Hydrogen bond-enabled heterolytic and homolytic peroxide activation within nonheme copper(ii)-alkylperoxo complexes

Article abstract of DOI:10.1021/acs.inorgchem.9b01898

To explore the reactivity of copper-alkylperoxo species enabled by the heterolytic peroxide activation, room-temperature stable mononuclear nonheme copper(II)-alkylperoxo complexes bearing a N-(2-ethoxyethanol)-bis(2-picolyl)amine ligand (HN₃O₂), [Cu^II(OOR)(HN₃O₂)]⁺ (R = cumyl or ^tBu), were synthesized and spectroscopically characterized. A combined experimental and computational investigation on the reactivity and reaction mechanisms in the phosphorus oxidation, C-H bond activation, and aldehyde deformylation reactions by the copper(II)-alkylperoxo complexes has been conducted. DFT-optimized structures suggested that a hydrogen bonding interaction exists between the ethoxyethanol backbone of the HN₃O₂ ligand and either the proximal or distal oxygen atom of the alkylperoxide moiety, and this interaction consequently results in the enhanced stability of the copper(II)-alkylperoxo species. In the phosphorus oxidation reaction, both experimental and computational results indicated that a phosphine-triggered heterolytic O-O bond cleavage occurred to yield phosphine oxide and alcohol products. DFT calculations suggested that (i) the H-bonding between the ethoxyethanol backbone and distal oxygen of the alkylperoxide moiety and (ii) the phosphine binding to the proximal oxygen of the alkylperoxide moiety engendered the heterolytic peroxide activation. In the C-H bond activation reactions, temperature-dependent reactivity of the copper(II)-alkylperoxo complexes was observed, and a relatively strong activation energy of 95 kcal mol^-1 was required to promote the homolytic peroxide activation. A rate-limiting hydrogen atom abstraction reaction of xanthene by the putative copper(II)-oxyl radical resulted in the formation of the dimeric copper product and the substrate radical that further underwent autocatalytic oxidation reactions to form an oxygen incorporated product. Finally, amphoteric reactivity of copper(II)-alkylperoxo complexes has been assessed by conducting kinetic studies and product analysis of the aldehyde deformylation reaction.

Full text of DOI:10.1021/acs.inorgchem.9b01898

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Hydrogen Bond-Enabled Heterolytic and Homolytic Peroxide
Activation within Nonheme Copper(II)-Alkylperoxo Complexes
†
‡,§
∥
,‡,#
,†
Hana Oh, Wei-Min Ching,
Jin Kim, Way-Zen Lee,*
and Seungwoo Hong*
†
Department of Chemistry, Sookmyung Women’s University, Seoul 04310, Korea
Department of Chemistry, National Taiwan Normal University, Taipei 11677, Taiwan
Instrumental Center, National Taiwan Normal University, Taipei 11677, Taiwan
Western Seoul Centre, Korea Basic Science Institute, Seoul 03759, Republic of Korea
Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiung 807, Taiwan
‡
§
∥
#
*
S Supporting Information
ABSTRACT: To explore the reactivity of copper-alkylperoxo
species enabled by the heterolytic peroxide activation, room-
temperature stable mononuclear nonheme copper(II)−
alkylperoxo complexes bearing a N-(2-ethoxyethanol)-bis(2-
II
+
picolyl)amine ligand (HN O ), [Cu (OOR)(HN O )] (R =
3
2
3
2
t
cumyl or Bu), were synthesized and spectroscopically
characterized. A combined experimental and computational
investigation on the reactivity and reaction mechanisms in the
phosphorus oxidation, C−H bond activation, and aldehyde
deformylation reactions by the copper(II)−alkylperoxo
complexes has been conducted. DFT-optimized structures
suggested that a hydrogen bonding interaction exists between the ethoxyethanol backbone of the HN O ligand and either the
3
2
proximal or distal oxygen atom of the alkylperoxide moiety, and this interaction consequently results in the enhanced stability of
the copper(II)−alkylperoxo species. In the phosphorus oxidation reaction, both experimental and computational results
indicated that a phosphine-triggered heterolytic O−O bond cleavage occurred to yield phosphine oxide and alcohol products.
DFT calculations suggested that (i) the H-bonding between the ethoxyethanol backbone and distal oxygen of the alkylperoxide
moiety and (ii) the phosphine binding to the proximal oxygen of the alkylperoxide moiety engendered the heterolytic peroxide
activation. In the C−H bond activation reactions, temperature-dependent reactivity of the copper(II)−alkylperoxo complexes
−
1
was observed, and a relatively strong activation energy of 95 kcal mol was required to promote the homolytic peroxide
activation. A rate-limiting hydrogen atom abstraction reaction of xanthene by the putative copper(II)-oxyl radical resulted in the
formation of the dimeric copper product and the substrate radical that further underwent autocatalytic oxidation reactions to
form an oxygen incorporated product. Finally, amphoteric reactivity of copper(II)−alkylperoxo complexes has been assessed by
conducting kinetic studies and product analysis of the aldehyde deformylation reaction.
INTRODUCTION
alkylperoxides (ROOH) within iron-hydroperoxo or -alkylper-
oxo species has been proposed in the catalytic cycle of
superoxide reductase enzymes (Scheme 1a, pathway iii).
■
Transition metal-mediated activation of peroxide has attracted
particular attention due to their catalytic and biological
relevance in versatile oxidative processes, including synthesis
of organic molecules, metabolism, and energy-related con-
15−19
Copper complexes bearing a peroxo ligand including copper-
hydroperoxo and alkylperoxo species have also been postulated
as key intermediates in copper monooxygenases such as
dopamine β-monooxygenase and lytic polysaccharide mono-
1
−7
versions.
In the heme system, it has been well-established
that the reaction between alkylperoxides and iron porphyrins
has involved either homolytic or heterolytic O−O bond
cleavage (Scheme 1a, pathways i and ii),
20−31
oxygenase.
An important aspect of these enzymes relied
8
−14
on the catalysis of the oxygenation and oxidation reactions in
dioxygen activation chemistry under biological and non-
biological conditions. One notable example is the peroxidation
of lipid by copper(II) in the presence of lipid hydro-
which
subsequently generates a catalytically active ferryl (FeO)
species capable of performing strong hydrocarbon oxygenation
reactions. For instance, the reactions of peroxides with typical
peroxidases were exclusively promoted in heterolytic cleavage
of peroxide and generated high-valent iron(IV)-oxo inter-
mediates coupled to a porphyrin π-radical cation or iron(V)-
oxo intermediates. Furthermore, apart from the O−O bond
cleavage, the proton-assisted loss of hydroperoxide (H O ) or
32−35
peroxide.
