Rigid pyridyl substituted NHC ligands, their Pd(0) complexes and their application in selective transfer semihydrogenation of alkynes

Article abstract of DOI:10.1002/aoc.1754

The synthesis of an air-stable series of Pd⁰ complexes with unsymmetric bidentate N-pyridine N-heterocyclic carbene ligands has been described. The carbenes were generated by synthesis of the silver(I) complexes from the imidazolium salts, foll

Full text of DOI:10.1002/aoc.1754

Full Paper
Received: 8 September 2010
Revised: 15 October 2010
Accepted: 2 November 2010
Published online in Wiley Online Library: 21 January 2011
(wileyonlinelibrary.com) DOI 10.1002/aoc.1754
Rigid pyridyl substituted NHC ligands, their
Pd(0) complexes and their application in
selective transfer semihydrogenation of
alkynes
Stefan Warsinka, Suzanne Bosmana, Jan J. Weigandb
and Cornelis J. Elseviera∗
The synthesis of an air-stable series of Pd0 complexes with unsymmetric bidentate N-pyridine N-heterocyclic carbene ligands
has been described. The carbenes were generated by synthesis of the silver(I) complexes from the imidazolium salts, followed
by transmetallation of the C-N ligands to obtain the target electron-rich zerovalent palladium compounds. The bidentate
coordination behaviour of the ligands was confirmed by 1H, 13C NMR and X-ray spectroscopy. The complexes are active
c
precatalysts for the selective transfer semihydrogenation of alkynes to Z-alkenes, with selectivities up to 99%. Copyright ꢀ
2011 John Wiley & Sons, Ltd.
Keywords: carbene complexes; organometallic compounds; palladium; transmetallation; hydrogenation
Introduction
moieties, we hope to gain insight in effects of the substituents on
the coordination chemistry and, ultimately, the performance of
the zerovalent palladium complexes in catalysis. As a probe for the
catalytic performance we employed the transfer hydrogenation
of alkynes to Z-alkenes, in which (NHC)Pd(0) complexes have
shown to be excellent catalysts.[14] This reaction is important in
organicchemistry, becauseitistheonlyselectiveandreproducible
methodofobtainingZ-alkenes.Thisreactionconstitutesavaluable
tool for materials sciences, as well as for the synthesis of
pharmaceuticals, fragrancechemicalsandnaturalproducts, where
selective synthesis of only one isomer is a prerequisite.It appeared
that the choice of the secondary nitrogen donor has a marked
influence on the results.[6,8]
We present here the synthesis of the imidazolium salts from
which the zerovalent palladium carbene complexes are generated
by intermediacy of their silver(I) complexes. The NHCs are then
transmetallated onto palladium(0). Finally, we verify whether (N-
pyridyl-NHC)Pd(0) complexes catalyze the transfer hydrogenation
of alkynes to Z-alkenes and compare their putative reactivity and
selectivity with previously investigated catalysts.
N-heterocyclic carbenes (NHCs) have been at the forefront of
organometallic chemistry and catalysis for a number of years[1]
and new developments are published regularly.[2] The high
stability they impart on their transition metal (TM) complexes
and the catalytic activity of those complexes make this class
of ligands an exceedingly valuable addition to the field.[3] The
strong donating character, combined with their ability to act
as acceptors as well, enables NHCs to ligate to a large number
of TMs in various oxidation states, as well as to many main
groupelements.[4] Nexttomonodentatecarbenes, alsohomo-and
heteropolydentate ligands containing NHCs and their complexes
are reported frequently.[5]
Our current interest lies in the development and synthesis
of heterobidentate NHC ligands containing a nitrogen moiety
as secondary donor.[6–8] By combining the strong carbon-based
donor with a more weakly coordinating nitrogen donor, we hope
to gain access to a class of catalysts which benefits from the
hemilabile behaviour of one of its ligands.[9] We endeavoured the
synthesis of electron rich palladium complexes bearing an NHC
ligand functionalized with a pyridine nitrogen donor, where the
pyridine ring is attached directly to the NHC. These ligands have
beenpreparedbefore,[10–12] butaninvestigationintotheinfluence
of the substituents on both the NHC and the nitrogen donor has
not been thoroughly performed. The pyridine functionality is a
commonly used motif in coordination chemistry,[13] and gives
rise to a five-membered chelate ring in combination with the
NHC scaffold. Because all non-metal atoms in the ring are sp2-
hybridized, a considerable amount of strain is expected, which is
likely to enhance the hemilability of the tethered C-N ligand.
By combining the NHC and the pyridine in one bidentate
ligand, varying the substituents on both the NHC and the pyridyl
∗
Correspondence to: Cornelis J. Elsevier, Molecular Inorganic Chemistry, van ‘t
Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park
904, 1090 GD Amsterdam, the Netherlands. E-mail: c.j.elsevier@uva.nl
a
Molecular Inorganic Chemistry, van ‘t Hoff Institute for Molecular Sciences,
University of Amsterdam, Science Park 904, 1090 GD Amsterdam, the
Netherlands
¨
¨
b
Institut fu¨r Anorganische und Analytische Chemie, Westfalische Universitat
Mu¨nster, Corrensstraße 28/30, 48149 Mu¨nster, Germany
c
Appl. Organometal. Chem. 2011, 25, 276–282
Copyright ꢀ 2011 John Wiley & Sons, Ltd.

Pd0 complexes bearing rigid N-pyridyl-NHC ligands in semihydrogenation
Scheme 2. Silver(I) carbene complex synthesis.
Scheme 1. Synthesis of imidazolium salts.
Results and Discussion
Imidazolium bromides. N-arylimidazoles 1–4[15] were quaternized
in two ways; heating with 2-bromopyridine a or 2-bromo-6-
methylpyridine b to obtain the imidazolium bromides 5 and
6,[11] or refluxing in 2-iodopropane to obtain 7 in high yields
(scheme 1). The latter method has to our knowledge not been
employed in this fashion, which is remarkable in view of the good
results and ease of the procedure.
