Palladium catalytic systems with hybrid pyrazole ligands in C-C coupling reactions. Nanoparticles versus molecular complexes

Article abstract of DOI:10.1039/c2cy20517c

This paper reports the comparison of the chemoselectivity of two different Pd catalytic systems, namely molecular and colloidal systems, in C-C coupling reactions. For this purpose, new hybrid pyrazole derived ligands containing alkylether, alkylthioether or alkylamino moieties have been synthesized and used to form Pd(ii) complexes and to stabilize Pd nanoparticles (Pd NPs). With the aim of studying the coordination mode of the ligands and further to understand their role in catalysis, both types of Pd species were characterized by appropriate techniques. In C-C coupling reactions promoted by different Pd colloidal systems, several reports evidenced that active species are molecular catalysts leached from Pd NPs. The most important feature of this work relies on the differences observed in the output of C-C coupling reactions, depending on the colloidal or molecular nature of the catalyst employed. Thus, molecular systems carry out typical Suzuki-Miyaura cross-coupling, together with the dehalogenation of the substrate in different proportions. In contrast, Pd NPs catalyze either Suzuki-Miyaura or C-C homocoupling reactions depending on the haloderivative used. Interestingly, Pd NPs catalyze the quantitative dehalogenation of 4-iodotoluene. Differences observed in the chemoselectivity of these two catalytic systems support that reactions carried out with Pd NPs stabilized with the hybrid pyrazole ligands employed here take place on the surface of the colloids. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2cy20517c

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Palladium catalytic systems with hybrid pyrazole
ligands in C–C coupling reactions. Nanoparticles versus
molecular complexes†
Cite this: Catal. Sci. Technol., 2013,
3, 475
a
a
a
a
´
Daniel Peral, Fernando Gomez-Villarraga, Xavier Sala, Josefina Pons,
J. Carles Bayon, Josep Ros, Miguel Guerrero, Laure Vendier,bc Pierre Lecante,d
a
a
bc
´
a
bc
´
Jordi Garc´ıa-Anton* and Karine Philippot*
This paper reports the comparison of the chemoselectivity of two different Pd catalytic systems, namely
molecular and colloidal systems, in C–C coupling reactions. For this purpose, new hybrid pyrazole
derived ligands containing alkylether, alkylthioether or alkylamino moieties have been synthesized and
used to form Pd(II) complexes and to stabilize Pd nanoparticles (Pd NPs). With the aim of studying the
coordination mode of the ligands and further to understand their role in catalysis, both types of Pd
species were characterized by appropriate techniques. In C–C coupling reactions promoted by different
Pd colloidal systems, several reports evidenced that active species are molecular catalysts leached from
Pd NPs. The most important feature of this work relies on the differences observed in the output of C–C
coupling reactions, depending on the colloidal or molecular nature of the catalyst employed. Thus,
molecular systems carry out typical Suzuki–Miyaura cross-coupling, together with the dehalogenation of
the substrate in different proportions. In contrast, Pd NPs catalyze either Suzuki–Miyaura or C–C
homocoupling reactions depending on the haloderivative used. Interestingly, Pd NPs catalyze the
quantitative dehalogenation of 4-iodotoluene. Differences observed in the chemoselectivity of these
two catalytic systems support that reactions carried out with Pd NPs stabilized with the hybrid pyrazole
ligands employed here take place on the surface of the colloids.
Received 23rd July 2012,
Accepted 21st September 2012
DOI: 10.1039/c2cy20517c
www.rsc.org/catalysis
investigated in the last few years. This comes not only from
their fundamental and technological scientific interest,1–5 but
Introduction
The synthesis of nanometre-sized colloidal particles (NPs),
especially those of the noble metals, has been intensively
also from their specific properties which are clearly different
from those of the bulk metals due to their small sizes and high
surface-to-volume ratios,6 which consequently make them
interesting candidates for various applications. Among them,
catalysis has emerged in the last few years as one of the most
relevant applications of metal nanoparticles.7
It is well-known that the properties of noble metal NPs are
mainly determined by their size, shape and composition.8–10
Therefore, the application of size- and shape-control methodo-
logies for the synthesis of NPs has attracted a great deal of
attention.11–15 Among others, the decomposition of organo-
metallic precursors under appropriate conditions represents
a
´
´
`
`
Departament de Quımica, Unitat de Quımica Inorganica, Universitat Autonoma de
Barcelona, 08193, Bellaterra, Barcelona, Spain. E-mail: jordi.garciaanton@uab.es;
Fax: +34 935812477
b CNRS, LCC (Laboratoire de Chimie de Coordination du CNRS), 205, route de
Narbonne, F-31077 Toulouse, France. E-mail: Karine.Philippot@lcc-toulouse.fr;
Fax: +33 5 61553003
c
´
Universite de Toulouse, UPS, INPT, LCC, F-31077 Toulouse, France
d
´
CNRS, CEMES (Centre d’Elaboration de Materiaux et d’Etudes Structurales),
29 rue J. Marvig, F-31055 Toulouse, France
† Electronic supplementary information (ESI) available: Crystallographic data for
compounds C1–C4, complete catalytic data, HR-TEM micrographs of all Pd NPs, an efficient way to obtain well-defined metallic nanoparticles.4,16
WAX, SEM-FEG and DLS measurements for selected Pd NPs, NOESY NMR
However, the surface modification of these nanoparticles is
spectrum of N3, DOSY experiments for in situ prepared complexes
crucial to develop applications in different areas such as bio-
‘‘[Pd(0)(dba)(L1)2]’’ and ‘‘[Pd(0)(dba)(L3)]’’, ORTEP drawings of the two non-
technology or catalysis.17–21 In this respect, the research carried
equivalent molecules of C1–C3, discussion of the NMR studies for C1–C4. CCDC
out during the last decade has evidenced the importance of
understanding the surface chemistry of NPs.22–26 In particular,
836431 (C1), 836430 (C2), 836429, (C3) and 836432 (C4). For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c2cy20517c
c
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functionalized metal nanoparticles by addition of ligands have haloarenes as substrates. In order to study and compare their
been used as catalysts, whose activities and selectivities were effect on the catalytic reactivity, the same new family of hybrid
shown to be influenced by the nature of the stabilizing ligands.27–29 pyrazolic ligands was used to prepare both molecular and
Palladium plays a key role in many industrial applications, colloidal catalytic systems.
especially those related to hydrogen storage and the reduction
of pollutants.30,31 Concerning palladium nanoparticles, they
have been mainly used as catalysts in olefin hydrogenation
Results and discussion
and carbon–carbon coupling reactions that are also catalyzed
by organometallic Pd complexes and Pd heterogeneous
catalysts.32–39 The interest in nanocatalysis has considerably
increased given that nanocatalysis appears to be one of the
most promising solutions towards efficient reactions under
mild, and in some cases environmentally benign, conditions
in the context of green chemistry.40–43
Synthesis of the N-substituted pyrazole ligands (L1–L4)
The following alkylether, alkylthioether or alkylamino
N-substituted pyrazole ligands (Fig. 1) have been prepared:
3,5-dimethyl-1-[2-(octyloxy)ethyl]-1H-pyrazole (L1), 3,5-dimethyl-
1-[2-(octylthio)ethyl]-1H-pyrazole (L2), 3,5-dimethyl-1-[2-(octyl-
amino)ethyl]-1H-pyrazole (L3) and 3,5-dimethyl-1-[2-(dioctylamino)-
ethyl]-1H-pyrazole (L4). Compounds L1–L3 are here reported for
the first time, while L4 was prepared according to the synthesis
described by some of us in a previous report.78
For the synthesis of L1, 1-bromooctane, 3,5-dimethyl-1-(2-
hydroxyethyl)-1H-pyrazole79 and NaH were refluxed in THF for
12 h. Ligands L2 and L3 were prepared by the reaction of 3,5-
dimethyl-1-(2-toluene-p-sulfonyloxoethyl)-1H-pyrazole,79 sodium
hydroxide and 1-octanethiol in water for L2 or 1-octylamine
in a mixture of water–THF (1 : 1) for L3. All these ligands have
been fully characterized by standard techniques.
