Modular synthesis of bidentate triazolyl-functionalized N-heterocyclic carbenes and their palladium complexes

Article abstract of DOI:10.1021/om100435x

New heterobidentate N-heterocyclic carbene-triazolyl ligands and several of their palladium(II) complexes have been synthesized in a modular fashion using click chemistry. These complexes are the first examples where triazolyl-substituted NHCs exhibit bidentate behavior, which was confirmed by NMR and X-ray diffraction studies. The synthesis of the complexes could be achieved in relatively few steps by introducing the diversity at a late stage in the synthesis. The complexes are active precatalysts in the transfer semihydrogenation of alkynes to Z-alkenes, with activity and selectivity depending on the triazolyl substituent and the NHC nitrogen substituent. Selectivities as high as 99% were observed.

Full text of DOI:10.1021/om100435x

Organometallics 2010, 29, 3109–3116 3109
DOI: 10.1021/om100435x
Modular Synthesis of Bidentate Triazolyl-Functionalized N-Heterocyclic
Carbenes and Their Palladium Complexes
Stefan Warsink,† Ruben M. Drost,† Martin Lutz,‡ Anthony L. Spek,‡ and
Cornelis J. Elsevier*,†
†Molecular Inorganic Chemistry, van’t Hoff Institute for Molecular Sciences, University of Amsterdam,
Nieuwe Achtergracht 166, Amsterdam, The Netherlands, and ‡Faculty of Science, Bijvoet Center for
Biomolecular Research, Utrecht University, Utrecht, The Netherlands
Received May 6, 2010
New heterobidentate N-heterocyclic carbene-triazolyl ligands and several of their palladium(II)
complexes have been synthesized in a modular fashion using click chemistry. These complexes are the
first examples where triazolyl-substituted NHCs exhibit bidentate behavior, which was confirmed by
NMR and X-ray diffraction studies. The synthesis of the complexes could be achieved in relatively few
steps by introducing the diversity at a late stage in the synthesis. The complexes are active precatalysts in the
transfer semihydrogenation of alkynes to Z-alkenes, with activity and selectivity depending on the triazolyl
substituent and the NHC nitrogen substituent. Selectivities as high as 99% were observed.
Introduction
the choice of an appropriate hemilabile donor, an increase in
complex stability and selectivity can be obtained without loss of
activity.16 The secondary donor atoms employed are most often
phosphorus, oxygen, and nitrogen. Through these donors, a
large range of steric and electronic effects can be imparted on the
metal center.
N-Heterocyclic carbenes (NHCs) are versatile and widely
applied ligands, with strong electron-donating capacities.1-4
Their use leads to electron-rich and often (air-)stable complexes,
due to the strong carbon-metal bond.5,6 Complexes of NHCs
with transition metals (TMs) are routinely applied in catalytic
transformations such as (transfer) hydrogenation, C-C and
C-heteroatom couplings, and polymerization reactions.7,8 Be-
sides the monodentate carbene systems, a significant amount of
the reported complexes involve homo- and heteropolydentate
ligands. By functionalizing NHCs with secondary donors, dis-
symmetric bidentate complexes can be obtained.9-15 Through
We have pursued the synthesis of low-valent palladium
complexes incorporating one strong carbon donor and one
weak nitrogen donor with excellent results.17,18 For these
complexes a marked difference in the trans-positions as well
as an influence on complex stability and selectivity in cata-
lysis are observed. We are interested in the use of various
secondary nitrogen donors to complement the NHC, with
the goal of obtaining stable, yet catalytically active com-
plexes. Even though a lot of different donor moieties have
been reported, one intuitively obvious motif has not yet been
reported as a ligating moiety. The 1,2,3-triazolyl group has
gained popularity through the “click” chemistry coined by
Sharpless and Meldal.19-21 This N-heterocycle can be selec-
tively and reliably synthesized from an alkyne and an azide
under copper(I) catalysis.22-24 Because of the ease of the
*To whom correspondence should be addressed. E-mail: c.j.elsevier@
uva.nl.
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2010 American Chemical Society
Published on Web 06/22/2010
pubs.acs.org/Organometallics

3110 Organometallics, Vol. 29, No. 14, 2010
Warsink et al.
Scheme 2. Cu-Catalyzed Cycloaddition to Form the
1,2,3-Triazole Moiety
Scheme 1. Synthesis of N-(Prop-2-ynyl)imidazolium Salts
protocol and the high functional group tolerance, click
chemistry is frequently employed in the synthesis of libraries.
However, in organometallic and coordination chemistry,
this versatile building block has not yet found the widespread
use it has in other fields of chemistry. Most applications of
click chemistry involve the use of the triazole as a linker,25-27
except examples like Clickphine and others.28,29 Further-
more, click chemistry is inherently suited for use in combi-
natorial research, which could make optimization in a
catalytically active group of complexes a more facile process.
By combining the NHC and the triazole motifs in one
ligand, we hope to generate a valuable new ligand type. We
here report the modular synthesis and structural characteri-
zation of functionalized N-aryl-N0-propargyl imidazolium
bromides and N-aryl-N0-triazolyl imidazolium bromides and
their complexes with palladium(II), as well as a preliminary
catalytic screening in the transfer hydrogenation of alkynes
to Z-alkenes.
with the principles of click chemistry is the presence of the
cationic charge of the imidazolium salt. With stoichiometric
amounts of copper sulfate and sodium ascorbate in acetonitrile,
the products 5-8 were obtained in acceptable to excellent yields
(60-99%) and in high purity (Scheme 2).
The 1H NMR signal for the imidazolium proton in CDCl3 is
generally found around 10.0 ppm, except in compound 8,
where the electron-poor N-aryl substituent induces a downfield
shift to 10.86 ppm. The best indication of successful conversion
1
is obtained from the signal of the triazolyl moiety. In the H
NMR, the triazolyl signal is observed between 8.5 and 9.0 ppm,
with a notable exception for compound 5e, where the electron-
withdrawing para-trifluoromethylphenyl substituent induces a
chemical shift of 9.44 ppm. For all ligand precursors, the
methylene linker shows a slight downfield shift compared to
the starting materials 1-4.
Results and Discussion
An alternative approach to construct this ligand scaffold
could be to first synthesize the triazole, which is then coupled
to an imidazole. After several trials with propargyl bromide,
it appeared that in our hands the cycloaddition suffered from
side reactions.25 Also propargyl alcohol as a building block
did not give a viable route toward the desired triazolyl-
substituted imidazolium salts. We abandoned this alterna-
tive for the above-mentioned, more reliable protocol.
Silver(I) NHC Complexes. With the ligand precursors in
hand, the synthesis of the NHC-silver(I) complexes was
attempted first. The preference for this reaction is under-
standable, as the desired complexes are often air stable, and
generally no solvent pretreatment or additional base is
required when silver(I) oxide is used to generate the carbene
complexes.32-34 Deprotonation is observed at C-2 of the
NHC, and other acidic protons in the structure are generally
not affected by the silver(I) oxide. However, the resulting
complexes 9a-c were highly unstable and could be observed
only in solution. The main proof of complex formation is the
disappearance of the C-2 proton signal in the 1H NMR and
the observation of the carbene carbon at 182.2 ppm in the 13C
NMR (for 9b). On removal of the dichloromethane solvent,
the complexes reverted to the imidazolium salts (Scheme 3).
One complex that could be isolated in good yield was the
uncyclized silver(I) alkynyl-carbene complex 10, which was
stable for short periods under an inert atmosphere (Scheme 4).
The carbene carbon of 10 has a resonance at 183.8 ppm in
the 13C NMR, which is a normal value for silver(I)-carbene
species. On the basis of this signal, and the fact that the
Ligand Synthesis. Easy access to the NHC-triazolyl biden-
tate ligands was accomplished in essentially two steps by first
reacting N-aryl imidazoles30 with propargyl bromide in reflux-
ing acetonitrile, providing the alkynyl-imidazolium salts 1-4 in
near quantitative yields, either by precipitation from or by
concentration of the reaction mixture (Scheme 1).25
1
The products could easily be identified by their H NMR
spectra, where the signal characteristic for the imidazolium
proton is observed at 9.57 ppm (for 1 in DMSO-d6) or between
10.5 and 11.2 ppm (in CDCl3). The propargyl substituent gives
signals around 5.7 ppm for the CH2 moiety and around
2.8 ppm for the alkynyl proton.
The propargyl imidazolium salts 1-4 were then reacted with
several azides31 in the presence of catalytic amounts of copper-
(II) sulfate and sodium ascorbate (NaAsc) to form the 1,2,3-
triazolyl moiety. However, these conditions did not give the
products in sufficient yield. A possibility for this discrepancy
(25) Sommer, W. J.; Weck, M. Langmuir 2007, 23, 11991–11995.