All compounds were obtained pure and in high yields after
washing the precipitated crude products with diethyl ether.
The characteristic signal of the imidazolium proton is observed
between 11.4 ppm and 10.8 ppm in the 1H NMR.
(NHC)Silver(I) complexes. After the ligand precursors were
obtained, the N-heterocyclic carbenes were generated using
a well described method.[16–18] In this instance, we employed
silver(I) oxide as the base and as the metal precursor under mild
conditions, giving the target complexes in high yields for this
scaffold (scheme 2). It has to be noted that the exact structure of
the NHC silver complexes is hard to deduce, as dynamic exchange
processes can take place in solution, and the solid state structure
is influenced by the method of isolation.[17] We depict these
complexes as monocarbene silver(I) halide species, as this shows
the one to one ratio of the NHC and the silver ion.
The protocol for the synthesis of these silver(I) complexes
is simple without being deleterious to the yield or purity of
the complexes, because there is no need for inert solvents or
atmosphere with these ligands. It has to be noted that the
less sterically encumbering substituents on the ligand in 10a
and 10b probably cause the complex to exist predominantly as
the silver(I) biscarbene species in solution, as opposed to the
monocarbene complex. As the former is charged, its solubility
in the dichloromethane reaction solvent is presumably lower
than that of the latter; hence a precipitate is sometimes seen.
Further dilution of the reaction mixture and sufficient washing of
the celite pad ensures high isolated yields of the products. The
disappearance of the signal for the imidazolium proton in the
1H NMR and the appearance of a peak for the carbene carbon
Scheme 3. Synthesis of zerovalent Pd(NHC) complexes.
between 175 ppm and 183 ppm in the 13C NMR are indicative of
product formation.
For the complexes 8b and 9b, a slight broadening of the
carbene signal was observed, indicating that slow exchange of the
carbene ligands takes place in solution. However, the coupling of
the carbene carbon with the silver ion was not resolved.[19] This
observation is supported by the fact that this broadening is only
observed with the complexes bearing the most bulky ligands. This
causes the dynamic ligand exchange to be slower than in the other
complexes bearing less bulky ligands.
(NHC)Palladium(0) complexes. The silver(I) NHC complexes
were used for transmetallation to the target palladium(0) com-
plexes, which was performed according to a previously reported
protocol.[6] In this reaction the relatively air-stable palladium pre-
cursor palladium(bis-tert-butyldiazabutadiene)(maleic anhydride)
(Pd(tBuDAB)(ma)) was used;[20] the precipitation of the silver(I)
halide is the driving force for the reaction. The pure products were
obtained after filtration of the reaction mixture by precipitation
with pentane (scheme 3).
Yields for this transformation are excellent, except for the N-
isopropyl substituted NHC ligands in 10a and 10b, where only
decomposition was observed. We believe that this is mostly due
to the fact that the isopropyl substituent is a more electron-
donating group than the Mes or DiPP substituents, which causes
a higher electron density on the palladium center. As these
zerovalent palladium complexes containing two donor ligands
are already very electron-rich, a destabilizing effect is the likely
result. The electronic influence of the ligand on the palladium
can be countered by changing the other ligand so the electron-
densityatpalladium islowered;weroutinelyusetheelectron-poor
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S. Warsink et al.
Scheme 4. Less electron-rich complexes can be isolated.
Scheme 5. Catalytic transfer hydrogenation of 1-phenyl-1-propyne.
maleic anhydride to stabilize the complex. However, even more
electron-poor alkenes like tetracyanoethylene (TCNE) are also
used.[20] When the silver(I) complex 10b was reacted with a
precursor bearing this alkene, the product 14 could be obtained
in reasonable yield (scheme 4).
All spectral data are congruous with the results obtained for
complexes 11 and 12, except for the value of the carbene carbon
in the 13C NMR, which is found at 179.7 ppm, which is a higher
field shift by about 15 ppm. We have observed a similar effect
before when employing TCNE,[6] and relate this to the fact that
the extremely electron-poor alkene renders the palladium only
formally zerovalent. The chemical shift of the carbene carbon is
closer to that of a divalent complex.[12]
Figure 1. Displacement ellipsoid plot (50% probability level) of 13b. H-
atoms are omitted for the sake of clarity. Selected bond lengths (´Å) and
angles (◦): Pd1-C1 2.042(3), Pd1-N3 2.173(2), Pd1-C20 2.068(3), Pd1-C21
2.104(3), C20-C21 1.434(4); C1- Pd1-N3 77.73(9), C20-Pd1-C21 40.21(12),
N1-C1-Pd1 143.3(2), N2-C1-Pd1 113.55(18).
X-ray structure determination. To unequivocally establish the
structure of this type of complexes, a crystal structure analysis
of complex 13b was performed. X-ray quality crystals were
obtained by slow evaporation of a dichloromethane solution
of 13b (figure 1).
Table 1. Catalytic transfer hydrogenation of 1-phenyl-1-propyne
Initial TOF
(mol sub/
mol cat/h)
Selectivity
Z/E/alkane
(%)
Full
conversion
reached (h)
entry
complex
The complex crystallizes in the triclinic spacegroup P −1 with
four complex molecules in the asymmetric cell. On viewing the
molecular structure it is apparent that the bidentate coordination
of the C-N ligand causes the carbene ring, the pyridine ring and the
palladium atom to lie virtually in one plane. Also the two carbon
atoms of the maleic anhydride double bond lie in this plane. The
bond length between palladium and the carbene carbon (Pd1-
C1) is 2.042(3) Å, which is a normal value for palladium(0) NHC
complexes.[6] The Pd1-N3 bond length is 2.173(2) Å, which to our
knowledge is the shortest palladium nitrogen bond in a zerovalent
complex bearing bidentate nitrogen donor functionalized NHC
ligands. The bite angle of the C-N ligand is 77.73(9)◦, which shows
the rigid nature of the ligand.[21] Because of the planar nature of
the coordination sphere around palladium, the difference in trans
influence of the pyridine nitrogen and carbene carbon donors
can be qualitatively gauged by the lengths of the bonds between
palladium and the maleic anhydride carbons. The bond trans to
the NHC (Pd1-C21) has a length of 2.104(3) Å, which is longer than
the bond trans to the nitrogen donor (Pd1-C20, 2.068(3) Å). This
indicates the stronger donating character of the carbene donor
over the pyridine donor.
