Pt-Pd nanoalloy for the unprecedented activation of carbon-fluorine bond at low temperature

Article abstract of DOI:10.1246/BCSJ.20200112

Carbon-fluorine bonds are considered the most inert organic functionality and their selective transformation under mild conditions remains challenging. Herein, we report a highly active Pt-Pd nanoalloy as a robust catalyst for the transformation of C-F bonds into C-H bonds at low temperature, a reaction that has hitherto often required harsh conditions. The alloying of Pt with Pd is crucial to promote the overall C-F bond. DFT calculations elucidated that the key step is the selective oxidative addition of the O-H bond of 2-propanol to a Pd center prior to C-F bond activation at a Pt site, which crucially reduces the activation energy of the C-F bond cleavage. Therefore, both Pt and Pd work independently but synergistically to promote the overall reaction.

Full text of DOI:10.1246/BCSJ.20200112

Advance Publication Cover Page
Pt-Pd Nanoalloy for the Unprecedented Activation of Carbon-Fluorine Bond at Low
Temperature
Raghu Nath Dhital, Keigo Nomura, Yoshinori Sato, Setsiri Haesuwannakij,
Masahiro Ehara,* and Hidehiro Sakurai*
Advance Publication on the web June 3, 2020
doi:10.1246/bcsj.20200112
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2020 The Chemical Society of Japan
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Publication manuscripts.

Pt-Pd Nanoalloy for the Unprecedented Activation of Carbon-Fluorine Bond at Low
Temperature
Raghu Nath Dhital, Keigo Nomura, Yoshinori Sato, Setsiri Haesuwannakij, Masahiro Ehara,4*
1
1
2
1,3
and Hidehiro Sakurai1*
1Division of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871.
2Graduate School of Environmental Studies, Tohoku University, Aoba 6-6-20, Aramaki, Aoba-ku, Sendai 980-8579.
3School of Molecular Science & Engineering, Vidyasirimedhi Institute of Science and Technology (VISTEC), 555 Moo 1 Payupnai,
Wangchan, Rayong 21210 Thailand.
4Institute for Molecular Science Okazaki, Aichi 444-8787.
E-mail: hsakurai@chem.eng.osaka-u.ac.jp
Hidehiro Sakurai
Hidehiro Sakura received PH.D. Degree from The University of Tokyo in 1994.
Abstract
Carbon-Fluorine bonds are considered the most inert
difficulties, the fact that
a
significant number of
5
a, 6
pharmaceuticals
and functional materials contain fluorinated
organic functionality and their selective transformation under
mild conditions remains challenging. Herein, we report a
highly active Pt-Pd nanoalloy as a robust catalyst for the
transformation of C-F bonds into C-H bonds at low temperature,
a reaction that has hitherto often required harsh conditions. The
alloying of Pt with Pd is crucial to promote the overall C-F
bond. DFT calculations elucidated that the key step is the
selective oxidative addition of the O-H bond of 2-propanol to a
Pd center prior to C-F bond activation at a Pt site, which
crucially reduces the activation energy of the C-F bond
cleavage. Therefore, both Pt and Pd work independently but
synergistically to promote the overall reaction.
moieties has fueled intense interest in the activation of C-F
bonds.
Keywords: Pt/Pd nanoalloy, C-F bond activation,
protodefluorination
1
. Introduction
Homogeneous mixtures of two metals, also known as
alloys, are commonly used as catalysts for a number of
1
applications . Alloying two metals may give rise to unique
2
catalytic activities and novel properties. Alloying of Pd with
other metals often improves its catalytic activity, stability, and
product selectivity owing to “synergistic effects” with the
2
b,3
foreign metallic atoms present in the single particles.
Previously, we have shown an example of the unique catalytic
activity of Au-Pd nanoalloy for the Ullmann coupling of
chloroarenes at low temperature. As part of our ongoing efforts
Scheme 1. Different approaches to cleavage C-F bond
4
The simplest modification of the C-F bond is its
transformation into a C-H bond, hereafter referred to as
to extend the utility of nanoalloys in terms of unique reactivity,
7
we herein
report that
a
poly(N-vinylpyrrolidone)
hydrodefluorination (HDF; Scheme 1a). This process often
(
PVP)-stabilized Pt-Pd nanoalloy serves as an effective and
involves the use of harsh reductants at high temperature or
electrochemical conditions, which leads to a wide mechanistic
diversity. Aryl fluorides are defluorinated by homogeneous or
heterogeneous transition-metal-catalyzed hydride transfer in the
robust catalyst for the activation of the C-F bonds in aryl
fluorides at low temperature (Scheme 1b).
The high electronegativity of fluorine induces
a
8
significant ionic bond character, which results in the strongest
and shortest sigma bond to carbon. Therefore, C-F bonds are
among the most inert functionalities and their transformation at
presence of hydride sources. Fluorophilic reagents, including
boron/aluminum hydrides and silanes, are also used for the
defluorination of aryl fluorides, where the fluoride is replaced
5
9
low temperature remains challenging.
Despite these
with hydride. Electron-rich metal centers are preferred for the
activation of C-F bonds and many transition-metal complexes

