Palladium nanoparticles in Suzuki cross-couplings: Tapping into the potential of tris-imidazolium salts for nanoparticle stabilization

Article abstract of DOI:10.1002/adsc.201100574

Inspired by the proclivity of various palladium sources to form nanoparticles in imidazolium-based ionic liquids, we now report that tris-imidazolium salts bearing hexadecyl chains and a bridging mesitylene moiety are potent stabilizers of palladium nanoparticles efficiently prepared via a Chaudret-type hydrogenation of the bis(dibenzylideneacetone)palladium(0). The palladium nanoparticles have been isolated in pure form and characterized by ¹H nuclear magnetic resonance, transmission electron microscopy, electron diffraction and dynamic light scattering. The new materials proved effective in Suzuki cross-coupling at a loading of 0.2% palladium. Thus, using a tris-imidazolium iodide-palladium material, a series of biaryl products has been prepared starting from aryl bromides and some activated chlorides. The possibility that this catalytic activity might be due to the formation of palladium N-heterocyclic carbenes has been addressed through solid state ¹³C NMR and the synthesis of an imidazolium analogue in which the acidic 2-H was replaced with a methyl group. Copyright

Full text of DOI:10.1002/adsc.201100574

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DOI: 10.1002/adsc.201100574
Palladium Nanoparticles in Suzuki Cross-Couplings: Tapping
into the Potential of Tris-Imidazolium Salts for Nanoparticle
Stabilization
Marc Planellas,a Roser Pleixats,a,* and Alexandr Shafira,*
a
Department of Chemistry, Universitat Autꢀnoma de Barcelona, 08193 Cerdanyola del Vallꢁs, Barcelona, Spain
Fax : (+34)-93-581-2477; e-mail: roser.pleixats@uab.cat or alexandr.shafir@uab.cat
Received: July 20, 2011; Revised: November 11, 2011; Published online: February 23, 2012
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201100574.
Abstract: Inspired by the proclivity of various palla- um. Thus, using a tris-imidazolium iodide-palladium
dium sources to form nanoparticles in imidazolium- material, a series of biaryl products has been pre-
based ionic liquids, we now report that tris-imidazoli- pared starting from aryl bromides and some activat-
um salts bearing hexadecyl chains and a bridging me- ed chlorides. The possibility that this catalytic activi-
sitylene moiety are potent stabilizers of palladium ty might be due to the formation of palladium N-het-
nanoparticles efficiently prepared via a Chaudret- erocyclic carbenes has been addressed through solid
type hydrogenation of the bis(dibenzylideneacet- state 13C NMR and the synthesis of an imidazolium
ACHTUNGTRENNUNGone)palladium(0). The palladium nanoparticles have analogue in which the acidic 2-H was replaced with
been isolated in pure form and characterized by a methyl group.
1H nuclear magnetic resonance, transmission electron
microscopy, electron diffraction and dynamic light
scattering. The new materials proved effective in Keywords: imidazolium salts; nanoparticles; palladi-
Suzuki cross-coupling at a loading of 0.2% palladi- um; Suzuki–Miyaura reaction
Introduction
Imidazolium-based ionic liquids (ILs) have been
largely used in the past decade in a wide range of cat-
Transition metal nanoparticles[1] have attracted much alytic reactions as green alternatives to volatile organ-
attention in the last decade as efficient catalysts[2] due ic solvents, as a consequence of some of their peculiar
to their unique properties based on quantum size physical and chemical properties, such as negligible
effect and high surface-to-volume ratio. Generally, vapor pressure, non-flammability and high thermal
the thermodynamic propensity of nanoparticles to ag- stability.[4] However, the non-innocent nature of imi-
glomerate to bulk metal can be prevented by the use dazolium ILs as solvents has been pointed out.[5] For
of a protective agent.[3] The stabilization of NPs in so- instance, in the presence of an appropriate base, the
lution can be achieved by electrostatic and/or steric deprotonation at the C-2 position of the imidazolium
protection (electrosteric stabilization). Different types salt takes place, generating N-heterocyclic carbenes
of capping agents have been developed such as poly- capable of acting as ligands. Even under neutral con-
mers, dendrimers, b-cyclodextrins, ionic and non-ionic ditions, imidazolium halides can give rise to palladi-
surfactants, in addition to nitrogen-, phosphorus- and um-carbene complexes in the presence of PdACTHUNTRGNE(UNG OAc)2.
sulfur-based ligands as well as several ionic com- In what may (or may not) be a related phenomenon,
pounds. The nature of the protecting shield deter- in situ generation of palladium nanoparticles from
mines, to a great extent, the solubility of the resulting palladium-carbene species has been observed in palla-
metal colloid. Moreover, in the case of catalytic appli- dium-catalyzed Heck reactions performed in ionic liq-
cations, the activity and selectivity of the nanocatalyst uids.[5b,6]
will depend not only on the relative abundance of dif- Dispersions of transition metal NPs (Pd, Pt, Rh,
ferent types of active sites but also on the concentra- Ru, Ir) in ILs have been prepared by reduction of
tion and type of stabilizers present in the medium.
