Hydroxylation of Unactivated C(sp3)-H Bonds with m-Chloroperbenzoic Acid Catalyzed by an Iron(III) Complex Supported by a Trianionic Planar Tetradentate Ligand

Article abstract of DOI:10.1021/acs.inorgchem.0c03469

Hydroxylation of cyclohexane with m-chloroperbenzoic acid was examined in the presence of an iron(III) complex supported by a trianionic planar tetradentate ligand. The present reaction system shows a high turnover number of 2750 with a high product selectivity of alcohol (93%). The turnover frequency was 0.51 s-1, and the second-order rate constant (k) for the C-H bond activation of cyclohexane was 1.08 M-1 s-1, which is one of the highest values among the iron complexes in the oxidation of cyclohexane so far reported. The present catalytic system can be adapted to the hydroxylation of substrates having only primary C-H bonds such as 2,2,3,3-tetramethylbutane as well as gaseous alkanes such as butane, propane, and ethane. The involvement of an iron(III) acyl peroxido complex as the reactive species was suggested by spectroscopic measurements of the reaction solution.

Full text of DOI:10.1021/acs.inorgchem.0c03469

pubs.acs.org/IC
Article
Hydroxylation of Unactivated C(sp3)−H Bonds with
m‑Chloroperbenzoic Acid Catalyzed by an Iron(III) Complex
Supported by a Trianionic Planar Tetradentate Ligand
Yuma Morimoto,* Shinichi Hanada, Ryusuke Kamada, Arisa Fukatsu, Hideki Sugimoto,
and Shinobu Itoh*
Cite This: https://dx.doi.org/10.1021/acs.inorgchem.0c03469
Read Online
sı
*
Metrics & More
Article Recommendations
Supporting Information
ACCESS
ABSTRACT: Hydroxylation of cyclohexane with m-chloroperbenzoic acid was examined in the presence of an iron(III) complex
supported by a trianionic planar tetradentate ligand. The present reaction system shows a high turnover number of 2750 with a high
product selectivity of alcohol (93%). The turnover frequency was 0.51 s−1, and the second-order rate constant (k) for the C−H bond
activation of cyclohexane was 1.08 M−1 s−1, which is one of the highest values among the iron complexes in the oxidation of
cyclohexane so far reported. The present catalytic system can be adapted to the hydroxylation of substrates having only primary C−
H bonds such as 2,2,3,3-tetramethylbutane as well as gaseous alkanes such as butane, propane, and ethane. The involvement of an
iron(III) acyl peroxido complex as the reactive species was suggested by spectroscopic measurements of the reaction solution.
INTRODUCTION
■
Selective hydroxylation of an aliphatic C−H bond is still a
challenging task because activation of inert C(sp3)−H bonds
generally requires harsh reaction conditions, which cause poor
product selectivity due to overoxidation of alcohol prod-
ucts.1−5 In the biological systems, heme enzymes are employed
in the alkane hydroxylation exhibiting a high alcohol product
Figure 1. (a) Chemical structure of L3− (fully deprotonated form of
LH3) and (b) crystal structure of 1 showing 50% probability thermal
ellipsoids. Hydrogen atoms and the countercation are omitted for
selectivity under mild conditions. The planar and electron-rich
character of the porphyrin (Por) ligand in the cytochrome
P450 enzymes facilitates formation of the highly reactive
clarity.
(Por•+)FeIVO intermediate, the so-called compound I, for
alkane hydroxylation reactions. Inspired by such a biological
we have synthesized and characterized an iron complex of L3−
catalyst, numerous iron complexes with planar and electron-
[(NEt4)[FeIII(L3)(Cl)] (1)] and found its prominent catalyst
rich ligands have been developed, and their catalytic activity
was examined in alkane hydroxylation reactions.6−8
Received: November 25, 2020
We have recently developed a planar trianionic ligand having
redox-noninnocent character, L3− (LH3 = 2,4-di-tert-butyl-6-
({(1E,2E)-3-[(3,5-di-tert-butyl-2-hydroxyphenyl)amino]-2-
nitroallylidene}amino)phenol; Figure 1a), which was used for
the synthesis of late-transition-metal [copper(II), nickel(II),
palladium(II), and rhodium(III)] complexes.9−12 In this study,
https://dx.doi.org/10.1021/acs.inorgchem.0c03469
© XXXX American Chemical Society
Inorg. Chem. XXXX, XXX, XXX−XXX
A

Inorganic Chemistry
pubs.acs.org/IC
Article
ability in the hydroxylation of alkanes including those having
only primary C−H bonds with m-chloroperbenzoic acid (m-
CPBA) under very mild conditions.
ferrocenium ion (Fc/Fc+) by cyclic voltammetry (CV; Figure
S2). The first oxidation potential is close to those of the Cu2+,
Ni2+, Pd2+, and Rh3+ complexes of L3− (−0.06, 0.04, 0.07, and
0.22 vs Fc/Fc+, respectively), indicating that the redox locus of
the first oxidation is on the ligand center irrespective of the
metal ion, as previously reported.9−11 Calculation of the one-
electron-oxidized complex [FeIII(L2•−)(Cl)] (2) with DFT
indicated that the complex has a quintet spin ground state
(Table S3). A significant amount of β-spin density was
localized on the ligand in the spin density map of 2 (Figure
S3), suggesting that the redox locus is the ligand rather than
the metal center, as suggested by the CV data. This spin
distribution also depicts the occurrence of antiferromagnetic
coupling of the ligand radical and the iron center in the high-
spin state.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of an Iron Complex.
The iron complex 1 was synthesized by treating LH3 with an
equimolar amount of FeCl2 in the presence of 4 equiv of
tetraethylammonium hydroxide (NEt4OH) in dichlorome-
thane (CH2Cl2) under air (for detailed procedures, see the
Supporting Information). Complex 1 was characterized by
electrospray ionization mass spectrometry (ESI-MS), UV−vis,
and electron paramagnetic resonance (EPR) spectra, and its
structure was confirmed by elemental analysis and single-
crystal X-ray structure analysis (Figures 1 and 2 and Table S1).
