Redox-Inactive Metal Ions That Enhance the Nucleophilic Reactivity of an Alkylperoxocopper(II) Complex

Article abstract of DOI:10.1021/acs.inorgchem.0c01109

The importance of redox-inactive metal ions in modulating the reactivity of redox-active biological systems is a subject of great current interest. In this work, the effect of redox-inactive metal ions (M3+ = Sc3+, Y3+, Yb3+, La3+) on the nucleophilic reactivity of a mononuclear ligand-based alkylperoxocopper(II) complex, [Cu(iPr2-tren-C(CH3)2O2)]+ (1), was examined. 1 was prepared by the addition of hydrogen peroxide and triethylamine to the solution of [Cu(iPr3-tren)(CH3CN)]+ (iPr3-tren = tris[2-(isopropylamino)ethyl]amine) via the formation of [Cu(iPr3-tren)(O2H)]+ (2) in methanol (CH3OH) at 30 °C. 1 was characterized using density functional theory (DFT) calculations and spectroscopic methods such as UV-vis, resonance Raman (rR), and electron paramagnetic resonance (EPR). DFT calculations support the electronic structure of 1 with an intermediate geometry between the trigonal-bipyramidal and square-pyramidal geometries, which is consistent with the observed EPR signal exhibiting a signal with g⊥ = 2.03 (A⊥ = 16 G) and g|| = 2.19 (A|| = 158 G). The Cu-O bond stretching frequency of 1 was observed at 507 cm-1 for 16O2 species (486 cm-1 for 18O2 species), and its O-O vibrational energy was determined to be 799 cm-1 for 16O2 species (759 cm-1 for 18O2 species) by rR spectroscopy. The reactivity of 1 was investigated in oxidative nucleophilic reactions. The positive slope of the Hammett plot (ρ = 2.3(1)) with para-substituted benzaldehydes and the reactivity order with 1°-, 2°-, and 3°-CHO demonstrate well the nucleophilic character of this copper(II) ligand-based alkylperoxo complex. The Lewis acidity of M3+ improves the oxidizing ability of 1. The modulated reactivity of 1 with M3+ was revealed to be an opposite trend of the Lewis acidity of M3+ in aldehyde deformylation.

Full text of DOI:10.1021/acs.inorgchem.0c01109

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Article
Redox-Inactive Metal Ions That Enhance the Nucleophilic Reactivity
of an Alkylperoxocopper(II) Complex
Bohee Kim, Seonghan Kim, Takehiro Ohta, and Jaeheung Cho*
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ABSTRACT: The importance of redox-inactive metal ions in modulating the
reactivity of redox-active biological systems is a subject of great current
interest. In this work, the effect of redox-inactive metal ions (M3+ = Sc3+, Y3+,
Yb3+, La3+) on the nucleophilic reactivity of a mononuclear ligand-based
alkylperoxocopper(II) complex, [Cu(iPr2-tren-C(CH3)2O2)]+ (1), was
examined. 1 was prepared by the addition of hydrogen peroxide and
triethylamine to the solution of [Cu(iPr3-tren)(CH3CN)]+ (iPr3-tren = tris[2-
(isopropylamino)ethyl]amine) via the formation of [Cu(iPr3-tren)(O2H)]+
(2) in methanol (CH3OH) at 30 °C. 1 was characterized using density
functional theory (DFT) calculations and spectroscopic methods such as
UV−vis, resonance Raman (rR), and electron paramagnetic resonance (EPR).
DFT calculations support the electronic structure of 1 with an intermediate
geometry between the trigonal-bipyramidal and square-pyramidal geometries,
which is consistent with the observed EPR signal exhibiting a signal with g⊥ = 2.03 (A⊥ = 16 G) and g|| = 2.19 (A|| = 158 G). The
Cu−O bond stretching frequency of 1 was observed at 507 cm−1 for 16O2 species (486 cm−1 for 18O2 species), and its O−O
vibrational energy was determined to be 799 cm−1 for 16O2 species (759 cm−1 for 18O2 species) by rR spectroscopy. The reactivity of
1 was investigated in oxidative nucleophilic reactions. The positive slope of the Hammett plot (ρ = 2.3(1)) with para-substituted
benzaldehydes and the reactivity order with 1°-, 2°-, and 3°-CHO demonstrate well the nucleophilic character of this copper(II)
ligand-based alkylperoxo complex. The Lewis acidity of M3+ improves the oxidizing ability of 1. The modulated reactivity of 1 with
M3+ was revealed to be an opposite trend of the Lewis acidity of M3+ in aldehyde deformylation.
