Functionalisable N-heterocyclic carbene-triazole palladium complexes and their application in the suzuki-miyaura reaction

Article abstract of DOI:10.1002/ejic.201301607

Seven functionalised N-heterocyclic carbene (NHC) ligands and their corresponding palladium(II) complexes have been synthesised with a triazole moiety as a modular and stable linkage between the catalytic centre and the secondary functional group. The complexes were prepared in good yields and fully characterised by NMR spectroscopy, mass spectrometry and X-ray crystallography. The complexes are active in the Suzuki-Miyaura cross-coupling reaction, with catalytic activities maintained whatever the triazolyl substituent. This opens the possibility for various functionalisations of NHC-palladium complexes. New functionalised N-heterocyclic carbene ligands and their corresponding palladium(II) complexes have been synthesised and fully characterised. The use of a triazole moiety as a modular and stable linkage between the catalytic centre and a targeted functionalisation does not damage the catalytic activity. Copyright

Full text of DOI:10.1002/ejic.201301607

FULL PAPER
DOI:10.1002/ejic.201301607
Functionalisable N-Heterocyclic Carbene–Triazole
Palladium Complexes and Their Application in the
Suzuki–Miyaura Reaction
Ganna Gogolieva,[a,b] Hélène Bonin,[a,b][‡] Jérôme Durand,[a,b]
Odile Dechy-Cabaret,*[a,b] and Emmanuel Gras*[a,b]
Dedicated to Dr. Bernard Meunier on the occasion of his official retirement from the CNRS
Keywords: Homogeneous catalysis / C–C coupling / Palladium / Carbene ligands / Nitrogen heterocycles
Seven functionalised N-heterocyclic carbene (NHC) ligands
and their corresponding palladium(II) complexes have been
synthesised with a triazole moiety as a modular and stable
linkage between the catalytic centre and the secondary func-
tional group. The complexes were prepared in good yields
and fully characterised by NMR spectroscopy, mass spec-
trometry and X-ray crystallography. The complexes are
active in the Suzuki–Miyaura cross-coupling reaction, with
catalytic activities maintained whatever the triazolyl substit-
uent. This opens the possibility for various functionalisations
of NHC–palladium complexes.
Reaction (SMR) has found numerous applications and has
been recognised for its efficiency, especially providing easy
access to unsymmetrical biaryl compounds.[1] Although
widely used, the SMR remains an area of intense research
efforts beyond activity and selectivity. Indeed mechanistic
studies are still under investigation.[2] However, in the cur-
rent era of sustainability there is a great need to imagine
new tools that could allow efficient tethering of the metal
complex to various substrates as this would enable the pos-
sibility of polyfunctional systems[3] or catalyst recovery.[4]
This second approach has been exemplified with a range of
supports (homogeneous or heterogeneous), to which li-
gands such as phosphines can be tethered.[5]
N-Heterocyclic carbenes (NHCs) are a class of appropri-
ate ligands for palladium-catalysed transformations since
NHC complexes show good activities and selectivities as
well as high thermal, air and moisture stabilities.[6]
Immobilisation of homogeneous NHC-based catalysts
on a support or in a phase (from which the reaction product
can be easily extracted) combines homogeneous control
(single-site molecularly-defined catalyst) and heterogeneous
advantages (catalyst recovery and reuse). Towards this goal,
a functionality is required on the NHC-ligand structure[7]
either for grafting on to the support[8] or for immobilisation
in the supporting phase.[9]
Introduction
Palladium-catalysed C–C bond formations have under-
gone huge developments recently. These developments open
the possibility for new synthetic strategies, by allowing ret-
rosynthetic disconnections that were previously not achiev-
able in a single step using conventional organic chemistry
methods. These new strategies probably represent one of
the most significant steps in the history of transition-metal-
assisted reactions as they exhibit high tolerances towards a
wide array of functional groups as well as excellent selectivi-
ties and efficiencies. Consequently Pd-catalysed C–C bond
formations have been applied in numerous fields (organic
synthesis, medicinal and materials chemistry, etc.) and they
have also proved amenable to enantioselective synthesis, be-
coming one of the most powerful elements of the synthetic
chemist’s toolbox. These developments were recognised by
the Nobel Prize jointly awarded to Heck, Neigishi and Su-
zuki in 2010. Among these reactions the Suzuki–Miyaura
[a] CNRS, LCC (Laboratoire de Chimie de Coordination),
205 route de Narbonne, B. P. 44099, 31077 Toulouse Cedex 4,
France
E-mail: odile.dechycabaret@ensiacet.fr
emmanuel.gras@lcc-toulouse.fr
http://www.univ-toulouse.fr/
[b] Université de Toulouse, UPS, INPT,
31077 Toulouse Cedex 4, France
Finally, the challenge is to design NHC ligands that both
meet the criteria imposed by the recovery approach and
confer the required properties for catalytic activity, while
retaining robustness (stability of the catalyst) and low cost
(easy chemical synthesis of the ligand).