metal salts or decomposition of organometallic com-
pounds in the zero oxidation state dissolved in the
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imidazolium-based ILs.[7] These dispersions have been especially interested in applying such nanocatalysts to
used directly in hydrogenation reactions of alkenes, the coupling of aryl bromides and chlorides, given
arenes and ketones[4e,8] and in carbon-carbon cross- that these are less reactive, but more affordable than
coupling reactions in the case of Pd NPs,[9] with the the aryl iodides.
aim of recycling the catalytic system. It is likely that
Following from studies on the preparation of a vari-
Pd NPs act as a reservoir for catalytically active mo- ety of Pd NP types[13] for use in C C bond forming re-
lecular species leached into the medium. A drawback actions, we report herein the synthesis of palladium
to using imidazolium-based ILs in certain palladium- nanoparticles stabilized by tris-imidazolium iodides
À
À
catalyzed C C bond forming reactions is the fact that and tetrafluoroborates and their catalytic activity in
salts formed stoichiometrically as by-products remain the Suzuki–Miyaura cross-coupling[14] reaction, in
in the IL layer and accumulate in successive runs, light of the importance of this transformation for the
having a deleterious effect on the effective recovery production of biaryls in the pharmaceutical, agro-
and reuse of the catalyst. Other detected problems chemical and fine chemical industries.[15]
are the leaching of palladium from the ionic phase
into the organic phase and the aggregation of Pd NPs
upon recycling (Ostwald ripening). The addition[10] of Results and Discussion
another external capping agent (a polymer, a ligand
or a support for the NPs immobilization) or, alterna- Tris-Imidazolium-Stabilized Pd Nanoparticles
tively, the presence[10h,11] of a coordinating group in
the imidazolium salt (such as cyano, amino, hydroxy, We followed our earlier reported procedure[12] to syn-
mercapto and carboxyl) is most often needed to en- thesize the tris-imidazolium salts 2a and 3a from the
hance the longevity of metal NPs in an IL medium.
Our interest in this topic stems from the recent
C3-symmetric tris-imidazole 1a (Scheme 1).
The alkylation of 1a[16] with 3 equivalents of 1-iodo-
work in our group on the preparation of mesitylene- n-hexadecane in refluxing acetonitrile afforded the
containing bis- and tris-imidazolium salts (with IÀ and tris-iodide 2a in 90% yield; subsequent anion meta-
À
BF4 counterions) and their use as ionic liquid crys- thesis with excess NaBF4 in ethanol/water mixture af-
tals.[12] These species bear long hexadecyl chains and, forded the corresponding tris-tetrafluoroborate 3a in
contrarily to imidazolium-based ionic liquids, are 77% yield. Although the anion exchange proved to
solids at room temperature. We reasoned that their be quite efficient, this treatment with NaBF4 was re-
particular structural features should render them effi- peated twice again to ensure complete removal of po-
cient in the stabilization of metal nanoparticles (elec- tential traces of iodide (see the Supporting Informa-
trosteric and steric stabilization), and that they could tion). The synthesis of both the iodide and the tetra-
be added in smaller amounts than the high-cost imi- fluoroborate salts (2a and 3a) was undertaken so as to
dazolium-based ILs. We also envisioned that the re- gauge the counterion effect on nanoparticle synthesis
sulting metal NPs could be isolated, characterized and and on their properties, including the catalytic activi-
then used in catalysis in a solvent of choice. We were ty. Additionally, the tris-iodide salt 2b bearing 2-Me
Scheme 1. Synthesis of the stabilizers 2a, 2b and 3a.
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Palladium Nanoparticles in Suzuki Cross-Couplings
Table 1. Preparation of Pd nanoparticles via hydrogenation of PdACHTNUTGRNEUNG
(dba)2.[a]
Entry
L (mmol)
mmol Pd[b]
% Pd[c]
Yield [%][d]
Ø [nm][e]
1
2
3
2a (0.423)
3a (0.425)
2b (0.051)
0.423
0.425
0.051
14.4
8.1
5.8
82
75
67
4.2Æ0.5
2.9Æ0.5
4.2Æ0.9
[a]
[Pd]=0.007 mol/L.
[b]
[c]
[d]
[e]
Based on % Pd in Pd
By ICP.
ACHTUNGRTEN(NUNG dba)2 determined by ICP.
[(mmol Pd in nanoparticles)/(initial Pd)]ꢃ100%.
Mean diameter determined by TEM.
imidazolium substituents was prepared from the me-
thylated tris-imidazole 1b (Scheme 1, Supporting In-
formation.)
