Selective oxidation of thioanisole with hydrogen peroxide using copper complexes encapsulated in zeolite: Formation of a thermally stable and reactive copper hydroperoxo species

Article abstract of DOI:10.1021/acscatal.7b04092

[Cu(terpy)]²⁺ complexes encapsulated into Na-Y zeolite ([Cu(terpy)]²⁺@Y) were prepared, and their catalytic activities for the oxidation of sulfides using hydrogen peroxide were investigated. Several spectroscopic results, as well as elemental analysis, demonstrated the formation of [Cu(terpy)]²⁺ complexes in supercages of Y-zeolite. [Cu(terpy)]²⁺@Y exhibited high selectivity for the oxidation of thioanisole into methylphenylsulfoxide when H₂O₂ was used. The kinetic study of this oxidation at the catalyst [Cu(terpy)]²⁺@Y suggests that the reaction of [Cu(terpy)]²⁺ species with H₂O₂ is the rate-determining step. The oxidation of thioanisole, benzene, and 2-phenylethylamine using [[Cu(terpy)]²⁺@Y]?, which was prepared from the reaction between [Cu(terpy)]²⁺@Y and H₂O₂, quantitatively proceeded to methylphenylsulfoxide, phenol, and 2-amino-1-phenylethanol, respectively. The reaction of [Cu(terpy)]²⁺@Y and H₂O₂ was found to yield thermally stable but active Cu^II-OOH species in [Cu(terpy)]²⁺@Y.

Full text of DOI:10.1021/acscatal.7b04092

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Article
Selective Oxidation of Thioanisole with Hydrogen Peroxide
using Copper Complexes Encapsulated in Zeolite; Formation of
a Thermally Stable and Reactive Copper Hydroperoxo Species
Syuhei Yamaguchi, Akinori Suzuki, Makoto Togawa, Maiko Nishibori, and Hidenori Yahiro
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04092 • Publication Date (Web): 15 Feb 2018
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ACS Catalysis
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Selective Oxidation of Thioanisole with Hydrogen Peroxide
using Copper Complexes Encapsulated in Zeolite; Formation
of a Thermally Stable and Reactive Copper Hydroperoxo
Species.
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Syuhei Yamaguchi,a Akinori Suzuki,a Makoto Togawa,a Maiko Nishibori,b and Hidenori Yahiro a*
a Department of Materials Science and Biotechnology, Graduate School of Science and Engineering, Ehime
University, Matsuyama 790-8577, Japan.
b Department of Energy and Material Sciences, Faculty of Engineering Sciences, Kyushu University, Kasuga,
Fukuoka 816-8580, Japan.
KEYWORDS: Cu complex, Zeolite, Oxidation, Hydrogen peroxide, Thioanisole.
ABSTRACT: [Cu(terpy)]2+ complexes encapsulated into Na-Y zeolite ([Cu(terpy)]2+@Y) were prepared and their catalytic
activity for the oxidation of sulfides using hydrogen peroxide were investigated. Several spectroscopic results, as well as
elemental analysis, demonstrated the formation of [Cu(terpy)]2+ complexes in supercages of Y-zeolite. [Cu(terpy)]2+@Y
exhibited high selectivity for the oxidation of thioanisole into methylphenylsulfoxide when using H2O2. The kinetic study
for this oxidation at the catalyst [Cu(terpy)]2+@Y suggests that the reaction of [Cu(terpy)]2+ species with H2O2 is the rate
determining step. The oxidation of thioanisole, benzene and 2-phenylethylamine using [[Cu(terpy)]2+@Y]*, which was
prepared from the reaction between [Cu(terpy)]2+@Y and H2O2, quantitatively proceeded to methylphenylsulfoxide,
phenol and 2-amino-1-phenylethanol, respectively. The reaction of [Cu(terpy)]2+@Y and H2O2 was found to yield
thermally stable but active CuII-OOH species in [Cu(terpy)]2+@Y.