[‡] Current address: Unité de Catalyse et Chimie du Solide, UMR
CNRS 8181, ENSCL, Bât. C7, Université des Sciences et
Technologies de Lille,
B. P. 90108, 59652 Villeneuve d’Ascq Cedex, France
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201301607
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Following our previous reports on user-friendly partners
The triazole-based imidazolium salts 4a–c, each bearing
involved in the Suzuki–Miyaura reaction,[10] we report one fluorous ponytail, were prepared by the copper-cata-
herein the design and modular synthesis of NHC-based pal- lysed Huisgen cycloaddition of the corresponding imid-
ladium complexes in which the functional group required azolium salts 2a–c with the fluorous azide 3[16] in the pres-
for catalyst recovery is linked to the catalytic centre by a ence of diisopropylamine (DIPA) in 2,2,2-trifluoroethanol
triazole moiety, as well as a preliminary catalytic screening (Scheme 2).[17]
in the Suzuki–Miyaura cross-coupling reaction. Here again
we have demonstrated ease of synthesis and handling to
access diversely functionalised complexes and their catalytic
efficiency has been investigated under simple and conve-
nient conditions.
Results and Discussion
Scheme 2. Synthesis of triazole-based imidazolium salts 4a–c.
The association of two different moieties in a unique mo-
lecule requires a highly modular chemical tethering strategy
exhibiting a wide tolerance towards a range of functional
groups and providing the desired coupling product in high
yield and regioselectivity. Among the various possible
“click” strategies, the Huisgen coupling between azides and
terminal alkynes to give 1,4-disubstituted 1,2,3-triazoles has
attracted much attention.[11] Two examples of triazole-
based NHC ligands have been reported in the litera-
ture[12,13] with the aim to build bidentate ligands, the tri-
azole moiety acting as a potential N-donor ligand rather
than as a neutral linkage.
In our case, we were attempting to design bifunctional
ligands using the triazole moiety as a chemical linkage be-
tween the NHC functionality, required for the catalytic ac-
tivity, and a second functionality introduced for another
purpose (such as bioconjugation or introduction of a coor-
dination site for a second metal). To allow for easy modula-
tion of the ligand structure, a convergent synthesis of the
NHC precursors was envisioned by means of the copper(I)-
catalysed alkyne–azide cycloaddition of various imid-
azolium salts bearing terminal alkyne groups with a range
of azides. Bellemin-Laponnaz et al. recently reported a sim-
ilar approach featuring a ruthenium-catalysed cycload-
dition directly on to alkyne-substituted PdII and PtII com-
plexes.[14]
Analogously, the triazole-based imidazolium salts 6a–c,
each bearing an aryl–NO2 moiety, were prepared by the
copper-catalysed Huisgen cycloaddition of 2a–c with the ar-
ylazide 5 in acetonitrile, whereas the triazole-based imid-
azolium salt 8b, bearing an aryl–CN moiety, was prepared
by the cycloaddition of 2b with the arylazide 7 in aceto-
nitrile (Scheme 3). All the triazole-based imidazolium salts
4a–c, 6a–c and 8b were obtained as solids in good to excel-
lent yields (70–99%). The formation of the triazole was
1
confirmed in the H NMR spectra by both the disappear-
ance of the alkynyl proton (around 3 ppm) and the observa-
tion of a singlet assigned to the proton of the 1,2,3-triazole
ring at 8.10–8.29 ppm for fluorous-tagged triazoles 4a–c
and at 9.13–9.21 ppm for aryl-substituted triazoles 6a–c
and 8b.
