Recyclable Pd(0)-Pd(II) composites formed from Pd(II) dimers with NHC ligands under Suzuki-Miyaura conditions

Article abstract of DOI:10.1016/j.jorganchem.2015.03.009

Dimeric complexes of the type [Pd(μ-X)X(NHC)]₂ were employed in the Suzuki-Miyaura cross-coupling leading to 2-methylbiphenyl. The excellent activity of the studied dimers is perfectly illustrated by a TOF of up to 10⁶ h^-1. Mechanistic investigations with the application of TEM, XPS, and mercury poisoning tests provided an insight into the nature of the catalytic process. Accordingly, the reduction of the dimeric palladium complex [Pd(μ-X)X(NHC)]₂ resulted in the formation of a composite containing Pd(0) nanoparticles protected by a layer of a Pd(II) species such as anions [PdBr₄]^2- or [PdBr₃(NHC)]^-. The in situ formation of Pd(0)-Pd(II) composites resulted in the high stability of the catalytic system, which was active in ten subsequent recycles without any additional protection.

Full text of DOI:10.1016/j.jorganchem.2015.03.009

Journal of Organometallic Chemistry 785 (2015) 92e99
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Recyclable Pd(0)-Pd(II) composites formed from Pd(II) dimers with
NHC ligands under SuzukieMiyaura conditions
a
a
a
b
ꢀ
Marta Gorna , Michał S. Szulmanowicz , Andrzej Gniewek , Włodzimierz Tylus ,
Anna M. Trzeciak a
a Faculty of Chemistry, University of Wrocław, 14 F. Joliot-Curie St., 50-383 Wrocław, Poland
,
*
b
_
ꢀ
Wrocław University of Technology, Institute of Inorganic Technology, 27 Wybrzeze Wyspianskiego, 50-370 Wrocław, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Dimeric complexes of the type [Pd(m-X)X(NHC)]2 were employed in the SuzukieMiyaura cross-coupling
Received 15 November 2014
Received in revised form
14 February 2015
Accepted 9 March 2015
Available online 17 March 2015
leading to 2-methylbiphenyl. The excellent activity of the studied dimers is perfectly illustrated by a TOF
of up to 106 ꢀ1. Mechanistic investigations with the application of TEM, XPS, and mercury poisoning
tests provided an insight into the nature of the catalytic process. Accordingly, the reduction of the
h
dimeric palladium complex [Pd(m-X)X(NHC)]2 resulted in the formation of a composite containing Pd(0)
nanoparticles protected by a layer of a Pd(II) species such as anions [PdBr4]2ꢀ or [PdBr3(NHC)]ꢀ. The in
situ formation of Pd(0)ePd(II) composites resulted in the high stability of the catalytic system, which was
active in ten subsequent recycles without any additional protection.
Keywords:
Nanoparticles
N-heterocyclic carbene
Palladium
© 2015 Elsevier B.V. All rights reserved.
SuzukieMiyaura reaction
Introduction
metal centers [6,7]. Such an effect can facilitate the oxidative
addition of an aryl halide or the transmetalation, important steps in
Palladium complexes bearing N-heterocyclic carbene ligands
(NHC) exhibit an exceptionally high catalytic activity in cross-
coupling reactions that are of practical importance in the synthe-
sis of pharmaceuticals, fine chemicals, or natural products [1]. In
this context, the SuzukieMiyaura cross-coupling should be
mentioned as an efficient method designed for the preparation of
non-symmetric biaryls [2,3].
Actually, monomeric palladium complexes have found applica-
tions in a broad range of CeC cross-coupling reactions, while
palladium dimers have received relatively less attention [4]. The
application of dimeric complexes as catalyst precursors offers some
additional advantages in comparison to monomeric analogs. The
dimeric structure can be split under catalytic reaction conditions
with the formation of coordinatively unsaturated intermediates
suitable for the efficient activation of the organic substrates. Simi-
larly, the high activity of PEPPSI-type complexes was explained by
the creation of a free coordination place after the dissociation of the
pyridine ligand [5].
the catalytic reaction.
The dimeric palladium complexes with bulky NHC ligands were
successfully applied in the SuzukieMiyaura [8], Heck, and Buch-
waldeHartwig [9] reactions, as well as in the cross-coupling of
Grignard reagents [10]. Palladium(II) hydroxide dimers were tested
as the catalysts in aryl amination and the SuzukieMiyaura re-
actions [11]. A dimeric acetate-bridged complex with a 1,2,3-
triazol-5-ylidene ligand catalyzed the hydroarylation of alkynes
with moderate efficiency [12]. Palladium dimers with phospho-
nium cations catalyzed the Heck reaction [13], whereas their analog
with the Bu4Nþ cation found application in the oxidation of alco-
hols [14]. The dimeric palladium complexes with small NHC ligands
were till now not tested as catalysts of CeC cross-coupling.
In this paper, we present studies on the dimeric PdeNHC
complexes of the type [Pd(
m-X)X(NHC)]2 (where NHC ¼ N-hetero-
cyclic carbene) in the SuzukieMiyaura reaction. The studies
focused on the mechanistic aspects and identification of the
palladium species formed under the catalytic reaction conditions.
Understanding of such transformations is essential for the design of
catalytic systems operating for a long time without deactivation.