Thus, understanding the structure, spectro-
scopic features and reactivity of copper-hydroperoxo and
Received: June 27, 2019
2
2
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b01898
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Inorganic Chemistry
Article
Scheme 1. Proposed Mechanisms of Peroxide Activation by
Mononuclear Metal Site, Recent Reports of Biomimetic
Mononuclear Copper-Alkylperoxo Models, and Proposed
Hydrogen Bonding (H-bonding) Effect on Copper-
Alkylperoxo Model
toward substrates having relatively weak C−H or O−H
47
bonds. Very recently, Cho and co-workers demonstrated
that copper(II)−alkylperoxo complexes were capable of
4
8
conducting nucleophilic reactions. Notwithstanding the
importance of the copper-alkylperoxo species due to their
proposed roles in biocatalysis, less is known about the intrinsic
reactivity of the copper(II)−alkylperoxo complexes via
heterolytic O−O bond cleavage due to their inherent
instability and sluggish reactivity in performing oxidative
transformation of substrates.
Recently, a strong relationship between hydrogen bonding
H-bonding) networks and the reactivity and/or stability of
(
copper(II)-hydroperoxo complexes has been highlighted by
49,50
Borovik and co-workers.
It has been proposed that a H-
bonding to the proximal O atom of the peroxo ligand resulted
in the stability of metal−oxygen species, whereas a H-bonding
to the distal O atom of the peroxo ligand produced a reactive
hydroperoxo ligand that enabled C−H bond activation of
5
1−57
substrates (Scheme 1c).
workers also demonstrated that copper(II)-hydroperoxo and
superoxo complexes featuring a proximal H-bonding displayed
an enhanced intrinsic stability and H atom abstraction
In this regard, Karlin and co-
-
58,59
reactivity.
However, to the best of our knowledge, there
is no report of a copper(II)−alkylperoxo complex with a H-
bonding to the distal or proximal O atom of the peroxo ligand.
Herein, we present the synthesis of copper(II)−OOR (R =
t
cumyl and Bu) complexes supported by a tetradentate ligand
containing a N,N-bis(2-picolyl)amine backbone with a pendant
ethoxyethanol side chain (HN O ), which is susceptible to
3
2
providing a H-bonding to the distal and/or proximal O atom
of the alkylperoxide moiety, and their spectroscopic character-
ization along with reactivity studies including oxygen atom
transfer, C−H bond activation, and aldehyde deformylation
reactions. Compared to other copper(II)−OOR complexes,
these intermediates exhibit much enhanced stability and
reactivity that provide new mechanistic understandings for
copper monooxygenases.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Copper(II) Com-
plex. The copper(II) complex coordinated with a HN O
3
2
II
ligand, [Cu (HN O )(CH CN)(H O)](ClO ) (1), was
3
2
3
2
4 2
synthesized and characterized by various spectroscopic
methods (see Experimental Section in the Supporting
Information). The crystal structure of 1 exhibits a tetragonal
geometry with a typical Jahn−Teller distortion, as expected for
9
d ions; an elongation of Cu−O
and Cu−OH2O distances
3 2
HN O
(
∼2.5 Å) along the z-axis and an average Cu−N distance of
-
alkylperoxo species is of critical importance in biochemical
∼2.0 Å were observed (Figure 1 and Tables S1 and S3). The
X-band electron paramagnetic resonance (EPR) spectrum of 1
research and catalytic oxidation chemistry.
In model chemistry, major advances have been achieved in
synthesizing and characterizing copper(II)−alkylperoxo spe-
reveals an axial signal with g
centered at 2.16 (A = 178 G)
∥ ∥
and g centered at 2.01, consistent with a tetragonally
⊥
3
6−46
9
cies
since the seminal study of Kitajima on the structural
elongated octahedral d copper(II) center (Figure S1).
Synthesis and Characterization of Copper(II)−Alkyl-
peroxo Complexes. The copper(II)−alkylperoxo complexes,
characterization of mononuclear copper(II)-cumylperoxo
complex bearing hydrotris(pyrazolyl)borate ligand was re-
36
II
+
CH
II
t
ported in 1993 (Scheme 1b). In addition, the reactivity
studies of copper(II)−alkylperoxo complexes was explored by
many research groups. Itoh and co-workers examined the
electrophilic reactivity of copper(II)−alkylperoxo complexes
[Cu (OOcumyl)(HN O )] (2 ) and [Cu (OO Bu)-
3 2
+
BH
(HN O )] (2 ), were synthesized by adding 2.0 equiv of
3
2
CH
alkylperoxide, cumene hydroperoxide (cumylOOH) for 2
and tert-butyl hydroperoxide ( BuOOH) for 2 , in the
presence of 2.0 equiv of triethylamine (TEA) in CH CN at
room temperature. The resulting green species were very stable
,
t
BH
39,43
that was triggered by O−O bond homolysis.
co-workers successfully characterized copper(II)- and copper-
III)-alkylperoxo complexes and investigated their reactivities
Tolman and
3
(
(t1/2 ∼ 2 days) at 293 K as compared to the previously
B
DOI: 10.1021/acs.inorgchem.9b01898
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Inorganic Chemistry
Article
EPR spectra of 2CH and 2 , recorded in a frozen solution of
BH
CH CN at 78 K, display axial signals at g = 2.31 (A = 198 G)
3
CH
BH
and 2.11 for 2 and g = 2.30 (A = 198 G) and 2.11 for 2 ,
z
respectively (Figures S2b and S3a). These large A values of
CH
BH
2
and 2 as compared to 1 imply that the geometries of
CH
BH
42
both 2 and 2 are closer to square planar; there might be
a weak interaction between the copper(II) center and the
ethoxyethanol moiety due to Jahn−Teller distortion. A positive
CH
mode ESI MS spectrum of 2 revealed a prominent ion peak
at m/z of 501.1, whose mass and isotope distribution patterns
II
+
correspond to [Cu (OOcumyl)(HN O )] (calculated m/z of
3
2
II
Figure 1. Crystal structure of [Cu (HN O )(CH CN)(H O)]-
CH
3
2
3
2
501.1; Figure S3b). When 2
l O OH, a four-mass-unit shift from m/z 501.1 to 505.1 was
was treated with cumy-
(
ClO ) (1), with thermal ellipsoids showing 50% probability. Two
18 18
4
2
perchlorate ions are omitted for clarity (see Tables S1 and S3).
observed (inset of Figure S3b). This result demonstrates that
CH
2
contains two oxygen atoms deriving from cumylperoxo
reported copper(II)−alkylperoxo complexes bearing picolyl-
CH
group. The rRaman spectrum of 2 , upon 441.6 nm
39,48,50
amine-based ligands,
allowing us to characterize them
excitation in CH CN at 233 K, exhibited four isotopically
3
with various spectroscopic methods, such as UV−vis, electro-
spray ionization mass (ESI MS), EPR, and resonance Raman
−1
sensitive bands at 880, 832, 594, and 516 cm , which shifted
to 850, 796, 584, and 502 cm upon O-labeling of 2
−1
18
CH
(
rRaman).