Complex Synthesis
Scheme 3. Synthesis of triazole-based imidazolium salts 6a–c and
8b.
Imidazolium salts 3-methyl-, 3-ethyl- and 3-vinyl-1-
(prop-1-ynyl)-1H-imidazolium bromides 2a–c, were ob-
tained by direct reaction of 3-bromoprop-1-yne with the
corresponding 1-methyl-, 1-ethyl- and or 1-vinylimidazoles
1a–c (Scheme 1).[15]
Formation of the palladium complexes was first at-
tempted by a transmetallation methodology,[18] but the syn-
thesis of the required silver–NHC complex led to insoluble
products that could never be properly characterised. Mov-
ing to a procedure described by Fukuyama,[9b] NHC–palla-
dium complexes 9a–c, 10a–c and 11b were prepared from
the corresponding triazole-based imidazolium salts with
Pd(OAc)2 and PPh3 in the presence of Et3N and either LiCl
or LiBr in acetonitrile. When using 5 mol-equiv. of Et3N
and 10 or 15 mol-equiv. of LiCl in the synthesis of the com-
plexes, mixed chloro/bromo complexes were obtained as ex-
pected for 9a–b, 10a–c and 11b. Surprisingly, 9c was ob-
Scheme 1. Synthesis of imidazolium salts 2a–c.
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tained as the dichloro complex quantitatively. We then
Single crystals of 10b suitable for X-ray analysis were ob-
moved to the use of LiBr, for which 30 mol-equiv. was re- tained by slow crystallisation from acetonitrile; the molecu-
quired to obtain 9a–b, 10a–c and 11b as pure dibromo com- lar structure of 10b in the solid state is depicted in Figure 1.
plexes. (Scheme 4). However, the dibromo complexes ap-
peared less easy to handle (apparently highly hygroscopic)
than the ones containing chlorides.
Scheme 4. Synthesis of triazole-based NHC–palladium(II) com-
plexes 9a–c, 10a–c and 11b.
Figure 1. ORTEP plot of 10b (50% probability; hydrogen atoms,
All the palladium(II) complexes were obtained as solids
except those on the methylene bridge and solvent molecule, omitted
for clarity) showing the C–H···Pd interaction. Selected bond
in good to excellent yields (73–99%). The formation of the
NHC-based palladium complexes was confirmed by the
disappearance of the acidic imidazolium proton at 8.92–
9.29 ppm in 4a–c and 9.28–9.69 ppm in 6a–c in the 1H
NMR spectra, as well as the appearance of a resonance
signal in the range of 155–176.5 ppm in the 13C NMR spec-
tra, ascribed to the carbenic carbon atom. This value is con-
lengths [Å] and angles [°]: Pd(1)–C(1) 1.977(3), Pd(1)–P(1)
2.2656(8), Pd(1)–Br(1) 2.4797(4), Pd(1)–Br(2) 2.4836(4); C(1)–
Pd(1)–P(1) 94.27(8), C(1)–Pd(1)–Br(2) 84.04(8), P(1)–Pd(1)–Br(2)
174.42(2),
C(1)–Pd(1)–Br(1)
172.30(9),
Br(1)–Pd(1)–Br(2)
92.892(13), P(1)–Pd(1)–Br(1) 89.43(2).