The Suzuki–Miyaura reaction belongs to an indispensable
set of palladium-catalyzed C–C coupling reactions and it is
nowadays one of the most important methods for the for-
mation of symmetrical and non-symmetrical biaryls,44–50 which
play an important role as organic intermediates for different
applications, including the preparation of biologically active
molecules.51–53 As far as noble metals are concerned, NPs
stabilized by polymers, micelles, and ligands have been used
as catalysts in Suzuki and other C–C coupling reactions.54–58 It
is worth noting that the introduction of ligands as NP stabi-
lizers is of special interest, because it focuses on the precise
molecular definition of the catalytic materials.59,60 In spite of
the numerous commercial nitrogen-based ligands used in
molecular palladium-catalyzed processes, these ligands have
not been extensively utilized to stabilize palladium nano-
particles. Moreover, there are very few reports on the surface
chemistry of palladium nanoparticles.61–64 In addition, certain
controversy about the nature of the catalytically active species
in Pd NP-catalyzed C–C coupling reactions has recently arisen.
Several authors have demonstrated that the catalytic activity
observed is due to molecular complexes leached from the
colloidal systems,65–69 while others have claimed that the
catalytic reactions occur on the surface of metal NPs.70–73
Furthermore, a recent report strengthens the controversy
between molecular or colloidal catalysts differentiation, as it
demonstrates the presence of up to 40% of Pd NPs in commer-
cial [Pd2(dba)3], one of the classical Pd precursors for catalytic
reactions promoted by Pd complexes.74
Synthesis of Pd(0) nanoparticles (Pd NPs)
Ligands L1–L4 were used as stabilizing agents to prepare
nanosized Pd particles (Scheme 1), according to a method
previously described.80 The synthesis of the Pd NPs was carried
out by reacting, in a Fischer–Porter reactor for 20 h and under
3 bar of dihydrogen, the chosen ligand (L1–L4) with [Pd2(dba)3]
(dba = dibenzylideneacetone) as a Pd source at the appropriate
molar ratio (ligand/Pd from 0.5 to 2).
For L1, slightly elongated particles were obtained, organized
into roughly spherical and homogeneous in size super-
structures. For all the L1/Pd molar ratios used (from 0.5 to 2),
the size of the NPs is quite similar, varying from 4.7(1.3) to
4.0(1.1) nm as observed by TEM analysis (Table 1, Fig. 2 and
Fig. S1, ESI†). These Pd NPs appear embedded in an organic
shell, which may result from a ligand excess. The size of these
superstructures varies depending on the ligand/Pd ratio
(Table 1). A size of 18(5) nm is observed for a L1/Pd ratio equal
Comparative studies on the ligand coordination chemistry
between metal nanoparticles and coordination compounds are
scarce although they could provide useful information for the
catalytic applications of palladium nanoparticles.75,76 In this con-
text, the design of efficient ligands to stabilize Pd NPs, and
furthermore the study of their influence on surface properties,
appear of high interest to develop novel applications in catalysis for
Fig. 1 Pyrazole derived ligands.
´
Pd NPs. Recently, Gomez et al. published a report that compares
the different behaviour of Pd molecular or colloidal systems in the
Suzuki–Miyaura reaction both with the same ligands, dicarboxi-
mides or alkylic amines. It is interesting to note that only Suzuki
coupling products were observed with both systems.77
Here we report a comparative study of Pd complexes and Pd
nanoparticles as catalysts in C–C coupling reactions using
Scheme 1 Synthesis of the Pd nanoparticles.
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Table 1 Stabilization of palladium nanoparticles by hybrid pyrazole derived
ligands L1–L4
Pd561(L2)24(THF)x, Pd1415(L3)149(THF)x, and Pd1415(L4)67(THF)x.
Taking into account that the ligands are situated only on the
surface of the NPs, ratios (%) of L per surface Pd atom are 24.6
(L1), 9.5 (L2), 30.3 (L3) and 13.6 (L4). It appears clearly that the
quantity of ligand at the surface of the Pd NPs varies depending
on the ligand. This can explain the differences observed by TEM
as well as the differences in the required amount of ligand to
obtain isolated NPs.
Wide-angle X-ray scattering (WAXS) measurements have
been performed on nanoparticles N1–N4 (those prepared with
L1–L4 in a ligand/Pd molar ratio equal to 1). Radial distribution
functions (RDFs) (Fig. S5, ESI†) for N1–N4 are very similar and
comparable to the function computed from a 3.5 nm face
centred cubic (fcc) model of bulk Pd. The average size of
crystalline domains is estimated to be in the 3.5–4 nm range,
Nanoparticles,
d (nm)
Superstructures,
D (nm)
L
[L]/[Pd] ratio
L1
L1
L1
L2
L2
L2
L3
L3
L3
L4
L4
L4
0.5
1.0
2.0
0.5
1.0
2.0
0.5
1.0
2.0
0.5
1.0
2.0
4.7(1.3)
4.0(1.1)
4.2(0.9)
3.4(0.9)
2.7(0.7)
2.7(0.5)
3.8(1.1)
3.5(0.9)
3.3(0.9)
3.3 and 5.7
3.5(1.0)
3.2(1.0)
18(5)
32(12)
41(10)
—
—
—
—
—
—
Layer of NPs
66(11)
—
to 0.5, which is smaller than those obtained for the L1/Pd ratio in agreement with the sizes obtained from TEM (4.0, 2.7, 3.5,
of 1 and 2, 32(12) and 41(10) nm, respectively. This difference in and 3.5 nm, for N1–N4 respectively), indicating that the NPs are
the size of the superstructure can be related to the proportion single crystals.
of extra ligand, which is not directly coordinated to the surface
Scanning electron microscopy-field emission gun (SEM-FEG)
of the particles, and thus may form an organic shell around the experiments showed a good correlation in the mean size of the
particles and give rise to the superstructures, as previously spherical superstructures (Fig. S6, ESI†) in comparison with
observed for other pyrazole-alkyl ether ligands.81 As the propor- TEM.
tion of extra ligand increases at higher ligand/metal ratios, this
could explain the larger superstructures observed for L1/metal mean size of the nano-objects in solution. Only the super-
ratios of 1 and 2. structures were observed, with a good correlation with TEM
DLS measurements82,83 were carried out to determine the
In contrast to L1, the L2 ligand allowed us to obtain isolated values (Fig. S7, ESI†). For example, for the N4 system, DLS
spherical Pd NPs, presumably due to the presence in this ligand showed a mean size of objects of 79 nm, very close to the size
of a sulfur atom with better coordinating properties toward Pd measured by TEM, 66(11) nm for the superstructures.
than the oxygen atom of L1. NPs of the same size (ca. 2.7 nm)
NMR solution studies could not be performed due to the
were obtained for L2/M ratios of 1 and 2. However, slightly poor solubility of the systems in common deuterated solvent,
larger NPs, 3.4(0.9) nm, were obtained when less ligand (L2/M except for N3. For this system, NOESY NMR studies show that
ratio 0.5) was used in the synthesis (Fig. 2 and Fig. S2, ESI†).
the ligand is coordinated to the surface of the Pd NPs (Fig. S8,
When a L3/Pd ratio of 0.5 was used, elongated worm-like ESI†).