group reported the first instance of the modulated reactivity of
an iron−peroxo complex with redox-inactive metal ions in
electrophilic and nucleophilic reactions.18
INTRODUCTION
■
It is regarded that redox-inactive metal ions play a key role in
biological and biomimetic processes. In photosystem II, water
is oxidized to dioxygen (O2) by an oxygen-evolving complex
(OEC), which is composed of the Mn4CaO5 cluster.1−3 It is
suggested that a redox-inactive Ca2+ ion plays a role in O−O
bond formation to generate O2 in OEC.4,5 Elucidation of the
role of redox-inactive metal ions has attracted much interest in
the past 10 years.6,7 To investigate the role of redox-inactive
metal ions in transition-metal reactive oxygen adducts such as
metal−oxo, −peroxo, and −superoxo species, a number of
metal−oxygen adducts binding to redox-inactive metal ions
have been prepared and characterized in biomimetic
chemistry.8−10 In particular, it has been demonstrated that
redox-inactive metal ions acting as Lewis acids affected the
various oxidation reactions. Binding of secondary metal ions
such as Ca2+, Mg2+, Zn2+, Lu3+, Y3+, Al3+, and Sc3+ to high-
valent metal−oxo complexes improves the oxidizing power in
O-atom transfer as well as electron transfer.11−15 In addition,
catalysts binding redox-inactive metal ions lead to a rise in the
catalytic efficiency in sulfoxidation and epoxidation.16,17
Although the role of secondary metal ions with metal reactive
oxygen complexes has been widely explored, the scope of the
study is still limited to metal−oxo species. Recently, Nam’s
Ligand-based alkylperoxide binding metal complexes have
been proposed as intermediates, which functionalize the
aliphatic C−H bond.19−21 In synthetic chemistry, Suzuki and
co-workers have shown the first crystal structures of [Ni2(Me-
tpa-CH2O2)2]2+ (Scheme 1a)19 and [Cu(Me-tpa-CH2O2)]+
(Scheme 1b)20 through oxidation of the methyl group of the
Me2-tpa ligand. Although well-characterized ligand-based
alkylperoxometal complexes were recognized as intermediates
in ligand oxidation, the reactivity of the ligand-based
alkylperoxometal species toward external substrates has not
been revealed. On the other hand, it is well demonstrated that
the alkylperoxocopper(II) complexes can conduct electrophilic
reactions through O−O bond cleavage, which affords putative
Received: April 16, 2020
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of 4 equiv of triethylamine (TEA) in CH3OH at 30 °C afforded a
green solution. [Cu(iPr2-tren-C(CH3)218O2)]+ (1-18O) was prepared
by adding 10 equiv of H218O2 (72 μL, 95% 18O-enriched, 2.2%
H218O2 in water) to a solution containing [Cu(iPr3-tren)(CH3CN)]-
(ClO4)2 (1 mM) and 4 equiv of TEA in CH3OH at 30 °C. UV−vis
[CH3OH; λmax, nm (ε, M−1 cm−1)]: 367 (710), 650 (160), 810 (110)
(Figure 1a).
Scheme 1. Representations of the Ligand-Based
Alkylperoxometal Complexes: X-ray Structures for (a)
[Ni2(Me-tpa-CH2O2)2]2+ and (b) [Cu(Me-tpa-CH2O2)]+
a
and (c) DFT Structure for 1
a
Color code: gray, C; blue, N; red, O, yellow, Ni; green, Cu.
copper(II)−oxyl radical or copper(III)−oxo species.22,23 Very
recently, it was also reported that alkylperoxocopper(II)
complexes carry out aldehyde deformylation through cleavage
of the Cu−O bond.24
Herein, we report the nucleophilic reactivity of a
mononuclear alkylperoxocopper(II) complex, [Cu(iPr2-tren-
C(CH3)2O2)]+ (1; Scheme 1c), toward various aldehydes.