The palladium centre is coordinated by an NHC carbon
sistent with the values of around 160 ppm reported for atom, a triphenylphosphine phosphorus atom and two cis
mixed NHC and triphenylphosphine palladium(II) com- bromide ions, in a distorted square-planar geometry. The
plexes.[9b,19] 31P NMR spectroscopy of every complex (δ ≈ functionalised NHC ligand is thus coordinated to palla-
26 ppm) was also in full accordance with the 31P chemical dium in a monodentate fashion by the carbenic carbon with
shifts reported for such carbene–phosphine complexes.
the imidazolylidene ring perpendicular to the coordination
1
Notably, in the H NMR spectra the methylene protons plane. The palladium–carbene distance Pd–C(1)
located between the imidazolium and the triazole rings of [1.977(3) Å] and the palladium–phosphine distance Pd–P
the ligand precursors appeared as singlets whereas an AX [2.2656(8) Å] are similar to the reported values for cis
splitting pattern (Δν between 180 and 300 Hz) with a cou- NHC–PPh3–PdII complexes (Pd–C 1.97–1.99 Å and Pd–P
pling constant around 15 Hz was observed in the spectra of 2.25–2.27 Å)[19,20] and shorter than those of analogous trans
the complexes. Such a splitting pattern has also been ob- NHC–PPh3–PdII complexes (Pd–C 2.03 Å and Pd–P 2.31–
served by Elsevier and co-workers for NHC–triazole com- 2.34 Å).[21]
plexes and has been identified as evidence of the bidentate
The hypothesised monodentate nature of the NHC li-
nature of the ligand, which prohibits rotation upon com- gand is clearly confirmed by the crystallographic structure
plexation.[13] Although from these data it could be sug- of 10b, for which a triazole–palladium interaction is unam-
gested that under our reaction conditions a bidentate biguously excluded [d(N–Pd) = 4.479(3) Å; torsion angle
NHC–triazole complex might also be accessed, we were (N1–N2–N3–Pd): 73.91(14)° when 180° would be expected
concerned by two facts: (1) no free triphenylphosphine is in case of triazole coordination]. However, one can observe
detected by 31P NMR (δ ≈ –5 ppm); (2) the chemical shift from the structure that the complex exhibits an anagostic
difference between these two protons is larger than that ob- interaction[22] between one proton of the methylene bridge
served by Elsevier and co workers. Indeed the chemical shift and palladium [d(H–Pd) = 2.85 Å, angle (C–H–Pd): 112°,
values were around 0.5 ppm for complexes 9a–c bearing a Δν = 480 Hz], as already reported for NHC–palladium
perfluorinated chain and around 1.2 ppm for aryl-substi- complexes.[23] This could explain the NMR splitting pattern
tuted complexes 10a–c and 11b. Moreover it must be observed for the two protons of the methylene bridge. A
pointed out that the solvent exhibited a significant effect on similar but smaller splitting (Δν = 240 Hz) is also observed
Δν, a decrease in its value being observed when switching for the methylene protons of the ethyl moiety that also lay
from chloroform to acetonitrile. Considering this, we hy- close to the metal centre [bond length (H–Pd): 2.98 Å, angle
pothesise that the chemical nonequivalence of the two (C–H–Pd): 112°]. Of note, the values of these two splittings
methylene protons might arise from another interaction.
depend on both the nature of the NMR solvent used and
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the temperature at which the NMR experiment is carried
Although NHC–palladium complexes are well known for
out, as expected for interactions mainly of an electrostatic their robustness, we explored the heterogeneous or homo-
nature. This splitting is also observed for the methylene pro- geneous nature of the catalytic species. We first noted that,
tons of the ethyl moiety in complexes 9b and 11b.
under the conditions of the catalytic tests, the solutions re-
To further study the role of the triazole on the palladium mained yellow and clear with no induction period being
coordination and on the properties of the complex, the syn- observed and no black precipitate appearing in the reaction
thesis of an analogous complex without the triazole ring mixture. Moreover, upon mercury-poisoning experi-
was envisioned with 1-ethyl-3-benzyl-imidazolium bromide ments,[24] neither the efficiency nor the rates of the reactions
(12a) as the ligand precursor (Scheme 5). Unfortunately, the were affected (see Supporting Information). These experi-
procedure used for all the triazole-based complexes failed ments tend to indicate the homogeneous nature of this ca-
in this case and the transmetallation procedure led to an talysis and confirm the robustness of these NHC–metal
insoluble product preventing analysis by NMR spec- complexes.
troscopy. This may be indicative of a polymeric structure.