NPs were obtained with a homogeneous mean diameter of
3.8(1.1) nm, but with a non-homogeneous length (around
Synthesis of Pd(II) complexes (C1–C4)
10 nm). A similar behaviour has been previously observed by Even if there might be differences between the coordination
some of us in Pd NP systems stabilized by hexadecylamine.80 mode of the L1–L4 ligands in molecular complexes and nano-
They also form small aggregates of 10–50 units (Fig. 2 and Fig. S3, particles (isolated atoms versus surfaces of atoms), Pd(II)
ESI†). When the L3/Pd ratio was increased to 1 or 2, isolated complexes, C1–C4 hereafter, were prepared and characterized.
spherical NPs were observed, although some of them coalesced We thus believed that the structure of these complexes, by
into elongated particles with a mean size around 3.5 nm.
showing the preferential coordination modes of L1–L4 ligands,
Finally, with L4, isolated spherical Pd NPs with an average would help to rationalize the influence of the ligands on the
size of 3.2(1.0) nm were obtained at a L4/Pd ratio of 2, although stabilization and the size control of the Pd NPs.
some of them coalesced to form aggregates with a worm shape,
Palladium complexes C1–C4 were prepared by mixing
as happened with L3. When a L4/Pd ratio equal to 1 was used, CH2Cl2 solutions of [PdCl2(CH3CN)2] with the corresponding
Pd NPs with a size of 3.5(1.0) nm were obtained. They are ligands. They were fully characterized by standard techniques,
aggregated into spherical superstructures with a size of including X-ray analyses of monocrystals obtained from
ca. 66 nm, although around these superstructures some iso- Et2O–CH2Cl2 solutions of C1–C4. The C4 complex, [PdCl2(L4)],
lated particles are present. Such superstructures were also was previously reported as a complex where L4 behaves as an
obtained at a L4/Pd ratio of 0.5, but in this case, a continuous N,N-bidentate chelating ligand.78 Concerning complexes
layer was observed. The NPs embedded in this mosaic display a C1–C3, L1 forms a [PdCl2(L1)2] complex, while L2 and L3 lead
bimodal size distribution with average sizes of ca. 3.3 nm and to a [PdCl2(L)] type complex.
5.7 nm (Fig. 2 and Fig. S4, ESI†).
In complex C1, L1 behaves as an N-monodentate ligand,
Elemental microanalyses (C, H, N and Pd) of the NPs (L/Pd coordinating the palladium centre only through the azine
ratio equal to 1; N1–N4) have been performed and are in nitrogen of the pyrazole ring, with the ether group remaining
agreement with the following stoichiometries: Pd2057(L1)158(THF)x, uncoordinated (Fig. 3 and Fig. S9, ESI†). On the other hand, in
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Fig. 2 HR-TEM micrographs and the corresponding size-histograms of Pd nanoparticles synthesized as following: (A) [L1]/[Pd] = 1.0; (B) [L2]/[Pd] = 1.0; (C) [L3]/[Pd] =
1.0; (D) [L4]/[Pd] = 1.0.
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Fig. 3 ORTEP drawing of [PdCl2(L1)2] (C1) showing all non-hydrogen atoms and
the atom numbering scheme; 50% probability amplitude displacement ellipsoids
are shown. Only one of the two non-identical molecules present in the X-ray
crystal structure is shown. Both of them are shown in the ESI.†
C2–C4, the pyrazolic ligands L2–L4 act as chelating N,Y-donors
(Y = S (C2), Y = N (C3, C4)), forming a six-membered metallo-
cycle (Fig. 4 and Fig. S10–S11, ESI†). In all cases (C1–C4), the
geometry around the palladium centre is square planar with
very small distortions. The distances and angles are in agree-
ment with those reported in the literature for similar palladium
compounds with a square planar geometry (Table S1, ESI†).84
NMR data of C1–C4 confirm that the structure of the
complexes observed in the X-ray analysis is preserved in
solution (NMR studies of C1–C4, ESI†).
Catalytic experiments
The catalytic performance of the Pd molecular complexes
(C1–C4) and the Pd NPs (N1–N4) in C–C coupling reactions
was investigated to evaluate the influence of the L1–L4 ligands
on both catalytic systems.
Catalytic studies with Pd molecular complexes
Fig. 4 ORTEP drawing of (A) [PdCl2(L2)] (C2), (B) [PdCl2(L3)] (C3), and (C)
[PdCl2(L4)] (C4) showing all non-hydrogen atoms and the atom numbering scheme;
50% probability amplitude displacement ellipsoids are shown. For complexes C2
and C3, only one of the two non-identical molecules present in the X ray crystal
structure is shown. For these complexes, both of them are shown in the ESI.†
Palladium complexes of ligands L1–L4 were evaluated as cata-
lysts for the Suzuki–Miyaura reaction (Scheme 2).85–89
Previously described C1–C4 complexes on one hand and, on
another hand, complexes formed in situ from [PdCl2(CH3CN)2]
and [Pd2(dba)3] precursors in the presence of one or two molar
equivalents of ligands L1–L4 were used as catalysts. Moreover, Table 2, while Table S3 in the ESI† collects the full set of
in the case of Pd(0) complexes, DOSY NMR90,91 experiments performed experiments.
were performed to corroborate the coordination of the ligand to
the metal centre (Fig. S12–S13, ESI†).
None of the catalysts used was able to activate 4-chloroto-
luene and, as expected, 4-iodotoluene led to better conversions
4-Halogenotoluene derivatives (4-chlorotoluene, 4-bromo- than 4-bromotoluene. In addition to the cross-coupled product
toluene or 4-iodotoluene) and phenylboronic acid were selected (4-methylbiphenyl; BT), both halogenoderivatives yielded a
as substrates in order to differentiate the cross-coupling pro- variable amount of toluene arising from the dehalogenation
duct from the homocoupling of two identical aryl moieties. In of the substrates. This was the only side product observed in a
order to optimize the catalytic reaction conditions, the effect of significant amount. The dehalogenation of aryl halides as a
the solvent, the base, and the substrate on catalyst molar ratio side reaction of the Suzuki–Miyaura coupling has been reported
was studied (Table S2, ESI†). This study led to the following before.92,93 In these cases, the hydrogen arises from a primary
t
conditions: BuOK/PhB(OH)2/halogenotoluene/Pd = 5000/3125/ or secondary alcohol used as solvent: the alkoxide coordinates
2500/1 with a [Pd] = 0.1 mM. A mixture of DMF–water (4/1) was the Pd(II) ion, suffering subsequent b-elimination. This
used since all reagents and catalysts are soluble in this media. produces the palladium hydride species, which reductively
Selected results achieved with molecular catalysts are shown in eliminates with the already coordinated aryl moiety. In our
c
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Interestingly, the dehalogenation was more favoured with
4-iodotoluene than with the bromo derivative. Therefore, even
if the preformed C1–C4 complexes presented higher conver-
sions with iodotoluene than with bromotoluene, the chemo-
selectivities in the cross-coupling products were clearly higher
with the latter. Overall outputs of the cross-coupling products
reach values close to 50% yield both with bromo- and iodo-
derivatives in the case of C3 catalyst (Table 2, entries 3 and 13,
respectively). A similar yield is achieved with C2 and iodoto-
luene (Table 2, entry 2).
When the preformed catalysts C1–C4 were used with 4-iodo-
toluene, complexes C2 and C3 yielded the best results in terms
of turnover numbers (TON in the cross-coupling reaction equal
to 1000–1200 in 6 h; Table 2 entries 2 and 3). These TON were
slightly worse than those achieved with the [PdCl2(CH3CN)2]
and [Pd2(dba)3] precursors without any pyrazolic ligand (TON
ca. 1500 in 6 h; Table 2 entries 5 and 6, respectively) which were
very fast catalysts, but also showed poor selectivity in the cross-
coupled product (ca. 50%). Surprisingly, an equimolar amount
of [PdCl2(CH3CN)2] and L3 (a mixture that should lead to C3)
rendered a better chemoselectivity (88%) than the preformed
complex and an overall output comparable to that of the ligand
free catalysts (Table 2, entry 7).
Scheme 2 Plausible catalytic cycles for the Suzuki–Miyaura coupling and
dehalogenation reactions.44,89,93,94
When 4-bromotoluene was used, conversions were low,
except in the case of complex C3 (52% conversion and TON
ca. 1200 in 6 h; Table 2, entry 13). In situ catalysts prepared from
case, DMF instead of an alcohol was employed as a solvent. ligand L3 and Pd(II) (Table 2, entry 17) or Pd(0) (Table 2, entry 19)
Nevertheless, DMF is also capable of acting as a hydrogen yielded worse results with bromotoluene than the preformed
source, as it was recently reported94 and illustrated in complex C3.
