Alkynyl-functionalized imidazolium for "click" dendrimer functionalisation and palladium nanoparticle stabilization

Article abstract of DOI:10.1002/ejic.201403045

Functionalization of imidazolium salts (IMSs) that allow easy derivation of molecular or solid supports are called for in various applications. Here, the synthesis of an alkyne-containing IMS can be achieved in three steps from 2,4,6-trimethylaniline, and

Full text of DOI:10.1002/ejic.201403045

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DOI:10.1002/ejic.201403045
Alkynyl-Functionalized Imidazolium for “Click”
Dendrimer Functionalisation and Palladium Nanoparticle
Stabilization
Christophe Deraedt,[a] Martin d’Halluin,[a] Stephanie Lesturgez,[b]
Lionel Salmon,[c] Graziella Goglio,[b] Jaime Ruiz,[a] and
Didier Astruc*[a]
Keywords: Dendrimers / Nanostructures / Click chemistry / Homogeneous catalysis / Palladium
Functionalization of imidazolium salts (IMSs) that allow easy
derivation of molecular or solid supports are called for in
various applications. Here, the synthesis of an alkyne-con-
taining IMS can be achieved in three steps from 2,4,6-tri-
methylaniline, and this IMS can be further successfully func-
tionalized by using either Sonogashira or a “click” CuAAC
reaction that was applied to dendritic IMS synthesis giving
high yields. Applications are illustrated for Pd nanoparticle
stabilization by the IMS dendrimer for Suzuki–Miyaura
catalysis.
currently functionalized by using the CuAAC reaction (Fig-
ure 1).
Introduction
Imidazolium salts (IMSs) derived from imidazole
through alkylation of both nitrogen atoms, leading to ion
pairs, are widely used in chemical and biological fields, for
instance as anticancer and antimicrobial drugs.[1] They are
also well known for their ionic-liquid properties and are
used as electrolytes or green solvents because of their low
vapor pressure and high chemical stability.[2] Another key
property is their deprotonation to N-heterocyclic carbenes
(NHCs) such as 1 or bis(imidazolidines), which have many
applications in organic synthesis.[3] NHCs have been widely
used as strong σ donor ligands in metal complexes such as
Ru metathesis catalysts,[4,5] for which functionalizable ver-
sions are useful for recycling purposes. Thus the synthesis
of various NHCs is of interest, especially NHCs that are
modified with “clickable” termini for easy functionalization
of materials. Along this line, we targeted the synthesis of
the new IMS 2 containing an alkyne terminus that is, for
example, easily “clickable” by using a copper(I) alkyne-
azide cycloaddition (CuAAC) reaction[6] or can participate
in Sonogashira C–C cross coupling.[7] Molecular com-
pounds and nanomaterials with azido termini, easily
accessible from halogeno derivatives, are most often readily
available. Consequently dendrimers, polymers, and nano-
particles (silica, iron oxide) terminated by azido groups are
Figure 1. NHC 1 used in Ru metathesis catalysts and the targeted
IMS 2.
Dendrimers[8] that correspond to molecular micelles[9]
occupy a privileged place among branched macromolecules
because they are multifaceted monodisperse macro-
molecules, and their supramolecular properties have poten-
tial applications in medicinal chemistry[10] and nanoscience.
In this latter area, they are often used as sensors,[11] green
catalysts[12] or stabilizers of nanoparticles (NP).[13]
We[11b,11c,14] and others[15] have explored the useful func-
tionalization of azido-terminated dendrimers for various
syntheses and applications including stabilization of PdNPs
for catalysis,[16] and this method has now been applied here
to functionalize dendrimers with IMS termini.
[a] ISM, UMR CNRS 5255, Univ. Bordeaux,
351 Cours de la Libération, 33405 Talence Cedex, France
http://astruc.didier.free.fr/welcome.htm
Results and Discussion
Synthesis of the Propargyl IMS 2
[b] ICMCB, UPR CNRS N°9048,
87 Albert Schweitzer, 33608 Pessac Cedex, France
[c] LCC, CNRS,
The synthesis of 2 was successfully achieved in only three
steps from commercial products with an over yield of 35%
(Scheme 1).
