatively high
6
3
0
an inert atmosphere. After the deep-red solution was stirred at room
barriers for the rebound and desaturation reactions reflect the
temperature for 8 h, the reaction was quenched with D O (5 mL). The
2
lack of driving force in these reactions. The desaturation
−1
crude product was filtered and washed with copious amounts of H
2
O.
reaction is only 9.4 kcal mol exergonic (in the S = 1 spin
1
H NMR confirmed the >99% deuteration of DHA-d . 1-Cl·
4
state), and the rebound process is even endergonic by 21.1 kcal
(
CH CN)2 and 2-Cl·CH CN·H O were prepared according to
3 3 2
16
−1
mol . While these values (and TSs; see the SI, Table S8) are
literature methods.
considerably lower in the S = 2 state because of favorable
Physical Methods. UV−vis spectra were recorded on a Hewlett-
Packard 8453 diode-array spectrophotometer equipped with a
UNISOKU Scientific Instrument for low-temperature experiments.
Cold spray ionization time-of-flight mass spectrometry (CSI-TOF
MS) spectra were collected on a JMS-T100CS (JEOL) mass
spectrometer equipped with a CSI source. Typical measurement
conditions are as follows: needle voltage, 2.2 kV; orifice 1 current, 50−
2
8
exchange interactions between the unpaired spins, as
mentioned earlier, spin flip to the S = 2 state should still not
be able to compete with dissociation. These results are all in
agreement with the results discussed in the Experimental
III
Section, such as the observation of Cr OH as the metal
product together with 45% organic product (vide supra).
5
00 nA; orifice 1 voltage, 0−20 V; ring lens voltage. 10 V; ion source
temperature, 5 °C; spray temperature, −30 °C. The CSI-TOF MS of 3
was observed by directly infusing the reaction solution into the ion
source through a precooled tube under high air gas pressure. Product
analysis was performed with an Agilent Technologies 6890N gas
chromatograph and a Thermo Finnigan (Austin, TX) FOCUS DSQ
CONCLUSIONS
Mechanistic studies of the C−H bond activation of hydro-
carbons by nonheme chromium(III) superoxo and chromium-
■
(
IV) oxo complexes bearing a common supporting ligand, 1
16
(
dual-stage quadrupole) mass spectrometer interfaced with a Finnigan
and 2, were performed under stoichiometric conditions. In
the C−H bond activation by the chromium(IV) oxo complex,
we have shown that the C−H bond activation reaction occurs
via a H-atom-abstraction mechanism. We have also demon-
strated that the C−H bond activation does not occur via the
conventional H-atom-abstraction/oxygen-rebound mecha-
1
FOCUS gas chromatograph (GC−MS). H NMR spectra were
measured with a Bruker DPX-400 spectrometer. The effective
magnetic moments were determined using the modified H NMR
method of Evans at −10 °C. A WILMAD coaxial insert (sealed
capillary) tubes containing the blank acetonitrile-d solvent (with 1.0%
1
31
3
tetramethylsilane, TMS) only was inserted into the normal NMR
tubes containing the complexes (8 mM) dissolved in acetonitrile-d3
(with 0.05% TMS). The chemical shift of the TMS peak (and/or
5
−7
nism.
Instead, dissociation of the substrate radical formed
via H-atom abstraction from the substrate is more favorable
F
dx.doi.org/10.1021/ic402831f | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
solvent peak) in the presence of the paramagnetic metal complexes
was compared to that of the TMS peak (and/or solvent peak) in the
inner coaxial insert tube. The effective magnetic moment was
ASSOCIATED CONTENT
Supporting Information
X-ray crystallographic data in CIF format, experimental and
DFT details, Figure S1, Table S1−S16, DFT-calculated
coordinates, and the full reference for ref 35 (as ref S3). This
material is available free of charge via the Internet at http://
pubs.acs.org.
■
*
S
1/2
calculated using the equation μ = 0.0618(ΔνT/2f M) , where f is
the oscillator frequency (MHz) of the superconducting spectrometer,
T is the absolute temperature, M is the molar concentration of the
metal ion, and Δν is the difference in frequency (Hz) between the two
31
reference signals.
X-ray Crystallography. Single crystals of 3-Cl·CH CN·H O were
3
2
picked from solutions by a nylon loop (Hampton Research Co.) on a
AUTHOR INFORMATION
Corresponding Authors
E-mail: jaeheung@dgist.ac.kr.
E-mail: wwnam@ewha.ac.kr.
■
handmade copper plate mounted inside a liquid N Dewar vessel at ca.