INTRODUCTION
whereas an active species is too unstable to be studied
thoroughly.2-4
From a synthetic chemistry and biological point of
view, mononuclear copper-hydroperoxo species have
been considered as important active species for the partial
oxidation of organic substrates when using hydrogen
peroxide.1,2 Although a few well-characterized CuII-OOH
complexes have been described,2,3 not a great deal is
known about their intrinsic reactivity and reaction
mechanism. Masuda et al.2,4 reported the catalytic activity
of a CuII-OOH species for the selective oxidation of
thioanisole to methylphenylsulfoxide. They demonstrated
that the thermal stability of the species could be
controlled by molecular design of the coordination
geometry around the copper ion, and that the complex
From the standpoint of environmental issues, the
development of green chemistry procedures and
environmentally friendly technologies are crucial to
achieve a sustainable growth. The immobilization of
transition metal complexes in zeolite has attracted much
interest for the development of catalysts that combine the
advantages of homogeneous and heterogeneous
catalysis.6-11 We previously reported that iron-bipyridine
complexes
encapsulated
into
Na-Y
zeolite
([Fe(bpy)3]2+@Y) possess high catalytic activity for the
oxidation of cyclohexene and benzene using hydrogen
peroxide to form their corresponding alcohols with high
selectivity.7 Furthermore, the maximum value of catalytic
activity for the oxidation of benzene with H2O2 was
achieved when the volume ratio of solvents (CH3CN and
H2O) was equal (1:1).7c To this date, many have reported
the immobilization of copper complexes in zeolite and
their catalytic activity for the oxidation of sulfides,8
benzenes,9 alcohols,10 and other substrates11 using H2O2.
However, little is known about their reactivity and
reaction mechanism.
with
a square-planar geometry exhibited the best
performance among the tested complexes. Karlin et al.5
reported that a CuII-OOH species can perform oxidative
N-dealkylation reactions on a pyridyl ligand with pendant
dimethylamine and dibenzylamine moieties as substrate,
resulting in the corresponding secondary amine and
aldehyde (formaldehyde and benzaldehyde), derived from
the oxidized methyl and benzyl groups. However, there is
little information regarding the thermally stable but
active CuII-OOH species because normally no substrate
reacts with a thermally stable isolated CuII-OOH species,
The design concept of copper-terpyridine complexes
encapsulated in Y-type zeolite ([Cu(terpy)]2+@Y) is shown
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in Scheme S1 (terpy = 2,2’:6',2''-terpyridine). Based on the
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filtration, then washed with water and methanol using a
Soxhlet extractor, and finally dried at room temperature
under vacuum to give [Cu(terpy)]2+@Y as a light-blue
powder. Elemental analysis: C, 5.67; H, 2.22; N, 1.39; Na,
5.29; Cu, 2.13; and Al, 8.37 wt%.
report by Masuda et al. 2,4, it is expected that this complex,
which has a square-planar geometry, would be active for
the oxidation of organic substrates in presence of H2O2. In
this study, we report the synthesis of the [Cu(terpy)]2+@Y
catalyst and its characterization using different methods.
Also, its catalytic activity on the oxidation of thioanisole
with H2O2 was studied. Furthermore, the preparation and
characterization of an activated catalyst were also
investigated. This catalyst, [[Cu(terpy)]2+@Y]*, was
obtained after the reaction of [Cu(terpy)]2+@Y with H2O2;
its activity on the oxidation of thioanisole, benzene, 2-
phenylethylamine was evaluated.
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Preparation of [Cu(terpy)(CH3CN)](ClO4)2. This
complex was synthesized for comparison with
[Cu(terpy)]2+@Y. 2,2’:6',2''-terpyridine (0.10 g, 0.4 mmol)
was added into a methanol solution (40 mL) of Cu(ClO4)2
∙ 6H2O (0.158 g, 0.4 mmol) and stirred at room
temperature for 1 h. After evaporation of the methanol, a
blue powder was formed. Then, the precipitate was
recrystallized from CH3CN/diethylether to yield needle-
like blue crystals. Elemental analysis: C16H14Cl2CuN4O8∙
0.25H2O (541.27) calcd. C 37.72, H 2.70, N 10.35; found C
37.63, H 2.61, N 10.17.
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EXPERIMENTAL SECTION
General. Na ion-exchanged Y-type zeolite (Na-Y) with
SiO2/Al2O3 = 5.5 was acquired from Tosoh Co. All the
materials used were of analytical grade, commercially
available from Aldrich, Wako, and TCI.
