Tuning the Reactivity of Chromium(III)-Superoxo Species by Coordinating Axial Ligands

Article abstract of DOI:10.1021/acs.inorgchem.5b02068

Metal-superoxo species have attracted much attention recently as key intermediates in enzymatic and biomimetic oxidation reactions. The effect(s) of axial ligands on the chemical properties of metal-superoxo complexes has never been explored previously. In this study, we synthesized and characterized chromium(III)-superoxo complexes bearing TMC derivatives with pendant pyridine and imidazole donors, such as [Cr^III(O₂)(TMC-Py)]²⁺ (1, TMC-Py = 4,8,11-trimethyl-1-(2-pyridylmethyl)-1,4,8,11-tetraazacyclotetradecane) and [Cr^III(O₂)(TMC-Im)]²⁺ (2, TMC-Im = 4,8,11-trimethyl-1-(2-methylimidazolmethyl)-1,4,8,11-tetraazacyclotetradecane). The reactivity of chromium(III)-superoxo complexes binding different axial ligands, such as 1, 2, and [Cr^III(O₂)(TMC)(Cl)]⁺ (3, TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), was then investigated in C-H bond activation and oxygen atom transfer reactions. Kinetic studies revealed that the reactivity of the Cr(III)-superoxo complexes depends on the axial ligands, showing the reactivity order of 1 > 2 > 3 in those electrophilic oxidation reactions. It was also shown that there is a good correlation between the reactivity of the chromium(III)-superoxo complexes and their redox potentials, in which the redox potentials of the chromium(III)-superoxo complexes are in the order 1 > 2 > 3. DFT calculations reproduced the reactivity order between 1 and 3 in both C-H bond activation and oxygen atom transfer reactions, and the latter reaction is described using orbital interactions. The calculations are also in agreement with the experimentally obtained redox potentials. The present results provide the first example showing that the reactivity of metal-superoxo species can be tuned by the electron-donating ability of axial ligands bound trans to the metal-superoxo moiety.

Full text of DOI:10.1021/acs.inorgchem.5b02068

Article
pubs.acs.org/IC
Tuning the Reactivity of Chromium(III)-Superoxo Species by
Coordinating Axial Ligands
Yi Re Goo,† Annada C. Maity,† Kyung-Bin Cho,† Yong-Min Lee,† Mi Sook Seo,† Young Jun Park,†
Jaeheung Cho,*,‡ and Wonwoo Nam*,†
†Department of Chemistry and Nano Science, Ewha Womans University, Seoul 03760, Korea
‡Department of Emerging Materials Science, DGIST, Daegu 42988, Korea
S
* Supporting Information
ABSTRACT: Metal-superoxo species have attracted much attention recently
as key intermediates in enzymatic and biomimetic oxidation reactions. The
effect(s) of axial ligands on the chemical properties of metal-superoxo
complexes has never been explored previously. In this study, we synthesized
and characterized chromium(III)-superoxo complexes bearing TMC
derivatives with pendant pyridine and imidazole donors, such as [CrIII(O2)-
(TMC-Py)]2+ (1, TMC-Py = 4,8,11-trimethyl-1-(2-pyridylmethyl)-1,4,8,11-
tetraazacyclotetradecane) and [CrIII(O2)(TMC-Im)]2+ (2, TMC-Im = 4,8,11-
trimethyl-1-(2-methylimidazolmethyl)-1,4,8,11-tetraazacyclotetradecane).
The reactivity of chromium(III)-superoxo complexes binding different axial
ligands, such as 1, 2, and [CrIII(O2)(TMC)(Cl)]+ (3, TMC = 1,4,8,11-
tetramethyl-1,4,8,11-tetraazacyclotetradecane), was then investigated in C−H
bond activation and oxygen atom transfer reactions. Kinetic studies revealed
that the reactivity of the Cr(III)-superoxo complexes depends on the axial
ligands, showing the reactivity order of 1 > 2 > 3 in those electrophilic oxidation reactions. It was also shown that there is a good
correlation between the reactivity of the chromium(III)-superoxo complexes and their redox potentials, in which the redox
potentials of the chromium(III)-superoxo complexes are in the order 1 > 2 > 3. DFT calculations reproduced the reactivity order
between 1 and 3 in both C−H bond activation and oxygen atom transfer reactions, and the latter reaction is described using
orbital interactions. The calculations are also in agreement with the experimentally obtained redox potentials. The present results
provide the first example showing that the reactivity of metal-superoxo species can be tuned by the electron-donating ability of
axial ligands bound trans to the metal-superoxo moiety.
structural and spectroscopic properties of the metal-superoxo
intermediates in the enzymatic reactions.6,7 A notable example
is the crystal structure of a synthetic nonheme iron(III)-
superoxo complex reported recently.8 Nonetheless, the
chemistry of the metal-superoxo species is less clearly
understood and remains to be elusive.