Complex 1 is of a ligand-based alkylperoxo moiety through C−
H bond activation of one of the isopropyl groups (Scheme S1).
Kinetic studies including Hammett analysis and a reactivity
order in the oxidation of a series of alkylaldehydes demonstrate
the nucleophilic character of 1. Moreover, the oxidative
nucleophilic reactivity of 1 was modulated by the Lewis acidity
of redox-inactive metal ions such as Sc3+, Y3+, Yb3+, and La3+.
EXPERIMENTAL SECTION
■
Materials. All chemicals obtained from Aldrich Chemical Co. were
of the best available purity and were used without further purification
unless otherwise indicated. The solvents acetonitrile (CH3CN),
methanol (CH3OH), and diethyl ether (Et2O) were passed through
solvent purification columns (JC Meyer Solvent Systems) prior to use.
H218O2 (95% 18O-enriched and 2.2% H218O2 in water) was purchased
from ICON Services Inc. (Summit, NJ). The tris[2-(isopropylamino)-
ethyl]amine (iPr3-tren) ligand and redox-inactive metal triflates
M(CF3SO3)3 (M = Sc3+, Y3+, Yb3+, La3+) were obtained from Aldrich
Chemical Co.
Figure 1. (a) UV−vis spectrum of 1 in CH3OH at 30 °C. The inset
shows the rR spectra of 1 (16 mM) prepared with H216O2 (red line)
and H218O2 (blue line) in CH3OH at −30 °C. (b) X-band EPR
spectrum (red line) and simulation (black line) of 1 in frozen
CH3OH at 113 K.
Reactivity Studies. All reactions were performed in a 1 cm UV
cuvette by monitoring UV−vis spectral changes of the reaction
solutions, and rate constants were determined by fitting the changes
in absorbance at 380 nm for 1 in the absence and presence of redox-
inactive metal ions (M3+ = La3+, Yb3+, Y3+, Sc3+). Reactions were run
at least in triplicate, and the data reported represent the average of
these reactions. In situ generated 1 (2 mM) with and without M3+ (3
mM) was used in reactivity studies, such as the oxidation of
cyclohexylcarboxaldehyde (CCA) in CH3OH at 30 °C (Figures 2−4).
After completion of the reactions, pseudo-first-order fitting of the
kinetic data allowed us to determine kobs values. Products, formed in
the oxidation of CCA by 1 with and without M3+ in CH3OH at 30 °C,
were analyzed by injecting the reaction mixture directly into a gas
chromatograph. All peaks of products 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 calibration curves.
Computational Details. Theoretical calculations were performed
with density functional theory (DFT)25 using the Gaussian 09
package.26 The B3LYP functional was employed for all geometry
optimizations with double-ζ basis set 6-31G* for all atoms.27 All
calculations, including optimizations, were performed in a solvent
(methanol) using the CPCM scheme.28 Optimized geometries were
visualized with Gaussview 6. The root-mean-square deviation
(RMSD) value (0.13 Å) supports the credibility of the DFT
calculations, where the RMSD value was previously obtained by
comparing the optimized geometry of [Cu(iPr3-tren)(CH3CN)]2+
Instrumentation. UV−vis spectra were recorded on a Hewlett-
Packard 8454 diode-array spectrophotometer equipped with a
UNISOKU scientific instrument for low-temperature experiments or
with a circulating water bath. Electrospray ionization mass
spectrometry (ESI-MS) spectra were collected on a Waters (Milford,
MA) Acquity SQD quadrupole mass instrument by infusing samples
directly into the source using a manual method. The spray voltage was
set at 2.5 kV and the capillary temperature at 80 °C. Resonance
Raman (rR) spectra were obtained using a liquid-nitrogen-cooled
charge-coupled detector (CCD-1024×256-OPEN-1LS; Horiba Jobin
Yvon, Kyoto, Japan) attached to a 1 m single polychromator (MC-
100DG; Ritsu Oyo Kogaku, Saitama, Japan) with a 1200 grooves/mm
holographic grating. An excitation wavelength of 405 nm was
provided by a diode laser (LM-405-PLR-40-2, Ondax, Inc., Monrovia,
CA), with 10 mW power at the sample point. All measurements were
performed with a spinning cell at −30 °C. Raman shifts were
calibrated with indene, and the accuracy of the peak positions of the
Raman bands was 1 cm−1. Electron paramagnetic resonance (EPR)
spectra were obtained on a JEOL JES-FA200 spectrometer. EPR spin
quantification was carried out using a spin quantification program,
JEOL v 2.8.0. v2 series. The spectral simulation was carried out using
a simulation software, JEOL AniSimu/FA, version 2.4.0. Product
analysis was performed on a Thermo Fisher Trace 1310 gas
chromatograph system equipped with a flame ionization detector.