2
−
40 °C and mounted on a goniometer head in a N cryostream. Data
2
*
collections were carried out on a Bruker SMART APEX II CCD
*
diffractometer equipped with a monochromator in the Mo Kα (λ =
0
.71073 Å) incident beam. The CCD data were integrated and scaled
Author Contributions
These authors contributed equally.
Notes
‡
using the Bruker SAINT software package, and the structure was
32
solved and refined using SHELXTL V 6.12. H atoms were located in
the calculated positions except H1 in a hydroxyl group, which was
found from the Fourier difference map. All non-H atoms were refined
The authors declare no competing financial interest.
with anisotropic thermal parameters. Crystal data for 3-Cl·CH CN·
3
ACKNOWLEDGMENTS
The research was supported by NRF/MEST of Korea through
the CRI (Grant 2-2012-1794-001-1) and GRL (Grant 2010-
0353) programs (to W.N.), Basic Research Program (Grant
2010-0002558 to M.S.S.), and the R&D programs (Grants 13-
BD-0403 and 2013K2A2A4000610 to J.C.).
H O: C H Cl CrN O : monoclinic, P2 , Z = 2, a = 7.35270(10) Å, b
■
2
16 38
2
5
2
1
=
9.7682(2) Å, c = 14.9867(2) Å, β = 94.8750(10)°, V = 1072.49(3)
3
−1
−3
Å , μ = 0.803 mm , ρ
= 1.410 g cm , R1 = 0.0235, wR2 = 0.0497
calcd
for 5146 unique reflections, 252 variables. CCDC-942471 for 3-Cl·
0
CH CN·H O contain the supplementary crystallographic data for this
3
2
paper. These data can be obtained free of charge via www.ccdc.cam.ac.
uk/data_request/cif (or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.; fax (+44) 1223-
REFERENCES
3
36-033 or e-mail deposit@ccdc.cam.ac.uk).
Generation and Characterization of 3-Cl·CH CN·H O. The
■
(
1) (a) Solomon, E. I.; Brunold, T. C.; Davis, M. I.; Kemsley, J. N.;
3
2
II
Lee, S.-K.; Lehnert, N.; Neese, F.; Skulan, A. J.; Yang, Y.-S.; Zhou, J.
Chem. Rev. 2000, 100, 235−349. (b) Meunier, B., Ed. Metal−Oxo and
Metal−Peroxo Species in Catalytic Oxidations; Springer: Berlin, 2000.
c) Abu-Omar, M. M.; Loaiza, A.; Hontzeas, N. Chem. Rev. 2005, 105,
227−2252. (d) Nam, W. Acc. Chem. Res. 2007, 40, 465. and review
articles in the special issue. (e) Gunay, A.; Theopold, K. H. Chem. Rev.
treatment of 2, which was obtained by the reaction of [Cr (TMC)-
(
Cl)](Cl)·(CH CN) (23.1 mg, 0.05 mmol), excess O , and 1 equiv of
3 2 2
16b
PPh in CH CN (1 mL) at −10 °C, with 10 equiv of CHD afforded
3
3
(
2
formation of a light-yellow solution. Single crystals suitable for X-ray
analysis were obtained from layering of diethyl ether on top of the
reaction solution at −40 °C. Yield: 19.5 mg (80%). UV−vis [λ, nm (ε,
−
1
−1
2
010, 110, 1060−1081. (f) Warren, J. J.; Tronic, T. A.; Mayer, J. M.
M
5
cm )] in CH CN at −10 °C (Figure 1a): 360 (330), 440 (160),
3
Chem. Rev. 2010, 110, 6961−7001. (g) Cho, J.; Sarangi, R.; Nam, W.
Acc. Chem. Res. 2012, 45, 1321−1330. (h) Huynh, M. H. V.; Meyer, T.
J. Chem. Rev. 2007, 107, 5004−5064. (i) Mayer, J. M. Acc. Chem. Res.
40 (110). CSI-MS in CH CN at −10 °C (Figure 3a): m/z 360.2110
3
for 3 (calcd m/z 360.1748).
Reactivity Studies. All reactions were run by monitoring UV−vis
spectral changes of reaction solutions, and rate constants were
determined by fitting changes in absorbance at 603 nm for 2.
Reactions were run at least in triplicate, and the data reported
represent the average of these reactions. Typically, complex 2,
prepared by treating 1 with 1 equiv of PPh in CH CN at −10 °C,
2
(
011, 44, 36−46.