−1
(
Figure 2b). These bands in the range of 800−900 cm are
The UV−vis spectra of the obtained copper(II)−alkylperoxo
assignable to the mixed O−O/C−O/C−C vibrations of the
complexes exhibited ligand-to-metal charge transfer bands at
3
cm ) for 2 along with weak d−d transition bands at 557 (ε
=
−1
cumylperoxo group, the band at 594 cm to the Cu−O
−
1
−1
CH
−1
90 (ε = 1800 M cm ) for 2 and 403 nm (ε = 900 M
−1
vibration, and the band at 516 cm to the C−C−O
−
1
BH
39,49
deformation mode of the cumylperoxo moiety.
Despite
−
1
−1
CH
−1
−1
130 M cm ) for 2 and 595 nm (ε = 110 M cm ) for
39
multiple attempts, the improved stability did not allow us to
grow single crystals suitable for X-ray crystallography.
BH
2
, respectively (Figure 2a and Figure S2a). The X-band
Density Functional Theory (DFT) Calculations. In
order to gain more insight into the intramolecular H-bonding
interaction between the HN O ligand and cumylperoxo
3
2
CH
CH
moiety in 2 , DFT calculations on 2 without and with
intramolecular H-bonding were performed. The 2a, 2b, and 2c
were constructed from the crystal structure of [Cu (HN O )-
II
3
2
(
CH CN)(H O)](ClO ) (1) by simply replacing the
3 2 4 2
coordinated CH CN with a cumylperoxo group. In addition,
3
the intramolecular H-bonding interactions were introduced
into 2b and 2c. The optimized structures of 2a, 2b, and 2c are
illustrated in Figures S4 and 3a and b, respectively, and the
important bond distances and angles of all optimized structures
are tabulated in Table S5. The relative energies of 2a, 2b, and
2
c are also displayed in Figure S5. The relative energies
diagram shows that 2a is energetically unfavorable. Interest-
ingly, the energy of 2c is slightly lower (ca. 3.2 kcal/mol) than
CH
Figure 2. (a) UV−vis spectrum of 2 obtained from the reaction of
1
(1.0 mM) with cumylOOH (2.0 mM) in the presence of TEA (2.0
CH
mM) in CH CN at 293 K (blue trace for 2 and black trace for 1).
3
CH
Inset shows the visible spectrum of 2 in the region of 400−800 nm.
II 16 16
+
(
b) rRaman spectra of [Cu ( O Ocumyl)(HN O )] (4.0 mM,
3 2
CH 16
II 18 18
+
2
2
2
- O, black trace) and [Cu ( O Ocumyl)(HN O )] (4.0 mM,
3 2
CH 18
- O, blue trace) obtained from the reaction of 1 (4.0 mM) with
16
16
18 18
18
.0 equiv of cumyl O OH and cumyl O OH (>90% O enriched)
Figure 3. DFT optimized structure of 2CH, (a) 2b, and (b) 2c and X-
ray crystal structure of (c) 1-N3 and (d) 1-Bn showing 50%
probability ellipsoids. Colors for the structures: Cu, orange; N, blue;
O, red; C, pale gray; H, white.
in the presence of 2.0 equiv of TEA in CH CN at 233 K (λ = 441.6
3
ex
nm), respectively. The red trace indicates the corresponding
1
6
18
O − O isotope difference spectrum. Asterisks denote the solvent
2
2
band.
C
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Inorganic Chemistry
Article
that of 2b, suggesting that the H-bonding interaction between
was first investigated in the oxygen atom transfer reaction.
the HN O ligand and the proximal oxygen atom of the
Upon the addition of triphenylphosphine (PPh ) to CH CN
3
2
3
3
CH
BH
cumylperoxo group is more pronounced and two conforma-
tions could interconvert to each other in the solution phase
due to a small energy gap. Notably, the Mulliken spin density
analysis indicates that the distal oxygen atom loses its spin
density through the H-bonding interaction (Figure S6). The
higher spin density of the proximal oxygen atom suggests
heterolytic O−O bond cleavage would be carried out in
phosphorus oxidation.
solutions of 2 and 2 , the UV−vis absorption bands at 390
CH BH
and 557 nm for 2 and 403 and 595 nm for 2 were
decreased (Figure 4a and b). The rate constants (kobs),
Synthesis and Characterization of Copper(II)- Com-
plexes with and without H-Bonding Interaction. To
mimic the H-bonding between the HN O ligand and the
3
2
alkylperoxo group seen in the computed structures of 2b and
c, we synthesized two modified copper(II) complexes. First,
2
the addition of sodium azide to the solution of 1 yielded a
copper(II) complex with the binding of two azido ligands,
II
[
Cu (HN O )(N ) ] (1-N ; Figure 3c and Table S3), to
3
2
3 2
3
examine the structural influence by the H-bonding interaction.