1
2
3
4
5
6
11a
11b
12c
12d
15
83.7
35.9
68.5
20.4
24.9
81.4
99/1/0
96/3/1
95/4/1
97/2/1
95/4/1
93/5/2
2
6
3
10
24
2.3
16
Reaction conditions: 1% Pd-complex, 150 mM alkyne, 750 mM triethy-
lammonium formate and 150 mM p-xylene (internal standard) in 14 ml
refluxing acetonitrile. Selectivity measured at full conversion. Initial
TOF was determined with the initial linear part of the reaction profiles.
In previous research we have found that mono- and hetero-
bidentate (NHC)palladium(0) complexes are excellent catalysts for
the conversion of the alkyne to the Z-alkene.[14] This transforma-
tion shows a high chemo- and stereoselectivity which is unique to
this type of complexes. With a secondary amine donor tethered
to the NHC, selectivities of more than 99% towards Z-1-phenyl-
1-propene were observed.[6] The results of our screening with
differently substituted N-pyridyl-NHC ligands are summarized in
table 1, together with results from complexes reported earlier
(scheme 6).[6,8]
Catalytic transfer hydrogenation. As a probe for the activity and
selectivity of the complexes 11–12, transfer hydrogenation of
1-phenyl-1-propyne was performed (scheme 5).
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Appl. Organometal. Chem. 2011, 25, 276–282

Pd0 complexes bearing rigid N-pyridyl-NHC ligands in semihydrogenation
affinity of its N-donor is apparently lower than that of the alkene
product. This results in isomerization of the Z- to the E-alkene
after full conversion of the substrate and hence an apparent lower
selectivity.
Experimental Section
General procedures and instrumentation
Allreactionsinvolvingpalladiumcomplexeswereperformedusing
standardSchlenktechniquesunderanatmosphereofdrynitrogen.
Solventsweredistilledusingstandardprocedures.[22] Allchemicals
were used as received, with the exception of palladium bis-t-
butyldiazabutadiene maleic anhydride, which was synthesized
according to a literature procedure.[20] NMR measurements were
performedonVarianMercury300(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 high-frequency shifts relative to the
external TMS reference. High resolution mass spectra were
recordedonaJEOLJMSSX/SX102Afour-sectormassspectrometer;
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 3KeV. During the high resolution FAB-MS
measurements 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.
Scheme 6. Other catalysts used in transfer semihydrogenation.
Indeed, all Pd(N-pyridyl-NHC) complexes are very selective and
fast catalysts in the transfer hydrogenation of 1-phenyl-1-propyne.
The difference between the substitution patterns is also visible;
the complexes with a methyl substituted pyridyl donor show a
much lower TOF than their unsubstituted counterparts (entries
2 and 4 compared with entries 1 and 3). The aryl substituent on
the NHC also has an influence on the TOF, but it is much less
pronounced; the Mes substituent (entries 1 and 2) shows a higher
rate than the DiPP substituent (entries 3 and 4). The selectivities
are excellent for all four complexes, but it has to be noted that
isomerization and overreduction occur for all complexes after full
consumption of the alkyne. Apparently, the pyridyl donor does
not displace the product alkene from the palladium. The affinity
of the alkyne for palladium exceeds that of the alkene product,
which is why the sidereactions only occur after full conversion
of the substrate. Careful monitoring of the reaction is therefore
necessary. When these results are compared to those obtained
employing complexes bearing other chelating nitrogen donors on
the NHC (entry 5 and 6), the selectivity is similarly high. However,
a profound difference in the TOF and the behaviour after full
consumptionofthesubstrateisnoted.Whereascomplex16shows
the same overreduction and isomerisation as observed with 11
and 12, the slower catalyst 15 does not give any overreduction
after full conversion. This is attributed to the higher donor strength
of the tethered tertiary amine donor compared to the pyridine or
the triazole donors. This implies a stronger coordination, which
could also account for the lower reaction rate.
General synthesis of N-arylimidazoles
The imidazoles 1–4 were synthesized according to a literature
procedure.[15]
General synthesis of 1-aryl-3-pyridylimidazolium bromides
Compounds 5–6 were prepared according to a literature
procedure.[11]
1-mesityl-3-(2-pyridyl)-imidazolium bromide 5a
The product (1.17 g, 87%) was obtained as an off-white solid. 1H
NMR (CDCl3):δ = 11.38 (s, 1H, NCHN), 9.20 (d, 3J(HH) = 8.1 Hz, 1H,
6-pyr-H), 8.91 (d, 3J(HH) = 1.8 Hz, 1H, CH), 8.53 (dd, 3J(HH) = 6.6 Hz,
4J(HH) = 1.5 Hz, 1H, 3-pyr-H), 8.11 (dd, 3J(HH) = 6.6 Hz, 3J(HH) =
1.5 Hz, 1H, 4-pyr-H), 7.49 (m, 1H, 5-pyr-H), 7.32 (d, 3J(HH) = 1.8 Hz,
1H, CH), 7.03 (s, 2H, m-aryl-H), 2.33 (s, 3H, p-aryl-CH3), 2.17 (s, 6H,
o-aryl-CH3).