2) (a) Denisov, I. G.; Makris, T. M.; Sligar, S. G.; Schlichting, I.
Chem. Rev. 2005, 105, 2253−2277. (b) van Eldik, R. Coord. Chem. Rev.
007, 251, 1649−1662. (c) Shaik, S.; Hirao, H.; Kumar, D. Acc. Chem.
2
Res. 2007, 40, 532−542. (d) Bruijnincx, P. C. A.; van Koten, G.; Klein
Gebbink, R. J. M. Chem. Soc. Rev. 2008, 37, 2716−2744. (e) Krebs, C.;
Fujimori, D. G.; Walsh, C. T.; Bollinger, J. M., Jr. Acc. Chem. Res. 2007,
40, 484−492. (f) Nam, W. Acc. Chem. Res. 2007, 40, 522−531.
(3) (a) Ortiz de Montellano, P. R. Cytochrome P450: Structure,
Mechanism, and Biochemistry, 3rd ed.; Kluwer Academic/Plenum
Publishers: New York, 2005. (b) Shaik, S.; Hirao, H.; Kumar, D. Acc.
Chem. Res. 2007, 40, 532−542. (c) Green, M. T. Curr. Opin. Chem.
Biol. 2009, 13, 84−88. (d) Rittle, J.; Green, M. T. Science 2010, 330,
933−937.
(4) (a) Borovik, A. S. Chem. Soc. Rev. 2011, 40, 1870−1874. (b) de
Visser, S. P.; Rohde, J.-U.; Lee, Y.-M.; Cho, J.; Nam, W. Coord. Chem.
Rev. 2013, 257, 381−393.
(5) (a) Groves, J. T. J. Chem. Educ. 1985, 62, 928−931. (b) Groves, J.
T. Proc. Natl. Acad. Sci. U. S. A. 2003, 100, 3569−3574. (c) Groves, J.
T. J. Inorg. Biochem. 2006, 100, 434−447.
(6) (a) Groves, J. T.; McClusky, G. A. J. Am. Chem. Soc. 1976, 98,
859−861. (b) Groves, J. T.; Van Der Puy, M. J. Am. Chem. Soc. 1976,
98, 5290−5297. (c) Hjelmeland, L. M.; Aronow, L.; Trudell, J.
Biochem. Biophys. Res. Commun. 1977, 76, 541−549. (d) Gelb, M. H.;
Heimbrook, D. C.; Malkonen, P.; Sligar, S. G. Biochemistry 1982, 21,
370−377.
(7) (a) Ogliaro, F.; Harris, N.; Cohen, S.; Filatov, M.; de Visser, S. P.;
Shaik, S. J. Am. Chem. Soc. 2000, 122, 8977−8989. (b) Guallar, V.;
Baik, M.-H.; Lippard, S. J.; Friesner, R. A. Proc. Natl. Acad. Sci. U. S. A.
2003, 100, 6998−7002. (c) Kamachi, T.; Yoshizawa, K. J. Am. Chem.
3
3
was directly used in kinetic studies by adding appropriate amounts of
substrates. After the reactions were completed, pseudo-first-order
fitting of the kinetic data allowed us to determine the kobs values. The
purity of the substrates was checked with GC and GC−MS prior to
use. Products were analyzed by injecting the reaction mixture directly
into GC and GC−MS. Products were identified by comparison with
authentic samples, and product yields were determined by comparison
against standard curves prepared with authentic samples and using
decane as an internal standard.
DFT Calculations. Calculations were performed at the B3LYP/6-
3
3
3
(
11+G(d,p)//LANL2DZ(6-311G* on Cl) level,3 with solvent
4
acetonitrile) modeled through the CPCM scheme (also included
during the optimizations), using and as implemented in the Gaussian
35
0
9 package. The energies reported are electronic energies only
because free-energy calculations in solvent-included optimized systems
2
6,36
introduce a formal error
(nevertheless, calculated ΔG values are
reported in the SI). Spin contamination issues were investigated using
a restricted open-shell approach where applicable as well as spin-
37
projected DFT. Even though there are arguments about using the
38
latter procedure, it remains one of the few DFT tools available to use
when spin contamination is suspected and the restricted open-shell
approach is not applicable. We have also used single-point
39
B2PLYP //UBLYP to evaluate the spin contamination because
double hybrids may yield more accurate results with regard to this
40
issue.
G
dx.doi.org/10.1021/ic402831f | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Soc. 2003, 125, 4652−4661. (d) Scho
̈
neboom, J. C.; Cohen, S.; Lin,
H.; Shaik, S.; Thiel, W. J. Am. Chem. Soc. 2004, 126, 4017−4034.
