Direct cross-coupling between alkenes and tetrahydrofuran with a platinum-loaded titanium oxide photocatalyst

Article abstract of DOI:10.1039/c8cy00129d

A Pt-loaded TiO₂ photocatalyst successfully catalyzed the direct cross-coupling between various alkenes and tetrahydrofuran (THF) without any additional oxidizing agent. The reaction between cyclohexene and THF gave three cross-coupling products, namely, 2-cyclohexyltetrahydrofuran (A), 2-(cyclohex-2-en-1-yl)tetrahydrofuran (B) and 2-(cyclohex-1-en-1-yl)tetrahydrofuran (C), along with gaseous hydrogen. The mechanistic study revealed that these products were formed through different individual mechanisms: successive addition of two radical species, a 2-tetrahydrofuranyl radical and a hydrogen radical, to the double bond of cyclohexene for A, coupling of a 3-cyclohexenyl radical and a 2-tetrahydrofuranyl radical for B, and 2-tetrahydrofuranyl radical addition and hydrogen radical elimination at the double bond of cyclohexene for C. Among these three mechanisms, those for B and C are dehydrogenative. In this photocatalytic reaction system, since the cyclohexene molecule has enough reactivity, due to the localized π electron density, the Pt nanoparticles loaded on the TiO₂ function not as a metal catalyst but as an electron receiver to enhance the charge separation, although the dehydrogenative cross-coupling of benzene with THF requires Pd metal catalysis.

Full text of DOI:10.1039/c8cy00129d

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Yamamoto, M. Yamamoto, T. Yoshida and H. Yoshida, Catal. Sci. Technol., 2018, DOI:
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ISSN 2044-4753
PAPER
Qingzhu Zhang et al.
Catalytic mechanism of C–F bond cleavage: insights from QM/MM
analysis of fluoroacetate dehalogenase
rsc.li/catalysis

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DOI: 10.1039/C8CY00129D
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ARTICLE
Direct cross-coupling between alkenes and tetrahydrofuran with
platinum-loaded titanium oxide photocatalyst
Received 00th January 20xx,
Accepted 00th January 20xx
a,b
c
d
Akanksha Tyagi,a Akira Yamamoto,
Muneaki Yamamoto, Tomoko Yoshida and Hisao
Yoshida*a,b
DOI: 10.1039/x0xx00000x
A
Pt-loaded TiO2 photocatalyst successfully catalyzed the direct cross-coupling between various alkenes and
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tetrahydrofuran (THF) without any additional oxidizing agent. The reaction between cyclohexene and THF gave three
cross-coupling products, namely, 2-cyclohexyltetrahydrofuran (A), 2-(cyclohex-2-en-1-yl)tetrahydrofuran (B) and 2-
(cyclohex-1-en-1-yl)tetrahydrofuran (C), along with gaseous hydrogen. The mechanistic study revealed that these products
were formed through the different individual mechanisms: successive addition of two radical species, 2-tetrahydrofuranyl
radical and hydrogen radical, to the double bond of cyclohexene for A, coupling of the 3-cyclohexenyl radical and 2-
tetrahydrofuranyl radical for B, and 2-tetrahydrofuranyl radical addition and hydrogen radical elimination at the double
bond of cyclohexene for C. Among these three mechanisms, those for B and C are dehydrogenative. In this photocatalytic
reaction system, since cyclohexene molecule has enough reactivity, due to the localized π electron density, the Pt
nanoparticles loaded on the TiO2 function as not a metal catalyst but an electron receiver to enhance the charge
separation, although the dehydrogenative cross-coupling of benzene with THF requires Pd metal catalysis.
more green and atom economical compared to the before
mentioned methods. However, the C–H bonds have quite low
reactivity, due to their high stability, which hinders their use in
the chemical reactions. To overcome this problem, most of the
dehydrogenative cross-coupling reactions involve an oxidizing
agent6 like hydrogen peroxide, tert-butyl hydroperoxide, and
tert-butyl peroxide along with a metal catalyst like Pd,7a Cu7b
or Fe.7c Although, some metal-free reactions are also
reported,8 an oxidant is still needed.
Recently, the use of photocatalysis is drawing much
attention towards the development of sustainable
dehydrogenative cross-coupling reactions for the C–C bond
formation under mild conditions.9 Further, employment of an
abundant, safe, and inexpensive heterogeneous photocatalyst
like TiO2 is attractive.10 Various organic molecules like
acetonitrile,10b,10d ether,10c,10e and other hydrocarbons11 can be
oxidized to make radical species and protons by the
photogenerated holes on the TiO2 photocatalyst. The radical
species react to give products while the protons are reduced
by the photogenerated electrons to form hydrogen gas. Thus,
the TiO2 photocatalyst can bring out a dehydrogenative cross-
coupling reaction without any external oxidizing agent under
mild conditions.
1. Introduction
Reactions for carbon–carbon (C–C) bond formation are
absolutely necessary to synthesize large organic compounds
from readily available small molecules.1 Majority of the widely
employed C–C bond formation reactions involve pre-
functionalized
molecules
like
alkylboranes
(BR3),2
organostannanes (SnR4),3 or those having leaving groups like
perfluoroalkylsulfonates (OSO2R)4 and so on, which are
eliminated during the reaction thus generating a lot of waste.
Moreover, these molecules need to be pre-synthesized,
making the reaction inefficient and uneconomical. Hence, new
techniques are required to make C–C bonds efficiently. An
ideal case would involve two C–H bonds to make the new C–C
bond with a concomitant release of hydrogen. Such reactions
are called dehydrogenative cross-coupling reactions.5 As
hydrogen is the sole by-product of these reactions, they are
In the present work, we have employed a TiO2 photocatalyst
for the oxidant-free direct cross-coupling between alkenes and
tetrahydrofuran (THF). Alkenes and ethers are a versatile class
of compounds and represent an abundant chemical feedstock
available for organic synthesis. Alkenes activated with
functional groups are often used in organic syntheses like in
Ullmann reaction and Heck reaction, but, a few reports exist
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for the simple alkenes devoid of any functionality. Ethers, catalysts, where M designates the loaded metal oVnieAwlA2rOtic3leaOnndlinxe
DOI: 10.1039/C8CY00129D
often used as solvents in organic chemistry, are also found in denotes its loading amount in weight%.