(26) Li, H.; Cheng, F.; Duft, A. M.; Adronov, A. J. Am. Chem. Soc.
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N. K.; Wang, L.; Duran, R. S.; Ciaramitaro, D. A.; Bedford, C. D.
J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 3748–3756.
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Hiemstra, H.; Reek, J. N. H.; van Maarseveen, J. H. Org. Lett. 2006, 8,
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M.; Spek, A. L.; van Koten, G.; Klein Gebbink, R. J. M. Dalton Trans.
2007, 1273–1276.
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Article
Organometallics, Vol. 29, No. 14, 2010 3111
Scheme 3. Ag(I)-NHC Complexes Are Not Isolable, But Are
Formed in Solution
very bulky 2,6-diisopropylphenyl substituents on both the
NHC and the triazolyl donor, could a trace amount of
1
monodentate coordinated product be observed in the H
NMR. Furthermore, the triazolyl-CH shows a resonance
between 9.31 and 8.37 ppm. These two extreme values
correspond to complexes bearing electron-poor and elec-
tron-rich substituents on the triazolyl moiety, respectively.
The carbene carbon is found between 176 and 181 ppm in the
13C NMR, which is a value commonly observed for divalent
palladium(NHC) complexes.1
In most cases, the pure compounds could be precipitated
from the reaction mixture with pentane. If minor impurities
were present, the complexes could easily be purified in air by
column chromatography over basic alumina with a methanol-
chloroform gradient. This demonstrates the high stability of
the product complexes 11-14, especially viewed relative to the
silver complexes 9a-c and the palladium complex 15 (vide
infra). Apparently a subtle interplay between the strong donor
capability of the NHC and the weaker donicity of the triazolyl
moiety as well as the metal center determines the stability
of the isolated complexes. Their formation is not affected by
these effects, as in all cases the products were observed in
solution.
Scheme 4. The Silver(I)-Alkynyl NHC Complex Is Moderately
Stable
Scheme 5. Direct Synthesis of Palladium Allyl NHC Complexes
To lend more weight to this hypothesis, freshly prepared
alkynyl-substituted silver-NHC complex 10 was also reacted
with palladium allyl chloride dimer in THF to obtain the
monodentate palladium-NHC complex 15. The desired
compound was formed in quantitative yield, but decom-
posed on isolation. However, the click reaction could con-
secutively be performed on the palladium-alkynyl complex
in the same pot. This way, C-N-ligated complexes 11-14
can be obtained from one common precursor complex in
reasonable yields (40-60%), reducing the number of reac-
tions required for assembly of a library of precatalysts
(Scheme 6). The complexation is the only step for which
the yield is moderate, but the overall yields are acceptable.
This one-pot procedure shows the high potential of this
modular approach toward diversification of the complexes.
X-ray Crystallography. In order to be completely certain of
the coordinative behavior of our complexes, we attempted to
grow X-ray quality crystals. However, all attempts to obtain
single crystals of the halide complexes failed. After exchange
of the chloride in 11c for the noncoordinating triflate anion
to obtain complex 16, we were able to grow crystals by slow
evaporation of a dichloromethane solution (Figure 1).
The coordinative properties of the ligand in 16 do not
change on anion exchange, as no significant changes are
propargyl signals are not shifted significantly from the
imidazolium salt 1, a silver-alkyne interaction is not likely.
In mass spectrometry experiments, a bis-carbene silver spe-
cies that corresponds to the cationic part of 10 was detected.
This is a commonly observed structural motif for silver(I)
NHC complexes.
Palladium Complexes. When the silver(I) NHC complex 9c
was treated with η3-allylpalladium chloride dimer in situ, the
product complex 11c could be obtained only in a low yield of
49%. The only widely applied alternative for the silver-assisted
NHC formation is deprotonation of the imidazolium salts to
obtain the free carbene, which is most often treated with a TM
precursor complex in situ. We were pleased to find that none of
the ligand precursors 5-8 seemed to suffer any ill effects when
submitted to the strongly basic conditions associated with this
protocol. In contrast to earlier experiments, potassium tert-
butoxide as base in THF smoothly gave formation of the target
tethered NHC-triazolyl ligands. Subsequent metalation with
η3-allylpalladium chloride dimer yielded the desired palladium
complexes 11-14 in high yields (Scheme 5). It has to be noted
that in our hands the complexation failed with other palladium
precursors such as divalent PdCl2(COD) andPd(Me)Cl(COD)18
and zerovalent precursors such as Pd(tBuDAB)(ma).17
In the 1H NMR spectrum of 11-14 the bidentate nature of
the ligand could clearly be deduced from the splitting of the
methylene linker signal into an AB pattern caused by dis-
symmetrization of the protons. Only in complex 12, with
1
observed in the H and 13C NMR spectra. The complex
crystallizes in the triclinic space group P1 with one indepen-
dent cationic palladium moiety and one triflate anion. The
allyl fragment is disordered in the crystal, confirming that
both the syn- and anti-conformers are present. The X-ray
crystal structure shows that the ligand is coordinated in a
bidentate fashion; along with the NMR experiments this is
strong evidence of the chelating properties of the ligand. The
complex adopts a square-planar geometry, with the NHC-
triazolyl ligand forming a six-membered chelate ring with the
TM center. The ligand has a bite angle of 86.41(10)°, a small
deviation from the ideal 90°. The carbene palladium distance
is 2.044(3) A, which is a length commonly reported for NHC-
palladium allyl complexes.35 The distance between N(3) of
(35) Danopoulos, A. A.; Tsoureas, N.; Macgregor, S. A.; Smith, C.
Organometallics 2007, 26, 253–263.

3112 Organometallics, Vol. 29, No. 14, 2010
Warsink et al.
Scheme 6. Modular Approach to the Synthesis of NHC-Palladium Allyl Complexes
Scheme 7. Transfer Hydrogenation of Alkynes Catalyzed by
Pd(NHC)allyl Complexes
Table 1. Catalytic Transfer Hydrogenation of
1-Phenyl-1-propynea
selectivity
Z/E/alkane (%)
entry
precatalyst
TOFinit
1
2
3
4
5
6
7
8
11a
11b
11c
11d
11e
12
76.8
81.4
55.3
62.9
60.1
115.5
59.4
54.7
94/5/1
93/5/2
94/5/1
94/5/1
94/5/1
85/13/2
92/7/1
93/6/1
13
14
a Reaction conditions: 1% Pd complex, 150 mM alkyne, 750 mM
triethylammonium formate, and 150 mM p-xylene (internal standard) in
15 mL of refluxing acetonitrile. Selectivity at full conversion reported.
Figure 1. Displacement ellipsoid plot (50% probability level) of the
cationic part of 16 in the crystal. H atoms and the triflate anion are
omitted for the sake of clarity. Only the major form of the disordered
allyl ligand is shown. Selected bond lengths (A) and angles (deg):
Pd(1)-C(1) = 2.044(3), Pd(1)-N(3) = 2.098(2), Pd(1)-C(17) =
2.186(3), Pd(1)-C18A=2.163(6), Pd(1)-C18B=2.132(8), Pd(1)-
C(19)=2.111(3); C(1)-Pd(I)-N(3)=86.41(10).
Catalytic Transfer Hydrogenation. As a probe for catalytic
activity of 11-14, the partial transfer hydrogenation of
alkynes to Z-alkenes developed in our laboratories was used
as a tool (Scheme 7).17,37
This remarkable transformation shows the unique reactivity
of the NHC-palladium(0) systems;37 1-phenyl-1-propyne is
converted to Z-1-phenyl-1-propene with a selectivity of 96%
using an in situ generated Pd(0) complex bearing a monodentate
NHC. Additionally, the chemoselectivity is very high, showing a
high functional group tolerance. When a tethered secondary
donor is introduced onto the NHC ligand, the selectivity could
be increased to 99%, albeit at the cost of reaction rate.17 An
interesting result was that the divalent palladium species 11-14
did show significant catalytic activity and selectivity, where all
earlier experiments indicated that a zerovalent palladium source
was a prerequisite for this protocol. Most probably, reductive
elimination of the auxiliary ligands takes place, leading to a
zerovalent active species stabilized by the coordinating solvent.