The UV−vis spectra of 1-N exhibited ligand-to-metal charge
3
−
1
−1
transfer bands at 384 (ε = 3000 M cm ) with weak d−d
transition bands at 600 nm (ε = 235 M− cm ; Figure S7a). A
1
−1
negative mode ESI MS spectrum of 1-N revealed a prominent
3
ion peak at an m/z of 533.0, whose mass and isotope
II
distribution patterns correspond to [Cu (HN O )-
3
2
−
(
N ) (ClO )] (calculated m/z of 533.0; inset of Figure
3 2 4
S7a). The X-band EPR spectra of 1-N , recorded in a frozen
3
solution of CH CN at 78 K, display axial signals at g = 2.30 (Az
3
=
174 G) and 2.06 (Figure S7b). The coordination of two
azido ligands constructed a distorted octahedral geometry due
to the bending of two pyridines in the HN O ligand
3
2
framework (e.g., ∠N1−Cu1−N3 of 165.8° for 1 vs ∠N1−
Cu1−N3 of 103.5° for 1-N ). Interestingly, the H-bonding
3
from the ethoxyethanol moiety to the proximal N atom of an
azido ligand was observed similar to that between ethox-
yethanol moiety to the proximal O atom seen in the computed
2
c (Figure 3a). Second, we modified the HN O ligand by
3 2
incorporating a benzyl group into ethoxyethanol moiety in
order to eliminate the H-bonding interaction as revealed in the
II
X-ray structure of [Cu (N O-OBn)Br](ClO ) (1-Bn; Figure
3
4 2
3
d and Tables S2 and S4). A positive mode ESI MS spectrum
of 1-Bn exhibited a prominent ion peak at an m/z of 539.1,
whose mass and isotope distribution patterns correspond to
Figure 4. UV−vis spectral changes observed in the reactions of (a)
II
+
CH
BH
[
Cu (N O-OBn)(ClO )] (calculated m/z of 539.1; Figure
2
(0.25 mM, blue line) with PPh (5.0 mM) and (b) 2 (0.25
3
3
4
mM, blue line) with PPh (5.0 mM) in CH CN at 293 K. Insets show
S8a, inset). The X-band EPR spectra of 1-Bn, recorded in a
3
3
CH
the time courses monitored (a) at 390 nm for the decay of 2 and
b) at 403 nm for the decay of 2 . (c) Plot of log (k ) against the σ
value of para-X-substituted PPh for the reaction of 2 (Table S6).
frozen solution of CH CN at 78 K, displays axial signals at g =
3
BH
(
rel P
2
.36 and 2.07 (Figure S8b). As expected, the formation of
BH
3
copper(II)−alkylperoxo species was not observed upon the
addition of cumylOOH to the solution containing 1-Bn and
TEA (Figure S8a). Based on Scheme 1c, it is plausible that the
removal of the H-bonding interaction results in the instability
of copper(II)−alkylperoxo under identical reaction conditions.
Also, the DFT calculation result predicted that the formation
determined by pseudo-first-order fitting of the kinetic data for
CH
BH
the disappearance at 390 nm for 2 and 403 nm for 2 ,
increased proportionally with the increase of PPh concen-
3
−
1
tration. The second-order rate constants (k ) of 7.5 × 10
2
−
1
−1
CH
−1 −1
BH
2
a without an intramolecular H-bonding interaction is
M
s
for 2 and 3.5 M
s
for 2 were determined
energetically disfavored. This clearly illustrates the importance
of H-bonding on the stability of the copper(II)−alkylperoxo
complexes. In sum, the square planar geometry imposed by the
ethoxyethanol moiety is required for the H-bonding interaction
between HN O and alkylperoxo ligands; this effectively
corroborated the formation and stability of the copper(II)−
alkylperoxo complexes.
(Figures S9 and S10c). The reactivity difference could be
rationalized by two factors: (i) the steric demands imposed by
the alkyl group of two peroxides since the phosphorus
oxidation was presumably triggered by the PPh attack on
3
the alkylperoxo ligand and (ii) their electrochemical properties
3
2
CH
because the smaller oxidation potential (E ) value of 2
ox
+
BH
+
(0.61 V vs Fc/Fc ) than that of 2 (0.49 V vs Fc/Fc ) may
Phosphorus Oxidation Reactions by Copper(II)-
Alkylperoxo Complexes. The reactivity of 2 and 2
result due to the smaller oxidizing power (Figure S11). Direct
CH
BH
addition of PPh to 1 did not afford any spectral changes
3
D
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Inorganic Chemistry
Figure S12); therefore, we could rule out the possibility of the
Article
(
was then explored in the C−H bond activation reactions.
CH
ligand exchange between the alkylperoxo ligand and PPh . It is
Upon the addition of xanthene to a CH CN solution of 2 ,
3
3
noteworthy that other copper(II)−alkylperoxo complexes
accommodating picolylamine-based ligands without H-bonds
such as TPA and BPA did not exhibit any reactivity toward
the absorption band at 390 nm remained unchanged at 293 K,
indicating that 2 is inactive toward xanthene at ambient
temperature. However, it has been well-documented that the
self-decomposition of copper(II)−alkylperoxo complexes at an
elevated temperature caused homolytic peroxide activation;
subsequent formation of acetophenone from the cumyloxyl
radical via β-scission (Scheme 3, pathway a) and copper(II)-
6
0,61
PPh3.
complexes was demonstrated by determining the ρ value of
3.0 in the oxygen atom transfer reaction of para-X-
The electrophilic character of the copper(II)−OOR
−
BH
substituted PPh (X = OMe, Me, H, and Cl) by 2 (Figure
3
4
c and Figure S10). The Eyring plots of the temperature-
BH
CH
dependent kinetic studies for 2
activation parameters of ΔS and ΔH for the oxygen atom
transfer reactions (Figures S13 and S14). Large negative
entropy values (ΔS of −55 J K mol for 2 and −98 J K
mol for 2 ) suggested that the phosphorus oxidation by 2
and 2
revealed the
Scheme 3. Proposed Mechanisms in C−H Bond Activation
‡
‡
CH
Reactions by 2
‡
−1
−1
BH
−1
−
1
CH
occurred via a biomolecular mechanism.
Product analysis of the reaction solution of the phosphorus
CH
oxidation reactions by 2
revealed the formation of
triphenylphosphine oxide in over 95% yield. Interestingly,
HPLC analysis of the organic products revealed the formation
of cumyl alcohol in over 95% yield and a trace amount of
18
CH
acetophenone. The oxidation of PPh with O-labeled 2
3
revealed both triphenylphosphine oxide and cumyl alcohol
18
products containing ∼90% of O (Figure S15). These results
strongly suggest a direct oxygen atom transfer from the
oxyl species was capable of activating the C−H bonds (Scheme
copper(II)-OOR unit to PPh via the heterolytic peroxide
62,63
3
3, pathway b).
