Tuning the hydrogen donor/acceptor behavior of ionic liquids in Pd-catalyzed multi-step reactions

Article abstract of DOI:10.1016/j.catcom.2014.10.011

Palladium nanoparticles stabilized by thioether-phosphine ligands (1-3) were used as catalytic precursors in Heck-Mizoroki cross-coupling and CC reduction reactions in ionic liquids. The ionic liquid [EMI][MeHPO₃] (A) exhibited an important hydrogen donor/acceptor behavior depending on the reaction conditions. Tandem or parallel pathways for C-C coupling and H-transfer reactions were observed, leading to the coupled product trans-4-phenyl-3-buten-2-one (I) and the corresponding reduced partner 4-phenylbutan-2-one (II) pointing to a molecular-like catalytic reactivity of the process. When the reaction was carried out under hydrogen pressure and low temperature, the reduction to II was inhibited by the presence of thioether-phosphine ligands.

Full text of DOI:10.1016/j.catcom.2014.10.011

Catalysis Communications 63 (2015) 56–61
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Tuning the hydrogen donor/acceptor behavior of ionic liquids in
Pd-catalyzed multi-step reactions
Angela M. López-Vinasco a, Itzel Guerrero-Ríos a, Isabelle Favier b,c, Christian Pradel b,c, Emmanuelle Teuma b,c
,
Montserrat Gómez b,c, , Erika Martin
⁎
a,⁎⁎
a
Depto. de Química Inorgánica, Facultad de Química, Universidad Nacional Autónoma de México, Av. Universidad 3000, 04510 México D.F., Mexico
Université de Toulouse, UPS, LHFA, 118 route de Narbonne, 31062 Toulouse Cedex 9, France
CNRS, LHFA, UMR 5069, 31062 Toulouse Cedex 9, France
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 July 2014
Received in revised form 30 September 2014
Accepted 15 October 2014
Available online 23 October 2014
Palladium nanoparticles stabilized by thioether-phosphine ligands (1–3) were used as catalytic precursors in
Heck–Mizoroki cross-coupling and C_C reduction reactions in ionic liquids. The ionic liquid [EMI][MeHPO3]
(A) exhibited an important hydrogen donor/acceptor behavior depending on the reaction conditions. Tandem
or parallel pathways for C–C coupling and H-transfer reactions were observed, leading to the coupled product
trans-4-phenyl-3-buten-2-one (I) and the corresponding reduced partner 4-phenylbutan-2-one (II) pointing
to a molecular-like catalytic reactivity of the process. When the reaction was carried out under hydrogen pressure
and low temperature, the reduction to II was inhibited by the presence of thioether-phosphine ligands.
© 2014 Elsevier B.V. All rights reserved.
Keywords:
Palladium nanoparticles
Thioether-phosphines
Ionic liquids
Heck–Mizoroki reaction
H-transfer reaction
1. Introduction
phosphorous ligands in the ionic liquid [EMI][MeHPO3] (A) [23]. We
found a unique catalytic system for a one-pot multi-step process of
One of the main applications of metal nanoparticles (MNP) relies on
their ability to achieve homo- or heterogenous catalytic transformations
[1–4]. The modulation of size, shape and catalytic performance of MNP
is one of the main challenges in this area [5–8]; nevertheless, the
surface-like reactivity of the MNP with organic molecules cannot be
ruled out, since it has been found that PdNP activate C–S and C–P
bonds' cleavage [9]. Stabilization of MNP can be achieved by the fitting
combination of ligands and ionic liquids (ILs). It is well-established
that the presence of ionic liquids in catalytic processes makes possible
a highly efficient product separation together with successful nano-
catalyst recycling [10–16]. Some of the ligands that have been employed
in MNP in ILs include phenantroline [17], bipyridines [18], pyridines
[19] and amines [20,21]. The most commonly used ILs stem from
imidazolium cations resulting in highly stabilized and well-structured
MNP, where the possible formation of NHC-carbene species on the
nanoparticle surface represents a main drawback [22]. In general, IL cat-
ions are considered as non-innocent ligands in many reactions, whereas
IL anions are mistakenly considered as ‘spectators’. In a previous study,
we investigated the reaction chemistry of palladium nanoparticles with
Heck–Mizoroki reaction followed by reduction of the coupled product
in a sequential way (Scheme 1) as a result of a cooperative effect be-
tween the ligand and the ionic liquid.