In the reactivity studies of metal-oxygen species, one of the
frequently discussed subjects is the role of axial ligands in
oxidation reactions by metal-oxygen species, since it has been
shown that the axial ligands of heme enzymes, such as
cytochromes P450 and peroxidases, play a significant role in
tuning the enzyme’s ability in oxidation reactions by high-valent
iron(IV)-oxo porphyrin π-cation radical species (compound I).9
Thus, the axial ligand effect on the reactivities of high-valent
iron(IV)-oxo species has been investigated intensively with
synthetic iron-oxo complexes of heme and nonheme ligands,
thereby demonstrating that the axial ligand indeed affects the
reactivities of the iron-oxo complexes dramatically in hydrogen
atom transfer (HAT) and oxygen atom transfer (OAT)
INTRODUCTION
■
Synthetic metal-oxygen complexes, such as metal-superoxo,
-peroxo, -hydroperoxo, and -oxo, have attracted much attention
in the communities of bioinorganic and biological chemistry,
since these metal-oxygen species are key intermediates in the
dioxygen activation and oxidation reactions by metalloenzymes
and their biomimetic compounds.1,2 A large number of high-
valent metal-oxo complexes bearing heme and nonheme ligands
have been synthesized and investigated intensively in various
oxidation reactions for the past several decades.2 Metal-peroxo
and -hydroperoxo complexes have also been synthesized
successfully and characterized spectroscopically and/or struc-
turally, and their reactivities have been well-demonstrated in
nucleophilic and electrophilic oxidation reactions.2g,3
Metal-superoxo species have been recently proposed as
active oxidants in enzymatic reactions, such as nonheme iron
enzymes (e.g., isopenicillin N synthase, myo-inositol oxygenase,
homoprotocatechuate 2,3-dioxygenase, and cysteine dioxyge-
nases) and copper enzymes (e.g., peptidylglycine-α-amidating
monooxygenase and dopamine β-monooxygenase).4,5 In
biomimetic studies, a number of metal-superoxo complexes
have been synthesized and characterized to understand the
Received: September 8, 2015
© XXXX American Chemical Society
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reactions.10,11 The axial ligand effect has also been demon-
strated in oxidative nucleophilic reactions by iron(III)- and
manganese(III)-peroxo complexes, in which binding of
electron-rich axial ligands increases the nucleophilic reactivity
of the metal(III)-peroxo complexes.12 However, such an axial
ligand effect has never been explored in metal-superoxo species,
probably due to the lesser availability of thermally stable and
well-characterized metal-superoxo complexes for reactivity
studies.
reported previously for the synthesis of 3.13 The intermediates
1 and 2 were metastable (e.g., ca. 5% decay for 1 h at −10 °C),
allowing us to use them for spectroscopic characterization and
reactivity studies.
The UV−vis spectrum of 1 shows an intense band at 320 nm
(ε = 2100 M−1 cm−1) and several weak bands at 455 nm (ε =
130 M−1 cm−1), 527 nm (ε = 170 M−1 cm−1), 605 nm (ε = 60
M−1 cm−1), 624 nm (ε = 50 M−1 cm−1), and 670 nm (ε = 30
M−1 cm−1) (Figure 1a). The CSI-MS of 1 exhibits a prominent
Recently, we reported the synthesis and spectroscopic and
structural characterization of a thermally stable chromium(III)-
superoxo complex, [CrIII(O2)(TMC)(Cl)]+ (3, TMC =
1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane)
(Scheme 1).13 This Cr(III)-superoxo complex was reactive
Scheme 1. Schematic Drawing of Macrocyclic Ligands and
Chromium(III)-Superoxo Complexes Used in This Study
enough to be used in the reactions of HAT (e.g., the oxidation
of xanthene, dihydroanthracene (DHA), cyclohexadiene
(CHD), and fluorene) and OAT (e.g., the oxidation of
phosphines and sulfides).13 In the present study, we
synthesized chromium(III)-superoxo complexes bearing TMC
derivatives with pendant pyridine and imidazole donors, such as
[CrIII(O2)(TMC-Py)]2+ (1, TMC-Py = 4,8,11-trimethyl-1-(2-
pyridylmethyl)-1,4,8,11-tetraazacyclotetradecane) and
[CrIII(O2)(TMC-Im)]2+ (2, TMC-Im = 4,8,11-trimethyl-1-(2-
methylimidazolmethyl)-1,4,8,11-tetraazacyclotetradecane)
(Scheme 1), and used these Cr(III)-superoxo complexes in the
investigation of the axial ligand effect on the reactivity of metal-
superoxo species. We now report experimental and theoretical
results showing that the reactivity of metal-superoxo species is
significantly affected by the axial ligands coordinated trans to
the metal-superoxo moiety in HAT and OAT reactions.
Figure 1. (a) UV−vis spectrum of 1 (0.50 mM) in CH3CN at −10 °C.
Inset shows solution IR spectra of 1 prepared with 16O2 (red line) and
18O2 (blue line). (b) CSI-MS of 1 in CH3CN at −10 °C. Insets show
the observed isotope distribution patterns for {Cr(16O2)(TMC-
Py)(ClO4)}+ (lower) and {Cr(18O2)(TMC-Py)(ClO4)}+ (upper).
The peaks at m/z of 484.2 and 501.2 marked with asterisk are
assigned to {Cr(TMC-Py)(ClO4)}+ and {Cr(OH)(TMC-Py)-
(ClO4)}+, respectively.
ion peak at a mass-to-charge ratio (m/z) of 516.2 (Figure 1b),
whose mass and isotope distribution pattern correspond to
{Cr(O2)(TMC-Py)(ClO4)}+ (calculated m/z of 516.2). When
the reaction was carried out with isotopically labeled 18O2, a
mass peak corresponding to {Cr(18O2)(TMC-Py)(ClO4)}+
appeared at m/z of 520.2 (calculated m/z of 520.2) (Figure
1b, inset). The four mass unit shift on substitution of 16O with
18O indicates that the source of two oxygen atoms in 1 is
molecular O2. Compound 1 is EPR silent, and the effective
magnetic moment of 1 (μeff = 3.3 μB at −40 °C) indicates the
spin state of S = 1 resulting from strong antiferromagnetic
coupling between the CrIII (“d3”, S = 3/2) and the superoxide (S
RESULTS AND DISCUSSION
■
The starting materials, [CrII(TMC-Py)]2+ and [CrII(TMC-
Im)]2+, were prepared by reacting CrCl2 with TMC-Py and
TMC-Im, respectively, in CH3CN under an inert atmosphere.
Blue crystalline powders were isolated by adding NaClO4
[Supporting Information (SI) Experimental Section for
experimental details, and Figure S1 for UV−vis and cold
spray ionization mass spectra (CSI-MS)]. The chromium(III)-
superoxo complexes, [CrIII(O2)(TMC-Py)]2+ (1) and
[CrIII(O2)(TMC-Im)]2+ (2), were then synthesized by
bubbling O2 into the solutions of [CrII(TMC-Py)]2+ and
[CrII(TMC-Im)]2+, respectively, in CH3CN at −10 °C, as
13−15
1
= /2).