Generation and Characterization of [Cu(iPr2-tren-C-
(CH3)2O2)]+ (1). The treatment of [Cu(iPr3-tren)(CH3CN)](ClO4)2
(1 mM) with 10 equiv of hydrogen peroxide (H2O2) in the presence
B
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+
Figure 3. (a) Hammett plot of ln krel against σp of benzaldehyde
derivatives in the reaction of 1 with para-substituted benzaldehydes.
The krel values were calculated by dividing kobs of p-X-Ph-CHO (X =
F, H, Cl, CF3) by kobs of benzaldehyde in CH3OH at 30 °C. (b)
Second-order rate constants determined in the reactions of 1 with
pentanal (1°-CHO; circles), 2-methylbutanal (2°-CHO; squares), and
pivalaldehyde (3°-CHO; triangles) in CH3OH at 30 °C.
Figure 2. Reactions of 1 with CCA in CH3OH. (a) UV−vis spectral
changes of 1 (2 mM) with 80 equiv of CCA at 30 °C. The inset shows
the time course of the absorbance at 380 nm. (b) Plot of kobs against
the CCA concentration to determine a second-order rate constant for
1 at 30 °C. (c) Plot of second-order rate constants against 1/T to
determine the activation parameters.
with the crystal structure of [Cu(iPr3-tren)(CH3CN)]2+.29 Popula-
tions were obtained from Mulliken analysis. Molecular orbital
compositions and overlap populations between molecular fragments
were calculated using QMForge.30 Time-dependent DFT (TD-DFT)
calculations were carried out with 150 roots.31
RESULTS AND DISCUSSION
■
Synthesis and Characterization of 1. The
alkylperoxocopper(II) complex 1 was generated in the reaction
of [Cu(iPr3-tren)(CH3CN)]2+ with 10 equiv of H2O2 and 4
equiv of TEA in CH3OH at 30 °C via the formation of
[Cu(iPr3-tren)(O2H)]+ (2; see the Experimental Section and
Scheme S1). Very recently, we reported the formation of 2,
which was metastable at −40 °C.29 Complex 2 decomposes
readily to 1 in CH3OH at 30 °C (see the Experimental Section
and Scheme S1).
Figure 4. Reactions of 1 in the presence of La3+ with CCA in CH3OH
at 30 °C. (a) Plot of kobs against the CCA concentration to determine
a second-order rate constant for 1 in the presence of La3+. (b) Plot of
log k2 values of the deformylation reaction with CCA by 1 in the
presence of Sc3+, Y3+, Yb3+, and La3+.
The green solution of complex 1 exhibited three absorption
bands at about λmax = 367 nm (ε = 710 M−1 cm−1), 650 nm (ε
= 160 M−1 cm−1), and 810 nm (ε = 110 M−1 cm−1) in the
UV−vis spectrum (Figure 1a). The absorption spectrum of 1 is
very similar to that of the reported ligand-based
alkylperoxocopper(II) complex, [Cu(Me-tpa-CH2O2)]+
(Scheme 1b).20 The rR spectrum of 1 was observed by using
405 nm excitation in CH3OH at −30 °C (Figure 1a, inset).