The one-electron-oxidized complex 2 can be generated by
chemical oxidation using a one-electron oxidant, tris(4-
bromophenyl)ammoniumyl cation [(4-BrC6H4)3N•+; Figure
S4a]. The spectral changes in the titration ended when an
equimolar amout of (4-BrC6H4)3N•+ was added to the solution
of 1 (Figure S4b). Complex 2 shows a broad absorption band
in the near-IR (NIR) region attributable to the one-electron-
oxidized ligand L2•−, which is a common spectral feature of the
one-electron-oxidized complexes of this ligand.9−11 The EPR
spectral change observed upon the addition of (4-
BrC6H4)3N•+ was also consistent with formation of the one-
electron-oxidized species with an integer spin state (Figure
2b).15
Catalytic Activity of the Iron Complex 1 in the
Oxidation of Hydrocarbons with m-CPBA. With the iron
complex 1 in our hands, we first examined hydroxylation of
cyclohexane using m-CPBA as an oxidant.16−18 The reaction
was started by adding 1 (23 μM) to a 3/1 (v/v) CH2Cl2/
acetonitrile (MeCN) solution of cyclohexane (0.47 M)
containing m-CPBA (0.18 M) at 30 °C under a N2
atmosphere. After 2 h, we obtained cyclohexanol as the
major product, where the turnover number (TON) was 2750
(the product yield was 55% based on m-CPBA; Table 1, entry
5). Cyclohexanone and ε-caprolactone were detected as
byproducts (1% and 3% yield based on m-CPBA, respectively),
so that the alcohol product selectivity was 93%.19 From m-
CPBA, chlorobenzene was produced as much as the oxidation
products of cyclohexane (∼60%).
At the initial stage of the reaction, the amount of the
products increased linearly with the reaction time (Figure 3a),
from which the initial reaction rate (V, M s−1) was determined
from the slope. The reaction rate V increased linearly with an
increase of the concentration of 1 added in the system (Figure
3b). Then, the turnover frequency (TOF) was calculated to be
0.51 0.01 s−1. The V value depended on the cyclohexane
concentration but was independent of the concentration of m-
CPBA (Figure 3c−f). Thus, the reaction rate can be expressed
as eq 1, which indicates that the turnover limiting step (TLS)
of the catalytic reaction is the substrate oxidation step by a
reactive species generated from 1 and m-CPBA.
Figure 2. (a) UV−vis spectra of 1 (50 μM; black line) and its one-
electron-oxidized complex, 2, generated by the reaction with an
equimolar amount of (4-BrC6H4)3NSbF6 (red line) in CH2Cl2 at −50
°C. (b) EPR spectra of 1 (1.0 mM) before (black line) and after (red
line) the addition of an equimolar amount of (4-BrC6H4)3NSbF6
measured at 80 K in frozen CH2Cl2. The asterisk denotes the signal
due to the remaining(4-BrC6H4)3N•+.
+
There is a tetraethylammonium cation (NEt4 ) in the unit cell,
indicating that the complex has a monoanionic charge. Thus,
the ligand LH3 is fully deprotonated, making the trianionic
form L3−, and the iron center is trivalent iron (FeIII) with a
chloride (Cl−) axial ligand. Compared to the Por ligand
system, L3− provides a narrower square-planar donor-atom
arrangement (Figure S1a,b), allowing the central metal ion to
be displaced from the mean-square plane of the donor atom set
(0.607 Å). Because of such a structural feature induced by the
ligand, the iron center may prefer the five-coordinate square-
pyramidal structure.
Strong π-electron conjugation between the two phenolate
moieties through the β-diketiminate backbone may also induce
the planar structure of the ligand moiety, which is somewhat
different from the salen ligand systems providing a distorted
trigonal-bipyramidal coordination environment (Figure S1c).
The EPR signals of 1 (g = 6.7, 5.2, and 1.98) were assigned to
5
those of a high-spin (S = /2) iron complex with a slightly
rhombic environment (Figure 2b).13,14 Density functional
theory (DFT) of 1 at the B3LYP/6-31G(d) level of theory
supported the presence of a sextet spin ground state and a
thermodynamically accessible quartet spin state (+5.5 kcal
mol−1; Table S2). The low-spin state (S = 1/2) was found to be
highly unstable (+21.9 kcal mol−1) compared to the former
ones.
V(M s−1) = TOF[1] = k[1][cyclohexane]
(1)
From eq 1, the second-order rate constant (k, M−1 s−1) was
determined to be 1.08 M−1 s−1, which was one of the largest
values reported to date for cyclohexane hydroxylation among
the iron complex catalysts under these temperature conditions
(Table S4).20−26
The first and second oxidation potentials of 1 were
determined to be 0.11 and 0.76 V versus ferrocene/
B
https://dx.doi.org/10.1021/acs.inorgchem.0c03469
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
Table 1. Oxidation of Hydrocarbons with m-CPBA
f
Catalyzed by 1
Figure 3. (a) Time courses of product formation in the oxidation of
cyclohexane (0.47 M) with m-CPBA (89 mM) catalyzed by 1 (black,
12 μM; red, 18 μM; blue, 23 μM; green, 29 μM) in 3/1 (v/v)
CH2Cl2/MeCN at 30 °C. The sum of cyclohexanol, cyclohexanone,
and ε-caprolactone was counted as the product amount. (b) Plot of
the reaction rate V against the concentration of 1. (c) Time courses of
product formation in the oxidation of cyclohexane (black, 0.11 M;
red, 0.23 M; blue, 0.35 M; green, 0.47 M) by m-CPBA (89 mM)
catalyzed by 1 (24 μM) in 3/1 (v/v) CH2Cl2/MeCN under N2 at 30
°C. (d) Plots of V against the cyclohexane concentration. (e) Time
course of product formation in the oxidation of cyclohexane (0.47 M)
with various amounts of m-CPBA (black, 89 mM; red, 133 mM; blue,
178 mM; green, 223 mM) catalyzed by 1 (24 μM) in 3/1 (v/v)
CH2Cl2/MeCN under N2 at 30 °C. (f) Plots of V against the
concentration of m-CPBA.
a
b
Based on the oxidant. [1] = 23 μM; [cyclohexane] = 0.47 M.
c
d
e
[butane] = 0.32 M. pressure of ropane = 0.91 MPa. pressure of
f
ethane = 4.1 MPa. Conditions: [substrate] = 2.7 M, [m-CPBA] =
0.18 M, [1] = 36 μM, in 3/1 (v/v) CH2Cl2/MeCN for 2.0 h at 30 °C
under a N2 atmosphere.
The catalytic activity of 1 was further examined in the
oxidation of other hydrocarbon substrates under the same
conditions (Table 1). The benzylic positions of cumene and
toluene were oxidized to the corresponding alcohol products
with TONs of 1280 and 207, respectively (Table 1, entries 1
and 2). The lower TON and product selectivity observed in
the toluene hydroxylation could be ascribed to its higher C−H
bond dissociation energy (BDEC−H) compared to that of
cumene (vide infra). Cyclic alkanes such as cyclooctane and
cycloheptane were also readily oxidized to the corresponding
alcohols in higher TONs (3070 and 2050, respectively)
compared to that of cyclohexane (entries 3 and 4), which is
consistent with their lower BDEC−H and larger number of
equivalent methylene protons in the compounds. Formation of
cyclooctanone (TON = 81) and cycloheptanone (133) was
observed, whereas the corresponding lactones were not
detected.27−30 Notably, 2,2,3,3-tetramethylbutane (TMB),
containing only primary C−H bonds, was oxidized to the
corresponding alcohol (1-hydroxy-2,2,3,3-tetramethylbutane,
TMB-OH), where the TON of 1 reached 171 (entry 6).