many biologically active natural products.12 Thus, the
development of new methodologies for the incorporation of 2.2. Characterization of catalysts
ether moiety into large organic compounds is extensively
A CO pulse adsorption technique was used to determine the
dispersion and average particle size of Pt and Pd nanoparticles
deposited on the surface of the TiO2 and Al2O3 samples. X-ray
Absorption Fine Structure (XAFS) revealed the electronic state
of Pt and Pd nanoparticles in the fresh and used M(x)/TiO2 and
M(x)/Al2O3 samples. The Pt LIII-edge XAFS measurements were
carried out at BL9C of Photon Factory at Institute of Material
Structure Science, High Energy Accelerator Research
studied.13 Although some works were reported on the metal
catalysed cross-coupling between alkenes and ethers,14
a
photocatalytic reaction remains unexplored. The present work
is the first example of an oxidant-free direct cross-coupling
between alkenes and THF with a TiO2 photocatalyst. The
reaction between cyclohexene and THF was investigated by
various techniques to propose the reaction mechanism and
elucidate the role of both the photocatalyst and the loaded
noble metal nanoparticles. In addition, we compared the
present reaction system with the photocatalytic cross-coupling
of an aromatic and alkane molecule with THF, to clarify the
differences in the reaction mechanism and role of metal
nanoparticles in these photocatalytic reactions. This
comprehensive study has allowed us to fully understand the
photocatalytic cross-coupling of different hydrocarbons with
ether molecules. The findings will contribute to the
development of efficient catalysts for the photocatalytic
dehydrogenative cross-coupling reactions.
Organization (KEK-PF, Tsukuba, Japan) using
a Si(111)
monochromator. The metal foil was measured in
a
transmission mode while the catalyst samples were measured
in a fluorescence mode. The Pd LIII-edge spectra were recorded
at the beam line 6N1 at the Aichi Synchrotron Radiation Centre
of Knowledge Hub Aichi (Toyota, Japan) in a fluorescence X-ray
yield mode with a silicon drift detector (Vortex Electronics)
using a Ge(111) monochromator. Athena software was used to
analyse the obtained spectra.15
2.3. Procedure for the photocatalytic activity tests
All the chemicals were of analytical grade and used without
further purification; cyclohexene (TCI Research Chemicals,
>99%), THF (Wako Pure Chemicals, 99%), cyclopentene
(Nacalai Tesque, 98%), 1-hexene (TCI Research Chemicals,
>99%), cyclohexene-d10 (CDN Quality Stable Isotopes, 98%)
and THF-d8 (Euriso-Top, 99.5%). For the quantification, 2-
2. Experimental
2.1. Preparation of catalysts
In this study, we have employed various metal-loaded TiO2
photocatalysts, prepared by
a photodeposition method.
cyclohexyltetrahydrofuran (A), the major product of the
Powder TiO2 sample was supplied from the Catalysis Society of
Japan as JRC-TIO-8 (anatase phase, specific surface area, 338
m2/g). 4 g of this sample was dispersed in 300 mL ion-
exchanged water and stirred for 30 min under photoirradiation
from a xenon lamp (PE300 BUV). Then, 100 mL methanol and
the aqueous solution of metal precursor, like H2PtCl6·6H2O for
Pt (Wako Pure Chemicals, 99.9%), RhCl3·3H2O (Kishida
Chemicals, 99%) for Rh, PdCl2 (Kishida Chemicals, 99%) for Pd,
and HAuCl2·4H2O for Au (Kishida Chemicals, 99.9%), were
added and stirred for 15 min in the dark and 1 h under the
photoirradiation. The resultant suspension was filtered with
suction, washed with ion-exchanged water and dried at 325 K
in an electric oven to get M(x)/TiO2 samples, where M refers to
the metal loaded on TiO2 and x denotes its loading amount in
weight%.
We have also used alumina (Al2O3) supported metal
catalysts, prepared by an impregnation method as follows. The
Al2O3 sample obtained from the Catalysis Society of Japan as
JRC-ALO-7 (γ phase, specific surface area, 204 m2/g) was
grounded to a fine powder and 4 g of it was dispersed in 100
mL ion-exchanged water. Then, after 10 min of stirring,
aqueous solution of metal precursor like PdCl2 and
H2PtCl6.6H2O was added. The suspension was stirred at room
temperature for 1 h, and heated at 373 K with stirring to
evaporate it to dryness. The solid so obtained was dried
overnight at 323 K and then powdered to obtain M(x)/Al2O3
photocatalytic cross-coupling between cyclohexene and THF,
was synthesized as reported in the literature (See ESI for the
detailed procedure).16 The photocatalytic cross-coupling
reaction tests between alkenes and THF were carried out in a
Pyrex test tube (70 mL) under argon atmosphere as follows: at
first, the M(x)/TiO2 sample (typically, 50 or 100 mg) was
irradiated with the xenon lamp through an optical long pass
filter, allowing light of wavelength more than 350 nm, for 1 h.
Then, the test tube was sealed with a silicon septum and
purged with argon gas for 10 min, followed by the addition of
alkene and THF, and stirring with light irradiation for the
desired time. After the reaction, a part of the gaseous phase
was collected in a gastight syringe and analysed by a GC-TCD
(Shimadzu GC-8A). The liquid phase was diluted with ethanol,
filtered by a syringe with a PTFE filter, and analysed by a GC-
MS (Shimadzu, GCMS-QP5050A).
3. Results and discussion
3.1. Catalyst characterization
3.1.1. CO pulse adsorption
Table 1 shows the results of the CO pulse adsorption
experiments on various M(0.1)/TiO2 and M(0.1)/Al2O3 (M: Pd
and Pt) samples. The results revealed that well dispersed Pd
and Pt nanoparticles of 2.7–5.6 nm size on the TiO2 surface
(Table 1, entries 1 and 2) and 1.9–3.2 nm on the Al2O3 support
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(Table 1, entries 3 and 4) were obtained. Compared to the Pd
nanoparticles, the Pt nanoparticles had a higher dispersion and
smaller size on these samples (Table 1).
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DOI: 10.1039/C8CY00129D
(d)
(c)
3.1.2. X-Ray absorption fine structure (XAFS)
The electronic state of the Pt and Pd nanoparticles in the
M(x)/TiO2 and M(x)/Al2O3 samples was determined by XAFS.