Our earlier investigations showed that monomeric palladium
complexes with small NHC ligands, of the type [PdX2(NHC)2], are
very active in the SuzukieMiyaura cross-coupling [15]. It was also
evidenced that soluble Pd(II) complexes and Pd(0) nanoparticles,
For dimeric precursors, a bimetallic catalytic mechanism can
also be considered, which makes possible a synergetic effect of both
* Corresponding author. Tel.: þ48 71 3757253.
E-mail address: anna.trzeciak@chem.uni.wroc.pl (A.M. Trzeciak).
http://dx.doi.org/10.1016/j.jorganchem.2015.03.009
0022-328X/© 2015 Elsevier B.V. All rights reserved.

ꢀ
M. Gorna et al. / Journal of Organometallic Chemistry 785 (2015) 92e99
93
formed from these PdeNHC precursors, participated in the catalytic
process [15]. In this paper we studied transformations of PdeNHC
dimers containing NHC ligands with different steric properties.
Moreover, we investigated stability of the catalytic systems based
on PdeNHC dimers in several subsequent SuzukieMiyaura runs. In
the previous reports [8] very high catalytic activity of PdeNHC
dimers was demonstrated in a single reaction however possibility
of catalyst recycling was not discussed.
was dissolved in CH2Cl2 (5 mL) and water (3e10 mL) was added to
remove unreacted SIMes.HCl. Organic phase was separated and
hexane (5 mL) was slowly added. After 24 h the product was filtered
off and dried in vacuum. Yield: 50%.
Crystals suitable for X-ray analysis were obtained by slow
evaporation of the CH2Cl2/hexane solution.
1H NMR (500 MHz; CDCl3;TMS):
d
¼ 6.92 (s, 8H, ArH), 6.77 (s,
8H, NCH2eNCH2), 2.47 (s, 12H, p-ArCH3), 1.94 (s, 24H, oArCH3)
13C NMR (125 MHz; CDCl3; TMS):
d
¼ 170.9 (s, N2C), 137.5 (s, NC),
Experimental
136.1 (s, o-CCH3), 135.7 (s, p-CCH3), 128.8 (s, CH), 122.4 (s, CH2),
21.25 (s, o-CH3), 18.9 (s, p-CH3) elemental analysis calcd (%)
Synthesis of [Pd(
m
-X)X(NHC)]2 complexes
-Br)Br(bmim-y)]2
C42H52Cl4N4Pd2: C 52.14, H 5.42, N 5.79; found: C 51.15, H 5.09, N
5.30.
Synthesis of [Pd(
m
Pd(OAc)2 (0.49 g, 0.22 mmol) and [bmim]Br (0.106 g, 0.48 mmol)
were placed in a Schlenk tube. The resulting mixture was heated to
55 ꢁC along with magnetic stirring for 1.5 h. The color changed from
red to orange and homogeneous phase was observed. After cooling
down the mixture to RT, CHCl3 was added and the orange solution
was purified on silica-gel column. The obtained solution was
evaporated and the resulting oily residue was dissolved in CH2Cl2.
Next, hexane (10 mL) was added and the two-phase mixture was
left at RT. After 24 h the precipitated product was filtered off and
dried in vacuo. Yield: 45%.
SuzukieMiyaura reaction procedure
SuzukieMiyaura reactions were carried out in a 50 mL Schlenk
tube in an air atmosphere. The solid substrates: NaHCO3 (1.7 mmol)
or KOH (1.2 mmol) and phenylboronic acid (1.15 mmol, 0.133 g) or
NaBPh4 (0.30 mmol, 0.095 g) were weighed and placed directly in
the Schlenk tube. Next, 5 mL of the solvent (ethylene glycol or 2-
propanol or 2-propanol/water 1/1) and 2-bromotoluene (1 mmol,
0.118 mL) were added. After heating the substrates to the required
temperature (40 or 100 ꢁC), the palladium complex (8 ꢂ 10ꢀ3 to
1 ꢂ 10ꢀ6 mmol) was added. The Schlenk tube was closed with a
rubber stopper, and the reaction mixture was stirred at 40 or 110 ꢁC.
After 1 h, the reactor was quickly cooled down to room temperature
and the organic products were extracted with 3 ꢂ 3 þ 1 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 dodec-
ane as an internal standard. The products were identified by
GCeMS (Hewlett Packard 5971A).
Crystals suitable for X-ray analysis were obtained by slow
evaporation of the CH2Cl2/hexane solution.
1H NMR (500 MHz; CDCl3; TMS):
d
¼ 6.89 (m, 4H, CH), 4.52 (br,
4H, NCH2), 4.13 (s, 6H, CH3) 2.04 (m, 4H, CH2), 1.48 (m, 4H, CH2) 1.04
(t, 6H, 3J(H,H) ¼ 7.32 Hz, 6H; CH3);
13C NMR (100 MHz, CDCl3, TMS):
d
¼ 144.6 (N2CH), 123.4 (NCH),
122.07 (NCH), 51.1 (NCH3), 38.5 (NCH2), 32.3 (CH2), 19.8 (CH2), 13.7
(CH3) elemental analysis calcd (%) for C16H28Br4N4Pd2: C 23.76, H
3.49, N 6.93; found: C 24.05, H 3.30, N 6.80.