We observed that the presence of ligands such as 1 (Scheme 2) in
combination with ionic liquid A triggered tandem reactions to obtain
phenylbutanones whose skeleton is commonly found in fragrances.
With the aim to get a deeper insight in the influence of thioether-
phosphine ligands (L) on the stabilization of nanoparticles and their
role in multi-step catalytic reactions in ionic liquids, we turned our atten-
tion to the study of different substituents on the thioether moiety; in
addition to the aryl-derivative 1, we explored the reactivity of palladium
nanoparticles stabilized by L (PdNPL, L = 1–3) employing a long chain
hydrocarbon (2) to promote stabilization by steric effects, and an ethyl-
enic chain containing a OH group (3) in order to favor the interaction
with the ionic liquid. Herein, we report a systematic study of the influence
of both thioether-phosphine-based ligands and the ionic liquid nature on
the stabilization of PdNP as well as on the combined Heck–Mizoroki
cross-coupling reaction followed by reduction of the alkene function.
2. Experimental section
⁎
Corresponding author. Tel.:+33 561557738; fax: +33 561558204.
2.1. General methods
⁎⁎ Correspondence to: E. Martin, Depto. de Química Inorgánica, Facultad de Química,
Universidad Nacional Autónoma de México, Av. Universidad 3000, 04510 Mexico D.F.,
Mexico. Tel./fax:+52 5556223720.
A description of all the equipments reagents used can be found in
Appendix A.
E-mail addresses: gomez@chimie.ups-tlse.fr (M. Gómez), erikam@unam.mx (E. Martin).
http://dx.doi.org/10.1016/j.catcom.2014.10.011
1566-7367/© 2014 Elsevier B.V. All rights reserved.

A.M. López-Vinasco et al. / Catalysis Communications 63 (2015) 56–61
57
Scheme 1. One-pot sequential Heck–Mizoroki cross-coupling/reduction reaction.
2.2. Synthesis of palladium nanoparticles PdNPL (L = 1–3): General
methodology
temperature (90, 120 or 150 °C) for 16 h. Products were extracted from
the IL phase with cyclohexane (5 × 5 mL), the organic solvent phase was
filtered over celite and volatiles were removed under reduced pressure.
The reaction mixture was analyzed by 1H NMR in order to determine
the conversion of the process.
[Pd2(dba)3]·CHCl3 (0.05 mmol of Pd, 23 mg) and the appropriate
ligand (0.01 mmol, ratio Pd/L = 1/0.2) were placed in a Fisher–Porter
bottle. 5 mL of the corresponding IL (IL = A or B; A = [EMI][MeHPO3],
B = [BMP][NTf2]) was introduced under an argon atmosphere. The sys-
tem was then pressurized with H2 (3 bar) and stirred at 50 °C overnight,
leading to a black solution. After replacing the residual H2 pressure by
argon, a sample was taken for TEM and GC analyses. The solution was
washed with pentane (3 × 20 mL) and dried under reduced pressure.
The organic phase was concentrated and analyzed by 1H NMR, proving
the absence of free ligand.
2.5. Catalytic experiments: hydrogenation. Reduction of
trans-4-phenyl-3-buten-2-one (I)
A mixture of trans-4-phenyl-3-buten-2-one (146 mg, 1.2 mmol)
and 1 mL of preformed PdNPL in A (I/Pd = 100; L = 1, 2 or 3) were
placed in a 45 mL stainless steel reactor. The system was perfectly closed
and vacuum was applied for 15 min. The reactor was stirred and heated
to the desired temperature (30, 50 or 80 °C) and charged with pressure
of hydrogen (20 or 40 bar). After the desired time (16 or 24 h), the re-
actor was cooled to room temperature, followed by depressurization.