Figure 1 shows normalized Pt LIII-edge XANES spectra of the
Pt(0.1)/TiO2 (Figure 1b) and Pt(0.1)/Al2O3 (Figure 1c) samples
Table 1. Metal dispersion and particle size on the M(0.1)/TiO2
and M(0.1)/Al2O3 samples
(b)
(a)
11.5
11.53
11.56
11.59
11.62
11.65
Entry
Sample
Metal dispersion
(%)
Particle size
(nm)
X-ray energy (keV)
1
2
3
4
Pt(0.1)/TiO2
Pd(0.1)/TiO2
Pt(0.1)/Al2O3
Pd(0.1)/Al2O3
42.6
19.8
56.7
34.5
2.7
5.6
1.9
3.2
Fig. 1. Normalized Pt-LIII edge XANES spectra of (a) a Pt
foil, (b) the Pt(0.1)/TiO2 sample recovered after the
reaction test, (c) the Pt(0.1)/Al2O3 sample recovered
after the reaction test, and (d) PtO2 powder.
recovered after the reaction test along with the reference
samples of Pt foil (Figure 1a) and PtO2 (Figure 1d). The
Pt(0.1)/TiO2 sample (Figure 1b) recovered after the reaction
test exhibited similar spectrum to that of the Pt foil with small
white line intensity at 11.55 keV, and it was clearly different
from that of the PtO2 sample. Meanwhile, the Pt(0.1)/Al2O3
sample (Figure 1c) recovered after the reaction test showed
slightly larger white line intensity than that of the Pt foil but
not similar to that of the PtO2 sample. These results suggest
that the Pt species on both the samples were metallic, but the
latter was slightly electron deficient. Figure 2 shows the
normalized Pd LIII-edge XANES spectra of the fresh samples of
Pd(1.0)/TiO2 (Figure 2a) and Pd(1.0)/Al2O3 (Figure 2c), and the
Pd(1.0)/Al2O3 sample reduced by hydrogen at 673 K (Figure
2b). The spectra of the fresh Pd(1.0)/TiO2 sample (Figure 2a)
and reduced Pd(1.0)/Al2O3 sample (Figure 2b) showed similar
white line intensity, meaning that the Pd species on the two
samples was similarly metallic. On the other hand, the white
line intensity in the fresh Pd(1.0)/Al2O3 sample (Figure 2c) was
larger than Pd(1.0)/TiO2 sample (Figure 2a), meaning that the
Pd species was slightly oxidized on Al2O3 than TiO2. This result
would originate from the acidic property of the Al2O3 support
or the smaller size of the metal particles on the Al2O3 support.
Thus, XAFS results revealed that the Pt and Pd nanoparticles
on these samples were basically metallic but those on TiO2
sample were in a more reduced state than those on Al2O3
sample. This would originate from the electron transfer at the
(c)
(b)
(a)
3150
3180
3210
3240
X-ray energy (eV)
Fig. 2. Normalized Pd-LIII XANES spectra of (a) the fresh
Pd(1.0)/TiO2 sample, (b) the Pd(1.0)/Al2O3 sample
reduced by H2 at 673 K, and (c) the fresh Pd(1.0)/Al2O3
sample.
metal-semiconductor junction.
3.2. Photocatalytic activity tests
3.2.1. Homocoupling reactions
Table 2, entries 1 and 2 show the results of homocoupling of
cyclohexene and THF, respectively, with the Pt(0.1)/TiO2
sample and photoirradiation. The reaction of cyclohexene
without THF gave a dehydrogenative homocoupling product,
3,3'-bi(cyclohexene) (1a), and hydrogen (not shown) in the
liquid and the gas phase, respectively (Table 2, entry 1). This
reaction produced a large amount of hydrogen, which is why
its amount could not be determined accurately. The selective
formation of 1a can be reasoned by comparing the bond
dissociation energies of the C–H bonds in cyclohexene, since
the allylic position has the lowest C–H bond dissociation
energy among the vinylic (457.7 kJ/mol), allylic (349.5 kJ/mol)
and homoallylic (415.1 kJ/mol) C–H bonds (Table S1, entry 1).17
So, it should be easier to generate the allylic cyclohexenyl
radical, which would give the product 1a. On the other hand,
the reaction of THF with the Pt(0.1)/TiO2 sample in the
absence of cyclohexene gave the following products in the
liquid phase: octahydro-2,2'-bifuran (1b), and octahydro-2,3'-
bifuran (1c), and 2-butyltetrahydrofuran (1d) (Table 2, entry 2).
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1b and 1c were the dehydrogenative homocoupling products ether, the formation of diethyl ether radical species under
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DOI: 10.1039/C8CY00129D
of THF, while 1d might have been formed by the photoirradiation with the TiO2 photocatalyst was evidenced by
deoxygenative reduction of 1b. Hydrogen was the sole product ESR spectroscopy.10c As all other ethers were proposed to
observed in the gaseous phase, but, due to the same reasons follow the same mechanism, the reported result can be
as mentioned before, its amount could not be determined extended to the present system, and thus, it is proposed that
accurately. When these homocoupling reaction tests were the THF molecule is oxidized by the photogenerated holes on
done without the Pt(0.1)/TiO2 sample or light irradiation, no the TiO2
products were obtained, implying that the formation of these
homocoupling products was a photocatalytic reaction.
In our previous report on the photocatalytic
dehydrogenative cross-coupling between benzene and diethyl
Table 2. The results of the photocatalytic cross-coupling reaction between cyclohexene and THF with the Pt(0.1)/TiO2 sample in various
reaction conditions a
Entry
Reactants
(ml)
Cross-coupling products
Homocoupling products
Cross-coupling selectivity
(%)
(μmol) b
(μmol)
f
g
Cy
THF
0.0
A
0.0
B c
0.0
C c
0.0
1a d
7.2
1b+1c+1d e
SCy
STHF
1
2
2.0
0.0
1.0
0.5
0.2
3.0
0.5
0.5
0.5
0.5
0.0
n.a. h
n.a.
81
n.a.
n.a.