Catalytic oxidation of thioanisole. The catalytic
oxidation was evaluated in a glass tube reactor at 50 °C
under air. The typical procedure was as follows: catalyst
(17 ꢀmol), MeCN solvent (10 mL), thioanisole (8.5 mmol),
and 30% aqueous hydrogen peroxide (0.42 mmol) were
Instrumentation. The powder X-ray diffraction (XRD)
patterns of the catalysts were collected on a Rigaku
MiniFlex II diffractometer using CuKα radiation. UV–vis
spectra were recorded on a Hitachi U-4000 spectrometer
for solid samples or a Shimadzu U-1200 for liquid
samples. The ESR measurements were performed at room
placed
into
the
tube.
After
the
reaction,
triphenylphosphine was added as a quencher and o-
dichlorobenzene as an internal standard into the reactor.
The reaction solution was analyzed via GC with a flame
ionization detector (FID). The products were identified by
comparison to authentic samples. The turnover number
(TON) was calculated as the ratio of product per copper
(TON [‒] = Product [mol]/Cu [mol]).
temperature with
a
JEOL JES-FA200S X-band
spectrometer. Before the ESR measurements, the sample
was evacuated at room temperature. The ESR spectral
line-shape simulation was carried out using the SimFonia
software (Bruker). The FT-IR spectra of the samples with
KBr powder were recorded using a PerkinElmer Spectrum
One spectrometer. The specific surface area was
determined via the BET analysis (Belsorp-mini, BEL
Japan) for the adsorption-desorption measurements using
N2 as adsorbent at −196 °C. Gas chromatograph (GC)
analysis was performed on a Shimadzu GC-14B with a
flame ionization detector equipped with a DB-1MS
capillary column (internal diameter = 0.25 mm, length =
30 m) using a polar liquid phase. The X-ray absorption
spectroscopy (XAS) spectra, including X-ray absorption
near-edge structure (XANES) and extended x-ray
absorption fine structure (EXAFS) of Cu K-edge, were
measured at room temperature in the transmission mode
at the Kyushu University Beamline (BL06) in the Kyushu
Synchrotron Light Research Center (SAGA-LS), Tosu,
Japan. An appropriate amount of the samples and 80 mg
of boron nitride powder were mixed and pressed into
pellets a 10-mm diameter. Synchrotron radiation was
monochromated by a Si (111) double crystal; a cylindrical
mirror coated with rhodium was used to eliminate higher
harmonics. The data were analyzed using ATHENA and
ARTEMIS, a suite of IFEFFIT software programs.12
Pretreatment
of
catalysts
with
H2O2
([[Cu(terpy)]2+@Y]*
and [Cu-Y]*).
When
[Cu(terpy)]2+@Y (0.050 g, 17 ꢀmol) reacted with 30%
aqueous hydrogen peroxide (0.42 mmol) in acetonitrile
(10 mL) for 3 h at room temperature, the color of the
catalyst in the suspension changed from light-blue to
light-green. The light-green powder, [[Cu(terpy)]2+@Y]*,
was filtrated and then washed and centrifuged three
times with acetonitrile to remove unreacted hydrogen
peroxide (Scheme S2). The same method was used for the
light-blue Cu-Y powder; however, the color remained the
same for the activated [Cu-Y]*. The partial oxidation of
thioanisole without hydrogen peroxide was performed at
50 °C under air as a reference reaction to evaluate the
catalytic activity of [[Cu(terpy)]2+@Y]* and [Cu-Y]*. The
typical conditions were: catalyst (17.0 ꢀmol Cu atoms in
catalysts), thioanisole (8.5 mmol), and acetonitrile (10
mL).
RESULTS AND DISCUSSION
Characterization of [Cu(terpy)]2+@Y. The estimated
molecular diameter of [Cu(terpy)]2+ is 8.1–9.8 Å.13 The Y-
type zeolite, which belongs to the faujasite family, has
cavities with a diameter of ca. 13.0 Å, the so-called
supercages.14 These supercages are connected to each
other by tunnels or windows with a diameter of maximum
ca. 7.4 Å.14 The estimated diameter of the complex is
larger than the window size but smaller than the
supercage, thus it can be synthesized inside the
supercage.