To further study the influence of the catalyst’s substitu-
This result shows that the triazole ring plays a critical role tion pattern on the reaction course, reactions carried out
in obtaining a stable monomeric palladium(II) complex, under the same conditions for all catalysts were stopped at
even if the triazole is not directly involved in the palladium the same time before full conversion and the relative activi-
coordination. Indeed, all the triazole-based NHC–palladi- ties of the complexes were evaluated by comparison of the
um(II) complexes 9a–c, 10a–c and 11b were obtained as air- 4-bromoacetophenone conversions, determined by HPLC,
stable solids despite a low steric bulk on the nitrogen atoms in each different assay (Table 1).
of the imidazolydene.
Table 1. Relative catalytic activity of the triazole-based NHC–pal-
ladium(II) complexes 9a–c, 10a–c and 11b in the Suzuki–Miyaura
coupling of (4-methoxyphenyl)boronic acid with 4-bromoaceto-
phenone.
Entry
Catalyst[a]
Conversion[b]
1
2
3
4
5
6
7
9a
9b
54%
87%
61%
37%
68%
51%
82%
Scheme 5. Failure in the synthesis of a nontriazole-based NHC–
palladium(II) complex from 12a.
9c
10a
10b
10c
11b
Catalytic Tests
[a] Reaction conditions: 4-bromoacetophenone 1.0 mmol, (4-meth-
oxyphenyl)boronic acid 1.2 mmol, catalyst loading 0.05 mol-%,
K3PO4 1.0 mmol, DMF/H2O (4:1) 1 mL, 100 °C, in air, 15 min. [b]
Determined by HPLC percentage area.
The palladium-catalysed Suzuki–Miyaura cross-coupling
reaction (Scheme 6) was chosen as a model reaction to eval-
uate the catalytic properties of these complexes. Initially, the
complexes were tested for activity in the coupling of (4-
methoxyphenyl)boronic acid with an easy substrate, 4-
bromoacetophenone.
Several conclusions can be drawn from these results.
Considering the substituent pattern of the NHC ring, the
ethyl moiety appeared to be more active than its methyl or
vinyl counterparts [9b (entry 2) more active than 9a and 9c
(entries 1 and 3) and 10b (entry 5) more active than 10a
and 10c (entries 4 and 6)] and was thus kept for the rest of
the study. Comparing the functionalities added by way of
the triazole ring to these NHC-based complexes, catalysts
9a–c, each bearing a fluorous ponytail, appeared more
active than the corresponding 10a–10c and 11b catalysts,
bearing aryl–NO2 or –CN moieties (entry 1 vs. entry 4, en-
try 2 vs. entries 5 and 7, entry 3 vs. entry 6). Finally, the
low influence of the structures of the complexes on their
Scheme 6. Suzuki–Miyaura coupling of (4-methoxyphenyl)boronic
acid with 4-bromoacetophenone catalysed by triazole-based NHC–
palladium(II) complexes.
The results were encouraging with all the catalysts 9a–c, catalytic activities indicates that the triazole group is an
10a–c and 11b showing good activities at 100 °C. When adequate linkage between the NHC–Pd centre and the
using 3 mol-equiv. of K3PO4 as a base and DMF/water functionality required for any other purpose: whatever the
(4:1) as a solvent (5 mL per mmol of substrate) full conver- triazolyl substituents, all complexes 9a–c, 10a–c and 11b al-
sion of the bromide was obtained in 15 min with 1 mol-% lowed for significant 4-bromoacetophenone conversion (37
catalyst loading [turnover frequency (TOF) Ͼ 400 h–1]. to 87%) within only 15 min and at a 500 ppm catalyst load-
When decreasing the base quantity to 1 mol-equiv., the sol- ing.