The results for the hydrogenation of our model substrate
1-phenyl-1-propyne are depicted in Table 1.
the triazolyl donor and Pd is 2.098(2) A; this is rather long
compared to other donor moieties, reflecting the weaker
donating ability of the triazole. For example, in a palladium
complex bearing a picolyl-functionalized NHC, the distance
between palladium and the secondary nitrogen donor is
2.042(8) A.36 In an analogous system with a 2-pyridyl-functio-
nalized NHC, the distance between the nitrogen donor atom
and palladium is 2.113(4) A; this is a longer distance because
the described system has a more strained geometry, with a less
flexible secondary donor causing a decrease in orbital over-
lap (bite angle 78.44(16)°).35 In a comparable monodentate
triazole palladium complex, 2.1664(17) A was reported for
the Pd-N distance.29 Finally, the different trans-influence of
the donors in the bidentate ligand was qualitatively gauged in
the different bond lengths between the palladium center and
the distal allyl carbons. The Pd(1)-C(17) distance of the allyl-
carbon in trans-position to the carbene is 2.186(3) A, which is
significantly longer than that of the allyl-carbon trans to the
triazolyl donor (2.111(5) A).
Several conclusions can be drawn from the results. First,
all precatalysts show high Z-selectivity at full conversion.
Precatalyst 12, with two bulky 2,6-diisopropylphenyl sub-
stituents, shows the lowest selectivity toward the Z-alkene,
but still it amounts to 85%, with mostly E-alkene as the
(36) Barczak, N. T.; Grote, R. E.; Jarvo, E. R. Organometallics 2007,
26, 4863–4865.
(37) Hauwert, P.; Maestri, G.; Sprengers, J. W.; Catellani, M.; Elsevier,
C. J. Angew. Chem., Int. Ed. 2008, 47, 3223–3226.

Article
Organometallics, Vol. 29, No. 14, 2010 3113
byproduct (entry 6). The selectivity toward the desired
product and the turnover frequency seem to be related; a
high rate diminishes the selectivity. Apparently, a bulky
substituent on the NHC gives a higher TOF; for the triazolyl
donor, the electronic properties of its substituent seem to
determine the rate of the reaction to some degree. For the
para-anisyl and DiPP substituents, a beneficial effect is seen
(entries 2 and 1, respectively). However, the adamantyl
substituent gives the lowest TOF of all experiments (entry 3),
showing the need for an aryl substituent on the triazole donor.
For all complexes it is necessary to monitor the progress of the
reaction, as side reactions occur after full consumption of the
alkyne. After 24 h, all reactions showed between 60% and 70%
Z-1-phenyl-1-propene with roughly equal amounts of E-1-
phenyl-1-propene and propylbenzene as byproduct.
high-frequency shifts relative to the external TMS reference.
High-resolution mass spectra were recorded on a JEOL JMS
SX/SX102A four-sector mass spectrometer; mass samples were
loaded in a matrix solution (3-nitrobenzyl alcohol) onto a
stainless steel probe and bombarded with xenon atoms with
an energy of 3 keV. During the high-resolution FAB-MS mea-
surements a resolving power of 10 000 (10% valley definition)
was used. Gas chromatography analyses were performed with a
Carlo Erba HRGC 8000 Top instrument using a DB-5 capillary
column and p-xylene as internal standard.
Synthesis of N-Mesitylimidazole 1. A literature procedure30
was modified as follows: instead of purifying the crude product
by column chromatography, the product (11.9 g, 64%) was
obtained as off-white crystals by recrystallization from THF.
General Procedure for the Synthesis of [(1-(Prop-2-ynyl)-3-
aryl)imidazolium] Bromides. A 0.3 M solution of N-arylimida-
zole was heated to reflux, after which 2.2 equiv of propargyl
bromide (80% wt solution in toluene) was added. The solution
was stirred at reflux for 2 days. The reaction mixture was then
Conclusions
concentrated under reduced pressure to yield the product.
[(1-(Prop-2-ynyl)-3-mesityl)imidazolium] Bromide, 1.
We have synthesized a series of NHC-triazolyl ligand
precursors with various substitution patterns via click chemi-
stry and successfully ligated them in a bidentate fashion to
divalent palladium, resulting in very stable CNHC-N che-
lates. These complexes constitute the first example of NHC-
ligand scaffolds, the weakly donating triazolyl moiety of
which is employed as a secondary donor. A delicate balance
was observed for the stability of the synthesized complexes:
apparently the donicity of the triazole influences the stability
of the palladium complexes to a high degree. This is a large
contrast to our earlier research,17,18 where the amine donor
moiety stabilized the palladium irrespective of substitution
pattern of the ligands or oxidation state of the transition
metal. This difference demonstrates the importance of the
nature of the secondary donor and supports the research
performed in this area. Next to the coordinative behavior of
the ligands itself, catalytic transfer hydrogenation of 1-phen-
yl-1-propyne was performed as a probe for the influence of
the ligand on palladium. The complexes showed a relatively
high activity, and the initial selectivity for the desired Z-1-
phenyl-1-propene was high. However, products due to iso-
merization and further hydrogenation were formed after full
consumption of the substrate. This shows that the affinity of
the triazole donor for the palladium is lower than that of the
Z-alkene product, which can then be isomerized or reduced
to give the byproduct. High selectivities can be obtained if
the reaction were to be monitored closely.
25
The
product was obtained as a white solid in 99% yield (5.62 g). 1H
NMR (DMSO-d6): δ 9.57 (s, 1H, NCHN), 8.13 (s, 1H, CH), 7.99
(s, 1H, CH), 7.15 (s, 2H, aryl-H), 5.31 (s, 2H, CH2), 3.91 (s, 1H,
CCH), 2.50 (s, 3H, p-aryl-CH3), 2.01 (s, 6H, o-aryl-CH3).
[(1-(Prop-2-ynyl)-3-(2,6-diisopropylphenyl))imidazolium] Bro-
mide, 2. The product was obtained as a white solid in 99%
yield (2.78 g). 1H NMR (CDCl3): δ 10.51 (dd, 4J(HH)=1.8 Hz,
4J(HH) = 1.8 Hz, 1H, NCHN), 8.05 (dd, 3J(HH) = 1.8 Hz,
4J(HH)=1.8 Hz, 1H, CH), 7.59 (t, 3J(HH)=8.1 Hz, 1H, p-aryl-
H), 7.36 (d, 3J(HH)=8.1 Hz, 1H, m-aryl-H), 7.28 (dd, 3J(HH)=
1.8 Hz, 4J(HH)=1.8 Hz, 1H, CH), 5.86 (d, 4J(HH)=2.7 Hz, 2H,
CH2), 2.85 (t, 4J(HH)=2.7 Hz, 1H, CCH), 2.33 (dq, 3J(HH)=6.9
Hz, 3J(HH)=6.9 Hz, 2H, iPr-CH), 1.24 (d, 3J(HH)=6.9 Hz, 6H,
iPr-CH3), 1.17 (d, 3J(HH)=6.9 Hz, 6H, iPr-CH3). 13C NMR (CD-
Cl3): δ 145.4 (o-aryl-C), 137.9 (NCN), 132.1 (i-aryl-C), 130.0 (p-
aryl-C), 124.8 (m-aryl-C), 124.2 (CH), 122.8 (CH), 78.0 (alkyne),
74.4 (alkyne), 40.8 (CH2), 28.7 (iPr-CH), 24.4 (iPr-CH). MS-
(FABþ, m/z): calcd for C18H23N2 [M - Br]þ 267.1861; found
267.1857.
[(1-(Prop-2-ynyl)-3-(4-methoxyphenyl))imidazolium] Bromide,
3. The product was obtained as a white solid in 99% yield (0.66
g). 1H NMR (CDCl3): δ 10.97 (dd, 4J(HH)=1.8 Hz, 4J(HH)=1.8
Hz, 1H, NCHN), 7.70 (dd, 3J(HH)=1.8 Hz, 4J(HH)=1.8 Hz, 1H,
CH), 7.64 (d, 3J(HH)=6.9 Hz, 2H, aryl-CH), 7.54 (dd, 3J(HH)=
1.8 Hz, 4J(HH)=1.8 Hz, 1H, CH), 7.05 (d, 3J(HH)=6.9 Hz, 2H,
aryl-CH), 5.63 (d, 4J(HH)=2.4 Hz, 2H, CH2), 3.86 (s, 3H, OMe),
2,70 (t, 4J(HH)=2.4 Hz, 1H, CCH). 13C NMR (CDCl3): δ=161.1
(p-aryl-C), 136.1 (i-aryl-C), 127.3 (NCHN), 123.6 (o-aryl-C), 122.0
(CH), 120.7 (CH), 115.7 (m-aryl-C), 77.9 (alkyne), 77.2 (alkyne),
55.8 (OMe), 40.4 (CH2). MS(FABþ, m/z): calcd for C13H13N2O
[M - Br]þ 213.1028; found 213.1031.
