Dissimilar catalytic behavior of molecular or colloidal palladium systems with a new NHC ligand

Article abstract of DOI:10.1039/c7dt02729j

In this work, we describe the synthesis of a new N-heterocyclic carbene (NHC) ligand, derived from a hybrid pyrazole-imidazolium scaffold, namely 1-[2-(3,5-dimethylpyrazol-1-yl)ethyl]-3-((S)-1-phenylethyl)-3H-imidazol-2-ylidene (L). This ligand has been used as a stabilizer for the organometallic synthesis of palladium(0) nanoparticles (Pd NPs). L presents a better stabilizing effect than its pre-carbenic HLCl counterpart, allowing the formation of isolated Pd NPs while HLCl yields aggregated ones. Additionally, molecular Pd(ii) coordination compounds of L and HLCl were synthesized and characterized to better understand the coordination modes of these ligands. Both molecular and colloidal Pd systems have been further tested in catalytic C-C coupling processes. Three different types of reactions have been observed depending on the catalytic system: (i) the Suzuki-Miyaura reaction takes place with Pd molecular complexes; (ii) a secondary reaction, the dehalogenation of the substrate, is always detected and (iii) the C-C homocoupling between two molecules of bromoarenes is observed with colloidal catalysts.

Full text of DOI:10.1039/c7dt02729j

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Villarraga, J. De Tovar, M. Guerrero, P. Nolis, T. Parella, P. Lecante, N. Romero, L. Escriche, R. Bofill, J.
Ros, X. SALA, K. Philippot and J. García-Antón, Dalton Trans., 2017, DOI: 10.1039/C7DT02729J.
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PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
rsc.li/dalton

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Dissimilar Catalytic Behavior of Molecular or Colloidal Palladium
Systems with a New NHC Ligand
Received 00th January 20xx,
Accepted 00th January 20xx
[
a,†]
[a,†]
[b,c]
[d]
Fernando Gómez-Villarraga , Jonathan De Tovar , Miguel Guerrero , Pau Nolis , Teodor
[
d]
[e]
[a]
[a]
[a]
[a]
Parella, Pierre Lecant[eb,,c] Nuria Romero , Lluís Escriche , Roger Bofill , Josep Ros , Xavier
DOI: 10.1039/x0xx00000x
[a]
[a]
Sala , Karine Philippot *, and Jordi García-Antón *
www.rsc.org/
In this work, we describe the synthesis of a new N-heterocyclic carbene (NHC) ligand, derived from a hybrid pyrazole-
imidazolium scaffold, namely 1-[2-(3,5-dimethylpyrazol-1-yl)ethyl]-3-((S)-1-phenylethyl)-3H-imidazol-2-ylidene (L). This
ligand has been used as a stabilizer for the organometallic synthesis of palladium(0) nanoparticles (Pd NPs). L presents a
better stabilizing effect than the pre-carbenic HLCl counterpart, allowing the formation of isolated Pd NPs while HLCl yields
aggregated ones. Additionally, molecular Pd(II) coordination compounds of L and HLCl were synthesized and characterized
to better understand the coordination modes of these ligands. Both molecular and colloidal Pd systems have been further
tested in catalytic C-C coupling processes. Three different types of reactions have been observed depending on the
catalytic system: i) the Suzuki-Miyaura reaction takes place with Pd molecular complexes; ii) a secondary reaction, the
dehalogenation of the substrate, is always detected and iii) the C-C homocoupling between two molecules of bromoarenes
is observed with colloidal catalysts.
with metal pre-catalysts, imidazolium salts are the most
frequently employed as the core structures for NHCs given
Introduction
During the last few years, the use of N-heterocyclic
carbenes (NHCs) has emerged as primary ligands in catalysis
and beyond. Amongst other catalytic processes,
organometallic compounds containing these ligands have been
applied in the cross-coupling reactions of un-activated
their straightforward syntheses and feasible tuning
5
properties.
Different methodologies can be followed to obtain and
stabilize NPs in different media while controlling their size,
shape and composition, which, in turn, can tune their unique
1
substrates. This can be explained not only by their remarkably
2
strong σ-binding and steric tunability, but also because they
6
properties. The use of coordinating ligands is of particular
interest to prepare MNPs because it gives the possibility to
modulate their surface properties as known for the synthesis
are able to stabilize highly unusual and hitherto elusive
reactive species such as metal nanoparticles (MNPs) thanks to
7
of molecular complexes. As a consequence, ligand-stabilized
3
their stability under oxidative conditions once coordinated.
8
MNPs became very attractive for applications in catalysis.
Thus, NHCs have become ligands of paramount importance in
nanochemistry given their advantageous behavior for the
4
stabilization and functionalization of MNPs. Although
MNPs not only are able to catalyze reactions also catalyzed by
molecular complexes (i.e. C-C coupling or hydrogenation of
olefins) but also reactions that cannot be catalyzed by
molecular complexes (i.e. hydrogenation of arenes).
different families of NHCs have been largely explored in
various important organic transformations when combined
Furthermore, palladium is the most versatile transition
metal in chemical catalytic reactions since many of these
a.
Departament de Química, Unitat de Química Inorgànica, Facultat de Ciències,
Universitat Autònoma de Barcelona, 08193-Bellaterra, Barcelona, Spain. Fax:
9
processes cannot be catalyzed by other transition metals.
*
+34) 93 581 3101. E-mail: Jordi.GarciaAnton@uab.es
(
CNRS; LCC (Laboratoire de Chimie de Coordination du CNRS), 205, route de
Thus, Pd(0) NPs have shown large catalytic efficiency in C-C
b.
coupling
reactions
specially
when
non-aggregated
10
*
Narbonne, F-31077 Toulouse, France. Fax: (+33) 5 61553003. E-mail:
Karine.Philippot@lcc-toulouse.fr
homogeneous NPs of 1-4 nm size are used as catalysts.
The Suzuki-Miyaura reaction is one of the most important
11
c.
Université de Toulouse; UPS, INPT ; LCC ; F-31077 Toulouse, France
Servei de Ressonància Magnètica Nuclear, Facultat de Ciències, Universitat
Autònoma de Barcelona, 08193 Cerdanyola del Vallès, Barcelona, Catalonia,
Spain
CNRS, CEMES (Centre d'Elaboration de Matériaux et d'Etudes Structurales), 29 rue
J. Marvig, F-31055 Toulouse, France.
These authors equally contributed to this work.
d.
cross-coupling processes from an industrial point of view. It
allows to easily obtain biaryl products, important
intermediates in the organic syntheses of various target
products ranging from performance materials to
e.