vent volume to 1 mL per mmol of substrate and the catalyst
Furthermore, as depicted in Table 2, complex 9b has
loading to 0.05 mol-%, reactions were completed within an been evaluated in the Suzuki–Miyaura cross-coupling reac-
hour for all the catalysts (TOF ≈ 2000 h–1).
tion of various substrates to illustrate the versatility of its
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Table 2. Catalytic tests with 9b in the Suzuki–Miyaura cross-coupling reaction of various substrates.
catalytic activity. It is noteworthy that catalyst loading the Suzuki–Miyaura cross-coupling reaction, irrespective of
could be easily decreased to 50 ppm [entry 1 turnover the identity of the substituent on the triazole group. This
number (TON) 16000, TOF 4570 h–1] while maintaining work represents a simple and straightforward strategy for
good activity in the coupling of 4-bromoacetophenone with various functionalisations of NHC–palladium complexes.
(4-methoxyphenyl)boronic acid, achieving full conversion Further structural studies of these new functionalised
within 2 hours at 100 °C (isolated yield 80%). Similar ac- NHC-based ligands and their complexes are currently un-
tivity was observed for 4-bromonitrobenzene (entry 2, TON derway for various catalytic applications.
1780, TOF 710 h–1). Switching to an electron-rich aryl
bromide did not lower the reaction efficiency as exemplified
by the results obtained with 4-bromotoluene (entry 3, TON
Experimental Section
1500, TOF 900 h–1). Interestingly, variation in the nature of
General Methods: All commercially available reagents were used as
received. Unless otherwise stated, all reactions were carried out un-
the boronic acid partner was also well tolerated as naphth-
alene-1-boronic acid could also be coupled to 4-bromo-
der Argon with Schlenk techniques. Dichloromethane and pentane
acetophenone with a catalytic activity in the same range
were dried under N2 with a solvent purification system (SPS). Ace-
(entry 4, TON 1920, TOF 1150 h–1). Although a partial
tonitrile was distilled under Argon from CaH2. NMR spectra were
coupling is also observed for aryl chlorides such as 4-
chloroacetophenone and 4-chlorotoluene, with formation
of the product being observed after 2 h, the conversion re-
mains incomplete after 20 h at 100 °C with a 1% catalyst
loading. Therefore these complexes have not been further
considered for this application.
recorded at 25 °C with a Bruker Avance 300, 400 Ultrashield,
DPX300 or Fourier 300 Ultrashield apparatus. 1H NMR and
13C{1H} NMR chemical shifts are referenced to the solvent signal.
31P{1H} NMR chemical shifts are referenced to an external stan-
dard (85% aqueous H3PO4). 19F{1H} NMR chemical shifts are
referenced to an external standard (CFCl3). For atom number as-
signments for NMR spectroscopy, see Supporting Information.
HRMS analyses were carried out with a Xevo G2 QTOF spectrom-
eter (Waters). Hereafter the typical synthetic procedures, exem-
plified on the preparation of ligands 4b and 6b and the correspond-
ing complexes 9b and 10b, are presented. Characterisation of all
the triazole-based imidazolium salts 4a–c, 6a–c and 8b and all the
palladium complexes 9a–c, 10a–c and 11b are detailed in Support-
ing Information.
Conclusions
In summary, an easy to perform synthesis of new func-
tionalised N-heterocyclic carbene precursors and their cor-
responding palladium(II) complexes has been established.
This gave access to an array of organometallic complexes,
which have been fully characterised. The introduction of a
triazole moiety in the vicinity of the coordination sphere
of the metal eases the isolation of air- and moisture-stable
complexes despite the relatively nonbulky imidazolylidene
substituents.
Ligand Synthesis: A mixture of 2b (1 mol-equiv.), fluorous azide 3
or aryl azide 5 (1.0 mol-equiv.), diisopropylamine (1.0 equiv.), CuI
catalyst (5 mol-%) and trifluoroethanol (5 mL/mmol of imid-
azolium) was stirred overnight under an argon atmosphere at
25 °C. The reaction mixture was then filtered and concentrated.
The resulting precipitate was washed with Et2O and dried under
vacuum to afford the corresponding fluorous ligand 4b or nitro-
substituted ligand 6b.