Angew. Chem., Int. Ed. 2005, 44, 4235−4239. (d) Sastri, C. V.; Lee, J.;
Oh, K.; Lee, Y. J.; Lee, J.; Jackson, T. A.; Ray, K.; Hirao, H.; Shin, W.;
Halfen, J. A.; Kim, J.; Que, L., Jr.; Shaik, S.; Nam, W. Proc. Natl. Acad.
Sci. U. S. A. 2007, 104, 19181−19186. (e) England, J.; Martinho, M.;
(
(
e) de Visser, S. P. J. Am. Chem. Soc. 2006, 128, 9813−9824.
f) Comba, P.; Maurer, M.; Vadivelu, P. J. Phys. Chem. A 2008, 112,
1
3028−13036. (g) Hirao, H.; Que, L., Jr.; Nam, W.; Shaik, S. Chem.
Farquhar, E. R.; Frisch, J. R.; Bominaar, E. L.; Mu
Angew. Chem., Int. Ed. 2009, 48, 3622−3626. (f) England, J.; Guo, Y.;
Farquhar, E. R.; Young, V. G., Jr.; Munck, E.; Que, L., Jr. J. Am. Chem.
Soc. 2010, 132, 8635−8644. (g) Xue, G.; Hont, R. D.; Munck, E.; Que,
L., Jr. Nat. Chem. 2010, 2, 400−405. (h) Seo, M. S.; Kim, N. H.; Cho,
K.-B.; So, J. E.; Park, S. K.; Clemancey, M.; Garcia-Serres, R.; Latour,
̈
nck, E.; Que, L., Jr.
Eur. J. 2008, 14, 1740−1756. (h) Geng, C.; Ye, S.; Neese, F. Angew.
Chem., Int. Ed. 2010, 49, 5717−5720.
̈
(
8) (a) Wu, X.; Seo, M. S.; Davis, K. M.; Lee, Y.-M.; Chen, J.; Cho,
̈
K.-B.; Pushkar, Y. N.; Nam, W. J. Am. Chem. Soc. 2011, 133, 20088−
20091. (b) Cho, K.-B.; Wu, X.; Lee, Y.-M.; Kwon, Y. H.; Shaik, S.;
́
Nam, W. J. Am. Chem. Soc. 2012, 134, 20222−20225. (c) Cho, K.-B.;
Shaik, S.; Nam, W. J. Phys. Chem. Lett. 2012, 3, 2851−2856. (d) Hong,
S.; So, H.; Yoon, H.; Cho, K.-B.; Lee, Y.-M.; Fukuzumi, S.; Nam, W.
Dalton Trans. 2013, 42, 7842−7845.
J.-M.; Shaik, S.; Nam, W. Chem. Sci. 2011, 2, 1039−1045.
(20) (a) Mayer, J. M. Acc. Chem. Res. 1998, 31, 441−450. (b) Borovik,
A. S. Acc. Chem. Res. 2005, 38, 54−61. (c) Gardner, K. A.; Kuehnert, L.
L.; Mayer, J. M. Inorg. Chem. 1997, 36, 2069−2078. (d) Matsuo, T.;
Mayer, J. M. Inorg. Chem. 2005, 44, 2150−2158. (e) Lam, W. W. Y.;
Yiu, S. M.; Yiu, D. T. Y.; Lau, T. C.; Yip, W. P.; Che, C. M. Inorg.
Chem. 2003, 42, 8011−8018. (f) Goldsmith, C. R.; Cole, A. P.; Stack,
T. D. P. J. Am. Chem. Soc. 2005, 127, 9904−9912.
(
9) (a) Prigge, S. T.; Eipper, B. A.; Mains, R. E.; Amzel, L. M. Science
004, 304, 864−867. (b) Chen, P.; Solomon, E. I. Proc. Natl. Acad. Sci.
U. S. A. 2004, 101, 13105−13110. (c) Klinman, J. P. J. Biol. Chem.
006, 281, 3013−3016. (d) Rolff, M.; Tuczek, F. Angew. Chem., Int.
Ed. 2008, 47, 2344−2347.
10) (a) Bollinger, J. M., Jr.; Krebs, C. Curr. Opin. Chem. Biol. 2007,
1, 151−158. (b) van der Donk, W. A.; Krebs, C.; Bollinger, J. M., Jr.
Curr. Opin. Struct. Biol. 2010, 20, 673−683.