With compounds 2a, 2b and 3a in hand, we tested
the ability of the tris-imidazolium salts to stabilize
palladium nanoparticles. Although chemical reduction
of Pd(II) salts remains the most widely used method
for the synthesis of Pd(0) nanoparticles,[1] a careful
and laborious search for optimal reaction conditions
is often necessary. In the present case, we were also
interested in avoiding basic conditions and a Pd(II)
precursor, which could give rise to the formation of
carbene species. The organometallic approach, con-
sisting in the reduction and subsequent displacement
of a ligand from an M(0) organometallic precursor,
developed by Chaudret and co-workers[17] offers
a more straightforward and reliable alternative for
Figure 1. ACTHNUTRGNEUG(N Top) HR-TEM image and particle size distribu-
the controlled synthesis of metal nanoparticles with tion diagram for 2a-Pd; (bottom) electron diffraction pattern
for 2a-Pd.
narrow size distribution. In fact, the generation of
metal nanoparticle dispersions in ionic liquids using
this approach has been reported.[18] In collaboration core of the nanoparticles, dynamic light scattering
with Chaudret and Philippot, we have previously suc- (DLS) was used to estimate the total diameter in so-
ceeded in the preparation of ruthenium[19] and plati- lution. This was found to be Ø=10.6Æ2.1 nm for 2a-
num[20] nanoparticles in the presence of heavily fluori- Pd, 7.7Æ1.9 nm for 2b-Pd and 13.0Æ3.6 nm for 3a-Pd
nated protecting agents. We, therefore, opted for this (see Figure 2). In all cases, the resulting homogeneous
methodology, which, in the case of palladium, consists black suspension was filtered through a nylon mem-
[21]
in hydrogenating Pd
G
in the presence of the brane filter (Milli-Pore filter, 0.45 mm) to eliminate re-
appropriate stabilizer. This process was carried out in sidual palladium black, the solvent was removed
the presence of salts 2a, 3a and 2b overnight in under vacuum and the black residue was washed with
a stirred THF solution at 3 bar H2. The results are diethyl ether to eliminate the 1,5-diphenylpentan-3-
shown in Table 1.
one side-product. The insoluble residue was further
In all three cases, a Pd:L ratio of 1:1 and a [Pd] of purified by washings with methanol and additional
about 0.007M were used to obtain palladium nano- portions of diethyl ether. After centrifugation and de-
particles of a mean diameter of 2.9–4.2 nm (deter- cantation, the resulting black solid was examined by
mined by TEM, see Figure 1) in good yields. Given high resolution transmission electron microscopy
that TEM in our case is only sensitive to the metal (HR-TEM) (Figure 1). In both cases, the presence of
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analogous bis-imidazolium salts previously reported
by our group.[12]
The counterion effect was also evident in the ap-
1
pearance of the H NMR spectra of the iodide salts
2a and 2b and the tetrafluoroborate 3a (see the Sup-
1
porting Information). While the H NMR spectrum of
the tris-tetrafluoroborate 3a-Pd was very similar to
that of the free stabilizer, the spectra of the tris-io-
dides 2a-Pd and 2b-Pd showed a complex spectral
pattern with broad and ill-defined signals, most pro-
nounced in the region corresponding to the imidazoli-
um ring (7–10 ppm). In general, this significant altera-
tion of the stabilizer NMR signals in metal nanoparti-
cles (with respect to the free stabilizer) is in line with
a stronger stabilizer-nanoparticles interaction.[23] Thus,
1
the broadened H NMR signals in the tris-iodide salts
might suggest a much greater interaction of the imida-
zolium cations with the metal surface than that found
in the tetrafluoroborate 3a-Pd. Initially, the possibility
Figure 2. Dynamic light scattering size distribution (nm) for
a) 2a-Pd Ø=10.6Æ2.1; b) 3a-Pd Ø=13.0Æ3.6. Logarithmic
scale is used.
À
of a strong interaction between the BF4 anion and
the nanoparticles surface was tested by NMR. How-
the corresponding stabilizer was confirmed via IR and ever, we found that the 11B NMR signals appear at
1H NMR and the metal content in the samples was the same chemical shift in both the 3a and 3a-Pd;
determined by inductively coupled plasma (ICP). similarly, the 19F NMR spectra of both 3a and 3a-Pd
Electron diffraction (ED) confirmed the presence of show the same two isotopomeric signals (10B, I=3,
palladium(0) with a face-centered cubic (fcc) struc- 19% natural abundance and 11B, I =3/2, 81% natural
ture (Figure 1). The synthesis method was found to be abundance). A possible insight into the exceptional
reproducible in all cases. The newly obtained nano- behavior of material 2a-Pd was glimpsed from the
materials were soluble in toluene, THF, CH2Cl2 and solid state (CP-MAS) 13C NMR spectra of the stabil-
CHCl3, and insoluble in hexane, diethyl ether, metha- izers 2a and 3a and the corresponding Pd nanoparti-
nol and ethyl acetate. The tris-iodide 2a was some- cles 2a-Pd and 3a-Pd (Figure 3). While no important
what more efficient as a stabilizer, giving 82% yield change in the carbon resonances was observed in
of 2a-Pd, as opposed to 75% for tris-BF4 3a-Pd.[22] It going from 3a to 3a-Pd, a significant difference was
was also found that the methylated salt 2b was less ef- indeed observed between 2a and 2a-Pd. The change
ficient than the 2-H derivative 2a, giving a 67% yield in 2a-Pd consisted in a considerable reduction of the
of 2b-Pd. The presence of three imidazolium rings C-2 carbon peak, accompanied by the appearance of
was beneficial in nanoparticles stabilization, with a very broad resonance at ~167 ppm, consistent with
lower yields of Pd nanoparticles obtained when using the formation of an N-heterocyclic carbene (NHC)-
Figure 3. Comparison of the 13C CP-MAS solid state NMR spectra of the stabilizers 2a and 3a with their corresponding pal-
ladium nanoparticle materials.