Experimental Section
[(1-(Prop-2-ynyl)-3-(4-trifluoromethylphenyl))imidazolium] Bro-
mide, 4. The product was obtained as a white solid in 99% yield
(0.94 g). 1H NMR (CDCl3): δ 11.14 (dd, 4J(HH) = 1.8 Hz,
General Procedures and Instrumentation. All reactions invol-
ving transition metal complexes were performed using standard
Schlenk techniques under an atmosphere of dry nitrogen.
Solvents were distilled using standard procedures.38 All chemi-
cals were obtained from commercial suppliers and were used as
received, with the exception of N-aryl imidazoles 2-4,30 aryl
azides,31 and palladium allyl chloride dimer,39 which were
synthesized according to literature procedures. NMR measure-
ments were performed on Varian Mercury 300 (1H, 300.13
MHz; 13C, 75.47 MHz), Bruker DRX 300 (1H, 300.11 MHz;
13C, 75.47 MHz), and Varian Inova 500 (1H, 499.86 MHz; 13C,
125.70 MHz) spectrometers. 13C NMR spectra were measured
with 1H decoupling. Positive chemical shifts (δ) are denoted for
3
4J(HH)=1.8 Hz, 1H, NCHN), 8.03 (d, J(HH)=8.4 Hz, 2H,
aryl-H), 7.92 (dd, 3J(HH)=1.8 Hz, 4J(HH)=1.8 Hz, 1H, CH),
7.84 (d, 3J(HH)=8.4 Hz, 2H, aryl-H), 7.84 (dd, 3J(HH)=1.8 Hz,
4J(HH) = 1.8 Hz, 1H, CH), 5.62 (d, 4J(HH)=2.4 Hz, 2H, CH2),
2.76 (t, 4J(HH) = 2.4 Hz, 1H, CCH). 13C NMR (CDCl3): δ =
137.1 (NCHN), 136.5 (p-aryl-C), 132.6 (CF3), 128.0 (aryl-C),
125.0 (i-aryl-C), 123.1 (CH), 122.8 (aryl-C), 120.9 (CH), 78.4
(alkyne), 73.9 (alkyne), 40.7 (CH2). MS(FABþ, m/z): calcd for
C13H11N2BrF3 [M þ H]þ 331.0058; found 331.0057.
General Procedure for the Cycloaddition of Alkynyl-Imidazo-
lium Salts with Azides. Copper sulfate pentahydrate (4 mmol)
and 1.5 equiv of sodium ascorbate were suspended to 0.1 M in
acetonitrile. The mixture was stirred for 20 min at 60 °C, after
which 1 equiv of alkynyl-imidazolium salt was added and stirred
for an hour. Azide (1 equiv) was then added in a small volume of
(38) Perrin, D. D.; Armagero, L. F. Purification of Laboratory
Chemicals; Pergamon Press: Oxford, 1998.
(39) Angelici, R. J. Inorganic Syntheses; Wiley: New York, 1990;
Vol. 28, p 342.

3114 Organometallics, Vol. 29, No. 14, 2010
Warsink et al.
acetonitrile. The reaction was stirred at 60 °C for 2 days. The
reaction mixture was filtered after cooling to room temperature
and the solvent removed under reduced pressure. The residue
was dissolved in 50 mL of dichloromethane and was washed
with 3 ꢀ 30 mL of water. The organic layer was dried over
MgSO4 and filtered, and the solvent was evaporated to yield the
product.
(NCHN), 134.3 (o-Mes-C), 131.9 (CF3), 130.6 (i-Mes-C), 129.9
(m-Mes-C), 129.1 (triazole-C), 127.1 (aryl-C), 125.2 (i-aryl-C),
123.7 (CH), 123.3 (CH), 120.6(aryl-C), 77.8(CH2), 44.8(triazole-
CH), 21.1 (p-aryl-CH3), 17.8 (o-aryl-CH3). MS(FABþ, m/z):
calcd for C22H21F3N5 [M - Br]þ 412.1749; found 412.1742.
[1-((4-(2,6-Diisopropylphenyl)-1H-1,2,3-triazolyl)methylene-
3-(2,6-diisopropylphenyl))imidazolium] Bromide, 6. The product
was obtained as an orange solid in 93% yield (0.23 g). 1H NMR
(CDCl3): δ 9.89 (s, 1H, NCHN), 8.58 (s, 1H, triazole-H), 8.20 (s,
1H, CH), 7.53 (m, 2H, 2 p-aryl-H), 7.30 (m, 4H, 4 m-aryl-H),
7.23 (s, 1H, CH), 5.68 (s, 2H, CH2), 2.32 (m, 2H, iPr), 2.13 (m,
2H, iPr), 1.16-1.00 (m, 24H, iPr). 13C NMR (CDCl3): δ 145.6,
145.3 (2 o-aryl-C), 137.6 (NCHN), 132.7, 132.0 (2 i-aryl-C),
131.1, 130.1 (2 p-aryl-C), 128.5 (triazole-C), 124.7 (m-aryl-C),
124.7, 124.2 (2 CH), 123.9 (m-aryl-C), 77.4 (CH2), 44.7 (triazole-
CH), 28.7, 28.5 (2 iPr), 24.5, 24.3, 24.1, 23.5 (4 iPr). MS(FABþ,
m/z): calcd for C30H40N5 [M - Br]þ 470.3284; found 470.3278.
[1-((4-(2,6-Diisopropylphenyl)-1H-1,2,3-triazolyl)methylene-
3-(4-methoxyphenyl))imidazolium] Bromide, 7. The product was
obtained as a yellow solid in 60% yield (93 mg). 1H NMR
(CDCl3): δ 10.17 (s, 1H, NCHN), 8.50 (s, 1H, triazole-H), 7.96
[1-((4-(2,6-Diisopropylphenyl)-1H-1,2,3-triazolyl)methylene-
3-mesityl)imidazolium] Bromide, 5a. The product was obtained
as a yellow solid in 96% yield (1.92 g). 1H NMR (DMSO-d6): δ
9.63 (s, 1H, NCHN), 8.62 (s, 1H, triazole-H), 8.14 (s, 1H, CH),
7.98 (s, 1H, CH), 7.58 (t, 3J(HH)=7.5 Hz, 1H), 7.40 (d, 3J(HH)=
7.5 Hz, 2H), 7.15 (s, 2H, Mes), 5.78 (s, 2H, CH2), 2.32 (s, 3H,
p-aryl-CH3), 2.04 (dq, 3J(HH)=6.9 Hz, 3J(HH)=6.9 Hz, 2H, iPr-
CH), 1.99 (s, 6H, o-aryl-CH3), 1.06 (d, 3J(HH)=6.9 Hz, 6H, iPr-
i
CH3), 1.04 (d, 3J(HH) = 6.9 Hz, 6H, Pr-CH3). 13C NMR
(DMSO-d6): δ 145.0 (o-aryl-C), 140.0 (p-Mes-C), 137.7 (NCN),
134.0 (o-Mes-C), 132.6 (i-aryl-C), 132.4 (m-Mes-C), 131.1 (p-
aryl-C), 130.9 (i-Mes-C), 129.0 (m-aryl-C), 127.0 (triazole-C),
123.7, 123.2 (2 CH), 54.7 (CH2), 44.2 (triazole-CH), 27.7 (iPr-
CH), 23.5, 23.4 (iPr-CH3), 20.4 (o-Mes-CH3), 16.5 (p-aryl-CH3).
MS(FABþ, m/z): calcd for C27H33BrN5 [M]þ 508.1903; found
508.1909.
3
(s, 1H, CH), 7.63 (d, J(HH) = 8.1 Hz, 2H, aryl-H), 7.49 (t,
3J(HH) = 7.5 Hz, 1H, p-aryl-H), 7.28 (d, 3J(HH) = 7.5 Hz, 2H,
m-aryl-H), 7.25 (s, 1H, CH), 7.06 (d, 3J(HH) = 8.1 Hz, 2H, aryl-
H), 6.10 (s, 2H, CH2), 3.86 (s, 3H, OMe), 2.14 (m, 2H, 2 iPr), 1.11
(m, 12H, 2 iPr). 13C NMR (CDCl3): δ 161.1 (p-aryl-C), 146.1 (o-
DiPP-C), 135.7 (NCN), 132.9 (i-DiPP-C), 131.1 (p-DiPP-C),
129.0 (m-DiPP-C), 127.6 (CH), 125.9 (triazole-C), 124.2 (aryl-
CH), 123.7 (CH), 121.4 (i-aryl-C), 115.9 (aryl-CH), 77.6 (OMe),
56.1 (CH2), 45.0 (triazole-CH), 28.7 (iPr-CH), 24.5, 24.3 (iPr-
CH3). MS(FABþ, m/z):calcdfor C25H30N5O[M-Br]þ 416.2450;
found 416.2449.