have been developed in combination with electron donating,
please see the Supporting Information.
1
0
bulky, and nucleophilic ligands. Recent developments in
1
1
hydrodehalogenation reactions involve the use of 2-propanol,
HCOOK, or (C
1
2
13
2 5 2
H ) CHONa as greener reductants or
hydride sources in combination with nucleophilic
1
4
ligand-bearing metals. Some examples have been recently
1
5
reported where HDF proceeds at ambient temperature.
In view of the key role that ligands play in the
1
0
homogeneous catalysis promoted by metal catalysts, we
developed a conceptually different approach to activate C-F
bonds using a heterobimetallic system to mimic the steric
and/or electronic environment provided by ligands in a single
2
b
particle.
2
. Experimental
2
.1 General. All chemicals and solvents were used as received
without further purification unless otherwise noticed.
Hexachloroplatinate (H PtCl •4H Tanaka Kikinzoku),
palladium chloride (PdCl , FUJIFILM Wako Pure Chemical)
2
4
2
O
2
and poly(N-vinylpyrrolidone) (PVP-K-30) (Kishida chemicals)
were used as precursors for the preparation of monometallic
and bimetallic nanoclusters (NCs). Ethyl acetate, ether and
hexane were obtained from FUJIFILM Wako Pure Chemical.
Cesium carbonate was obtained from Aldrich. Other bases such
as NaOH, KOH, and anhydrous t-BuOK were obtained from
the FUJIFILM Wako Pure Chemical. Aryl fluorides were
obtained from TCI. Ultrapure water (Milli-Q, 18.2 MΩ) was
used in all experiments. All glassware was cleaned by freshly
prepared aqua-regia (3:1 mixture of conc. HCl and conc.
3
HNO ) and rinsed by Milli-Q (18.2 MΩ) water before
preparation of catalysts.
2
x y
.2 Preparation of Pt Pd nanoalloy. 160 mg (1.25 mmol,
monomer unit of polymer) of PVP (K-30) was placed in a hard
glass test tube (φ=42 mm) and dissolved in ethanol water
mixed solvent (32 mL ethanol+6 mL water). To the solution
was added required amount of aqueous PdCl
2 2 6
and H PtCl
(from 12.5 mM stock solution) solution (1:58 metal to
monomer unit of polymer) and final ratio of ethanol/water was
made 4:1. The resulting solution was stirred (in organic
synthesizer, EYELA, PPS-2510) for 15 min at 25 °C under
argon. The solution was then refluxed at 92 °C for 3 hours
under vigorous stirring (1000 rpm) under argon atmosphere.
The color of the mixture turned from pale yellow to brown,
indicating the formation of bimetallic clusters. The
x y
thus-obtained Pt Pd :PVP clusters were subsequently dialyzed
using membrane filter with a cut-off molecular weight of 10
kDa (Vivaspin 15R, United Kingdom) at 4000 rpm to remove
the inorganic impurities and organic solvent, which is a crucial
treatment to enhance the stability of the clusters against
coalescence. The dialyzed hydrosol of Pt
x y
Pd was dried using a
lyophilizer and then vacuum at 45 °C for 3 h.
For the preparation of other catalysts, please see the Supporting
Information.
Figure 1. TEM images and corresponding histogram plots of a)
Pt1.0 NCs, b) Pt0.8Pd0.2 c) Pt0.5Pd0.5 d) Pt0.2Pd0.8 and e) Pd1.0
NCs.
2
.3 Transmission electron microscopy analysis (TEM). A
2
.4 Typical procedure for de-fluorination of aryl fluorides.
drop of aqueous Pt1.0 or Pd1.0 or Pt Pd nanoalloy (1 mM) was
x
y
De-fluorination reaction was carried out using an organic
synthesizer (EYELA, PPS-2510). Aryl fluoride (0.25 mmol),
bases (200 mol%, 0.50 mmol) and catalyst (2 atom%) were
placed in a test tube (φ=20 mm) under argon conditions. To the
mixture, anhydrous 2-propanol was added and the solution was
stirred vigorously (1300 rpm) at required temperature for
desired times under argon (from balloon). Reaction was
quenched by HCl (1 M) and the product was extracted with
placed on the carbon coated copper grid followed by vacuum
drying. The sample was then analyzed using a transmission
electron microscope (TEM). By counting more than 300
particles, histograms were plotted and the average diameters (in
nanometers, nm) of the series of clusters were calculated. The
observed TEM images and corresponding histogram plots of
monometallic and bimetallic catalysts are shown in Figure 1.
For the STEM-EDS measurements and other characterization,