Scheme 2. Partial decomposition of DMF in basic media leads
In summary, best conversions both for iodo- and bromo-
to carbon monoxide and a Pd(II) coordinated dimethylamide, toluene were achieved with molecular catalysts prepared with
which via b-elimination generates the palladium hydride inter- ligands L2 and L3, likely because these ligands present the best
mediate. To corroborate the role of DMF in the catalytic cycle, coordinative abilities. Consequently, palladium nanoparticles
we have performed an additional experiment using 1,4-dioxane N2 and N3, stabilized with these two ligands were thoroughly
and water (4 : 1) as solvents (Table S2, ESI†) and no toluene investigated in the Suzuki–Miyaura cross-coupling reaction,
from the dehalogenation pathway was observed.
using Pd/C as a reference nanocatalyst.
Table 2 Suzuki–Miyaura reactions with palladium molecular complexesa
X = I
X = Br
Catalyst
L ([L]/[Pd])b Entry Conv.c (%) Chem. BTd (%) Yield BTe (%) Entry Conv.c (%) Chem. BTd (%) Yield BTe (%)
C1
C2
C3
C4
1
2
3
4
5
6
7
8
9
43
85
74
61
57
58
70
58
49
88
74
66
82
26
48
43
34
58
49
56
44
42
38
11
12
13
14
15
16
17
18
19
20
19
26
52
25
30
39
26
32
23
34
94
84
91
93
82
95
80
85
72
70
18
22
47
23
25
37
21
27
17
24
48
[PdCl2(CH3CN)2]
[Pd2(dba)3]
[PdCl2(CH3CN)2] L3 (1/1)
[PdCl2(CH3CN)2] L3 (2/1)
[Pd2(dba)3]
[Pd2(dba)3]
—
—
100
100
64
59
64
L3 (1/1)
L3 (2/1)
10
46
a
Reaction conditions: 1 ꢀ 10ꢁ3 mmol of Pd, 2.5 mmol of 4-halogenotoluene, 3.1 mmol of phenylboronic acid, 5.0 mmol tBuOK and 0.5 mmol of
b
naphthalene as internal standard in 8.0 mL of DMF and 2.0 mL of H2O. Temperature 100 1C. Ligand and equivalents of ligand added.
c
d
e
Conversion after 6 h reaction. Chemoselectivity in 4-methylbiphenyl (BT). Yield of 4-methylbiphenyl (BT).
c
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Catalytic studies with Pd nanocatalysts N1–N4
Regardless of the homogenous or heterogeneous nature of
the catalyst, the homocoupling of arylhalides requires a stoi-
chiometric amount of a reductive reagent. Electropositive
metals (Al, Zn, etc.), as well as organic reductants such as
(Me2N)2CQC(NMe2)2, combined with Ni or Pd catalysts, have
been used in this reaction.100 Interestingly, in many cases, DMF
is used as solvent for these reactions. Since it has been
demonstrated that DMF, in the presence of a base, can act as
a source of hydrides,94 it is possible that this solvent works as
the stoichiometric source of electrons to reduce the Pd(II) to
Pd(0) in arylhalide homocoupling, via HX reductive elimination
promoted by the base. Furthermore, a reasonable mechanism
for this reaction should involve a multimetallic catalyst, such as
a metallic nanoparticle surface, in order not to invoke Pd(IV)
species arising from double oxidative addition of ArX to a Pd(0).
In a metallic surface, two oxidative additions of ArX to two
vicinal Pd(0) will lead to two Pd(II) centers. Subsequent halide
to hydride conversion, by reaction with DMF, followed by
two-center reductive elimination of the HX and biaryl will
For this study nanoparticles N1–N4, prepared with a pyrazole
ligands/Pd metal ratio equal to 1.0, were used. Nanoparticles
stabilized by ligands L2 and L3 (N2 and N3, respectively)
yielded the best results, and those will be discussed here
(Table 3). A complete set of results for N1–N4 is collected in
Table S4 (ESI†). As in the case of molecular Pd complexes,
4-chlorotoluene was not reactive with these nanoparticles.
When 4-iodotoluene was used as a substrate with N2 and N3
catalysts (0.4 mol% of Pd), complete conversions were achieved
in 6 h and the formation of the cross-coupling product,
4-methylbiphenyl (BT), was observed as the main component
of the reaction, together with variable amounts of toluene
(Table 3, entries 1 and 2, respectively). Pd/C (Table 3, entry 3)
yielded similar chemoselectivity to N2 and N3 (ca. 65%), but
with a significant lower conversion. With the N3 nanocatalyst, a
higher load of the substrate was tested (0.04 mol%), leading to
a remarkable improvement in the reaction output (100% con-
version in 6 h and 90% chemoselectivity, Table 3, entry 4).
When 4-iodotoluene was replaced by 4-bromotoluene, N2 and regenerate the Pd(0) surface.
N3 nanocatalysts (0.4 mol% of Pd) lead to 4,40-dimethylbiphenyl
In order to confirm that the phenylboronic acid does not play
(TT), arising from the 4-bromotoluene homocoupling, as the only any role in the homocoupling reaction, this was carried out in the
biarylic product formed. Toluene was the only side product under absence of the boron reagent, leading in the case of N3 to a
these conditions, with no traces of the cross-coupled product remarkable increase in the yield of the homocoupled product (TT;
4-methylbiphenyl (BT), Table 3, entries 8 and 9, respectively. Pd 70% chemoselectivity and 110 TON in 6h, Table 3, entry 13).
catalyzed halogenoaryl homocoupling has been investigated in Interestingly, when 4-iodotoluene was used in the absence of
recent years as an alternative to the classical Ullmann reaction that phenylboronic acid, complete conversion of the halogeno-
requires stoichiometric amounts of copper and harsh conditions.95 derivative to toluene was observed (Table 3, entries 5 and 6). A
As a consequence, both palladium complexes96,97 and Pd supported recent publication by H. Fujihara et al. reports that Suzuki–
in different materials,98 as well as nanoparticles,99 have been used Miyaura coupling reactions can be catalyzed by Pd NPs at room
for this reaction. Nevertheless, none of them show enough effi- temperature.101 In this context, we have carried out the homo-
ciency for practical applications yet, because of the low turnover coupling reaction of 4-bromobenzene catalyzed by N3 at different
numbers achieved and the narrow substrate applicability.
temperatures. At 25 1C, no homocoupled product was observed,
Table 3 C–C coupling catalytic reactions with N2–N3 nanocatalysts and Pd/Ca
X = I
X = Br
[PhB(OH)2]/
[Pd]
Conv.b
(%)
BTc
(%)
TTc
(%)
PhMec
(%)
Yield
Conv.b
(%)
BTc
(%)
TTc
(%)
PhMec
(%)
Yield
Cat.
Entry
BTe (%)
Entry
TTf (%)
N2
N3
Pd/C
N3d
N2
N3
Pd/C
315
315
315
3125
—
—
—
1
2
3
4
5
6
7
100
100
31
100
100
100
100
69
61
65
90
—
—
—
—
—
—
—
—
—
1
31
39
35
69
61
20
90
0
0
0
8
9
10
11
12
13
14
12
37
84
—
18
64
83
—
—
17
—
—
—
—
14
53
46
—
24
70
41
86
47
37
—
76
30
59
2
20
39
—
4
45
34
10
100
100
99
a
t
Reaction conditions: 1 ꢀ 10ꢁ2 mmol of Pd, 2.5 mmol of 4-halogenotoluene, 5.0 mmol of BuOK and 0.5 mmol of naphthalene as internal
b
c
standard in 8.0 mL of DMF and 2.0 mL of H2O. Temperature 100 1C. Conversion after 6 h reaction. Product distribution after 6 h reaction.
d
e
f
4-Halogenotoluene/tBuOK/Pd = 2500/5000/1. Yield of 4-methylbiphenyl (BT). Yield of 4,40-dimethylbiphenyl (TT).
c
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Table 4 Homocoupling catalytic reactions with the N3 nanocatalysta
Sel. in biaryl
productc (%)
TON for biaryl
productd
Yield for biaryl
product (%)
Substrate
Entry
[Substrate]/[Pd]
Conv.b (%)
1
2
250
500
500
86
92
23
52
99
72
112
455
93
45
91
17
3e
4
5
250
500
64
29
70
46
112
67
45
13
6
7
250
500
84
49
51
44
107
108
43
22
8
9
250
500
57
46
62
61
88
140
35
28
10
11
250
500
36
21
0
0
0
0
0
0
12
13
250
500
47
20
37
41
43
41
17
8
14
15
250
500
91
90
13
10
30
45
12
9
16
17
250
500
21
15
78
71
41
53
16
11
a
t
Reaction conditions: 1 ꢀ 10ꢁ2 mmol of Pd atoms, 1 eq. BuOK versus substrate and naphthalene as internal standard in 8.0 mL of DMF and
b
c
d
2.0 mL of H2O at 100 1C. Conversion after 6 h reaction. Selectivity towards the biaryl product. Turnover number for the biaryl product.
e
100 equivalents of Hg added after 10 minutes of reaction.
and at 50 1C only 21% of biphenyl was obtained, whereas at
100 1C the reaction yielded 91% of biphenyl (Table 4, entry 2).