[1-((4-(4-Methoxyphenyl)-1H-1,2,3-triazolyl)methylene-3-mesi-
tyl)imidazolium] Bromide, 5b. The product was obtained as a
brown solid in 99% yield (1.92 g). 1H NMR (CDCl3): δ 10.33 (s,
1H, NCHN), 9.02 (s, 1H, triazole-H), 8.09 (s, 1H, CH), 7.68 (d,
3JHH=9 Hz, 2H, aryl-H), 7.12 (s, 1H, CH), 6.98 (d, 3JHH=9 Hz,
2H, aryl-H), 6.96 (s, 2H, aryl-H), 6.18 (s, 2H, CH2), 3.84 (s, 3H,
OMe), 2.31 (s, 3H, p-aryl-CH3), 2.02 (s, 6H, o-aryl-CH3). 13C
NMR (CDCl3): δ 160.2 (p-aryl-C), 141.7 (p-Mes-C), 138.1
(NCN), 134.4 (o-Mes-C), 130.9 (aryl-CH), 130.4 (i-Mes-C),
130.1 (m-Mes-C), 125.2 (triazole-C), 123.6 (CH), 123.3 (CH),
122.6 (i-aryl-C), 115.0 (aryl-CH), 77.5 (OMe), 55.9 (CH2), 44.7
(triazole-CH), 21.3 (o-aryl-CH3), 18.4 (p-aryl-CH3). MS(FABþ,
m/z): calcd for C22H24N5O [M - Br]þ 374.1981; found 374.1979.
[1-((4-Adamantyl-1H-1,2,3-triazolyl)methylene-3-mesityl)imi-
dazolium] Bromide, 5c. The product was obtained as a light
brown foam in 72% yield (0.72 g). 1H NMR (MeCN-d3): δ 8.82
(s, 1H, NCHN), 8.10 (s, 1H, triazole-H), 7.71 (s, 1H, CH), 7.44
(s, 1H, CH), 7.11 (s, 2H, aryl-H), 5.57 (s, 2H, CH2), 2.35 (s, 3H,
p-aryl-CH3), 2.24 (m, 9H, R/β-Ad), 2.02 (s, 6H, o-aryl-CH3),
1.80 (m, 6H, γ-Ad). 13C NMR (MeCN-d3): δ 142.9 (p-aryl-C),
138.3 (NCN), 136.3, (o-aryl-C), 132.6 (i-aryl-C), 131.0 (m-aryl-
C), 125.7 (CH), 124.9 (CH), 123.1 (triazole-C), 68.2 (1-Ad-C),
61.6 (CH2), 46.6 (triazole-CH), 44.1 (2-Ad-C), 37.0 (4-Ad-
C), 31.1 (3-Ad-C), 21.7 (p-aryl-CH3), 18.1 (o-aryl-CH3). MS-
(FABþ, m/z): calcd for C25H32N5 [M - Br]þ 402.2658; found
402.5648.
[1-((4-Mesityl-1H-1,2,3-triazolyl)methylene-3-mesityl)imidazo-
lium] Bromide, 5d. The product was obtained as a brown solid in
85% yield (89 mg). 1H NMR (CDCl3): δ 10.08 (s, 1H, NCHN),
8.66 (s, 1H, triazole-H), 8.16 (s, 1H, CH), 7.13 (s, 1H, CH), 6.98
(s, 2H, aryl-H), 6.94 (s, 2H, aryl-H), 6.24 (s, 2H, CH2), 2.32 (s,
6H, 2 p-aryl-CH3), 2.03 (s, 6H, o-aryl-CH3 NHC), 1.89 (s, 6H,
o-aryl-CH3 triazole). 13C NMR (CDCl3): δ 141.5, 140.4 (2
p-aryl-C), 137.4 (NCHN), 134.7, 134.2 (2 o-aryl-C), 133.0 (i-
aryl-C NHC), 130.6 (i-aryl-C triazole), 129.9, 129.2 (2 m-aryl-
C), 128.0 (triazole-C), 123.7, 123.1 (2 CH), 77.4 (CH2), 44.7
(triazole-CH), 21.1 (2 p-aryl-CH3), 17.7, 17.4 (2 m-aryl-CH3).
MS(FABþ, m/z): calcd for C24H28N5 [M - Br]þ 386.2345;
found 386.2348.
[1-((4-(2,6-Diisopropylphenyl)-1H-1,2,3-triazolyl)methylene-
3-(4-trifluoromethylphenyl))imidazolium] Bromide, 8. The pro-
duct was obtained as a green solid in 63% yield (0.32 g). H
1
NMR (CDCl3): δ 10.86 (s, 1H, NCHN), 8.57 (s, 1H, triazole-H),
8.01 (s, 1H, CH), 7.90-7.65 (m, 4H, aryl-H), 7.48 (t, 3J(HH) =
7.5 Hz, 1H, p-aryl-H), 7.35-7.18 (m, 3H, CH and m-aryl-H),
6.19 (s, 2H, CH2), 2.13 (m, 2H, 2 iPr), 1.10 (m, 12H, 2 iPr). 13
C
NMR (CDCl3): δ 145.9 (o-DiPP-C), 143.1 (p-aryl-C), 136.4
(NCHN), 132.7 (i-DiPP-C), 131.1 (p-DiPP-C), 127.9 (m-DiPP-
C), 127.1 (aryl-CH), 126.8 (i-aryl-C), 126.4 (triazole-C), 123.9
(aryl-CH), 123.0 (CH), 120.8 (CH), 77.3 (CH2), 45.2 (triazole-
CH), 28.5 (iPr-CH), 24.2 (iPr-CH3). MS(FABþ, m/z): calcd for
C25H27F3N5 [M - Br]þ 454.2219; found 474.2227.
[Bis((1-(prop-2-ynyl)-3-mesityl)imidazol-2-ylidene)silver(I)]sil-
ver(I) Dibromide, 10. 25 A Schlenk flask was charged with 1 and 1
equiv of silver(I) oxide under a N2 atmosphere and suspended in
a 1:1 dichloromethane/THF mixture to 0.1 M. The suspension
was stirred for 2 days and filtered over Celite, and the solvent
was removed in vacuo. Traces of THF were co-evaporated twice
with dichloromethane. The product is obtained as a light brown
1
foam in 84% yield (0.233 g). H NMR (CD2Cl2): δ 7.44 (d,
3J(HH) = 1.8 Hz, 1H, CH), 7.04 (d, 3J(HH) = 1.8 Hz, 1H, CH),
7.03 (s, 2H, aryl-H), 5.05 (d, 4J(HH) = 2,7 Hz, 2H, CH2), 2.62 (t,
4J(HH) = 2.7 Hz, 1H, CCH), 2.34 (s, 3H, p-aryl-CH3), 2.00 (s,
6H, o-aryl-CH3). 13C NMR (CD2Cl2): δ 183.8 (NCN), 141.8 (p-
aryl-C), 137.7 (i-aryl-C), 137.0 (o-aryl-C), 131.4 (m-aryl-C),
125.2 (CH), 123.0 (CH), 78.7 (alkyne), 78.0 (alkyne), 43.5
(CH2), 23.0 (p-aryl-CH3), 19.6 (o-aryl-CH3). MS(FABþ, m/z):
calcd for C30H32N4Ag [NHC2Ag]þ 555.1678; found 555.1675.
General Procedure for Synthesis of NHC-Triazolyl Palladium
Allyl Chloride Complexes. Imidazolium salt, potassium tert-
butoxide (1.1 equiv), and palladium allyl chloride dimer (0.5
equiv) were charged into a Schlenk flask and suspended in THF
to 0.1 M. The reaction mixture was stirred for 4 h, filtered over
Celite, and concentrated in vacuo. The crude product was
dissolved in dichloromethane and precipitated by addition of
pentane. Because of a small amount of halide scrambling,
elemental analysis of the complexes did not give reliable results.