11) (a) Kunishita, A.; Ertem, M. Z.; Okubo, Y.; Tano, T.; Sugimoto,
2
2
(
21) Qin, K.; Incarvito, D. I.; Rheingold, A. L.; Theopold, K. H. J.
Am. Chem. Soc. 2002, 124, 14008−14009.
22) We rule out the possibility that the formation of the Cr OH
(
1
III
(
IV
II
product does not result from comproportionation of Cr O and Cr
see the SI, Figure S1), as we have shown in the reactions of nonheme
iron(IV) oxo and manganese(IV) oxo complexes. See refs 8a and 8b.
23) It is of interest to note that 2 is inactive in OAT reactions (i.e.,
(
(
H.; Ohkubo, K.; Fujieda, N.; Fukuzumi, S.; Cramer, C. J.; Itoh, S.
Inorg. Chem. 2012, 51, 9465−9480. (b) Peterson, R. L.; Himes, R. A.;
Kotani, H.; Suenobu, T.; Tian, L.; Siegler, M. A.; Solomon, E. I.;
Fukuzumi, S.; Karlin, K. D. J. Am. Chem. Soc. 2011, 133, 1702−1705.
(
the oxidation of PPh and sulfides). This is how 2 was prepared by
3
reacting 1 with either PPh or thioanisole. See ref 16b.
3
(
24) Cho, J.; Woo, J.; Han, J. E.; Kubo, M.; Ogura, T.; Nam, W.
Chem. Sci. 2011, 2, 2057−2062.
25) The S = 1 ground state of 1 was confirmed by using the H
(
c) Kunishita, A.; Kubo, M.; Sugimoto, H.; Ogura, T.; Sato, K.; Takui,
T.; Itoh, S. J. Am. Chem. Soc. 2009, 131, 2788−2789. (d) Maiti, D.;
Lee, D.-H.; Gaoutchenova, K.; Wurtele, C.; Holthausen, M. C.;
1
(
̈
NMR spectroscopic method of Evans; the magnetic moment is 3.0(2)
μB. See the Experimental Section for details.
Narducci Sarjeant, A. A.; Sundermeyer, J.; Schindler, S.; Karlin, K. D.
Angew. Chem., Int. Ed. 2008, 120, 88−91. (e) Maiti, D.; Fry, H. C.;
Woertink, J. S.; Vance, M. A.; Solomon, E. I.; Karlin, K. D. J. Am.
Chem. Soc. 2007, 129, 264−265. (f) Cramer, C. J.; Tolman, W. B. Acc.
(
26) Cho, K.-B.; Kim, E. J.; Seo, M. S.; Shaik, S.; Nam, W. Chem.
Eur. J. 2012, 18, 10444−10453.
27) Gould, I. R.; Turro, N. J.; Zimmt, M. B. In Advances in Physical
(
̈
Chem. Res. 2007, 40, 601−608. (g) Wurtele, C.; Gaoutchenova, E.;
Organic Chemistry; Gold, V., Bethell, D., Eds.; Academic Press: New
York, 1984; Vol. 20, pp 1−53.
Harms, K.; Holthausen, M. C.; Sundermeyer, J.; Schindler, S. Angew.
Chem., Int. Ed. 2006, 45, 3867−3869. (h) Itoh, S. Curr. Opin. Chem.
Biol. 2006, 10, 115−122.
(28) Shaik, S.; Chen, H.; Janardanan, D. Nat. Chem. 2011, 3, 19−27;
Nat. Chem. 2011, 4, 511.
29) Armarego, W. L. F., Perrin, D. D., Eds. Purification of Laboratory
Chemicals; Pergamon Press: Oxford, U.K., 1997.
30) Goldsmith, C. R.; Jonas, R. T.; Stack, T. D. P. J. Am. Chem. Soc.
002, 124, 83−96.
(
12) (a) Lee, Y.-M.; Hong, S.; Morimoto, Y.; Shin, W.; Fukuzumi, S.;
(
Nam, W. J. Am. Chem. Soc. 2010, 132, 10668−10670. (b) Mukherjee,
A.; Cranswick, M. A.; Chakrabarti, M.; Paine, T. K.; Fujisawa, K.;
(
2
Mu
̈
nck, E.; Que, L., Jr. Inorg. Chem. 2010, 49, 3618−3628.
(
13) (a) Chung, L. W.; Li, X.; Hirao, H.; Morokuma, K. J. Am. Chem.