Products were extracted from the IL phase with cyclohexane (5 ×
5 mL), the organic solvent phases were filtered over celite and
volatiles were removed under reduced pressure. The reaction mixture
was analyzed by 1H NMR to determine the conversion of the process.
2.3. Catalytic experiments: Heck–Mizoroki cross-coupling/hydrogen
transfer concomitant reactions
Iodobenzene (204 mg, 1 mmol) and butenone (0.1 mL, 1.2 mmol)
were added to 1 mL of preformed PdNPL in A (PhI/Pd = 100; L = 1, 2
or 3; A = [EMI][MeHPO3]). The mixture was stirred at the selected tem-
perature (90, 120 or 150 °C) for 16 h. Products were extracted from the
IL phase with cyclohexane (5 × 5 mL), the organic solvent phase was fil-
tered over celite and volatiles were removed under reduced pressure.
The reaction mixture was analyzed by 1H NMR in order to determine
the conversion of the process.
In the case of PdNP1/B, (B = [BMP][NTf2]) NEt3 (0.35 mL, 2.5 mmol)
and isopropanol (77 μL, 1 mmol) were also added as base and hydrogen
transfer reagent, respectively, and the reaction mixture was treated as
described above.
3. Results and discussion
3.1. Synthesis of PdNPL in ionic liquids
Ligands 1–2 were prepared as we reported elsewhere [9] and 3 was
obtained from the thiophenol derivative and bromoethanol as a white
solid and was fully characterized by conventional spectroscopic tech-
niques (see Appendix A).
PdNPL (L = 1–3) were prepared from decomposition of
[Pd2(dba)3]·CHCl3 by molecular hydrogen under mild conditions
(50 °C, 3 bar H2) using [EMI][MeHPO3] (A) and [BMP][NTf2] (B) for
comparative purposes (Scheme 2).
2.4. Catalytic experiments: hydrogen transfer. Reduction of
trans-4-phenyl-3-buten-2-one (I)
In all cases, the obtained black solutions were analyzed by TEM
(Fig. 1) and EDX (see Appendix A). The ligands exhibited a strong influ-
ence on the size, distribution and stabilization of nanoparticles. Well-
dispersed spherical nanoparticles with 2D arrays were obtained for
A 50 mL Schlenck tube was charged with trans-4-phenyl-3-buten-2-
one (I) (146 mg, 1.2 mmol) and 1 mL of preformed PdNPL in IL (I/Pd =
100; L = 1, 2 or 3; IL = A or B). The mixture was stirred at the desired
Scheme 2. Synthesis of palladium nanoparticles (PdNPL) in ionic liquids.

58
A.M. López-Vinasco et al. / Catalysis Communications 63 (2015) 56–61
Table 1
Heck–Mizoroki cross-coupling/H-transfer concomitant reactions using PdNPL in ILs.a
Entry
Catalytic system
T (°C)
Conv. (%)b
I/II
1
2
3
4
5
6
7c
8
9
PdNP1/A
PdNP1/A
PdNP2/A
PdNP2/A
PdNP3/A
PdNP3/A
PdNP1/B
PdNP/Ad
PdNP1/A
90
120
90
120
90
120
120
120
150
61
100
71
100
61
100
100
100
100
60/40
80/20
50/50
78/22
40/60
73/27
97/3
92/8
20/80
a
Reaction conditions: iodobenzene/butenone/Pd, 1/1.2/0.01 mmol.
Determined by 1H NMR.
Addition of 2.5 mmol of Et3N and 1.0 mmol of iPrOH.