Encouraged by such a high catalytic activity of 1, we further
examined the oxidation of gaseous alkanes such as n-butane,
propane, and ethane (entries 7−9). In the case of n-butane
(0.32 M), 2-butanol and 2-butanone were obtained with
TONs of 378 and 40, respectively, after 2 h (entry 7). The
hydroxylation reactions of propane and ethane were conducted
under pressurized conditions (0.91 and 4.1 MPa for propane
and ethane, respectively). Although we could not directly
determine the concentration of the substrates in these
reactions, those values were estimated as 1.4 M and 62 mM,
respectively, using Henry’s law from the saturated concen-
trations of these substrates under normal pressure in 3/1 (v/v)
CD2Cl2/CD3CN determined by NMR at 298 5 K (Figure
S5). In both cases, we observed formation of the
corresponding alcohol and carbonyl products as listed in
Table 1 (entries 8 and 9).
In Figure 4 is shown the rate dependence on the BDEC−H of
the substrates. Cumene (83.2 kcal mol−1), toluene (89.7 kcal
C
https://dx.doi.org/10.1021/acs.inorgchem.0c03469
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
Scheme 1. Possible Mechanism for the C−H Bond Cleavage
Step in the Catalytic Reaction
an equimolar amount of chlorobenzene was obtained as
mentioned above (vide supra). It is well-known that the m-
chlorobenzoloxyl radical readily undergoes a decarboxylation
reacion to give chlorobenzene and carbon dioxide (for a more
detailed discussion, see below).35 Because the kinetic studies
indicated that the substrate oxidation is the slowest step in the
catalytic cycle, the spectrum of the reaction solution must
exhibit a spectroscopic feature of a reactive species. The
absorption band of 1 (30 μM) at 415 nm (black line spectrum)
immediately disappeared upon the addition of m-CPBA (180
mM) into the solution in the presence of cyclohexane (100
mM), leaving an absorption band around 375 nm (red line
spectrum in Figure 5). Afterward, this band gradually decayed
Figure 4. Plot of 2.3RT log(k′) against the substrate BDEC−H values
for the catalytic oxidation of hydrocarbons by m-CPBA catalyzed by 1
(1, cumene; 2, toluene; 3, cyclooctane; 4, cycloheptane; 5,
cyclohexane; 6, TMB).
mol−1), cyclooctane (95.7 kcal mol−1), cycloheptane (96.5 kcal
mol−1), and TMB (101 kcal mol−1) were employed for the
plot, in addition to cyclohexane (99.5 kcal mol−1).31 The k
values for the reactions were determined from the V values
obtained from the initial stages of the reactions (Figure S6)
and eq 1.32 In Table S5, the normalized second-order rate
constants k′ are listed, which were obtained by dividing k with
the number of equivalent C−H bonds in the substrate. Then,
the 2.302RT log(k′) values, where R is the gas constant (kcal
mol−1 K−1) and T is the absolute reaction temperature (303.2
K), were plotted against BDEC−H, as shown in Figure 4, in
which the slope was determined to be −0.15. Such a shallow
slope illustrates that the transition state is closer to the
reactants than the products.33 The shallow slope also indicates
that the effect of the substrate BDEC−H is not as significant in
the present catalytic reaction. By using this slope value, we can
estimate the normalized reaction rate constant (k′) for the
oxidation of cyclohexanol (BDEC−H = 92.8 kcal mol−1) as 0.4
M−1 s−1, which is 5 times larger than the normalized reaction
rate constant (k′) for the oxidation of cyclohexane (0.090 M−1
s−1). Cyclohexane has 12 equivalent C−H bonds; therefore,
the reaction rate constant (k = 12k′ = 1.08 M−1 s−1) becomes
larger than that for cyclohexanol. Moreover, the concentration
of cyclohexane (substrate) is much higher than that of
cyclohexanol (product).34 Thus, a high alcohol product
selectivity was attained in the present catalytic system. The
kinetic isotope effect (KIE) value for cyclohexane hydrox-
ylation was determined to be 3.0 using cyclohexane-d12. The
plot for log k′ against the KIE value suggested that substrate
C−H homolysis is the rate-determining step in the present
catalytic oxidation reaction.
Figure 5. Spectral changes observed upon the addition of m-CPBA to
a solution of 1 (black), initially giving 3 (red, after 10 s of mixing) and
the final product (blue, after 400 s) at 303 K. Inset: X-band (9.560
GHz) EPR of a mixture of 1 (0.50 mM) and m-CPBA (180 mM)
frozen after 5 s of mixing at 303 K measured at 100 K. Those spectra
were obtained in the presence of cyclohexane (2.7 M) in a mixed
solvent of 3/1 (v/v) CH2Cl2/MeCN.
to give the final spectrum (blue line spectrum), in which
neither the absorption band due to the ligand radical (vide
supra) in the NIR region nor the d−d band characteristic of a
high-valent (IV or V) iron center exists.20−26,36−41 The decay
process was largely decelarated at −30 °C, as shown in Figure
S7, but no other absorption band due to a different
intermediate was observed.
Although the intermeidate exhibited a featureless UV−vis
spectrum, it exhibited distinct EPR signals at g = 4.3 and 2.0
(Figure 5, inset), which are different from those of the reported
iron(V) oxido complexes.23,37−42 Thus, the spectrum was
Possible Reactive Intermediate. In the present rection
system, two plausible reaction pathways can be drawn for the
C−H bond cleavage process, as shown in Scheme 1. One is a
concerted mechanism, in which the iron(III)−m-CPBA adduct
complex 3 directly reacts with the substrate (path A), and
another one is a stepwise mechanism, where complex 3 initially
undergoes O−O bond cleavage to generate an iron oxido
intermediate, 4, which then reacts with the substrate (path B).