†
1
2
Electronic
DOI: 10.1039/x0xx00000x
Supplementary
Information
(ESI)
available:
See pharmaceuticals. Although noble metal complexes are
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usually employed as catalysts for this reaction, MNPs merit superstructures do not have a preferential shape and their size
also to be studied since playing with stabilizing ligands may varies from 50 to 250 nm. For /Pd ratios of 0.3, 0.5 and 1.0,
only isolated NPs were observed (Figure 1b-d). In all these
L
1
3
tune their catalytic performance.
Our research group has focused on discerning the catalytic cases, NPs are spherical and present similar mean sizes in the
behavior of colloidal and molecular palladium systems in C-C range 2.1 – 2.4 nm. These similar sizes indicate that a
coupling reactions. For example, we have recently studied Pd minimum L/Pd molar ratio of 0.3 is needed to have stable
molecular/colloidal systems containing new hybrid pyrazole isolated NPs, but the addition of a higher ligand/metal ratio
derived ligands with alkylether, alkylthioether or alkylamino does not have a significant effect on the size of the NPs.
moieties leading to different catalytic output depending on the
For comparative purposes, the stabilization of the NPs has
1
4
system. Here we present improved Pd systems based on a also been attempted with the protonated ligand HLCl (see HR-
new NHC-pyrazole hybrid ligand. Interestingly, as it will be TEM images in Figure 2 and SEM-FEG images in the Supporting
described hereafter, different behaviors were observed in C-C Information, Figure S7b-d). In this case, molar ratio 0.1 failed
coupling catalysis that illustrates the possibility to optimize the to give stable NPs. For molar ratios 0.3 and 0.5 Pd NPs
catalytic system according to the final aim.
appeared organized into superstructures, and for the molar
ratio 1.0, a mixture of isolated and aggregated NPs was
observed. The mean sizes of these NPs are 3.0(0.7), 3.2(0.9)
and 2.1(0.9) nm for HLCl / Pd molar ratios of 0.3, 0.5 and 1.0,
respectively. Therefore, with HLCl a higher ratio of ligand is
needed to successfully stabilize the NPs. From these results
Results and Discussion
Synthesis of the ligand (L)
For the synthesis of 1-[2-(3,5-dimethylpyrazol-1-yl)ethyl]-3- and given the main difference of the two ligands (
(S)-1-phenylethyl)-3H-imidazol-1-ium chloride (HLCl), 1-(2- carbene while HLCl is not deprotonated), we believe that the
chloroethyl)-3,5-dimethyl-1H-pyrazole and 1-(S)- stabilization of the Pd NPs takes place by the coordination of
phenylethyl)imidazole were heated solventless at 120 ˚C for through the carbene (M-C) and pyrazolic (M-N) groups to the
L is a
(
L
(
64
h. The deprotonation of HLCl to yield 1-[2-(3,5- surface of the Pd NPs whereas HLCl would only coordinate
through the pyrazolic group (Scheme 1).
The presence of the ligands or HLCl in N1 or N2 (those
catalytic amount of BuOK in anhydrous dichloromethane prepared with
and HLCl, respectively, in a ligand/Pd molar
Scheme 1). Both and HLCl have been fully characterized by ratio equal to 0.5) has been demonstrated by IR spectroscopy
NMR and IR spectroscopies as well as ESI-MS (Figures S1-S6 in (Figure S8 and Experimental Section), and ICP-OES and
dimethylpyrazol-1-yl)ethyl]-3-((S)-1-phenylethyl)-3H-imidazol-
-ylidene ( ) was achieved by reacting HLCl with NaH and a
2
L
L
t
L
(
L
the Supplementary Information).
elemental analyses. The Pd / Ligand ratios found are Pd561L37
and Pd1415HLCl93 for N1 and N2, respectively.
Wide-angle X-ray scattering (WAXS) measurements have
been performed on nanoparticles N1 and N2. The two radial
distribution functions (RDFs) (Figure 3a) are very similar and in
good agreement with the fcc structure of bulk Pd. However,
significant distances can be observed at least up to 4 nm, thus
in disagreement with the sizes obtained from HR-TEM (2.1(0.9)
and 3.2(0.9) for N1 and N2, respectively). Indeed, when
compared with the RDF computed (Figure 3b) from a spherical
model 2.0 nm in diameter, the agreement does not go beyond
Synthesis of Pd(0) nanoparticles (Pd NPs)
Ligand
organometallic approach in mild reaction conditions. The
syntheses have been carried out by reacting [Pd (dba) ] (dba =
dibenzylideneacetone) and ([ ]/[Pd] molar ratios varying
L has been used to prepare Pd NPs according to the
1
5
2
3
L
L
from 0.1 to 1.0) in anhydrous dichloromethane for 18 h in a
Fisher-Porter reactor, under 3 bar of dihydrogen at room
temperature. Finally, Pd NPs were precipitated and washed
with cold pentane and dried under reduced pressure.
1.5 nm. A much better result can be obtained with a dual size
TEM (transmission electron microscopy) and SEM (scanning
electron microscopy) analyses (after deposition/drying of a
drop of crude colloidal solutions onto a holey carbon-covered
copper grid) were performed with all obtained samples. The
results showed that the Pd NPs are aggregated into
model, including a small amount (10%) of much bigger
spherical particles (5 nm in diameter). Actually, this dual size
model is likely itself oversimplified and different combinations
including a large majority of 2 nm particles and a minority of
bigger ones should probably lead to the same level of
agreement. This result, however, is not in contradiction with
TEM results taking into account not only the mean sizes but
also the relatively large tail on the bigger size side.
superstructures when a L/Pd ratio of 0.1 is used (HR-TEM
images are given in Figure 1a and SEM-FEG images in the
Supporting Information, Figure S7a). The single NPs are
spherical and have a mean size of 3.6(0.9) nm but the
2
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Figure 2. HR-TEM micrographs and the corresponding size-
histograms of Pd nanoparticles stabilized with HLCl synthesized as
following: (a) [HLCl]/[Pd] = 0.3; (b) [HLCl]/[Pd] = 0.5; (c) and (d)
[
HLCl]/[Pd] = 1.0.
Synthesis and characterization of Pd(II) molecular complexes (C1-
C2)
Pd(II) molecular complexes of
L and HLCl have been
synthesized in order to better understand the coordination
properties of these ligands (Scheme 1).
