Suzuki-Miyaura and Hiyama coupling catalyzed by PEPPSI-type complexes with non-bulky NHC ligand

Article abstract of DOI:10.1016/j.molcata.2016.03.022

Palladium PEPPSI-type complexes with small NHC ligand were applied as catalyst precursors in cross-coupling reactions forming a wide range of non-symmetric biaryls with high yields. The recycling of the in situ formed palladium composite was successfully performed in 10 subsequent runs. Mechanistic studies confirmed a labile character of the N-ligands coordinated to palladium by ligand exchange monitored using ¹H NMR. The formation of Pd(0) NPs during the catalytic process was evidenced by TEM.

Full text of DOI:10.1016/j.molcata.2016.03.022

Journal of Molecular Catalysis A: Chemical 418 (2016) 9–18
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Suzuki–Miyaura and Hiyama coupling catalyzed by PEPPSI-type
complexes with non-bulky NHC ligand
Marta Osin´ ska, Andrzej Gniewek, Anna M. Trzeciak∗
Faculty of Chemistry, University of Wrocław, 14 F. Joliot-Curie St., 50-383 Wrocław, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Palladium PEPPSI-type complexes with small NHC ligand were applied as catalyst precursors in cross-
coupling reactions forming a wide range of non-symmetric biaryls with high yields. The recycling of the
in situ formed palladium composite was successfully performed in 10 subsequent runs. Mechanistic stud-
ies confirmed a labile character of the N-ligands coordinated to palladium by ligand exchange monitored
using 1H NMR. The formation of Pd(0) NPs during the catalytic process was evidenced by TEM.
© 2016 Elsevier B.V. All rights reserved.
Received 11 December 2015
Received in revised form 1 March 2016
Accepted 8 March 2016
Available online 17 March 2016
Keywords:
Suzuki-Miyaura cross- coupling
Hiyama reaction
Palladium
PEPPSI-type complexes
Nanoparticles
1. Introduction
over, the purification of final biaryls from silicon byproducts can be
easier than from organoboron compounds.
Palladium complexes bearing bulky NHC ligands, two halides
and a labile 3 -chloro-pyridine, were introduced by Organ as Pyri-
dine Enhanced Precatalyst Preparation Stabilization and Initiation
(PEPPSI) complexes [1,2] . Air and moisture stable PEPPSI com-
The studies presented in this paper were focused on the reac-
tivity of PEPPSI-type complexes in Suzuki-Miyaura and Hiyama
reactions leading to the same products, non-symmetric biaryls. As
catalyst precursors, we selected PEPPSI-type complexes with dif-
ferent N ligands, such as pyridines and amines. Considering the
fact that the original PEPPSI catalysts contained bulky NHC ligands,
we focused on the smaller bmim-y (1-butyl-3-methylimidazol-2-
ylidene) NHC ligand, which was coordinated to palladium in the
catalyst precursors. We were encouraged by very good results
obtained in the Suzuki–Miyaura reaction for Pd(NHC) 2Br2 and
[Pd(NHC)Br2]2 precursors with small NHC ligands [31,32]. It was
supposed that the bmim-y ligand would be able to efficiently
stabilize catalytically active forms of palladium, either molecular
complexes or nanoparticles. Two complexes with more sterically
demanding NHC ligands, SIMes and IPr, were also studied for com-
parison under the same conditions.
plexes have found many applications as catalysts of C C and C
N
cross-coupling reactions due to their ready activation under the
reaction conditions [3–7] . In addition, PEPPSI complexes can be
easily obtained and are easy to handle in contrast to other Pd-
NHC precursors [8–15]. Recently the new preparations of PEPPSI
complexes have been reported [16,17].
There are many examples of the application of PEPPSI complexes
in the Suzuki–Miyaura reaction, in which their excellent catalytic
activity and versatility are clearly evidenced [18–24]. In contrast,
their applications in the Hiyama coupling are rather scarce [25–27].
This is surprising because these two methods, the Suzuki–Miyaura
and the Hiyama reactions, present alternative pathways leading to
biaryl compounds and, therefore, it could be interesting to esti-
mate their relative applicability under similar reaction conditions.
The expected advantages of the Hiyama procedure consist in the
low cost, the good availability and stability of the organosilicon
reagents, and their environmentally friendly nature [28–30]. More-
2. Results and discussion
2.1. Synthesis and structure of PEPPSI-type complexes
The PEPPSI-type complexes studied in this paper are presented
in Fig. 1.
∗
In order to obtain these complexes, we employed a reaction
between a palladium dimer [Pd (bmim-y)X2]2 and an N-ligand. A
Corresponding author.
E-mail address: anna.trzeciak@chem.uni.wroc.pl (A.M. Trzeciak).
http://dx.doi.org/10.1016/j.molcata.2016.03.022
1381-1169/© 2016 Elsevier B.V. All rights reserved.

10
M. Osin´ska et al. / Journal of Molecular Catalysis A: Chemical 418 (2016) 9–18
N
N
N
N
Cl
Cl
Pd
L
Br
Br
Pd
L
Pd(bmim-y)Cl2L
Pd(bmim-y)Br2L
N
N
N
NH2
N
N
L =
N
N
Cl
Cl-py
CN
CN-py
Me-py
py
TPA
TEA
Me-py-N
N
N
N
N
Cl
Cl
Pd
Cl
Cl
Pd
N
N
Cl
Cl
Pd(SIMes)Cl2(Cl-py)
Pd(IPr)Cl2(Cl-py)
Fig. 1. PEPPSI-type complexes.