Reduction of [Pd(m-Br)Br(bmim-y)]2
Synthesis of [Pd(
The method was the same as for [Pd(
52%
1H NMR (500 MHz; CDCl3; TMS):
m-Cl)Cl(bmim-y)]2
m
-Br)Br(bmim-y)]2. Yield:
0.083 g (0.10 mmol) [Pd(m
-Br)Br(bmim-y)]2 and 0.068 g KOH
(1.2 mmol) and 5 mL 2-propanol were placed in a Schlenk tube. The
resulting mixture was heated to 40 ꢁC along with magnetic stirring
for 1 h. The color changed from orange to silverblack. The mixture
was centrifuged and decanted. Precipitated product was washed
two times with 5 mL CH2Cl2, centrifuged and dried under nitrogen
stream. elemental analysis found (%): C 6.78, H 0.91, N 0.35;
d
¼ 6.8 (m, 4H, CH), 4.5 (br, 4H,
NCH2), 4.1 (s, 6H, CH3), 2.0 (m, 4H, CH2), 1.4 (m, 4H, CH2), 1.0 (t, 6H,
CH3, 3J(H,H) ¼ 7.3 Hz, 6H, CH3)
13C NMR (100 MHz, CDCl3, TMS):
d
¼ 144.5 (N2CH), 122.6 (NCH),
121.1 (NCH), 50.1 (NCH3), 37.4 (NCH2), 31.4 (CH2), 19.1 (CH2), 12.7
(CH3) elemental analysis calcd (%) for C16H28Cl4N4Pd2: C 30.45, H
4.47, N 8.88, found; C 30.62, H 4.47, N 9.15.
Transmission electron microscopy
Synthesis of [Pd(
The method was the same as for [Pd(
42%
m
-Br)Br(emim-y)]2
TEM measurements were carried out using a FEI Tecnai G2 20 X-
TWIN electron microscope operating at 200 kV. To the sample
(1e2 mg) of reduced palladium complex 2 mL of methanol were
added and the resulted mixture was sonicated for 5 min. Specimens
for TEM studies were prepared by putting a droplet of a colloidal
suspension on a copper grid followed by evaporation the solvent
under IR lamp for 15 min. The nanoparticle size distributions were
determined by counting the size of approximately 250 palladium
nanoparticles from several TEM images obtained from different
places of the TEM grids. The size distribution plots were fitted using
Gauss curve approximation.
m
-Br)Br(bmim-y)]2. Yield:
Crystals suitable for X-ray analysis were obtained by slow
evaporation of the CH2Cl2/hexane solution.
1H NMR (500 MHz; CDCl3; TMS):
d
¼ 1.64 (t, 6H, CH3,
JHH ¼ 7.3 Hz), 4.15 (s, 6H, NCH3), 4.64 (q, 4H, CH2, JHH ¼ 7.3 Hz), 7.01
(d, 2H, NCH, J(H,H) ¼ 1.8 Hz), 7.00 (d, 2H, NCH, J(H,H) ¼ 1.8 Hz)
13C NMR (125 MHz; CDCl3; TMS):
CH), 121.95 (s, CH), 46.68 (s, NCH3), 38.71 (CH2), 16.08 (s, CH3)
elemental analysis calcd. (%) for C12H20Br4N4Pd2: C 19.15, H 2.68, N
7.44; found: C 20.11, H 2.87, N 7.06;
d
¼ 143.9 (s, N2C), 124.25 (s.
Crystallographic data collection and refinement
Synthesis of [Pd(m-Cl)Cl(SIMes)]2
Pd(OAc)2 (0.110 g, 0.50 mmol) and SIMes.HCl (0.170 g,
0.50 mmol) were dissolved in THF (15 mL). The resulting mixture
was heated to 55 ꢁC along with magnetic stirring for 2 h. After
cooling down the mixture to RT, it was purified on silica-gel col-
umn. The resulting orange solution was transferred to the Schlenk
tube and the solvent was removed under vacuum. The oily residue
Single crystals suitable for X-ray measurements were mounted
on glass fibers in silicone grease, cooled to 100 K in a nitrogen gas
stream [16], and the diffraction data were collected on a Kuma KM-
4 CCD diffractometer with graphite monochromated Mo-K
ation (
¼ 0.71073 Å). The structures were subsequently solved
using direct methods and developed by full leasts-quares
a radi-
l

ꢀ
M. Gorna et al. / Journal of Organometallic Chemistry 785 (2015) 92e99
94
Table 1
Crystallographic data.
Complex
[Pd(m
-Br)Br(emim-y)]2
[Pd(m
-Br)Br(bmim-y)]2
[Pd(m-Br)Br(SIMes)]2
Formula
Mr
[Pd2Br4(C6H10N2)2]$CH2Cl2
837.69
[Pd2Br4(C8H14N2)2]
808.86
[Pd2Cl4(C21H26N2)2]$CHCl3
1086.84
Crystal system
Space group
a [Å]
Monoclinic
I2/a
Monoclinic
P21/n
Monoclinic
P21/c
14.880 (4)
7.759 (3)
20.983 (5)
105.00 (3)
2340.0 (12)
4
7.525 (3)
14.085 (6)
11.344 (4)
108.51 (3)
1140.1 (8)
2
14.290 (3)
11.887 (2)
30.984 (9)
116.37 (3)
4716 (2)
b [Å]
c [Å]
b
[ꢁ]
V [Å3]
Z
4
Dx [g/cm3]
Radiation
Reflections
Parameters
Rint
2.378
Mo-K
12,458
146
0.025
0.025
0.055
3.0e25.0
8.60
2.356
1.531
Mo-Ka
65,064
517
0.107
0.089
0.113
3.0e27.5
1.19
a
Mo-K
9244
120
0.047
0.051
0.134
a
R1 (I > 2
s)
wR2 (I > 2
s
)
q
m
[ꢁ]
3.0e27.5
8.60
[mmꢀ1
]
T [K]
Color, shape
Size [mm]
100 (2)
Plate, yellow
0.15 ꢂ 0.15 ꢂ 0.08
100 (2)
Plate, yellow
0.10 ꢂ 0.10 ꢂ 0.05
100 (2)
Needle, yellow
0.20 ꢂ 0.10 ꢂ 0.10
R2
N
X
X-
X
R1
X
+
+ Pd(OAc)2
N
N
N
Pd
Pd
no solvent
R2
N
R1
R1
X
N
R2
Fig. 1. Synthesis of [Pd(m-X)X(NHC)]2 complexes.