The solution infrared (IR) spectrum of 1 was recorded in
CH3CN at −10 °C. Compound 1 prepared under an 16O2
atmosphere exhibits an isotopically sensitive band at 1179
cm−1, which shifts to 1113 cm−1 when 18O2 is used (Figure 1a,
inset). The observed vibrational change is consistent with its
assignment as an O−O stretching vibration on the basis of its
16−18Δν value of 66 cm−1
(
16−18Δνcalc = 67 cm−1). Comparable
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signals were reported in the spectra of end-on chromium(III)-
superoxo complexes, such as [Cr(O2)(cyclam)(H2O)]2+
(1134/1145 (doublet) cm−1), [Cr(O2)(H2O)5]2+ (1166
cm−1), and 3 (1170 cm−1).13,16,17 However, the values are
higher than those of side-on chromium(III)-superoxo com-
plexes (1027−1104 cm−1).14,18 The DFT calculated 16O−16O
bond vibration frequency for 1 is slightly off at 1156 cm−1, but
on the other hand, so is its 18O−18O vibration (1089 cm−1); i.e.,
the DFT calculated 16−18Δνcalc = 67 cm−1 matches the
experiments. On the basis of the spectroscopic characterization
and the calculations presented above, 1 is assigned as a
chromium(III) complex with an “end-on” superoxo group,
[CrIII(O2)(TMC-Py)]2+.
1.7(2) × 10−2 s−1 (SI, Figure S3, inset). The pseudo-first-order
rate constant increased proportionally with the concentration of
CHD, giving a second-order rate constant (k2) of 2.2(2) × 10−1
M−1 s−1 (Figure 2a). Similarly, a second-order rate constant of
Spectroscopic properties of 2 are similar to those of 1. The
UV−vis spectrum of 2 exhibits characteristic bands at 315 nm
(ε = 2100 M−1 cm−1), 450 nm (ε = 170 M−1 cm−1), 515 nm (ε
= 170 M−1 cm−1), 600 nm (ε = 60 M−1 cm−1), 628 nm (ε = 70
M−1 cm−1), and 660 nm (ε = 70 M−1 cm−1) (SI, Figure S2a).
The CSI-MS of 2 shows a prominent signal at m/z of 519.2 (SI,
Figure S2b), corresponding to {Cr(O2)(TMC-Im)(ClO4)}+
(calculated m/z of 519.2). The mass peak shifted to m/z of
523.2 (SI, Figure S2b, inset) when 18O2 was used. Evan’s
method analysis of the 1H NMR data indicates that 2 (μeff = 3.4
μB at −40 °C) has the spin state of S = 1,15 which is EPR silent.
The solution IR spectrum of 2 in CH3CN shows an isotopically
sensitive band at 1196 cm−1, which shifts to 1128 cm−1 on 18O-
substitution (16−18Δν = 68 cm−1; SI, Figure S2a, inset). The
O−O stretching frequency of 2 is comparable to those of 1
(1179 cm−1) and 3 (1170 cm−1).13a,17
The reactivities of chromium(III)-superoxo complexes
bearing different axial ligands were investigated in HAT
reactions (Scheme 2, left). Upon addition of CHD (40 equiv;
▲
Figure 2. Kinetic studies of the reaction of 1 (red line with red ), 2
■
●
(blue line with blue ), and 3 (black line with black ) with external
substrates in CH3CN at −10 °C. Plots of kobs against the concentration
of CHD (a) and PPh3 (b) to determine second-order rate constants.
Scheme 2. Possible Mechanisms for the HAT and OAT
Reactions of 2 with CHD (left) and PPh3 (right),
Respectively
8.9(7) × 10−2 M−1 s−1 was determined in the reaction of 2 and
CHD in CH3CN at −10 °C (Figure 2a and SI, Figure S5).
Hence, the order of the second-order rate constants for the
different ligands was determined to be 1 > 2 > 3 (Table 1 and
Figure 2a).13a Product analysis of the final reaction mixture
revealed the formation of [CrIII(OH)(TMC-Im)]2+ (SI, Figure
S6) and benzene (79(5)%).
Table 1. Second-Order Rate Constants in Substrate
Oxidations and Electrochemical Data for Chromium(III)-
Superoxo Complexes, 1−3, in CH3CN at −10 °C
k2 (M−1 s−1
)
E1/2 (V)
vs SCE
Epc (V)
vs SCE
complex
CHD
PPh3
1
2
2.2(2) × 10−1
8.9(7) × 10−2
2.7(3) × 10−2
2.6(2) × 102
8.1(7) × 10
8.4(6) × 10−1
−0.17
−0.37
−0.52
−0.36
−0.42
−0.55
a
a
3
a
The values obtained in this study are the same as those reported in
ref 13.
80 mM) to 1 in CH3CN at −10 °C, the characteristic UV−vis
absorption bands of 1 disappeared through pseudo-first-order
decay (SI, Figure S3), and product analysis of the reaction
solution revealed the formation of benzene (85(5)%). In
addition, [CrIII(OH)(TMC-Py)]2+ was formed as a product,
and [CrIII(18OH)(TMC-Py)]2+ was produced when 1-18O was
used (SI, Figure S4). The pseudo-first-order fitting of the
kinetic data, monitored at 527 nm, yielded the kobs value of
The axial ligand effect was also investigated for the OAT
reactions by the chromium(III)-superoxo complexes (Scheme
2, right). Addition of triphenylphosphine (PPh3) to the
solutions of 1 and 2 caused the decay of the intermediates
with the concomitant formation of the corresponding
chromium(IV)-oxo complexes as detected by UV−vis and
CSI-MS (SI, Figures S7 and S8). The product analysis of the
reaction solutions with HPLC and 31P NMR revealed the
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formation of OPPh3 in a quantitative yield (Experimental
Section). The pseudo-first-order rate constant increased
proportionally with the concentration of PPh3, giving k2 values
of 2.6(2) × 102 M−1 s−1 for 1 and 8.1(7) × 10 M−1 s−1 for 2
(Table 1 and Figure 2b), indicating that OAT from 1 to PPh3 is
∼3 times faster than that from 2 to PPh3. Further, by
comparing the second-order rate constants of 1 and 2 to that of
3 under the identical reaction conditions, we were able to
demonstrate that 1 and 2 are about 100 times or more reactive
than 3 in the oxidation of the phosphine and the reactivity
order of the chromium(III)-superoxo complexes is 1 > 2 > 3
(Table 1 and Figure 2b). Thus, these results demonstrate that
the reactivity of the chromium(III)-superoxo complexes in
OAT reactions is modulated by the axial ligands bound trans to
the superoxo group.