Reaction conditions: 0.01 mmol [Pd], 1.1 mmol 2-Br-toluene, 1.1 mmol PhB(OH)2, 1.1 mmol KOH, 2-propanol/water (1/1) 5 mL, 70 ◦C, each run 1 h.
similar strategy was also used for preparation of Pd(NHC)Cl2(TEA)
complexes with NHC = IPr or SIPr [21] .
Such a procedure was successfully performed with a low excess
of the N ligand, at the [N-ligand]/[Pd] ratio of 1, in contrast to
the original method described for the synthesis of PEPPSI-type
complexes utilizing neat pyridine as the reaction medium [2]. The
palladium complexes presented in Fig. 1 were isolated with yields
of 70–95% and characterized by spectroscopic methods.
The molecular structure of compound Pd(bmim-y)Cl2(Cl-py)
has been determined by single crystal X-ray diffraction (Fig. 3). The
Pd atom in this compound is four-coordinated in a slightly distorted
square-planar environment (Table 1). All the bond distances are
within standard ranges.
The palladium complex Pd(bmim-y)Br2(CN-py) was also syn-
thesized according to the one-step procedure starting from PdCl2
[2] (Scheme 1); however, the product was isolated with ca. 10%
yield. The synthetic method elaborated for PEPPSI with bulky NHC
ligands probably does not work properly for bmim-y.
Fig. 3. Molecular structure and atom numbering scheme of Pd(bmim-y)Cl2(Cl-py).
Displacement ellipsoids are drawn at the 30% probability level and H atoms are
shown as small spheres of arbitrary radii.
Cl-py
PdCl2 + [bmim]Cl
Pd(bmim-y)Cl2(Cl-py)
10%
K2CO3
80oC, 16 h
N
Scheme 1. Synthesis of Pd(bmim-y) Cl2(Cl-py) according to [2].
Br
Br
L
L
Br
N
Br
N
N
Pd
Pd
Pd
N
N
Br
Br
2.2. The Suzuki–Miyaura reaction
The reactions between bromotoluene and phenylboronic acid
or phenyltrimethoxysilane were selected as a model reactions for
testing the activity of our precursors (Fig. 4).
Fig. 2. Synthesis of PEPPSI-type complexes.

M. Osin´ska et al. / Journal of Molecular Catalysis A: Chemical 418 (2016) 9–18
11
H3C
CH3
CH3
OH
B
Si(OMe)3
+
Br
Br
OH
+
Suzuki-Miyaura reaction
Hiyama reaction
Fig. 4. Suzuki-Miyaura and Hiyama reactions.
temperature increased to 110 ◦C, the difference between small and
bulky NHC ligands became invisible, all complexes providing high
substrate conversions.
Table 1
Selected bond lengths (Å) and angles (◦) for Pd(bmim-y)Cl2(Cl-py).
Pd1-C1
Pd1-N3
Pd1-Cl1
Pd1-Cl2
1.963(3)
2.132(2)
2.3038(10)
2.3141(10)
The significant difference in activity observed at a lower temper-
ature indicated that a higher concentration of catalytically active
form is available in the case of Pd complex with bulky NHC ligand.
When the temperature was raised to 110 ◦C, an effect of bulkiness
of NHC was much smaller suggesting that Pd active species was
formed at the similar rate. Formation of the active Pd(0) species
involves two important steps, namely dissociation of N-ligand and
reduction of Pd(II) to Pd(0). These two steps could be influenced
by properties of NHC ligand. For example, XPS studies shown that
only ca. 50% of Pd was reduced in the case of smaller NHC ligand
and as a result its activity was lower.
The influence of the base on the reaction course is relatively
small, and similar results were obtained with KOH, NaOH, and
NaHCO3 in ethylene glycol. In the presence of NaOH, the results
were slightly better, and in some cases the conversion exceeded
90% (Table 2).
After optimizing the reaction conditions, we started to inves-
tigate the substrate scope of our reaction. PEPPSI-type complexes
afforded the efficient coupling of different aryl bromides (Table 3),
and in most cases the yield was higher than 90%, with the exception
of 3-bromo-4-hydroxybenzonitrile, which did not react. The reac-
tion showed a good tolerance of different groups on the aromatic
ring.
When aryl chlorides were used as substrates, coupling products
were formed with a lower yield (Table 4). This was expected on the
basis of the higher values of the C Cl bond energy with respect to
C–Br. Nevertheless, good results were obtained for chlorobenzene
and 1-chloro-2-nitrobenzene.
C1-Pd1-Cl1
C1-Pd1-Cl2
C1-Pd1-N3
N3-Pd1-Cl1
N3-Pd1-Cl2
Cl1-Pd1-Cl2
88.02(9)
87.41(8)
174.07(10)
92.65(7)
92.26(7)
174.24(2)
All the compounds were active in 2-propanol at 40 ◦C, forming
up to 65% of the coupling product. Better results, over 80% of 2-
methylbiphenyl, were achieved after the introduction of water to
the reaction solution at the same temperature (Table 2). Clearly,
the presence of water positively influenced the reaction course,
probably as a result of the better solubility of phenylboronic
acid. Interestingly, under these conditions, the steric hindrance
of the NHC ligand practically did not influence the reaction
course. For example, almost the same yield, 84–87%, was obtained
using either complexes with bulky NHC ligands, Pd(IPr)Cl2(Cl-py)
or Pd(SIMes)Cl2(Cl-py), or complexes with smaller NHC ligands,
Pd(bmim-y)Cl2(Cl-py). Similarly, pyridines and amines performed
very similarly despite the presence of different substituents. These
observations suggest the transformation of the palladium precur-
sors to similar forms during the catalytic reaction. Interestingly,
under lower catalyst loadings (0.1, 0.01 mol% Pd) the results were
still good and the coupling product was formed with ca. 80% yield
(Table 2).
At the temperature of 40 ◦C, in ethylene glycol, it was possi-
ble to see the positive effect of bulky NHC ligands on the reaction
course (Table 2). Thus, the yields of 64% and 77% were obtained
with precursors bearing bulky ligands, Pd(SIMes)Cl2 (Cl-py) and
Pd(IPr)Cl2(Cl-py), while precursors containing smaller bmim-y lig-
ands produced only 11–14% of the product. However, after the
2.3. The Hiyama reaction
The Hiyama coupling presents an alternative procedure mak-
ing it possible to obtain substituted biaryls (Fig. 2). Under our
fluoride-free conditions, with ethylene glycol as the solvent and
Table 2
Results of Suzuki −Miyaura reaction in different solvents.
2-propanol
40 ◦C, 1 h
2-propanol/water (1/1)
40 ◦C, 1 h
ethylene glycol
40 ◦C, 1 h
ethylene glycol
110 ◦C, 1 h
ethylene glycol
110 ◦C, 1 h NaOH
Catalyst
Conversion%
Pd(bmim-y)Br2(TEA)
Pd(bmim-y)Cl2(Cl-py)
Pd(bmim-y)Br2(TPA)
Pd(bmim-y)Br2(Cl-py)
Pd(bmim-y)Br2(Me-py-N)
Pd(bmim-y)Br2(Me-py)
Pd(bmim-y)Cl2(TEA)
Pd(bmim-y)Br2(CN-py)
Pd(SIMes)Cl2(Cl-py)
Pd(IPr)Cl2(Cl-py)
41.0
59.0
82.0 (80.0c)
86.0 (82.0c)
58.0 (55.0c)
12.0
11.0
14.0
81.0
75.0
79.0
91.0
50.0
86.3
92.0
59.0
45.0
80.0
59.9
89.0 (86.0c)
58.0 (54.0c)
84.0 (80.0c)
89.0 (87.0c)
87.0 (86.0c)
84.0 (83.0c)
85.0
85.0
88.0
69.0
86.0
75.0
82.0 (77.5a, 79.9b)
92.0
87.0
92.8
26.0
65.0
64.0
77.0
Reaction conditions: 0.01 mmol [Pd], 1 mmol 2-Br-Toluene, 1.2 mmol KOH, 1.1 mmol PhB(OH)2, solvent 5 mL.
a
0.001 mmol [Pd].
0.0001 mmol [Pd].
isolated yield.
b
c

12
M. Osin´ska et al. / Journal of Molecular Catalysis A: Chemical 418 (2016) 9–18
Table 3
Results of Suzuki-Miyaura reaction for different aryl bromides.
Ar-Br
Pd(bmim-y)Br2-(Cl-py)
110 ◦
Pd(bmim-y)Br2- (CN-py)
Pd(SIMes)Cl2(Cl-py)
C
110 ◦
C
110 ◦
C
40 ◦
C
4-bromotoluene
90.0
100.0
100.0
100.0
92.0
98.0
96.3
0.0
87.0
98.0
87.0
96.3
39.0
94.0
98.0
0.0
92.0
97.0
85.0
96.1
85.0
99.0
84.0
0.0
3-bromo-4-methylbenzonitrile
1-bromo-4-nitrobenzene
2-bromo-5-nitrotoluene
2-bromoanisole
2-bromo-4-nitrotoluene
4-bromobenzaldehyde
3-bromo-4-hydroxybenzonitrile
99.0
60.0
50.0
99.0
0.0
Reaction conditions: 1 h, 40 ◦C or 110 ◦C, 0.01 mmol [Pd], 1.2 mmol NaOH, 1 mmol Ar-Br, 1.1 mmol PhB(OH)2; ethylene glycol 5 mL.
Table 4
Results of Suzuki-Miyaura reaction for aryl chlorides.
Ar-Cl
Pd(bmim-y)Br2(CN-py)
Pd(IPr)Cl2(Cl-py)
Pd(SIMes)Cl2(Cl-py)
chlorobenzene
43
62
0
0
7
0
0
75
0
5
83
1-chloro-2-nitrobenzene
1-chloro-3-nitrobenzene
4-chloro-2-nitrotoluene
chloroanisole
13
28
2
8
Reaction conditions: 24 h, 110◦ C, ethylene glycol, 1.2 mmol NaOH, 0.01 mmol [Pd], 1.1 mmol PhB(OH)2.
Table 5
only chlorobenzene and 1-chloro-2- nitrobenzene were efficiently
coupled (Table 6).
Results of Hiyama reaction in different solvents.
Catalyst
conversion (%)
2.4. 1H NMR studies of N ligand exchange
ethylene glycol 1 h, 110 ◦
C
water 4.5 h, 120 ◦
C
Pd(bmim-y)Cl2(Cl-py)
Pd(bmim-y)Br2(Cl-py)
Pd(bmim-y)Br2(CN-py)
Pd(bmim-y)Br2(Me-py)
Pd(bmim-y)Br2(TEA)
Pd(bmim-y)Br2(TPA)
Pd(SIMes)Cl2(Cl-py)
Pd(IPr)Cl2(Cl-py)
94.0
90.0
96.0
82.0
91.0
81.5
87.0
97.0
57.0
30.0
The high activity of PEPPSI-type palladium precursors in the
Suzuki–Miyaura and the Hiyama reactions prompted us to study
their structural properties and, in particular, their transformations
under the catalytic reaction conditions.
30.0
60.0
On the basis of literature reports, it can be expected that N
ligands coordinated to palladium will be labile and easily exchange-
able [1,2,33,34]. Thus, the exchange of the CN-py ligand in the
Pd(bmim-y)Br2(CN-py) complex was studied in a CDCl3 solu-
tion using 1H NMR. Three solutions were analyzed containing
palladium complexes and 2-, 4-, and 6-fold excesses of 2-amino-4-
methylpyridine (Me-py-N). 1H NMR spectra were recorded after 5,
15, 30, and 60 min. Already in the first spectrum, there appeared sig-
nals at 5.4 and 8.3 ppm, indicating the coordination of 2-amino-4-
methylpyridine to palladium. At the same time, a signal at 8.8 ppm
confirmed the presence of free CN-py. The analysis of the inten-
sity of signals originating from coordinated (8.3 ppm) and free
(7.9 ppm) Me-py-N made it possible to conclude that the yield of
Pd(bmim-y)Br2(Me-py-N) formed was 28%, 36%, and 95% in solu-
tions containing a 2-, 4-, and 6-fold excess of the entering ligand
(Fig. 5). The substitution of ligands was fast on the time scale of the
experiment, and practically no further changes were observed in
solutions stored 5 or 45 min. Moreover, the concentration of new
palladium complexes also remained unchanged after a few days.
In similar experiments performed at a two-fold excess of the
entering N ligand with respect to palladium, it was found that CN-
py in Pd(bmim-y)Br2 (CN-py) could be completely substituted by
0.0
0.0
Reaction conditions: 0.01 mmol [Pd], 1 mmol 2-Br-toluen, 1.5 mmol PhSi(OMe)3,
2 mmol NaOH.
Table 6
Results of Hiyama reaction for different bromides and chlorides with
Pd(bmim)Br2(CN-py).
Substrate
conversion (%)
4-bromotoluene
60.0
89.0
92.0
93.0
53.0
88.0
93.0
93.0a
50.0a
0a
3-bromo-4-methylbenzonitrile
1-bromo-4-nitrobenzene
2-bromo-5-nitrotoluene
2-bromoanisole
2-bromo-4-nitrotoluene
4-bromobenzaldehyde
chlorobenzene
1-chloro-2-nitrobenzene
1-chloro-3-nitrobenzene
4-chloro-2-nitrotoluene
1.0a
Reaction conditions: 0.01 mmol [Pd], 1 mmol 2-Br-toluene, 1.5 mmol PhSi(OMe)3,
2 mmol NaOH, ethylene glycol 5 mL, 110 ◦C 1 h, a 24 h.
Pd(bmim-y)Br2(CN-py) + 6 Me-py-N
Pd(bmim-y)Br2(Me-py-N) + CN-py
95%
2-bromotoluene as the substrate, the Hiyama procedure appeared
slightly better than the Suzuki–Miyaura reaction with the yield of
product ca. 90% (Table 5).
Fig. 5. Exchange of N-ligands studied by 1H NMR.
Interestingly, the Hiyama coupling also worked in water as a
solvent although palladium complexes were not soluble in that
medium (Table 5). Very good results were also obtained using
differently substituted aryl bromides as substrates in the Hiyama
coupling (Table 6). Similarly as in the Suzuki-Miyaura reaction,
Pd(bmim-y)Br(C2H4O2)
m/z 385.1
Pd(bmim-y)2(Ph)
m/z 461
Pd(bmim-y)Br(MeC6H4)(C2H3O2)
m/z 473
Fig. 6. Selected palladium species identified by ESI–MS.

M. Osin´ska et al. / Journal of Molecular Catalysis A: Chemical 418 (2016) 9–18
13
Pd 3d
amount
BE (eV)
Pd form (%)
334.96 ; 340.22
336.17 ; 341.43
337.72 ; 348.92
Pd(0)
PdO
Pd(II)
38.5
9.2
52.3
344 342 340 338 336 334
Binding Energy, eV
Fig. 7. XPS data of the Pd species isolated after Suzuki-Miyaura reaction catalyzed by Pd(bmim-y)Br2(CN-py).
Fig. 8. TEM micrograph of Pd species isolated after Suzuki-Miyaura reaction catalyzed by Pd(bmim-y)Br2(CN-py) at 0.1 mol%.
Fig. 9. TEM micrograph of Pd species isolated after Suzuki-Miyaura reaction catalyzed by Pd(bmim-y)Br2(py) at 0.01 mol%.
Fig. 10. TEM micrograph of Pd species isolated after Suzuki-Miyaura reaction catalyzed by Pd(IPr)Br2(Cl-py).

14
M. Osin´ska et al. / Journal of Molecular Catalysis A: Chemical 418 (2016) 9–18
Table 7
100
90
80
70
60
50
40
30
20
10
0
Crystallographic data for Pd(bmim- y)Cl2(Cl-py).