The silver carbene transmetallation reaction is a general
procedure for the preparation of palladium – carbene
1
6
complexes and was considered to obtain the Pd complex
with . The silver complex of CAg) was prepared by reacting
L
L (
1
silver dioxide and HLCl in anhydrous dichloromethane. The H
NMR spectrum of this complex shows that the pre-carbene
proton of the imidazolium salt is no longer present, thus
confirming the deprotonation of the imidazolium group and
formation of the Ag-NHC bond (Figure 4 and Figure S9). Also,
the formation of CAg has been confirmed by ESI-MS and IR
spectroscopy (Figures S10 and S11, respectively). The next step
consisted in the reaction of CAg with [PdCl (cod)] in anhydrous
2
Figure 1. HR-TEM micrographs and the corresponding size-
histograms of Pd nanoparticles stabilized with L synthesized as
following: (a) [L]/[Pd] = 0.1; (b) [L]/[Pd] = 0.3; (c) [L]/[Pd] = 0.5; (d)
L]/[Pd] = 1.0.
dichloromethane (Scheme 1) to obtain the carbenic Pd
complex [PdCl L] (C1). The absence of the pre-carbene proton
2
1
in the H-NMR spectrum of C1 confirms the formation of the
[
Pd–NHC bond (Figure 4 and Figure S12). The formation of the
complex has been attested by HR-ESI-MS and IR spectroscopy
(Figures S13 and S14, respectively). Moreover, the NMR data
also evidence the chelation of the ligand to the metallic centre
through the NHC and pyrazolic groups. The rigidity imposed by
this coordination mode provokes that the two protons of each
CH2 in the N-CH CH -N moiety (H8 and H ) become
2
2
9
diastereotopic, resulting in four visible signals assigned to a
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single hydrogen atom each (see high-resolution NMR spectra in the ROESY spectrum. Figure 5 and S12d show the 2D ROESY
1
13
of C1 including H and C NMR spectroscopy chemical shift experiment applied on C1, where chemical exchange cross
assignments in Figures S12a and b) It is also noticeable that peaks are detected with negative phase (blue color), while ROE
some proton signals split, which is single quote labeled in cross peaks are positive phased (red color). Thus, protons H
Figure S12a. This splitting is due to conformational exchange and H in the ethyl chain undergo a pronounced chemical
between a major and a minor component with a ratio of environment exchange, with cross peaks situated notably
.
8
9
approximately 70/30, as deduced from signal integration.
separated from the spectrum diagonal. This conformational
exchange might be explained on the basis of a slow
conformational equilibrium of the new seven-membered ring
4 6 7
formed Other protons (H , H , H , H13, H14 and H16) also exhibit
chemical exchange cross peaks, which are closer to the
spectrum diagonal, indicating that chemical exchange is taking
place between less differentiated chemical environments. This
latter effect could be explained as a side-effect from the
mentioned seven ring conformational exchange. On the other
side, neither benzyl ring protons H -H nor H15 show any
1
8
22
chemical exchange cross peaks, suggesting that these protons
are situated far enough from the conformational exchange
described.
Figure 3. WAXS measurements on Pd nanoparticles for N1 and
N2 and comparison with Pd fcc in reciprocal space (a) and real
space including simulated from a model including only 2 nm
particles based on the Pd fcc structure (lower) and simulated
from a model including 2 nm particles plus 10% of 5 nm
particles (upper) (b).
In order to study further this chemical exchange process, a
2
D ROESY (Rotating frame Overhauser Effect SpectrocopY)
1
7
spectrum was recorded. Briefly explained, in this NMR
experiment ROE and occasionally chemical exchange cross
1
8
Scheme 1. Synthetic preparation of L and HLCl and their
peaks are observed. The former are indicative of through
space distance proximity, and the latter are detected when
protons experience distinct chemical environments, which is
commonly the case when a conformational equilibrium is
present. The distinction of the above mentioned different
cross peaks families is possible from the opposite signal phase
corresponding Ag and Pd molecular complexes (
C
Ag
,
C1 and C2
)
and Pd nanoparticulate systems (N1 and N2).
On the other hand, the direct reaction of HLCl with PdCl
acetonitrile yielded the complex [PdCl
2
in
3
(HL)] (C2, Scheme 1),
4
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which has been characterized by NMR and IR spectroscopies complex. These interactions are responsible for the expansion
and ESI-MS (Figures S15-S17), thus confirming its in the crystallographic a direction.
stoichiometry. Interestingly, the existence of the protonated
+
1
ligand HL is confirmed by the singlet H NMR spectroscopy
resonance appearing at 9.03 ppm for H11 pre-carbene proton
(Figure S15).
Figure 5. 2D ROESY spectrum of C1 registered at 600 MHz (CDCl
98K). Arrows pointing negative cross peaks (blue color) indicate
chemical exchange between H and H protons.
3
,
1
Figure 4. H-NMR of HLCl
,
L
and CAg registered at 360 MHz
2
(CDCl , 298K) and C1 registered at 600 MHz (CDCl , 298K).
3 3
8
9
Crystal and molecular structure of trichloro[(S)-3-(2-(3,5-dimethyl-
H-pyrazol-1-yl)ethyl)-1-(1-phenylethyl)-1H-imidazol-3-ium-
1
1
κ N]palladium(II) (C2)
Single crystals of C2 suitable for X-Ray diffraction analysis were
obtained by slow evaporation of a concentrated acetonitrile
solution of the complex at RT. The structure of C2 consists of
monomeric [PdCl (HL)] units (Figure 6a), linked by Van der
3
Waals forces (Figure 6b). The palladium center is coordinated
by three terminal chloride ligands and one pyrazolic nitrogen
from
a
1-[2-(3,5-dimethylpyrazol-1-yl)ethyl]-3-((S)-1-
phenylethyl)-3H-imidazol-1-ium cation in a slightly distorted
square-planar geometry. Crystallographic data is listed in Table
S1, and selected bond lengths and angles data are gathered in
Table S2. The positive charge generated by the imidazolium
cation is canceled out by the negative charge provided by the
third chloride ion coordinated to Pd(II), thus forming a
zwitterionic structure.
There are three complexes reported in the literature with a
PdCl N core (terminal chloride ligands and pyrazolic
3
1
9-21
nitrogen).
The Pd-N and Pd-Cl bond lengths here observed
are comparable to the values found in the literature, and the
N-Pd-Cl and Cl-Pd-Cl bond angles slightly deviate from the
square planar angles (Table S2).