1-16O exhibited two isotope-sensitive bands at 799 and 507
C
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cm−1, which shifted to 759 and 486 cm−1 in 1-18O. The bands
at 799 and 507 cm−1 can be assigned as O−O and Cu−O
stretching vibrations on the basis of 16−18Δ values of 40 and 21
(CH2CH2NHCH(CH3)2)2}(CHOO)]+ (Figure S6). A sec-
ond-order rate constant (k2) for the oxidation of CCA by 1
was obtained to be 5.5(4) × 10−2 M−1 s−1 from a linear
correlation of the first-order rate constants against the
concentration of CCA (Figure 2b). The activation parameters
were determined by the reaction temperature dependence of
the k2 value for the oxidation of CCA with 1 to be ΔH⧧ =
59(1) kJ mol−1 and ΔS = −75(2) J mol−1 K−1 in the range of
233−258 K (Figure 2c). A bimolecular mechanism for the
oxidative reaction of 1 with CCA is suggested by the observed
negative entropy and second-order kinetics.
cm−1
(
16−18Δ(calcd) = 48 and 23 cm−1 for a diatomic
harmonic oscillator), respectively. These results are compara-
ble to [Cu(Me-tpa-CH2O2)]+ [(v(O−O) = 789 and 804 cm−1;
v(Cu−O) = 498 cm−1).20
The EPR spectrum of a frozen solution of 1 at 113 K shows
a signal with g⊥ = 2.03 (A⊥ = 16 G) and g|| = 2.19 (A|| = 158
G), which indicates an intermediate geometry between the
trigonal-bipyramidal and square-pyramidal geometries (Figure
1b).32,33 The results were further supported by DFT
calculations (vide infra). Spin quantification found that the
EPR signal corresponds to 93(7)% of the total copper content
in the sample (see the Experimental Section).
In the oxidation reaction of aromatic aldehydes, the
nucleophilic reactivity of 1 was further examined. The
+
Hammett plot of the first-order rate constant against σp
gave a linear correlation in the reaction of 1 and p-X-Ph-
CHO (X = F, H, Cl, CF3), affording a ρ value of 2.3(3)
(Figure 3a). After the reaction was completed, product analysis
revealed the formation of benzoic acid in a quantitative yield
(93(5)%). The positive ρ value implicates a nucleophilic
character of 1. The reactivity of 1 was also investigated by
using primary (1°-CHO), secondary (2°-CHO), and tertiary
(3°-CHO) aldehydes (Figure 3b). The observed reactivity
order of 1°-CHO > 2°-CHO > 3°-CHO supports a
nucleophilic character for 1. Plausible mechanisms for
aldehyde oxidation by 1 are described in Scheme S2.
Nucleophilic Reaction of 1 in the Presence of Redox-
Inactive Metal Ions. Interestingly, the nucleophilic reactivity
of 1 was modulated by the addition of redox-inactive metal
ions to the solution of 1. Upon the addition of 1.5 equiv of
La3+ to the solution of 1 with CCA, the oxidative reaction was
facilitated (Figure S7). Kinetic studies of 1 in the presence of
La3+ with CCA exhibited a pseudo-first-order reaction profile
and the first-order rate constants increased proportionally with
the CCA concentration (Figure 4a). Product analysis revealed
the formation of cyclohexene (75(2)%) in the reaction
solution. The nucleophilic character of 1 in the existence of
La3+ was also explored in the reaction of p-X-Ph-CHO (X =
OCH, CH3, F, H, Cl). The obtained positive ρ value (2.3(1))
indicates that 1 in the presence of La3+ has a nucleophilic
character (Figure S8). Benzoic acid was obtained in 90(10)%
yield from the reaction solution of 1 with La3+ and
benzaldehyde in CH3OH. A series of redox-inactive metal
ions (M3+ = Sc3+, Y3+, Yb3+, La3+) were adopted to investigate
the effect of Lewis acidity in the nucleophilic reaction with 1
and CCA (Table S4 and Figure S9). The observed reactivity
order is Sc3+ < Y3+ < Yb3+ < La3+, which corresponds to an
opposite trend of Lewis acidity of M3+ (Figure 4b).36 Strong
Lewis acidity of M3+ would cause the withdrawal of electron
density from the alkylperoxo moiety, resulting in a decrease of
the nucleophilicity of 1 compared to the reaction with weak
Lewis acidity of M3+.