comparisons (in terms of calculated reaction energies, vide
infra). Interestingly, the one-electron reduced species of both 1
and 3 resulted in chromium(III)-peroxo species instead of
chromium(II)-superoxo. The preferred spin states are, in both
3
cases, high-spin (S = /2). The electron affinities (i.e., the
energy difference between the chromium(III)-peroxo and
chromium(III)-superoxo structures) are 94.4 kcal/mol for 1
and 87.1 kcal/mol for 3. The difference of 7.3 kcal/mol
corresponds to 0.32 V for one-electron transfer, in good
agreement with the experimental CV difference of 0.35 V
between 1 and 3.
Then, we attempted to reproduce the observed reactivity
with DFT as well. The experimental reactivity order of the
chromium(III)-superoxo complexes is 1 > 2 > 3, and 1 is 8-fold
and 310-fold more reactive than 3 in HAT and OAT reactions,
respectively (Table 1). Using the Eyring equation, these
differences correspond to only about 1.1 and 3.0 kcal/mol in
energy barrier differences at −10 °C, respectively. These
differences are within error margins of DFT when compared
directly to experiments. However, the comparisons here would
be between two theoretically calculated rates on very similar
structures using the same computational protocol, presumably
raising the accuracy somewhat. Hence, if the calculations could
support the experimental results by yielding the same reactivity
trend, the combined results of experiments and theory would
be convincing. Figure 4 shows the S = 1 transition states found
The cyclic voltammograms (CVs) of chromium(III)-super-
oxo complexes were investigated to gain insight into the axial
ligand effect on the observed reactivities of the chromium-
superoxo moiety. The redox potentials of chromium(III)-
superoxo complexes were determined to be E1/2 = −0.17 V
versus SCE for 1, E1/2 = −0.37 V versus SCE for 2, and E1/2
=
−0.52 V versus SCE for 3 (Table 1) by the increase of scan rate
because the cyclic voltammograms are all irreversible (SI,
Figure S9). However, the data clearly show that the reduction
peak potentials (Epc) shift in the negative direction upon
binding of more basic axial ligands, yielding the reduction peak
potential order of 1 (−0.36 V vs SCE) > 2 (−0.42 V vs SCE) >
3 (−0.55 V vs SCE). The negative shifts observed are
consistent with the introduction of electron-rich axial ligands
trans to the superoxo group, resulting in the decrease of the
reduction peak potentials (or redox potentials) of the
chromium(III)-superoxo moiety. Such an axial ligand effect
was observed in the reactivity of iron(IV)-oxo complexes.11a
Interestingly, we observed a good linear correlation between
the Epc values of the chromium(III)-superoxo complexes and
the reaction rates of CHD and PPh3 oxidation (Figure 3). To
Figure 4. Examples of transition states for 1 (S = 1) in HAT (left) and
OAT (right) reactions. Atom colors used are red for oxygen, blue for
nitrogen, chrome for chromium, gray for carbon, green for
phosphorus, and white for hydrogen.
Figure 3. Correlation between reactivities and reduction peak
potentials of chromium(III)-superoxo complexes. Plots of log k2
in HAT and OAT reactions with 1. The S = 1 state of 1 is lower
in energy at the reactant state than its S = 2 equivalent (by 4.7
kcal/mol). However, both spin states are approximately at the
same energy level at the transition states (Figure 5 and SI,
Tables S3 and S4). We have earlier reported the DFT-
calculated HAT transition states for the CHD oxidation by 3 at
17.5 and 18.7 kcal/mol in S = 1 and S = 2 spin states,
respectively, above the S = 1 ground state using the same
computational protocol.13c Here, the HAT transition states
obtained for 1 are at 16.5 and 17.0 kcal/mol in respective spin
states above the S = 1 ground state. Hence, the difference of 1.0
kcal/mol between the barriers of 1 and 3 in the S = 1 state
(which are the lowest in energy, Figure 5) shows a good
■
determined in the oxidation of CHD (blue line with blue ) and
●
PPh3 (red line with red ) against the reduction peak potentials of 1−
3 in CH3CN at −10 °C.
conclude the reactivity trend of the chromium(III)-superoxo
complexes, the one having the most positive potential shows
the fast rates in the electrophilic oxidation reactions.
To correlate these results with theory, one-electron reduced
species of 1 and 3 were calculated with density functional
theory (DFT) calculations (see also SI, Tables S1−S15).
Compound 2 was excluded from this theoretical study as the
differences to 1 and 3 were considered too small for meaningful
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is likely not compensated by the small energy gained (0.4 kcal/
mol) compared to performing this reaction in the S = 1 state.
Thus, the energy barrier is assumed to be 10.0 kcal/mol (S = 1)
for 1 and 13.3 kcal/mol (S = 1) for 3. Therefore, the difference
of 3.3 kcal/mol also corresponds nicely to the experimentally
derived 3.0 kcal/mol difference.
The nature of the O−O bond breaking reaction can be
deduced from orbital analysis as well as the Mulliken spin
density distributions (SI, Tables S5 and S6). The chromium-
(III)-superoxo valence orbitals were described previously,13c
and the S = 1 configuration was described as three singly α-
occupied orbitals on Cr (dxy, dxz, and dyz) in combination with a
singly β-occupied orbital π*y on the superoxo moiety (Figure
6). This latter orbital has an α occupation in the case of S = 2.
Figure 6. Orbital interactions during OAT interaction of the
chromium(III)-superoxo species with P(CH3)3. The substrate lone-
pair sp3 orbital forms bonding [π*y + sp3lp] and antibonding [π*y −
lp
sp3lp] orbitals with π*y, which upon O−O bond cleavage results in the
O−P bonding orbital σy and the antibonding σ*y orbital. The electron
in σ*y is subsequently used to fill the lone-pair orbitals of the O bound
to P.