On the basis of these well-established
cleavage (Scheme 2). To examine the possibility of the direct
CH
reports, the reactivity of 2 in the C−H bond activation
reaction was examined at different temperatures. The natural
decay rate was increased with an increase of temperature
Scheme 2. Proposed Mechanisms in Phosphorous
CH
Oxidation Reactions by 2
(
Figure S18); the activation energy (E ) of homolytic peroxide
a
−
1
CH
activation was determined to be 94 kJ mol for 2 and 95 kJ
mol for 2 (Figure S19). It would be reasonable to consider
−
1
BH
that these E values would present the sum of the energy costs
a
for the homolytic cleavage of peroxide and for the break of the
H-bonding between the ethoxyethanol moiety and alkylperoxo
ligand, i.e., generally, H-bonding energy is in the range of 10 to
−
1
−1
3
0 kJ mol and at 30.5 kJ mol in the present case on the
64
oxidation of PPh by alkyl hydroperoxides, we also conducted a
basis of DFT calculation. The addition of xanthene to the
solution of 2 resulted in the disappearance of a UV−vis
absorption band of 2 at 390 nm (Figure 5a). Interestingly,
3
t
CH
phosphorus oxidation reaction with only BuOOH. By
CH
comparing the reaction rate and the product yield of
BH
t
phosphorus oxidation reactions using 2
and BuOOH
the decay rate was linearly increased with an increase of the
(
Figures S13 and S16), we confirmed that the triphenylphos-
concentration of xanthene at 343 K, and a k value of 2.0 ×
2
t
−1
−1 −1
phine oxide formed by free BuOOH is almost negligible (less
than 5%) under identical reaction conditions. The copper
product was then detected by EPR and ESI MS spectroscopies.
The X-band EPR spectrum of the copper product exhibited
signals at g = 2.20, and 2.03, indicating the oxidation state of
10
M
s
was determined (Figure S20a). The k values in
2
the oxidation of other substrates, such as 9,10-dihydroan-
thracene (DHA) and 1,4-cyclohexadiene (CHD), were also
determined (Figure S20 and Table S7), and a linear correlation
between the k values and the C−H bond dissociation energies
2
+
2 for the Cu product (Figure S17a). The ESI MS spectrum of
(BDEs) of substrates was evidenced (Figure 4b). In addition, a
the reaction solution exhibited a prominent mass peak at m/z
kinetic isotope effect (KIE) value of 7.1(1) was obtained in the
xanthene oxidation by 2 (Figure S21 and Table S7). A large
KIE value together with a good correlation between the
CH
of 349.1 and 612.0 corresponding to the mass and isotope
II
+
+
distribution pattern of the deprotonated [Cu (N O2)]
3
I
(
(
calculated m/z of 349.1) and [Cu (HN O )(PPh )]
reaction rates and the C−H BDEs of substrates suggest that H-
3
2
3
CH
calculated m/z of 612.1), respectively (Figure S17b).
atom abstraction from the C−H bond of substrates by 2 is
CH
However, the sulfoxidation of thioanisole by 2
did not
the rate-determining step.
occur under identical reaction conditions.
Taken together, the phosphorus oxidation occurred via
In addition, the copper product was successfully recrystal-
lized as shown in Figure 6 (Tables S2 and S4). Unlike other
copper(II) complexes supported by a picolylamine-based
heterolytic O−O bond cleavage of the copper(II)−alkylperoxo
39,47
complexes. Presumably, the PPh -promoted phosphorus
ligand,
the cyclization of the copper(II) complex occurred
3
oxidation reaction by 2 would take place when the H-bonding
interaction was localized at the distal O-atom of the
alkylperoxo ligand (Scheme 2).
after the homolytic peroxide cleavage (Scheme 3, pathway c);
the deprotonation of the ethoxyethanol group of the HN O
3
2
ligand would provoke the dimerization of copper(II)
II
C−H Bond Activation Reactions by Copper(II)-
complexes, [Cu (N O2)] (ClO ) (5; Figure S22). The
3
2
4 2
CH
BH
Alkylperoxo Complexes. The reactivity of 2 and 2
formation of the hydroxide ion was proposed via H-atom
E
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Inorganic Chemistry
Article
monitored (Figure 5a, inset). An increase of the reaction
temperature from 293 to 353 K resulted in the shortening of
the induction period accompanied by an acceleration of
sigmoidal formation of xanthone (Figure S23). Furthermore,
the addition of a radical scavenger such as bromotrichloro-
CH
methane (CCl Br) yielded an immediate decay of 2 , and a
3
trace amount (less than 5%) of xanthone was detected
(
Scheme 4 and Figure S24). When the xanthene oxidation
Scheme 4. Exogenous Factors Such as Temperature, Radical
Scavenger, and O Affecting the C−H Bond Activation
2
CH
Reactions by 2
Figure 5. (a) UV−vis spectral changes observed in the reaction of
reaction was carried out under a N atmosphere, the trace
amount of xanthone was detected by either UV−vis spectros-
copy or HPLC, indicating that oxygen incorporation into
2
CH
2
(1.0 mM, blue line) and xanthene (10 mM) in CH CN at 343 K.
3
Inset shows the time courses monitored at 390 nm (blue circle) due
CH
to the decay of 2 and 337 nm (red circle) due to the formation of
xanthone was originated from air (e.g., the presence of O ;
2
xanthone. (b) Plot of log k ′ against the C−H BDE of substrates for
18
CH
2
Figure S25). Finally, the use of O-labeled 2 under identical
reaction conditions revealed the O-incorporated xanthone
and O-incorporated acetophenone with over 90% yields
CH
the reactions of 2 . Second-order rate constants, k , were determined
2
16
and then adjusted for reaction stoichiometry to yield k ′ based on the
2
18
number of equivalent target C−H bonds of substrates (see data in
(
Figure S26). These results supported that the in situ generated
Table S7 and Figure S20).
hydroxide ligand did not rebound to the substrate radical but
to deprotonated ethanol group of the HN O ligand. In
3
2
addition, the autocatalytic radical chain reaction was induced
by the homolytic activation of alkylperoxide within the
copper(II)−OOR complexes in the presence of dioxygen to
yield xanthone as a final organic product.