In compound C2, chlorine atoms (Cl2 and Cl3, Figure 6b)
form three different hydrogen bonds: C3-H3A···Cl2 (2.707 Å,
1
46.11
hydrogens from the ethylene group of an adjacent C2 unit, and Figure 6. ORTEP drawing of C2, showing the atom-numbering
C11-H11···Cl2 (2.840 Å, 138.45 ), interacting with a hydrogen scheme; 50% probability amplitude displacement ellipsoids are
˚) and C15-H15···Cl3 (2.935 Å, 154.30˚) interacting with
˚
atom from the imidazole aromatic ring of another adjacent C2 shown for all non-hydrogen atoms (a) and hydrogen interactions
present in the unit cell of C2 (b).
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a
Table 1. SuzukiꢀMiyaura reactions with palladium catalysts.
b
c
c
c
Conv.
BT
TT
PhMe
(%)
49
59
29
Yield BT
(%)
Yield TT
(%)
Entry
Catalyst
X
2
[PhB(OH) ]/[Pd]
(%)
(%)
51
41
71
93
81
47
0
(%)
1
2
3
4
5
6
7
8
9
C1
C2
C1
C2
N1
N2
N1
N2
N1
N2
N1
I
I
Br
Br
I
I
I
I
Br
Br
Br
Br
3125
3125
3125
3125
3125
3125
0
100
82
28
0
0
0
0
0
0
0
0
51
37
83
72
51
34
20
43
81
47
0
0
3
0
0
0
0
0
0
0
46
7
100
100
100
56
66
36
19
53
100
56
44
56
17
28
0
0
5
7
0
0
3125
3125
0
34
13
60
26
1
1
1
0
1
2
3
0
0
72
36
N2
0
0
a
ꢀ3
t
Reaction conditions: 1·10 mmol of Pd, 2.5 mmol of 4ꢀhalogenotoluene, 5.0 mmol BuOK and 0.5 mmol naphthalene as internal standard in 8.0 mL of DMF
2
and 2.0 mL of H O. Temperature 100º C.
b
Conversion after 6 h reaction.
Chemoselectivity in 4ꢀmethylbiphenyl (BT), 4,4’ꢀdimethylbiphenyl (TT) and toluene (PhMe), respectively.
c
1
4
Catalytic Experiments with Pd molecular or colloidal systems
C1 and C2 Pd complexes (with and HLCl ligands,
literature on Pd molecular complexes. Also, 4-bromotoluene
yields BT as the major product, while the amount of the
dehalogenation product (toluene) significantly rises when 4-
iodotoluene is used.
L
respectively) as well as N1 and N2 Pd nanoparticles have been
evaluated as catalysts for the Suzuki-Miyaura reaction (Scheme
2
2,23
Recently, V. P. Ananikov et al. have demonstrated the
formation of Pd NPs when molecular Pd complexes with NHC
2
, Table 1).
-halogenotoluene
4
derivatives
(4-chlorotoluene,
4-
2
5
ligands are used in the Mizoroki-Heck reaction. In our case,
even if it is not a definitive proof, TEM control analyses of the
solutions after catalysis with C1 or C2 did not show the
presence of Pd nanoparticles. In any case, Hg poisoning tests
carried out to these systems (vide infra) confirm the molecular
nature of the real catalyst when C1 or C2 are used.
bromotoluene or 4-iodotoluene) and an excess of
phenylboronic acid were chosen as substrates in order to
easily distinguish between cross-coupling and homocoupling
products. Under the catalytic reaction conditions applied
1
4
(Scheme 2) , two different by-products can be formed beside
the expected cross-coupled product (4-methylbiphenyl; BT) as
follows: i) toluene (PhMe) arising from the dehalogenation of
the substrates and ii) 4,4'-dimethylbiphenyl (TT), resulting
from the 4-bromotoluene homocoupling. Once optimized, 4-
halogenotoluene (substrate/Pd = 2500), phenylboronic acid
When 4-iodotoluene was used as
a substrate with
nanocatalysts N1 and N2, complete conversions were achieved
in 6 h in both cases. In the case of N1, the formation of the
cross-coupling product (4-methylbiphenyl; BT) was mainly
observed together with a small amount of toluene (Table 1,
entry 5). Formation of toluene rises to 53% in the case of N2
(PhB(OH)2/Pd = 3125 or 0), tBuOK (base/Pd = 5000) were
employed in a DMF/water mixture (4/1) as solvent. As a
general trend, higher conversions were obtained with 4-
iodotoluene compared to 4-bromotoluene in both molecular
and colloidal systems (Table 1), as expected from the
respective C(Ph)-X bond energies (X=Br, 84 kcal/mol; X=I, 67
(Table 1, entry 6). On the contrary, N1 and N2 behave similarly
against 4-bromotoluene, with the formation of 4,4'-
dimethylbiphenyl (TT), arising from the homocoupling
reaction, together with toluene and small amounts of BT
(Table 1, entries 9 and 10). Furthermore, for the two
2
4
kcal/mol). Unfortunately, 4-chlorotoluene could not be
activated probably due to the higher C(Ph)-Cl bond energy
halogenotoluene substrates, both conversion
and
2
4
chemoselectivities are higher for the Pd NP/L system (N1) than
value (97.1 kcal/mol) and the Suzuki-Miyaura coupling
reaction was unsuccessful.
for the Pd NP/HLCl system (N2). These differences can be
explained by the binding of the carbene group at the Pd NP
surface in the case of N1 compared to N2. Indeed, the
different coordination mode (by C and N in N1 and only by N in
N2) may tune both electronic and steric properties at the
Interestingly, in the presence of the molecular catalysts C1 and
C2, toluene was the only side product observed in a significant
amount using both 4-bromotoluene and 4-idodotoluene as
substrates (Table 1, entries 1-4). Homocoupling reaction does
not take place, as otherwise predictable according to the
surface and consequently the catalytic behavior of N1
.
6
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and N2 after catalysis, respectively) is similar to those found
for the same NPs prior to the catalytic tests (2.1(0.9) and
3.2(0.9) nm for N1 or N2, respectively). Even if no
restructuration of the NPs seems to have taken place (which
could indicate a leaching / deposition mechanism through the
2
9
Ostwald rippening process ), it is important to note that for
N2 the NPs appear isolated after catalysis, in contrast with the
aggregation observed prior catalysis.
Secondly, ICP-MS analyses of the solutions after catalysis for
entries 9 and 10 in Table 1 have been carried out in order to
know the amount of Pd leached from the NPs to the solution.
The concentration of Pd in these solutions are 0.17 and 1.8
ppm for N1 or N2, respectively, meaning that 0.7% and 15% of
Pd has leached to the solution for N1 or N2, respectively. The
differences on the percentage of leached Pd can be explained
by the different coordinative properties of the ligands on each
Scheme 2. Suzuki-Miyaura, homocoupling and dehalogenation
reactions performed in the presence of Pd complexes C1 and
C2 and Pd NPs N1 and N2
.