ethyl acetate or ether (4x10 mL). The extracted organic layer
was diluted up to 50 mL and the content of products was
quantified using gas chromatography. Hexadecane was used as
the internal standard. For the confirmation of products, the
contrast, a physical mixture of Pt:PVP and Pd:PVP did not
exhibit any catalytic activity, which indicates that a bimetallic
surface is indispensable for the reaction (entry 9).
Table
1.
Comparison
of
the
Catalytic
6 3
crude was dissolved in acetone-d or CDCl and analyzed by
Defluorination of 1a.
NMR. The product was isolated by preparative thin layer
chromatography (WAKO gel B-5F; ethyl acetate and hexane as
eluent).
For the details of the kinetic studies, please see the Supporting
Information.
2
.5 General information for theoretical studies. The reaction
mechanism of this catalytic cycle proposed in Scheme 2 was
examined using density functional theory (DFT) calculations
Entrya Catalyst
Time (h) Yield (%)b
1
6
with M06-L functional. The model clusters of Pt
7 6
Pd and
1
2
3
4
5
6
7
8
9c
Au1.0:PVP
24
24
24
24
24
24
28
40
24
0
Pt11Pd
2
in nearly icosahedral structures which are usually stable
and less reactive were adopted for calculating the reaction
pathways and simulating the Pt0.5Pd0.5 and Pt0.8Pd0.2 cases,
Pd1.0:PVP
0
Pt1.0:PVP
0
respectively. The results obtained for the Pt
mostly presented and those for the Pt11Pd NC were essentially
the same as Pt Pd NC and not reported here, except for some
cases specified. The relativistic effective core potential (RECP)
7 6
Pd NC were
2
Au0.5Pd0.5:PVP
Au0.5Pt0.5:PVP
Pt0.2Pd0.8:PVP
Pt0.5Pd0.5:PVP
Pt0.8Pd0.2:PVP
Pt1.0:PVP
0
7
6
0
1
7
with LANL2DZ basis sets were adopted for Pt and Pd, while
1
8
0
the 6-31G(d,p) basis sets were used for other atoms. The
stable geometrical structures and the spin states of bare
bimetallic NCs were examined using the Birmingham genetic
97
95
0
1
9
algorithm (GA) using Gupta potential followed by the DFT
calculations. All the DFT calculations were conducted using
2
0
Gaussian09 suit of programs.
The structure and spin state of bare Pt
+
Pd1.0:PVP
7
6
Pd NC were examined
using the GA and DFT calculations. Nearly symmetric
icosahedral structures were obtained and the alloy structure was
more stable than the phase separated structure. The structure
a
b
Reactions conditions: 0.25 mmol of 1, 2 mL of 2-propanol. Yield
was determined by using gas chromatography, hexadecane was
used as external standard. cPd1.0:PVP (1 atom%)+Pt1.0:PVP (1
atom%) was used.
with nonet spin state was found to be most stable in bare Pt
7 6
Pd
NC. The present computational protocol with M06-L was found
2
1
to be suitable for metal NC with high spin state previously.
The method was extended to a wide variety of aryl
fluorides in order to verify the generality and scope of the
current protocol. As shown in Table 2, aryl fluorides bearing
different functional groups were successfully converted into the
hydroarenes in excellent yield. In particular, inactive Ar-F