[1-((4-(4-Trifluoromethylphenyl)-1H-1,2,3-triazolyl)methylene-
3-mesityl)imidazolium] Bromide, 5e. The product was obtained as
anorangesolidin78%yield (0.13 g). 1H NMR (CDCl3): δ9.81(s,
1H, NCHN), 9.44 (s, 1H, triazole-H), 8.12, s, 1H, CH), 8.05 (d,
3J(HH) = 7.2 Hz, 2H, aryl-H), 7.76 (d, 3J(HH) = 7.2 Hz, 2H,
aryl-H), 7.13 (s, 1H, CH), 6.97 (s, 2H, aryl-H), 6.21 (s, 2H, CH2),
2.32 (s. 3H, p-aryl-CH3), 2.05 (s, 6H, o-aryl-CH3). 13C NMR
(75 MHz, CDCl3): δ 141.5 (p-Mes-C), 139.0 (p-aryl-C), 137.7

Article
[1-((4-(2,6-Diisopropylphenyl)-1H-1,2,3-triazolyl)methylene-
3-mesityl)imidazol-2-ylidenepalladium(II)allyl] Chloride, 11a.
Organometallics, Vol. 29, No. 14, 2010 3115
o-aryl-CH3 NHC), 2.02 (s, 3H, o-aryl-CH3 NHC), 1.99 (s, 6H,
o-aryl-CH3 triazole). 13C NMR (CD2Cl2): δ 177.6 (NCN), 141.1
(p-aryl-C triazole), 139.5 (p-aryl-C), 136.5 (i-aryl-C), 135.5,
135.1 (o-aryl-C), 134.8 (o-aryl-C triazole), 132.7 (i-aryl-C tria-
zole), 129.3 (m-aryl-C triazole), 129.1, 128.9 (m-aryl-C), 127.0
(CH), 123.4 (CH), 122.0 (triazole-C), 117.8 (allyl), 71.7 (CH2),
62.3 (allyl), 50.2 (allyl), 45.6 (triazole-CH), 22.3 (p-aryl-CH3
triazole), 20.8 (p-aryl-CH3), 17.8, 17.6 (o-aryl-CH3), 17.2 (o-
aryl-CH3 triazole). MS(FABþ, m/z): calcd for C27H32N5Pd
[M - Cl]þ 532.1693; found 532.1706.
The product was obtained as a yellow solid in 92% yield
1
(0.212 g). H NMR (CD2Cl2): δ 8.37 (s, 1H, triazole-H), 7.80
(d, J(HH)=1.8 Hz, 1H, CH), 7.59 (t J(HH)=7.8 Hz, 1H,
3
3
3
p-aryl-H), 7.31 (d, J(HH) = 1.8 Hz, 2H, m-aryl-H), 7.07 (d,
3J(HH)=1.8 Hz, 1H, CH), 7.05 (s, 1H, Mes-H), 6.97 (s, 1H,
Mes-H), 5.92 (d 2J(HH)=15.5 Hz, 1H, CH2), 5.59 (d, 2J(HH)=
15.5 Hz, 1H, CH2), 5.07 (m, 1H, allyl-H), 4.07 (d, 3J(HH)=7.5
Hz, 1H, allyl-H), 3.10 (d, 3J(HH) = 6.3 Hz, 1H, allyl-H), 3.04 (d,
3J(HH) = 13.5 Hz, 1H, allyl-H), 2.33 (s, 3H, p-Mes-CH3), 2.17
[1-((4-(4-Trifuoromethylphenyl)-1H-1,2,3-triazolyl)methylene-
3-mesityl)imidazol-2-ylidenepalladium(II)allyl] Chloride, 11e.
The product was obtained as an orange solid in a yield of 94%
(0.138 g). 1H NMR (CD2Cl2): δ 9.31 (s, 1H, triazole-H), 8.12 (d,
3J(HH) = 8.4 Hz, 2H, aryl-H), 7.88 (d, 3J(HH) = 8.4 Hz, 2H, aryl-
H), 7.76 (s, 1H, CH), 7.04 (s, 1H, CH), 7.02 (s, 3H, Mes-H), 5.97 (d,
2J(HH)=15.9 Hz, 1H, CH2), 5.73 (d, 2J(HH)=15.9 Hz, 1H, CH2),
5.23 (m, 1H, allyl-CH), 4.41 (m, 1H, allyl-CH2), 4.09 (m, 1H, allyl-
CH2), 3.32 (m, 1H, allyl-CH2), 3.00 (m, 1H, allyl-CH2), 2.36 (s, 3H,
i
(m, 2H, Pr-H), 2.12 (s, 3H, o-Mes-CH3), 2.02 (s, 3H, o-Mes-
CH3), 1.85 (d, 3J(HH) = 12 Hz, 1H, allyl-H), 1.12 (d, 3J(HH)=
2.1 Hz, 6H, iPr-CH3), 1.10 (d, 3J(HH) = 2.1 Hz, 6H, iPr-CH3).
13C NMR (CD2Cl2): δ 180.5 (NCN), 143.5 (o-aryl-C), 139.3 (p-
Mes-C), 136.6 (i-Mes-C), 135.9 (o-Mes-C), 135.3 (o-Mes-C),
133.3 (i-aryl-C), 131.2 (p-aryl-C), 129.2 (m-Mes-C), 129.0 (m-
Mes-C), 127.6 (m-aryl-C), 124.1 (CH), 122.9 (CH), 122.1
(triazole-C), 115.7 (allyl), 78.0 (allyl), 71.9 (CH2), 49.4 (allyl),
46.0 (triazole-CH), 28.7 (iPr-CH, br), 24.1 (iPr-CH3), 24.0 (iPr-
CH3), 23.9 (iPr-CH3), 23.8 (iPr-CH3), 21.1 (p-Mes-CH3), 18.2
(o-Mes-CH3), 17.9 (o-Mes-CH3). MS(FABþ, m/z): calcd for
C30H55N5Pd [M - Cl]þ 574.2162; found 574.2168.
p-aryl-CH3), 2.05 (s, 3H, o-aryl-CH3), 2.01 (s, 3H, o-aryl-CH3). 13
C
NMR (CD2Cl2): δ 177.6 (NCN), 142.9 (p-Mes-C), 139.6 (p-aryl-
C), 136.4 (i-Mes-C), 135.5, 135.0 (m-Mes-C), 131.2 (CF3), 129.1,
128.9 (o-Mes-C), 127.3 (aryl-CH), 125.4 (i-aryl-C), 123.7 (triazole-
C), 123.2 (CH), 122.1 (CH), 121.2 (aryl-CH), 117.8 (allyl), 72.0
(CH2), 62.5 (allyl), 50.6 (allyl), 45.5 (triazole-CH), 20.9 (p-Mes-
CH3), 17.9, 17.7 (o-Mes-CH3). MS(FABþ, m/z): calcd for C25H25-
F3N5Pd [M - Cl]þ 558.1097; found 558.1116.
[1-((4-(4-Methoxyphenyl)-1H-1,2,3-triazolyl)methylene-3-me-
sityl)imidazol-2-ylidenepalladium(II)allyl] Chloride, 11b. The
product was obtained as a beige solid in a yield of 93% (0.10
g). 1H NMR (CD2Cl2): δ 8.80 (s, 1H, triazole-H), 7.71 (d,
3
3J(HH) = 15 Hz, 2H, aryl-H), 7.56 (d J(HH) = 2.5 Hz, 1H,
[1-((4-(2,6-Diisopropylphenyl)-1H-1,2,3-triazolyl)methylene-
3-(2,6-diisopropylphenyl))imidazol-2-ylidenepalladium(II)allyl]
Chloride, 12. The product was obtained as a brown solid in a
CH), 7.04 (d, 3J(HH) = 15 Hz, 2H, aryl-H), 6.98 (s, 2H, Mes-H),
6.97 (d 3J(HH) = 2.5 Hz, 1H, CH), 5.85 (d, 2J(HH) = 15.0 Hz,
1H, CH2), 5.76 (d, 2J(HH) = 15.0 Hz, 1H, CH2), 5.07 (m, 1H,
allyl), 4.15 (d, 3J(HH) = 12.5 Hz, 1H, allyl), 3.86 (s, 3H), 3.21 (d,
3J(HH) = 10.5 Hz, 1H, allyl), 3.03 (d, 3J(HH) = 22.5 Hz, 1H,
allyl), 2.33 (s, 3H, p-Mes-CH3), 2.12 (s, 3H, o-Mes-CH3), 2.03 (s,
1
yield of 99% (0.258 g). H NMR (CD2Cl2): δ 8.63 (s, 1H, tria-
zole-H), 8.03 (s, 1H, CH), 7.59 (t, 3J(HH)=7.8 Hz, 1H, p-aryl-H),
7.53 (t, 3J(HH)=7.8Hz, 1H, p-aryl-H), 7.36 (d, 3J(HH) = 7.8 Hz,
2H, m-aryl-H), 7.32 (d, 3J(HH)=7.8 Hz, 2H, m-aryl-H), 7.13 (s,
1H, CH), 6.06 (d, 2J(HH)=15.9 Hz, 1H, CH2), 5.81 (d, 2J(HH)=
15.9 Hz, 1H, CH2), 5.22 (m, 1H, allyl-CH), 4.38 (m, 1H, allyl-
CH2), 3.99 (m, 1H, allyl-CH2), 3.33 (m, 1H, allyl-CH2), 2.91 (m,
1H, allyl-CH2), 2.45 (m, 2H, iPr), 2.20 (m, 2H, iPr), 1.30-1.00 (m,
24H, iPr). 13C NMR (CD2Cl2): δ 177.3 (NCN), 146.2 (o-aryl-C),
145.8 (o-aryl-C), 136.3 (i-aryl-C NHC), 132.3 (i-aryl-C triazole),
131.7 (p-aryl-C NHC), 130.4 (p-aryl-C triazole), 127.6 (m-Aryl-C
NHC), 124.2 (m-aryl-C triazole), 124.1 (CH), 123.9 (CH), 123.1
(triazole-C), 118.4 (allyl), 71.9 (CH2), 62.0 (allyl), 50.6 (allyl), 45.4
(triazole-CH), 28.7 (iPr-CH NHC), 28.5 (iPr-CH triazole), 28.3
(iPr-CH NHC), 25.0 (iPr-CH3 NHC), 24.8 (iPr-CH3 NHC), 24.1
(iPr-CH3 triazole), 23.5 (iPr-CH3 triazole), 23.3 (iPr-CH3 NHC),
23.0 (iPr-CH3 NHC). MS(FABþ, m/z): calcd for C33H44N5Pd
[M - Cl]þ 616.2632; found 616.2646.