DFT Calculations. The optimized geometric and electronic
structures of 1 were calculated by DFT at the spin-unrestricted
B3LYP theory level (see Computational Details). The
optimized structure of 1 revealed an intermediate geometry
between the trigonal-bipyramidal and square-pyramidal geo-
metries with τ = 0.52, which is similar to that of [Cu(Me-tpa-
CH2O2)]+ (τ = 0.51;20 Scheme 1b,c and Table S1 and Figure
S1),34 and its axial site is occupied by the ligand-based
alkylperoxo group. The calculated O−O bond distance (1.47
Å) showed a similar observation with [Cu(Me-tpa-CH2O2)]+
(1.47 Å) and Kitajima’s alkylperoxocopper(II) complex (1.46
Å).20,35 TD-DFT calculations exhibited that the electronic
transitions of 1 are in agreement with the experimental UV−vis
spectrum. The calculated absorption spectrum displayed an
intense peak at 385 nm and multiple weak peaks in the range
from 550 to 750 nm (Figure S2). The peak at 385 nm
indicated a form of ligand-to-metal charge transfer from mixed
orbitals among the supporting ligand (25%), alkylperoxide
(44%), and d orbitals in Cu (32%) to σ* orbitals between π*
2
orbitals in alkylperoxide (21%) and the dz orbital in Cu (68%;
Figure S3). The multiple minor weak peaks in the 550−750
nm range are assigned to d−d transitions. The singly occupied
molecular orbital (SOMO) for 1 was observed to be combined
2
with the π* orbital in alkylperoxide and dz orbitals in Cu. The
calculated spin-density distribution of the SOMO indicates
that over half a radical belongs to the Cu center and the rest is
contained in the alkylperoxide and ligand with a doublet state
(Figure S4 and Table S3). Thus, all of the spectroscopic data
and DFT calculations clearly support that 1 is assigned as an
alkylperoxocopper(II) intermediate between the trigonal-
bipyramidal and square-pyramidal geometries.
Reactivity Study of 1. It is well-known that metal−
hydroperoxo and −alkylperoxo complexes exhibit electrophilic
reactivity through O−O bond cleavage, affording metal−oxyl
radical or metal−oxo species. However, the nucleophilic
reactivity of CuII-OOR (R = H, cumyl, and tert-butyl)
complexes has also been reported in recent years.24,29 The
electrophilic reactivity of 1 was examined with thioanisole and
cyclohexadiene. By addition of the substrates to 1 in CH3OH
at 30 °C, 1 remained intact, and only trace amounts of
products were detected in the reaction solution (Figure S5).
We also investigated the nucleophilic reaction of 1 with
CCA. Upon the addition of 80 equiv of CCA to 1 in CH3OH
at 30 °C, the characteristic absorption bands of 1 decreased
with a first-order decay profile (Figure 2a). Product analyses of
the reaction solution of 1 with CCA showed cyclohexene
(87(10)%) as a major organic product and an iminated
formatocopper(II) complex, [Cu{N(CH2CH2NC(CH3)2)-
Plausible Mechanisms. On the basis of kinetic studies of
1, the proposed mechanisms for the aldehyde oxidation by 1
with and without Lewis acids are shown in Scheme 2. In the
absence of Lewis acids, aldehyde oxidation by 1 would be
initiated by the nucleophilic attack of the proximal O atom of
the alkylperoxo moiety in 1 to the C atom of the CO group
in aldehydes through Cu−O bond cleavage. Then, O−O bond
cleavage of the peroxyhemiacetal-like intermediate produces
the products. A similar mechanism has been proposed in the
nucleophilic oxidative reaction of the alkylperoxocopper(II)
complex.24 Very recently, Tolman’s group demonstrated an
alternative pathway for the nucleophilic reaction of copper-
D
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Scheme 2. Proposed Mechanism for the Reaction of 1 and
Aldehydes in the Absence (blue) and Presence (red) of
Redox-Inactive Metal Ions (M3+)
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c01109.