compounds containing a deactivating group showed better
reactivity than those bearing activating substituents. For
For more details about the calculations, please see the
Supporting Information.
3
. Results and Discussion
A series of PVP-stabilized monometallic nanoparticles
and bimetallic nanoalloys was prepared via a wet chemical
approach (Supporting Information). Typically, the transmission
electron microscopy (TEM) and scanning transmission electron
microscopy (STEM) images of Pt-Pd nanoalloys (Figure 1)
show a approximately uniform distribution and crystallinity.
The energy dispersive X-ray spectroscopy (EDS) pattern of
Pt0.5Pd0.5:PVP (Figure S2) shows the stoichiometric presence of
Pt and Pd (50% on average) in the particles, which is consistent
with the observed experimental inductively coupled
plasma-atomic emission spectrometry (ICP-AES) (Table S1).
The EDS/STEM images show that both Pt (red dots) and Pd
example,
1
4
the
-fluoro-4-methoxybenzene (1a), fluorobenzene (1b), and
-fluorobenzoic acid (1c) (Figure S6) follow the order 1a > 1b
1c, which strongly indicates that C-F bond activation step is
observed
reaction
rates
for
>
not the rate determining step and that the reaction mechanism
does not involve an aromatic nucleophilic substitution reaction.
To gain further insight into the mechanism, the reaction of 1a
was carried out in deuterium-labeled 2-propanol at 45 °C using
t-BuOK as the base. The nearly-identical initial rates for the
consumption of 1a at all reaction times (Figure S7) suggest that
the reaction is zero order with respect to 1a, ruling out the
oxidative addition of Ar-F as the rate determining step. The
hydrogen source for the product from 1a was investigated using
various deuterated 2-propanol solvents (Table 3).
Characterization of the product resulting from the reaction with
D1 (entry 1) confirmed the substitution of fluoride by
deuterium (60% deuterium incorporation), indicating that the
alcoholic proton of 2-propanol is the major hydrogen source. In
addition, 92% of the incorporation in the product was observed
(green dots) species (Figure S1) are mixed in a single particle
that forms a nanoalloy.
Our investigation started by monitoring the defluorination
of 4-fluoroanisole (1a) in 2-propanol as the hydrogen source at
2
7 °C, and results are shown in Table 1. The defluorinated
product, anisole (2a), was not obtained using pure metal (Au,
Pt, Pd) particles and bimetallic (Au0.5-Pd0.5 Au0.5-Pt0.5,
,
Pt0.2Pd0.8) nanoalloys (entries 1-6). To our delight, the
Pt0.5Pd0.5:PVP(mean size 2.7±0.3 nm) showed remarkable
catalytic activity and the conversion into 2a was accomplished
after 28 h (entry 7). With other bimetallic combinations, such
as nanoalloy containing 20% of Pd (Pt0.8Pd0.2:PVP, mean size
from the reaction with (CD
3 2
) CDOH (D7), further confirming
that the source of hydrogen is the alcoholic proton (entry 2).
2
.1±0.2 nm), the yield of 2 reached 95% after 40 h (entry 8). In