Conclusion
We have successfully synthesized zerovalent palladium complexes
bearing heterobidentate pyridine functionalized NHC ligands and
maleic anhydride via their silver(I) complexes. All complexes were
obtained in excellent yields and purities. Only the palladium
complexes with ligands containing N-isopropyl substituents could
not be isolated. However, when a more electron-poor alkene was
used as the ancillary ligand, the C-N ligated complex could be
obtained in a reasonable yield. This implies that the high electron
density on palladium was deleterious to the stability of the maleic
anhydridecomplex. Allstablecompoundswerefullycharacterized
by NMR and MS measurements and a representative palladium(0)
complex was analyzed by X-ray crystallography. Screening of
four of the complexes in the catalytic transfer hydrogenation
of 1-phenyl-1-propyne to Z-1-phenyl-1-propene showed that all
complexes are good catalysts for this conversion, with high TOFs
and excellent selectivities. As a secondary donor, the 2-pyridyl
moiety is not the ideal choice for this catalytic conversion, as the
1-mesityl-3-(6-methyl-2-pyridyl)-imidazolium bromide 5b
The product (3.27 g, 95%) was obtained as an off-white powder.
1H NMR (CDCl3):δ = 11.10 (s, 1H, NCHN), 8.97 (s, 1H, CH), 8.94 (d,
3J(HH) = 7.5 Hz, 1H, 3-pyr-H), 7.96 (dd, 1H, 3J(HH) = 7.5 Hz, 3J(HH)
3
= 7.8 Hz, 4-pyr-H), 7.33 (d, J(HH) = 7.8 Hz, 1H, 5-pyr-H), 7.30 (s,
1H, CH), 7.04 (s, 2H, m-aryl-H), 2.59 (s, 3H, 6-pyr-CH3), 2.34 (s, 3H,
p-aryl-CH3), 2.17 (s, 6H, o-aryl-CH3). 13C NMR (CDCl3):δ = 160.3
(6-pyr-C), 146.5 (2-pyr-C), 142.9 (4-pyr-CH), 142.3 (p-aryl-C), 135.5
(o-aryl-C), 132.6 (i-aryl-C), 132.1 (NCHN), 131.0 (m-aryl-CH), 121.6,
126.1, 126.6 (2 CH, 5-pyr-CH), 114.1 (3-pyr-CH), 25.3 (6-pyr-CH3),
22.6 (p-aryl-CH3), 18.6 (o-aryl-CH3). MS(FAB+): m/z = 278.1656 for
C18H20N3 [M − Br]+.
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S. Warsink et al.
1-(2,6-diisopropylphenyl)-3-(2-pyridyl)-imidazolium bromide
6a
General procedure for the synthesis of silver(I) N-heterocyclic
carbene complexes
The product (0.53 g, 96%) was obtained as a brown solid. 1H NMR
(CDCl3):δ = 11.01 (s, 1H, NCHN), 9.30–9.34 (m, 2H, CH, 6-pyr-H),
8.51 (d, 3J(HH) = 5.1 Hz, 1H, 3-pyr-CH), 8.14 (dd, 3J(HH) = 8.1 Hz,
3J(HH) = 5.1 Hz, 1H, 4-pyr-CH), 7.57 (t, 3J(HH) = 8.1 Hz, 1H, p-aryl-
CH), 7.49 (dd, 3J(HH) = 4.8 Hz, 3J(HH) = 8.1 Hz, 1H, 5-pyr-CH), 7.34
(m, 3H, m-aryl-CH, CH), 2.41 (dq, 3J(HH) = 6.9 Hz, 3J(HH) = 6.9 Hz,
Complexes 8–10 were synthesized according to a literature
procedure.[16]
[1-mesityl-3-(2-pyridyl)-imidazol-2-ylidene] silver(I) bromide
8a
i
3
i
3
The product (0.23 g, 89%) was obtained as a brown solid. 1H NMR
(CD2Cl2):δ = 8.52 (d, 3J(HH) = 4.5 Hz, 1H, 6-pyr-CH), 8.31 (d, 3J(HH)
= 8.7 Hz, 1H, 3-pyr-CH), 8.12 (m, 1H, CH), 7.91 (m, 1H, 4-pyr-CH),
7.39 (dd, 3J(HH) = 6.6 Hz, 3J(HH) = 5.7 Hz, 1H, 5-pyr-CH), 7.08 (d,
3J(HH) = 1.8 Hz, 1H, CH), 6.96 (s, 2H, m-aryl-CH), 2.33 (s, 3H, p-aryl-
CH3), 2.02 (s, 6H, o-aryl-CH3). 13C NMR (CD2Cl2):δ = 182.4 (NCN),
150.7 (2-pyr-C), 148.5 (6-pyr-CH), 139.7 (p-aryl-C), 139.3 (4-pyr-
CH), 135.8 (i-aryl-C), 134.8 (o-aryl-C), 129.4 (m-aryl-CH), 123.9 (CH),
123.4 (5-pyr-CH), 120.1 (CH), 115.2 (3-pyr-CH), 20.9 (p-aryl-CH3),
17.6 (o-aryl-CH3). MS(FAB+): m/z = 635.1900 for C34H34N6Ag [M
− Br]+.
2H, Pr-CH), 1.28 (d, J(HH) = 6.9 Hz, 6H, Pr-CH3), 1.17 (d, J(HH)
i
= 6.9 Hz, 6H, Pr-CH3). 13C NMR (CDCl3):δ = 148.7 (6-pyr-CH),
145.9 (2-pyr-C), 145.1 (o-aryl-C), 141.4 (4-pyr-CH), 135.4 (i-aryl-C),
132.3 (NCHN), 130.0 (p-aryl-CH), 125.7 (5-pyr-CH), 124.9 (m-aryl-
CH), 123.8 (CH), 121.0 (CH), 116.6 (3-pyr-CH), 28.9 (iPr-CH), 24.3
(iPr-CH3).