■
sı
Tables S1−S4, Schemes S1 and S2, and Figures S1−S9
(PDF)
AUTHOR INFORMATION
Corresponding Author
■
Jaeheung Cho − Department of Emerging Materials Science,
DGIST, Daegu 42988, Korea; orcid.org/0000-0002-2712-
4295; Email: jaeheung@dgist.ac.kr
Authors
Bohee Kim − Department of Emerging Materials Science,
DGIST, Daegu 42988, Korea
Seonghan Kim − Department of Emerging Materials Science,
DGIST, Daegu 42988, Korea
Takehiro Ohta − Picobiology Institute, Graduate School of Life
Science, University of Hyogo, Hyogo 679-5148, Japan;
orcid.org/0000-0003-4140-5293
(II)−superoxo species in the presence of water via the
formation of a copper(II)−enolate complex, which was
generated by deprotonation of α-C−H of the aldehyde with
the copper intermediate.37 We added a proton to 1 to figure
out the ability of protonation in 1. However, 1 remained intact
without exhibiting any UV−vis spectral changes upon the
addition of H(CF3SO3) to the solution of 1. In addition, any
difference of reactivity in the absence and presence of H+ in the
reaction with CCA was not observed. From these results, we
may rule out the reaction pathway including enolate species
from deprotonation of the aldehyde by 1.
On the other hand, aldehyde oxidation by 1 in the presence
of Lewis acids easily takes place via a second metal-assisted
Cu−O bond cleavage compared to that in the absence of Lewis
acids. Redox-inactive metal ions facilitate ring cleavage through
withdrawal of the electron density of the Cu−O bond, where
the nucleophilicity of 1 is increased after Cu−O bond cleavage.
Then, the various Lewis acids bound to the proximal O atom
of the alkylperoxo moiety pull the electron density from the
alkylperoxo moiety to different degrees, modulating the degree
of nucleophilic character of the alkylperoxo group in its attack
on the C of aldehydes and bringing about a reactivity order
with an opposite trend of the Lewis acidity (Figure 4b). DFT
calculations are ongoing to investigate the detailed mechanism.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c01109
Author Contributions
B.K. and J.C. conceived and designed the experiments; B.K.,
S.K., and T.O. performed the experiments; B.K., S.K., and J.C.
analyzed the data; B.K., S.K., and J.C. cowrote the paper.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the NRF funded by the Ministry
of Science, ICT and Future Planning (Grants
2019R1A2C2086249, 2018R1A5A1025511, and
2016M3D3A01913243 to J.C.) of Korea and the Ministry of
Education, Culture, Sports, Science and Technology of Japan
through the “Strategic Young Researcher Overseas Visits
Program for Accelerating Brain Circulation” (to T.O.).
DEDICATION
■
CONCLUSIONS
■
This paper is dedicated to the memory of Professor Takashi
Ogura (deceased July 23, 2017).
We have successfully prepared the mononuclear
alkylperoxocopper(II) complex 1, derived from the reaction
of the copper(II) precursor complex with H2O2 and TEA via
the formation of a hydroperoxocopper(II) intermediate.
Complex 1 exhibits characteristic features for the ligand-
based alkylperoxocopper(II) complex in UV−vis, rR, and EPR
analyses. These spectroscopic data show 1 is of an intermediate
geometry between the trigonal-bipyramidal and square-
pyramidal geometries, which is well matched with theoretical
calculations. 1 is capable of performing aldehyde oxidation
reactions including aldehyde deformylation. Importantly, the
nucleophilic reactivity, which was evidenced by a positive
Hammett ρ value in the benzaldehyde oxidation, was enhanced
by the addition of redox-inactive metal ions (M3+ = La3+, Yb3+,
Y3+, Sc3+). A good correlation between the reaction rates of 1
in the aldehyde deformylation and the Lewis acidity of M3+
was observed in the order of La3+ > Yb3+ > Y3+ > Sc3+. Thus,
the oxidative nucleophilic reactivity of 1 can be modulated by
the redox-inactive metal ions.
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