In order to determine the influence of phenylboronic acid in C-
C coupling reactions catalyzed by the colloidal systems,
catalytic experiments were also carried out in absence of the
boron reagent. Besides that BT was not detected, it is
remarkable that full conversion to the dehalogenated product
system. For N1, the carbene and pyrazolyl groups from
L can
coordinate to the surface of the NPs, and better stabilize them
from Pd leaching than in N2, where only the pyrazolyl group
from HLCl can coordinate the surface of the NPs.
Moreover, the leached molecular Pd species can be
responsible of the small amount of C-C heterocoupling
reaction observed in entries 9 and 10 of Table 1 (5% and 7%,
respectively).
(PhMe) is observed in the case of the reaction carried out with
4-iodotoluene catalyzed by N1 (Table 1, entry 7). However,
when 4-bromotoluene was employed a significant increase in
the yield of TT homocoupling product is observed for both
nanocatalysts (from 34% to 60%; Table 1, entries 9 and 11,
respectively, for N1, and from 13% to 26% for N2, entries 10
and 12). When performed with the molecular complexes, the
same reaction of 4-halotoluene in the absence of
phenylboronic acid reagent did not yield to homocoupling
products at all.
All in all, these experiments indicate that that C1 and C2 act as
molecular catalysts in Suzuki-Miyaura C-C heterocoupling and
N1 and N2 are the real active species for C-C homocoupling
reaction.
Experimental Section
A recurrent discussion with Pd NPs as catalysts for C-C coupling
reactions is whether the real catalyst are the NPs or molecular
General procedure and reagents
1
Pd species leached from them. There are several publications
0
All manipulations were carried out under an argon
or atom-leaching atmosphere using standard Schlenk tubes or Fischer–Porter
2
6
that support either a surface-based
2
7
mechanism. In this context, three different tests have been reactors and vacuum line techniques, or in a glove-box.
carried out to assess the nature of the active species during [Pd (dba) ] was purchased from Strem Chemicals, and
2
3
3
0
31
the catalytic process. First of all, in the mercury poisoning [PdCl (CH CN) ] and [PdCl (cod)] (cod = 1,5-cyclooctadiene)
2
3
2
2
2
8
test, 100 equivalents of Hg were added to the catalytic were prepared as described in the literature. Solvents were
mixtures after 10 min of reaction. For molecular systems (C1 purchased from SDS and dried through a purification machine
and C2) the conversions were not affected, while for the (MBraun MB SPS-800) or distilled prior to use: tetrahydrofuran
colloidal systems (N1 and N2) the catalytic reactions ceased and diethyl ether over sodium/benzophenone, and pentane, n-
completely. These data suggest that C1 and C2 act as real hexane, acetonitrile and dichloromethane over calcium
molecular catalysts when starting with them, whereas Pd NPs hydride.
are the active species when N1 and N2 are introduced in the
Elemental analyses (C, H, and N) were carried out by the
catalysis. This is in accordance with the different results Chemical Analyses Service of the Universitat Autònoma de
observed in catalysis depending on the molecular or colloidal Barcelona on a Eurovector 3011 instrument. The Pd weight
nature of the introduced catalysts. However, the mercury percentages or concentrations were analyzed by the Chemical
poisoning test is not definitive, since the amalgam of Hg and Analyses Service of the Universitat Autònoma de Barcelona
Pd from the NPs could prevent potential Pd molecular species using an inductively coupled plasma mass spectrometry (ICP-
from leaching. For this reason, additional experiments have MS) Agilent 7500ce model system or an inductively coupled
been performed with systems from entries 9 and 10 in Table 1 plasma optical emission spectrometry (ICP-OES) Perkin-Elmer
(Suzuki-Miyaura reaction conditions,
p-bromotoluene as the Optima 4300DV model system. Infrared spectra were run on a
haloderivative and N1 or N2 as the catalyst, respectively). First Perkin-Elmer FT spectrophotometer, series 2000, as KBr pellets
-
1
1
13
of all, TEM grids were prepared after the catalytic or polyethylene films in the range 4000-150 cm . H NMR,
C
1
experiments, observing the existence of Pd NPs (Figure S18). { H} NMR, HSQC, COSY, DOSY and NOESY spectra for all
The mean size of these NPs (2.7(0.7) and 3.3(0.6) nm for N1 compounds but C1 were recorded on Bruker AVANCE 360 and
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ARTICLE
00 NMR spectrometers in CDCl
Journal Name
4
3
solutions at room 1H, H4], 5.76 [q, 1H, 3J = 7.0 Hz, H15], 7.00, 7.23 [2br, 1H each
temperature. For C1, NMR spectroscopy experiments were one, H13, H14], 7.30 [br, 5H, H18-H22], 10.32 [s, 1H, H11].
performed on a Bruker Avance 600 spectrometer (Bruker (DMSO, 360 MHz, 298K) δ: 1.81 [d, 3H, 3J = 7.0 Hz, H16], 1.99
Biospin, Rheinstetten, Germany) equipped with TXI HCN z-grad [s, 6H, H6 and H7], 4.42 [t, 2H, 3J = 5.0 Hz, H9], 4.58 [t, 2H, 3J =
probes. The temperature for all measurements was set to 5.0 Hz, H8], 5.73 [s, 1H, H4], 5.83 [q, 1H, 3J = 7.0 Hz, H15], 7.36
2
98 K. The NMR spectroscopy experiments performed for the [br, 5H, H18-H22], 7.72, 7.92 [2br, 1H each one, H13, H14],
1
1
13
13
3
structural characterization were the standard H, C, COSY, 9.39 [s, 1H, H11]. C{ H}-NMR: (CDCl , 91 MHz, 298K) δ:
ROESY, multiplicity-edited HSQC and HMBC experiments (see 10.7,13.4 [C6 ,C7], 21.1 [C16], 47.9 [C9], 49.2 [C8], 59.9 [C15],
Supporting Information). All chemical shifts values (δ) are 105.5 [C4], 120.4, 123.0 [C13, C14], 126.8, 129.4, 137.8 [C17-
given in ppm. Electrospray ionization mass spectra (SI-MS) C22], 136.6 [C11], 140.5, 148.7 [C3, C5]. (DMSO-d6, 91 MHz,
were obtained with an Esquire 3000 ion trap mass 298K) δ: 10.2, 13.3 [C6 ,C7], 20.6 [C16], 47.2 [C9], 48.8 [C8],
spectrometer from Bruker Daltonics.