1-(2,6-diisopropylphenyl)-3-(6-methyl-2-pyridyl)-
imidazolium bromide 6b
The product (0.25 g, 89%) was obtained as a brown solid. 1H NMR
(CDCl3):δ = 10.81 (s, 1H, NCHN), 9.41 (s, 1H, CH), 9.02 (d, 3J(HH) =
3
3
8.4 Hz, 1H, 3-pyr-CH), 7.99 (dd, J(HH) = 8.4 Hz, J(HH) = 7.8 Hz,
1H, 4-pyr-CH), 7.58 (t, 3J(HH) = 7.8 Hz, 1H, p-aryl-CH), 7.32 (m, 4H,
CH, m-aryl-CH, 5-pyr-CH), 2.59 (s, 3H, 6-pyr-CH3), 2.41 (dq, 3J(HH)
= 6.9 Hz, 3J(HH) = 6.6 Hz, 2H, iPr-CH), 1.28 (d, 3J(HH) = 6.6 Hz, 6H,
iPr-CH3), 1.17 (d, 3J(HH) = 6.9 Hz, 6H, iPr-CH3). 13C NMR (125 MHz,
CDCl3):δ = 157.7 (6-pyr-C), 144.1 (2-pyr-C), 143.4 (o-aryl-C), 140.6
(4-pyr-CH), 137.5 (i-aryl-C), 133.8 (NCHN), 131.3 (p-aryl-CH), 129.1
(5-pyr-CH), 124.2 (m-aryl-CH), 123.7 (CH), 120.2 (CH), 112.2 (3-
pyr-CH), 27.7 (iPr-CH), 23.9 (6-pyr-CH3), 23.3 (iPr-CH3). MS(FAB+):
m/z = 320.2122 for C21H26N3 [M − Br]+.
[1-mesityl-3-(6-methyl-2-pyridyl)-imidazol-2-ylidene] silver(I)
bromide 8b
The product (0.38 g, 83%) was obtained as a brown solid. 1H NMR
(CD2Cl2):δ = 8.06 (s, 1H, CH), 7.92 (d, 3J(HH) = 8.1 Hz, 1H, 3-pyr-H),
3
4
7.79 (m, 1H, 4-pyr-H), 7.26 (dd, J(HH) = 7.8 Hz, J(HH) = 2.7 Hz,
1H, 5-pyr-H), 7.22 (s, 1H, CH), 7.08 (s, 2H, m-aryl-H), 2.60 (s, 3H,
6-pyr-CH3), 2.35 (s, 3H, p-aryl-CH3), 2.02 (s, 6H, o-aryl-CH3). 13C
NMR (CD2Cl2):δ = 183.2 (NCN), 159.1 (6-pyr-C), 150.0 (2-pyr-C),
139.9 (4-pyr-CH), 139.7 (i-aryl-C), 136.2 (p-aryl-C), 135.0 (o-aryl-C),
129.5 (m-aryl-CH), 123.6 (CH), 119.8 (5-pyr-CH), 111.7 (3-pyr-CH),
24.1 (6-pyr-CH3), 21.1 (p-aryl-CH3), 17.8 (o-aryl-CH3). MS(FAB+):
m/z = 384.0630 for C18H19N3Ag [M − Br]+.
General synthesis of imidazolium iodides 7
The pyridylimidazole was stirred in 10 equivalents of 2-iodo-
propane at reflux for 16 hours. The product precipitates from the
reaction mixture and was washed with diethyl ether until the
supernatant was colourless.
[1-(2,6-diisopropylphenyl)-3-(2-pyridyl)-imidazol-2-ylidene]
silver(I) bromide 9a
The product (0.33 g, 86%) was obtained as a brown solid. 1H NMR
(CD2Cl2):δ = 8.60(m, 1H, 4-pyr-H), 8.10(s, 1H, CH), 7.97(d, 3J(HH)=
7.5 Hz, 1H, 6-pyr-H), 7.29–7.54 (m, 5H, 5-pyr-H, 3-pyr-H, m-aryl-H,
p-aryl-H), 7.26 (s, 1H, CH), 2.42 (septet, 2H, 3J(HH) = 6.9 Hz, iPr-CH),
1.12–1.31 (m, 12H, iPr-CH3).
1-isopropyl-3-(2-pyridyl)-imidazolium iodide 7b
The product (0.90 g, 87%) was obtained as a beige solid. 1H NMR
(CDCl3):δ = 11.10 (s, 1H, NCHN), 8.57 (d, 3J(HH) = 7.8 Hz, 1H, 6-pyr-
C), 8.50 (d, 3J(HH) = 1.8 Hz, 1H, 3-pyr-CH), 8.32 (s, 1H, CH), 8.03 (dd,
4
3J(HH) = 7.8 Hz, J(HH) = 1.8 Hz, 1H, 5-pyr-CH), 7.68 (s, 1H, CH),
[1-(2,6-diisopropylphenyl)-3-(6-methyl-2-pyridyl)-imidazol-
2-ylidene] silver(I) bromide 9b
3
i
7.45 (m, 1H, 4-pyr-CH), 5.24 (septet, J(HH) = 6.9 Hz, 1H, Pr-CH),
1.72 (d, 3J(HH) = 6.6 Hz, 12H, iPr-CH3). 13C NMR (CDCl3):δ = 149.1
(4-pyr-CH), 145.9 (5-pyr-CH), 140.5 (3-pyr-CH), 133.4 (NCHN), 125.3
(6-pyr-CH), 121.1 (CH), 119.5 (CH), 115.2 (2-pyr-CH), 54.4 (iPr-CH),
23.3 (iPr-CH3). MS(FAB+): m/z = 188.1186 for C11H14N3 [M − I]+.