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Palladium Nanoparticles in Suzuki Cross-Couplings
Pd interaction.[24] We, therefore, conclude that due to ganic and inorganic bases showed that potassium hy-
the Àenhanced basicity of IÀ, as compared to that of droxide, the acetate salts and triethylamine were inef-
BF4 , the formation of a surface carbene is more fa- ficient (entries 1–4) and that the process could be im-
vored in the 2a than in 3a.
proved somewhat by employing potassium phosphate,
cesium fluoride and even sodium bicarbonate (en-
tries 5–7). Finally, excellent yields of 6 were achieved
using the carbonate bases (entries 8–10).
Use of 2-Pd in Suzuki–Miyaura Cross-Coupling
Given that highest yields were achieved using anhy-
Material 2a-Pd was chosen for development of the drous K2CO3, we went on to examine the perfor-
coupling method, with the catalytic activity of these mance of this carbonate base in several additional sol-
nanoparticles first being tested in a model Suzuki vent systems (Table 3 and the Supporting Informa-
cross-coupling between p-bromoacetophenone, 4, and tion). It was found that replacing THF with toluene in
phenylboronic acid, 5, to give p-acetylbiphenyl, 6. The the solvent mixture allowed for the reaction time to
tests were conducted using a 0.2 mol% loading of pal- be reduced from 24 h to just 30 min (Table 3, entries 1
ladium, 2 equivalents of base and an excess of the and 2). Toluene alone led to a slower reaction, al-
boronic acid (1.5 equivalents). First, we sought to de- though in this case raising the temperature to 1308C
termine the optimal base to use in the reaction. Based once again allowed the coupling to be complete in
on some previous results, our initial base screen was 30 min (Table 3, entries 3 and 4). A TEM analysis of
performed in an 80:20 THF:H2O solvent mixture at the Pd NP material isolated after the reaction (from
908C for 15–24 h (Table 2). The test of a series of or- Table 3, entry 2, see Supporting Information) revealed
Table 2. Initial base screen in a model Suzuki cross-coupling catalyzed by nanoparticles 2a-Pd.[a]
Entry
Base
Yield [%] of 6[b]
Entry
Base
Yield [%] of 6[b]
1
2
3
4
5
KOH
<3
35
42
8
6
7
8
9
CsF
55
62
93
98
96
NaOAc
CsOAc
NBu3
NaHCO3
Na2CO3
K2CO3
Cs2CO3
K3PO4
54
10
[a]
In closed vessels (40 mL) in a multireactor using 2.5 mmol of 4, 3.75 mmol of 5 (1.5 equiv.), 5 mmol base (2 equiv.), in
2.5 mL of solvent; reaction times not optimized.
GC yields corrected to internal n-C11H24.
[b]
Table 3. Solvent effect in the coupling of 4 and 5.[a]
Entry
Solvent
Temperature T [oC]
Time t [h]
Yield [%] of 6
1
2
3
THF-H2O (80:20)
toluene-H2O (80:20)
toluene
toluene
toluene-H2O (80:20)
toluene-H2O (80:20)
90
90
90
130
90
90
24
0.5
22
0.5
0.5
24
98
95 (85)[b,c]
80
95
46
94
4
5[d]
6[d]
[a]
As in Table 2 (see Experimental Section and Supporting Information).
Isolated yield.
A second cycle performed with this material required 7 h for complete conversion.
Using 3a-Pd.
[b]
[c]
[d]
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Marc Planellas et al.
Table 4. Suzuki cross-coupling between p-chloroacetophenone (7) and phenylboronic acid (5), catalyzed by 2a-Pd.[a]
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Entry
Base
Solvent
Temperature T [8C]
Time t [h]
Yield [%] of 6[b]
1
2
3
4
5
6
7
8
9
K2CO3
K2CO3
KOH
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
toluene
toluene
toluene
toluene-H2O (90:10)
toluene-H2O (80:20)
toluene-H2O (80:20)
toluene-H2O (70:30)
THF-H2O (80:20)
90
46
26
26
26
16
40
16
26
26
6
14
1
51
130
130
130
130
130
130
130
130
70
92 (81)[c]
63
5
<5
[BDMIM]
G
[a]
In closed vessels (40 mL) in a multireactor using 2.5 mmol of 7, 3.75 mmol of 5 (1.5 equiv.), 5 mmol of base (2 equiv.) in
2.5 mL of solvent.
GC yields corrected to internal n-C11H24.
Isolated yield in parenthesis.