58.5 [C15], 105.0 [C4], 121.3, 123.2 [C13, C14], 126.5, 128.7,
Specimens for TEM/HR-TEM and SEM-FEG analyses were 129.0, [C17-C22], 136.0 [C11] 139.4, 146.8 [C3, C5]. IR (ATR)
-1
prepared by slow evaporation of a drop of crude colloidal cm : 3040, 2978 ν (C-H), 1552 (ν (C=C), ν (C=N))ar, 1455
solution deposited under argon onto holey carbon-covered (δ(C=C), δ (C=N))ar, 1160 δ (C-H)ip, 703 δ (C-H)oop. MS (ESI):
23 4
copper grids. TEM/HR-TEM analyses were performed at the m/z (%) 295.2 (100.0%) [C18H N Cl-Cl-].
Servei de Microscopia de la UAB with a JEOL JEM 2010 electron
microscope working at 200 kV with a resolution point of 2.5 Ǻ. 1-[2-(3,5-Dimethylpyrazol-1-yl)ethyl]-3-((S)-1-phenylethyl)-
SEM-FEG analyses were performed at the Service Commun de 3H-imidazol-2-ylidene (L). Under argon atmosphere, sodium
Microscopie Electronique de l′Université Paul Sabatier in hydride (60% dispersion in mineral oil, 124.1 mg, 3.10 mmol)
Toulouse with an MEB JSM6700F microscope. The size was washed with anhydrous pentane twice (2 x 15 mL) and the
distributions were determined via manual analysis of enlarged residual solvent was evaporated under reduced pressure.
micrographs by measuring ca. 200 particles on a given grid to Then, a catalytic amount of potassium tert-butoxide (95%, 10
obtain a statistical size distribution and a mean diameter.
mg, 0.08 mmol) and anhydrous dichloromethane (5 mL) was
Data collection for WAXS was performed at the CEMES- added. A solution of 1-[2-(3,5-dimethylpyrazol-1-yl)ethyl]-3-
CNRS (Toulouse) on small amounts of powder. All samples ((S)-1-phenylethyl)-3H-imidazol-1-ium chloride (HLCl, 513.2
were sealed in 1 mm diameter Lindemann glass capillaries. The mg, 1.55 mmol) in anhydrous dichloromethane (20 mL) was
t
measurements of the X-ray intensity scattered by the samples added to the dispersion of NaH/KO Bu. The resulting mixture
irradiated with graphite monochromatized MoKα (0.071069 was stirred overnight at room temperature. The reaction crude
nm) radiation were performed using a dedicated two-axis was then filtered through a small plug of celite and the solvent
diffractometer. The measurement time was 15 h for each was removed under reduced pressure. The product was
sample. Scattering data were corrected for polarization and obtained as a sticky dark orange solid. Yield: 333 mg, 73%.
1
absorption effects, then normalized to one Pd atom and
H-NMR: (CDCl
3
3
, 400 MHz, 298K) δ: 2.00 [s, 3H, H
or H ], 4.55 [t, 2H,
], 5.65 [q, 1H, J = 7.0
6 7
or H ],
Fourier transformed to obtain the RDFs. To make comparisons 2.02 [d, 3H, J = 7.0 Hz, H16], 2.16 [s, 3H, H
6
7
3
3
3
with the crystalline structure in real space, a model was
generated from bulk Pd parameters. The classic Debye’s Hz, H15], 5.71 [s, 1H, H
function was then used to compute intensity values, Hz, H13, H14], 7.31-7.42 [2m, 5H, H18-H22]. C{ H}-NMR: (CDCl
subsequently Fourier transformed in the same conditions than 100 MHz, 298K) δ: 10.7,13.5 [C ,C ], 21.2 [C16], 48.0 [C ], 49.2
], 120.4, 123.0 [C13, C14], 126.8, 129.5,
37.8 [C17-C22], 140.5, 148.8 [C ,C ], signal for [C11] was not
J = 5.0 Hz, H
9
], 5.01 [t, 2H, J = 5.0 Hz, H
8
3
4
], 6.77, 6.90 [2d, 1H each one, J = 2.0
1
3
1
3
,
6
7
9
the experimental ones.
8 4
[C ], 60.2 [C15], 105.6 [C
1
3
5
-1
Synthesis and characterization of the ligands
observed. IR (ATR) cm : 3033, 2960 ν(C-H), 1552 (ν(C=C),
ν(C=N))ar, 1454 (δ(C=C), δ (C=N))ar, 1015 δ (C-H)ip, 700 δ (C-
H)oop.
1
-[2-(3,5-Dimethylpyrazol-1-yl)ethyl]-3-((S)-1-phenylethyl)-
H-imidazol-1-ium chloride (HLCl) mixture of 1-(2-
3
.
A
2
chloroethyl)-3,5-dimethyl-1H-pyrazole (0.942 g, 5.94 mmol)
0
3
2
Synthesis and characterization of the Pd(II) or Ag(I) complexes
and 1-(S)-(phenylethyl)imidazole (1.023 g, 5.94 mmol) was
heated and stirred at 120
˚
C for 64 hours. After cooling to Chloro[(S)-3-(2-(3,5-dimethyl-1H-pyrazol-1-yl)ethyl)-1-(1-
room temperature, the product was dissolved in phenylethyl)-1H-imidazol-2-ylidiene)]silver(I), (CAg). A mixture
dichloromethane (5 mL) and precipitated with diethylether (50 of 1-[2-(3,5-dimethylpyrazol-1-yl)ethyl]-3-((S)-1-phenylethyl)-
mL). To lead the complete precipitation, the system was stored 3H-imidazol-1-ium chloride (HLCl, 342 mg, 1.03 mmol) and
overnight at –31°C. The product was washed with diethylether silver(I) oxide (99%, 240 mg, 1.03 mmol) in anhydrous
(
3 x 30 mL) and dried under vacuum. The product was dichloromethane (25 mL) was stirred at room temperature for
obtained as a sticky orange solid. Yield: 1.79 g, 91%. 24 hours in the dark. Then, the reaction crude was filtered
Anal. Calcd. for C18 Cl·1.5H O: C, 60.41; H, 7.32; N, through celite. The filtrate solvent was removed under vacuum
5.66. Found: C, 60.54; H, 7.27; N, 16.02. 1H-NMR: (CDCl , 360 and the product was dried completely. The product was
MHz, 298K) δ: 1.89 [s, 3H, H6 or H7], 1.92 [br, 3H, H16], 2.06 obtained as a sticky grey-brown solid. Yield: 361 mg, 80%.
s, 3H, H6 or H7], 4.44 [br, 2H, H9], 4.87 [br, 2H, H8], 5.61 [s,
H
23
N
4
2
1
3
[
8
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ARTICLE
3
Anal. Calcd. for C18
H
22AgClN
4
: C, 49.39; H, 5.07; N, 12.80.