The product (0.75 g, 95%) was obtained as a brown solid. 1H NMR
(CD2Cl2):δ = 8.03 (s, 1H, CH), 7.84 (m, 1H, 4-pyr-H), 7.54 (t, 1H,
3J(HH) = 3.6 Hz, p-aryl-CH), 7.24–7.44 (m, 4H, m-aryl-H, 3-pyr-H,
5-pyr-H), 7.21 (s, 1H, CH), 2.66 (s, 3H, 6-pyr-CH3), 2.51 (m, 2H, iPr-
i
CH), 1.12–1.27 (m, 12H, Pr-CH3). 13C NMR (CD2Cl2):δ = 183.4
(NCN), 159.3 (6-pyr-C), 150.0 (2-pyr-C), 145.9 (o-aryl-C), 139.6
(4-pyr-CH), 135.5 (i-aryl-C), 130.8 (p-aryl-CH), 124.7 (CH), 124.5
(m-aryl-CH),123.7(5-pyr-CH),119.7(CH),111.9(3-pyr-CH),28.5(iPr-
CH), 28.4 (6-pyr-CH3), 24.3 (iPr-CH3). MS(FAB+): m/z = 426.1101
for C21H25N3Ag [M − Br]+.
1-isopropyl-3-(6-methyl-2-pyridyl)-imidazolium iodide 7b
The product (1.72 g, 89% based on the imidazole) was obtained as
a yellow crystalline solid. 1H NMR (CDCl3):δ = 11.04 (s, 1H, NCHN),
3
8.33 (s, 1H, CH), 8.31 (d, J(HH) = 8.1 Hz, 1H, 3-pyr-CH), 7.89 (dd,
3J(HH)= 8.1 Hz, 3J(HH)= 7.8 Hz, 1H, 4-pyr-CH), 7.66(s, 1H, CH), 7.28
(d, 3J(HH) = 7.8 Hz, 1H, 5-pyr-CH), 5.26 (septet, 3J(HH) = 6.3 Hz, 1H,
[1-isopropyl-3-(2-pyridyl)-imidazol-2-ylidene] silver(I) iodide
10a
3
i
iPr-CH), 1.72 (d, J(HH) = 6.3 Hz, 6H, Pr-CH3). 13C NMR (75 MHz,
CDCl3):δ = 159.1 (4-pyr-CH), 145.1 (5-pyr-CH), 140.4 (3-pyr-CH),
133.0 (NCHN), 128.4 (6-pyr-C), 121.14 (CH), 119.4 (CH), 111.7 (2-
pyr-C), 54.2 (iPr-CH), 24.1 (6-pyr-CH3), 23.3 (iPr-CH3). MS(FAB+):
m/z = 202.1344 for C12H16N3 [M-I]+.
The product (0.14 g, 92%) was obtained as a yellow solid. 1H NMR
(CD2Cl2):δ = 8.46 (d, 3J(HH) = 6.0 Hz, 1H, 6-pyr-H), 8.14 (d, 3J(HH)
= 7.5 Hz, 1H, 3-pyr-H), 7.87 (dd, 3J(HH) = 6.0 Hz, 3J(HH) = 4.5 Hz,
c
wileyonlinelibrary.com/journal/aoc
Copyright ꢀ 2011 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2011, 25, 276–282

Pd0 complexes bearing rigid N-pyridyl-NHC ligands in semihydrogenation
1H, 5-pyr-H), 7.80 (d, 3J(HH) = 1.8 Hz, 1H, CH), 7.30–7.46 (dd,
[1-(2,6-diisopropylphenyl)-3-(2-pyridyl)-imidazol-2-ylidene]
palladium(0) maleic anhydride anhydride 12a
3
3
3J(HH) = 7.5 Hz, J(HH) = 4.5 Hz, 1H, 4-pyr-H), 7.17 (d, J(HH) =
1.8 Hz, 1H, CH), 4.94 (septet, 3J(HH) = 6.6 Hz, 1H, iPr-CH), 1.52 (d,
3J(HH) = 6.6 Hz, 6H, iPr-CH3). 13C NMR (CD2Cl2):δ = 174.9 (NCN),
148.8 (2-pyr-C), 148.0 (6-pyr-CH), 137.6 (4-pyr-CH), 123.6 (5-pyr-
CH), 120.1 (CH), 117.9 (CH), 115.5 (3-pyr-CH), 55.0 (iPr-CH), 23.8
(iPr-CH3). MS(FAB+): m/z = 294.0166 for C11H13N3Ag [M − I]+.
The product (49 mg, 91%) was obtained as an ochre solid. 1H
NMR (CD2Cl2):δ = 8.77 (d, 3J(HH) = 5.1 Hz, 1H, 6-pyr-CH), 8.09 (dd,
3J(HH) = 8.4 Hz, 4J(HH) = 1.5 Hz, 1H, 5-pyr-H), 7.74 (s, 1H, CH), 7.61
(d, 3J(HH) = 7.8 Hz, 1H, 3-pyr-H), 7.55 (t, 1H, 3J(HH) = 7.5 Hz, p-aryl-
3
H), 7.44 (m, 1H, 4-pyr-H), 7.36 (d, J(HH) = 7.5 Hz, 2H, m-aryl-H),
7.19 (s, 1H, CH), 3.48–3.53 (broad s, 2H, ma), 2.37–2.68 (m, 2H,
iPr-CH), 1.05–1.42 (m, 12H, iPr-CH3). 13C NMR (CD2Cl2):δ = 198.3
(NCN), 173.4 (CO), 152.5 (2-pyr-C), 151.9 (6-pyr-CH), 145.6 (m-
aryl-C), 144.9 (m-aryl-C), 140.7 (4-pyr-CH), 135.4 (i-aryl-CH), 130.0
(p-aryl-C), 125.5 (o-aryl-C), 124.1 (CH), 123.7 (CH), 115.1 (5-pyr-CH),
111.0 (3-pyr-CH), 42.8 (alkene), 40.4 (alkene), 28.4 (iPr-CH), 24.1
(iPr-CH3), 23.6 (iPr-CH3), 23.0 (iPr-CH3), 22.3 (iPr-CH3). MS(FAB+):
m/z = 411.0938 for C20H23N3Pd [M − C4H2O3]+. Anal. Calcd. for
C24H25N3O3Pd C, 56.53; H, 4.94; N, 8.24. Found: C, 56.08; H, 4.91; N,
8.37.