205 Route de Narbonne, 31077 Toulouse Cedex, France
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201403045.
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Scheme 1. Synthesis of the alkyne-containing imidazolium 2.
This rather modest yield is related to the known first step Moreover, the infrared spectra of 4 and 2 are a little bit
involving the reaction on a relatively large scale between different. The appearance of two new intense peaks at
2,4,6-trimethylaniline and 2,3-dibromopropanol over 19 1631 cm–1 and 1059 cm–1 in the infrared spectrum of 2 are
–
hours at 120 °C under nitrogen to form 2,3-bis(mesityl- characteristic of C=N and BF4 groups, respectively (Fig-
amino)-1-propanol (3) in only 44.6% yield.[17] The hydroxy ure 3).
group of 3 is then functionalized upon reaction with prop-
After obtaining 2, two different reactions were tested: the
argyl bromide in THF (16 h at 40 °C) in order to introduce Sonogashira C–C cross coupling and the CuAAC “click”
the alkyne terminal group in the IMS, leading to the desired reaction. The Sonogashira reaction between 2 and iodo-
2,3-bis(mesitylamino)-1-propoxypropargyl (4) in 96.6% benzene was carried out in THF in the presence of DIPA
yield. The appearance of two new signals at δ = 2.41 ppm as a base, and [PdCl2(PPH3)2] (8 mol.-%) and CuI (8 mol.-
(triplet) and at δ = 4.10 ppm (dd) in the 1H NMR spectrum %) as cocatalysts at 40 °C for 1 day. The final product 5
of the final product shows the success of the reaction and was purified by precipitation from ether and recovered in
the presence of the alkyne moiety (Figure 2).
only 71% yield, mainly due to the formation of side prod-
ucts (Scheme 2). Compound 2 also reacts with benzyl azide
in THF in the presence of CuBr (60 mol.-%) as a catalyst
at 40 °C over two days. The desired product 6 was recovered
after precipitation from ether in 90.5% yield (Scheme 2).
Since the CuAAC reaction leads to a better yield than the
Sonogashira reaction and is more convenient, the “click”
reaction was selected to functionalize the dendritic core.
The “click” reaction between 2 and the nona-azido core
7 was performed in THF at 45 °C (Scheme 3). An amount
of 60% CuBr per branch is necessary to complete the
“click” reaction between 2 and 7, probably because of more
or less stoichiometric coordination of the copper ions of the
catalyst by the 1,2,3-triazole ligand and trapping inside the
dendrimer. This inhibition effect has already been noted in
“click” dendrimer construction when the CuAAC “click”
reaction is conducted in the presence of a simple copper
salt as a catalyst rather than a copper(I) catalyst[11b] con-
taining a polydentate nitrogen ligand.[14,18] The end of the
reaction was determined by infrared spectroscopy, the dis-
appearance of the vN3 band at 2097 cm–1 being observed
after two days.
1
Figure 2. H NMR spectra of 4 (up) and 2 (down).
The infrared spectrum reveals the presence of the alkyne
with the absorption bands at vCϵC = 2113 cm–1 and vCϵC
= 3268 cm–1 (Figure 3). The last step of the synthesis of the
The imidazolium dendrimer 8 was obtained as a brown
propargyl IMS 2 involves the reaction of 4 with triethyl shiny powder in 85% yield, which indicates the easy use and
orthoformate and NaBF4 at 140 °C for 2.5 h yielding 2 in high reactivity of 2 in CuAAC catalysis. The dendrimer 8
1
81.0% yield (Scheme 1). Compound 2 was obtained as a was characterized by H and 13C NMR and infrared spec-
1
brown solid and fully characterized by H and 13C NMR troscopy and by mass spectrometry. The mass spectrum is
spectroscopy, mass spectrometry and infrared spectroscopy. difficult to analyse, as usual, when there are many counter
1
The H NMR spectrum of 2 indicates the presence of a anions (nine counter anions in this case). The weight Mw
new deshielded proton at δ = 7.92 ppm (Figure 2). This new of 8 is 5685 + 23 (Na), and peaks around this molecular
signal corresponds to the imidazolium proton localized be- weight are observed in this area, especially the peak at m/z
tween the two nitrogen atoms. The signals of the protons = 5710, which is close to the actual molecular weight.
of the mesityl and the terminal alkyne remain unchanged. Surprisingly, 8 is neither soluble in water nor in alcoholic
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Figure 3. Infrared spectra of 4 (top) and 2 (bottom).