Figure 5. Energy profiles for the HAT reaction from CHD (upper)
and OAT reaction to P(CH3)3 (lower) by 1 and 3. Full lines are S = 1
spin state curves, while dashed lines represent S = 2 spin states.
Transition state values for the HAT reaction from CHD by 3 are taken
from ref 13c.
The orbital interactions are more clearly understood by
investigating the two-step reaction of the S = 2 state first
from a conceptual point of view. The first step involves
interaction between the phosphorus sp3 lone-pair orbital with
π*y on the O2 moiety. This creates a bonding and antibonding
orbital pair [π*y + sp3 ] and [π*y − sp3 ] (Figure 6). Upon
lp
lp
O−O bond breaking, the bonding and antibonding σy/σ*y
orbitals are kept between O−P; σy is doubly occupied, while
σ*y is singly occupied. The lone electron in σ*y will be
relocated to fill the lone-pair (lp) p-orbitals of the phosphor
bound oxygen. In the S = 1 state, all this will occur
simultaneously. This conceptual view can be made more
complicated by invoking participation of metal d orbitals as well
as hybridization in the reaction. Figure S10 in SI shows a more
detailed view of the orbital transformations.
correlation to the experimentally derived 1.1 kcal/mol
difference. To gauge how much the different charges of the
axial ligands affect the system, we added a counterion
([ClO4]−) to the modeling of S = 1 reactivity of 1 with
CHD, thus making the total system charge the same as in 3
(i.e., 1+). The differences in reaction energies (Table S3),
Mulliken spin density distribution (Table S8), and geometries
(Table S13) between the systems with and without [ClO4]−
were found to be negligible. We therefore conclude that,
despite the different ligand charges, the charge difference by
itself does not contribute to the reactivity differences.
For the OAT reaction, the reactions of 1 and 3 with a model
substrate P(CH3)3 were also calculated by DFT. The S = 1 and
S = 2 spin states again show similar transition state heights
(Figure 5 and SI, Tables S2 and S3). For the S = 2 reaction,
there is a prereaction step where the substrate P donates about
0.6 spin to the superoxo moiety of 1 or 3, forming something
akin to CrIII-peroxo species before the OAT reaction itself.
However, these prereaction species are only stable at low basis
set level, and are not minima when energy is corrected by larger
basis set (see Methods Section). Also, even though the
transition state heights are similar, the end CrIVO product is
clearly preferred in its S = 1 spin state. Therefore, in order to
utilize the S = 2 transition state, the system has to flip one
electron spin at the beginning of the reaction, and do it once
more after the reaction as well. The probability of this occurring
CONCLUSION
■
We have synthesized and characterized mononuclear
chromium(III)-superoxo complexes bearing different axial
ligands. We have also shown that the reactivity of the
chromium(III)-superoxo complexes depends on the electron-
donating ability of the axial ligands in electrophilic oxidation
reactions; the observed reactivity order of 1 > 2 > 3 indicates
that the electrophilicity of the chromium(III)-superoxo moiety
can be tuned by the electron-donating ability of the axial
ligands. The redox potential order of 1 > 2 > 3, determined by
CV measurements, provides the definitive evidence supporting
that the electron-rich chromium(III)-superoxo species is less
reactive in the electrophilic oxidation reactions, as observed in
high-valent metal-oxo species.2,11a DFT calculations on 1 and 3,
as well as their one-electron reduced species, confirm the above
CV results. Also, the reactivity calculations surpassed the
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by bubbling 18O2 into a solution containing [Cr(TMC-Py)](ClO4)2 in
CH3CN at −10 °C. Spectroscopic data, including UV−vis, CSI-MS,
and solution IR, were reported in Figure 1. Caution! Perchlorate salts
are potentially explosive and should be handled with care!
Generation and Characterization of 2. Bubbling O2 directly
through a blue solution of [Cr(TMC-Im)](ClO4)2 (5.8 mg, 10 μmol)
in CH3CN (2.0 mL) at −10 °C produces a violet intermediate,
[Cr(O2)(TMC-Im)]2+ (2). [Cr(18O2)(TMC-Im)]2+ was also prepared
by bubbling 18O2 into a solution containing [Cr(TMC-Im)](ClO4)2 in
CH3CN at −10 °C. Spectroscopic data, including UV−vis, CSI-MS,
and solution IR, were reported in SI, Figure S2.
Reactivity Studies and Product Analysis. All reactions were run
by monitoring UV−vis spectral changes of reaction solutions, and rate
constants were determined by fitting the changes in absorbance at 527
nm for 1 and 515 nm for 2. Reactions were run at least in triplicate,
and the data reported represent the average of these reactions. After
the completion of reactions, pseudo-first-order fitting of the kinetic
data allowed us to determine kobs values. Products formed in the
oxidation of PPh3 and CHD by 1 and 2 in CH3CN at −10 °C were
analyzed by 1H NMR and HPLC. Quantitative analysis was conducted
on the basis of comparison between standard curves and authentic
samples. In the oxidation of PPh3 and CHD by 1, OPPh3 (93(5)%)
and benzene (85(5)%) were obtained as a major product. OPPh3
(90(6)%) and benzene (79(5)5%) were formed as a sole product in
the oxidation of PPh3 and CHD by 2, respectively. Product yields in
the oxidation of PPh3 were also confirmed by 31P NMR measure-
ments.
expectations as the rate differences match closely to the
experimental rates. The calculations also show that the
transition states of both S = 1 and S = 2 states lie close to
each other, in principle allowing the reaction to proceed in both
spin states. Inspection of the participating orbitals shows that
this is because the only difference between the spin states is in
the π*y orbital (α or β spin), and at least in the initial stages of
the reaction, no chromium orbitals are involved in both the
spin states. Despite this, a reaction in the S = 1 spin state would
avoid potentially complicated spin flips, particularly in the OAT
reaction. We are currently investigating the axial ligand effect(s)
on the chemical properties and reactivities of a mononuclear
nonheme iron(III)-superoxo complex reported recently.8
METHODS SECTION
■
Materials. All chemicals obtained from Aldrich Chemical Co. were
of the best available purity and used without further purification unless
otherwise indicated. Solvents were dried according to published
procedures and distilled under Ar prior to use.19 18O2 (98% 18O-
enriched) was purchased from ICON Services Inc. (Summit, NJ).