[1-isopropyl-3-(6-methyl-2-pyridyl)-imidazol-2-ylidene]
silver(I) iodide 10b
The product (0.30 g, 88%) was obtained as a white solid. 1H NMR
(CD2Cl2):δ = 7.78 (dd, 3J(HH) = 7.8 Hz, 3J(HH) = 8.1 Hz, 1H, 4-pyr-
CH), 7.76 (d, 3J(HH) = 2.1 Hz, 1H, CH), 7.59 (d, 3J(HH) = 8.1 Hz,
1H, 5-pyr-CH), 7.26 (d, 3J(HH) = 2.1 Hz, 1H, CH), 7.21 (d, 3J(HH)
= 7.8 Hz, 1H, 3-pyr-CH), 4.87 (septet, 3J(HH) = 6.6 Hz, 1H, iPr-CH),
2.53 (s, 3H, 6-pyr-CH3), 1.55 (d, 3J(HH) = 6.6 Hz, 6H, iPr-CH3). 13C
NMR (CD2Cl2):δ = 181.4 (NCN), 158.4 (6-pyr-C), 150.1 (2-pyr-C),
139.6 (4-pyr-CH), 119.6 (CH), 118.6 (CH), 112.9 (5-pyr-CH), 111.6 (3-
pyr-CH), 55.1 (iPr-CH), 24.0 (6-pyr-CH3), 23.5 (iPr-CH3). MS(FAB+):
m/z = 308.0320 for C12H15N3Ag [M-I]+.
[1-(2,6-diisopropylphenyl)-3-(6-methyl-2-pyridyl)-imidazol-
2-ylidene] palladium(0) maleic anhydride anhydride
12b
The product (0.10 g, 94%) was obtained as an ochre solid. 1H
General synthesis of palladium(0) N-heterocyclic carbene
complexes
3
NMR (CD2Cl2):δ = 7.95 (d, J(HH) = 7.8 Hz, 1H, 5-pyr-H), 7.69 (s,
1H, CH), 7.56 (dd, 3J(HH) = 7.8 Hz, 3J(HH) = 7.8 Hz, 1H, p-aryl-H),
7.35–7.41 (m, 4H, m-aryl-H, 3-pyr-H, 4-pyr-H), 7.16 (s, 1H, CH), 3.53
(d, 3J(HH) = 3.9 Hz, 1H, CH), 3.41 (d, 3J(HH) = 3.9 Hz, 1H, ma), 2.88
(s, 3H, 6-pyr-CH3), 2.53 (m, 2H, iPr-CH), 1.10–1.33 (m, 12H, iPr-CH3).
13C NMR (CD2Cl2):δ = 192.4 (NCN), 173.2 (CO), 172.0 (CO), 161.4
(6-pyr-C), 151.7 (2-pyr-C), 145.5 (o-aryl-C), 144.7 (o-aryl-C), 140.5 (4-
pyr-CH), 135.5 (p-aryl-CH), 129.9 (i-aryl-C), 125.5 (m-aryl-C), 124.4
(m-aryl-CH), 123.5 (CH), 115.2 (5-pyr-CH), 112.9 (CH), 107.7 (3-pyr-
CH), 41.8 (alkene), 40.1 (alkene), 29.1 (iPr-CH), 28.6 (iPr-CH), 28.2
(6-pyr-CH3), 24.4 (iPr-CH3), 23.2 (iPr-CH3), 22.8 (iPr-CH3), 22.3 (iPr-
CH3). MS(FAB+): m/z = 425.1088 for C21H25N3Pd [M − C4H2O3]+.
Anal. Calcd. for C25H27N3O3Pd C, 57.31; H, 5.19; N, 8.02. Found: C,
57.77; H, 4.91; N, 8.24.
Complexes 11–12 and 14 were synthesized according to a
literature procedure.[6]
[1-mesityl-3-(2-pyridyl)-imidazol-2-ylidene] palladium(0)
maleic anhydride anhydride 11a
The product (0.30 g, 96%) was obtained as a yellow solid. 1H NMR
(CD2Cl2):δ = 8.75 (d, 1H, 3J(HH) = 7.5 Hz, 6-pyr-H), 8.07 (dd, 3J(HH)
= 5.4 Hz, 4J(HH) = 7.5 Hz, 1H, 5-pyr-H), 7.70 (d, 1H, 3J(HH) = 1.8 Hz,
CH), 7.57 (d, 1H, 3J(HH) = 7.5 Hz, 3-pyr-H), 7.42 (dd, 3J(HH) = 7.5 Hz,
3
3J(HH) = 5.4 Hz, 1H, 4-pyr-H), 7.10 (d, J(HH) = 1.8 Hz, 1H, CH),
7.07 (broad s, 2H, m-aryl-CH), 3.55 (broad s, 1H, ma), 3.50 (broad s,
1H, ma), 2.40 (s, 3H, p-aryl-CH3), 2.06 (s, 6H, o-aryl-CH3). 13C NMR
(CD2Cl2):δ = 192.7 (NCN), 173.1 (CO), 172.3 (CO), 152.4 (2-pyr-CH),
151.9 (6-pyr-C), 140.82 (4-pyr-CH), 139.0 (p-aryl-C), 135.6 (i-aryl-C),
134.7 (o-aryl-C), 133.9 (o-aryl-C), 129.4 (m-aryl-CH), 128.6 (m-aryl-
CH), 124.4 (CH), 123.7 (5-pyr-CH), 115.5 (CH), 111.0 (3-pyr-CH), 42.2
(alkene), 39.9 (alkene), 20.9 (p-aryl-CH3), 17.6 (o-aryl-CH3), 17.4
(o-aryl-CH3). MS(FAB+): m/z = 369.0464 for C17H17N3Pd [M −
C4H2O3]+. Anal. Calcd. for C21H19N3O3Pd C, 53.92; H, 4.09; N, 8.98.
Found: C, 54.07; H, 4.35; N, 9.12.