54
2.0
3.0
3.0
3.0
0.5
3.0
3.0
3.0
3.0
0.0
1.7
2.0
1.3
0.7
20.4
4.4
0.0
0.0
0.0
1.5
1.4
0.9
0.4
12.5
3.8
0.0
0.0
0.0
1.4
1.6
0.8
0.4
10.7
3.1
0.0
0.0
0.0
1.1
0.6
0.2
6.8
10.2
1.3
0.0
0.0
4.0
3.9
3
4
12.2
15.8
0.0
89
29
5
93
16
6
18
100
7 i
8 j
9 k
10 l
39.6
31.6
0.0
80
52
90
26
n.a
n.a
n.a
n.a
0.0
a
Reaction conditions: Desired amount of cyclohexene (Cy) and THF (1.0 ml of cyclohexene corresponds to 9.8 mmol, 1.0 ml of THF corresponds to 12
mmol), and 50 mg of the Pt(0.1)/TiO2 sample were used for the reaction, the reaction time was 1 h, the wavelength of the light was ≥ 350 nm, I = 40
b
mW/cm2 (at 365±20 nm), and the reaction temperature was 303 K. The amount of the cross-coupling products A, B, and C was determined from the
calibration curve of A, which was synthesized in the manner mentioned in ESI.16 c The assignments of the structures of the peaks 2 and 3 are based on the
discussion in ESI and the results of the isotopic experiments. However, there is no concrete evidence for these assignments and thus they should not be
d
considered as the definite assignments. The amount of 1a was tentatively determined from the calibration curve of an authentic sample of 1,1’-
bi(cyclohexane). e The amount of 1b–1d was tentatively determined from the calibration curve of A. f Selectivity (%) for the cross-coupling products based
on cyclohexene, SCy = [100 × (total amount of A, B, and C)]/(Total amount of the products from cyclohexene). g Selectivity (%) for the cross-coupling
h
i
products based on THF, STHF = [100 × (total amount of A, B, and C)]/(Total amount of the products from THF). Not applicable. The reaction time was
increased to 12 h. j 100 mg of the Pt(0.1)/TiO2 sample was used for the reaction. k The reaction was done without the Pt(0.1)/TiO2 sample. l The reaction
was done in the dark.
photocatalyst. Further, in another previous study from our radical species would combine to give the corresponding
laboratory, the oxidation of cyclohexene to radical species by homocoupling products, while the protons would be reduced
the TiO2 photocatalyst was evidenced by ESR spectroscopy.10d by the photoexcited electrons to hydrogen radicals, followed
So, it was proposed that the photogenerated holes on the TiO2 by the formation of hydrogen gas.
photocatalyst could oxidize both cyclohexene and THF to their
corresponding radicals and proton (equations 1 and 2). These
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3.2.2. Cross-coupling between cyclohexene and THF
ARTICLE
cyclohexene and THF gave three new peaks, hereafter referred
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Table 2 also shows the results of the reaction tests for the to as peak 1, peak 2, and peak 3
in the inDcrOeIa: 1s0in.1g03o9r/dCe8rCYo0f0t1h2e9iDr
photocatalytic cross-coupling between cyclohexene and THF retention time in the total ion-chromatograph (TIC) of GC-MS
with the Pt/TiO2 sample and photoirradiation in various
reaction conditions. The reactions performed with a mixture of
Table 3. Result of the photocatalytic cross-coupling between cyclohexene and THF with various M(0.1)/TiO2 samples a
Entry
Sample
Cross-coupling products
Homocoupling products
Cross-coupling selectivity b
(%)
(μmol)
(μmol)
A
B
C
D
1a
1b+1c+1d
31.6
1.5
SCy
89
STHF
26
71
34
59
46
1
2
Pt(0.1)/TiO2
Pd(0.1)/TiO2
Rh(0.1)/TiO2
Au(0.1)/TiO2
TiO2
4.4
0.8
2.6
0.6
0.5
3.8
0.2
2.3
0.4
0.3
3.1
0.3
1.8
0.3
0.3
0.0
2.5
0.0
0.0
0.0
1.3
0.0
0.9
0.0
0.2
100
88
3
13.0
0.9
4
5 c
100
1.3
85
a 0.5 ml (4.9 mmol) of cyclohexene, 3 ml (36 mmol) of THF, and 100 mg of the M(0.1)/TiO2 sample were used for the reaction, other conditions were the
same as shown in Table 2. b Selectivity (%) for the cross-coupling products based on cyclohexene, SCy = [100 × (total amount of A, B, C, and D)]/(Total
amount of the products from cyclohexene). Selectivity (%) for the cross-coupling products based on THF, STHF = [100 × (total amount of A, B, C, and
D)]/(Total amount of the products from THF). c The reaction was done with the pristine TiO2 sample.
(Figure S1a) along with the homocoupling products of the two amount of the Pt/TiO2 sample increased the amount of the
reactants. Based on their TIC area, the peak
peak, while the peak and the peak had similarly smaller based on THF (Table 2, entry 8). Further, the products were
areas. Comparing with the retention time in TIC and the mass not obtained in the reactions performed without light or the
spectrum of the synthesized authentic sample, the peak was Pt/TiO2 sample, confirming that these cross-coupling reactions
clearly assigned to 2-cyclohexyltetrahydrofuran ( ), the major were photocatalyzed (Table 2, entries 9 and 10). Since the
1 was the major cross-coupling products, albeit with a lowered selectivity
2
3
1
A
cross-coupling product. As discussed in the ESI and supported mixture of 0.5 ml cyclohexene and 3 ml THF gave the largest
by the results of the isotopic experiments (discussed later), the amount of the cross-coupling products with high selectivity
peak
yl)tetrahydrofuran
yl)tetrahydrofuran (
2
and the peak
3
was assigned to 2-(cyclohex-2-en-1- based on cyclohexene (SCy) and appreciable selectivity based
and 2-(cyclohex-1-en-1- on THF (STHF), it was chosen as the standard concentration of
C), respectively, as the dehydrogenative the two reactants (Table 2, entry 4).
(B)
cross-coupling products. To maximize the amount of the cross-
coupling products and achieve high selectivity based on both 3.2.3. Screening of M(0.1)/TiO2 samples
reactants (SCy and STHF), the amount of the two reactants was
Table 3 shows the result of the screening of different metals
varied. The homocoupling of THF was the major process loaded on the TiO2 photocatalyst (Table 3, entries 1–4). Among
occurring in the reactions performed with an excess of THF them, the Pt loaded TiO2 sample (Pt/TiO2) exhibited the
(Table 2, entries 3–5). This side reaction could be completely highest activity for the formation of the cross-coupling
suppressed by conducting the experiment with cyclohexene in products (Table 3, entry 1). The reaction performed with the
excess, although it also decreased the amount of the cross- Pd(0.1)/TiO2 sample produced a new product peak
coupling products (Table 2, entry 6). The amount of all to 2-phenyltetrahydrofuran ( ), which was probably a product
products increased drastically upon increasing the reaction of successive dehydrogenation of the cross-coupling products,
time to 12 h, where the cross-coupling selectivity was high due or (Table 3, entry 2). The reaction performed with the
4, assigned
D
A
,
B
C
to a larger increase in the cross-coupling products than the pristine TiO2 photocatalyst gave only a few products (Table 3,
homocoupling products (Table 2, entry 7). Increasing the entry 5), suggesting that the Pt loading was necessary to
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obtain high product yields. The effect of the loading amount of Pt/TiO2 sample gave the highest yield of the cross-coupling
View Article Online
DOI: 10.1039/C8CY00129D
Pt on TiO2 was also studied (Table S4). The amount of the products, further experiments were done with this sample.