[Cr(O2)(TMC)(Cl)](Cl)·2CH3CN was prepared according to the
literature method.13a The TMC-Py and TMC-Im ligands, and
[Cr(TMC-Py)](ClO4)2 and [Cr(TMC-Im)](ClO4)2, were prepared
according to the literature method (see also SI Experimental Section
for experimental details).20
Theoretical Methods. Geometry optimizations and frequency
calculations have been done on an unrestricted UB3LYP/LanL2DZ
level,22 except for Cl and P atoms which were assigned 6-311G*.
Solvent (acetonitrile) effects were also added during optimizations
through the CPCM scheme.23 Single-point energy corrections has
been done with the 6-311+G(d,p) basis set. Gaussian 0924 was used
throughout. Although the free energies (including dispersion25) were
calculated and included in SI, Tables S1−S5, we tend to favor bare
electronic energies due to additional uncertainties introduced during
free energy corrections.26 Hence, all the energies mentioned in this
text are electronic energies only.
Instrumentation. 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 mass spectra (CSI-MS) were collected on a
JEOL JMS-T100CS spectrometer. Solution infrared (IR) spectra were
obtained on a Remspec 626 spectrometer in CH3CN at −10 °C under
Ar atmosphere. Electrochemical measurements were performed on a
CH Instrument (CHI630B) electrochemical analyzer in deaerated
CH3CN in the presence of 0.1 M Bu4NPF6 as a supporting electrolyte.
A conventional three-electrode cell was used with a platinum working
electrode (surface area of 0.3 mm2) and a platinum wire as a counter
electrode. The platinum working electrodes (BAS) were routinely
polished with BAS polishing alumina suspension and rinsed with
CH3CN before use. The measured potentials were recorded as a
function of Ag/Ag+ (0.01 M) reference electrode. All potentials (vs
Ag/Ag+) were converted to values versus SCE by adding 0.29 V.21 All
electrochemical measurements were performed under Ar atmosphere.
1H and 31P NMR spectra for spin state measurements and product
analyses, respectively, were measured with a Bruker AVANCE III 400
spectrometer. The effective magnetic moments were determined using
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b02068.
Experimental and theoretical sections, and DFT details
and coordinates (PDF)
15
1
the modified H NMR method of Evans at −40 °C. A WILMAD
coaxial insert (sealed capillary) tubes containing the blank acetonitrile-
d3 solvent (with 1.0% TMS) only was inserted into the normal NMR
tubes containing the complexes (4.0 mM) dissolved in acetonitrile-d3
(with 0.03% TMS). The chemical shift of the TMS peak (and/or
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 moments were
calculated using the equation, μ = 0.0618(ΔνT/2fM)1/2, where f is the
oscillator frequency (MHz) of the superconducting spectrometer, T is
the absolute temperature, M is the molar concentration of the
chromium complexes, and Δv is the difference in frequency (Hz)
between the two reference signals.15c CW-EPR spectra were taken at
5.0 K using an X-band Bruker EMX-plus spectrometer equipped with a
dual mode cavity (ER 4116DM). Low temperatures were achieved and
controlled using an Oxford Instruments ESR900 liquid He quartz
cryostat with an Oxford Instruments ITC503 temperature and gas flow
controller. Product analysis was conducted with Waters Pump Series
P580 equipped with a various wavelength UV-200 detector (HPLC).
Generation and Characterization of 1. Bubbling O2 directly
through a blue solution of [Cr(TMC-Py)](ClO4)2 (5.8 mg, 10 μmol)
in CH3CN (2.0 mL) at −10 °C produces a violet intermediate,
[Cr(O2)(TMC-Py)]2+ (1). [Cr(18O2)(TMC-Py)]2+ was also prepared
AUTHOR INFORMATION
Corresponding Authors
*E-mail: jaeheung@dgist.ac.kr.
*E-mail: wwnam@ewha.ac.kr.
Author Contributions
■
Y.R.G., A.C.M., and K.-B.C. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
W.N. acknowledges the financial support from the NRF of
KOREA through the CRI (NRF-2012R1A3A2048842) and
GRL (NRF-2010-00353). K.-B.C. and Y.J.P. acknowledge
Ministry of Science, ICT and Future Planning (NRF-
2013R1A1A2062737 and NRF-2013R1A1A2061528, respec-
tively). J.C. acknowledges the NRF (2014R1A1A2056051), the
Ministry of Science, ICT and Future Planning (DGIST R&D
Program 15-BD-0403
&
15-HRLA-02, KCRC
F
DOI: 10.1021/acs.inorgchem.5b02068
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Chen, H.; Janardanan, D.; de Visser, S. P.; Shaik, S.; Nam, W. Chem.
Commun. 2012, 48, 2189−2191.
(8) Hong, S.; Sutherlin, K. D.; Park, J.; Kwon, E.; Siegler, M. A.;
Solomon, E. I.; Nam, W. Nat. Commun. 2014, 5, 5440.
2014M1A8A1049320, and 2015M3D3A1064890), and the
Ministry of Oceans and Fisheries (Marine Biotechnology
Program 20150220) of Korea.
(9) (a) Green, M. T. Curr. Opin. Chem. Biol. 2009, 13, 84−88.
(b) Green, M. T.; Dawson, J. H.; Gray, H. B. Science 2004, 304, 1653−
1656. (c) Rittle, J.; Green, M. T. Science 2010, 330, 933−937.
(d) Yosca, T. H.; Rittle, J.; Krest, C. M.; Onderko, E. L.; Silakov, A.;
Calixto, J. C.; Behan, R. K.; Green, M. T. Science 2013, 342, 825−829.
(10) (a) Takahashi, A.; Yamaki, D.; Ikemura, K.; Kurahashi, T.;
Ogura, T.; Hada, M.; Fujii, H. Inorg. Chem. 2012, 51, 7296−7305.