[1-isopropyl-3-(6-methyl-2-pyridyl)-imidazol-2-ylidene]
palladium(0) tetracyanoethylene 14
The product (0.10 g, 72%) was obtained as a dark orange solid. 1H
NMR (CD2Cl2):δ = 8.02 (dd, 1H, 3J(HH) = 7.8 Hz, 3J(HH) = 8.1 Hz,
4-pyr-CH), 7.62 (d, 3J(HH) = 2.1 Hz, 1H, CH), 7.45 (d, 3J(HH) = 7.8 Hz,
31H, -pyr-H), 7.38 (d, 3J(HH) = 8.1 Hz, 1H, 5-pyr-H), 7.29 (d, 3J(HH)
3
i
= 2.1 Hz, 1H, CH), 4.85 (septet, J(HH) = 6.6 Hz, 1H, Pr-CH), 2.99
(s, 3H, 6-pyr-CH3), 1.64 (d, 3J(HH) = 6.6 Hz, 6H, iPr-CH3). 13C NMR
(CD2Cl2):δ = 179.7 (NCN), 161.2 (6-pyr-C), 151.7 (2-pyr-C), 142.5
(4-pyr-CH), 123.7 (CH), 119.4 (CH), 117.0 (5-pyr-CH), 115.8 (TCNE),
114.9 (TCNE), 108.5 (3-pyr-CH), 55.6 (iPr-CH), 27.9 (6-pyr-CH3), 23.4
(iPr-CH3). MS(FAB+): m/z = 436.0512 for C18H16N7Pd [M + H]+.
Anal. Calcd. for C27H25N7O3Pd C, 58.54; H, 4.55; N, 17.70. Found: C,
58.36; H, 4.89; N, 18.03.
[1-mesityl-3-(6-methyl-2-pyridyl)-imidazol-2-ylidene]
palladium(0) maleic anhydride anhydride 11b
The product (50 mg, 91%) was obtained as a pale yellow solid.
1H NMR (CD2Cl2):δ = 7.89–7.95 (m, 1H, 5-pyr-H), 7.68 (s, 1H,
CH), 7.39 (m, 2H, 3-pyr-H, 4-pyr-H), 7.01–7.13 (m, 3H, CH, m-aryl-
CH), 3.46 (broad s, 2H, ma), 2.87 (s, 3H, 6-pyr-CH3), 2.42 (s, 3H,
p-aryl-CH3), 2.07 (s, 6H, o-aryl-CH3). 13C NMR (CD2Cl2):δ = 191.7
(NCN), 174.0 (CO), 173.4 (CO), 161.3 (6-pyr-C), 151.9 (2-pyr-C), 140.8
(4-pyr-CH), 139.1 (p-aryl-C), 135.8 (i-aryl-C), 134.1 (o-aryl-C), 129.8
(m-aryl-CH), 128.7 (m-aryl-CH), 124.4 (CH), 122.9 (CH), 115.8 (5-pyr-
CH), 107.8 (3-pyr-CH), 41.3 (alkene), 39.9 (alkene), 28.3 (6-pyr-CH3),
17.9 (o-aryl-CH3), 17.6 (o-aryl-CH). MS(FAB+): m/z = 383.0623 for
C18H19N3Pd [M − C4H2O3]+. Anal. Calcd. for C22H22N3O3Pd C,
54.84; H, 4.39; N, 8.72. Found: C, 54.63; H, 4.21; N, 9.05.
X-ray data collection, reduction, and refinement
A Suitable single crystal of 11b was coated with Paratone-N oil,
mounted using a glass fibre pin and frozen in the cold nitrogen
stream of the goniometer. X-ray diffraction data was collected on
a Bruker AXS APEX CCD diffractometer equipped with a rotation
anodeat153(2)Kusinggraphite-monochromatedMoKα radiation
(λ = 0.71073 Å) with a scan width of 0.3◦ and exposure time of
c
Appl. Organometal. Chem. 2011, 25, 276–282
Copyright ꢀ 2011 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

S. Warsink et al.
35 sec. The generator setting was 50 kV and 180 mA. Diffraction
data was collected over the full sphere and the frames were
integrated using the Bruker SMART[23] software package using
the narrow frame algorithm. Data were corrected for absorption
effects using the SADABS routine (empirical multi-scan method).
Atomic scattering factors for non-hydrogen elements were taken
from the literature tabulations.[24] The structure solution was
found by using direct methods as implemented in the SHELXS-97
package[25] and was refined with SHELXL-97[26] against F2 using
first isotropic and later anisotropic thermal parameters for all
non-hydrogen atoms. C–H atom positions were calculated and
allowed to ride on the carbon atom to which they are bonded,
assuming C–H bond length of 0.95 Å for aromatic protons and
0.98 Å for methyl groups. H-atom temperature factors were fixed
at 1.20 (arom-H) or 1.50 (CH3) times the isotropic temperature
factor of the C atom to which they are bonded. The H-atom
contributions were calculated but not refined. In the case of the
aromatic protons of the maleic anhydride moiety the protons
(H21, H22) were refined isotropically. The locations of the largest
peaks in the final difference Fourier map calculated as well as the
magnitude of the residual electron densities in each case were
of no chemical significance. 11b (C22H21N3O3Pd): FW = 481.82,
triclinic, space group P − 1, Z = 4, a = 9.9708(4), b = 12.1777(5),
c = 17.6335(7), α = 80.365(1), β = 76.437(1), γ = 75.693(1),
V = 2003.17(14), F000 = 976, T = 153(2), µ = 0.927, 21223
reflections collected, 9931 reflections unique (Rint = 0.0277), 7729
reflections observed (F > 2σ (F)). The final was R1 = 0.0339 and
wR2 = 0.0845(alldata). CCDC789503containsthesupplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre:
www.ccdc.cam.ac.uk/data request/cif.
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Acknowledgments
We thank the Netherlands Organization for Scientific Research
(NWO) for financial support within the bi-national research
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