Moreover it has been shown that the triazole does not
coordinate the metal. Nevertheless the complexes exhibited
interesting structural features such as a Pd–HC anagostic
interaction (involving a proton on the methylene bridge)
evidenced through a crystallographic study. It must also be
pointed out that the introduction of the triazole group did
1-Ethyl-3-[4-methyl-1-(1H,1H,2H,2H-perfluororoctyl)-1H-1,2,3-tri-
azole]imidazolium Bromide (4b): Beige solid (1.45 g, 87%). 1H
NMR (400 MHz, CD3CN): δ = 9.21 (br. s, 1 H, H2), 8.29 (s, 1 H,
H10), 7.59 (d, J4,5 = 1.8 Hz, 1 H, H4), 7.49 (d, J5,4 = 1.8 Hz, 1 H,
not preclude the catalytic activities of these complexes in H5), 5.58 (s, 2 H, H8), 4.80–4.76 (t, J11,12 = 7 Hz, 2 H, H11), 4.27–
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4.21 (q, J6,7 = 7.3 Hz, 2 H, H6), 3.00–2.89 (tt, J11,12 = 7 Hz, J = 100 °C until full conversion of the arylbromide. The reaction mix-
19 Hz, 2 H, H12), δ = 1.50–1.47 (t, J6,7 = 7.3 Hz, 3 H, H7) ppm.
13C{1H} NMR (100.6 MHz, [D6]DMSO): δ = 141.0 (C9), 136.4
(C2), 125.6 (C10), 122.9 (C4), 122.8 (C5), 119.2, 118.0, 111.9, 111.7,
ture was cooled to room temperature, after which water was added.
The mixture was then extracted with diethyl ether and the organic
extracts dried with MgSO4, filtered and concentrated in vacuo. Pu-
111.2, 109.3 (C13–18), 44.8 (C8), 44.30 (C11), 42.4 (C6), 30.7 (C12), rification by flash chromatography gave the pure cross-coupled
15.5 (C7) ppm. 19F{1H} NMR (376 MHz, CD3CN): δ = –81.6 (3 product. The spectroscopic data were in agreement with those pre-
F), –114.7 (2 F), –122.4 (2 F), –123.4 (2 F), –124.0 (2 F), –126.7 (2
viously reported for 4-methoxy-4Ј-acetyl-biphenyl,[25] 4-methoxy-
4Ј-nitrobiphenyl,[26] 4-methoxy-4Ј-methylbiphenyl[25,27] and 4Ј-(1-
naphthyl)-acetophenone.[28]
F) ppm. ESI-MS: m/z = 524 [M+].
1-Ethyl-3-[4-methyl-1-(4-nitrophenyl)-1H-1,2,3-triazole]imidazolium
Bromide (6b): Brown solid (0.712 g, 99%). 1H NMR (400 MHz,
[D6]DMSO): δ = 9.38 (s, 1 H, H2), 9.18 (s, 1 H, H10), 8.50 (d, J13,12
= 9.2 Hz, 2 H, H13), 8.24 (d, J12,13 = 9.2 Hz, 2 H, H12), 7.89–7.87
(m, 2 H, H4, H5), 5.70 (s, 2 H, H8), 4.25 (q, J6,7 = 7.3 Hz, 2 H,
H6), 1.44 (t, J7,6 = 7.3 Hz, 3 H, H7) ppm. 13C{1H} NMR
(75.5 MHz, [D6]DMSO): δ = 147.4 (C14), 142.6 (C9), 141.0 (C11),
136.6 (C2), 126.1 (C13), 124.2 (C5), 123.2 (C10), 122.9 (C4), 121.4
(C12), 44.9 (C8), 44.0 (C6), 15.5 (C7) ppm. ESI-MS: m/z = 299.0
[M+].