Preparation of the [Cu(terpy)]2+@Y catalyst. Na-Y
zeolite (5.0 g) was ion-exchanged via a conventional
method using an aqueous solution (300 mL) of Cu(NO3)2∙
3H2O (0.36 g, 1.5 mmol) to yield copper(II) ion-exchanged
Y-type zeolite (Cu-Y). The obtained Cu-Y (1.0 g) was
refluxed in a methanol solution (100 mL) of 2,2’:6',2''-
terpyridine (0.0734 g, 1.6 mmol) for 20 h, followed by
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ICP-AES and elemental analysis of [Cu(terpy)]2+@Y
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were carried out after the sample was dissolved in a HF
solution (Table S1). The [Cu(terpy)]2+@Y sample
contained 2.1 wt% copper(II) ions, 5.7 wt% C atoms, and
1.4 wt% N. This indicates that the terpy/Cu ratio is ca. 1.0,
suggesting that the Cu ion in zeolite Y was coordinated
with one terpy ligand to yield the [Cu(terpy)]2+ ion. BET
surface area of [Cu(terpy)]2+@Y (579 m2g−1) remarkably
decreased as compared to that of Cu-Y (796 m2g−1). In the
TG/DTA results (Fig. S1), the exothermic peaks derived
from the combustion of terpy ligands in the complex were
observed around 350 °C for [Cu(terpy)(CH3CN)](ClO4)2 +
Na-Y and 500 °C for [Cu(terpy)]2+@Y. The thermal
stability of the ligand in the latter was higher than in the
former because the zeolite framework protected terpy
from heat. BET and TG/DTA results suggest that complex
ions were located within the zeolite cages.
The IR spectra of [Cu(terpy)]2+@Y, Cu-Y, Na-Y, and
[Cu(terpy)(CH3CN)](ClO4)2 were recorded (Fig. S2). The
perchlorate complex showed sharp bands in the range of
1200–1600 cm−1, Fig. S2(d), which can be assigned to the
ν(C−N) and ν(C−C) of the pyridine ring in terpy.15 Similar
ν(C−N) bands were observed for [Cu(terpy)]2+@Y, Fig.
S2(a). For this complex, the IR peaks corresponding to
[Cu(terpy)]2+ were observed even after Soxhlet extraction,
suggesting that these ions were present in the supercage
of zeolite Y. The presence of free terpy embedded in Cu-Y
was ruled out because uncoordinated terpy that has a
kinetic diameter smaller than the pore opening of zeolite
Y would be easily removed during Soxhlet extraction. IR
spectra of [Cu(terpy)]2+@Y, Cu-Y, and Na-Y showed
intense bands in the ranges of 1000–1200 cm−1 and 700–
800 cm−1 respectively assigned to the νas(OTO) and
νs(OTO) bands, which are derived from the zeolite
framework.
The XRD patterns of [Cu(terpy)]2+@Y, Cu-Y, and Na-Y
are shown in Fig. 1. This figure demonstrates that the
zeolite Y structure remained after the introduction of the
metal-complex. It has been reported that the empirically
derived relationship between the relative peak intensities
of the (220) and (311) reflections, represented by I220 and
I311, in the XRD pattern confirms the formation of a large
metal complex ion in the supercage of faujasite-type
zeolites: I220 > I311 for the original zeolite Y and I220 < I311 for
the zeolite containing large complexes.7 As can be seen in
Fig. 1, I220 (2θ = 10°) was greater than I311 (2θ = 12°) for Cu-Y,
and Na-Y (Figs. 1(b) and 1(c)), the opposite was observed
for [Cu(terpy)]2+@Y (Fig. 1(a)). This is one of evidences of
the formation of copper-complex ions within the
supercage.
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Figure 1. XRD patterns of (a) [Cu(terpy)]2+@Y, (b) Cu-Y,
and (c) Na-Y.
The
UV–vis
diffuse
reflectance
spectra
of
[Cu(terpy)]2+@Y, Cu-Y, and Na-Y are shown in Fig. 2. No
absorption was observed from the ultraviolet to visible
region in the absence of metal complexes. The spectrum
of [Cu(terpy)]2+@Y presented two absorption bands at
680 and 336 nm, which can be assigned to a d-d and a
π-
π* transition of the terpy ligand. Similar bands were
observed for [Cu(terpy)(CH3CN)](ClO4)2 + Na-Y (Fig.