refinement on F2. Structural solution and refinement was carried
out using SHELX suite of programs [17]. Analytical absorption cor-
rections were performed with CrysAlis RED [18]. C, N, Cl, Br and Pd
atoms were refined anisotropically. All H atoms were positioned
geometrically and refined isotropically using a riding model with a
common fixed isotropic thermal parameter. The molecular struc-
ture plots were prepared using ORTEP-3 program [19]. Crystal data
and selected details of structure determination are summarized in
Table 1.
solution. Four dimeric complexes were prepared; their composition
was confirmed by elemental analyses, and a further structural
characterization was based on spectroscopic and X-ray results.
In the solid state, three complexes were characterized by the X-
ray single crystal analysis: [Pd(
ethyl-3-methylimidazol-2-ylidene), [Pd(
y ¼ 1-butyl-3-methylimidazol-2-ylidene), and [Pd(
(SIMes 1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-
ylidene). In all these complexes, the PdeC bond length is in the
m
-Br)Br(emim-y)]2 (emim-y ¼ 1-
-Br)Br(bmim-y)]2 (bmim-
-Cl)Cl(SIMes)]2
m
m
¼
Results and discussion
Synthesis and structure of palladium dimers [Pd(
m-X)X(NHC)]2
The dimeric complexes of the type [Pd( -X)X(NHC)]2 were ob-
m
tained in reaction of Pd(OAc)2 with ionic liquids (NHC·HX) under
conditions similar to those reported in the literature [20] (Fig. 1). In
a modified procedure, the first step of the synthesis was performed
without a solvent, by mixing substrates, followed by the treatment
of the resulting mixture with THF. Such a protocol enabled the in-
crease of the yield of the product compared to that obtained in
Table 2
Characteristic bond lengths (Å) observed in [Pd(m-X)X(NHC)]2 complexes.
Complex
PdeC
Pd-(m
-X)
Pd-X
[Pd(
[Pd(
[Pd(
m
m
m
-Br)Br(emim-y)]2
-Br)Br(bmim-y)]2
-Cl)Cl(SIMes)]2
1.952 (4)
1.924 (7)
1.947 (5)
1.944 (5)
2.5255 (8)
2.4988 (8)
2.332 (2)
2.397 (2)
2.402 (2)
2.328 (2)
2.4189 (8)
2.4056 (8)
2.294 (2)
2.302 (2)
Fig. 2. The molecular structure and atom numbering scheme of [Pd(m-Br)Br(emim-
y)]2. Displacement ellipsoids are drawn at the 30% probability level and H atoms are
shown as small spheres of arbitrary radii. Disordered parts with lower occupancy (45%)
are represented by dashed lines. Unlabeled atoms are generated by symmetry oper-
ations [symmetry code: (i) 1 ꢀ x, ꢀy, 1 ꢀ z]. For clarity, solvent molecules have been
omitted.

ꢀ
M. Gorna et al. / Journal of Organometallic Chemistry 785 (2015) 92e99
95
The crystal structure of [Pd(m-Br)Br(emim-y)]2 (Fig. 2) shows a
substitutional disorder of the ethyl and methyl groups with 55/45%
occupancy factors. The coordination geometry of the palladium
atoms is, as expected, square-planar. However, there are observed
small distortions of the angles between the coordinated ligands of
about 3ꢁ due to the influence of the bridging
lecular structure of [Pd(
the structure of [Pd( -Br)Br(emim-y)]2.
The X-ray structure of [Pd( -Cl)Cl(SIMes)]2 (Fig. 4) shows more
m-Br atoms. The mo-
m
-Br)Br(bmim-y)]2 (Fig. 3) is very similar to
m
m
significant distortions of the coordination geometry around the Pd
atoms. It is especially important to notice the dihedral angle of
28.5ꢁ between the planes formed by Pd1/C11/Cl1/Cl3/Cl4 and Pd2/
C21/Cl2/Cl3/Cl4. In [Pd(m-Br)Br(emim-y)]2 and [Pd(m-Br)Br(bmim-
y)]2, all the respective atoms were located in a planar arrangement.
Catalytic activity of palladium dimers in the SuzukieMiyaura
reaction
Fig. 3. The molecular structure and atom numbering scheme of [Pd(m-Br)Br(bmim-
y)]2. Displacement ellipsoids are drawn at the 30% probability level and H atoms are
shown as small spheres of arbitrary radii. Unlabeled atoms are symmetrically depen-
dent via inversion center [symmetry code: (i) 1 ꢀ x, 1 ꢀ y, 1 ꢀ z].