H D
observed kinetic isotope effect (k /k =2.41) indicates that the
Table 2. Substrate Scope
hydrogen at the 1-position of 2-propanol participates in the
rate-determining step of the overall process, although this
hydrogen is not mainly introduced in the product (Figure S7).
H D
In contrast, we observed an inverse KIE (k /k =0.93), where
the hydrogen stems from the alcoholic proton (-OD).
Subsequently, the initial rate of consumption of 1a was found
to depend linearly on the concentration of the catalyst (first
order with respect to the catalyst) (Figure S8), providing
additional evidence that the oxidative addition step occurs on
2
2
the surface of the alloy. In addition, the concentration of both
metals after the reaction was confirmed by ICP-AES to be
under the detection limit (<40 ppb). To elucidate the reaction
mechanism, we performed DFT calculations. Scheme 2 shows
the proposed mechanism for the catalytic cycle and Figure 2
provides the calculated free energy profile using a Pt-Pd model
cluster. Both Pt and Pd sites are present on the cluster surface,
which is supported by the EDS and XRD data and genetic
algorithm calculations. The oxidative addition of 2-propanol
under concomitant O-H dissociation occurs selectively at the
Pd sites prior to the C-F activation with an energy barrier of
1
8.3 kcal/mol. This step is an endothermic equilibrium process
prior to the high energy barrier process, which would explain
the inverse KIE (Figure S7). Owing to the effect of
electron-donative adsorption at nearby Pd site, C-F activation
2
3
occurs selectively at the Pt sites with an energy barrier of 18.1
kcal/mol, which is sufficiently low for the bond activation even
at room temperature: this step is thus not the rate-determining
step. The C-F activation prior to the O-H oxidative addition, on
the other hand, requires 26.3 kcal/mol, while the energy barrier
for C-F activation without adsorption of KOH on the Pt-Pd
cluster is 23.5 kcal/mol. Potassium ion coordinates to C-F and
supports to abstract F atom. These results indicate that the O-H
dissociation of 2-propanol and the adsorption of KOH as well
as the activation of the cluster surface under similar reaction
conditions, are crucial for the activation of the C-F bond.
The LUMO of the C-F activation transition state is
delocalized over the Pt-C bond in anti-phase, indicating that the
reduction of the energy barrier should be attributed to a
stabilization of the LUMO. In contrast, the HOMO is
delocalized over the Pt-Pd cluster and related to its
nucleophilicity. The energy level of HOMO is almost constant
even after co-adsorption, suggesting that the oxidative addition
of 2-propanol does not affect the subsequent oxidative addition
aReactions conditions: 0.25 mmol of 1, 2 mL of 2-propanol, and 200
b
mol% of KOH under argon. Yield determined by GC using hexadecane or
(Figure S15). Since the migration of H easily occurs from Pd to
anisole as the internal standard. c300 mol% of KOH was used. dIsolated
Pt, the reductive elimination of benzene follows through a
low-energy barrier, which is exothermic by 17.2 kcal/mol. The
obtained results clearly demonstrate that the hydrogen atom
introduced into the product is derived mainly from the
alcoholic proton of 2-propanol, although experimentally H/D
scrambling was observed due to multiple sites on the same
particle surface.
e
yield. Pt0.8Pd0.2:PVP was used as catalyst.
Table 3. Reactions in Deuterated 2-Propanol
Finally, the -H transfers from the adsorbed 2-propoxy
group is followed by elimination of H O to complete the
2
catalytic cycle. Notably, 2-propanol acts as the reductant via a
-H transfer to the cluster surface, where it is oxidized into
acetone, as confirmed experimentally (Figure S10) as well as
the formation of KF (Figure S11). These results are also
consistent with another kinetic observation (Figure S9). In short,
the aforementioned theoretical results and derived mechanism
show excellent agreement with the experimental findings. The
oxidative addition of 2-propanol on Pd occurs before Pt
activates the C-F bond. The most important thing to note is that
Pd and Pt play cooperative roles and the bimetallic effects stand
in sharp contrast to the conventional behavior observed for
b
c
Entry
2-PrOH
Yield (2a) (%)
H:D (%)
1
(CH
(CD
(CD
3
3
3
)
2
)
2
)
2
CHOD (D
CDOH (D
CDOD (D
1
7
8
)
)
)
97
98
55
40:60
92:8
3:97
2
3
aReaction conditions: 0.15 mmol of 1a and 0.8 mL of 2-propanol.
b
c
Yield determined by GC using hexadecane as the external standard. The
H/D ratio was determined by GC-MS.
Therefore, we refer to this reaction as “protodefluorination”
instead of hydrodefluorination (HDF) (Scheme 1b). The
2
4
Au-Pd alloys.

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7 6
Pd model.
4
. Conclusion
In conclusion, we have demonstrated that Pt-Pd nanoalloy
exhibits superior catalytic activity for the activation of C-F
bonds at room temperature. The present reaction (C-F
activation by Pt-Pd) seems similar to our previously reported
C-Cl activation by Au-Pd, but the two reaction mechanisms are
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4
totally different. In the latter case, the catalytic center is Pd
and the major role of Au is to anchor the Pd centers onto the
bimetallic surface to avoid leaching (deactivation) processes, in
4
a,
addition to accelerate the migration process of the Cl atoms.
1
8
In contrast, in the current reaction, the C-F activation occurs
at the Pt sites, but this is only possible once the neighboring Pd
atoms are activated upon oxidative addition of 2-propanol,
which also provides the coupling partner “H” for the
exothermic reductive elimination. Therefore, Pt and Pd play
independent and cooperative roles to accomplish the effective
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Acknowledgement
This work was supported by JST(ACT-C) project
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