H
7
], 2.01 [s, 3H, H
6
or H
], 5.75 [s, 1H, H
7.40 [2m, 5H, H18-H22], 7.64, 7.83 [2br, 1H each one, H13, H14],
7
], 4.38 [t, 2H, J = 5.0 Hz, H
9
], 4.54 [t,
1
3
Found: C, 49.92; H, 5.14; N, 12.47. H-NMR: (CDCl
98K) δ: 1.77 [s, 3H, H or H ], 1.81 [br, 3H, H16], 2.18 [s, 3H, H
or H ], 4.29 [br, 2H, H
3
, 360 MHz, 2H, J = 5.0 Hz, H
8
4
], 5.79 [br, 1H, H15], 7.32,
2
6
7
6
1
3
1
7
9
], 4.54 [br, 2H, H
8
], 5.68 [s, 1H, H
4
], 5.76 9.03 [s, 1H, H11]. C{ H}-NMR: (DMSO-d6, 91 MHz, 298K) δ:
,C ], 20.6 [C16], 47.1 [C ], 48.9 [C ], 58.5 [C15],
], 121.3, 123.2 [C13, C14], 126.4, 128.7, 129.0, 139.4
[
br, 1H, H15], 6.43, 6.80 [2br, 1H each one, H13, H14], 7.23, 7.32 10.2,13.3 [C
6
7
9
8
1
3
1
[
2br, 5H, H18-H22]. C{ H}-NMR: (CDCl
3
, 91 MHz, 298K) δ: 105.0 [C
], 51.9 [C ], 60.9 [C15], [C17-C22], 135.8 [C11], 139.4, 146.9 [C
], 118.5, 122.2 [C13, C14], 126.6, 128.7, 129.2, 139.7 2984 ν(C-H), 1555 (ν(C=C), ν(C=N))ar, 1453 (δ(C=C), δ(C=N))ar,
17-C22], 140.2, 148.9 [C ,C ], signal for [C11] was not 1152 δ(C-H)ip, 704 δ(C-H)oop. MS (ESI): m/z (%) 399.1
4
-1
1
0.6,13.7 [C
05.5 [C
6
,C
7
], 21.4 [C16], 49.3 [C
9
8
3 5
,C ]. IR (ATR) cm : 3091,
1
4
[C
3
-
5
1
-
observed. IR (ATR) cm : 3087, 2977 ν(C-H), 1551 (100.0%)
ν(C=C),ν(C=N))ar, 1448 (δ(C=C),δ(C=N))ar, 1219 δ(C-H)ip, 700 [C18
[C18
H
23
N
4
Cl
3
Pd-Cl -2HCl],
437.1
(26.5%)
-
Pd-Cl -HCl], 473.0 (25.7%) [C18
-
Cl Pd-Cl ].
(
H
23
N
4
Cl
3
H
23
N
4
3
-
22 4
δ(C-H)oop. MS (ESI): m/z (%) 401.1 (100.0%) [C18H N ClAg-Cl ].
Synthesis of Pd/L and Pd/HLCl nanoparticles
The general procedure for the preparation of palladium
nanoparticles is detailed through the case of [ ] / [Pd] = 0.5
Figure 2a and Scheme 1). The procedure was similar for all
other samples.
Pd (dba) ] (200 mg, 0.22 mmol) and
Dichloro[(S)-3-(2-(3,5-dimethyl-1H-pyrazol-1-yl)ethyl)-1-(1-
1
phenylethyl)-1H-imidazol-2-ylidiene)-κ N]palladium(II), (C1).
L
A mixture of
CAg (242 mg, 0.55 mmol) and dichloro(1,5-
(
cyclooctadiene)palladium(II) (158 mg, 0.55 mmol) in
dichloromethane (30 mL) was kept under vigorous stirring for
[
2
3
L (28.6 mg, 0.11
24 hours at room temperature in the dark. The reaction crude
mmol) were dissolved in a Fischer-Porter reactor in previously
degassed anhydrous dichloromethane (200 mL) under argon at
was filtered through celite. Then, the filtrate solvent was
evaporated under vacuum and the resulting solid was washed
with hexane (30 mL). The residue was purified by flash column
chromatography (silica gel 60 Å) using ethyl acetate:diethyl
1
96 K. The mixture was pressurized under 3 bar of dihydrogen
and left to room temperature under vigorous stirring. The
color of the solution turned after 1 h from purple to black. The
hydrogen pressure and the temperature were maintained for
ether 4:1 as eluent and then by
chromatography (silica gel
a
second column
Å) using
60
1
8 h. After that period of time, the colloidal solution was black
dichloromethane:hexane 9:1 as eluent. The product was
and homogeneous. The hydrogen was evacuated and a drop of
the crude colloidal solution was deposited under argon on a
holey carbon-covered copper grid using a paper filter under
the grid for TEM and SEM analysis. Then, the colloidal solution
was concentrated to ca. 10 mL. The addition of cold pentane
obtained as a pale yellow powder. Yield: 133 mg, 51%.
1
H-NMR: (CDCl
br, 3H, H16], 2.70 [s, 3H, H
H, H ], 6.27 [br, 2H, H ], 6.08, 6.55 [2br, 1H each one, H13
14], 6.89 [br, H15], 7.39, 7.62 [2br, 5H, H18-H22]. C{ H}-NMR:
CDCl , 150 MHz, 298K) δ: 10.9,14.5 [C ,C ], 19.5 [C16], 50.1
], 51.1 [C ], 59.2 [C15], 106.4 [C ], 118.5, 123.4 [C13, C14],
27.4, 128.5, 128.8 [C17-C22], 139.6, 144.1 [C , C ], signal for
11] was not observed. IR (ATR) cm : 3097, 2978 ν(C-H), 1555
ν(C=C), ν (C=N))ar, 1452 (δ(C=C), δ(C=N))ar, 1180 δ(C-H)ip,
3
, 600 MHz, 298K) δ: 1.61 [s, 3H, H
6
or H
7
], 2.02
[
6
or H ], 5.71 [s, 1H, H
7
4
], 5.87 [br,
2
9
8
,
1
3
1
H
(
20 mL) allowed the precipitation of the particles as a black
(
3
6
7
solid, which was washed with pentane (3 x 20 mL) and dried
under reduced pressure. The filtrated pentane from NPs
precipitation was slightly yellow, from dba elimination. This
was corroborated by 1H-NMR experiments of the dried
[C
9
8
4
1
3
5
-1
[C
(
pentane solution. N1 Anal. Found (wt%): Pd, 66.0; N, 5.34 for a
-1
37. IR (ATR, cm ): 3026, 2939 ν (C-H),
-
6
94 δ(C-H)oop. HRMS (ESI): m/z 399.0808 [C18
H
22
N
4
Cl
2
Pd-Cl -
stoichiometry of Pd561
L
HCl].