S3(b)) and [Cu(terpy)(CH3CN)](ClO4)2 (Fig. S4(a)) in
CH3OH. Despite having the same concentration of
[Cu(terpy)]2+, the intensity of the
[Cu(terpy)]2+@Y
was larger
π
-
π
*
band in
than in
[Cu(terpy)(CH3CN)](ClO4)2 + Na-Y (Fig. S3). For the
[Cu(terpy)(OH2)](CF3SO3)2 single crystal reported by
Castro et al.,12 the terpy ligands of [Cu(terpy)]2+ are close
to each other due to
π
-
π
stacking between the pyridine
stacking
rings of the ligands. The intermolecular
π-π
interaction of terpy in [Cu(terpy)]2+@Y was inhibited by
immobilization of the complex into the supercage,
resulting in the increased intensity of the band assigned
to the π-π* transition.
Figure 2. UV–vis diffuse reflectance spectra of (a)
[Cu(terpy)]2+@Y, (b) Cu-Y, and (c) Na-Y.
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The ESR spectra of the [Cu(terpy)]2+@Y and Cu-Y
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not corrected in this study. A strong peak at around 1.5 Å
corresponds to a Cu-N bond. Table S2 indicates the
results of EXAFS curve fittings of [Cu(terpy)]2+@Y and
[Cu(terpy)(CH3CN)](ClO4)2. The Cu-N interatomic
distance of [Cu(terpy)]2+@Y (1.97 Å) was equivalent to
that of [Cu(terpy)(CH3CN)](ClO4)2 (1.98 Å).
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samples evacuated at 150 °C for 2 h are shown in Fig. 3.
The characteristic ESR signals of isolated Cu2+ ion (I=3/2)
were observed for [Cu(terpy)]2+@Y and Cu-Y; however,
the determined ESR parameters of [Cu(terpy)]2+@Y (g// =
2.24, g⊥ = 2.05, and |A//| = 171 G) were clearly different
from those of Cu-Y (g// = 2.40, g⊥ = 2.06, and |A//| = 123 G).
The hyperfine splitting constant of [Cu(terpy)]2+@Y was
larger than that of Cu-Y. In addition, the ESR parameters
of [Cu(terpy)]2+@Y and Cu-Y were respectively similar to
those of [Cu(terpy)(CH3CN)](ClO4)2 (g// = 2.27, g⊥ = 2.05,
|A//| = 163 G) and [Cu(OH2)6](ClO4)2 (g// = 2.43, g⊥ = 2.08,
|A//| = 108 G) in CH3CN at 77 K (Fig. S5(a)).
All the results suggest that [Cu(terpy)]2+ complex ions
with a square-planar geometry could be formed in the
supercages of Y-type zeolite.
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These results led us to the conclusion that the
coordination environment around copper in Cu-Y was
changed from a six-coordinated distorted octahedral
geometry to a four-coordinated square-planar one after
the introduction of terpy.16 The present ESR spectra gave
no information of the kind of ligands in the coordination
sphere of Cu. It was reported that Cu ion introduced in Y-
zeolite is coordinated with 6 water molecules to give
octahedral geometry at ambient atmosphere and that
upon evacuation at high temperature, the octahedral
geometry was distorted by the change in the ligand from
the oxygen of water molecule to the lattice oxygen in
zeolite framework.17 Combining above facts with the
elemental analysis and TG-DTA result in this study, the
Cu ion in [Cu(terpy)]2+@Y is probably coordinated with
three N atoms in terpy and one O atom in water (or one
O atom in zeolite framework).
Figure 4. (A) Cu K-edge XANES and (B) Fourier
transformation of k3-weighted EXAFS of (a) [Cu(terpy)]2+@Y,
(b) [Cu(terpy)(CH3CN)](ClO4)2, (c) Cu-Y, and (d) Cu foil. F.T.
range was fixed to 3-12 Å for both EXAFS spectra.
Catalytic activities for the oxidation of thioanisole.
Masuda et al.2,4 investigated the thioanisole oxidation
activity of Cu complexes with H2O2 as oxidant at a 10- or
100-fold thioanisole to H2O2. Such conditions, where the
amount of substrate is much larger than that of oxidant,
are generally used in the re-oxidation of sulphoxides to
sulphones. Therefore, similar conditions were used in this
report.
Fig. 5 shows the time course for the oxidation of
thioanisole
with
H2O2
and
over
heterogeneous
homogeneous
[Cu(terpy)]2+@Y
[Cu(terpy)(CH3CN)](ClO4)2 catalysts under air. No
catalytic activity was observed with Na-Y and without a
catalyst under the same experimental conditions. For
[Cu(terpy)]2+@Y and [Cu(terpy)(CH3CN)](ClO4)2, the
amount of methylphenylsulfoxide increased with time,
although the production of methylphenylsulfone by re-
oxidation of the sulfoxide was not observed. The reaction
rate of [Cu(terpy)(CH3CN)](ClO4)2 was faster than that of
[Cu(terpy)]2+@Y, though the catalytic activity of the latter
was comparable to that of the former in steady state. It is
worth noting that [Cu(terpy)]2+@Y was conveniently
separated by filtration and reused under similar
conditions. The [Cu(terpy)]2+@Y catalyst was used at least
five times without significant loss of catalytic activity and
selectivity (Figure S7 and Table S3).
Figure 3. ESR (X band, RT, solid) spectra of (a) Cu-Y and
(b) [Cu(terpy)]2+@Y.
Fig. 4(A) shows the Cu K-edge XANES spectra of
[Cu(terpy)]2+@Y together with the reference compounds
[Cu(terpy)(CH3CN)](ClO4)2, Cu-Y and Cu foil. The
spectrum of [Cu(terpy)]2+@Y was very similar to that of
[Cu(terpy)(CH3CN)](ClO4)2, but was clearly different from
those of Cu-Y and Cu foil. The edge position (measured at
the half-height of the edge jump) of [Cu(terpy)]2+@Y
(8988.5 eV) was almost the same as that of
[Cu(terpy)(CH3CN)](ClO4)2 (8988.9 eV), indicating that
Cu ions in both compounds have the same electronic
states. Fig. 4(B) shows the Fourier transformation of k3-
weighted EXAFS oscillations (Fig. S6). The phase shift was
The kinetic study for the oxidation of thioanisole
showed adequate zero-order plots for the amount of
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thioanisole (1.70−17.0 mol) consumed (Figure S8). In
addition, the first-order dependence of the reaction rate
on the amount of [Cu(terpy)]2+@Y catalyst (3.40−34.0
mol) (Figure S9) and on the amount of H2O2 (0.42−1.26
mol) (Figure S10) was observed. Therefore, the reaction
rate can be expressed as v = ktotal[Cu][H2O2]. This suggests
that the reaction of [Cu(terpy)]2+ species with H2O2 to
form CuII-OOH species is the rate determining step
(Scheme S3).
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Figure 6. UV–vis diffuse reflectance spectra of
[Cu(terpy)]2+@Y (solid line) and [[Cu(terpy)]2+@Y]* (dashed
line). Inset: difference spectrum of [[Cu(terpy)]2+@Y]* and
[Cu(terpy)]2+@Y.
When H2O2 was added to [Cu(terpy)]2+@Y and Cu-Y,
the former changed from blue to green, whereas the latter
exhibited no change. The [Cu(terpy)]2+@Y] obtained after
the reaction with H2O2 ([[Cu(terpy)]2+@Y]*) was used for
the oxidation of thioanisole (Figs. 7 and S12). For
[[Cu(terpy)]2+@Y]*, the maximum value of turnover
number respect to Cu (TON) reached the unity after 16 h.
On the other hand, the [Cu-Y]* sample exhibited no
catalytic activity at all (Fig. S11). The oxidation of benzene
and 2-phenylethylamine over [[Cu(terpy)]2+@Y]* also
quantitatively proceeded (Fig. 7) under the same
conditions than for thioanisole; phenol and 2-amino-1-
phenylethanol were selectively obtained. It should be
noted that the time to reach the maximum value (TON ~
1) for oxidation of benzene (240 h) was much longer than
for methylphenylsulfoxide (16 h) and 2-phenethylbenzene
(48 h) (Fig. 7). This means that the activity of
[[Cu(terpy)]2+@Y]* lasted for at least 240 h at 50 °C. These
results indicate that active and thermal stable CuII-OOH
species were generated in [Cu(terpy)]2+@Y].
Figure 5. Time courses for the oxidation of thioanisole with
hydrogen peroxide to yield methylphenylsulfoxide over
[Cu(terpy)]2+@Y (a) and [Cu(terpy)(CH3CN)](ClO4)2 (b).