In both mechasnims, O−O bond homolysis formally takes
place to generate a m-chlorobenzoloxyl radical because nearly
D
https://dx.doi.org/10.1021/acs.inorgchem.0c03469
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
5
assigned to that of an iron(III) complex 3 in an S = /2 spin
state with rhombic distortion.43 An isotropic signal at g = 2.005
was ascribable to an organic radical, but its spin amount was
neglegibly small.44−46 A decrease of the EPR signal after 400 s
of mixing of 1 and m-CPBA was consistent with the spectral
change observed in the UV−vis−NIR spectrum.
energies of 3−5 were −23.91, −1.33, and −15.83 kcal mol−1,
respectively. The phenolate oxygen in the ligand accepts a
proton from m-CPBA to help in the formation of the
acylperoxido complex, and ligand protonation may lift up the
oxidation potential of the ligand. The iron(V) oxido complex
(5) seems accessible from 3; however, the formation of
chlorobenzene in the experiment suggests that O−O bond
heterolysis requires large activation energies.
An isotope-labeling experiment further supported the
oxidation mechanism by 3. An addition of H218O (2.8 M) to
the reaction mixture did not produce cyclohexanol-18O,
implying that the reaction does not go through the formation
of a metal oxido species generated via O−O bond cleavage in
the m-CPBA adduct intermediate (Figure S9) because such a
metal oxido species usually shows rapid oxygen exchange
reactivity with water molecules.47−49
Previously, Nam and co-workers reported that an iron(III)
acylperoxido complex supported by a tetradentate cyclic
ligand, 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclotridecane
(13-TMC), showed alkane hydroxylation reactivity.48 They
also reported an iron(IV) oxido complex supported by the
same ligand 13-TMC by using iodosylbenzene as an oxidant.
The iron(IV) oxido complex showed reactivity toward
substrates comparable to that of the iron(III) acylperoxido
complex; the iron(IV) oxido complex oxidizes 1,4-cyclo-
hexadiene (BDEC−H = 78 kcal mol−1) with a rate constant of
5.4 M−1 s−1, whereas the iron(III) acylperoxido complex
oxidizes triphenylmethane (BDEC−H = 81 kcal mol−1) with a
rate constant of 1.3 M−1 s−1.50 For a nickel system, Nakazawa,
Hikichi, and co-workers reported that an m-CPBA adduct
complex of nickel(II) showed reactivity toward weak C−H
substrates such as 1,4-cyclohexadiene.51 It should also be noted
that a copper peroxido adduct itself can act as the active
oxidant for the aliphatic C−H bond activation without forming
the corresponding oxyl or oxido species.52,53
Despite our great efforts, we did not detect mass peaks of
either 3 or another iron oxido species by cold spray ionization
mass spectrometry (CSI-MS). On the basis of these
observations, complex 3 is considered to be the reactive
species for C−H abstraction from the substrates, generating an
organic substrate radical as mentioned above. After the
hydrogen-transfer process, the generated alkyl radical may
accept a hydroxyl group from the iron center to produce the
alcohol product (rebound mechanism, Scheme 2a). Alter-
natively, the alkyl radical accepts a hydroxyl group from
another molecule of m-CPBA (Scheme 2a).
Scheme 2. Reactions after H• Abstraction from a Substrate
Overall Catalytic Oxidation Process. As described in
Scheme 1, the reaction between the active oxidant and m-
CPBA would generate an acyloxyl radical (ArCOO•). This
radical spontaneously decomposed to a phenyl radical and
carbon dioxide at a rate of ∼105 s−1 as mentioned above.35
Acyloxyl radicals are also known to abstract a hydrogen atom
based on the large bond formation energy of O−H in a
carboxylic acid at a rate comparable to that of decarboxylation
(Scheme 2b). In the catalytic reaction, chlorobenzene was
obtained as the major byproduct; thus, decarboxylation of
ArCOO• is the dominant process in our system. The phenyl
radical produced can also abstract a hydrogen atom from the
substrate. Therefore, two equimolar amounts of alkyl radicals
are generated by m-CPBA.
When an alkyl radical accepts the hydroxyl group from the
iron complex, the alcohol formation process completes. Alkyl
radicals are also known to directly react with m-CPBA to
produce ArCOO• and start a chain reaction.35 To examine the
chain length, we conducted catalytic cyclohexane hydroxyla-
The assignment of the intermediate complex was also
supported by DFT calculations. Possible structures of iron(III)
acylperoxido (3), iron(IV) oxido (4), and iron(V) oxido (5)
complexes are calculated at the UB3LYP/6-31G(d) level of
theory (Figure S8). In the evaluation, the chloride-dissociated
and ligand-protonated forms of the complexes were considered
by changing their spin states. Among the possible structures, a
six-coordinate iron(III) acylperoxido complex in the S = /2
spin state with a protonated ligand has the lowest electronic
energy; on the basis of complex 1 and m-CPBA, the formation
1
tion under air, where the product amount decreased to /3
compared to the reaction under an inert N2 atmosphere. The
alcohol product selectivity decreased as well; the TONs for
cyclohexanol, cyclohexanone, and ε-caprolactone were 481, 78,
and 427, respectively. A change in the product selectivity under
air indicates the formation of a cyclohexyl radical (Cy•) as an
intermediate of alkane hydroxylation, which is instantly
captured by molecular oxygen to form a cyclohexyl peroxy
radical (CyOO•; ∼109 M−1 s−1; Scheme 2c).54 The other
5
E
https://dx.doi.org/10.1021/acs.inorgchem.0c03469
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
8A or GC-2010 equipped with a GL Science InertCapWAX capillary
column (30 m × 0.25 mm). Gas chromatography−mass spectrometry
(GC−MS) measurements were performed on a Shimadzu GS-MS QP
2010 Plus instrument. All of the manipulations of air- and moisture-
sensitive compounds were carried out in a glovebox (KK-011-AS,
Korea Kiyon product) filled with N2 ([O2] < 1 ppm; [H2O] < 1
ppm). The temperature of the glovebox was kept at 30−32 °C.
Elemental analyses were performed using a PerkinElmer or Fisons
Instruments EA1108 elemental analyzer. All of the experiments
regarding the reactivity of the iron complex were conducted under a
N2 atmosphere unless otherwise noted. Hydroxylation reactions of
gaseous substrates, propane and ethane, were conducted in a high-
pressure reactor equipped with a high-performance liquid chromato-
graph pump to introduce liquid samples into a pressurized reaction
vessel at the designed time (Taiyo-system, Co., Ltd.). Substrate gas
was supplied to the reactor with continuous flow, and the pressure of
the reactor was controlled with a vent valve. The reaction temperature
were monitored with a thermometer set in the reactor and controlled
with a feedback circuit.
putative radical carrier, a phenyl radical, is also reported to
react with an oxygen molecule.55 This experiment suggests the
occurrence of a radical chain reaction. However, the moderate
effect of oxygen on the product amount (∼1/3) indicates that
the chain-transfer process should not be efficient in the
reaction conditions. As shown in Figures 3 and S6, the time
course of alcohol production did not show the induction
period in this system, also denying the radical-chain-reaction
mechanism as the major process for catalytic alcohol
formation. The decrease in the alcohol product selectivity in
the presence of molecular oxygen can be explained as follows:
dimerization of CyOO• forms tetroxide, followed by Russell
rearrangement to produce cyclohexanol and cyclohexanone in
a 1:1 ratio (Scheme 2d).56 The generated cyclohexanone
converted to ε-caprolactone via the Baeyer−Villiger reaction
(Scheme 2e).
Synthesis of (Et4N)[FeIII(L)(Cl)] (1). A CH2Cl2 solution (1 mL)
containing LH3 (222 mg, 400 μmol) and FeCl2 (50.8 mg, 400 μmol)
was treated with NEt4OH [1.6 mmol, 1.2 mL (aqueous 20%)], and
the mixture was stirred for 1 h at room temperature under air. The
solvent was removed by evaporation, and the residue was recrystal-
lized from CH2Cl2/n-hexane to give single crystals suitable for X-ray
crystallographic analysis (145 mg, 55% yield). Anal. Calcd for
C39H62FeN4O4·2.5H2O: C, 59.50; H, 8.58; N, 7.12. Found: C, 59.38;
H, 8.50; N, 7.24.
SUMMARY
■
In summary, we have newly developed an iron complex
supported by a planar N2O2 ligand with trianionic charge
exhibiting a high catalytic activity toward alkane hydroxylation
with m-CPBA. The presented system efficiently cleaves C−H
bond of cyclohexane with high TOF (0.51 s−1) and TON
(2750) and a high alcohol product selectivity of 93%. The TLS
of the present catalytic reaction is the C−H bond activation
step by a reactive oxidant, presumably the m-CPBA adduct 3 of
the iron complex. The second-order rate constant (k) was 1.08
M−1 s−1, which is currently one of the largest values in
cyclohexane hydroxylation by iron complexes. This highly
active catalytic system was applicable to the oxidation of
gaseous alkane substrates such as n-butane and propane and,
more importantly, to the oxidation of the primary C−H bond
in TMB and ethane. Spectroscopic and DFT studies suggested
that the iron(III) acylperoxido complex 3 is the reactive
species of the catalytic system rather than an iron(IV) or
iron(V) oxido complex. This study has extended the limit of
catalytic hydroxylation reactions with molecular catalysts.
X-ray Structure Determination. A single crystal was mounted
on a CryoLoop (Hamptom Research Co.) with mineral oil, and all X-
ray data were collected on a Rigaku R-AXIS RAPID diffractometer
using filtered Mo Kα radiation. The structures were solved by direct
methods (SIR2008) and expanded using Fourier techniques. Non-
hydrogen atoms were refined anisotropically by full-matrix least
squares on F2. The CIF file of the structure has an A-level alert
(plat201) regarding refinement; however, the atoms (Fe1, Cl1, etc.)
pointed out in the alert are on the mirror plane of the crystal.
Therefore, some of the thermal anisotropy factors become zero.
Hydrogen atoms were attached on the carbon atoms at idealized
positions and not refined. All structures in the final stages of
refinement showed no movement in the atom positions. The
calculations were performed using Single-Crystal Structure Analysis
Software, version 3.8 (Rigaku Corp., The Woodlands, TX, 2000−
2006). The crystallographic parameters are summarized in Table S1.
Atomic coordinates, thermal parameters, and intramolecular bond
distances and angles are deposited in the Supporting Information.
Catalytic Hydroxylation. Experiments were conducted with
screw vials (ϕ12 × H35) sealed with silicone septum caps, and the
reactions was conducted in a 3/1 (v/v) CH2Cl2/MeCN mixed
solution with a volume of 400 μL. Reactions were started by adding 1
to a solution containing m-CPBA and the substrate. After the reaction
was quenched by passing the solution through a pen column charged
with aluminum, the products were analyzed using GC-FID. All peaks
of interest were identified by comparing the retention times with
those of the authentic samples. The products were quantified by
comparing their peak areas with that of an internal standard using a
calibration curve consisting of a plot of the mole ratio (moles of
organic compound/moles of internal standard) versus area ratio (area
of organic compound/area of standard). The TONs of the catalysts
were determined by dividing the amount of the product by the
amount of the catalyst. The yields of the products were determined
based on the oxidant concentration.
EXPERIMENTAL SECTION
■
General Procedures. The reagents and solvents used in this
study, except the ligands and complexes, were commercial products of
the highest available purity and were further purified by standard
methods, if necessary.57 Ligand LH3 was prepared according to the
reported procedures.9,10
1H NMR spectra were recorded on JEOL ECS400 and Bruker
AVANCE III HD NMR spectrometers. ESI- and CSI-MS measure-
ments were performed on a Bruker cryospray micrOTOFII
spectrometer. Electrochemical measurements were performed at 298
K using an Automatic Polarization System HZ-7000 (Hokutodenko,
Co., Ltd.) in deaerated CH2Cl2 containing tetra-n-butylammonium
hexafluorophosphate (0.10 M) as a supporting electrolyte. A
conventional three-electrode cell was used with a glassy-carbon
working electrode and a platinum wire counter electrode. The
measured potentials were recorded with respect to the Ag/AgNO3
reference electrode and converted versus Fc/Fc+ by adding 0.29 V.58
Electronic spectra were taken on a Jasco V-570 or Hewlett-Packard
8453 photodiode-array spectrophotometer equipped with a Unisoku
USP-203 thermostated cryostat cell holder (a desired temperature can
be fixed within 0.5 °C). EPR measurements were carried out on a
JEOL JES-TE200 or Bruker EMXmicro continuous-wave X-band
spectrometer. Samples were prepared in an EPR tube (d = 5.0ϕ) and
rapidly frozen in liquid nitrogen. The modulation width and
microwave power were set in not-over modulation and not-saturating
conditions, respectively. Flame-ionization-detected gas chromatog-
raphy (GC-FID) measurements were performed on a Shimadzu GC-
Catalytic Hydroxylation of n-Butane. Generally, the reaction was
conducted with a procedure similar to that for the alkane
hydroxylation experiment described above except for the introduction
of the substrate. Gaseous n-butane was introduced into a CD2Cl2/
CD3CN mixed solvent containing m-CPBA for 5 min after the
removal of dioxygen, and then 1 was injected into the reactor to
initiate the reaction.
F
https://dx.doi.org/10.1021/acs.inorgchem.0c03469
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
Catalytic Hydroxylation of Propane and Ethane. The reaction
was initiated by the addition of a CH2Cl2/MeCN (3.0 mL) mixed
solvent to a vessel containing solid m-CPBA (180 mM) and 1 under a
pressure of propane (0.91 MPa) or ethane (4.1 MPa) at 303(10) K.
After stirring for 2 h, the resulting mixture was quenched by passing it
through a pencil column charged with aluminum and analyzed by
GC−MS. The product amounts were determined by using calibration
curves made with the corresponding authentic samples.
Hideki Sugimoto − Department of Molecular Chemistry,
Division of Applied Chemistry, Graduate School of
Engineering, Osaka University, Osaka 565-0871, Japan
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c03469
Notes
Determination of the Deuterium KIE. The KIE on the
hydroxylation of cyclohexane was investigated by using an equimolar
mixture of cyclohexane and cyclohexane-d12 as the substrate. The KIE
value was obtained by measuring the peak area ratios of the
corresponding products derived from cyclohexane and cyclohexane-
d12, respectively, in GC−MS measurements.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by CREST, JST (JPMJCR16P1), and
by a Grant-in-Aid for Challenging Research (Exploratory)
(Grant 19K22201) from JSPS.
Isotope Labeling Using 18O-Labeled Water. To the catalytic
cyclohexane hydroxylation system was added 20 μL of H218O before
initiation of the reaction, and the resulting mixture was analyzed by
GC−MS. The product amount and molecular weight distribution
were compared to those of the control experiment employing H216O.
Theoretical Calculations. DFT calculations were performed by
using Gaussian09 (revision D.01; Gaussian, Inc.).59 Molecular
structures were optimized by using the UB3LYP functional with the
6-31G(d) basis set for all atoms on the ultrafine grid. For evaluation
of the relative electronic energies of the iron(III) acylperoxido
complexes, stabilization by solvation was analyzed with single-point
calculation using the polarizable continuum model (ε = 35.688, value
for MeCN) for the optimized geometry.60
REFERENCES
■
(1) Bergman, R. G. C−H Activation. Nature 2007, 446, 391.
(2) Newhouse, T.; Baran, P. S. If C−H Bonds Could Talk: Selective
C−H Bond Oxidation. Angew. Chem., Int. Ed. 2011, 50, 3362−3374.
(3) Godula, K.; Sames, D. C−H Bond Functionalization in Complex
Organic Synthesis. Science 2006, 312, 67−72.
(4) Sheldon, R. Metal-Catalyzed Oxidations of Organic Compounds:
Mechanistic Principles and Synthetic Methodology Including Biochemical
Processes; Elsevier Science, 2012.
(5) Mijs, W. J.; de Jonge, C. R. H. I. Organic Syntheses by Oxidation
with Metal Compounds; Springer, 2013.
(6) Gunay, A.; Theopold, K. H. C−H Bond Activations by Metal
Oxo Compounds. Chem. Rev. 2010, 110, 1060−1081.
(7) Zheng, C.; You, S.-L. Recent Development of Direct Asymmetric
Functionalization of Inert C−H Bonds. RSC Adv. 2014, 4, 6173−
6214.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c03469.
■
sı
(8) McMurray, L.; O’Hara, F.; Gaunt, M. J. Recent Developments in
Natural Product Synthesis Using Metal-Catalysed C−H Bond
Functionalisation. Chem. Soc. Rev. 2011, 40, 1885−1898.
(9) Takaichi, J.; Ohkubo, K.; Sugimoto, H.; Nakano, M.; Usa, D.;
Maekawa, H.; Fujieda, N.; Nishiwaki, N.; Seki, S.; Fukuzumi, S.; Itoh,
S. Copper Complexes of the Non-Innocent β-Diketiminate Ligand
Containing Phenol Groups. Dalton Trans. 2013, 42, 2438−2444.
(10) Morimoto, Y.; Takaichi, J.; Hanada, S.; Ohkubo, K.; Sugimoto,
H.; Fujieda, N.; Fukuzumi, S.; Itoh, S. Redox Behavior of Novel
Nickel and Palladium Complexes Supported by Trianionic Non-
Innocent Ligand Containing β-Diketiminate and Phenol Groups. J.
Porphyrins Phthalocyanines 2015, 19, 377−387.
(11) Fujita, D.; Sugimoto, H.; Shiota, Y.; Morimoto, Y.; Yoshizawa,
K.; Itoh, S. Catalytic C−H Amination Driven by Intramolecular
Ligand-to-Nitrene One-Electron Transfer through a Rhodium(III)
Centre. Chem. Commun. 2017, 53, 4849−4852.
(12) Fujita, D.; Sugimoto, H.; Morimoto, Y.; Itoh, S. Noninnocent
Ligand in Rhodium(III)-Complex-Catalyzed C−H Bond Amination
with Tosyl Azide. Inorg. Chem. 2018, 57, 9738−9747.
(13) Que, L. Physical Methods in Bioinorganic Chemistry: Spectroscopy
and Magnetism; University Science Books, 2010.
Supporting experiments (PDF)
Accession Codes
CCDC 2020480 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
Yuma Morimoto − Department of Molecular Chemistry,
Division of Applied Chemistry, Graduate School of
Engineering, Osaka University, Osaka 565-0871, Japan;
orcid.org/0000-0002-5209-266X; Email: y-morimoto@
chem.eng.osaka-u.ac.jp
Shinobu Itoh − Department of Molecular Chemistry, Division
of Applied Chemistry, Graduate School of Engineering, Osaka
University, Osaka 565-0871, Japan; orcid.org/0000-
0002-3711-2378; Email: shinobu@chem.eng.osaka-u.ac.jp
(14) Petasis, D. T.; Hendrich, M. P. In Methods in Enzymology; Qin,
P. Z., Warncke, K., Eds.; Academic Press, 2015; Vol. 563, pp 171−
208.
(15) Hendrich, M. P.; Debrunner, P. G. Integer-Spin Electron
Paramagnetic Resonance of Iron Proteins. Biophys. J. 1989, 56, 489−
506.
Authors
Shinichi Hanada − Department of Material and Life Science,
Division of Advanced Science and Biotechnology, Graduate
School of Engineering, Osaka University, Osaka 565-0871,
Japan
Ryusuke Kamada − Department of Molecular Chemistry,
Division of Applied Chemistry, Graduate School of
Engineering, Osaka University, Osaka 565-0871, Japan
Arisa Fukatsu − Department of Material and Life Science,
Division of Advanced Science and Biotechnology, Graduate
School of Engineering, Osaka University, Osaka 565-0871,
Japan
(16) Groves, J. T.; Haushalter, R. C.; Nakamura, M.; Nemo, T. E.;
Evans, B. J. High-Valent Iron-Porphyrin Complexes Related to
Peroxidase and Cytochrome P-450. J. Am. Chem. Soc. 1981, 103,
2884−2886.