(b) Kumar, D.; Sastry, G. N.; de Visser, S. P. Chem. - Eur. J. 2011, 17,
6196−6205. (c) Takahashi, A.; Kurahashi, T.; Fujii, H. Inorg. Chem.
2009, 48, 2614−2625. (d) Song, W. J.; Ryu, Y. O.; Song, R.; Nam, W.
JBIC, J. Biol. Inorg. Chem. 2005, 10, 294−304. (e) Gross, Z.; Nimri, S.;
Barzilay, C. M.; Simkhovich, L. JBIC, J. Biol. Inorg. Chem. 1997, 2,
492−506.
REFERENCES
■
(1) (a) Nam, W. Acc. Chem. Res. 2007, 40, 465 and review articles in
the special issue. (b) Holm, R. H.; Solomon, E. I. Chem. Rev. 2014,
114, 3367−3368 and review articles in the special issue . (c) Ortiz de
Montellano, P. R. Cytochrome P450: Structure, Mechanism, and
Biochemistry, 3rd ed.; Kluwer Academic/Plenum Publishers: New
York, 2005. (d) Kovaleva, E. G.; Lipscomb, J. D. Nat. Chem. Biol. 2008,
4, 186−193. (e) Ortiz de Montellano, P. R. Chem. Rev. 2010, 110,
932−948. (f) McQuarters, A. B.; Wolf, M. W.; Hunt, A. P.; Lehnert, N.
Angew. Chem., Int. Ed. 2014, 53, 4750−4752.
(2) (a) Nam, W. Acc. Chem. Res. 2007, 40, 522−531. (b) Bruijnincx,
P. C. A.; van Koten, G.; Klein Gebbink, R. J. M. Chem. Soc. Rev. 2008,
37, 2716−2744. (c) Gunay, A.; Theopold, K. H. Chem. Rev. 2010, 110,
1060−1081. (d) Borovik, A. S. Chem. Soc. Rev. 2011, 40, 1870−1874.
(e) Costas, M. Coord. Chem. Rev. 2011, 255, 2912−2932.
(f) Hohenberger, J.; Ray, K.; Meyer, K. Nat. Commun. 2012, 3, 720.
(g) Cho, J.; Sarangi, R.; Nam, W. Acc. Chem. Res. 2012, 45, 1321−
1330. (h) Nam, W.; Lee, Y.-M.; Fukuzumi, S. Acc. Chem. Res. 2014, 47,
1146−1154. (i) Ray, K.; Pfaff, F. F.; Wang, B.; Nam, W. J. Am. Chem.
Soc. 2014, 136, 13942−13958.
(3) (a) Cho, J.; Jeon, S.; Wilson, S. A.; Liu, L. V.; Kang, E. A.;
Braymer, J. J.; Lim, M. H.; Hedman, B.; Hodgson, K. O.; Valentine, J.
S.; Solomon, E. I.; Nam, W. Nature 2011, 478, 502−505. (b) Liu, L.
V.; Hong, S.; Cho, J.; Nam, W.; Solomon, E. I. J. Am. Chem. Soc. 2013,
135, 3286−3299. (c) Kim, Y. M.; Cho, K.-B.; Cho, J.; Wang, B.; Li, C.;
Shaik, S.; Nam, W. J. Am. Chem. Soc. 2013, 135, 8838−8841. (d) So,
H.; Park, Y. J.; Cho, K.-B.; Lee, Y.-M.; Seo, M. S.; Cho, J.; Sarangi, R.;
Nam, W. J. Am. Chem. Soc. 2014, 136, 12229−12232.
(11) (a) 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.; Shaik, S.;
Nam, W. Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 19181−19186.
(b) Hirao, H.; Que, L., Jr.; Nam, W.; Shaik, S. Chem. - Eur. J. 2008, 14,
1740−1756. (c) Dhuri, S. N.; Seo, M. S.; Lee, Y.-M.; Hirao, H.; Wang,
Y.; Nam, W.; Shaik, S. Angew. Chem., Int. Ed. 2008, 47, 3356−3359.
(12) (a) Wertz, D. L.; Valentine, J. S. Struct. Bonding (Berlin) 2000,
97, 37−60. (b) Annaraj, J.; Cho, J.; Lee, Y.-M.; Kim, S. Y.; Latifi, R.; de
Visser, S. P.; Nam, W. Angew. Chem., Int. Ed. 2009, 48, 4150−4153.
(13) (a) Cho, J.; Woo, J.; Nam, W. J. Am. Chem. Soc. 2010, 132,
5958−5959. (b) Cho, J.; Woo, J.; Nam, W. J. Am. Chem. Soc. 2012,
134, 11112−11115. (c) Cho, K.-B.; Kang, H.; Woo, J.; Park, Y. J.; Seo,
M. S.; Cho, J.; Nam, W. Inorg. Chem. 2014, 53, 645−652.
(14) Qin, K.; Incarvito, C. D.; Rheingold, A. L.; Theopold, K. H.
Angew. Chem., Int. Ed. 2002, 41, 2333−2335.
(15) (a) Evans, D. F. J. Chem. Soc. 1959, 2003−2005. (b) Loliger, J.;
̈
Scheffold, R. J. Chem. Educ. 1972, 49, 646−647. (c) Evans, D. F.;
Jakubovic, D. A. J. Chem. Soc., Dalton Trans. 1988, 2927−2933.
(16) (a) Bakac, A. J. Am. Chem. Soc. 1997, 119, 10726−10731.
(b) Vasbinder, M. J.; Bakac, A. Inorg. Chem. 2007, 46, 2921−2928.
(17) We have reported the O−O stretching frequency of 3 (1170/
1104 cm−1) using resonance Raman (rRaman) in ref 13a. We calculate
here the O−O vibration frequencies for 3 with DFT as (1157/1091);
i.e., 16−18Δνcalc = 66 cm−1 matches the experiments. However, we
could not obtain the rRaman data of 1 and 2 because of laser
decomposition. The O−O stretches of 1 (1179/1113 cm−1) and 2
(1196/1128 cm−1) were obtained by solution IR spectroscopy. For
comparison, the O−O stretch frequency of 3 (1170/1096 cm−1),
which is almost identical to the value obtained from rRaman, was also
obtained using solution IR.