Reaction conditions: Cu in catalysts (17 ꢀmol), thioanisole (8.5
°
mmol), 30% aqueous H2O2 (0.42 mmol), CH3CN (10 mL), 50 C,
air atmosphere. TON [‒] = Product [mol]/Cu[mol].
To further examine the formation of CuII-OOH species
in this system, the reaction between [Cu(terpy)]2+@Y and
H2O2 was investigated in detail. The UV–vis band due to
LMCT from OOH‒ to Cu 2-5,18,19 was observed at ca. 380 nm
for
[[Cu(terpy)]2+@Y]*
after
the
reaction
of
[Cu(terpy)]2+@Y and H2O2 (Fig. 6). The ESR parameters of
[[Cu(terpy)]2+@Y]* (g// = 2.24, g⊥ = 2.06, |A//| = 180 G)
indicated that the mononuclear CuII center maintained a
square planar geometry (Fig. S11). These kinetic and
spectroscopic results suggest that the mononuclear CuII-
OOH species was probably generated in the zeolite. The
observation of resonance Raman peaks assigned to O-O
stretching vibration would provide evidence of the
formation of CuII-OOH species. However, unfortunately,
no resonance Raman peaks (laser excitation wavelength
of 406.7 nm) was observed due to very strong Rayleigh
scattering derived from the zeolite frames.
The reason why the catalytically active CuII-OOH
species is thermally stable in zeolitic media is still
unclear. However, a similar phenomenon has been
reported: the “cage effects,” where the zeolitic media
enhances the stability of unstable radical species. 20 For
example, it was demonstrated that the radical cations,
+
[(CH3)3N]+ and [(CH3)3NCH2] , in a sodalite cage of
SAPO-37, a β-cage of SAPO-42, and in the main channel
of Al-offretite are stable even at room temperature.21
Onaka et al. reported the long-term storage of unstable
monomeric formaldehyde in Na-X and Na-Y at ambient
temperature.22
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experiments were performed at Kyushu University Beamline
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4
(SAGA-LS /BL06) with the proposal of No. 2012IK009. We
are grateful to Dr. S. Yanagisawa and Prof. T. Ogura
(University of Hyogo) for measurement of resonance Raman
spectrum.
5
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Figure 7. Reaction activity of [Cu(terpy)]2+@Y]* for the
oxidation of thioanisole to methylphenylsulfoxide,
phenyethylamine to 2-amino-1-phenylethanol, and benzene
to phenol. Substrate (8.5 mmol), Cu (17.0 ꢀmol), CH3CN (10
mL), 50 °C. TON [‒] = Product [mol]/Cu[mol].
CONCLUSION
[Cu(terpy)]2+ complex ions with square-planar geometry
were prepared within supercages of Na-Y. The sulfoxide
selective oxidation of thioanisole using H2O2 proceeded at
[Cu(terpy)]2+@Y. The recycling test of this catalyst
demonstrated no degradation in neither the catalytic
activity nor in the selectivity after at least five runs. From
the kinetic study, the reaction rate can be expressed as v =
ktotal[Cu][H2O2]. This suggests that the formation of Cu-
OOH is the rate-determining step. For the oxidation of
thioanisole, 2-phenethylamine, and phenol over
[[Cu(terpy)]2+@Y]*, which was prepared by the reaction
between [Cu(terpy)]2+@Y and H2O2, the maximum value
of TON was ca. 1. It was found that active and stable Cu-
OOH species were formed in [Cu(terpy)]2+@Y.
ASSOCIATED CONTENT
Supporting Information
Synthetic and analytical details, Figures S1-11 including TG-
DTA, IR, UV–vis, ESR, EXAFS spectra, catalytic reaction
results, Tables S1-3, and Schemes S1-3. This material is
available free of charge via the
http://pubs.acs.org.
internet at
AUTHOR INFORMATION
Corresponding Author
* hyahiro@ehime-u.ac.jp
ORCID
Syuhei Yamaguchi: 0000-0002-6669-3597
Hidenori Yahiro: 0000-0002-0415-1725
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ACKNOWLEDGMENT
This work was supported by JSPS KAKENHI Grant Numbers
JP25820394 and JP16K06855, and CREST, JST. The XAFS
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