(17) Nagataki, T.; Tachi, Y.; Itoh, S. NiII(TPA) as an Efficient
Catalyst for Alkane Hydroxylation with m-CPBA. Chem. Commun.
2006, 4016−4018.
(18) Nagataki, T.; Ishii, K.; Tachi, Y.; Itoh, S. Ligand Effects on NiII-
Catalysed Alkane-Hydroxylation with m-CPBA. Dalton Trans. 2007,
1120−1128.
G
https://dx.doi.org/10.1021/acs.inorgchem.0c03469
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
(19) This catalyst system could not utilize hydrogen peroxide, tert-
butyl hydroperoxide, and iodosylbenzene instead of m-CPBA.
(20) Biswas, A. N.; Puri, M.; Meier, K. K.; Oloo, W. N.; Rohde, G.
(37) Lyakin, O. Y.; Zima, A. M.; Samsonenko, D. G.; Bryliakov, K.
P.; Talsi, E. P. Epr Spectroscopic Detection of the Elusive FevO
Intermediates in Selective Catalytic Oxofunctionalizations of Hydro-
carbons Mediated by Biomimetic Ferric Complexes. ACS Catal. 2015,
5, 2702−2707.
T.; Bominaar, E. L.; Munck, E.; Que, L. Modeling Taud-J: A High-
̈
Spin Nonheme Oxoiron(IV) Complex with High Reactivity toward
C−H Bonds. J. Am. Chem. Soc. 2015, 137, 2428−2431.
(38) Van Heuvelen, K. M.; Fiedler, A. T.; Shan, X.; De Hont, R. F.;
(21) Seo, M. S.; Kim, N. H.; Cho, K.-B.; So, J. E.; Park, S. K.;
Meier, K. K.; Bominaar, E. L.; Munck, E.; Que, L. One-Electron
̈
Oxidation of an Oxoiron(IV) Complex to Form an [OFeVNR]+
Center. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 11933−11938.
(39) Panda, C.; Debgupta, J.; Díaz Díaz, D.; Singh, K. K.; Sen Gupta,
S.; Dhar, B. B. Homogeneous Photochemical Water Oxidation by
Biuret-Modified Fe-TAML: Evidence of Fev(O) Intermediate. J. Am.
Chem. Soc. 2014, 136, 12273−12282.
́
Clemancey, M.; Garcia-Serres, R.; Latour, J.-M.; Shaik, S.; Nam, W. A
Mononuclear Nonheme Iron(IV)-Oxo Complex Which Is More
Reactive Than Cytochrome P450 Model Compound I. Chem. Sci.
2011, 2, 1039−1045.
(22) Morimoto, Y.; Shimaoka, Y.; Ishimizu, Y.; Fujii, H.; Itoh, S.
Direct Observation of Primary C-H Bond Oxidation by an Oxido-
Iron(IV) Porphyrin π-Radical Cation Complex in a Fluorinated
Carbon Solvent. Angew. Chem., Int. Ed. 2019, 58, 10863−10866.
(23) Ghosh, M.; Singh, K. K.; Panda, C.; Weitz, A.; Hendrich, M. P.;
Collins, T. J.; Dhar, B. B.; Sen Gupta, S. Formation of a Room
Temperature Stable Fev(O) Complex: Reactivity toward Unactivated
C−H Bonds. J. Am. Chem. Soc. 2014, 136, 9524−9527.
(40) Serrano-Plana, J.; Oloo, W. N.; Acosta-Rueda, L.; Meier, K. K.;
̃ ̈
Verdejo, B.; García-Espana, E.; Basallote, M. G.; Munck, E.; Que, L.;
Company, A.; Costas, M. Trapping a Highly Reactive Nonheme Iron
Intermediate That Oxygenates Strong CH Bonds with Stereo-
retention. J. Am. Chem. Soc. 2015, 137, 15833−15842.
(41) de Oliveira, F. T.; Chanda, A.; Banerjee, D.; Shan, X.; Mondal,
(24) Kaizer, J.; Klinker, E. J.; Oh, N. Y.; Rohde, J.-U.; Song, W. J.;
S.; Que, L.; Bominaar, E. L.; Munck, E.; Collins, T. J. Chemical and
̈
Spectroscopic Evidence for an FeV-Oxo Complex. Science 2007, 315,
835−838.
Stubna, A.; Kim, J.; Munck, E.; Nam, W.; Que, L. Nonheme Feivo
̈
Complexes That Can Oxidize the C−H Bonds of Cyclohexane at
Room Temperature. J. Am. Chem. Soc. 2004, 126, 472−473.
(42) Lyakin, O. Y.; Bryliakov, K. P.; Britovsek, G. J. P.; Talsi, E. P.
Epr Spectroscopic Trapping of the Active Species of Nonheme Iron-
Catalyzed Oxidation. J. Am. Chem. Soc. 2009, 131, 10798−10799.
(43) The rhombic symmetry of zero-field splitting (E/D) can be
estimated to be around 0.07 based on the g values. An expected signal
around g = 7.5 could not be observed in these temperature conditions.
(44) The g value is different from those of •OH (g∥=2.06; g⊥=2.008)
and the acyl radical (giso ∼ 2.012), which possess high reactivity
toward alkane substrates.
́
(25) Company, A.; Prat, I.; Frisch, J. R.; Mas-Balleste, D. R.; Gu
̈
ell,
́
M.; Juhasz, G.; Ribas, X.; Mu
̈
nck, D. E.; Luis, J. M.; Que, L., Jr.;
Costas, M. Modeling the cis-Oxo-Labile Binding Site Motif of Non-
Heme Iron Oxygenases: Water Exchange and Oxidation Reactivity of
a Non-Heme Iron(IV)-Oxo Compound Bearing a Tripodal
Tetradentate Ligand. Chem. - Eur. J. 2011, 17, 1622−1634.
(26) Kudrik, E. V.; Afanasiev, P.; Alvarez, L. X.; Dubourdeaux, P.;
́
Clemancey, M.; Latour, J.-M.; Blondin, G.; Bouchu, D.; Albrieux, F.;
Nefedov, S. E.; Sorokin, A. B. An N-Bridged High-Valent Diiron−Oxo
Species on a Porphyrin Platform That Can Oxidize Methane. Nat.
Chem. 2012, 4, 1024−1029.
(45) Tripathi, S. C.; Kadam, R. M.; Kumar, M.; Mittal, J. P.; Mishra,
S. P. EPR Investigations on Radiation Induced Chemical Trans-
formations in Pd(ClO4)2/i-PrOH/HClO4 System from 77 to 300 K.
Spectrochim. Acta, Part A 2005, 62, 1107−1113.