The structurally characterized PdeNHC dimers were employed
in the SuzukieMiyaura reaction leading to 2-methylbiphenyl
(Fig. 5).
Preliminary tests showed that the studied dimers offered a very
ꢁ
good yield of the SuzukieMiyaura product already at 40 C in 2-
propanol or a 2-propanol/water mixture (Table 3).
Under these conditions monomeric complexes [PdX2(NHC)2]
were not active [15]. When NaHCO3 was used instead of KOH they
formed 40e63% of 2-methylbiphenyl [15].
Therefore, in order to compare the efficiency of the PdeNHC
dimeric complexes with that of the [PdX2(NHC)2] precursors [15],
ethylene glycol was used as a solvent in further experiments. The
results of the optimization of the reaction conditions with respect
to the palladium amount are presented in Table 4.
The studied complexes produced 80e100% of 2-methylbiphenyl
when used in as small amounts as 10ꢀ7 mol. Even higher yield,
close to 100%, was noted in reactions with NaBPh4 as a substrate.
The excellent activity of the studied dimers is perfectly illustrated
by TON values of up to 106. As the reaction time was fixed as 1 h, the
TOF values are of the same order as when expressed in hꢀ1. Inter-
estingly, very good results were obtained for the precursor con-
taining the bulky SIMes ligand as well as for analogs bearing the
less hindered emim-y or bmim-y ligands. In fact, the catalytic re-
sults obtained with application of PdeNHC dimers were compa-
rable or slightly better than those achieved for PdeNHC monomeric
precursors [15]. This is in particular seen in reactions performed
with the lowest concentration of palladium (0.0001 mol%). The
yield obtained with monomeric PdeNHC complexes was 2e76%,
and 86% was obtained with [Pd(IMes)2Cl2]. The studied dimers
provided 81e90% under the same conditions. Importantly, the
presented systems perfectly operated in air atmosphere without
any catalyst pretreatment.
Fig. 4. The molecular structure and atom numbering scheme of [Pd(m-Cl)Cl(SIMes)]2.
Displacement ellipsoids are drawn at the 30% probability level and H atoms are shown
as small spheres of arbitrary radii.
range of 1.92e1.95 Å (Table 2) and is not influenced by the type of
substituent in the imidazolium ring. This length is characteristic for
the dimeric PdeNHC complexes [21e23]. In contrast, the PdeC bond
length in the trans isomers of the monomeric PdeNHC complexes is
slightly longer and is around 2.02 Å [15].
Recycling experiments
To prove the stability of our system, we performed several
consecutive reactions using one portion of [Pd(m-Br)Br(bmim-y)]2.
After each reaction, the mixture was centrifuged and the black
H3C
CH3
CH3
-
Br
B(OH)2
[PdX2(NHC)]2
base
Br
BPh3
[PdX2(NHC)]2
base
+
+
Fig. 5. Catalytic reactions under study.

ꢀ
96
M. Gorna et al. / Journal of Organometallic Chemistry 785 (2015) 92e99
Table 3
Table 5
Yield of 2-methylbiphenyl obtained in the SuzukieMiyaura cross-coupling with
[Pd( -X)X(NHC)]2 catalysts.
Size of Pd(0) nanoparticles formed in ethylene glycol and in the SuzukieMiyaura
reaction.
m
Catalyst
Yield [%]
Complex
Average diameter of Pd(0) nanoparticles [nm]
2-Propanol
2-Propanol/water
Ethylene glycol
SuzukieMiyaura
reaction
[Pd(
[Pd(
m
m
-Br)Br(bmim-y)]2
-Cl)Cl(bmim-y)]2
82
69
84
88
[Pd(
[Pd(
[Pd(
m
m
m
-Br)Br(bmim-y)]2
-Br)Br(emim-y)]2
-Cl)Cl(SIMes)]2
5.7
5.5
4.4
2.9
3.0
4.4
Reaction conditions: [Pd] 1% mol; 2-Br-toluene 1 mmol, PhB(OH)2 1.1 mmol, KOH
1.2 mmol, 1 h, 40 ꢁC.
2-propanol, KOH
Table 4
[PdX2(NHC)]2
[Pd]red
40oC
Yield of 2-methylbiphenyl obtained in the SuzukieMiyaura cross-coupling with
different amounts of [Pd(
m
-X)X(NHC)]2 catalysts.
Fig. 7. Reduction of PdeNHC dimer.