(
̈
31) (a) Evans, D. F. J. Chem. Soc. 1959, 2003−2005. (b) Loliger, J.;
Soc. 2011, 133, 20076−20079. (b) Cho, K.-B.; Chen, H.; Janardanan,
D.; de Visser, S. P.; Shaik, S.; Nam, W. Chem. Commun. 2012, 48,
Scheffold, R. J. Chem. Educ. 1972, 49, 646−647. (c) Evans, D. F.;
Jakubovic, D. A. J. Chem. Soc., Dalton Trans. 1988, 2927−2933.
2
189−2191. (c) Chen, H.; Cho, K.-B.; Lai, W.; Nam, W.; Shaik, S. J.
Chem. Theory Comput. 2012, 8, 915−926.
14) (a) Chung, L. W.; Li, X.; Sugimoto, H.; Shiro, Y.; Morokuma, K.
(
32) Sheldick, G. M. SHELXTL/PC for Windows XP, version 6.12;
Bruker AXS Inc.: Madison, WI, 2001.
33) (a) Becke, A. D. Phys. Rev. A 1988, 38, 3098−3100. (b) Becke,
(
(
J. Am. Chem. Soc. 2008, 130, 12299−12309. (b) Hirao, H.; Morokuma,
K. J. Am. Chem. Soc. 2009, 131, 17206−17214. (c) Chung, L. W.; Li,
X.; Sugimoto, H.; Shiro, Y.; Morokuma, K. J. Am. Chem. Soc. 2010,
A. D. J. Chem. Phys. 1993, 98, 1372−1377. (c) Becke, A. D. J. Chem.
Phys. 1993, 98, 5648−5652. (d) Lee, C.; Yang, W.; Parr, R. G. Phys.
Rev. B 1988, 37, 785−789.
1
2
(
(
32, 11993−12005. (d) Hirao, H.; Morokuma, K. J. Am. Chem. Soc.
(
34) (a) Barone, V.; Cossi, M. J. Phys. Chem. A 1998, 102, 1995−
001. (b) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. J. Comput.
Chem. 2003, 24, 669−681.
35) Frisch, M. J. et al. Gaussian 09, revisions B.01 and D.01;
Gaussian Inc.: Wallingford, CT, 2009.
36) Ho, J.; Klamt, A.; Coote, M. L. J. Phys. Chem. A 2010, 114,
3442−13444.
37) Yamaguchi, K.; Jensen, F.; Dorigo, A.; Houk, K. N. Chem. Phys.
Lett. 1988, 149, 537−542.
38) Wittbrodt, J. M.; Schlegel, H. B. J. Chem. Phys. 1996, 105,
010, 132, 17901−17909.
2
15) Lai, W.; Shaik, S. J. Am. Chem. Soc. 2011, 133, 5444−5452.
16) (a) Cho, J.; Woo, J.; Nam, W. J. Am. Chem. Soc. 2010, 132,
(
5
958−5959. (b) Cho, J.; Woo, J.; Nam, W. J. Am. Chem. Soc. 2012,
134, 11112−11115.
(
1
(
(
17) We found that 3 is a decomposed product of 2, which is
confirmed by electrospray ionization mass spectrometry (data not
shown).
(
18) (a) Luo, Y.-R. Handbook of bond dissociation energies in organic
(
compounds; CRC Press: New York, 2003. (b) Roth, J. P.; Mayer, J. M.
Inorg. Chem. 1999, 38, 2760−2761. (c) Lam, W. W. Y.; Man, W.-L.;
Lau, T.-C. Coord. Chem. Rev. 2007, 251, 2238−2252.
6
(
574−6577.
39) Grimme, S. J. Chem. Phys. 2006, 124, 034108.
(
40) Menon, A. S.; Radom, L. J. Phys. Chem. A 2008, 112, 13225−
(
19) (a) Price, J. C.; Barr, E. W.; Glass, T. E.; Krebs, C.; Bollinger, J.
1
3230.
M., Jr. J. Am. Chem. Soc. 2003, 125, 13008−13009. (b) Kaizer, J.;
Klinker, E. J.; Oh, N. Y.; Rohde, J.-U.; Song, W. J.; Stubna, A.; Kim, J.;
Mu
̈
nck, E.; Nam, W.; Que, L., Jr. J. Am. Chem. Soc. 2004, 126, 472−
473. (c) Oh, N. Y.; Suh, Y.; Park, M. J.; Seo, M. S.; Kim, J.; Nam, W.
H
dx.doi.org/10.1021/ic402831f | Inorg. Chem. XXXX, XXX, XXX−XXX