(46) Yamauchi, S.; Hirota, N.; Takahara, S.; Misawa, H.; Sawabe, K.;
Sakuragi, H.; Tokumaru, K. A Time-Resolved EPR Study on
Photodecomposition of Dibenzoyl Peroxides in Carbon Tetrachlor-
ide. J. Am. Chem. Soc. 1989, 111, 4402−4407.
(27) The Baeyer−Villiger reaction with cyclohexanol is reported to
be much faster than that with cyclooctenone and cycloheptanone.
Nucleophilic attack on the carbon sp2 of cyclohexanone tends to show
relatively low activation energy due to the strain energy of the ring.
See refs 28−30.
(28) Friess, S. L.; Frankenburg, P. E. Further Observations on the
Reaction of Cyclanones with Perbenzoic Acid. J. Am. Chem. Soc. 1952,
74, 2679−2680.
(29) Starcher, P. S.; Phillips, B. Synthesis of Lactones 1. J. Am. Chem.
Soc. 1958, 80, 4079−4082.
(30) Wu, Y.-D.; Houk, K. N.; Paddon-Row, M. N. Effect of
Torsional Strain and Electrostatic Interactions on the Stereochemistry
of Nucleophilic Additions to Cyclohexanone and Related Systems.
Angew. Chem., Int. Ed. Engl. 1992, 31, 1019−1021.
(31) Luo, Y.-R. Comprehensive Handbook of Chemical Bond Energies;
CRC Press: Boca Raton, FL, 2007.
(32) We chose the reaction conditions not to change the TLS of the
reaction by reducing the substrate concentration with a decrease of
the BDE of the substrates.
(33) Xue, X.-S.; Ji, P.; Zhou, B.; Cheng, J.-P. The Essential Role of
Bond Energetics in C−H Activation/Functionalization. Chem. Rev.
2017, 117, 8622−8648.
(34) The rate ratio of the reaction for alkane hydroxylation over
alcohol oxidation (VA/K) can be explained by the rate constants for
each reaction, kalkane and kalcohol, and their concentrations, [alkane] and
[alcohol], as follows: VA/K = kalkane[alkane]/kalcohol[alcohol]. The final
ratio of alcohol and ketone can be obtained by the time integral of
(47) Nam, W.; Lim, M. H.; Lee, H. J.; Kim, C. Evidence for the
Participation of Two Distinct Reactive Intermediates in Iron(III)
Porphyrin Complex-Catalyzed Epoxidation Reactions. J. Am. Chem.
Soc. 2000, 122, 6641−6647.
́
(48) Wang, B.; Lee, Y.-M.; Clemancey, M.; Seo, M. S.; Sarangi, R.;
Latour, J.-M.; Nam, W. Mononuclear Nonheme High-Spin Iron(III)-
Acylperoxo Complexes in Olefin Epoxidation and Alkane Hydrox-
ylation Reactions. J. Am. Chem. Soc. 2016, 138, 2426−2436.
(49) We cannot exclude the possibility of much faster substrate
oxidation than oxide ligand exchange after O−O bond cleavage.
(50) Hong, S.; So, H.; Yoon, H.; Cho, K.-B.; Lee, Y.-M.; Fukuzumi,
S.; Nam, W. Reactivity Comparison of High-Valent Iron(IV)-Oxo
Complexes Bearing N-Tetramethylated Cyclam Ligands with Differ-
ent Ring Size. Dalton Trans. 2013, 42, 7842−7845.
(51) Nakazawa, J.; Terada, S.; Yamada, M.; Hikichi, S. Structural
Characterization and Oxidation Reactivity of a Nickel(II) Acylperoxo
Complex. J. Am. Chem. Soc. 2013, 135, 6010−6013.
(52) Kunishita, A.; Ertem, M. Z.; Okubo, Y.; Tano, T.; Sugimoto,
H.; Ohkubo, K.; Fujieda, N.; Fukuzumi, S.; Cramer, C. J.; Itoh, S.
Active Site Models for the CuA Site of Peptidylglycine α-
Hydroxylating Monooxygenase and Dopamine β-Monooxygenase.
Inorg. Chem. 2012, 51, 9465−9480.
VA/K
.
(53) Tano, T.; Ertem, M. Z.; Yamaguchi, S.; Kunishita, A.;
Sugimoto, H.; Fujieda, N.; Ogura, T.; Cramer, C. J.; Itoh, S.
Reactivity of Copper(II)-Alkylperoxo Complexes. Dalton Trans. 2011,
40, 10326−10336.
(54) Maillard, B.; Ingold, K. U.; Scaiano, J. C. Rate Constants for the
Reactions of Free Radicals with Oxygen in Solution. J. Am. Chem. Soc.
1983, 105, 5095−5099.
(35) Bravo, A.; Bjorsvik, H.-R.; Fontana, F.; Minisci, F.; Serri, A.
Radical Versus “Oxenoid” Oxygen Insertion Mechanism in the
Oxidation of Alkanes and Alcohols by Aromatic Peracids. New
Synthetic Developments. J. Org. Chem. 1996, 61, 9409−9416.
(36) McDonald, A. R.; Que, L. High-Valent Nonheme Iron-Oxo
Complexes: Synthesis, Structure, and Spectroscopy. Coord. Chem. Rev.
2013, 257, 414−428.
H
https://dx.doi.org/10.1021/acs.inorgchem.0c03469
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
(55) Russell, G. A.; Bridger, R. F. The Reactivity of Phenyl Radicals
toward Molecular Oxygen. J. Am. Chem. Soc. 1963, 85, 3765−3766.
(56) Russell, G. A. Deuterium-Isotope Effects in the Autoxidation of
Aralkyl Hydrocarbons. Mechanism of the Interaction of Peroxy
Radicals1. J. Am. Chem. Soc. 1957, 79, 3871−3877.
(57) Armarego, W. L. F. Purification of Laboratory Chemicals;
Elsevier Science, 2017.
(58) Mann, C. K.; Mann, C. K.; Barnes, K. K. Electrochemical
Reactions in Nonaqueous Systems; Marcel Dekker, 1970.
(59) 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.; 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.
Gaussian09, revision D.01;Gaussian, Inc.: Wallingford, CT, 2009.
̌
(60) Miertus, S.; Scrocco, E.; Tomasi, J. Electrostatic Interaction of a
Solute with a Continuum. A Direct Utilizaion of Ab Initio Molecular
Potentials for the Prevision of Solvent Effects. Chem. Phys. 1981, 55,
117−129.
I
https://dx.doi.org/10.1021/acs.inorgchem.0c03469
Inorg. Chem. XXXX, XXX, XXX−XXX