Scheme 2. IMS 2 involved in both Sonogashira and CuAAC reactions.
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Scheme 3. Synthesis of the imidazolium dendrimer 8 by CuAAC “click” chemistry.
solvents. On the other hand, it is freely soluble in aceto- probably reduced most of the PdII species. The new catalyst
nitrile, acetone as well as dimethylformamide. was tested for the Suzuki–Miyaura C–C cross-coupling re-
As an application, the stabilization of palladium nano- action that has become one of the most known and power-
particles (PdNPs) with 8 using its intradendritic triazolyl ful Pd0 reactions allowing the synthesis of biaryl com-
rings was probed. In previous examples, PdNPs stabilized pounds, such as natural products, pharmaceuticals, poly-
by dendrimers were synthesized in aqueous solution when mers, etc.[19]
the dendrimer is water soluble, and this solution was di-
Thus coupling between bromobenzene and phenyl-
rectly used as a catalytic medium for C–C coupling.[13c] boronic acid was carried out in water at 80 °C over 24 h
Given the insolubility of 8 in water, the stabilization of with 0.3 mol.-% of the Pd catalyst. The presence of TBAB
PdNPs with 8 was probed, in this case, in acetonitrile for as a phase transfer agent is mandatory for total conversion
further isolation of the dendrimer-stabilized PdNPs in the (isolated yield: 94%). The reaction was performed on four
solid state. Indeed, an acetonitrile solution of a stoichiomet- others substrates leading to quantitative yields; see Equa-
ric amount of 9 equiv. of [Pd(OAc)2] per dendrimer was tion (1). The catalyst loses half of its activity after the first
added to the acetonitrile solution of 8, provoking an instan- run. The mechanism of such homogeneous Suzuki–
taneous color change from brown/orange to intense green. Miyaura reactions catalyzed by PdNPs usually involves Pd
Then, reduction of triazole-coordinated PdII to Pd0 was leaching from the PdNP catalyst subsequent to oxidative
carried out by addition of NaBH4 as a solid (6 equiv. per addition of the aryl halide followed by active catalysis by
Pd atom) to the 8/Pd(OAc)2 solution leading to a direct solvent-coordinated Pd atoms, and no catalyst recycling is
change of color from green to brown/black that is charac- efficient without support.[13c–13e,16c]
teristic of PdNPs. This was followed by isolation of the
solid after solvent removal. TEM microscopy of the PdNPs
revealed a size between 2.0 nm and 6.0 nm (average size:
4Ϯ1.2 nm), but another small population with sizes around
16 nm was also observed (Figure 4).
(1)
Conclusion and Outlook
The successful synthesis of a new alkyne-containing IMS
2 has been achieved, and this IMS could be easily function-
alized using both the Sonogashira and the “click” CuAAC
Figure 4. PdNPs stabilized by dendrimer 8 and analyzed by TEM
reactions. Easy functionalization by the CuAAC reaction
(A), their distribution histogram (B) and isolation at the solid state
has been conducted with a nona-branched dendritic core 7
in high yield, showing that the functional IMS allows easy
(C).