A study of the substrate scope of N3 nanoparticles in the
homocoupling reaction was then undertaken. Working at 0.4
In the presence of phenylboronic acid and 4-bromotoluene, and 0.2 mol% Pd, N3 was used with different substituted
the Pd/C catalyst (Table 3, entry 10) yielded a mixture of the bromoaryl derivatives (Table 4). The results indicate that con-
cross-coupled product (BT; 17%) together with the homo- versions and selectivities towards the homocoupled product
coupling one (TT; 46%) and toluene (PhMe; 37%). In the were not improved when the substrate concentration was
absence of boronic acid (Table 3, entry 14), a fair conversion doubled, except in the case of bromobenzene, for which the
was achieved with this catalyst, but toluene was this time the turnover number increased four times, since complete selectivity
major reaction product (59%). Therefore, N3 nanoparticles was achieved at 0.2 mol% Pd (Table 4, entry 2), while at 0.4 mol%
show better selectivity towards the homocoupling reaction than it was only 52% (Table 4, entry 1). Except for 4-bromotoluene
the Pd/C catalyst. The homocoupling reaction of 4-bromoto- and 3,5-bis(trifluoromethyl)bromobenzene (Table 4, entries 4/5
luene catalyzed by the molecular complex C3 in the absence of and 8/9, respectively) for which the TON decreases and
phenylboronic acid has also been tested but no homocoupled increases ca. 50%, respectively, for the rest of substrates
product was observed.
the TON of the homocoupling reaction after 6 h did not
c
482 Catal. Sci. Technol., 2013, 3, 475--489
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significantly change with the increase of substrate concen- Hg were added and the reaction was stopped after a total time
tration, roughly suggesting a zero order dependence on the sub- of 6 hours. At this point, the conversion increased to 72%,
strate concentration. Furthermore, the reaction output showed hinting that C3 acts as a real molecular catalyst.
to be very sensitive to the nature and position of the substi-
In order to gain further insight into the mechanism of the
tuents of the bromoarene. As a general trend, substitutions in homocoupling reaction catalyzed by nanoparticles, the reaction
the aryl ring reduced the TON, since bromobenzene yielded the was carried out with equimolar amounts of 4-bromotoluene
best result, suggesting that the reaction is very sensitive to and bromobenzene in the absence of phenylboronic acid.
steric constraints of the substituents. This can be attributed to Results are summarized in Table 5. In all cases, the three
an initial interaction between the phenyl ring of the bromoder- expected coupling products were obtained, namely biphenyl
ivative and the surface of the Pd nanoparticle that leads to C–Br (BB), 4-methylbiphenyl (BT) and 4,40-dimethylbiphenyl (TT), as
bond activation (oxidative addition step). Therefore, any sub- well as the dehalogenation products (benzene and toluene).
stituent, even at the para position, would have a negative effect. Also in this case, N3 nanoparticles yielded the best conversion
For similar steric hindrance, electron withdrawing groups, such (Table 5, entry 3). Interestingly, although bromobenzene dis-
as CF3, led to better results than the electron donating ones, but appeared from 1.1 to 1.7 times faster than bromotoluene,
the sterically hindered ortho-bromotrifluoromethylbenzene did similar proportions of dehalogenation products (toluene and
not produce the homocoupled product (Table 4, entries 10 benzene) were observed. This indicates that the formation of
and 11). 2-Bromopyridine showed a fair selectivity in the the arene from the metal-aryl intermediate is probably faster
homocoupled product, but conversions were low (Table 4, for the tolyl fragment than for the phenyl one. Furthermore, the
entries 16 and 17).
distribution of homocoupling products shows higher propor-
In order to determine the nature of the active catalytic tion of products containing the phenyl than tolyl fragment
species (molecular complex formed from the nanocatalyst or moieties, suggesting a faster reductive elimination step from
Pd NPs) in the homocoupling reaction, a mercury poisoning the metal surface for the phenyl group in the homocoupling
test was carried out (Table 4, entry 3).102 Specifically, 100 equiva- reaction. In summary, both the oxidative addition of the
lents of Hg were added to the homocoupling of bromobenzene haloderivative and the reductive elimination of the metal-aryl
with N3 10 min after the addition of the catalyst (0.2 mol% Pd). fragment leading to the homocoupling product seem to favour
At this point, the conversion was 23%. After 6 h, the conversion bromobenzene over bromotoluene. However the latter seems to
remained at 23%, in contrast to the yield observed for the same be faster in the dehalogenation process.
reaction performed without mercury addition (92% conversion,
Table 4, entry 2). These data are most consistent with Pd NPs observed depending on the catalytic system (Scheme 3). The
being the active species in the catalytic reaction. Suzuki–Miyaura reaction takes place with Pd molecular complexes
In conclusion, three different catalytic reactions have been
The Hg poisoning test was also performed with molecular containing L1–L4. The same reaction is also seen with Pd NPs
complexes. Specifically, we carried out the Suzuki–Miyaura stabilized with these ligands but only when 4-iodotoluene
reaction between bromobenzene and phenylboronic acid cata- is used as the substrate (Scheme 3A). A secondary reaction,
lyzed by C3. After 10 minutes, an aliquot of the reaction was taken the dehalogenation of the substrate, is always observed
and analyzed by GC (12% of conversion). Then, 100 equivalents of (Scheme 3B). Finally, the C–C homocoupling between two
Table 5 Cross-coupling catalytic reactions with N1–N4 nanocatalystsa
Catalyst
Entry
Conv. Bb (%)
Conv. Tc (%)
% Homocouplingd (%BB, %BT, %TT)
% Dehalogenatione (%PhH, %PhMe)
N1
N2
N3
N4
1
2
3
4
5
22
18
61
17
21
13
17
38
11
15
36 (44, 42, 14)
35 (31, 43, 26)
49 (43, 41, 16)
13 (46, 38, 16)
50 (38, 46, 16)
64 (52, 48)
65 (48, 52)
51 (49, 51)
87 (57, 43)
50 (54, 46)
Pd/C
a
t
Reaction conditions: 1 ꢀ 10ꢁ2 mmol of Pd atoms, 1.3 mmol of 4-bromotoluene, 1.3 mmol of bromobenzene, 5.0 mmol of BuOK, 0.5 mmol of
naphthalene as internal standard, 8.0 mL of DMF, 2.0 mL of H2O, 100 1C, 6 h. BB = biphenyl; BT = 4-methylbiphenyl; TT = 4,40-dimethylbiphenyl.
b
c
d
Conversion of benzene after 6 h reaction. Conversion of toluene after 6 h reaction. Selectivity towards homocoupling products (product
e
distribution). Selectivity towards dehalogenation products (product distribution).
c
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Specimens for TEM/HR-TEM and SEM-FEG analyses were
prepared by slow evaporation of a drop of crude colloidal
solution deposited under argon onto holey carbon-covered
copper grids. TEM/HR-TEM analyses were performed at Servei
Microscopia de la UAB using a JEOL JEM 2010 electron micro-
scope working at 200 kV with a resolution point of 2.5 Å.
SEM-FEG analyses were performed at the ‘‘Service Commun de
´
Microscopie Electronique de l’Universite Paul Sabatier’’ using a
MEB JSM6700F microscope. The size distributions were deter-
mined via manual analysis of enlarged micrographs by measuring
ca. 200 particles on a given grid to obtain a statistical size
distribution and a mean diameter.
Data collection for WAXS was performed at the CEMES-CNRS
(Toulouse) on small amounts of powder. All samples were
sealed in 1 mm diameter Lindemann glass capillaries. The
Scheme 3 Catalytic reactions.
molecules of bromoarenes is observed with colloidal catalysts measurements of the X-ray intensity scattered by the samples
N1–N4 (Scheme 3C).
irradiated with graphite monochromatized MoKa (0.071069 nm)
The different chemoselectivity observed for the reaction with radiation were performed using a dedicated two-axis diffracto-
bromoarenes between the molecular and colloidal systems meter. Measurement time was 15 h for each sample. Scattering
points out the heterogeneous nature of the catalytic species data were corrected for polarization and absorption effects, then
when using pyrazole-stabilized Pd NPs. To confirm this hypo- normalized to one Pd atom and Fourier transformed to obtain
thesis, C–C coupling reactions using [Pd2(dba)3] with or without the RDFs. To make comparisons with the crystalline structure in
free pyrazolic ligands as catalysts (the most similar species to real space, a model was generated from bulk Pd parameters. The
those that could be obtained from the leaching of molecular classic Debye’s function was then used to compute intensity
complexes from the Pd NPs) were also performed. Under these values subsequently Fourier transformed under the same condi-
conditions, these catalysts, as well as the rest of the molecular tions as the experimental ones.
catalytic systems (C1–C4), did not produce any trace of the
homocoupling product (Table 2, entries 6, 9, 10, 16, 19 and 20).
Synthesis and characterization of the ligands
3,5-Dimethyl-1-[2-(octyloxy)ethyl]-1H-pyrazole (L1). A sus-
pension of 1.08 g of sodium hydride (60%, 27.0 mmol) in
Experimental section
General procedure and reagents
5 mL of dry tetrahydrofuran was added to a solution of 2.00 g
(14.3 mmol) of 1-(2-hydroxyethyl)-3,5-dimethylpyrazole79 and
All manipulations were carried out under an argon atmosphere 2.5 mL (14.1 mmol) of 1-bromooctane in 25 mL of tetrahydro-
using a standard Schlenk tube or a Fischer–Porter reactor and furan. The solution was stirred under reflux for 12 h. After
vacuum line techniques, or in a glove-box. [Pd2(dba)3] was cooling to room temperature, 10 mL of water were added
purchased from Strem Chemicals, palladium on activated dropwise to destroy the excess NaH. The solution pH was
carbon, 10% Pd (Pd/C), was purchased from Sigma-Aldrich neutralized with diluted HCl. The solvents were then evapo-
and [PdCl2(CH3CN)2] was prepared as described in the literature.103 rated under reduced pressure. Then, 25 mL of dichloromethane
Ligand L4 and complex C478 were synthesized following were added and the insoluble NaBr was filtered off. The solvent
methodologies previously described. Solvents were purchased was evaporated and the desired product was purified by chro-
from SDS and dried through a purification machine (MBraun matography (silica gel 60 Å) using ethyl acetate as the eluent.
MB SPS-800) or distilled prior to use: THF, diethyl ether and The product was obtained as pale yellow oil.
n-hexane over sodium/benzophenone, and pentane and
L1. Yield: 36% (1.3 g). Anal. calcd for C15H28N2O: C, 71.38; H,
11.18; N, 11.10. Found: C, 71.50; H, 11.56; N, 11.09%. MS m/z
dichloromethane over calcium hydride.
Elemental analyses (C, H, and N) were carried out by the (%) = 275.1 (100%) [L1 + Na]+. IR (KBr, cmꢁ1): 2924, 2855
`
staff of Chemical Analyses Service of the Universitat Autonoma [n(C–H)al], 1554 [(n(CQC), n(CQN))ar], 1461 [(d(CQC),
de Barcelona on a Eurovector 3011 instrument. Infrared spectra d(CQN))ar], 1115 [n(C–O–C)], 773 d(C–H)oop]. 1H NMR (CDCl3
were run on a Perkin Elmer FT spectrophotometer, series solution, 360 MHz, 298 K) d: 5.76 (s, 1H, CH(pz)), 4.12 (t, 2H,
2000 cmꢁ1 as KBr pellets or polyethylene films in the range of 3J
=
5.8 Hz, NpzCH2CH2O), 3.72 (t, 2H, 3J
=
5.8 Hz,
1
3
4000–150 cmꢁ1. H NMR, 13C {1H} NMR, HSQC, COSY, DOSY NpzCH2CH2O), 3.34 (t, 2H, J = 6.6 Hz, NpzCH2CH2OCH2), 2.24
and NOESY spectra were recorded on a Bruker AVANCE360 (s, 3H, CH3(pz)), 2.21 (s, 3H, CH3(pz)), 1.48 (br, 2H,
NMR spectrometer in CDCl3 solutions at room temperature. All OCH2CH2C5H10CH3), 1.24 (br, 10H, OCH2CH2C5H10CH3), 0.87
chemical shift values (d) are given in ppm. Electrospray mass (t, 3H, 3J = 7.2 Hz, OCH2CH2C5H10CH3), ppm. 13C{1H} NMR
spectra were obtained using an Esquire 3000 ion trap mass (CDCl3 solution, 91 MHz, 298 K) d: 147.6 (pz-C), 140.1 (pz-C),
spectrometer from Bruker Daltonics.
104.9 (CH(pz)), 71.6 (NpzCH2CH2OCH2), 70.0 (NpzCH2CH2OCH2),
c
484 Catal. Sci. Technol., 2013, 3, 475--489
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48.8 (NpzCH2CH2OCH2), 32.0–26.1 (OCH2CH2C5H10CH3), 14.2 (NpzCH2CH2NHCH2), 32.0–22.4 (NHCH2CH2C5H10CH3), 14.3
(OCH2CH2C5H10CH3), 13.6, 11.3 (CH3(pz)) ppm.
(NHCH2CH2C5H10CH3), 13.6, 11.2 (CH3(pz)) ppm.
3,5-Dimethyl-1-[2-(octylthio)ethyl]-1H-pyrazole (L2). 2.8 mL
Synthesis and characterization of complexes [PdCl2(L)] (L =
of 1-mercaptooctane (98.5%, 15.9 mmol) were added to a L1 (C1); L = L2 (C2); L = L3 (C3)). A CH2Cl2 solution (10 mL) of
mixture of 5.18 g of 2-(3,5-dimethyl-1H-pyrazol-1-yl)ethyl-4- the corresponding ligand (0.096 g, 0.38 mmol for L1; 0.051 g,
methylbenzenesulfonate79 (17.6 mmol) and 0.79 g of sodium 0.19 mmol for L2; 0.048 g, 0.19 mmol for L3) was added to a
hydroxide (97%, 19.2 mmol) in 25 mL of distilled water. Then, solution of 0.050 g (0.19 mmol) of [PdCl2(CH3CN)2] in 10 mL of
the mixture was refluxed for 8 hours. After cooling to room CH2Cl2. The solution was kept under vigorous stirring for
temperature, the reaction mixture was extracted with CH2Cl2 24 hours. Then, the solvent volume was reduced to 5 mL and
(3 ꢀ 25 mL). The collected organic layers were dried with dry hexane was added dropwise to induce precipitation. Once
anhydrous sodium sulphate and removed in vacuo to give a the precipitate had formed, it was filtered off, washed twice
brown oil that was purified by chromatography (silica gel 60 Å) with 5 mL of dry hexane and dried under vacuum. The com-
using ethyl acetate as the eluent. The product was obtained as plexes were obtained as yellow-orange solids.
pale yellow oil.