Catalyst
[Pd] [mol%]
PhB(OH)2
Yield [%]
NaBPh4
Yield [%]
[Pd(
[Pd(
[Pd(
m
m
m
-Br)Br(bmim-y)]2
-Br)Br(emim-y)]2
-Cl)Cl(SIMes)]2
0.8
83
90
86
87
97
90
100
90
99
99
98
99
99
99
98
90
98
of Pd(0) in the reaction course. It was found that heating of the
dimeric complexes [Pd( -X)X(NHC)]2 in ethylene glycol resulted in
0.01
0.0001
0.8
0.01
0.0001
0.8
m
the formation of Pd(0) nanoparticles with the average diameter of
4.4e5.7 nm, as estimated by TEM. Interestingly, when the dimeric
complexes were heated in ethylene glycol in the presence of all of
the SuzukieMiyaura reactants, slightly smaller nanoparticles were
formed (Table 5). Thus, considering the decrease in size, it can be
supposed that the nanoparticles were solubilized in contact with
the components of the reaction mixture and formed molecular
palladium species. Possibility of transformation of Pd(0) nano-
particles into Pd(II) complexes under catalytic reaction conditions
was also discussed by other authors [20]. It can also be deduced
from the size of nanoparticles that their aggregation is diminished
in the presence of SuzukieMiyaura substrates. Accordingly, Pd(0)
nanoparticles produced in situ can be considered a reservoir of
soluble catalytically active palladium forms [15]. In agreement with
such a supposition in a case of studied dimers was the result of the
Hg(0) test, in which the inhibiting effect was not observed at a 100-
fold excess of Hg(0) in relation to palladium. It is accepted in the
literature that Hg(0) interacts with Pd(0) or an underligated
palladium species forming an amalgam which does not contribute
to the catalytic process. When the inhibition is not observed, it
might be proposed that Pd(0) nanoparticles do not represent the
catalytically active form [21].
0.01
0.0001
81
Reaction conditions: [Pd] 0.8e0.0001% mol; 2-Br-toluene
NaBPh4 1.15 mmol, NaHCO3 1.7 mmol, 1 h, 110 ꢁC.
1
mmol, PhB(OH)2,
precipitate containing palladium was washed with 2-propanol,
dried, and used in the next run. It is worth noting that experiments
were performed in air and the recovered catalyst was also stored in
contact with air between reactions. Although a significant amount
of palladium was lost during the experiments, in particular soluble
palladium species were washed out, ten runs were successfully
performed (Fig. 6). The yield of 2-methylbiphenyl changed from
91% in the first reaction to 55% in the last one, indicating a very high
stability of the system. In fact, there is the first example of the
efficient recycling of the not-stabilized PdeNHC catalyst.
Formation of Pd(0) nanoparticles
During catalytic experiments, the formation of black palladium
was observed, indicating palladium reduction. Several attempts
were undertaken to explain the course of this process and the role
Reduction of Pd(II) precursors
In order to learn more about the nature of Pd(0) nanoparticles in
the studied systems, additional experiments with the reduction of
the dimeric complex [Pd(m-Br)Br(bmim-y)]2 were undertaken. It
100
80
60
40
20
0
was found that warming the dimer [Pd(m-Br)Br(bmim-y)]2 in 2-
propanol at 40 ꢁC did not cause palladium reduction. However,
the same procedure carried out in the presence of KOH resulted in
the formation of a black solution with some amount of a precipitate
(Fig. 7).
Table 6
The yield of the SuzukieMiyaura cross-coupling in ethylene
glycol catalyzed by [Pd]red obtained according to Fig. 7.
Amount of Pd [mol%]
Yield [%]
0.25
0.5
0.75
1
91
90
91
93
66
1
2
3
4
5
6
7
8
9
10
run
2
Fig. 6. Consecutive SuzukieMiyaura reactions performed with [Pd(m-Br)Br(bmim-y)]2
recovered after each run. Reaction conditions: [Pd] 1% mol; 2-Br-toluene 1 mmol,
Reaction conditions: 2-Br-toluene
1
mmol; PhB(OH)2
PhB(OH)2 1.1 mmol, KOH 1.1 mmol, 1 h, 70 ꢁC, 2-propanol/water 1/1.
1.15 mmol, KOH 1 mmol, ethylene glycol 5 mL, 110 ꢁC, 1 h.

ꢀ
M. Gorna et al. / Journal of Organometallic Chemistry 785 (2015) 92e99
97
Pd 3d
BE [eV]
Pd form
Amount
[%]
334.76
340.02
337.45
342.71
336.17
341.43
Pd(0)
Pd-Br
Pd-O
62.4
27.8
9.8
344 342 340 338 336 334
Binding Energy, eV
Fig. 8. XPS data for the [Pd]red obtained by reduction of [Pd(m-Br)Br(bmim-y)]2 in 2-propanol/KOH.
Fig. 9. TEM data for the [Pd]red obtained by reduction of [Pd(m-Br)Br(bmim-y)]2 in 2-propanol/KOH.
A sample of the reduced palladium ([Pd]red) was obtained by
heating the [Pd( -Br)Br(bmim-y)]2 complex with a 10-fold excess
Pd(II) to Pd(0) was expected; however, it was not the case, and,
according to XPS data, not only Pd(0) but also Pd(II) species were
found in the analyzed sample. Representative XPS data of the
sample of [Pd]red are presented in Fig. 8. Two forms of Pd(II) were
observed, namely ones containing PdeBr and PdeO bonds. The
presence of PdeBr bondings is consistent with XPS Br 3d spectrum
showing binding energy (BE) of Br 3d5/2 68 eV, very close to that
estimated for [Bu4N]2[PdBr4], 67.95 eV [23]. The presence of Pd(0)
nanoparticles was also confirmed by TEM measurements in the
same sample (Fig. 9).
m
ꢁ
of KOH at 40 C in 2-propanol for 1 h. Interestingly, the resulting
black powder efficiently catalyzed the SuzukieMiyaura coupling,
leading to 93% product yield in 1 h Table 6 presents the results of
the catalytic reactions performed with the application of different
amounts of reduced palladium, [Pd]red. High yields of the product
were noted for 0.25e1% mol of palladium, while for 2% mol, the
yield of 2-methylbiphenyl decreased to 66%. It may be assumed
that the faster agglomeration of palladium probably inhibited the
catalytic reaction at a higher concentration [22].