The formation of these bigger PdNPs is due to the derivatization of supports. This dendrimer 8 is potentially
evaporation of the solvent, leading to nanoscale aggrega- usable as a ionic liquid and, moreover, 2, 5, 6 or 8 should
tions. X-ray diffraction (XRD) analysis reveals the presence be sources of carbenic species for metal coordination when
of Pd0 in the catalyst, indicating that the excess of NaBH4 they are in the presence of a base such as tBuOK or
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KHMDS.[20] Finally, the IMS 2 has been shown to be a
good precursor of catalytically active PdNPs.
brown solid was washed several times with pentane and Et2O and
then was dried under reduced pressure for a few hours, and 2.77 g
of 2 was obtained (81% yield). H NMR (200 MHz. CDCl3): δ =
1
Ar-CH3 2.24–2.39 (m, 18 H), CϵCH 2.43 (m, 1 H), CH2 3.5–3.7
(m, 2 H), CH2 4.19 (d, 2 H), CH2 4.25–4.38 (m, 1 H), CH2 4.63
(m, 1 H), CH 5.12 (m, 1 H), H aromatic 6.93 (s, 4 H), H imid-
azolium 7.92 (s, 1 H) ppm. 13C NMR (50.33 MHz. CDCl3): δ =
17.70, 18.28, 21.14 (CH3 Ar), 52.82, 58.62, 63.83, 66.28 (3ϫ CH2,
1ϫ CH), 76.02, 78.2 (C ethylenic), 128.39, 130.15, 130.35, 130.64,
135.46, 135.59, 136.10, 140.50, 140.76 (C aromatic), 158.53 (C
imidazolium) ppm.
Experimental Section
Purification Procedures
Biphenyl: Bromobenzene and phenylboronic acid: simple flash
chromatography with petroleum ether as the mobile phase and
silica as the stationary phase.
4-Nitrobiphenyl: Reaction between 1,4-bromonitrobenzene and
phenylboronic acid: flash chromatography with 95% petroleum
ether/5% dichloromethane as mobile phase and silica as stationary
phase.
4-{[(1-Benzyl-1H-1,2,3-triazol-4-yl)methoxy]methyl}-1,3-dimesityl-
4,5-dihydro-1H-imidazol-3-ium Tetrafluoroborate (Test for the
CuAAC “Click” Reaction): Benzyl azide (0.75 mmol, 1 equiv.) and
2 (470 mg, 0.78 mmol, 1.05 equiv.) were dissolved in distilled THF
(5 mL) in a Schlenk tube, CuBr (48 mg, 60 mol.-%) was added, and
the reaction was maintained at 40 °C for 36 h. At the end of the
reaction, water (20 mL) and ammonia solution (1 mL, 37%) were
added, and the medium was stirred for 10 min. The aqueous phase
was extracted 3 times with CH2Cl2 (20 mL). The organic phases
were combined and washed 3 times with water (20 mL) dried with
Na2SO4, the solvent was evaporated, and 230 mg of the desired
4-Biphenylcarbaldehyde: Reaction between bromobenzaldehyde
and phenylboronic acid: flash chromatography with 95% petro-
leum ether/5% dichloromethane as the mobile phase and silica as
the stationary phase.
4-Acetylbiphenyl: Reaction between 4-bromoacetophenone and
phenylboronic acid: flash chromatography with 95% petroleum
ether / 5% diethyl ether as the mobile phase and silica as the
stationary phase.
1
“click” product was obtained (90.5% yield). H NMR (200 MHz,
CDCl3): δ = Ar-CH3 2.18–2.32 (m, 18 H), CH2 3.52 (d, 1 H), CH2
3.73 (dd, 1 H, 2J = 11.4, 3J = 2 Hz), CH2 4.34 (dd, 1 H, 2J = 12 Hz
3J = 7.3 Hz), CH2 4.46–4.66 (m, 3 H), CH 5.04 (m, 1 H), CH2-
triazole 5.48 (s, 2 H), H benzyl 6.7–7 (m, 5 H), H aromatic 7.32 (s,
4 H); H-triazole 7.71 (s,1 H), H imidazolium 7.85 (s, 1 H) ppm.
13C NMR (50.33 MHz, CDCl3): δ = 17.62, 18.40, 21.21 (CH3 Ar),
52.93, 54.35, 64.25, 66.49 (3ϫ CH2, 1ϫ CH), 123.95, 128.38,
128.60, 128.90, 129.29, 130.19, 130.52, 135.02, 135.43, 136.18,
140.80, 140.93 (C Aromatic + 2 C triazole), 158.01 (C imid-
azolium) ppm.