C1. Yield: 93% (0.12 g). Anal. calcd for C30H56Cl2N4O2Pd: C,
L2. Yield: 49% (2.1 g). Anal. calcd for C15H28N2S: C, 67.11; H, 52.82; H, 8.27; N, 8.21. Found: C, 52.58; H, 8.15; N, 8.01%. MS
10.51; N, 10.43. Found: C, 66.82; H, 10.93; N, 10.34%. MS m/z m/z (%) = 647.3 (100%) [C1 ꢁ Cl]+. IR (KBr, cmꢁ1): 2917, 2854
(%) = 291.1 (100%) [L2 + Na]+. IR (KBr, cmꢁ1): 2924, 2855 [n(C–H)al], 1556 [(n(CQC), n(CQN))ar], 1469 [(d(CQC),
[n(C–H)al], 1555 [(n(CQC), n(CQN))ar], 1462 [(d(CQC), d(CQN))ar], 1124 [n(C–O–C)], 811 d(C–H)oop]. 1H NMR (CDCl3
d(CQN))ar], 1030 [n(C–O–C)], 775 d(C–H)oop]. 1H NMR (CDCl3 solution, 360 MHz, 298 K) d: 5.86 (s, 2H, CH(pz)), 4.99 (t, 2.4H,
solution, 360 MHz, 298 K) d: 5.78 (s, 1H, CH(pz)), 4.13 (t, 2H, 3J = 5.5 Hz, NpzCH2CH2O syn/anti), 4.92 (t, 1.6H, 3J = 5.5 Hz,
3
3J = 7.2 Hz, NpzCH2CH2S), 2.90 (t, 2H, 3J = 7.2 Hz, NpzCH2CH2S), NpzCH2CH2O syn/anti), 4.38 (t, 2.4H, J = 5.5 Hz, NpzCH2CH2O
3
2.40 (t, 2H, 3J = 7.5 Hz, NpzCH2CH2SCH2), 2.26 (s, 3H, CH3(pz)), syn/anti), 4.29 (t, 1.6H, J = 5.5 Hz, NpzCH2CH2O syn/anti), 3.41
3
2.21 (s, 3H, CH3(pz)), 1.53 (q, 2H, 3J = 7.5 Hz, SCH2CH2C5H10CH3), (t, 4H, J = 6.6 Hz, NpzCH2CH2OCH2), 2.87, 2.78, 2.31 (s, 12H,
1.25 (br, 10H, SCH2CH2C5H10CH3), 0.87 (t, 3H, 3J = 7.1 Hz, CH3(pz) syn/anti), 1.51 (br, 4H, OCH2CH2C5H10CH3), 1.24
SCH2CH2C5H10CH3), ppm. 13C{1H} NMR (CDCl3 solution, (br, 20H, OCH2CH2C5H10CH3), 0.88 (t, 3H, 3J = 7.1 Hz,
91 MHz, 298 K) d: 147.6 (pz-C), 139.2 (pz-C), 105.0 (CH(pz)), OCH2CH2C5H10CH3), ppm. 13C{1H} NMR (CDCl3 solution,
48.9 (NpzCH2CH2SCH2)), 32.5 (NpzCH2CH2SCH2), 32.3 (NpzCH2- 91 MHz, 298 K) d: 150.2, 149.7, 144.7, 144.5 (pz-C), 107.6,
CH2SCH2)), 31.9–22.8 (SCH2CH2C5H10CH3), 14.2 (SCH2CH2C5- 107.5 (CH(pz)), 71.8 (NpzCH2CH2OCH2), 69.3 (NpzCH2CH2OCH2),
H10CH3), 13.5, 11.3 (CH3(pz)) ppm.
50.2 (NpzCH2CH2OCH2), 32.0–22.8 (OCH2CH2C5H10CH3), 14.3
(OCH2CH2C5H10CH3), 15.0, 12.2 (CH3(pz)) ppm.
3,5-Dimethyl-1-[2-(octylamino)ethyl]-1H-pyrazole (L3).
A
mixture of 2.00 g of 2-(3,5-dimethyl-1H-pyrazol-1-yl)ethyl-4-
C2. Yield: 90% (0.076 g). Anal. calcd for C15H28Cl2N2PdS: C,
methylbenzenesulfonate (6.8 mmol) and 1.62 g of sodium 40.41; H, 6.33; N, 6.28. Found: C, 39.98; H, 6.20; N, 6.16%. MS
hydroxide (97%, 39.3 mmol) and 7.4 mL of 1-octylamine m/z (%) = 373.1 (100%) [C2 ꢁ HCl ꢁ Cl]+. IR (KBr, cmꢁ1): 2924,
(99%, 44.3 mmol) was refluxed for 24 hours in 80 mL of 2853 [n(C–H)al], 1554 [(n(CQC), n(CQN))ar], 1466 [(d(CQC),
distilled water : tetrahydrofuran (1 : 1). After cooling to room d(CQN))ar], 1038 [n(C–O–C)], 790 d(C–H)oop]. 1H NMR (CDCl3
temperature, the solvent was partially removed in vacuo until solution, 360 MHz, 223 K) d: 6.01 (s, 1H, CH(pz)), 5.20, 4.71
25 mL remained. Then, the mixture was extracted with CH2Cl2 (2 br, 1H each, NpzCHHCH2S), 3.48, 3.13 (2 br, 1H each,
(3 ꢀ 25 mL). The collected organic layers were dried with NpzCH2CHHS), 2.62 (s, 3H, CH3(pz)), 2.49, 2.13 (2 br, 1H each,
anhydrous sodium sulphate and removed in vacuo to give a NpzCH2CH2SCH2), 2.36 (s, 3H, CH3(pz)), 1.85 (br, 2H,
brown oil that was purified by chromatography (silica gel 60 Å) SCH2CH2C5H10CH3), 1.21 (br, 10H, SCH2CH2C5H10CH3), 0.85
using ethyl acetate : absolute ethanol (4 : 1) as the eluent. The (br, 3H, SCH2CH2C5H10CH3) ppm. 13C{1H} NMR (CDCl3
product was obtained as pale yellow oil.
solution, 91 MHz, 298 K) d: 152.9 (pz-C), 143.0 (pz-C), 108.9
L3. Yield: 41% (0.7 g). Anal. calcd for C15H29N3: C, 71.66; H, (CH(pz)), 49.9 (NpzCH2CH2SCH2)), 40.1 (NpzCH2CH2SCH2), 34.8
11.63; N, 16.71. Found: C, 71.44; H, 11.58; N, 16.26%. MS m/z (NpzCH2CH2SCH2)), 31.8–22.7 (SCH2CH2C5H10CH3), 15.4
(%) = 252.1 (100%) [L3 + H]+. IR (KBr, cmꢁ1): 3303 [n(N–H)], (CH3(pz)), 14.2 (SCH2CH2C5H10CH3), 12.4 (CH3(pz)) ppm.
2923, 2853 [n(C–H)al], 1552 [(n(CQC), n(CQN))ar], 1460
C3. Yield: 91% (0.074 g). Anal. calcd for C15H29Cl2N3Pdꢂ
[(d(CQC), d(CQN))ar], 1126 [n(C–O–C)], 773 d(C–H)oop]. 0.5H2O: C, 41.16; H, 6.91; N, 9.60. Found: C, 40.94; H, 6.73;
1H NMR (CDCl3 solution, 360 MHz, 298 K) d: 5.77 (s, 1H, N, 9.40. MS m/z (%) = 426.1 (100%) [C3 ꢁ Cl + CH3OH]+; 356.2
CH(pz)), 4.76 (t, 2H, 3J = 6.3 Hz, NpzCH2CH2NH), 3.01 (t, 2H, (30%) [C3 – HCl – Cl]+. IR (KBr, cmꢁ1): 3157 [n(N–H)], 2926,
3J = 6.3 Hz, NpzCH2CH2NH), 2.60 (t, 2H, 3J = 7.4 Hz, 2855 [n(C–H)al], 1555 [(n(CQC), n(CQN))ar], 1469 [(d(CQC),
NpzCH2CH2NHCH2), 2.23 (s, 3H, CH3(pz)), 2.20 (s, 3H, CH3(pz)), d(CQN))ar], 1149 [n(C–O–C)], 786 d(C–H)oop]. 1H NMR (CDCl3
1.83 (br, 1H, NH), 1.46 (br, 2H, NHCH2CH2C5H10CH3), 1.25 (br, solution, 360 MHz, 298 K) d: 5.92 (s, 1H, CH(pz)), 5.70, 4.32
10H, NHCH2CH2C5H10CH3), 0.87 (t, 3H, 3J
= 7.1 Hz, (2 br, 1H each, NpzCHHCH2NH), 3.51, 2.76 (2 br, 1H each,
NHCH2CH2C5H10CH3), ppm. 13C{1H} NMR (CDCl3 solution, NpzCH2CHHNH), 2.61 (s, 3H, CH3(pz)), 2.28 (s, 3H, CH3(pz)), 1.96
91 MHz, 298 K) d: 147.7 (pz-C), 139.3 (pz-C), 105.0 (CH(pz)), (br, 2H, NpzCH2CH2NHCH2), 1.59 (br, 2H, NHCH2CH2C5H10CH3),
49.8 (NpzCH2CH2NHCH2), 49.6 (NpzCH2CH2NHCH2), 48.3 1.47 (br, 1H, NH), 1.25 (br, 10H, NHCH2CH2C5H10CH3), 0.87
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(t, 3H, 3J = 7.1 Hz, NHCH2CH2C5H10CH3) ppm. 13C {1H} NMR with an electron impact ionization detector and a HP-5 column
(CDCl3 solution, 91 MHz, 298 K) d: 152.8 (pz-C), 142.0 (pz-C), 108.2 (5% diphenylpolysiloxane and 95% dimethylpolysiloxane). The
(CH(pz)), 55.3 (NpzCH2CH2NHCH2), 50.1 (NpzCH2CH2NHCH2), 49.3 mass spectra of the catalytic products are in agreement with
(NpzCH2CH2NHCH2), 31.9–22.8 (NHCH2CH2C5H10CH3), 15.1 those published in the literature.109–112
(CH3(pz)), 14.2 (NHCH2CH2C5H10CH3), 11.9 (CH3(pz)) ppm.