cross-coupling products did not vary with Pt loading so much
(Table S4, entries 1–3), hence all experiments were carried out
with a constant loading amount of 0.1 weight%. As the
25
20
15
10
5
100
80
60
40
20
0
50
40
30
20
10
0
(a)
(b)
0
0
3
6
9
12
15
0
3
6
9
12
15
Reaction time / h
Reaction time / h
Figure 3. Time course of the yielded amount of (a) the cross-coupling products and (b) the homocoupling products in the
photocatalytic reaction between cyclohexene and THF with the Pt(0.1)/TiO2 photocatalyst. Reaction conditions: 0.5 ml of
cyclohexene, 3 ml of THF, and 50 mg of the Pt(0.1)/TiO2 photocatalyst were used; other conditions were same as Table 3.
Open square
1b 1c 1d.
A
, open circle
B, open triangle
C, filled circle SCy, filled diamond STHF, filled square 1a, and filled triangle
+
+
3.2.4. Time course
were mentioned in the ESI. The three alkenes, i.e.,
Figure 3 shows the time course for the photocatalytic cross- cyclohexene, cyclopentene, and 1-hexene, exhibited different
coupling between cyclohexene and THF with the Pt(0.1)/TiO2 reactivity in the photocatalytic cross-coupling. From the yield
photocatalyst. The reaction conditions were the same as of the cross-coupling products, it appeared that cyclic alkenes
shown in Table 3. The amount of all cross-coupling products might exhibit
steadily increased with time, meaning that the Pt(0.1)/TiO2
photocatalyst exhibited long time activity (Figure 3a). Further,
the product distribution was almost constant throughout the
experiments. As the products A–C had similar structures, one
might think that some of them are formed by a successive
reaction such as hydrogenation or dehydrogenation, but the
results of time course experiments denied this possibility and
revealed that all were formed by an independent route. The
homocoupling of cyclohexene increased steadily with time
(Figure 3b), which slightly decreased the selectivity of the
cross-coupling products based on cyclohexene (SCy) (Figure 3a).
On the other hand, the homocoupling of THF showed an initial
increase but the rate slowed down later (Figure 3b), which is
the reason why the selectivity of the cross-coupling products
based on THF (STHF) improved with time (Figure 3a). Thus, the
time course of the photocatalytic cross-coupling between
cyclohexene and THF revealed that each cross-coupling
product were formed by an independent route.
Scheme 1 The photocatalytic cross-coupling of different
alkenes with THF with the Pt/TiO2 photocatalyst.
higher reactivity than acyclic alkenes towards the
photocatalytic cross-coupling with THF.
3.3. Mechanistic studies
3.3.1. Isotopic experiments
The photocatalytic reaction between cyclohexene and THF
was performed with deuterated reagents so as to elucidate the
mechanism of the photocatalytic reactions between alkenes
and THF (Table 4). The homocoupling of cyclohexene, to give
1a, did not proceed in the reaction carried out with deuterated
cyclohexene (cyclohexene-d10), suggesting that the cleavage of
the C–H bond of cyclohexene was the rate determining step
(RDS) of the homocoupling reaction of cyclohexene (Table 4,
entries 1 and 2). As a C–D bond is slightly stronger than a C–H
3.2.5. Scope of alkenes
To study the scope of this reaction, we examined some
alkene molecules, like cyclopentene (Table S5) and 1-hexene
(Table S6), for their photocatalytic cross-coupling with THF.
Both substrates reacted with THF to give the corresponding
cross-coupling products (Scheme 1, 2A and 3A). More details
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bond, replacement of cyclohexene with cyclohexene-d10 will of the C–H bond of THF was the RDS of these products’
View Article Online
make its oxidation difficult and decrease the amount of 1a
which is a normal kinetic isotope effect (KIE). The amount of cyclohexene increased in this reaction, which could be
the homocoupling products of THF, 1b 1d, was reduced in the explained by considering that the hole-catalysed oxidation of
,
formation in this condition. InterestinglyD, OthI:e10h.1o0m39o/cCo8uCpY0li0n1g29oDf
–
reaction performed with cyclohexene and deuterated THF cyclohexene
(THF-d8) (Table 4, entries 1 and 3), indicating that the cleavage
Table 4. Isotopic experiments with deuterated compounds of cyclohexene and THF a
b
Entry
Deuterated reactant
Products / μmol
kH/kD
C
A
B
C
1a
1b-1d
1.15
0.94
0.88
A
-
B
-
1a
-
1b-1d
-
1
2
3
None
cyclohexene-d10
THF-d8
0.32
0.37
0.44
0.46
0.15
0.54
0.39
0.15
0.48
0.26
trace
0.46
-
0.8
0.7
3.0
0.8
2.6
0.8
n.a.
0.6
1.2
1.3
a
0.2 ml (1.9 mmol) of cyclohexene, 0.3 ml (3.6 mmol) of THF, and 30 mg of the Pt(0.1)/TiO2 sample were used for the reaction, other conditions
were same as Table 1. b The kH/kD value for each product was determined from its amount produced in the entries 1 and 2 for the entry 2, and
those in the entries 1 and 3 for the entry 3, respectively.
and THF should be a competitive reaction. Since the oxidation and THF radical, while the formation of
C would include the
of THF-d8 by the photogenerated hole would be slightly slower addition of THF radical on the double bond of cyclohexene
than that of normal THF, more photogenerated holes would be followed by the elimination of a hydrogen radical from the
available for the oxidation of cyclohexene to increase the resultant radical transition state shown in Figure S3. Hence,
amount of 1a ultimately. Thus, the cleavage of the C–H bond in the formation of both
cyclohexene and THF was the RDS for their homocoupling of cyclohexene. However, for
products’ formation. cyclohexene molecule is cleaved, while for
In the photocatalytic cross-coupling reaction, the use of the transition state is cleaved. We propose that between these
cyclohexene-d10 increased the amount of , while it decreased two processes, the former would be more difficult. The
those of compounds and (Table 4, entries 1 and 2). The replacement of cyclohexene with cyclohexene-d10 would make
kH/kD values calculated for the compounds , and were both these processes difficult, as a C–D bond is stronger than a
0.8, 3.0, and 2.6, respectively, indicating an inverse Kinetic C–H bond. And, as proposed, the decrease would be more for
Isotope Effect (KIE) for and a normal KIE for and . Further, than , so that the value of the kH/kD for would be larger
the reaction with THF-d8 increased the amount of all three than (Table 4, entry 2). Thus, the results of isotopic
products (Table 4 experiments supported our assignment of the three peaks
entries 1 and 3). The kH/kD values were 0.7, 0.8, and 0.8 for the from the cross-coupling products to , and as mentioned
compounds , and , respectively, indicating an inverse KIE above.