Crystal Structure Determination: Diffraction data for 10b were col-
lected at low temperature (180 K) with an Agilent Technologies
GEMINI EOS diffractometer with graphite-monochromated Mo-
Kα radiation (λ = 0.71073 Å). The diffractometer was equipped
with an Oxford Instrument Cryojet cooler device. The structure
was solved by direct methods with SIR92.[29] All non-hydrogen
atoms were refined anisotropically by means of least-squares pro-
cedures on F2 with the aid of the program SHELXL-97.[30] The H
atoms were refined isotropically at calculated positions by a riding
model with their isotropic displacement parameters constrained to
1.5ϫ the equivalent isotropic displacement parameters of their
pivot atoms for terminal sp3 carbon and 1.2ϫ for all other carbon
atoms.
Complex Synthesis: The imidazolium-based ligand 4b or 6b (1 mol-
equiv.), PPh3 (1 mol-equiv.), triethylamine (5 mol-equiv.) and lith-
ium bromide (30 mol-equiv.) were added sequentially to a stirred
solution of Pd(OAc)2 (1 mol-equiv.) in acetonitrile (10 mL/mmol of
ligand) under argon. The resulting mixture was heated at 70 °C for
12 h under argon. The reaction mixture was then filtered and the
resulting precipitate was washed with water and dried under vac-
uum to afford the corresponding palladium complex 9b or 10b.
CCDC-939074 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Dibromo-{1-ethyl-3-[4-methyl-1-(1H,1H,2H,2H-perfluororoctyl)-
1H-1,2,3-triazole]imidazol-2-ylidene}triphenylphosphinepalla-
dium(II) (9b): Yellow solid (0.14 g, 75 %). 1H NMR (300 MHz,
CD3CN): δ = 8.10 (s, 1 H, H10), 7.59–7.45 (m, 15 H, PPh3), 6.86
(d, J4,5 = 2.0 Hz, 1 H, H4), 6.79 (d, J5,4 = 2.0 Hz, 1 H, H5), 5.51
(d, J8,8Ј = 15.3 Hz, 1 H, H8), 5.08 (d, J8Ј,8 = 15.3 Hz, 1 H, H8Ј),
4.86 (t, J11,12 = 7.0 Hz, 2 H, H11), 4.21–4.14 (dq, J6,7 = 7.3 Hz, J6,6Ј
= 13.4 Hz, 1 H, H6), 3.62–3.54 (dq, J6Ј,7 = 7.3, J6Ј,6 = 13.4 Hz, 1
H, H6Ј), 3.0–2.83 (tt, J12,11 = 7.0, J = 18.9 Hz, 2 H, H12), 1.22 (dd,
J7,6 = J7,6Ј = 7.3 Hz, 3 H, H7) ppm. 13C{1H} NMR (100.6 MHz,
[D6]DMSO): δ = 158.8 (C2), 140.8 (C9), 133.9 (d, J = 10.9 Hz,
PPh3), 131.1 (PPh3), 130.1 (d, J = 52.0 Hz, PPh3), 128.5 (d, J =
10.9 Hz, PPh3), 125.0 (C10), 122.1 (C5), 121.9 (C4), 45.2 (C6), 45.0
(C8), 41.8 (C11), 30.3 (t, J = 20.0 Hz, C12), 14.7 (C7) ppm. 19F{1H}
NMR (376 MHz, CD3CN): δ = –81 (3 F), –114.5 (2 F), –122.4 (2
F), –123.3 (2 F), –124.0 (2 F), –126.7 (2 F) ppm. 31P{1H} NMR
(122 MHz, [D6]DMSO): δ = 26.6 ppm. HRMS: calcd. [M – Br]+
972.0171; found 972.0189.
Supporting Information (see footnote on the first page of this arti-
cle): Text detailing the preparation and characterisation of all li-
gand precursors and palladium complexes, catalysis protocols and
X-ray crystallographic data.
Acknowledgments
The Agence Nationale de la Recherche (ANR), Research Grant
Switchcat 2010 JCJC7051, and the Centre National de la Recher-
che Scientifique (CNRS) are acknowledged for their support. The
authors also thank Sonia Mallet-Ladeira for the crystal structure
determination and Montserrat Gomez for scientific discussions.
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Received: December 20, 2013
Published Online: March 13, 2014
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