On the basis of the obtained results, it can be assumed that the
reduction of the PdeNHC complexes resulted in the formation of a
composite containing Pd(0) nanoparticles protected by a layer of a
cationic and an anionic palladium species, [PdBr(NHC)]þ and
The product of the reduction of [Pd(m-Br)Br(bmim-y)]2 in 2-
propanol, [Pd]red, was analyzed using the XPS and TEM methods
in order to estimate its composition. The complete reduction of
PdBr(NHC)+
NHC.H+
PdBr3(NHC)-
PdBr42-
NHC.H+
PdBr42-
PdBr3(NHC)-
PdBr42-
NHC.H+
PdBr42-
NHC.H+
Br
Br
Br
NHC
Br
PdBr(NHC)+
PdBr3(NHC)-
Pd
Pd
PdBr(NHC)+
PdBr3(NHC)-
NHC
NHC.H+
PdBr42-
NHC.H+
PdBr(NHC)+
PdBr42-
NHC
R
Pd L
R
Fig. 10. Plausible transformations of [Pd(m-Br)Br(bmim-y)]2 during SuzukieMiyaura conditions.

ꢀ
98
M. Gorna et al. / Journal of Organometallic Chemistry 785 (2015) 92e99
Fig. 11. TEM data for the palladium residue recovered after 10 runs of SuzukieMiyaura reactions catalyzed by [Pd(m-Br)Br(bmim-y)]2.
[PdBr3(NHC)]ꢀ. The protective shell can also contain imidazolium
cations [NHC$H]þ and [PdBr4]2ꢀ anions (Fig. 10). The presence of
NHC and Xꢀ ligands released when Pd(0) nanoparticles were
formed facilitated formation of anionic and cationic Pd(II) species.
A coreeshell structure presented on Fig. 10 is similar to that
proposed for the product obtained by the reduction of Pd(OAc)2 by
tetrabutylammonium acetate [24]. Under catalytic reaction condi-
tions the additional stabilization of Pd(0) nanoparticles by NHC li-
gands should also be considered. For example, the formation of the
self-assembled NHC monolayer on the surface of Au(0) nano-
particles was recently evidenced [25].
protonated forms (NHC$HX) are of fundamental importance for the
observed long-term activity of the palladium systems.
Acknowledgments
Financial support of National Science Foundation (NCN) with
grant 2012/05/B/ST5/00265 is gratefully acknowledged (MG, AG,
AMT).
Appendix A. Supplementary material
The palladium-containing residue isolated after ten recycling
experiments was analyzed for comparison using the TEM method.
TEM micrographs show small Pd(0) nanoparticles with a diameter
of ca. 4 nm, distributed uniformly without significant agglomera-
tion (Fig. 11). The size of the nanoparticles is very close to that
estimated after one SuzukieMiyaura reaction as well as after the
reduction of the precursor in 2-propanol/KOH. Considering the fact
that the recycling experiments (presented in Fig. 6) were per-
formed without any additional stabilizing agents, the high stability
of the nanoparticles against agglomeration is worth underlining. In
our opinion, the formation of Pd(0)-Pd(II) composites can ratio-
nalize the observed feature.
CCDC reference codes: 997186e997188 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
References
[1] a) N-heterocyclic carbenes in transition metal catalysis, in: F. Glorius (Ed.), N-
Heterocyclic Carbenes in Synthesis, Springer-Verlag, Berlin, Germany, 2007.
S.P. Nolan Ed., Wiley-VCH, New York, 2006;
b) C.M. Crudden, D.P. Allen, Coord. Chem. Rev. 248 (2004) 2247e2273;
€
c) W.A. Herrmann, C. Kocher, Angew. Chem. Int. Ed. 36 (1997) 2162e2187;
d) E.A.B. Kantchev, C.J. O'Brien, M.G. Organ, Aldrichim. Acta 39 (2006) 97e111;
e) S. Chowdhury, R.S. Mohan, L. Scott, Tetrahedron 63 (2007) 2363e2389;
f) N. Marion, S.P. Nolan, Acc. Chem. Res. 41 (11) (2008) 1440e1449;
g) N.M. Scott, S.P. Nolan, Eur. J. Inorg. Chem. (2005) 1815e1828;
h) F. Bellina, A. Carpita, R. Rossi, Synthesis 15 (2004) 2419e2440;
i) F.E. Hahn, M.C. Jahnke, Angew. Chem. Int. Ed. 47 (2008) 3122e3172;
j) W.J. Sommer, M. Weck, Coord. Chem. Rev. 251 (2007) 860e873;
k) K. Arentsen, S. Caddick, F.G.N. Cloke, Tetrahedron 61 (2005) 9710e9715.
[2] a) N. Miyaura, A. Suzuki, Chem. Rev. 95 (1995) 2457e2483;
b) A. Suzuki, J. Organomet. Chem. 576 (1999) 147e168;
c) N. Miyaura, Top. Curr. Chem. 219 (2002) 11e59.
[3] a) C. Torborg, M. Beller, Adv. Synth. Catal. 351 (2009) 3027e3043;
b) J. Magano, J.R. Dunetz, Chem. Rev. 111 (2011) 2177e2250.