[b]
[c]
nanoparticles that were smaller and more closely ag-
In most cases, the formation of 6 from both the
gregated than the original 2a-Pd. Consequently, the bromide 4 and chloride 7 was accompanied by small
use of this recycled material in the model Suzuki amounts of biphenyl (1–5% by GC) as a result of the
cross-coupling resulted in a somewhat longer time to homocoupling of the boronic acid 5. Only trace
reach completion (7 h).
Interestingly, under the conditions optimized for product could be detected in concentrated samples.
the tris-iodide 2a-Pd (80:20 toluene:H2O, Table 3, The protocol optimized for 2a-Pd NPs was then ex-
amounts of acetophenone, a dehydrohalogenation
entry 2), the corresponding tris-tetrafluoroborate ma- tended to the Suzuki coupling of a range of aryl bro-
terial 3a-Pd only gave a 46% yield of the desired mides and chlorides with several aryl- and heteroaryl-
product (Table 3, entry 5). In this latter case, a longer boronic acids. The results are compiled in Table 5.
reaction time (24 h) did lead to the complete conver- Unless otherwise stated, all reactions were carried out
sion of the starting material (Table 3, entry 6).
using 0.2 mol% of Pd and the reagent molar ratio
Next, we turned our attention to the coupling be- ArX/Ar’B(OH)2/K2CO3 of 1:1.5:2 using toluene-H2O
tween the more challenging p-chloroacetophenone, 7, (80:20) as the solvent at 908C.
and phenylboronic acid, 5 (Table 4). Attempts to
In testing the substrate scope, the catalyst 2a-Pd
carry out the reaction in toluene using K2CO3 only proved to be tolerant of a range of functional groups.
gave small amounts of the coupling products (en- Using phenylboronic acid 5, good yields were ob-
tries 1 and 2), and no improvement was observed by tained in the coupling of aryl bromides bearing alde-
employing a stronger base like KOH (entry 3). As hyde (entry 1), ketone (entries 2 and 3) as well as al-
was the case for the reactions with the aryl bromide cohol groups (entries 4 and 5), including the ortho-
4, significant improvement was achieved by using tol- substituted bromoindanol 11. The catalyst was also
uene-water as solvent mixtures (entries 4–7). Thus, tolerant of aromatic OH and NH2 substituents (en-
the best results were achieved, once again, using tries 6 and 7). As expected, an excellent yield was
a 80:20 toluene:water system, with a 70% yield being reached using the electron-rich boronic acid 18
reached after 16 h, and a 92% yield after 40 h at (entry 8). Coupling was more difficult in the case of
1308C (entries 5 and 6). The coupling proved ex- heterocyclic boronic acids (entries 9–11) due to ram-
tremely sluggish in THF:H2O (entry 8). Finally, we pant protodeboronation. Still, in some cases potassi-
were curious to see whether ionic liquids could serve um trifluoroborate salt could be used as an alternative
as suitable solvents for our imidazolium-based cata- to the boronic acid to obtain the coupling products
lyst. Disappointingly, the use of 1-butyl-2,3-dimethyl- (31 and 32, entries 10 and 11). Next, the coupling of
ACHTUNGTRENNUNGimidazolium tetrafluoroborate [BDMIM]ACHTUNTGERN[NUGN BF4] failed aryl chlorides was investigated. While in this case the
to lead to an active catalyst system (entry 9). This reaction was much more sluggish, activated ArCl did
lack of activity was likely the result of the observed indeed afford the desired biaryls albeit in modest
complete immiscibility of the catalyst with this ionic yields (entries 12 and 13). Still, in the case of the sub-
liquid.
strate 17 bearing two nitro groups an 85% yield of
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Palladium Nanoparticles in Suzuki Cross-Couplings
Table 5. Suzuki cross-coupling between aryl bromides and chlorides and aryl (heteroaryl) boronic acids catalyzed by 2a-Pd.[a]
Entry
1
ArX
Ar’B(OH)2
Ar-Ar’
Yield [%][b]
80
2
3
4
79
76
85
5
6
7
65
62
89
8
90
9
68
10
11[d]
39[c]
24[c]
47
12[d]
13[d]
24
85
14
[a]
In closed vessels (10 mL) using 2.5 mmol of ArX, 1.5 equiv. of Ar’B(OH)2, 2 equiv. K2CO3 in 2.5 mL of a 4:1 toluene-
H2O at 908C.
Isolated yield.
Using 1.2 equiv. HetBF3K.
At 1108C.
[b]
[c]
[d]
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Marc Planellas et al.
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Scheme 2. Double arylation of dibromoarene substrates
using 2a-Pd. Reaction conditions: as in Table 5.
the desired biaryl (35) was obtained after 3 h at 908C
(entry 14). Double arylation of dibromoarenes was
also possible under the same reaction conditions to
give products 36 and 37 in good yields (Scheme 2).
Additionally, a successful one-pot preparation of
the asymmetrically substituted terphenyl 40 was
Figure 4. Potential formation of Pd-NHC system from 2a.
achieved (Scheme 3) in an overall 60% isolated yield catalytic activity of imidazolium-stabilized nanoparti-
by selective monoarylation of 4-bromoiodobenzene, cles, their involvement in the case of the more chal-
38, with one equivalent of benzeneboronic acid at lenging substrates cannot be ruled out.
408C (intermediate 39), followed by the addition of
In fact, the potential of the tris-imidazolium salts to
one equivalent of 4-methoxybenzeneboronic acid. We act as both NP stabilizers and as ligands for soluble
found that the order of addition of areneboronic acids palladium carbene complexes may be relevant for the
was quite important, since when this order was re- mode of catalyst action. Specifically, this raises the
versed, the corresponding intermediate 4-bromo-4’- possibility that the nanoparticles act as reservoirs for
methoxy-1,1’-biphenyl reacted sluggishly in the soluble palladium species such as NHC-Pd.[26] We,
Suzuki coupling with benzeneboronic acid under the therefore, tested the catalytic activity of a mixture of
same conditions.
each of the stabilizers with PdACTHNGUTERNNU(G OAc)2 (Scheme 5). We
At this point, we wondered whether the active cata- found that for a coupling of p-bromoacetophenone 4
lyst might involve Pd-carbene species derived from with the phenylboronic acid 5, simply heating the re-
the imidazolium moieties. The question was addressed agent mixture in toluene:H2O (4:1, 908C) gives quan-
by testing the catalytic performance of palladium titative yields of 6 after only 15 min. In contrast, this
nanoparticles stabilized by compound 2b bearing an procedure failed for all three ligands when p-chloro-
Me group at the heterocyclic C-2 and in which the
formation of carbenoid would be disfavored catalyst activation, the coupling of 7 was then carried
(Table 1, Figure 4).[25]
out by first premixing the ligands with PdACTUHNGTRNENU(G OAc)2 in
ACHTUNGTRENaNUNG cetophenone 7 was used. In an attempt to favor the
a
Thus, the catalytic performance of 2b-Pd was com- DMSO (608C to 1108C).[27] In this case, while this
pared to that of 2a-Pd in the coupling between 4- procedure did afford an active catalyst using 2a (80%
bromo- and 4-chloroacetophenones 4 and 7 and phe- of 6 after 15 h), the method failed for 2b and 3a.
nylboronic acid 5 (Scheme 4). While the performance
The results described here likely suggest multiple
of both catalysts was similar in the case of the aryl ways in which imidazolium-based palladium species
bromide, the coupling of more challenging aryl chlo- may function as catalyst for Suzuki cross-coupling.
ride was only possible with the non-methylated stabil- We speculate that in the case of aryl bromides, the
izer 2a. Thus, it can be concluded that although the coupling may take place both at the nanoparticle sur-
presence of Pd-NHC species is not necessary for the face or at the leached palladium species with or with-
Scheme 3. Preparation of 4-methoxy-1,1’:4’,1’’-terphenyl.
658
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Adv. Synth. Catal. 2012, 354, 651 – 662

Palladium Nanoparticles in Suzuki Cross-Couplings
Scheme 4. Comparison of 2a-Pd and 2b-Pd as catalyst.
Scheme 5. Testing the in-situ formed imidazolium-derived catalysts in Suzuki cross-coupling.
out the intervention of the imidazolium stabilizer/ room temperature and were fully characterized. The
ligand. TEM analysis of the catalytic nanomaterial tris-iodide 2a-Pd proved to be an efficient catalyst for
after the first run (see Table 3, entry 2 and the Sup- the Suzuki cross-coupling reaction of aryl bromides
porting Information) points towards a dissolution pro- and activated aryl chlorides at a 0.2 mol% Pd loading.
cess. This conclusion is supported by the ability of 2b- The catalyst proved tolerant of a range of functional
based palladium systems, where carbene formation is groups, both for the aryl halides and the boronic acid
unlikely,[25] to promote the coupling of 4. On the coupling partners. Catalytic tests with a new 2-Me de-
other hand, in the case of the more challenging ArCl rivative 2b-Pd and with premixed combinations of
7, the fact that only 2a gives an active catalyst indi- PdACHTUNRGTNEUNG(OAc)2 with imidazolium salts suggest the forma-
cates that the coupling in this case takes place at the tion of Pd-NHC species in the 2-H imidazolium salt
Pd-NHC centers, and that the nanoparticles may then 2a-Pd. The solid state 13C CP MAS NMR spectrum of
serve as a reservoir for active catalytic species. In this 2a-Pd is also consistent with the presence of surface-
context, the decreased competence of the tetrafluoro- bound NHC carbenes.
borate 3a in both NP stabilization and catalysis will
be the subject of future work.
Experimental Section
Conclusions
General Remarks
Water Milli-Q was used in the preparation and purification
of compounds. THF was purchased from Scharlau and dried
over sodium/benzophenone and distilled prior to use. Cata-
lytic runs with magnetic stirring were carried out in a Rad-
leys Discovery System equipped with 12 sealed 40 mL glass
The previously reported tris-imidazolium salts 2a
(iodide) and 3a (tetrafluoroborate) were used as sta-
bilizers for palladium nanoparticles 2a-Pd and 3a-Pd.
Both materials were prepared in an efficient manner
via the hydrogenation of PdACHTUNGRTENUNG(dba)2 at 3 atm H2 at vessels. All NMR measurements were carried out at the
Adv. Synth. Catal. 2012, 354, 651 – 662
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
659

Marc Planellas et al.
FULL PAPERS
Servei de Ressonꢀncia Magnꢁtica Nuclear of the Universitat
Autꢂnoma de Barcelona. NMR spectra were recorded on
sodium sulfate and filtered. Solvent was partially removed
under vaccuum and excess diethyl ether was added to the
residue. A white powder precipitated, which was filtered
and washed with diethyl ether. It was recrystallized in
CH2Cl2-diethyl ether, affording the desired compound;
1
Bruker AC250 (250 MHz for H) or Avance360 (360 MHz
1
for H) spectrometers. Infrared spectra were recorded using
a Bruker Tensor 27 instrument equipped with an ATR
Golden Gate cell and a diamond window. ICP measure-
ments of palladium contents were carried out at the Servei
d’Anꢀlisi of the Universitat Autꢂnoma de Barcelona using
a Perkin–Elmer model Optima 4300 ICP-OES spectrometer.
Transmission electron microscopy (TEM) analyses were per-
formed at the Servei de Microscꢂpia of the Universitat Au-
tꢂnoma de Barcelona, using a JEOL JEM-2010 model at 200
kV. The TEM measurements were made by sonication of
the nanoparticulate material in THF for few minutes; then
a specially produced structureless carbon support film, wich
has a thickness of 4–6 nm, was placed in the solution for
a few seconds and dried before observation. The size distri-
butions were determined through a manual analysis of en-
larged micrographs by measuring a high number of particles
on a given grid to obtain a statistical size distribution and
a mean diameter (Gatan Digital Micrograph Program). Dy-
namic light scattering (DLS) measurements were carried
out at the ICN/CIN2 center (Bellaterra, Spain) in toluene
solution using a quartz cuvette on a Malvern ZetaSizer
Nano ZS instrument operating with a light source wave-
length of 532 nm and a fixed scattering angle of 1738 for
DLS measurements. A volume of 0.8 mL of the colloidal so-
lution of NPs was placed into a specific cuvette and the soft-
ware was arranged with the specific parameters of refractive
index and absorption coefficient of the material and the vis-
cosity of the solvent. Gas chromatographic analysis was ac-
complished using an Agilent Technologies 7890 A GC
system equipped with an Agilent HP-5 column (30 mꢃ
0.320 mmꢃ0.25 mm). The integrated areas and peak posi-
tions in the chromatograms were referenced internally to
undecane (Table 2, Table 3 and Table 4). Mass spectral anal-
yses of the Suzuki products were carried out using an Agi-
lent 6850 GC system equipped with a 5975C Triple Axis EI
Mass Detector. Melting points were determined using
a Reichert brand melting point apparatus and are uncorrect-
1
yield: 1.48 g (73%). H NMR (250 MHz, CDCl3): d=2.22 (s,
3H, imid-CH3), 2.49 (s, 3H, Ar-CH3), 5.04 (s, 2H, -CH2-),
6.20 (d, J=1 Hz, 1H, Himid), 6.82 (d, J=1 Hz, 1H, Himid);
13C NMR (62.5 MHz, CDCl3): d=13.5, 16.2, 44.9, 117.2,
127.5, 131.6, 138.9, 144.6; HR-MS (ESI): m/z=403.2605,
calcd. for C24H30N6 [M]+: 403.2605.
Synthesis of 1,1’,1’’-[(2,4,6-Trimethylbenzene-1,3,5-
triyl)tris(methylene)]tris(3-hexadecyl-2-methyl-1H-
imidazol-3-ium) Iodide (2b)
A stirred solution of 1’,1’’-[(2,4,6-trimethylbenzene-1,3,5-
triyl)tris(methylene)]tris(2-methyl-1H-imidazole) (500 mg,
1.24 mmol) and 1-iodohexadecane 95% (1.4 g, 4.09 mmol) in
CH3CN (20 mL) was heated to reflux for 72 h. The solvent
was partially removed under vacuum and excess diethyl
ether was added to the residue. A white powder precipitat-
ed, which was filtered and washed with diethyl ether to
1
afford 2b; yield: 1.36 g (75%). H NMR (250 MHz, CD2Cl2):
d=0.88 [t, J=6.5 Hz, 9H, -(CH2)15-CH3], 1.26 [m, 78H,
-(CH2)13-], 1.85 (m, 6H, -CH2CH2-imid), 2.40 (s, 9H, Ar-
CH3), 2.96 (s, 9H, imid-CH3), 4.08 (t, J=7.75 Hz, 6H,
-CH2CH2-imid), 5.56 (s, 6H, imid-CH2-Ar), 7.27 (d, J=2 Hz,
3H, Himid), 7.30 (d, J=2 Hz, 3H, Himid); 13C NMR
(100.6 MHz, CDCl3): d=13.2, 14.3, 19.0, 22.9, 26.72, 29.2,
29.5, 29.6, 29.7, 29.8, 29.84, 29.9, 30.1, 49.3, 121.7, 121.8,
129.1, 142.4, 143.6; HR-MS (ESI): calcd. for C72H129N6I (loss
of two IÀ): 1204.932, m/z=602.4656, calcd. for z=2:
1204.932/2=602.4656; anal. calcd. (%) for C72H129N6I3: C
59.25, H 8.91, N 5.76, I 26.08; found: C 58.32, H 8.94, N
5.68, I 25.56.