Synthesis reported in reference [23].
b
c
d
PdNP1/A and PdNP3/A. In contrast, PdNP2/A showed agglomerated
areas and a broad size distribution, indicating lower nanoparticle stabi-
lization in spite of the presence of a long alkyl chain on 2. The best dis-
persions and narrow distributions were obtained using the thioether-
phosphine containing a phenyl substituent, ligand 1, in both ILs, A and
B (Fig. 1a and d). In the absence of ligands, PdNP/A, displayed a well-
dispersed material with a substantially higher nanoparticles size
detected; in the case of 2, decane and decylphenylthioether could be
also observed, but in any case obtaining less than 6% fragmentation, in
contrast to the 28% attained in THF [9]. These results suggest that the
ionic liquid prevents the interaction between the metal surface and S–
C or P–C bonds, thus avoiding their activation through oxidative addi-
tion reactions.
(dm = 3.7
0.6 nm) compared to those observed when 1–3 were
3.2. Palladium-catalyzed Heck–Mizoroki cross-coupling and H-transfer
used as stabilizers (see Appendix A). Organic extracts from PdNPL/A so-
lutions were analyzed by GC to determine the stability of ligands during
the formation of nanoparticles (see Appendix A), since when PdNPL
were synthesized in THF in the presence of 1 or 2, we observed partial
fragmentation of ligands [9]. Using 1–3 as stabilizers, benzene was
reactions
With the aim to test the performance of palladium nanoparticles in
Heck–Mizoroki cross-coupling reaction using ionic liquids, screening
catalytic experiments were carried out using iodobenzene and
Fig. 1. TEM micrographs and size distribution histograms for: a) PdNP1/A; b) PdNP2/A; c) PdNP3/A and d) PdNP1/B.

A.M. López-Vinasco et al. / Catalysis Communications 63 (2015) 56–61
59
Table 2
1 at 120 °C) while the nature of L did not significantly affect the selectiv-
ity (Table 1, entries 1, 3, 5 and 2, 4, 6). Employing [BMP][NTf2] (B) as
ionic liquid or in the absence of ligand, I was quantitatively obtained
indicating that the ionic liquid A acts as hydrogen donor to form II by
a hydrogen transfer reaction from the anion [23]; in addition, the pres-
ence of the thioether-phosphine ligand (1–3) favored this H-transfer
(Table 1, entries 1–3 vs 8). When B was used, an external base was
required to form the Heck product and the attempt to promote the hy-
drogen donation using isopropanol as transfer agent, was unsuccessful
(Table 1, entry 7).
Hydrogen transfer reactions using PdNPsL in ILs.a
Entry
Catalytic system
T (°C)
Conv. (%)b
1
2
3
4c
5
6
7
8
PdNPL/A
PdNP1/A
PdNP1/B
PdNP1/A
PdNP1/A
PdNP2/A
PdNP3/A
PdNP-PPh3/Ad
90
120
120
120
150
150
150
150
b5
27
b5
b5
92
94
72
61
PdNPL were evaluated in the hydrogen transfer reaction to form the
hydrogenated product II from I using [EMI][MeHPO3] (A) as hydrogen
donor. In all cases, yellowish orange to brown solutions were observed
during the reduction reaction, pointing to the formation of palladium
molecular species. Results shown in Table 2 corroborate the role of A
as H-transfer agent (Table 2, entries 2, 5–8), which was inhibited
when Na2CO3 was added (Table 2, entry 4). Excellent conversions
were achieved at higher temperatures and the activity was related to
the nature of L, exhibiting the following trend: 1–2 N 3 N PPh3
(Table 2, entries 5–8). In the case of direct H-transfer, palladium species
were not active to produce II from I at 90 °C in contrast to the results ob-
tained in the apparent tandem reaction, where the selectivity was al-
most 1/1 for I/II (40–60% of II) at low temperature (Table 1, entries
1,3,5 vs Table 2, entry 1). This behavior indicates that both reactions,
Heck–Mizoroki cross-coupling and H-transfer, do not proceed in a se-
quential mode but they are competing reactions sharing the exact
same intermediate. At 120 °C, the yield obtained in the direct reduction
reaction; point out to a tandem coupling/reduction process (Table 1,
entry 2 vs Table 2, entry 2). In fact, when the reaction between
iodobenzene and butenone was carried out at 150 °C employing
PdNP1/A, full conversion and good selectivity toward the reduced prod-
uct II were achieved (Table 1, entry 9; I/II = 20/80).
a
Reaction conditions: trans-4-phenyl-3-buten-2-one/Pd = 1/0.01 mmol.
Determined by 1H NMR.
Addition of 2.5 mmol of Na2CO3.
b
c
d
Synthesis reported in reference [23].
butenone to form the coupled product I. As we previously observed
[23], phenylbutanone II was also generated under the reaction condi-
tions. Selected results are collected in Table 1.
At the end of the reaction, orange colored solutions were obtained
denoting that the reaction proceeded in a molecular fashion, where
PdNP acted as molecular catalyst reservoirs. The resulting solutions
were analyzed by TEM, revealing the absence of dispersed nanoparticles
in the medium.
In general, both conversion and selectivity toward the Heck coupling
product I increased at high temperatures (I/II = 1/1 at 90 °C; I/II = 4/
Scheme 3. Proposed mechanism for palladium-catalyzed multipath process in the ionic liquid [EMI][MeHPO3] (A).

60
A.M. López-Vinasco et al. / Catalysis Communications 63 (2015) 56–61
Table 3
homogeneous-like catalysts, an H-transfer pathway is discarded be-
cause at low temperatures this process does not occur as demonstrated
in Section 3.2. Therefore, we propose that an oxidative addition of mo-
lecular hydrogen on palladium takes place, and due to the presence of
L on the surface, the sequestration of Pd atoms occurs, leading to L-Pd-
hydride molecular species, responsible for the reduction of I to II, gener-
ating molecular LPd(0) complexes and restarting the catalytic cycle. The
effect of L on the catalytic performance (3 N 2 N 1) could be explained in
terms of the electronic donor properties of the sulfur substituent on
LPd(0) complexes favoring the dihydrogen oxidative addition.
Hydrogenation reactions of I using PdNPL in [EMI][MeHPO3] (A).a
Entry
Catalytic system
T (°C)
H2 (bar)
Conv.b (%)
1
2
PdNP1
PdNP
PdNP1
PdNP2
PdNP3
50
30
80
80
80
20
20
40
40
40
32
100
74
85
93
3c
4c
5c
4. Conclusions
a
Reaction conditions: trans-4-phenyl-3-buten-2-one/Pd =1/0.01 for 16 h.
b
c
Determined by 1H NMR.
For 24 h.
The Heck–Mizoroki and reduction multi-step process in the ionic
liquid [EMI][MeHPO3] (A) using PdNPL was explored; the palladium
nanoparticles stabilized by thioether-phosphine ligands generate cata-
lytically active molecular species which are involved in diverse reaction
pathways depending on both reaction temperature and hydrogen
donor/acceptor properties of the ionic liquid. When the ionic liquid
acts as H-acceptor, the Heck–Mizoroki reaction is favored; whereas as
H-donor, the hydrogen transfer reaction proceeds. A thorough analysis
of the catalytic results permitted to conclude that at relative low tem-
perature, the Heck–Mizoroki and H-transfer reactions, compete sharing
the same palladium intermediate. Meanwhile, at higher temperatures
both reactions complement each other through a tandem process;
therefore, it is possible to modulate the mechanism pathway and the
resulting product, coupled (I) or reduced (II), by selection of the appro-
priate ionic liquid and temperature range.
When no H-transfer reaction occurs, heterogeneous species are
responsible for the reduction of the Heck product in the presence of
dihydrogen under mild conditions; however, their performance dimin-
ished by the presence of ligands. In contrast, an increase in temperature
led to the formation of catalytically active molecular species, presum-
ably favored by thioether-phosphines. Under these conditions the
ionic liquid acts merely as solvent.
Taking into account the overall catalytic results, we propose that three
processes operate in the reaction between iodobenzene and butenone
and the reaction pathway depends on both the temperature and the
hydrogen donor/acceptor behavior of the ionic liquid (Scheme 3).
PdNPL work as precursor of molecular species which are formed by: ox-
idative addition of iodobenzene to the metallic surface generating Pd(II)
species a, via known mechanisms [24], or nanoparticle degradation by
temperature. Both processes are assisted by thioether-phosphine ligands
since they can bind to palladium in bi- or monodentate form, depending
on the metal coordinative requirements, leading to the formation of cat-
alytically active molecular species. The Heck–Mizoroki cross-coupling
cycle proceeds by the coordination of butenone to complex a, further
carbometallation produces the alkyl-palladium species c. Depending on
both, the temperature and the behavior of the ionic liquid, the hydrogen
transfer reaction occurs in a tandem mode or competes with the cross-
coupling reaction. At relative low temperature (right dotted box), the in-
termediate c is involved in the H-transfer path, where the anion
[MeHPO3]− acts as a hydrogen donor species giving f which leads to II
by reductive elimination, regenerating the Pd(0) species e. In a parallel
path, IL acts as hydrogen acceptor toward hydride complex d, which is
formed by β-elimination from c, affording I and e. In both pathways, spe-
cies e is stabilized by the thioether-phosphine ligand. When the temper-
ature increases, the Heck–Mizoroki reaction path is faster than the low-
temperature hydrogen transfer path (right dotted box) thus, the selectiv-
ity to I enhances. However, at even higher temperatures (left dotted box),
I is converted to II in a tandem fashion through the coordination of I to
Pd(0) species, generating an olefin complex g, which undergoes a proton
addition from the conjugated acid of [MeHPO3]− (species h). Subsequent
hydrometallation leads to alkyl-palladium species i, where hydrogen do-
nation by [MeHPO3]− followed by reductive elimination, gives II and re-
generates Pd(0) species e.
Taking into account that imidazolium-based ionic liquids used in this
work efficiently immobilize the catalyst, the scope of the coupling/
reduction process including the catalytic phase recycling, is currently
underway.
Acknowledgments
The authors thank the DGAPA-UNAM (Project IN 231211), the
CONACYT-CNRS (Project PCP 189474), the Centre National de la
Recherche Scientifique (CNRS) and the Université Paul Sabatier for
financial support. A. M. L.-V. thanks CONACYT for a PhD grant.
Appendix A. Supplementary data
3.3. Palladium-catalyzed hydrogenation reactions: homogeneous versus
heterogeneous catalysts
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2014.10.011. These data include MOL files
and InChiKeys of the most important compounds described in this
article.
In order to explore the influence of 1–3 on PdNPL reactivity as
heterogeneous catalysts, we carried out the reduction of coupled
product I under hydrogen pressure. As it is shown in Table 3, the pres-
ence of L on nanoparticles inhibited the trans-4-phenyl-3-buten-2-one
I reduction; therefore it was necessary to increase the temperature
and also the H2 pressure to improve the activity. The reaction proceeded
in a heterogeneous fashion at moderately harsh conditions (Table 3, en-
tries 1 and 2; black solutions were observed during all the reaction
time) meanwhile, at 80 °C and 40 bar of H2 pressure, orange colored
resulting solutions suggested the formation of molecular systems
which are favored by the ligand presence and high temperature
(Table 3, entries 3–5). TEM studies of the orange solution revealed the
absence of nanoparticles. In heterogeneous-like catalysts, the coordina-
tion of ionic liquid and the thioether-phosphine ligand hindered
the interaction between I and the metal surface. In the case of
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