Aldehyde Deformylation Reactions by Copper(II)-
Alkylperoxo Complexes. Recent findings on the oxidative
nucleophilic reactivity of copper(II)−alkylperoxo intermedi-
CH
ates intrigued us to investigate the reactivity of 2 in the
48
aldehyde deformylation reaction. The addition of 20 equiv of
cyclohexanecarboxaldehyde (CCA) to a CH CN solution of
3
CH
2
at 293 K afforded an immediate decay of the characteristic
band at 390 nm due to 2 . Since the decay rate was too fast
to evaluate even at 233 K, the pseudo-first-order rate constants
CH
Figure 6. Crystal structure of 3 with thermal ellipsoids showing 50%
probability. Two perchlorate ions, one CH CN molecule, and
were determined under stopped-flow kinetic conditions; a k
2
3
2
−1 −1
hydrogen atoms are omitted for clarity. Colors for the structures:
Cu, orange; N, blue; O, red; C, pale gray; H, white.
value of 3.8 × 10 M
s
was obtained at 233 K (Figure 7a
CH
BH
and b). This result indicates that 2 and 2 are exhibiting
amphoteric reactivity in nucleophilic and electrophilic
oxidative reactions. It has been recently reported that a
metal(III)-hydroperoxo complex can also perform both
abstraction from xanthene by the copper(II)-oxyl radical after
the homolytic peroxide cleavage, engendering the deprotona-
tion of the ethanol group of the HN O ligand. To confirm this
65−69
nucleophilic and electrophilic reactions.
Product analysis
3
2
unusual formation of a dimeric copper product, we added 1.1
equiv of hydroxide ions to complex 1, and the identical dimeric
of the reaction solutions revealed the exclusive formation of
cyclohexene (85(3)%), cumyl alcohol (60(10)%), and formate
5
was formed. Therefore, it is concluded that homolytic
(65(5)%; Scheme 5 and Figure S27). The copper product
CH
peroxide activation of the copper(II)−OOR complex occurred
as the reaction temperature increased.
analysis revealed that 2
was converted back to 1 after
aldehyde deformylation reaction (Figure S28). This result was
in sharp contrast with the previous report, in which the
exclusive formation of acetophenone was observed due to the
While the first-order kinetics were evidenced in the C−H
CH
bond activation reaction by 2 , an induction period was
48
observed when the time course of the formation of xanthone
O−O bond homolysis of the cumylperoxo ligand. We
with the absorption band at 337 nm (ε = 8000 M− cm ) was
1
−1
therefore proposed that the H-bonding between the
F
DOI: 10.1021/acs.inorgchem.9b01898
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
CH
was conducted (Figure 7c). The nucleophilic character of 2
was demonstrated by a ρ value of 2.6 in the reactions of 2
CH
with para-X-Ph-COCl. In the reactivity studies, we have shown
that the copper(II)−alkyperoxo complexes are capable of
conducting electrophilic and nucleophilic reactions under
identical reaction conditions. On the basis of the reaction
rates and Hammett plot, we propose that a nucleophilic attack
on the carbonyl group of aldehyde by the alkylperoxo moiety
CH
of 2
generates a putative formation of copper(II)−
peroxyhemiacetal-like species that subsequently undergo
heterolytic O−O bond cleavage to afford cumyl alcohol,
formate ions, and cyclohexene (Scheme 5).
CONCLUSION
■
In summary, copper(II)−alkylperoxo complexes bearing a
II
+
CH
picolylamine based ligand, [Cu (OOR)(HN O )] (2 for R
3
2
BH
t
=
cumyl; 2 h for R = Bu), were synthesized and
characterized by various spectroscopic methods. A pendant
ethoxyethanol side chain presumably creates a H-bonding
interaction with the proximal or distal O atom of the peroxo
CH
BH
ligand of 2
and 2 . This H-bonding interaction was
essential to forming stable copper(II)−alkylperoxo complexes
and to performing unusual oxygen atom transfer reactions via
heterolytic O−O bond cleavage. On the other hand, an
increase of the reaction temperature induced the homolytic
O−O bond cleavage of 2, resulting in the formation of
acetophenone and a copper(II)-oxyl radical, which was capable
of conducting the C−H bond activation reactions. In addition,
the nucleophilic reactivity of copper(II)−alkylperoxo com-
plexes was evidenced since these complexes can perform an
aldehyde deformylation reaction under stopped-flow kinetic
conditions. These results would give a clue for the elucidation
of the catalytic amphoteric reactivity of copper(II)−alkylper-
oxo complexes established by the H-bonding network.
EXPERIMENTAL SECTION
Figure 7. (a) UV−vis spectral changes observed in the reaction of
■
CH
2
(0.25 mM, blue line) and CCA (5.0 mM) in CH CN at 233 K.
Materials. All chemicals were obtained from Aldrich Chemical Co.
and used without further purification unless otherwise indicated.
Solvents were dried according to the reported procedures and distilled
3
Inset shows the time courses monitored at 390 nm for the decay of
CH
2
. (b) Plots of kobs against CCA concentrations to determine the k
2
70
18
18
18
CH
value in the reaction of 2 (0.25 mM) and CCA (5.0 mM) in
CH CN at 233 K. (c) Plot of log (k ) against the σ value of para-X-
under Ar prior to use. H2 O (95% O-enriched) and O (70%
2
1
8
O-enriched) were purchased from ICON Services Inc. (Summit, NJ,
3
rel
P
CH
18 18
18
substituted benzoyl chlorides for the reaction of 2 (Table S8).
USA). Doubly labeled cumyl O OH (90% O-enriched) was
1
8
synthesized by reacting cumene and O2 in hexane at 85 °C
71
according to the procedure of Finn and Sharpless. The nonheme
copper(II) complex was prepared according to the literature
Scheme 5. Proposed Mechanism of the Oxidation of CCA
CH
by 2
72
methods.
Caution! The peroxidized compound and perchlorate ion should be
treated as potential bombs. This can be done safely for relatively low levels
of peroxides and perchlorate ions (2 mM and 2.0 mL volume).