ed. Alugramꢄ SIL G/UV254 sheets (Macherey–Nagel) were Synthesis of Palladium(0) Nanoparticles Stabilized by
used for thin-layer chromatography. Column chromatogra-
phy was carried out using SDS brand silica gel with a grain
size of 35–70 mm and a pore size of 60 ꢅ. Compounds 1a,[16]
2a[12] and 3a[12] were prepared by adapting the previously de-
scribed procedures (see theSupporting Information). Due to
2a (2a-Pd) (entry 1 of Table 1)
A Fischer–Porter apparatus equipped with a magnetic stir-
bar was charged with 2a (598 mg, 0.423 mmol) and PdACHTUNGTRENNUNG(dba)2
(290.6 mg, 0.423 mmol Pd). The apparatus was closed and
the contents were subjected to three evacuate-refill cycles
with argon. THF (anhydrous, 60 mL) was added, the system
was purged with H2 three times, bringing the pressure of H2
to 3 atm. and then the mixture was allowed to stir at room
temperature for 18 h. The hydrogen was evacuated and the
solution was filtered through a nylon membrane Milli-Pore
filter. The solvent from the filtrate was evaporated and the
black solid residue was washed successively with diethyl
ether and methanol, in each washing the solid being recov-
ered by centrifugation and decantation of the supernatant
phase. The resulting black solid was dried under vacuum to
afford 2a-Pd; yield: 257 mg (82% with respect to initial pal-
ladium); Pd: 14.4% (ICP); size of the nanoparticles: 4.2Æ
0.5 nm (TEM).
the potential continuum in the composition, the amounts of
[21]
PdACHTUNGTRENNUNG(dba)2 used were calculated based on 17.5% Pd as de-
termined by ICP for the batch used.
Synthesis of 1,1’,1’’-[(2,4,6-Trimethylbenzene-1,3,5-
triyl)tris(methylene)]tris(2-methyl-1H-imidazole)
2-Methyl-1H-imidazole (1.32 g, 16.04 mmol) and potassium
hydroxide (3.67 g, 65.17 mmol) were dissolved in dimethyl
sulfoxide (40 mL) and the solution was stirred for 2 h at
room temperature, then 1,3,5-tris(bromomethyl)-2,4,6-trime-
thylbenzene (2.00 g, 5.01 mmol) was added. After stirring
for another 3 h at room temperature, an equivalent volume
of water was added to the mixture and it was extracted with
chloroform (4ꢃ40 mL). The combined organic phases were
washed with water (4ꢃ30 mL), dried over anhydrous
Nanoparticulate materials 2b-Pd and 3a-Pd were pre-
pared by analogous procedures (see Table 1, Scheme 4 and
the Supporting Information).
660
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Adv. Synth. Catal. 2012, 354, 651 – 662

Palladium Nanoparticles in Suzuki Cross-Couplings
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Catalyzed by 2a-Pd Nanoparticles (Table 5)
A screw-top sealable tube (10 mL) was charged with a mag-
netic stirbar, aryl halide (2.5 mmol), areneboronic acid
(3.75 mmol, 1.5 equiv.), K2CO3 (2 equiv.) and 2a-Pd NP
(0.2 mol% Pd). A mixture of toluene-water (4:1, 2.5 mL)
was added, the tube was sealed and heated to 908C for the
period of time indicated in the Supporting Information (the
consumption of aryl halide was monitored by GC). The tube
was then allowed to cool to room temperature and the sol-
vent was removed under vacuum. Diethyl ether was added
and stripped under vacuum to facilitate removal of trace sol-
vents. The residue was purified by column chromatography
(SiO2, hexane-ethyl acetate mixtures). See Supporting Infor-
mation for details and characterization of compounds.
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Supporting Information
This contains details of the synthesis and characterization of
stabilizers 2a, 2b and 3a and of the corresponding nanoparti-
cles 2a-Pd, 2b-Pd, 3a-Pd; HR-TEM images and particle size
distribution histograms for the Pd nanoparticles. Details of
catalytic runs and characterization data of the corresponding
cross-coupling products are also given therein.
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Acknowledgements
We acknowledge financial support from MICINN of Spain
(Project CTQ2009-07881/BQU),Consolider Ingenio 2010
(Project CSD2007-00006) and Generalitat de Catalunya
(Project SGR2009-01441). A. Shafir was supported through
a Ramꢃn y Cajal contract from the MEC, Spain (currently of
MCINN). We would like to thank Eudald Casals for valuable
help with DLS measurements.
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1
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