(4) (a) Bollinger, J. M., Jr.; Krebs, C. Curr. Opin. Chem. Biol. 2007,
11, 151−158. (b) van der Donk, W. A.; Krebs, C.; Bollinger, J. M., Jr.
Curr. Opin. Struct. Biol. 2010, 20, 673−683.
(5) (a) Prigge, S. T.; Eipper, B. A.; Mains, R. E.; Amzel, L. M. Science
2004, 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.
2006, 281, 3013−3016. (d) Rolff, M.; Tuczek, F. Angew. Chem., Int.
Ed. 2008, 47, 2344−2347.
(6) (a) Itoh, S. Acc. Chem. Res. 2015, 48, 2066. (b) Lee, J. Y.; Karlin,
K. D. Curr. Opin. Chem. Biol. 2015, 25, 184−193. (c) Pirovano, P.;
Magherusan, A. M.; McGlynn, C.; Ure, A.; Lynes, A.; McDonald, A. R.
Angew. Chem., Int. Ed. 2014, 53, 5946−5950. (d) Lee, J. Y.; Peterson,
R. L.; Ohkubo, K.; Garcia-Bosch, I.; Himes, R. A.; Woertink, J.; Moore,
C. D.; Solomon, E. I.; Fukuzumi, S.; Karlin, K. D. J. Am. Chem. Soc.
2014, 136, 9925−9937. (e) 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. (f) Kunishita,
A.; Kubo, M.; Sugimoto, H.; Ogura, T.; Sato, K.; Takui, T.; Itoh, S. J.
Am. Chem. Soc. 2009, 131, 2788−2789. (g) Maiti, D.; Lee, D.-H.;
(18) Cramer, C. J.; Tolman, W. B.; Theopold, K. H.; Rheingold, A. L.
Proc. Natl. Acad. Sci. U. S. A. 2003, 100, 3635−3640.
(19) Armarego, W. L. F.; Chai, C. L. L. Purification of Laboratory
Chemicals, 6th ed.; Pergamon Press: Oxford, U.K., 2009.
(20) (a) Thibon, A.; England, J.; Martinho, M.; Young, V. G., Jr.;
Frisch, J. R.; Guillot, R.; Girerd, J.-J.; Munck, E.; Que, L., Jr.; Banse, F.
̈
Gaoutchenova, K.; Wurtele, C.; Holthausen, M. C.; Sarjeant, A. A. N.;
̈
Angew. Chem., Int. Ed. 2008, 47, 7064−7067. (b) Halfen, J. A.; Young,
Sundermeyer, J.; Schindler, S.; Karlin, K. D. Angew. Chem., Int. Ed.
2008, 47, 82−85. (h) Cramer, C. J.; Tolman, W. B. Acc. Chem. Res.
2007, 40, 601−608. (i) Itoh, S. Curr. Opin. Chem. Biol. 2006, 10, 115−
122.
(7) (a) Schax, F.; Suhr, S.; Bill, E.; Braun, B.; Herwig, C.; Limberg, C.
Angew. Chem., Int. Ed. 2015, 54, 1352−1356. (b) Chiang, C.-W.;
Kleespies, S. T.; Stout, H. D.; Meier, K. K.; Li, P.-Y.; Bominaar, E. L.;
V. G., Jr. Chem. Commun. 2003, 2894−2895.
(21) Mann, C. K.; Barnes, K. K. In Electrochemical Reactions in Non-
Aqueous Systems; Marcel Dekker: New York, 1970.
(22) (a) Becke, A. D. Phys. Rev. A: At., Mol., Opt. Phys. 1988, 38,
3098−3100. (b) Becke, 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: Condens. Matter Mater. Phys. 1988,
37, 785−789. (e) Dunning, T. H., Jr.; Hay, P. J. Modern Theoretical
Chemistry; Schaefer, H. F., III, Ed.; Plenum: New York, 1976; pp 1−
28. (f) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270−283.
(g) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299−310.
(h) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284−298.
(23) (a) Barone, V.; Cossi, M. J. Phys. Chem. A 1998, 102, 1995−
2001. (b) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. J. Comput.
Chem. 2003, 24, 669−681.
Que, L., Jr.; Munck, E.; Lee, W.-Z. J. Am. Chem. Soc. 2014, 136,
̈
10846−10849. (c) Chen, H.; Cho, K.-B.; Lai, W.; Nam, W.; Shaik, S. J.
Chem. Theory Comput. 2012, 8, 915−926. (d) Chung, L. W.; Li, X.;
Hirao, H.; Morokuma, K. J. Am. Chem. Soc. 2011, 133, 20076−20079.
(e) Lee, Y.-M.; Hong, S.; Morimoto, Y.; Shin, W.; Fukuzumi, S.; Nam,
W. J. Am. Chem. Soc. 2010, 132, 10668−10670. (f) Mukherjee, A.;
Cranswick, M. A.; Chakrabarti, M.; Paine, T. K.; Fujisawa, K.; Munck,
E.; Que, L., Jr. Inorg. Chem. 2010, 49, 3618−3628. (g) Cho, K.-B.;
̈
G
DOI: 10.1021/acs.inorgchem.5b02068
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Inorganic Chemistry
Article
(24) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,
B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H.
P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.;
Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin,
K. N.; Staroverov, V. N.; Keith, T.; Kobayashi, R.; Normand, J.;
Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.;
Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.;
Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.;
Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.;
Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador,
P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.;
Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09,
Revision B.01 and D.01; Gaussian, Inc.: Wallingford, CT, 2009.
(25) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. J. Chem. Phys.
2010, 132, 154104.
(26) (a) For a discussion see: Cho, K.-B.; Kim, E. J.; Seo, M. S.;
Shaik, S.; Nam, W. Chem. - Eur. J. 2012, 18, 10444−10453. (b) Ho, J.;
Klamt, A.; Coote, M. L. J. Phys. Chem. A 2010, 114, 13442−13444.
H
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