Suzuki–Miyaura reactions. In a two-neck round-bottom flask
Synthesis of Pd/L nanoparticles. The procedure for the fitted with a reflux condenser and septum, 4-halogenotoluene
preparation of palladium nanoparticles is hereafter illustrated (2.5 mmol), phenylboronic acid (3.125 mmol), tBuOK (5.0 mmol),
through the case of [L1]/[Pd] = 1.0 (Table 1, Fig. 2a and and naphthalene (0.5 mmol) as internal standard were dissolved
Scheme 1). The procedure was similar for all other samples.
in DMF–H2O (10 mL, 8/2). Next, the palladium organometallic
150 mg of [Pd2(dba)3] (0.16 mmol) and the chosen quantity complex (1 ꢀ 10ꢁ3 mmol) with the appropriate quantity of ligand
of L1 (41 mg for [L1]/[Pd] = 1.0) were dissolved in a Fischer– L1–L4 or palladium nanoparticles (1 ꢀ 10ꢁ2 mmol of Pd atoms)
Porter reactor in THF (150 mL) under argon and vigorous were added. The solution was stirred vigorously and heated at
stirring at 196 K. The mixture was pressurized under 3 bar of 100 1C for 6 h under nitrogen. Then, the reaction crude was
dihydrogen and left at room temperature. The colour of the cooled to room temperature and the products were extracted
solution turned after 1 h from purple to black. The hydrogen with a mixture of diethyl ether–water (20 mL, 1/1). The organic
pressure and the temperature were maintained for 20 h. After phase was analyzed by GC and GC-MS.
that period of time, the colloidal solution was black and
Homocoupling and cross-coupling reactions. In a two-neck
homogeneous. The hydrogen was evacuated and a drop of the round-bottom flask fitted with a reflux condenser and septum,
crude colloidal solution was deposited under argon on a holey 4-halogenotoluene (2.5 mmol) or 4-bromotoluene (1.25 mmol)
carbon covered copper grid using a paper filter under the grid and bromobenzene (1.25 mmol), tBuOK (5.0 mmol) and
for TEM and SEM analyses. Then, the colloidal solution was naphthalene (0.5 mmol) as internal standard were dissolved
concentrated to ca. 5 mL. Addition of cold pentane (20 mL) in DMF–H2O (10 mL, 8/2). Next, the palladium nanoparticles
allowed the precipitation of the particles as a black solid which (1 ꢀ 10ꢁ2 mmol of Pd atoms) were added. The solution was
was washed with pentane (3 ꢀ 20 mL) and dried under reduced stirred vigorously and heated at 100 1C for 6 h under nitrogen.
pressure. The filtrated pentane from NPs precipitation was Then, the reaction crude was cooled to room temperature and
slightly yellow, showing dba elimination. This was corroborated the products were extracted with
a
mixture of diethyl
1
by H-NMR experiments of the dried pentane solution.
ether–water (20 mL, 1/1). The organic phase was analyzed by
Crystal structure determination of C1–C4. Data of compounds GC and GC-MS.
C2–C4 were collected at low temperature (180 K) on a Bruker
Kappa Apex II diffractometer using graphite-monochromated
Mo-Ka radiation (l = 0.71073 Å) and equipped with an Oxford
Conclusions
Cryosystems Cryostream Cooler Device. Data of C1 were A new family of hybrid pyrazole derived ligands containing
collected at low temperature (180 K) on a Gemini Agilent alkylether, alkylthioether or alkylamino moieties has been
diffractometer using a graphite-monochromated Mo-Ka radia- successfully used to prepare Pd(II) coordination compounds
tion (l = 0.71073 Å) and equipped with an Oxford Instrument and Pd NPs which were fully characterized. These two types of
Cooler Device. Crystallographic data for C1–C4 can be gathered species display highly differentiated catalytic behaviour in C–C
from Table S5 (ESI†).
coupling reactions. Both in Suzuki–Miyaura and homocoupling
The structures have been solved by direct methods using reactions, best results were obtained with homogeneous and
SIR92,104 and refined by means of least-squares procedures on heterogeneous catalysts containing L2 and L3 ligands. This is
a F2 with the aid of the program SHELXL97105 included in the probably due to the improved coordinative properties of these
software package WinGX version 1.63.106 The atomic scattering ligands compared to L1 and L4.
factors were taken from International Tables for X-ray Crystallo-
graphy.107,108 All hydrogen atoms were placed geometrically, observed in addition to the C–C coupling reaction. In particular,
and refined by using a riding model. when 4-iodotoluene was used as the substrate with N2 or N3
In all cases, the dehalogenation of the substrate was also
All non-hydrogen atoms were anisotropically refined, and in nanoparticles, in the absence of phenylboronic acid, the dehalo-
the last cycles of refinement a weighting scheme was used, genation product was obtained in quantitative yield.
where weights are calculated from the following formula:
Interestingly, Pd complexes catalyze typical Suzuki–Miyaura
reactions, whereas Pd NPs lead either to Suzuki–Miyaura or
2
w = 1/[s2(Fo2) + (aP)2 + bP] where P = (Fo2+2Fc )/3.
CCDC reference numbers 836431 (C1), 836430 (C2), 836429, C–C homocoupling reactions depending on the substrate (iodo
(C3) and 836432 (C4).† or bromo derivatives, respectively). These results point out that
Catalytic experiments. The quantification of the catalytic Pd-hybrid pyrazole molecular or colloidal species can drive the
reactions was carried out on a HP5890 Hewlett Packard gas selectivity of the reaction to the cross-coupling, the homo-
chromatograph equipped with a FID detector and a HP-5 column coupling or the dehalogenation product with the appropriate
(5% diphenylpolysiloxane and 95% dimethylpolysiloxane).
The products obtained from the catalytic reactions were
choice of the catalyst.
The fact that the chemoselectivity of the reaction depends
identified using a G1800A Hewlett Packard gas chromatograph on the molecular or colloidal nature of the catalyst strongly
c
486 Catal. Sci. Technol., 2013, 3, 475--489
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Paper
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Acknowledgements
This work has been supported by the project CTQ2011-26440 by
´
the Ministerio de Educacion y Cultura of Spain and MERQUINSA.
`
F.G.V. and D.P also acknowledge the Universitat Autonoma de
Barcelona for their pre-doctoral grants. The authors thank the
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H.-J. Freund, Angew. Chem., Int. Ed., 2003, 42, 5240–5243.
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Microscopy Service of the Universitat Autonoma de Barcelona for
technical assistance with TEM and HR-TEM. CNRS is also
thanked through the LTPMM-LEA 368 action.
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