[4] V.P. Ananikov, S.S. Zalesskiy, V.V. Kachala, I.P. Beletskaya, J. Organomet. Chem.
696 (2011) 400e405.
[5] C. Valente, S. Calimsiz, K.H. Hoi, D. Mallik, M. Sayah, M.G. Organ, Angew. Chem.
Int. Ed. 51 (2012) 3314e3332.
[6] V.P. Ananikov, I.P. Beletskaya, Organometallics 31 (2012) 1595e1604.
[7] A.S. Kashin, V.P. Ananikov, J. Org. Chem. 78 (22) (2013) 11117e11125.
[8] O. Diebolt, P. Braunstein, S.P. Nolan, C.S.J. Cazin, ChemComm (2008)
3190e3192.
[9] U.I. Tessin, X. Bantreil, O. Songis, C.S.J. Cazin, Eur. J. Inorg. Chem. (2013)
2007e2010.
[10] C.E. Hartmann, S.P. Nolan, C.S.J. Cazin, Organometallics 28 (2009) 2915e2919.
[11] J.D. Egbert, A. Chartoire, A.M.Z. Slawin, S.P. Nolan, Organometallics 30 (2011)
4494e4496.
[12] R. Saravankumar, V. Ramkumar, S. Sankararaman, Organometallics 30 (2011)
1689e1694.
Conclusions
Palladium(II) dimers, [Pd(m-X)X(NHC)]2, showed an excellent
catalytic activity in SuzukieMiyaura reactions, characterized by a
TOF of up to 106 hꢀ1. Transformations of the catalyst precursor to a
Pd(0)ePd(II) composite creates a very stable system, which is
highly efficient in several subsequent runs of the SuzukieMiyaura
reaction. During a catalytic reaction, the Pd(II) species present on
the surface of the composite can be easily transferred into the so-
lution and take part in the catalytic transformations. This is illus-
trated in Fig. 10 by the schematic activation of the substrate on a
single palladium ion leached from the composite. Pd(0) nano-
particles forming the core of the composite undergo solubilization
during the reaction, which is in fact catalyzed by molecular palla-
dium species. At the end of the catalytic cycle, palladium is reduced
and new nanoparticles are formed.
Therefore, the continuous transformations of Pd(II) to Pd(0) and
the oxidation of Pd(0) to Pd(II) have fundamental influence on the
reaction course, preventing the formation of inactive palladium
black. The catalytic system operating according to the described
model is active for more than one catalytic cycle and for a relatively
long time.
[13] A.R. Hajipour, G. Azizi, SynLett. 24 (2) (2013) 254e258.
[14] S. Bouquillon, A.M. d’Hardemare, M.-T. Averbuch-Puchot, F. Henin, J. Muzart,
Polyhedron (1999) 3511e3516.
[15] M.S. Szulmanowicz, A. Gniewek, W. Gil, A.M. Trzeciak, ChemCatChem 5 (2013)
1152e1160.
Interestingly, the stabilizing shell composed of Pd(II) species
protects the Pd(0) nanoparticles against agglomeration very well.
In our opinion, the presence of the NHC ligands as well as their
[16] J. Cosier, A.M. Glazer, J. Appl. Cryst. 19 (1986) 105e107.
[17] G.M. Sheldrick, Acta Crystallogr. Sect. A 64 (2008) 112e122.

ꢀ
M. Gorna et al. / Journal of Organometallic Chemistry 785 (2015) 92e99
99
ꢀ
ꢀ
[18] Oxford Diffraction, CrysAlis RED Version 1.171.33.55, Oxford Diffraction Ltd,
Wrocław, Poland, 2008.
[23] A. Gniewek, A.M. Trzeciak, J.J. Ziołkowski, L. Ke˛pinski, J. Wrzyszcz, W. Tylus,
J. Catal. 229 (2005) 332.
[19] L.J. Farrugia, J. Appl. Crystallogr. 30 (1997) 565.
[20] a) M. Perez-Lorenzo, J. Phys. Chem. Lett. 3 (2012) 167e174;
b) C.-J. Jia, F. Schüth, Phys. Chem. Chem. Phys. 13 (2011) 2457e2487;
c) A. Balanta, C. Godard, C. Claver, Chem. Soc. Rev. 40 (2011) 4973e4985.
[21] a) N.T.S. Phan, M. van der Sluys, C.W. Jones, Adv. Synth. Catal. 348 (2006)
609e679;
[24] a) V. Calo, A. Nacci, A. Monopoli, F. Montingelli, J. Org. Chem. 70 (2005)
6040e6044;
b) V. Calo, A. Nacci, A. Monopoli, A. Fornaro, L. Sabbatini, N. Cioffi, N. Ditaranto,
Organometallics 23 (2004) 5154e5158;
c) T.M. Reetz, M. Maase, Adv. Mater. 11 (1998) 773e777.
[25] C.M. Crudden, J.H. Horton, I.I. Ebralidze, O.V. Zenkina, A.B. McLean,
B. Drevniok, Z. She, H.-B. Kraatz, N.J. Mosey, T. Seki, E.C. Keske, J.D. Leake,
A. Rousina-Webb, G. Wu, Nat. Chem. 6 (2014) 409e414.
b) R.H. Crabtree, Chem. Rev. 112 (2012) 1536e1554.
[22] J.G. de Vries, Dalton Trans. (2006) 421e429.