1
564 (ν (C=C), ν (C=N))ar, 1462 (δ(C=C), δ (C=N))ar, 700 δ (C-
H)oop. N2: Anal. Found (wt%): Pd, 83.0; N, 2.90 for a
-1
stoichiometry of Pd1415HLCl93. IR (KBr, cm ): 3019, 2960 ν (C-
Trichloro[(S)-3-(2-(3,5-dimethyl-1H-pyrazol-1-yl)ethyl)-1-(1-
1
phenylethyl)-1H-imidazol-3-ium-κ N]palladium(II), (C2).
A
H), 1563 (ν (C=C), ν (C=N))ar, 1456 (δ(C=C), δ (C=N))ar, 1137 δ
mixture
of
1-[2-(3,5-dimethylpyrazol-1-yl)ethyl]-3-((S)-1-
(C-H)ip.
phenylethyl)-3H-imidazol-1-ium chloride (HLCl, 205 mg, 0.62
mmol) and palladium(II) chloride (99,9%, 110 mg, 0.62 mmol)
in acetonitrile (20 mL) was kept under vigorous stirring for 24
hours at room temperature. Then, the solution was filtered
through celite. The solvent volume was reduced to ca. 2 mL
and diethyl ether (30 mL) was added to induce precipitation.
The orange powder was filtered and washed with diethyl ether
Crystal Structure Determination of C2
Crystallographic data for compound C2 were collected at low
temperature (180 K) on a Bruker Kappa Apex II diffractometer
using a graphite-monochromated Mo-Kα radiation (λ =
.71073 Å) and equipped with an Oxford Cryosystems
0
Cryostream Cooler Device. Crystallographic data for C2 can be
gathered in the Supporting Information, Table S1 and S2.
The structures have been solved by Direct Methods using
(
3 x 30 mL), and dried under vacuum. The product was
obtained as an orange powder. The single crystals for X-ray
diffraction were obtained by slow evaporation of
a
3
3
SIR92 , and refined by means of least-squares procedures on
concentrated acetonitrile solution of the complex at room
temperature. Yield: 222 mg, 70%.
2
34
F with the aid of the program SHELXL97 included in the
3
5
software package WinGX version 1.63. The Atomic Scattering
Factors were taken from International Tables for X-Ray
Anal. Calcd. for C18
23 4 3
H N Cl Pd: C, 42.54; H, 4.56; N, 11.02.
1
Found: C, 42.03; H, 4.62; N, 10.76. H-NMR: (DMSO-d6, 360
3
6,37
3
MHz, 298K) δ: 1.80 [d, 3H, J = 7.0 Hz, H16], 1.98 [s, 3H, H
Crystallography.
All hydrogen atoms were placed
6
or
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Journal Name
geometrically, and refined by using a riding model. All non-
L is stronger and leads to nanoparticles with well-controlled
hydrogen atoms were anisotropically refined, and in the last size.
cycles of refinement a weighting scheme was used, where All those systems have been tested in catalytic C-C coupling
weights are calculated from the following formula: reactions. For colloidal systems (N1 N2), we observed an
improving effect, in terms of chemoselectivity and yield, of
Crystallographic data for the structural analyses have been containing NPs (N1) with respect to HLCl containing NPs (N2
,
2
2
2
2
2
w=1/[
σ
(Fo )+(aP) +bP] where P=(Fo +2Fc )/3.
L
)
deposited at the Cambridge Crystallographic Data Centre, as the result of the different coordination modes of the
CCDC, with reference number 1432563 (C2). A copy of this carbene group to the surface of the NPs. Interestingly, these
information may be obtained free of charge from The Director, systems are the only ones able to achieve the C-C
CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: +44 homocoupling reaction between two molecules of
1
223336033;
e-mail:
deposit@ccdc.cam.ac.uk
or bromoarenes or the complete dehalogenation reaction of
iodoarenes. The Suzuki-Miyaura reaction is favored with the
Pd molecular complexes.
http://www.htpp://ccdc.cam.ac.uk).
Catalytic Experiments
Taking into account the advantages of the organometallic
approach and the different attributes of these catalysts, we
believe these materials could find practical uses in C-C
coupling reactions in the future.
The quantification of the catalytic reactions was carried out
in an HP5890 Hewlett Packard gas chromatograph equipped
with
a
FID detector and an HP-5 column (5%
diphenylpolysiloxane and 95% dimethylpolysiloxane). The
products obtained in the catalytic reactions were identified
using a G1800A Hewlett Packard gas chromatograph with an
Acknowledgements
electron impact ionization detector and a HP-5 column (5% Support from MINECO (CTQ2015-64261-R and CTQ2015-
diphenylpolysiloxane and 95% dimethylpolysiloxane). The 64436-P) is gratefully acknowledged. F.G.V. and J.DeT.
mass spectra of the catalytic products are in agreement with acknowledge the Universitat Autònoma de Barcelona for their
3
8,39
those published in the literature.
Suzuki-Miyaura reactions. In a two-neck round-bottom flask Program. The authors thank the Microscopy Service of the
fitted with reflux condenser and
septum, 4- Universitat Autònoma de Barcelona for technical assistance
pre-doctoral grants. J.G.-A. acknowledges Serra Húnter
a
a
halogenotoluene (2.5 mmol), phenylboronic acid (3.125 with TEM and HR-TEM. CNRS is also thanked through the
t
mmol), BuOK (5.0 mmol), and naphthalene (0.5 mmol) as LTPMM-LEA 368 action.
internal standard were dissolved in DMF/H
2
O (10 mL, 4/1).
-3
Next, the palladium organometallic complex (1.10 mmol) or
-
3
Notes and references
palladium nanoparticles (1.10 mmol Pd atoms) were added.
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6
6
[
7
[
Conclusions
A new hybrid pyrazole-imidazol-2-ylidine ligand, 1-[2-(3,5-
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0 | J. Name., 2012, 00, 1-3
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Graphical Abstract and Synopsis
Molecular and colloidal palladium systems with a new NHC ligand show different catalytic
reactivity.