Instrumentation. UV−vis spectra were recorded on a Hewlett-
Packard Agilent 8453 UV−visible spectrophotometer equipped with a
T2/sport temperature controlled cuvette holder. Electrospray
ionization mass spectra (ESI MS) were collected on a Thermo
Finnigan (San Jose, CA, USA) LTQ XL ion trap instrument, by
infusing samples directly into the source at 5.0 μL/min using a syringe
pump. The spray voltage was set at 4.7 kV and the capillary
temperature at 200 °C. Electron paramagnetic resonance (EPR)
spectra were recorded at 78 K using an X-band Bruker EMX-plus
spectrometer equipped with a dual mode cavity (ER 4116DM). The
low temperatures were achieved and controlled by an Oxford
Instruments ESR900 liquid He quartz cryostat with an Oxford
Instruments ITC503 temperature and gas flow controller. The
experimental parameters for EPR measurements were as follows:
microwave frequency = 9.646 GHz, microwave power = 1.0 mW,
ethoxyethanol backbone and cumylperoxo ligand provoked the
O−O bond heterolysis rather than homolysis in the aldehyde
deformylation reaction. To scrutinize the nucleophilic
character of 2 , the reaction of 2 with para-substituted
benzoyl chlorides, para-X-Ph−COCl (X = OMe, Me, H, Cl),
CH
CH
4
modulation amplitude = 10 G, gain = 1 × 10 , modulation frequency
G
DOI: 10.1021/acs.inorgchem.9b01898
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
=
100 kHz, time constant = 40.96 ms, and conversion time = 85.00
Kinetic Measurements and Product Analysis. All reactions
were run in both 1 cm and 1 mm UV quartz cuvettes and followed by
monitoring UV−vis spectral changes of reaction solutions. Rate
constants were determined under pseudo-first-order conditions (e.g.,
ms. Product analyses were performed with an Agilent 1220 infinity II
high performance liquid chromatograph (HPLC), Agilent Technol-
ogies 6890N gas chromatograph (GC), Thermo Finnigan (Austin,
Texas, USA) FOCUS DSQ (dual stage quadrupole) mass
spectrometer interfaced with Finnigan FOCUS gas chromatograph
CH
BH
[substrate]/[2 ] or [2 ] > 10) by fitting the changes in absorbance
CH
BH
for the disappearance of bands at 390 nm for 2 and 403 nm for 2
,
1
1
(
GC-MS), and H NMR. H NMR spectra were measured with a
respectively, in the phosphorus oxidation and C−H activation
CH
BH
Bruker model digital AVANCE III 400 FT-NMR spectrometer.
Resonance Raman spectra were obtained using a liquid nitrogen
cooled CCD detector (CCD-1024 × 256-OPEN-1LS, HORIBA Jobin
Yvon) attached to a 1-m single polychromator (MC-100DG, Ritsu
Oyo Kogaku) with a 1200 grooves/mm holographic grating. An
excitation wavelength of 441.6 nm was provided by a He−Cd laser
reactions. The reactions of 2 and 2 with para-X-triphenylphos-
phine (X = OMe, Me, H, and Cl) were conducted in CH CN at 293
K. Temperature-dependent kinetic experiments were performed with
3
BH
CH
2
or 2 and triphenylphosphine (PPh
) in order to estimate
3
activation parameters in CH CN at 293 K. Substrates with varying
3
7
4
−1
BDE values, such as xanthene (75.5 kcal mol ), 9,10-dihydroan-
tracene (77 kcal mol ), and 1,4-cyclohexadiene (78 kcal mol ),
−
1
−1
(
Kimmon Koha, IK5651R-G, and KR1801C) with 20 mW power at
BH
the sample point. All measurements were carried out with a spinning
cell (1000 rpm) at 253 K. Raman shifts were calibrated with indene,
and the accuracy of the peak positions of the Raman bands was ±1
cm . Electrochemical measurements were performed on a CHI630B
electrochemical analyzer (CH Instruments, Inc.) in CH CN
containing 0.10 M Bu NPF (TBAPF ) as a supporting electrolyte
at 293 K. A conventional three-electrode cell was used with a
platinum working electrode (surface area of 0.30 mm ), a platinum
wire as a counter electrode, and a Ag/Ag electrode as a reference
were used in the C−H bond activation reactions by 2 in CH
343 K. The kinetic isotope effect (KIE) value for the reaction of 2
with xanthene was determined by comparing k values obtained in the
C−H and C−D bond activation reactions of xanthene and xanthene-
, respectively. k ′ values were obtained by dividing second-order rate
constants (k
3
CN at
BH
2
−
1
d
2
3
2
) with the number of equivalent target C−H bonds of
4
6
6
2
substrates. The kinetic experiments were run at least in triplicate, and
the data reported represent the average of these reactions. Fast
2
CH
+
reaction traces were collected at 390 nm due to 2 , using a 1.0 cm
optical path length of stopped-flow cell. Reaction traces were
electrode. The platinum working electrode was routinely polished
with BAS polishing alumina suspension and rinsed with acetone and
acetonitrile before use. The measured potentials were recorded with
collected, and the time dependence of the absorbance at 390 nm
−
1
was fitted with a single exponential function to give kobs (s ) under
pseudo-first-order conditions. The raw kinetic data were treated with
KinetAsyst 3 (Hi-Tech Scientific) and Specfit/32 Global Analysis
System software from Spectrum Software Associates. The oxidation
reaction rates of cyclohexanecarboxaldehyde (CCA) and para-X-
substituted benzoyl chloride by 2 were monitored by the decay of the
+
respect to a Ag/Ag (0.010 M) reference electrode.
II
Synthesis of [Cu (HN O )(CH CN)(H O)](ClO ) (1) and
3
2
3
2
4 2
II
[
(
Cu (HN O )(N ) ] (1-N ). The copper(II) complex, [Cu(HN O )-
CH CN)(H O)](ClO ) (1), was prepared by reacting equimolar
3 2 4 2
3 2 3 2 3 3 2
amounts of Cu(ClO ) ·6H O with 2-(2-(bis(pyridin-2-ylmethyl)-
amino)ethoxy)ethanol (HN O ) in CH CN at 293 K. The copper(II)
complex coordinated with two azido ligands, [Cu (HN O )(N ) ]
4
2
2
absorption band at 390 nm due to 2 in CH CN at 293 K.
3
2
3
3
II
The copper products in the reaction solutions were analyzed with
3
2
3 2
(
1-N ), was prepared by adding 2.2 equiv of tetrabutylammonium
EPR and ESI MS. Both X-band EPR and ESI MS spectra of the
3
CH
azide into the reaction solution of 1.