3
3H, o-Mes-CH3), 1.93 (d, J(HH) = 20.5 Hz, 1H, allyl). 13C
NMR (CD2Cl2): δ 176.6 (NCN), 160.9 (p-aryl-C), 139.9 (p-Mes-
C), 136.8 (i-Mes-C), 135.6 o-Mes-C), 135.2 (o-Mes-C), 129.6
(aryl-CH), 129.3 (m-Mes-C), 129.2 (m-Mes-C), 123.9, 123.2,
122.1 (2 CH, triazole-C), 122.8 (i-aryl-C), 119.0 (allyl), 115.2
(aryl-CH), 72.3 (CH2), 68.0 (allyl), 56.2 (OMe), 50.1 (allyl), 45.4
(triazole-CH), 21.2 (p-Mes-CH3), 18.0 (o-Mes-CH3), 17.9 (o-
Mes-CH3). MS(FABþ, m/z): calcd for C25H28ON5Pd [M - Cl]þ
520.1329; found 520.1344.
[1-((4-(Adamantyl)-1H-1,2,3-triazolyl)methylene-3-mesityl)imi-
dazol-2-ylidenepalladium(II)allyl] Chloride, 11c. The product
was obtained as a yellow solid in a yield of 94% (0.24 g). H
1
NMR (CD2Cl2): δ 8.54 (s, 1H, triazole-H), 7.78 (d, 3J(HH)=0.9
Hz, 1H, CH), 7.02 (s, 2H, aryl-H), 6.98 (d, 3J(HH)=0.9 Hz, 1H,
2
2
CH), 5.95 (d, J(HH)=15.0 Hz, 1H, CH2), 5.70 (d, J(HH)=
15.0 Hz, 1H, CH2), 5.14 (m, 1H, allyl-H), 4.27 (d, 3J(HH)=3.9
Hz, 1H, allyl-H), 3.17 (d, 3J(HH)=8.1 Hz, 1H, allyl-H), 3.02 (br
s, 1H, allyl-H), 2.37 (s, 3H, p-aryl-CH3), 2.28 (m, 9H, R/β-Ad),
2.09 (s, 3H, o-aryl-CH3), 2.02 (s, 3H, o-aryl-CH3), 1.90 (d,
3J(HH)=6 Hz, 1H, allyl-H), 1.83 (m, 9H, γ-Ad). 13C NMR
(CD2Cl2): δ 177.0 (NCN), 139.8 (p-aryl-C), 136.8 (i-aryl-C),
135.7 (o-aryl-C), 135.3 (o-aryl-C), 129.2 (m-aryl-C), 129.1 (m-
aryl-C), 123.5 (triazole-C), 122.0 (CH), 121.5 (CH), 118.4 (allyl),
71.7 (CH2), 68.1 (1-Ad-C), 62.3 (allyl), 50.6 (allyl), 45.6 (triazole-
CH), 42.8 (2-Ad-C), 35.8 (4-Ad-C), 29.8 (3-Ad-C), 21.1 (p-aryl-
CH3), 18.0, 17.8 (o-aryl-CH3). MS(FABþ, m/z): calcd for
C28H36N5Pd [M - Cl]þ 548.2006; found 548.2001.
[1-((4-(Mesityl)-1H-1,2,3-triazolyl)methylene-3-mesityl)imida-
zol-2-ylidenepalladium(II)allyl] Chloride, 11d. The product was
obtained as a yellow solid in a yield of 90% (90 mg). 1H NMR
(CD2Cl2): δ 8.49 (s, 1H, triazole-H), 7.82 (s, 1H, CH), 7.06 (s,
2H, aryl-H), 7.03 (s, 1H, CH), 7.01 (bs, 2H, aryl-H NHC), 6.02
(d, 2J(HH)=13.2 Hz, 1H, CH2), 5.73 (d, 2J(HH)=13.2 Hz, 1H,
CH2), 5.21 (m, 1H, allyl-CH), 4.33 (m, 1H, allyl-CH2), 4.05 (m,
1H, allyl-CH2), 3.27 (m, 1H, allyl-CH2), 2.95 (m, 1H, allyl-CH2),
2.38 (s, 3H, p-aryl-CH3), 2.36 (s, 3H, p-aryl-CH3), 2.05 (s, 3H,
[1-((4-(2,6-Diisopropylphenyl)-1H-1,2,3-triazolyl)methylene-
3-(4-methoxyphenyl))imidazol-2-ylidenepalladium(II)allyl] Chlo-
ride, 13. The product was obtained as a yellow solid in a yield of
91% (0.054 g). 1H NMR (CD2Cl2): δ 8.38 (s, 1H, triazole-H), 7.72
3
(s, 1H, CH), 7.55 (t, J(HH) = 12 Hz, 1H, p-aryl-H), 7.34 (d,
3J(HH)=9 Hz, 2H, aryl-H), 7.32 (d, 3J(HH)=12 Hz, 2H, m-aryl-
H), 7.02 (d, 3J(HH) = 9 Hz, aryl-H), 6.97 (s, 1H, CH), 5.94 (d,
2J(HH)=15.3 Hz, 1H, CH2), 5.77(d, 2J(HH)=15.3 Hz, 1H, CH2),
5.18 (m, 1H, allyl-CH), 4.29 (d, 3J(HH)=5.4 Hz, 1H, allyl-CH2),
3
3.87 (s, 3H, OMe), 3.27 (d, J(HH)=14.1 Hz, 1H, allyl-CH2),
2.3-2.1 (m, 3H, allyl-CH2, iPr-CH), 1.2-1.1 (m, 12 H, iPr-CH3).
13C NMR (CD2Cl2): δ 177.4 (NCN), 159.9 (p-aryl-C), 145.9 (o-
DiPP-C), 133.6 (i-DiPP-C), 131.5 (p-DiPP-C), 131.1 (m-DiPP-C),
126.9 (triazole-C), 124.1 (aryl-CH), 123.4 (CH), 123.0 (i-aryl-C),
122.1 (CH), 117.0 (allyl), 114.2 (aryl-CH), 70.7 (CH2), 63.2 (allyl),
55.8 (OMe), 51.9 (allyl), 46.0 (triazole-CH), 28.6 (iPr-CH), 24.1,
23.6 (iPr-CH3). MS(FABþ, m/z): calcd for C28H34N5OPd [M -
Cl]þ 562.1798; found 562.1799.