Chemical formula
C13H18Cl3N3Pd
Formula Mass
Crystal system
a/Å
b/Å
429.05
Triclinic
7.753(3)
10.146(4)
11.700(4)
80.79(3)
87.56(3)
69.11(3)
848.7(6)
100(2)
P–1
c/Å
␣/◦
␤/◦
ꢀ/◦
Unit cell volume/Å3
Temperature/K
1
2
3
4
5
6
7
8
9
10
Space group
run
No. of formula units per unit cell, Z
Absorption coefficient, ꢁ/mm−1
No. of reflections measured
No. of independent reflections
No. of parameters
Rint
Final R1 values (I > 2ꢂ(I))
Final wR(F2) values (I > 2ꢂ(I))
Final R1 values (all data)
Final wR(F2) values (all data)
Goodness of fit on F2
2
1.559
10 988
3883
Fig. 11. Results of recycling of experiment of Suzuki-Miyaura reaction by Pd(bmim-
y)Br2(py).
183
0.0341
0.0316
0.0855
0.0358
0.0871
1.100
py and Me-py. A partial substitution of CN-py by Me-py-N was con-
firmed by 1H NMR. In contrast, TEA does not substitute coordinated
CN-py. The second tested Pd-complex, Pd(bmim-y)Br2(Me-py),
reacted with CN-py producing ca. 50% of Pd(bmim-y)Br2(CN-py).
The obtained results confirmed that N ligands in PEPPSI-type
complexes in solution underwent exchange, whereas the Pd-NHC
bond was not attacked by an excess of the N ligand.
activity of the studied system in ethylene glycol compared to other
solvents.
2.5. ESI-MS studies of Pd intermediates
2.6. XPS and TEM studies
The ESI-MS method was used for the analysis of the soluble
forms of palladium generated from PEPPSI-type precursors under
the Suzuki–Miyaura conditions. Two complexes were selected for
these experiments, Pd(bmim-y)Br2(CN-py) and Pd(bmim-y)Br2(Cl-
py), and in both cases only palladium fragments without the
pyridine ligand were detected. In fact, the ESI-MS patterns obtained
in these studies were very similar to those measured earlier for
PdBr2(NHC)2 complexes under the Suzuki–Miyaura conditions [31]
. Accordingly, it can be assumed that catalytically active interme-
diate palladium species do not contain N ligands.
Besides palladium species analogous to those also identified
in earlier studies, ESI-MS spectra of the reaction mixture showed
new fragments in which ethylene glycol and the product of its
dehydrogenation were connected to palladium. Interestingly, such
fragments were not observed during the studies of PdBr2(NHC)2
complexes in ethylene glycol (Fig. 6). The presence of an ethylene
glycol fragment in the coordination sphere of palladium suggested
its double role in the catalytic system, as a ligand completing the
coordination sphere of palladium and as a reducing agent facili-
tating the reduction of Pd(II) to Pd(0). This can explain the high
According to the well accepted mechanism of cross-coupling
reactions, Pd (0) is considered a catalytically active form [35,36] .
In our system, the formation of Pd(0) during the Suzuki–Miyaura
reaction was evidenced by the XPS spectrum of the reaction mix-
ture measured after 1 h of the reaction at 70 ◦C with Pd(bmim-y)
Br2(CN-py) as the catalyst precursor. The spectrum showed typical
signals at 334.96 and 340.22 eV assigned to Pd(0) (Fig. 7).
Interestingly, the reduction of palladium was only partial, and
61.5% was still present as Pd(II). A fraction of Pd(II) contained two
forms, with Pd O bonds (9.2%) and Pd-Br bonds (52.3%). The pres-
ence of PdO can be explained by the oxidation of Pd(0) under
atmospheric conditions, whereas Pd-Br represents the starting
2−
complex or anionic species, such as [PdBr4]
or [PdBr3(NHC)]−,
formed during the catalytic process with PhBr as a substrate. The
formation of a palladium anionic species from Pd(0) nanopar-
ticles was observed earlier for the Heck reaction performed in
molten [Bu4N]Br [37]. It is worth noting that the formation of
Pd(0) from a PEPPSI-type complex was less efficient than from a
pyridine- free dimer, [PdBr2(bmim-y)]2. In the latter case, 62% of
Fig. 12. TEM results after recycling experiment of Suzuki-Miyaura reaction by Pd(bmim-y)Br2(py).

M. Osin´ska et al. / Journal of Molecular Catalysis A: Chemical 418 (2016) 9–18
15
R
N
N
R
N
R
+
N
R
N
R
R
N
N
R
R
N
N
Cl
N
N
O
HO-CH2-CH
Cl
Cl
Pd
N
N
N
Cl
Cl
Pd
Cl
HO-CH2-CH2-OH
R
N
N
Pd
HO-CH2-CH2-OH
N
N
R
Pd
L
X
KX
B(OH)3
R
N
N
KOH
B(OH)2
L
Pd
X
R
Fig. 13. Proposed mechanism of the Suzuki- Miyaura reaction in ethylene glycol.
Pd(II) was reduced to Pd(0) under the Suzuki reaction conditions,
whereas only ca. 40% was reduced here [32].
in the Pd precursor has only small influence on the size of Pd(0) NPs
formed during catalytic reaction.
The TEM analysis of the same palladium sample confirmed that
the Pd(0) existed in the form of Pd(0) NPs with a diameter of ca.
4 nm (Fig. 8).
2.7. Recycling PEPPSI-type catalysts
In order to get deeper knowledge about transformations of
Pd precursor during Suzuki-Miyaura reaction, TEM analysis was
done for the sample obtained at 0.01 mol% loading (Fig. 9). Inter-
estingly, the Pd(0) NPs were bigger and their diameter increased
to 7–8 nm. Moreover, a large part of nanoparticles has been
agglomerated.
TEM studies were also performed for the Pd species isolated
after Suzuki- Miyaura reaction catalyzed by the complex contain-
ing bulky NHC ligand, Pd(IPr) Br2(Cl-py) (Fig. 10). Here Pd(0) NPs
were slightly bigger than for smaller NHC, (Pd(bmim-y)Br2(CN-py))
with dominating size 5 nm. Moreover, some agglomerates were
also observed. However, the observed differences are not very large.
It can be concluded that the steric hindrance of NHC ligand present
In order to estimate the stability of
a catalytic system
based on a PEPPSI-type palladium complex, several consecutive
Suzuki–Miyaura reactions were performed with one portion of Pd
(bmim-y)Br2(py) (Fig. 11). After each run, the reaction mixture was
centrifuged, and the black solid residue was used for the next reac-
tion. Interestingly, ten reactions were successfully performed with
a 53–87% conversion to the product. It should also be underlined
here that the yield of the product did not decrease over time and
the differences observed between the runs can be explained by an
experimental error rather than by the deactivation of the catalyst.
The TEM analysis of the palladium residue isolated after ten
catalytic runs showed the presence of non-agglomerated Pd(0)

16
M. Osin´ska et al. / Journal of Molecular Catalysis A: Chemical 418 (2016) 9–18
nanoparticles 3–4 nm in diameter (Fig. 12). The obtained TEM pic-
ture was quite similar to that obtained after one catalytic cycle.
Although the presence of Pd(0) NPs suggests their involve-
ment in the catalytic process, the Hg(0) test did not confirm that
[38,39]. Thus, in a reaction performed in the presence of Hg(0),
conversion to 2- methylbiphenyl was 84%, whereas without Hg(0) it
was 85%. In agreement with this result, the homogeneous reaction
pathway could be proposed for the studied reaction. Catalytically
active soluble palladium species were formed from Pd(0) NPs under
the reaction conditions (Fig. 13). An additional confirmation of
this statement is the result of Hg(0) test performed at very low
loading of Pd, namely 0.01 mol%. In this case conversion to 2-
methylbiphenyl decreased from 79.9% to 63.0% as a result of Hg(0)
presence. The observed small decrease of conversion indicated
again on the dominating role of soluble Pd species in the catalytic
process.
4.2. Syntheses of Pd(bmim-y)X2(N-ligand)
A mixture of 0.104 mmol of [PdX2(bmim-y)]2 and 0.208 mmol
of pyridine or amine in 1–3 mL of CH2Cl2 was stirred for 1 h
at room temperature. During that time, the color changed from
orange to yellow. The solution was evaporated and 5–10 mL of
cold pentane or hexane was added to the resulting yellow oil. The
mixture was left for 24 h, and a yellow precipitate was formed.
The product was filtrated, washed with pentane or hexane, and
dried.
Pd(bmim-y)Br2(CN-py) calcd (%) C14H18Br2N4Pd: C 33.06; H
3.57; N 11.02; found C 33.29; H 3.98; N 9.28;
1H NMR: 9.3 (d, 2H, CH), 7.5 (d, 2H, CH, JHH = 6.9), 6.9 (m, 2H,
NCH), 4.4 (br, 2H, NCH2), 4.0 (s, 3H, CH3), 2.0 (m, 2H, CH2, JHH = 7.7),
1.4, (m, 2H, CH2, JHH = 7.4), 1.0 (t 3H, CH3, JHH = 7.3)
13C NMR: 152.8 (CH), 125.2 (CH), 114.5 (C N), 122.3 (N CH)
120.6 (N CH), 50.3 (N CH3), 37.4 (N CH2), 31.0, (CH2), 18.9 (CH2)
Pd(bmim-y)Br2(Me-py-N) calcd (%) C14H22Br2N4Pd: C 32.80; H
4.33; N 10.93; found C 33.76; H 4.22; N 11.14;
3. Conclusions
1H NMR: 8.2 (d, 1H, CH), 6.9 (m 2H, NCH), 6.5 (m, 1H, CH), 6.3
(m, 1H, CH), 5.4 (s, 2H, NH2), 4.5 (t, 2H, N CH2), 4.1 (s, 3H, N CH3),
2.3 (m, 3H, CH3), 2.1 (m, 2H, CH2), 1.50 (m, 2H, CH2), 1.05 (t, 3H,
CH3)
PEPPSI-type complexes with small bmim-y ligands efficiently
catalyze the Suzuki–Miyaura and the Hiyama reactions of substi-
tuted bromobenzenes and chlorobenzenes. The catalyst precursors
undergo structural modification under the reaction conditions, the
N ligand and being dissociated in the first step. Next, palladium, is
reduced to Pd(0) nanoparticles by ethylene glycol, which also acts
as the stabilizing agent preventing the agglomeration of nanopar-
ticles. The reduction of palladium was not complete, and some
amount of Pd(II) was present in the isolated Pd(0)–Pd(II) com-
posite. Interestingly, the Pd(0)–Pd(II) composite exhibited a high
catalytic activity in ten subsequent runs, what is not obvious for
unimmobilized palladium catalysts. Considering the mechanism of
the studied reactions, it should be assumed that Pd(0) NPs acted as
a reservoir of soluble Pd(II) responsible for the catalytic process.
As far as we know attempts to recycle PEPPSI-type catalysts
have not been reported earlier. This is probably because of their
homogeneous nature [34]. We have shown that PEPPSI-type com-
plexes with non- bulky NHC ligands formed Pd(0)-Pd(II) composite
which efficiently catalyze several subsequent runs. This is due to
the very well controlled leaching of soluble Pd species which are
catalytically active. The controlling role is played by NHC and N-
ligands which prevent agglomeration of Pd(0) NPs and recoordinate
to Pd(0) active complex. The high efficiency of the studied catalysts
is additionally proved in reactions employing concentration of Pd
as low as 0.01 mol%.
13C NMR: 158.0 (N C-NH2), 150.0 (C CH3) 148.6 (N CH),
147.55 (C N), 122.8 (N CH), 121.6 (N CH), 115.9(CH), 111.3
(C CH3), 50.9 (N CH3), 38.4 (N CH2), 32.2 (CH2), 20.9 (CH3) 19.9
(CH2), 13.7 (CH3)
Pd(bmim-y)Br2(py) calcd (%) C13H19Br2N3Pd: C 32.29; H 3.96;
N 8.69; found C 32.70; H 3.84; N 7.78;
1H NMR: 9.0 (d, 2H, CH), 7.7 (t, 2H, CH, JHH = 7.7), 7.3 (t, H, CH,
JHH = 7.4), 6.8 (m, 2H, NCH), 4.4 (2H, NCH2, JHH = 7.5), 4.0 (s, 3H, CH3),
2.0(m, 2H, CH2, JHH = 7.5), 1.4, (m, 2H, CH2, JHH = 7.5), 1.0 (t 3H, CH3,
JHH = 7.5)
13C NMR: 151.7 (N CH), 136.8 (CH), 123.7 (CH), 122.2 (N CH),
120.7 (N CH), 50.0 (N CH3), 37.4 (N CH2), 31.1 (CH2 CH2), 19.0
(CH2), 12.8 (CH3)
Pd(bmim)Br2(Me-py) calcd (%) C14H21Br2N3Pd: C 33.79; H
4.25; N 8.45; found C 34.19; H 3.76; N 7.81;
1H NMR: 8.8 (d, 2H, CH), 7.1 (d, 2H, CH), 6.8 (m, 4H, NCH), 4.4
(2H, NCH2, JHH = 7.5), 4.0 (s, 3H, CH3), 2.4(m, 2H, CH2, JHH = 7.5),
2.1 (s 3H, CH3, JHH = 7.5) 1.4, (m, 2H, CH2, JHH = 7.5), 1.0 (t 3H, CH3,
JHH = 7.5)
13C NMR: 151.0 (N CH), 148.88 (C CH3), 124.6 (CH), 122.1
(N CH), 120.6 (N CH), 50.1 (N CH3), 37.5 (N CH2), 31.2
(CH2 CH2), 20.1 (CH3), 19.1 (CH2), 12.8 (CH3)
The presented catalysts have in our opinion advantages over the
recently reported PEPPSI-type complexes which formed Pd(0) NPs
operating according to the heterogeneous pathway [22].
Pd(bmim)Br2(TEA) calcd (%) C14H29Br2N3Pd: C 33.26; H 5.78;
N 8.31; found C 33.4; H 5.99; N 8.02;
1H NMR: 6.7 (m, 4H, NCH), 4.3 (2H, NCH2, JHH = 7.5), 3.9 (s, 3H,
CH3) 2.9 (m, 6H, CH2, JHH = 7.1), 1.9 (m, 2H, CH2, JHH = 7.5), 1.4, (m,
2H, CH2, JHH = 7.55), 1.2 (t 3H, CH3, JHH = 7.5) 0.9 (t, 9H, CH3)
13C NMR: 144.6 (N2CH), 122.9 (N CH), 121.6 (N CH), 50.9
(N CH3), 48.3 (N CH2), 37.0 (N CH2), 32.0 (CH2), 20.0 (CH2), 13.7
(CH3), 10.2 (CH3)
4. Experimental
4.1. Measurements
1H and 13C NMR spectra were measured in CDCl3 on a Bruker
500 spectrometer.
Electrospray ionization mass spectra (ESI–MS) were recorded
on a Bruker Apex Ultra FT-ICR. The products of the catalytic exper-
iments were analyzed with a GC–MS (Hewlett Packard 8452A
instrument) using dodecane as an internal standard.
Pd(bmim-y)Br2(TPA) calcd (%) C17H35Br2N3Pd: C 37.28; H 6.44;
N 7.67; found C 36.84; H 6.07; N 7.57;
1H NMR: 6.8 (m, 2H, NCH), 4.3 (2H, NCH2, JHH = 7.5), 3.9 (s, 3H,
CH3), 2.8 (m, 6H, CH2),
2.0 (m, 2H, CH2, JHH = 7.5), 1.5 (m, 6H, CH2), 1.4 (m 2H, CH2), 1.0
(t 3H, CH3, JHH = 7.5). 0.9 (m, 9H, CH3, JHH = 7.7)
TEM measurements were performed using a FEI Tecnai G2 20 X-
TWIN electron microscope with a LaB6 cathode providing 0.25 nm
resolution. To the small sample of the catalyst, 2 mL of methanol
were added, and the resulting mixture was ultrasonically treated
for 5 min. Specimens for TEM studies were prepared by putting
a droplet of a colloidal suspension on a copper microscope grid
followed by evaporating the solvent under an IR lamp for 15 min.
13C NMR: 144.7 (N2CH) 122.9 (N CH), 121.6 (N CH), 57.3
(N CH2), 51.1 (N CH3), 37.8 (N CH2), 31.9 (CH2), 20.0 (CH2 CH2),
19.9 (CH2), 13.7 (CH3), 11.6 (CH3)
Pd(bmim-y)Br2(Cl-py) calcd (%) C13H18Br2ClN3Pd: C 30.14; H
3.50; N 8.11; found C 31.0; H 4.0; N 7.90;
1H NMR: 9.0 (d, 1H, CH), 8.9 (d, 1H, CH), 7.7 (d, 1H, CH), 7.2 (m,
1H, CH), 6.8 (m, 2H, NCH), 4.4 (2H, NCH2, JHH = 7.5), 4.0 (s, 3H, CH3)

M. Osin´ska et al. / Journal of Molecular Catalysis A: Chemical 418 (2016) 9–18
17
1.9 (m, 2H, CH2, JHH = 7.5), 1.4, (m, 2H, CH2, JHH = 7.5), 1.0 (t 3H, CH3,
JHH = 7.5)
tube. Next, 5 mL of the solvent (ethylene glycol), 2-bromotoluene
(1 mmol, 0.118 mL) and the palladium complex (1 × 10−5 mol) were
added. The Schlenk tube was closed with a rubber stopper, and
the reaction mixture was stirred at 110 ◦C. After 1 h, the reac-
tor was cooled down and the organic products were extracted
with 3 × 3 + 4 mL of n-hexane (5 min with intensive stirring). The
extracts (10 mL) were GC-FID analyzed (Hewlett Packard 5890)
with 0.076 mL of dodecane as an internal standard. The products
were identified by GC–MS (Hewlett Packard 5971A).
13C NMR: 150.9 (N CH), 149.6 (N CH), 136.9 (CH), 131.8 (CH),
124.1 (C Cl), 122.1 (N CH), 120.8 (N CH), 50.1 (N CH3), 37.6
(N CH2), 31.1 (CH2), 19.1 (CH2), 12.8 (CH3)
Pd(bmim-y)Cl2(Cl-py) calcd (%) C13H18Cl3N3Pd: C 36.39; H
4.23; N 9.79; found C 36.55; H 4.38; N 9.57;
1H NMR: 9.0 (d, 1H, CH), 8.9 (d, 1H, CH), 7.7 (d, 1H, CH), 7.2 (m,
1H, CH), 6.8 (m, 2H, NCH), 4.4 (2H, NCH2, JHH = 7.5), 4.1 (s, 3H, CH3)
1.9 (m, 2H, CH2, JHH = 7.5), 1.4, (m, 2H, CH2, JHH = 7.5), 1.0 (t 3H, CH3,
JHH = 7.5)
4.6. Isolation of 2-methylbiphenyl
13C NMR: 149.3 (N CH), 147.3 (N CH), 138.3 (CH), 132.8 (CH),
124.9 (C Cl), 123.2 (N CH), 121.7 (N CH), 50.1 (N CH3), 37.6
(N CH2), 32.8 (CH2), 19.9 (CH2), 13.3 (CH3)
To the post-reaction mixture 5 mL of 5% HCl were added to
remove inorganic side products. Organic products were extracted
with 3 × 5 mL of n-hexane. The extracts were filtered by short pad of
silica. Organic phases were combined, washed with water solution
of NaHCO3 and dried with Na2SO4. Next, solvent was evaporated
and crude products were separated on Silica 60 using eluent hex-
ane/ethyl acetate 5/1. Fractions of products were collected and
solvent was evaporated.
Pd(bmim-y)Cl2(TEA) calcd (%) C14H29Cl2N3Pd: C, 40.35; H,
7.01; N, 10.08; found C, 40.08; H, 6.44; N, 9.89;
1H NMR: 6.8 (m, 2H, NCH), 4.3 (2H, NCH2, JHH = 7.5), 3.9 (s, 3H,
CH3), 2.8 (m 6H, CH2), 2.0 (m, 2H, CH2, JHH = 7.5), 1.4 (m 2H, CH2),
1.0 (t 3H, CH3, JHH = 7.5) 0.9 (m, 9H, CH3, JHH = 7.7)
13C NMR: 144.7 (N2CH) 122.9 (N CH), 121.6 (N CH), 51.1
(N CH3), 37.8 (N CH2), 31.9 (CH2), 20.0 (CH2 CH2), 10.9 (CH2),
13.7 (CH3)
Acknowledgement
Pd(SIMes)Cl2(Cl-py) and Pd(IPr) Cl2(Cl-py) have been obtained
according to the literature method [40].
Financial support of National Science Foundation (NCN, Poland)
with grant 2012/05/B/ST5/00265 is gratefully acknowledged.
4.3. X-ray
Single crystals of Pd(bmim-y)Cl2(Cl-py) suitable for X-ray mea-
surements were mounted on a glass fiber in silicone grease,
cooled to 100 K in a nitrogen gas stream, and the diffraction data
were collected on a Kuma KM-4CCD diffractometer with graphite
monochromated Mo-K␣ radiation (␭ = 0.71073 Å). The structure
was subsequently solved using direct methods and developed by
full least-squares refinement on F2. Structural solution and refine-
ment was carried out using SHELX suite of programs [41]. Analytical
absorption corrections were performed with CrysAlis RED [42]. C, N,
Cl, and Pd atoms were refined anisotropically. The carbon-bonded
H atoms were positioned geometrically and refined isotropically
using a riding model with a common fixed isotropic thermal param-
eter. The molecular structure plots were prepared using ORTEP-3
program [43]. Crystal data and selected details of structure deter-
mination are summarized in Table 7.
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