for all three products. The observation of various KIE indicates The inverse KIE observed for the reaction carried out with
B
and
C
includes cleavage of a C–H bond
, the C–H bond of the
the C–H bond of
B
C
A
B
C
A
,
B
C
A
B
C
B
C
B
C
A
,
B
C
A
,
B
C
that the cleavage of the C–H bonds in both THF and cyclohexene and THF-d8 (Table 4, entry 3) can be explained by
cyclohexene influenced the RDS for the formation of each considering the competitive oxidation of cyclohexene and THF.
product, as discussed below.
As listed in Table 4, entry 2, the inverse KIE observed for
In the reaction mixture, since both homocoupling and cross-
coupling reactions take place competitively, the decrease of
A
during the reaction with cyclohexene-d10 can be attributed to THF radical formation rate due to the difficulty of the cleavage
the change in the hybridization of cyclohexene from sp2 to of the C–D bond would enhance the oxidation of cyclohexene
sp3.18 When the double bond of cyclohexene is attacked by leading to the formation of not only the homocoupling
THF radical, its hybridization changes from sp2 (the vinylic product, 1a, but also the cross-coupling products,
carbon in cyclohexene) to sp3 in the product, via a sp3 like in each mechanism mentioned in the next section.
radical transition state (shown in Figure S3). This process will
A, B, and C,
be accompanied by a change in the zero-point energies, which 3.3.2. Reaction mechanism
is smaller for a C–D bond (403.2 cm-1, which is the difference
The above observations are used to propose the following
of 586.8 cm-1 for sp2 and 990 cm-1 for sp3) than for a C–H bond three mechanisms for the formation of
A, B, and C in the
(550 cm-1, which is the difference of 800 cm-1 for sp2 and 1350 photocatalytic cross-coupling between cyclohexene and THF
cm-1 for sp3). Thus, due to small activation energy, the reaction (Figure 4).
performed with cyclohexene-d10 would be faster and hence
would produce larger amount of than the reaction under (1) Photoexcitation of the TiO2 photocatalyst generates holes
normal condition (Table 4, entry 2). Further, the normal KIE and electrons, where the holes move to the surface of the TiO2
observed for formation of compounds and indicated that photocatalyst and the electrons move to Pt nanoparticles. On
the cleavage of the C–H bond of cyclohexene would be the the TiO2 surface, the hole oxidizes a THF molecule to make a
RDS (Table 4, entry 2). As discussed later, the formation of THF radical species (2-tetrahydrofuryl radical) and a proton. (2)
would involve the coupling between an allylic cyclohexenyl This proton is reduced to hydrogen radical by the electrons
Figure 4a shows the proposed mechanism for the product A:
A
B
C
B
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present on the Pt nanoparticles. (3) The THF radical attacks a
vinylic sp2 carbon of a cyclohexene molecule to make a radical
View Article Online
DOI: 10.1039/C8CY00129D
transition state (
hybridization of the sp2 vinylic carbon in cyclohexene molecule
to the sp3 carbon in the product
, via the radical transition
state , gave an inverse KIE for the production of in the
reaction done with cyclohexene-d10 (Table 4, entry 2). Thus,
this step is the RDS for the formation of . (4) Finally, the
addition of a hydrogen radical to gives the product . The
is a one-photon process.
I, also Figure S3). The change in the
A
I
A
A
I
A
formation of
A
(a)
Figure 4. Three types of the proposed reaction mechanisms for
the formation of the cross-coupling products (a)
(c) in the reaction between cyclohexene and THF by the
Pt/TiO2 photocatalyst
A, (b) B, and
C
.
Figure 4b shows the proposed mechanism for the product B:
(1) The two photogenerated holes can oxidize both molecules
of cyclohexene and THF to generate their corresponding
radical species (a 2-tetrahydrofuryl radical and an allyl radical)
and two protons. Also, as evidenced by kinetic experiments
(Table 4, entry 2), the oxidation of cyclohexene is the RDS for
(b)
the formation of B. (2) The two protons would be reduced to
hydrogen radicals by the photogenerated electrons on Pt. (3)
The coupling of the 2-tetrahydrofuryl radical and allyl radicals
gives the product
B. (4) The hydrogen radical on the Pt
nanoparticles combine to give a hydrogen molecule. This
photocatalytic cross-coupling is a dehydrogenative two-photon
process.
Figure 4c shows the proposed reaction mechanism for the
formation of product
formation of the radical species, reduction of protons, the
formation of the transition state are the same as those for
the formation of . (4) But, in this case, elimination of a
takes place to re-generate the double
. The cleavage of the C–H bond of the
is
the RDS in this mechanism, which gives a normal KIE in the
reaction done with deuterated cyclohexene (Table 4 entry 2).
(5) Coupling of the hydrogen radicals on the Pt nanoparticles
C: The steps (1), (2), and (3) for the
I
A
hydrogen radical from
bond and give product
I
C
tertiary sp3 carbon of cyclohexene in the transition state
I
(c)
generates a hydrogen molecule. The formation of
dehydrogenative and one-photon process.
C is a
3.3.3. Role of Pt nanoparticles
The metal nanoparticles loaded on the TiO2 photocatalyst
have been proposed to function as an electron receiver and
decrease the recombination of the photogenerated electrons
and holes. However, as evidenced in some studies, these
metals can also participate catalytically in a photocatalytic
reaction, thus acting as a co-catalyst.10b-e,19 In this section, we
will discuss the role of the Pt nanoparticles on the TiO2
photocatalyst in the photocatalytic cross-coupling between
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alkenes and THF by doing some control experiments like, photocatalyst for the formation of the cross-coupling products,
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temperature controlled photocatalytic reaction tests, and
A–C
(Table 6, entries 1–3). The introducDtiOonI: 1o0f.10th39e/CM8C(xY)0/0A1l229OD3
reaction tests with a physical mixture of a photocatalyst and catalyst to the reaction mixture containing the pristine TiO2
an alumina supported metal catalyst. The details of these photocatalyst did not increase the amount of products so
experiments are mentioned in the ESI and the results are much, indicating that the metal nanoparticles did not
discussed below.