[1-((4-(2,6-Diisopropylphenyl)-1H-1,2,3-triazolyl)methylene-
3-(4-trifluoromethylphenyl))imidazol-2-ylidenepalladium(II)allyl]
Chloride, 14. The product was obtained as a dark yellow solid in
1
a yield of 97% (0.125 g). H NMR (CD2Cl2): δ 8.34 (s, 1H,

3116 Organometallics, Vol. 29, No. 14, 2010
Warsink et al.
triazole-H), 7.9-7.3 (m, 9H, DiPP, aryl, 2 CH), 6.0-5.8 (br m,
2H, CH2), 5.25 (m, 1H, allyl), 4.31 (m, 1H, allyl), 3.27 (m, 1H,
X-ray Crystal Structure Determination of 16. [C28H36N5Pd]-
(CF3O3S), fw = 698.09, colorless plate, 0.24 ꢀ 0.24 ꢀ 0.06 mm3,
triclinic, P1 (no. 2), a=9.5963(2) A, b=11.0135(2) A, c=16.1259(4)
A, R = 70.3568(10)°, β = 76.7089(11)°, γ = 67.2874(10)°, V =
1470.81(6) A3, Z = 2, Dx = 1.576 g/cm3, μ = 0.76 mm-1; 16 083
reflections were measured on a Nonius Kappa CCD diffractometer
with rotating anode (graphite monochromator, λ = 0.71073 A) up
to a resolution of (sin θ/λ)max =0.60A-1 at a temperature of 150(2)
K. Intensity integration was performed with HKL2000.40 The
SORTAV41 program was used for absorption correction and
scaling based on multiple measured reflections (0.85-0.96 correc-
tion range). A total of 5283 reflections were unique (Rint = 0.054),
of which 4604 were observed [I > 2σ(I)]. The structure was solved
with direct methods using the program SHELXS-97.42 The struc-
ture was refined with SHELXL-9742 against F2 of all reflections.
Non-hydrogen atoms were refined with anisotropic displacement
parameters. The coordinated allyl ligand was refined with a disorder
model with occupancies of 0.58:0.42 for the two disordered com-
ponents. All hydrogen atoms were introduced in calculated posi-
tions and refined with a riding model. A total of 392 parameters
were refined with 14 restraints concerning the disordered allyl ligand
(distance restraints and restraints to approximate isotropic behavior
of the displacement parameters). R1/wR2 [I > 2σ(I)]: 0.0337/
0.0831. R1/wR2 [all reflns]: 0.0404/0.0888. S = 1.060. Residual
electron density is between -0.76 and 0.92 e/A3. Geometry calcula-
tions and checks for higher symmetry were performed with the
PLATON program.43
i
i
allyl), 2.2-2.1 (m, 4H, allyl, Pr-CH), 1.3-1.0 (m, 12H, Pr-
CH3). 13C NMR (CD2Cl2): δ 179.8 (NCN), 145.9 (o-DiPP-C),
143.3 (p-aryl-C), 132.6 (i-DiPP-C), 132.1 (CF3), 131.3 (p-DiPP-
C), 131.1 (m-DiPP-C), 127.7 (triazole-C), 126.0 (aryl-CH), 125.4
(i-aryl-C), 124.0 (aryl-CH), 121.8 (CH), 120.5 (CH), 115.9
(allyl), 71.21 (CH2), 63.3 (allyl), 46.4 (allyl), 38.8 (triazole-
CH), 28.6 (iPr-CH), 24.0 (iPr-CH3), 23.7 (iPr-CH3). MS(FABþ,
m/z): calcd for C28H31F3N5Pd [M - Cl]þ 600.1566; found
600.1570.
[(1-(Prop-2-ynyl)-3-mesityl)imidazol-2-ylidenepalladium(II)allyl]
Chloride, 15. Freshly prepared 10 (0.183 g, 0.207 mmol) was
dissolved in 10 mL of THF. Then palladium allyl chloride dimer
(1 equiv) was added, upon which a white precipitate formed. The
reaction was stirred for 20 min and filtered over Celite, and the
yellow solution was concentrated in vacuo. The product was
obtained as a brown foam in 98% yield (0.18 g). 1H NMR
3
(CD2Cl2): δ 7.43 (d, J(HH) = 1.8 Hz, 1H, CH), 7.02 (s, 2H),
6.98 (d, 3J(HH)=1.8 Hz, 1H, CH), 5.30 (d, 4J(HH)=2.4 Hz, 2H,
CH2), 5.03 (m, 1H, allyl), 4.00 (dd, 3J(HH)=7.5 Hz, 3J(HH)=2.1
Hz, 1H, allyl), 3.28 (m, 1H, allyl), 2.94 (d, 3J(HH) = 13.5 Hz, 1H,
allyl), 2.58 (t, 4JHH= 2.4 Hz, 1H, CH), 2.33 (s, 3H, p-aryl-CH3),
2.04 (s, 6H, o-aryl-CH3), 1.87(d, 3J(HH) = 10.8 Hz, 1H, allyl). 13
C
NMR (CD2Cl2):δ187.2 (NCN), 139.9 (p-aryl-C), 136.4 (i-aryl-C),
135.3 (o-aryl-C), 129.3 (m-aryl-CH), 127.0 (CH), 122.9 (CH),
117.5 (allyl), 77.9, 74.5 (alkyne), 57.8 (allyl), 41.1 (CH2), 36.6
(allyl), 21.1 (p-aryl-CH3), 18.0 (o-aryl-CH3). MS(FABþ, m/z):
calcd for C18H21N2Pd [M - Cl]þ 371.0740; found 371.0752.
[1-((4-(Adamantyl)-1H-1,2,3-triazolyl)methylene-3-mesityl)-
imidazol-2-ylidenepalladium(II)allyl]trifluoromethane Sulfonate,
16. Silver triflate was added to a 0.1 M dichloromethane
solution of 11c (1 equiv). A white precipitate formed immedi-
ately, and the suspension was filtered over Celite. After removal
of the solvent in vacuo, the product (45 mg, 97%) was obtained
as a colorless solid. 1H NMR (CD2Cl2): δ 8.36 (s, 1H, triazole-
General Experimental Procedure for Catalytic Transfer Semi-
hydrogenation of Phenylpropyne Using Formic Acid As Hydrogen
Donor. A solution of 12 mL of MeCN containing 150 mM
37
1-phenyl-1-propyne, 750 mM triethylammonium formiate, and
150 mM p-xylene was heated to reflux. A solution of 1 mol %
catalyst in 2 mL of MeCN was added to the reaction mixture.
Periodically, samples were taken over 24 h. Reaction mixtures
were analyzed by GC to determine conversions. Reactions were
quenched by dilution with EtOH.
3
H), 7.63 (d, J(HH) = 2.1 Hz, 1H, CH), 7.03 (s, 2H, aryl-H),
7.01 (d 3J(HH) = 2.1 Hz, 1H, CH), 5.73 (d, 2J(HH) = 15.6 Hz,
1H, CH2), 5.43 (d, 2J(HH) = 15.6 Hz, 1H, CH2), 5.23 (m, 1H,
allyl), 4.44 (dd, 3J(HH) = 7.5 Hz, 2J(HH) = 2.4 Hz, 1H, allyl),
3.32 (d, 3J(HH) = 13.5 Hz, 1H, allyl), 2.73 (dd, 3J(HH) = 6.9
Hz, 2J(HH) = 2.4 Hz, 1H, allyl), 2.38 (s, 3H, p-aryl-CH3), 2.28
(m, 9H, R/β-Ad), 1.98 (m, 7H, o-aryl-CH3, allyl), 1.82 (m, 6H,
γ-Ad). 13C NMR (CD2Cl2): δ 176.9 (NCN), 139.8 (p-aryl-C),
136.8 (i-aryl-C), 135.6 (o-aryl-C), 129.2 (m-aryl-C), 123.5
(triazole-C), 122.0 (CH), 121.5 (CH), 118.4 (allyl), 71.7 (1-Ad-
C), 68.0 (allyl), 62.3 (CH2), 50.1 (allyl), 45.6 (triazole-CH), 42.9
(2-Ad-C), 34.3 (4-Ad-C), 25.9 (3-Ad-C), 21.0 (p-aryl-CH3), 18.0,
17.8 (o-aryl-CH3). The signal for the carbon of the triflate anion
was not observed. MS(FABþ, m/z): calcd for C28H36N5Pd [M -
OTf]þ 548.2006; found 548.1994.
Acknowledgment. This work is supported by the Dutch
Organization for Scientific Research (NOW) and is part
of a collaboration with the NSC in Taiwan. It was
financially supported under grant number 053.68.003.
We thank Jan Meine Ernsting and Han Peeters for their
invaluable help with NMR and MS experiments, respec-
tively.
Supporting Information Available: X-ray crystallographic
data in CIF format. This information is available free of charge
via the Internet at http://pubs.acs.org.
(41) Blessing, R. H. Acta Crystallogr., Sect. A: Found. Crystallogr.
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(43) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7–13.
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