Generation and Characterization of [Cu (OO Bu)(HN O )] (2 )
and [Cu (OOcumyl)(HN O )] (2 ). The copper(II) complexes 2
copper product formed in the reaction of 2 with PPh indicate that
3
II
t
+
BH
BH
the copper(II) complex was formed (Figure S17a). In the reaction
3
2
II
+
CH
CH
solution of 2 with xanthene, the copper(II) complex revealed the
3
2
CH
and 2 were prepared by treating the solution of [Cu(HN O )-
formation of the dimeric complex 3, which is consistent with X-ray
3
2
(
CH CN)(H O)](ClO ) (1) with 2.0 equiv of tert-butyl hydro-
crystallographic analysis (Figure 6 and Figure S28). ESI MS spectra of
3
2
4 2
t
CH
peroxide ( BuOOH) and 2.0 equiv of cumyl hydroperoxide
the copper product formed in the reaction of 2 with PPh exhibited
3
(
2
cumylOOH) in the presence of triethylamine (TEA) in CH CN at
93 K, respectively. The formation of 2 and 2 was confirmed by
mainly the deprotonated copper(II) starting complex (Figure S17b),
whereas both dimeric and monomeric products were detected as the
sole product in the reaction solution of 2 with xanthene (Figure
3
BH
CH
CH
monitoring the band growth at 403 and 390 nm, respectively, from
UV−vis spectral changes of the reaction solutions. The electron
paramagnetic resonance (EPR) spectra of 2 and 2 (1.0 mM)
were recorded at 78 K. The g values of 2 and 2 indicated a
rhombic symmetry. Resonance Raman (rRaman) spectroscopic
measurements were performed with 2
S28b).
BH
CH
Organic products formed in the reactions of 2CH with PPh
and
3
BH
CH
xanthene were analyzed by HPLC and GC-MS. The product yields
were determined by comparing the peak areas of sample products
against standard curves prepared with known authentic samples. The
CH
18
CH
and O-labeled 2
,
II 16 16
+
II 18 18
+
BH
[
Cu ( O Ocumyl)(HN O )] and [Cu ( O Ocumyl)(HN O )] ,
phosphorus oxidation reaction by 2 was performed by mixing 10
3
2
3
2
1
6
16
BH
which were generated by adding 2.0 equiv of cumyl O OH and
mM PPh
3
with 1.0 mM of 2
in CH CN at 293 K, and
3
18
18
18
triphenylphosphine oxide (PPh O) was obtained as a major
cumyl O OH (90% O-enriched) to a solution of 1 (4.0 mM) in
the presence of TEA in CH CN at 233 K, respectively. Upon
excitation of the laser wavelength at 441.6 nm, the isotopic shifts were
observed by comparing the vibrational bands of 2 and O-labeled
3
product in over 95% yield. The oxidation of xanthene was achieved
3
CH
by mixing 10 mM xanthene with 1.0 mM of 2 in CH CN at 343 K,
3
CH
18
and xanthone was obtained as a sole product in over 90% yield.
CH
CH
Organic products obtained in the reaction of 2 with CCA were
2
.
75
II
analyzed according to a modified literature procedure. An internal
Synthesis of [Cu (N O-OBn)(CH CN)](ClO ) (1-Bn). The N O-
3
3
4 2
3
standard of phenol (4.0 mM) was added to a DMSO-d solution
OBn ligand was synthesized according to a modified literature
procedure. To a DMF solution (10 mL) of the HN O ligand (0.14
6
CH
73
obtained in the reaction between 2 (2.0 mM) and CCA (10 mM),
3
2
and the mixture solution was transferred to a NMR sample tube. The
peaks were quantified by integrating the area, and the relative peak
area can be calculated as follows:
g, 0.50 mmol), NaH (0.03 g, 0.75 mmol) was added and stirred at
room temperature. After 10 min, benzyl bromide (0.09 mL, 0.75
mmol) was added to the reaction mixture and then stirred at room
temperature for 2 h. The reaction mixture was then quenched by
adding MeOH and diluted with an ice and water mixture. After
filtration of the reaction mixture, the product was extracted by ether
and a saturated aqueous solution of NaCl (brine). Finally, the solution
was dried over Na SO , filtered, and evaporated by rotary evaporator
to give compound N O-OBn as an orange oil (over 90%). The
copper(II) complex, [Cu(N O-OBn)(CH CN)](ClO ) (1-Bn), was
prepared by reacting an equimolar amount of Cu(ClO ) ·6H O with
N O-OBn in CH CN at 293 K.
single peak area at 8.3 ppm (formate)
relative peak area ratio =
triplet peak area at 7.2 ppm (phenol)
The relative peak area ratio was calculated to be 0.28. The
concentration of formate was obtained by using the calibration
curve shown in Figure S27.
2
4
3
3
3
4 2
1
8
CH 18
In the O-labeled oxidation reactions, 2 - O was generated by
4
2
2
1
8
18
reacting 1 (1.0 mM) with an equimolar amount of cumyl O OH in
3
3
H
DOI: 10.1021/acs.inorgchem.9b01898
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Inorganic Chemistry
Article
1
6
the presence of TEA (2.0 equiv) in CH CN at 293 K. The O and
and Technology of Taiwan (MOST 105-2113-M-003-005-
MY3 to W.-Z.L.).
3
18
CH 16
CH 18
O compositions in the oxidation of PPh by 2 - O and 2 - O
3
were analyzed by comparing the relative abundances of m/z = 121
and 277 for 16O products of 2-phenylpropan-2-ol and PPh O and
3
1
8
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■
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O and O compositions in the oxidation of xanthene by 2CH-18O
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AUTHOR INFORMATION
■
1
(
Corresponding Authors
E-mail: wzlee@ntnu.edu.tw.
E-mail: hsw@sm.ac.kr.
*
Sulfur Contributes to the Function of the Non-Heme Iron Enzyme
Superoxide Reductase. Acc. Chem. Res. 2007, 40, 501.
*
ORCID
(18) Namuswe, F.; Kasper, G. D.; Sarjeant, A. A. N.; Hayashi, T.;
Krest, C. M.; Green, M. T.; Moenne-Loccoz, P.; Goldberg, D. P.
Rational Tuning of the Thiolate Donor In Model Complexes of
Superoxide Reductase: Direct Evidence for a trans Influence In Fe −
OOR Complexes. J. Am. Chem. Soc. 2008, 130, 14189.
Wei-Min Ching: 0000-0002-5299-4479
Way-Zen Lee: 0000-0003-0053-1621
Seungwoo Hong: 0000-0001-7953-8433
III
Notes
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J.; Kovacs, J. A. Investigation of the Mechnism of Formation of a
Thiolate-Ligated Fe(III)-OOH. Inorg. Chem. 2011, 50, 1592.
The authors declare no competing financial interest.
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This work was supported by NRF of Korea through MSIP
NRF-2017R1C1B2002037 to S.H.) and Ministry of Science
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