participate catalytically in the reaction (Table 6, entries 1, 4
Since the reaction rate by a metal catalyst is expected to be and 5).
accelerated with temperature, the photocatalytic cross-
When the M(x)/Al2O3 function as a metal catalyst, a certain
coupling between cyclohexene and THF with the Pt(0.1)/TiO2 improvement could be observed, e.g., in the photocatalytic
photocatalyst was carried out at different temperatures. These reaction between benzene and THF to give 2-PTHF (Table S8)
experiments are in accordance to our previous studies.10c–e The and cyclohexane and THF to give 2-CTHF (Table S9). The
amount of the three cross-coupling products (
increased with temperature with a slight change of selectivity photocatalyst facilitated the formation of 2-PTHF (Table S8,
(Table S7, entries 1–4). The pseudo Arrhenius plot of these entries and 6). Similarly, both the addition of the
data (Figure S4a) provided the apparent thermal activation Pt(0.1)/Al2O3 sample and that of the Pd(0.1)/Al2O3 samples to
energies to be 29.3, 21.6, and 20.3 kJ/mol for the products the pristine TiO2 photocatalyst promoted the formation of 2-
, and , respectively (Table 5, entry 1). A similar trend was CTHF (Table S9, entries 1, 4, and 5). In these systems, the
A–C) was slightly addition of a reduced Pd(0.1)/Al2O3 sample to the pristine TiO2
1
A
,
B
C
observed for the reaction carried out with the pristine TiO2 metal nanoparticles on the TiO2 photocatalyst participated
photocatalyst, where the amount of the three cross-coupling catalytically in the photocatalytic reaction. So that these
products (A–C) was slightly increased with temperature (Table reactions were proposed to be a hybrid system of the TiO2
S7, entries 5–8). The apparent thermal activation energies photocatalysis and the metal catalysis. On the contrary, the
calculated from the pseudo Arrhenius plot of these data reaction between cyclohexene and THF was found to be a
(Figure S4b) were 23.2, 13.0, and 16.1 kJ/mol (Table 5, entry purely photocatalytic process and the metal nanoparticles did
2). These two sets of energy values for the two photocatalysts not catalyse any step except for the hydrogen evolution. Thus,
for the three cross-
in this reaction, the role of the Pt metal nanoparticles would
be restricted to receiving electrons
and promoting the reduction.
3.3.4. Cross-coupling of an aromatic, alkane, and alkene
molecule with THF
The results of the photocatalytic cross-coupling reaction of
cyclohexene with THF were compared to those of benzene and
cyclohexane with THF. The difference in the electronic
properties of these three hydrocarbons significantly affected
the reaction mechanism as well as the requirement for the
photocatalyst, especially the metal nanoparticles in the
M(x)/TiO2 photocatalysts.
The reaction between benzene and THF gave a single
product (Table S8). The previous work revealed that the
reaction between benzene and ether proceeds according to
the radical addition-elimination mechanism, where the
photogenerated THF radical attacked the benzene molecule.10c
The Pd(0.1)/TiO2 photocatalyst exhibited the highest activity
for this reaction, which was attributed to the well-known
catalytic activity of Pd metal in the substitution reaction of
aromatic compounds, i.e., the activation of benzene. In the
reaction mixture, THF molecules can be oxidized by the
photogenerated holes to form the radical species, while
benzene molecules cannot be oxidized. It is reported that
some kinds of radical species, such as surface OH radical,20a
and amide radical,20b can attack the benzene molecules to
form radical intermediates to give the functionalized products.
However, the THF radical and the benzene molecule cannot
react directly without the assistance of the Pd catalyst. The Pd
nanoparticles can activate the benzene molecule to react with
the THF radical, which allows the formation of the cross-
coupling products.
Table 5. Apparent thermal activation energies for the cross-
coupling products in the photocatalytic reaction between
cyclohexene and THF with the Pt/TiO2 photocatalyst and the
pristine TiO2 photocatalyst.
Entry
Catalyst
Apparent thermal activation energy /
kJ mol−1
A
B
C
1
2
Pt(0.1)/TiO2
TiO2
29.2
23.2
21.6
13.0
20.3
16.1
coupling products were similar to each other. And hence, the
contribution of metal catalysis in the photocatalytic cross-
coupling between cyclohexene and THF was ruled out. The
slight increase in the amount of the three products with
temperature might be related to some non-catalytic processes
like desorption of products from the photocatalyst surface and
so on. Thus, the formation of the three cross-coupling
products (A–C) was proposed to be a purely photocatalytic
process where the Pt nanoparticles loaded on the TiO2
photocatalyst function as an electron receiver that accept the
photoexcited electrons from the conduction band of TiO2 to
promote the whole reaction by decreasing their recombination
with the photogenerated holes, and also, to reduce protons to
form hydrogen molecules.
Next, the reaction tests were carried out with a physical
mixture of the TiO2 photocatalyst and an alumina supported
metal catalyst (M(x)/Al2O3). Since the metal nanoparticles
were not loaded on the TiO2 photocatalyst in these cases, they
could not function as an electron receiver and their possible
catalytic activity could be clarified. The results are shown in
Table 6. The Pt(0.1)/TiO2 photocatalyst was more active than
both the pristine TiO2 photocatalyst and the Pd(0.1)/TiO2
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On the other hand, the reaction between cyclohexane and carried out with physically mixed catalysts supported the
View Article Online
DOI: 10.1039/C8CY00129D
THF also gave a single product (Table S9). This reaction takes existence of this hybrid route (Table S9). However, as the
place through a radical-radical coupling mechanism mainly,10e amount of the product formed with the physically mixed
i.e., both cyclohexane and THF molecules are oxidized by the catalyst consisting of the TiO2 photocatalyst and the
photoexcited holes on the TiO2 photocatalyst to produce their Pt(0.1)/Al2O3 catalyst was much less than that obtained with
corresponding radical species, and they react together to give the Pt(0.1)/TiO2 photocatalyst alone, it can be proposed
the cross-coupling product. The Pt(0.1)/TiO2 photocatalyst that the main route was a photocatalytic process where the Pt
exhibited the highest activity for this reaction which was nanoparticles facilitated the reaction by acting as an electron
attributed to the efficiency of the Pt metal as an electron receiver.
receiver on the TiO2 photocatalyst to reduce the
Compared to these results, the reaction between
recombination between the photogenerated holes and cyclohexene and THF gave three products, each by an
electrons. Apart from this photocatalytic process, a minor individual mechanism. The formation of a mixture of these
route, involving both Pt thermal catalysis and TiO2 three cross- coupling products could be attributed to the
photocatalysis, was also confirmed. The reaction undergoes unique property of
between a photogenerated radical and a surface adsorbed
molecule on the Pt surface. The present results of the reaction
Table 6. The results of the photocatalytic cross-coupling reaction between cyclohexene and THF with different catalysts a
Entry
Catalyst
Cross-coupling products (μmol)
A
B
C
1
2
TiO2
0.5
1.3
4.4
0.4
0.6
0.2
0.8
3.8
0.3
0.2
0.1
0.6
3.0
0.2
0.2
Pd(0.1)/ TiO2
3
Pt(0.1)/ TiO2
4 b
5 b
TiO2 + Pd(0.1)/Al2O3
TiO2 + Pt(0.1)/Al2O3
a
Reaction conditions: 0.5 ml of cyclohexene, 3 ml of THF, and 50 mg of the photocatalyst were used, and the reaction time was 1 h; remaining
conditions were same as Table 2. b 50 mg of TiO2 photocatalyst and 50 mg of M(0.1)/Al2O3 catalyst were used.
alkene: one is the reactivity of the double bond that enables
both an addition reaction and a substitution reaction with the
THF radical as shown in Figures 4a and 4c, while another is the
4. Conclusions
high reactivity to form the stable allyl radical species which
could couple with the THF radical as shown in Figure 4b.
In this study, we found that a Pt-loaded TiO2 heterogeneous
photocatalyst can efficiently catalyze the direct cross-coupling
between various alkenes and THF without any external
oxidant. The reaction between cyclohexene and THF gave
three different cross-coupling products, each formed by an
independent reaction mechanism; addition of the
photogenerated THF radical to the double bond of
These results indicate that the different electronic
properties of the three hydrocarbons, aromatic, alkane, and
alkene, affect both the reaction mechanism and the role of the
metal nanoparticles on the TiO2 surface in their photocatalytic
cross-coupling with the ether. Although alkene molecules have
some features similar to both arene and alkane molecules,
they differ a lot in the photocatalytic cross-coupling with the
ether. The presence of localized π orbital in alkenes increases
their reactivity towards the attack of the ether radical,
although benzene with a delocalized π orbital needs activation
by Pd nanoparticles. Further, this localized electron density
decreases the C–H bond dissociation energy of the adjacent
allylic sp3C–H bonds, which results in easier oxidation by the
photogenerated holes, unlike the cyclohexane molecule where
the absence of any functional group makes the sp3C–H bond
unreactive. Thus, the efficiency of the photocatalytic cross-
coupling between the reactive alkenes and ethers might
predominantly depend on the separation of the
photogenerated electrons and holes on the TiO2 photocatalyst
and hence the contribution of the Pt metal nanoparticles
might be limited to an electron receiver.
cyclohexene to give
photon process), a direct coupling of the photogenerated
cyclohexene and THF radicals to give , 2-(cyclohex-2-en-1-
A, 2-cyclohexyltetrahydrofuran (a one-
B
yl)tetrahydrofuran (a two-photon process, dehydrogenative),
and substitution of the photogenerated THF radical to the
double bond of cyclohexene to give
C, 2-(cyclohex-1-en-1-
yl)tetrahydrofuran (a one-photon process, dehydrogenative).
The obtained results were compared with the photocatalytic
dehydrogenative cross-coupling of benzene, and cyclohexane
with THF and it was concluded that the role of the metal
nanoparticles varies with the hydrocarbon substrate. The
reaction of benzene with THF requires the assistance of Pd
metal catalysis, making the reaction to be a hybrid system of
TiO2 photocatalysis and metal catalysis. The coupling of
cyclohexene and THF, on the other hand, was proposed to be a
purely photocatalytic reaction where the contribution of Pt
metal nanoparticles loaded on the TiO2 photocatalyst was
limited to an electron receiver to facilitate the reaction.
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H. Yoshida, Catal. Sci. Technol., 2017,
7
, 2457; (e) A. Tyagi, A.
Conflicts of interest
There are no conflicts to declare.
View Article Online
Yamamoto, T. Kato, H. Yoshida, Catal. Sci. Technol., 2017, 7,
DOI: 10.1039/C8CY00129D
2616; (f) E. Wada, A. Tyagi, A. Yamamoto, H. Yoshida,
Photochem. Photobio. Sci., 2017, 16, 1744.
11 (a) H. Yoshida in Environmentally benign photocatalysts:
Applications of titanium oxide-based material, ed. Anpo, M.
and Kamat, P.V. Springer, 2010, pp-647; (b) V. Augugliaro, T.
Caronna, A. Di Paola, G. Marci, M. Pagilaro, G. Palmisano, L.
Palmisano, in Environmentally benign photocatalysts: TiO2-
based photocatalysis for Organic Synthesis, ed. Anpo, M. and
Kamat, P.V. Springer, 2010, pp-623.
Acknowledgements
The authors express their sincere thanks to Prof. Ken-ichi
Fujita and his laboratory members, Graduate School of Human
and Environmental Studies, Kyoto University, for their help and
guidance on the synthesis of the authentic sample of
12 C. E. Rye, D. Barker, Synlett, 2009, 3315.
compound A. The authors would also like to acknowledge the
13 (a) S. Y. Zhang, F. M. Zhang, Y. Q. Tu, Chem. Soc. Rev., 2011,
40, 1937; (b) A. Batra, P. Singh, K. N. Singh, Eur. J. Org.
Chem., 2017, 2017, 3739.
kindness of the laboratory members of Prof. T. Yoshida at
Graduate School of Engineering, Nagoya University, Japan, for
their help in recording the Pd LIII-edge XAFS measurements at
Aichi Synchrotron Radiation Center (Proposal no. 201603005).
The authors are thankful to the laboratory members of H.
Yoshida Laboratory at Kyoto University, for their help in
recording the Pt-LIII edge XAFS measurements at KEK-PF
(Proposal no. 2014G548). This study was funded by the JSPS
KAKENHI Grant Number 16K14477 as Grant-in-Aid for
Challenging Exploratory Research. A. Tyagi would like to thank
JICA for providing the scholarship under the IITH-JICA
Friendship project to pursue Ph.D. in Japan.
14 (a) D. Liu, C. Liu, H. Li, A. Lei, Chem. Commun., 2014, 50
,
3623; (b) Q. Wang, J. Lou, P. Wu, K. Wu, Z. Yu, Adv. Synth.
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