N,NЈ-Dimesityl-2,3-diamino-1-propanol (3): 2,3-dibromopropanol
(10 g, 45.9 mmol, 1 equiv.) and 2,4,6-trimethylaniline (16 g,
118 mmol, 2.6 equiv.) were introduced into a Schlenk tube. The re-
action medium was stirred at 120 °C for 20 h. The temperature was
then decreased to room temperature and an aqueous solution of
NaOH (50 mL of a 15 mol.-% solution) was added. Then CH2Cl2
(50 mL) was added, the organic phase was collected, and the
aqueous phase was extracted twice with CH2Cl2 (2ϫ 50 mL). The
organic phases were combined, dried with Na2SO4, and the solvent
was evaporated under reduced pressure. The crude product was
purified by silica chromatography column with Et2O/pentane (1:1),
and 6.7 g of 3 was obtained (44.6% yield).
Imidazolium Dendrimer 8: The azido-core 7 (100 mg, 0.066 mmol,
1 equiv.) and 2 (305 mg, 0.66 mmol, 1.1 equiv.) were dissolved in
N1,N2-Dimesityl-3-(prop-2-yn-1-yloxy)propane-1,2-diamine
Compound 3 (3.1 g, 9.48 mmol, 1 equiv.), propargyl bromide
(2.1 mL, 19 mmol, 2 equiv.) and distilled THF (40 mL) were placed the end of the reaction, a precipitate on the wall of the Schlenk
(4): distilled THF (5 mL) in a Schlenk flask, CuBr (60 mg, 60 mol.-%)
was added, and the reaction was maintained at 40 °C for 48 h. At
in a Schlenk tube and NaH (28.5 mmol, 3 equiv.) was slowly added.
The reaction was stirred at 40 °C for 16 h. At the end of the reac-
flask was observed. The product was filtered and washed with THF
several times. Acetone or acetonitrile was used to dissolve the prod-
tion and when the temperature had decreased to room temperature, uct and filtration through Celite was performed. The solvent was
water (20 mL) was added. The THF solvent was evaporated under
reduced pressure, and the water phase was extracted 3 times with
CH2Cl2 (50 mL). The organic phases were combined, dried with
Na2SO4, the solvent was evaporated under reduced pressure, and
3.3 g of the desired product 4 was obtained as a brown solid (96.6%
evaporated and 320 mg of the desired “click” product was obtained
(85% yield). H NMR of 8 in CD3CN (200 MHz, ppm): 8.70 (9 H
1
imidazolium), 8.20 (9 H, trz), 7.07–6.94 (37 H, Ar + 3 H core),
5.24–3.58 (81 H), 2.43–2.23 (162 H, CH3 mesityl), 1.81–0.68 (54
H, CH2CH2CH2Si); 0.05 [54 H, Si(CH3)2] ppm. 13C NMR of 8 in
yield). 1H NMR (200 MHz. CDCl3): δ = Ar-CH3 2.24–2.37 (m, 18 CD3CN (50 MHz): 160.45 (CH, imidazolim), 147.94 (Cq trz),
2
2
H), CϵCH 2.41 (t, 1 H, J = 2.4 Hz), CH2 3.08 (dd, 1 H, J = 12,
141.65–129.82 (aromatics + CH trz), 67.72–53.82 (3CH2, CH),
3J = 5.7 Hz), CH2 3.33 (dd, 1 H, 2J = 12, J = 5.6 Hz), CH + CH2 45.02 (CH2CH2CH2Si), 42.54 (SiCH2trz), 21.39–15.83 (CH3
+ 2ϫ NH 3.5–3.75 (m, 5 H), CH2 4.18 (dd, 1 H, J = 15.7, J = mesityl + CH2Si + CH2CH2Si) ppm.
2.5 Hz), CH2 4.23 (dd, 1 H, 2J = 15.7, 3J = 2.5 Hz), H aromatic
3
2
3
Correct elemental analysis could not be obtained because of the
insolubility of 8 in weakly polar solvents and the presence of inor-
ganic salts.
6.87 (s, 4 H) ppm. 13C NMR (50.33 MHz. CDCl3): δ = 18.52,
19.00, 20.74 (CH3 Ar), 50.75, 56.68, 58.66, 70.50 (3ϫ CH2 + 1ϫ
CH), 74.91, 79.62 (C ethylenic), 129.45, 129.60, 129.79, 130.09,
131.02, 131.46, 141.75, 143.77 (C Aromatic) ppm.
Synthesis of the PdNPs: Dendrimer 8 (63.3 mg, n = 0.0111 mmol,
Mw = 5685 gmol–1) was dissolved in CH3CN (3 mL) in a Schlenk
tube under nitrogen. Pd(OAc)2 (22.5 mg, 9 equiv.) was then added
1,3-Dimesityl-4-[(prop-2-yn-1-yloxy)methyl]-4,5-dihydro-1H-imid-
azol-3-ium Tetrafluoroborate (2): Compound 4 (2.7 g, 7.4 mmol,
1 equiv.), triethyl orthoformate (2.5 mL, 15 mmol, 2 equiv.) and to the CH3CN solution. The reaction was stirred for 1 h under
NH4BF4 (770 mg, 7.4 mmol, 1 equiv.) were placed in a Schlenk
tube. The reaction was stirred at 140 °C for 2.5 h under N2. At the
end of the reaction the temperature was decreased to 50 °C, and
the liquid was evaporated under reduced pressure. The resultant
nitrogen at 20 °C and NaBH4 (22.6 mg, 54 equiv.) was then added
quickly. The reaction medium was stirred for 2 min, and all the
solvent was evaporated in vacuo, leading to black PdNPs in the
solid state. Correct elemental analysis could not be obtained be-
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General Procedure for the Suzuki–Miyaura Coupling: The bromo-
arene (1 mmol), phenylboronic acid (1.5 mmol), K3PO4 (2 mmol),
the catalyst (0.3 mol-% i.e. 1.4 mg) and TBAB (1 mmol) were
placed in a Schlenk tube together with H2O (3 mL). The reaction
was stirred at 80 °C for 24 h after which the reaction mixture was
extracted twice with diethyl ether (Et2O, all the reactants and final
products were soluble in Et2O), the organic phase was dried with
Na2SO4, and the solvent was removed under vacuum. In parallel,
the reaction was checked by using TLC with petroleum ether as
1
eluent in nearly all the cases, and H NMR spectroscopy. Purifica-
tion by flash chromatography column was conducted with silica gel
as the stationary phase and petroleum ether as the mobile phase.
After each reaction, the Schlenk flask was washed with a solution
of aqua regia (3 volumes of hydrochloric acid for 1 volume of nitric
acid) in order to remove traces of Pd.
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Financial support from the Universities of Bordeaux and Toulouse
III, the Centre National de la Recherche Scientifique (CNRS) and
the Ministère de l’Enseignement Supérieur et de la Recherche (PhD
grant to C. D.) are gratefully acknowledged.
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Received: November 18, 2014
Published Online: ᭿
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Job/Unit: I43045
/KAP1
Date: 09-02-15 14:33:08
Pages: 7
www.eurjic.org
FULL PAPER
Dendrimer-Stabilized Nanoparticles
C. Deraedt, M. d’Halluin, S. Lesturgez,
L. Salmon, G. Goglio, J. Ruiz,
D. Astruc* ......................................... 1–7
An imidazolium salt containing an alkynyl
group was synthesized in three steps in
order to functionalize nanomaterials by
means of CuAAC and Sonogashira reac-
tions. For instance, a nona-azido dendritic
core could be “clicked” with the alkynyl-
containing imidazolium salt. The resulting
dendrimer allows the stabilization of
palladium nanoparticles that are active in
the Suzuki–Miyaura cross-coupling reac-
tion.
Alkynyl-Functionalized Imidazolium for
“Click” Dendrimer Functionalisation and
Palladium Nanoparticle Stabilization
Keywords: Dendrimers / Nanostructures /
Click chemistry / Homogeneous catalysis /
Palladium
Eur. J. Inorg. Chem. 0000, 0–0
7
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim