Bulky picolyl substituted NHC ligands and their Pd0 complexes

Article abstract of DOI:10.1002/ejic.201000768

Heterobidentate N-heterocyclic carbene-picolyl ligands with various substitution patterns and their palladium(0) complexes have been synthesized in excellent yields via their corresponding silver(I) complexes. These complexes are among the first examples where substitution next to the coordinating nitrogen is evaluated systematically. The complexes have been studied by NMR and X-ray diffraction, confirming their bidentate nature. The complexes are active precatalysts in the transfer semihydrogenation of alkynes to Z-alkenes, with activity and selectivity depending on the picolyl substituent to a high degree. Selectivities as high as 92 % were observed. The use of differently substituted secondary picolyl donors on NHC ligands leads to stable palladium(0) complexes. The complexes are active precatalysts for the selective transfer semihydrogenation of alkynes to Z-alkenes. Copyright

Full text of DOI:10.1002/ejic.201000768

FULL PAPER
DOI: 10.1002/ejic.201000768
Bulky Picolyl Substituted NHC Ligands and Their Pd0 Complexes
Stefan Warsink,[a] Carien M. S. van Aubel,[a] Jan J. Weigand,[b] Shiuh-Tzung Liu,[c] and
Cornelis J. Elsevier*[a]
Keywords: Carbene ligands / N ligands / Palladium / Hydrogen transfer / Hybrid ligands
Heterobidentate N-heterocyclic carbene-picolyl ligands with
various substitution patterns and their palladium(0) com-
plexes have been synthesized in excellent yields via their
corresponding silver(I) complexes. These complexes are
have been studied by NMR and X-ray diffraction, confirming
their bidentate nature. The complexes are active precatalysts
in the transfer semihydrogenation of alkynes to Z-alkenes,
with activity and selectivity depending on the picolyl substit-
among the first examples where substitution next to the coor- uent to a high degree. Selectivities as high as 92% were ob-
dinating nitrogen is evaluated systematically. The complexes
served.
nitrogen donors is well established for various TMs.[8–11]
However, relatively few reports have been published con-
cerning the substitution pattern of the picoline ring, espe-
cially on the position directly adjacent to the nitrogen.[12–14]
As this position has the largest steric influence on the coor-
dination properties of the secondary donor, we were inter-
ested to see the effects of introducing different groups. Here
we present the synthesis and structural characterization of
several N-picolyl-NHC ligands, as well as the synthesis of
their silver(I) and palladium(0) complexes. A screening of
the electron-rich palladium complexes as catalysts for the
transfer semi-hydrogenation of alkynes is also described.
Introduction
The use of N-heterocyclic carbenes (NHCs) in coordina-
tion chemistry and catalysis has grown tremendously in the
past decade. Their strong donating powers combined with
their ability to accept electron density as well makes this
class of compounds excellent ligands for a range of transi-
tion metals (TMs) in various oxidation states.[1] It is well
known that the introduction of multiple donors in a ligand
improves the stability of a complex by the chelate effect.
This approach has been successfully applied to NHCs as
well, giving rise to homopolydentate carbene ligands,[2] but
also to heteropolydentate scaffolds.[3] When a weaker sec-
ondary donor is introduced, the ligand can exhibit hemi-
labile behaviour, where the weaker donor can reversibly de-
and recoordinate. This generates a free coordination site at
the metal where for example a substrate can enter. After
reaction and dissociation of the product the secondary do-
nor can recoordinate to stabilize the catalyst.[4] With the
appropriate choice of the secondary donor, one may tune
the stability and catalytic activity of the complex. In pre-
vious publications, we have shown the effect of various ni-
trogen donors on the coordinative properties of the ligands
and the catalytic activity and selectivity of their zero- and
divalent palladium complexes.[5–7,26] The use of picoline as
donor in combination with NHC ligands is also known,
and the coordination chemistry of these flexible aromatic
Results and Discussion
Imidazolium Bromides. After synthesis of the N-arylimid-
azoles 1–4,[15,16] quaternization with 2-(bromomethyl)pyr-
idines was performed to obtain the imidazolium salts 5–8
in yields between 86% and 99% (Scheme 1).[10,12,13]
[a] Molecular Inorganic Chemistry, van ’t Hoff Institute for
Molecular Sciences, University of Amsterdam,
Science Park 904, Postbus 94157, 1090 GD Amsterdam, The
Netherlands
Fax: +31-20-525-5604
E-mail: c. j.elsevier@uva.nl
[b] Institut für Anorganische und Analytische Chemie,
Westfälische Universität Münster,
48149 Münster, Germany
[c] Department of Chemistry, National Taiwan University,
Taipei 106, Taiwan
Scheme 1. Synthesis of imidazolium bromides.
5556
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2010, 5556–5562

Bulky Picolyl Substituted NHC Ligands and Their Pd0 Complexes
For compounds 5 and 6 the characteristic signal for the
imidazolium proton was observed at a chemical shift be-
tween 10.0 and 10.4 ppm in the 1H NMR spectroscopy. For
7 and 8 this signal was observed at a significantly lower
field at δ = 11.0 and 11.5 ppm, respectively. For 8 this is
intuitively correct, but for 7 it needs to be kept in mind that
the methoxy substituent only has an inductive influence on
the imidazolium core, while a mesomeric influence is pro-
hibited. This then accounts for the deshielding of the imid-
azolium core, which is visible as a high-frequency shift.
Yields are consistently high and appear to be independent
of the substitution pattern.
(NHC)silver(I) Complexes. Subsequently, the carbenes
are generated from the imidazolium bromides by reaction
with silver(I) oxide.[5,6] This reliable protocol is routinely
applied and the method of choice to stabilize reactive
NHCs, while simultaneously giving access to a variety of
TM complexes by transmetallation.[17,18] Additionally, the
silver complexes are able to transfer the NHC to another
transition metal, which obviates the need for generation of
the free carbene. This has the advantage that rigorously air-
and moisture free reaction conditions are not necessary be-
cause the (NHC)silver(I) complexes can often be handled
in air. Our results are depicted in Scheme 2.
Scheme 3. Transmetallation to obtain Pd0 NHC complexes.
It appears that substituents on the ortho positions of the
NHC aryl group are crucial for the stability of the product.
Complexes 15 and 16 were formed in solution and could be
observed by 1H NMR, but rapid decomposition did not
allow full characterization of these complexes. In stark con-
trast, the complexes 13–14 were isolated in yields over 95%
and were obtained pure by precipitation with pentane. Prin-
cipal proof for coordination of the secondary picolyl donor
is the splitting of the methylene signal to an AB system in
1
the H NMR spectroscopy. However, due to ring inversion
of the chelating ligand, these signals are broadened at room
temperature. The carbene carbons of 13–14 are observed at
a chemical shift between 188.7 and 191.5 ppm in the 13C
NMR spectroscopy. This is significantly higher than the
values reported for divalent palladium complexes bearing
analogous ligands,[9,11,14] reflecting the higher electron den-
sity on palladium.
X-ray Crystallography. To establish the molecular struc-
ture of our complexes, we attempted to grow X-ray quality
crystals. However, it appeared that the complexes lacking a
substituent on the picolyl donor were not stable enough to
yield crystals. In contrast, when pentane was allowed to dif-
fuse into a concentrated solution of 13b in dichlorometh-
ane, suitable crystals were obtained (Figure 1).
Scheme 2. Synthesis of silver(I) NHC complexes.
The silver(I) complexes 9–12 were reproducibly obtained
in high yields and purities. Besides the disappearance of the
imidazolium signal in the proton NMR, the position of the
carbene carbon in the 13C NMR is indicative of carbene
generation. For these silver(I) complexes a signal between
181 and 184 ppm is observed, which compares well to re-
ported literature data.[18] The chemical change in the imid-
azolyl core also has an effect on the chemical shift of the
methylene linker. In the imidazolium salts 5–8 the 1H NMR
signal for these benzylic protons is observed between 5.94
and 6.25 ppm, whereas in the silver(I) complexes 9–12 they
show an upfield shift to a value between 5.40 and 5.56 ppm.
(NHC)palladium(0) Complexes. The silver(I) NHC
complexes 9–12 were employed for subsequent transmetal-
lation to an often employed precursor for zerovalent palla-
dium compounds.[19] Similar to our earlier report for a dif-
ferent type of ligand,[5] excellent results were obtained in
this instances as well (Scheme 3).
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 [°]: Pd–C1 2.030(2), Pd–C21 2.049(2), Pd–
C22 2.129(2), Pd–N3 2.183(2); C1–Pd1–N3 84.75(6).
Eur. J. Inorg. Chem. 2010, 5556–5562
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5557

C. J. Elsevier et al.
FULL PAPER
The coordination geometry around palladium is virtually Table 1. Catalytic transfer hydrogenation of 1-phenyl-1-propyne.[a]
planar, with the angles totalling 360°. The Pd–C1 bond has
Entry
Complex
Initial TOF
mol sub/mol cat./h
Selectivity Z/E/alkane
a length of 2.030(2) Å, which is a length commonly re-
ported for zerovalent palladium NHC complexes,[5] but
longer than for neutral PdII complexes bearing NHC-pico-
lyl ligands.[9] The length of the bond between the palladium
and the nitrogen donor is 2.183(2) Å, which is shorter than
that in related zerovalent complexes [2.2019(12) Å and
2.197(2) Å],[5] but comparable to divalent analogues that
have been reported.[9,11,14] The complex is Y-shaped, with a
bite angle of 84.75(7)° for the bidentate NHC-picolyl li-
gand. This is smaller than the bite angle for a similar ligand
where a value of 92.06(5)° was reported.[5] Apparently, the
ligand is not so flexible as to allow for the predicted coordi-
nation angle of 90°, an effect which has been noted be-
fore.[9,11,14] These N-picolyl-NHC ligands seem to be of in-
termediate rigidity. There is a significant difference between
the bond lengths of Pd–C21 and Pd–C22 [2.049(2) and
2.128(2) Å, respectively], showing the difference in trans in-
fluence of the carbene and the nitrogen donor, with the for-
mer being the stronger donor.
(%)
1
2
3
4
5
6
13a
13b
13c
13d
14a
14b
26.1
46.6
15.5
12.9
23.1
46.4
71:10:19
91:3:6
88:7:5
92:2:6
82:10:8
86:6:8
[a] Reaction conditions: 1 mol-% Pd complex, 150 mm alkyne,
750 mm triethylammonium formate and 150 mm p-xylene (internal
standard) in 14 mL refluxing acetonitrile. Selectivity measured at
full conversion, or after 24 h (entries 3 and 4).
or that it is caused by an electronic effect of the aryl substit-
uent. The structure of a related complex has been deter-
mined by X-ray diffraction.[14] It can be seen that the π-
system of the aryl substituent is oriented towards the metal,
possibly influencing the catalytic behaviour. The reaction
times for the fastest catalysts 13b and 14b are around 3 h,
but it was observed that significant amounts of the unde-
sired E-alkene and alkane were formed after full consump-
tion of the substrate (entries 2 and 5), which means that
close monitoring of the reaction progress is required. With
the unsubstituted analogues 13a and 14a full conversion
was reached after roughly 10 h, but side reactions occurred
to a lesser degree.
Catalytic Transfer Hydrogenation. As a probe for the
activity and selectivity of the complexes 13–14, transfer
hydrogenation of 1-phenyl-1-propyne was performed
(Scheme 4).
Conclusions
We have successfully synthesized picolyl-substituted
NHC ligands with varying bulk and electronic properties.
Thus far, a limited number of examples had been reported
where substituents are present on the secondary picolyl do-
nor. We have also synthesized their zerovalent palladium
Scheme 4. Transfer hydrogenation of 1-phenyl-1-propyne with
NHC-picolyl palladium complexes.
In previously reported research we have found that complexes in high yields and purities via the corresponding
mono- and heterobidentate (NHC)palladium(0) complexes silver(I) complexes. We found that ortho substituents on the
are excellent catalysts for the conversion of alkynes to the NHC aryl group are a necessity for isolation of the stable
corresponding Z-alkene.[20] This transformation shows a electron-rich complexes; for ligands without the required
high chemo- and stereoselectivity which is unique to this substitution the products were only observed in solution.
type of complexes. With a secondary amine donor, selectivi- All stable compounds were fully characterized and a repre-
ties of more than 99% towards (Z)-1-phenyl-1-propene sentative palladium complex has also been characterized by
were observed.[5] A significant influence of the nitrogen do- X-ray crystallography, showing the bidentate nature of the
nor on the activity and selectivity were found.[7] The results ligand and the difference in trans influence imparted by its
of our screening with differently substituted NHC-picolyl two distinct donors. The denticity was also inferred from
1
ligands are summarized in Table 1.
the H NMR, where splitting of the methylene signal into
The catalysts show a good to very good selectivity two doublets was observed. As a probe for the catalytic be-
towards the Z-alkene product, with the corresponding E- haviour of the complexes, transfer hydrogenation of 1-
alkene and alkane being the only byproducts. The substitu- phenyl-1-propyne was performed. It appeared that the sub-
tion on the secondary picolyl donor clearly has a large im- stitution on the picolyl donor has a larger influence on the
pact on the activity of the catalyst, and to a lesser extent results than the NHC substituent. For the unsubstituted
on the selectivity for (Z)-1-phenyl-1-propene. The unsubsti- picolyl donor, full conversion of the alkyne was generally
tuted complexes (entries 1 and 5) show a lower activity than observed after 10 h, with selectivities as high as 82% for
the methyl-substituted analogues (entries 2 and 6). How- the Z-alkene product. The use of methyl-substituted ligands
ever, when the substituent is larger (entries 3 and 4), a dra- resulted in faster and more selective catalysts; however, the
matic decrease of activity was observed; the conversion af- selectivity dropped after full consumption of the substrate,
ter 24 h did not exceed 80%. Further research has to show making monitoring mandatory. When aryl substituents
whether this is due to the steric bulk imparted by the ligand, were present on the picolyl donor, similar selectivities were
5558
www.eurjic.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2010, 5556–5562

Bulky Picolyl Substituted NHC Ligands and Their Pd0 Complexes
2 H, CH2), 2.47 (s, 3 H, pyr-CH3), 2.33 (s, 3 H, p-aryl-CH3), 2.07
(s, 6 H, o-aryl-CH3) ppm. 13C NMR (CDCl3): δ = 158.7 (6-pyr-C),
151.7 (2-pyr-C), 141.2 (p-aryl-C), 138.0 (4-pyr-C), 137.8 (NCN),
134.4 (o-aryl-C), 130.7 (i-aryl-C), 129.8 (m-aryl-C), 124.0 (CH),
123.5 (5-pyr-C), 122.7 (CH), 120.9 (3-pyr-C), 54.0 (CH2), 24.3 (pyr-
CH3), 21.1 (p-aryl-CH3), 17.6 (o-aryl-CH3) ppm. MS (FAB+, m/z):
calcd. for C19H22N3 [M – Br]+ 292.1814; found 292.1808.
observed, but the rate of reaction was diminished dramati-
cally. We have shown that the substitution pattern of the
secondary donor has a profound effect on catalytic behav-
iour and we will continue to study this subject, as well as
the use of different donor moieties.
{1-[2-(6-Phenyl)pyridyl]methylene-3-mesityl}imidazolium Bromide
(5c): The product (0.42 g, 94%) was obtained as a white powder.[12]
Experimental Section
3
1H NMR (CDCl3): δ = 10.21 (s, 1 H, NCHN), 7.96 [dd, J(H,H)
General Procedures and Instrumentation: All reactions involving
transition metal complexes were performed using standard Schlenk
techniques under an atmosphere of dry nitrogen. Solvents were dis-
tilled using standard procedures.[21] All chemicals were used as re-
ceived, except for the bis(tert-butyldiazabutadiene maleic anhy-
dride)–palladium complex, which was synthesized according to a
literature procedure.[19] NMR measurements 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 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 sam-
ples 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 measure-
ments 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.
4
= 8, 4J(H,H) = 1.6 Hz, 2 H, Ph], 7.94 [dd, 3J(H,H) = 8, J(H,H) =
1.6 Hz, 2 H, Ph], 7.89 [d, 3J(H,H) = 7.2 Hz, 1 H, 3-pyr-H], 7.81 [dd,
3J(H,H) = 7.2, 3J(H,H) = 7.2 Hz, 1 H, 4-pyr-H], 7.71 [d, 3J(H,H) =
7.6 Hz, 1 H, 5-pyr-H], 7.53–7.44 (m, 4 H, CH, Ph), 7.11 (s, 1 H,
CH), 6.92 (s, 2 H, aryl-H), 6.22 (s, 2 H, CH2), 2.30 (s, 3 H, p-aryl-
CH3), 1.96 (s, 6 H, o-aryl-CH3) ppm.
{1-[2-(6-Mesityl)pyridyl]methylene-3-mesityl}imidazolium Bromide
(5d): The product (0.45 g, 96%) was obtained as a white solid.[12]
1H NMR (CDCl3): δ = 10.22 (s, 1 H, NCHN), 8.00 (s, 1 H, CH),
3
3
7.95 [d, J(H,H) = 7.2 Hz, 1 H, 3-pyr-H], 7.81 [dd, J(H,H) = 7.2,
3J(H,H) = 7.2 Hz, 1 H, 4-pyr-H], 7.18 [d, J(H,H) = 7.2 Hz, 1 H,
3
5-pyr-H], 7.03 (s, 1 H, CH), 6.94 (s, 2 H, aryl-H), 6.88 (s, 2 H, aryl-
H), 6.19 (s, 2 H, CH2), 2.29 (s, 6 H, 2 p-aryl-CH3), 1.94 (s, 6 H, o-
aryl-CH3), 1.90 (s, 6 H, o-aryl-CH3) ppm.
[1-(2-Pyridyl)methylene-3-(2,6-diisopropylphenyl)]imidazolium Bro-
mide (6a): The product (0.94 g, 94%) was obtained as a red solid.[10]
1H NMR (CDCl3): δ = 10.16 [dd, 4J(H,H) = 1.8, 4J(H,H) = 1.8 Hz,
1 H, NCHN], 8.52 [d, 3J(H,H) = 3.9 Hz, 6-pyr-H], 8.26 [dd,
3J(H,H) = 1.8, 4J(H,H) = 1.8 Hz, 1 H, CH], 8.03 [d, 3J(H,H) =
3
4
7.5 Hz, 3-pyr-H], 7.78 [dt, J(H,H) = 7.5, J(H,H) = 1.8 Hz, 1 H,
4-pyr-H], 7.54 [t, 3J(H,H) = 7.8 Hz, 1 H, p-aryl-H], 7.31 [d, 3J(H,H)
= 7.8 Hz, 1 H, m-aryl-H], 7.29 (m, 1 H, 5-pyr-H), 6.25 (s, 2 H,
CH2), 2.31 [dq, 3J(H,H) = 6.9, 3J(H,H) = 6.9 Hz, 2 H, iPr-H], 1.22
General Synthesis of N-Arylimidazoles 1–4: 1-(2,6-Diisopro-
pylphenyl)imidazole (2), 1-(4-methoxyphenyl)imidazole (3)[15] and
1-(4-trifluoromethylphenyl)imidazole (4)[16] were synthesized ac-
cording to literature procedures. 1-(2,4,6-Methylphenyl)imidazole
(1) was synthesized according to a modified literature procedure.[15]
3
3
[d, J(H,H) = 6.9 Hz, 6 H, iPr-CH3], 1.14 [d, J(H,H) = 6.9 Hz, 6
H, iPr-CH3] ppm.
{1-[2-(6-Methyl)pyridyl]methylene-3-(2,6-diisopropylphenyl)}imid-
azolium Bromide (6b): The product (0.57 g, 99%) was obtained as
red crystals. 1H NMR (CDCl3): δ = 10.02 [dd, 4J(H,H) = 1.5,
1-Mesityl-1H-imidazole (1): The product (27.3 g, 73%) was ob-
tained as a light beige crystalline solid after recrystallization of the
1
crude product from tetrahydrofuran. H NMR (CDCl3): δ = 7.53
3
4
1.5 Hz, 1 H, NCHN], 8.24 [dd, J(H,H) = 1.5, J(H,H) = 1.5 Hz,
(s, 1 H, NCH), 7.28 (s, 1 H, NCH), 6.96 (s, 2 H, aryl-CH), 6.93 (s,
1 H, NCHN), 2.36 (s, 3 H, p-aryl-CH3), 2.01 (s, 6 H, o-aryl-CH3)
ppm.
1 H, CH], 7.72 [d, 3J(H,H) = 7.5 Hz, 1 H, 3-pyr-H], 7.62 [dd,
3
3J(H,H) = 7.5, 7.5 Hz, 1 H, 4-pyr-H], 7.52 [t, J(H,H) = 7.8 Hz, 1
3
H, p-aryl-H], 7.29 [d, J(H,H) = 7.8 Hz, 2 H, m-aryl-H], 7.14 [dd,
3J(H,H) = 1.5, 4J(H,H) = 1.5 Hz, 1 H, CH], 7.12 [d, 3J(H,H) =
7.5 Hz, 1 H, 5-pyr-H], 6.14 (s, 2 H, CH2), 2.46 (s, 3 H, pyr-CH3),
2.32 [dd, 3J(H,H) = 6.9, 6.9 Hz, 2 H, iPr], 1.20 [d, 3J(H,H) =
6.9 Hz, 6 H, iPr], 1.12 [d, 3J(H,H) = 6.9 Hz, 6 H, iPr] ppm. 13C
NMR (CDCl3): δ = 158.7 (6-pyr-C), 151.7 (2-pyr-C), 145.4 (o-aryl-
C), 138.2 (4-pyr-C), 137.9 (m-aryl-C), 137.8 (NCN), 131.9 (i-aryl-
C), 130.2 (p-aryl-C), 124.7 (CH), 124.1 (CH), 123.5 (5-pyr-C), 121.1
(3-pyr-C), 53.9 (CH2), 28.6 (iPr-CH), 24.4 (iPr-CH3), 24.2 (pyr-
CH3) ppm. MS (FAB+, m/z): calcd. for C22H28N3 [M – Br]+
334.2283; found 334.2284.
General Synthesis of Imidazolium Bromides 5–8: Compounds 1–4
were synthesized according to literature procedures.[10,12]
[1-(2-Pyridyl)methylene-3-mesityl]imidazolium Bromide (5a): The
product (1.11 g, 86%) was obtained as a dark pink solid.[10] 1H
4
4
NMR (CDCl3): δ = 10.34 [dd, J(H,H) = 1.5, J(H,H) = 1.5 Hz, 1
H, NCHN], 8.54 [d, 3J(H,H) = 4.8 Hz, 1 H, 6-pyr-H], 8.04 [dd,
3J(H,H) = 1.5, 4J(H,H) = 1.5 Hz, 1 H, CH], 7.88 [d, 3J(H,H) =
3
4
7.8 Hz, 1 H, 3-pyr-H], 7.71 [dt, J(H,H) = 7.8, J(H,H) = 1.5 Hz,
1 H, 4-pyr-H], 7.24 [dd, 3J(H,H) = 7.5 and 4J(H,H) = 4.8 Hz, 1 H,
5-pyr-H], 7.15 [dd, 3J(H,H) = 1.5, 4J(H,H) = 1.5 Hz, 1 H, CH],
7.00 (s, 2 H, aryl-H), 6.16 (s, 2 H, CH2), 2.37 (s, 3 H, p-aryl-CH3),
2.06 (s, 6 H, o-aryl-CH3) ppm.
[1-(2-Pyridyl)methylene-3-(4-methoxyphenyl)]imidazolium Bromide
(7): The product (0.88 g, 99%) was obtained as a red solid. 1H
4
4
NMR (CDCl3): δ = 11.02 [dd, J(H,H) = 2.1, J(H,H) = 2.1 Hz, 1
{1-[2-(6-Methyl)pyridyl]methylene-3-mesityl}imidazolium Bromide H, NCHN], 8.57 [dd, 3J(H,H) = 4.8, 1.8 Hz, 1 H, 6-pyr-H], 7.90
(5b): The product (0.49 g, 98%) was obtained as a orange/red solid.
[dd, 3J(H,H) = 7.8, 4J(H,H) = 1.2 Hz, 1 H, 3-pyr-H], 7.83 [dd,
1H NMR (CDCl3): δ = 10.18 [dd, 4J(H,H) = 1.5, 4J(H,H) = 1.5 Hz, 3J(H,H) = 2.1, J(H,H) = 2.1 Hz, 1 H, CH], 7.79 [ddd, J(H,H) =
4
3
1 H, NCHN], 8.02 [dd, 3J(H,H) = 1.5, 4J(H,H) = 1.5 Hz, 1 H, 7.8, 7.5, 4J(H,H) = 1.8 Hz, 1 H, 4-pyr-H], 7.65 [d, 3J(H,H) =
3
3
3
4
CH], 7.69 [d, J(H,H) = 7.8 Hz, 1 H, 3-pyr-H], 7.62 [t, J(H,H) =
9.0 Hz, 2 H, aryl-H], 7.58 [dd, J(H,H) = 2.1, J(H,H) = 2.1 Hz, 1
7.8 Hz, 1 H, 4-pyr-H], 7.12 [d, 3J(H,H) = 7.8 Hz, 1 H, 5-pyr-H], H, CH], 7.32 [ddd, J(H,H) = 7.5, 4.8, J(H,H) = 1.2 Hz, 1 H, 5-
3
4
3
7.11 [d, J(H,H) = 1.5 Hz, 1 H, CH], 6.99 (s, 2 H, Aryl-H), 6.07 (s,
pyr-H], 7.07 [d, 3J(H,H) = 9.0 Hz, 2 H, aryl-H], 5.95 (s, 2 H, CH2),
Eur. J. Inorg. Chem. 2010, 5556–5562
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5559

C. J. Elsevier et al.
3.86 (s, 3 H, OMe) ppm. 13C NMR (CDCl3): δ = 180.9 (p-aryl-C), 3J(H,H) = 1.6 Hz, 1 H, CH], 5.50 (s, 2 H, CH2), 2.31 (s, 6 H, 2 p-
FULL PAPER
152.8 (2-pyr-C), 149.8 (6-pyr-C), 137.6 (4-pyr-C), 136.1 (NHCN),
127.6 (i-aryl-C), 124.1 (CH), 124.0 (CH), 123.8 (5-pyr-C), 123.4
(aryl-C), 120.4 (3-pyr-C), 115.4 (aryl-C), 55.8 (OMe), 54.0 (CH2)
ppm. MS (FAB+, m/z): calcd. for C16H16N3OAg [M – Br]+
266.1293; found 266.1294.
aryl-CH3), 1.95 (s, 6 H, o-aryl-CH3), 1.91 (s, 6 H, o-aryl-CH3) ppm.
bis{[1-(2-Pyridyl)methylene-3-(2,6-diisopropylphenyl)]imidazol-2-
ylidene}silver(I) Silver(I) Dibromide (10a): The product (0.13 g,
85%) was obtained as a light yellow solid.[10] 1H NMR (CDCl3): δ
3
= 8.60 [d, J(H,H) = 4.2 Hz, 1 H, 6-pyr-H], 7.76 (m, 1 H, 5-pyr-
[1-(2-Pyridyl)methylene-3-(4-trifluoromethylphenyl)]imidazolium Bro- H), 7.6–7.0 (m, 7 H, pyr-H, CH, aryl-H), 5.52 (s, 2 H, CH2), 2.39
mide (8): The product (0.39 g, 97%) was obtained as a red solid.
(m, 2 H, iPr-CH), 1.20 (br. d, 6 H, iPr-CH3), 1.12 (br. d, 6 H, iPr-
1H NMR (CDCl3): δ = 11.52 [dd, 4J(H,H) = 1.8, 4J(H,H) = 1.8 Hz, CH3) ppm.
3
1 H, NCHN], 8.51 (m, 1 H, 6-pyr-H), 8.03 [d, J(H,H) = 8.1 Hz,
bis({1-[2-(6-Methyl)pyridyl]methylene-3-(2,6-diisopropylphenyl)}-
2 H, aryl-H], 7.94 [dd, 3J(H,H) = 1.8, 4J(H,H) = 1.8 Hz, 1 H, CH],
imidazol-2-ylidene)silver(I) Silver(I) Dibromide (10b): The product
3
4
7.91 [dd, J(H,H) = 1.8, J(H,H) = 1.8 Hz, 1 H, CH], 7.84 (m, 1
H, 3-pyr-H), 7.80 [d, 3J(H,H) = 8.1 Hz, 2 H, aryl-H], 7.72 (m, 1
H, 4-pyr-H), 7.27 (m, 1 H, 5-pyr-H), 5.94 (s, 2 H, CH2) ppm. 13C
NMR (75 MHz, CDCl3): δ = 152.0 (2-pyr-C), 149.8 (6-pyr-C),
137.9 (4-pyr-C), 137.0 (NCHN), 136.7 (p-aryl-C), 132.3 (CF3),
127.9 (i-aryl-C), 124.9, 124.2 (o- and m-aryl-C), 124.3 (5-pyr-C),
122.4 (CH), 122.3 (CH) 120.2 (3-pyr-C), 54.3 (CH2) ppm. MS
(FAB+, m/z): calcd. for C16H13F3N3 [M – Br]+ 304.1062; found
304.1067.
1
(0.27 g, 94%) was obtained as a red solid. H NMR (CD2Cl2): δ =
3
3
7.61 [dd, J(H,H) = 7.6, J(H,H) = 7.6 Hz, 1 H, 4-pyr-H], 7.48 [t,
3J(H,H) = 8.1 Hz, 1 H, p-aryl-H], 7.39 [d, J(H,H) = 1.8 Hz, 1 H,
3
3
3
CH], 7.28 [d, J(H,H) = 8.1 Hz, 2 H, m-aryl-H], 7.15 [d, J(H,H)
= 7.6 Hz, 1 H, 3-pyr-H], 7.1–7.0 (m, 3 H, CH, 5-pyr-H), 5.49 (s, 2
H, CH2), 2.52 (s, 3 H, pyr-CH3), 2.41 (m, 2 H, iPr-CH), 1.3–1.1
(m, 12 H, iPr-CH3) ppm. 13C NMR (CD2Cl2): δ = 184.1 (NCN),
158.9 (6-pyr-C), 154.4 (2-pyr-C), 146.0 (o-aryl-C), 137.4 (i-aryl-C),
137.2 (4-pyr-C), 137.3 (p-aryl-C), 130.4 (m-aryl-C), 124.1 (5-pyr-
C), 122.7 (CH), 122.2 (CH), 118.5 (3-pyr-C), 56.7 (CH2), 28.2 (iPr-
CH), 24.3 (pyr-CH3), 23.9 (iPr-CH3) ppm. MS (FAB+, m/z): calcd.
for C22H27N3Ag [M – Br]+ 440.1256; found 440.1250.
General Synthesis of Silver(I) N-Heterocyclic Carbene Complexes
9–12: In a typical experiment, 1 mmol of the imidazolium bromide
and 0.55 equiv. of silver(I) oxide were suspended in dichlorometh-
ane to 0.1 m and the reaction mixture was stirred for 16 h. It was
then filtered through a celite pad and concentrated in vacuo to
yield the product.
bis{[1-(2-Pyridyl)methylene-3-(4-methoxyphenyl)]imidazol-2-
ylidene}silver(I) Silver(I) Dibromide (11): The product (0.82 g, 85%)
1
was obtained as a red solid. H NMR (CD2Cl2): δ = 8.58 (m, 1 H,
3
Bis{[1-(2-Pyridyl)methylene-3-mesityl]imidazol-2-ylidene}silver(I)
6-pyr-H), 7.73 (m, 1 H, 4-pyr-H), 7.49 [d, J(H,H) = 9.0 Hz, 2 H,
Silver(I) Dibromide (9a): The product (0.20 g, 87%) was obtained
aryl-H], 7.37 (m, 1 H, 3-pyr-H), 7.36 [d, 3J(H,H) = 2.1 Hz, 1 H,
3
3
as a light brown solid.[10] 1H NMR (CDCl3): δ = 8.58 [d, J(H,H)
CH], 7.29 (m, 1 H, 5-pyr-H), 7.29 [d, J(H,H) = 2.1 Hz, 1 H, CH],
= 5.7 Hz, 1 H, 6-pyr-H], 7.72 [dt, 3J(H,H) = 7.5, 4J(H,H) = 1.8 Hz,
1 H, 4-pyr-H], 7.37 [d, 3J(H,H) = 2.1 Hz, 1 H, CH], 7.34 [d,
6.97 [d, 3J(H,H) = 9.0 Hz, 2 H, aryl-H], 5.49 (s, 2 H, CH2), 3.84
(s, 3 H, OMe) ppm. 13C NMR (CD2Cl2): δ = 181.4 (NCN), 159.8
3J(H,H) = 7.5 Hz, 1 H, 3-pyr-H], 7.27 (m, 1 H, 5-pyr-H), 6.96 (m, (6-pyr-C), 155.2 (2-pyr-C), 149.8 (p-aryl-C), 137.3 (3-pyr-C), 133.1
3 H, aryl-H, CH), 5.50 (s, 2 H, CH2), 2.32 (s, 3 H, p-aryl-CH3), (i-aryl-C), 1235.4 (aryl-CH), 123.4 (CH), 123.0 (5-pyr-C), 122.7
1.94 (s, 6 H, o-aryl-CH3) ppm.
(CH), 122.2 (4-pyr-C), 114.6 (aryl-CH), 57.3 (CH2), 55.6 (OMe)
ppm. MS (FAB+, m/z): calcd. for C16H15N3Ag [M – Br]+ 372.0266;
found 372.0260.
bis({1-[2-(6-methyl)pyridyl]methylene-3-mesityl}imidazol-2-ylidene)-
silver(I) Silver(I) Dibromide 9b: The product (0.27 g, 94%) was ob-
1
tained as a brown solid. H NMR (CD2Cl2): δ = 7.61 [dd, 3J(H,H)
bis{[1-(2-pyridyl)methylene-3-(4-trifluoromethylphenyl)]imidazol-2-
= 7.8, 3J(H,H) = 7.8 Hz, 1 H, 4-pyr-H], 7.37 [d, 3J(H,H) = 1.5 Hz, ylidene}silver(I) Silver(I) Dibromide (12): The product (0.10 g, 88%)
3
1
1 H, CH], 7.14 [d, J(H,H) = 7.8 Hz, 1 H, 3-pyr-H], 7.02–6.98 (m,
was obtained as a brown solid. H NMR (CD2Cl2): δ = 8.59 (m, 1
4 H, CH, aryl-H and 5-pyr-H), 5.40 (s, 2 H, CH2), 2.52 (s, 3 H,
H, 6-pyr-H), 7.84–7.77 (m, 5 H, aryl-H and 3-pyr-H), 7.74 (m, 1
pyr-CH3), 2.36 (s, 3 H, p-aryl-CH3), 1.97 (s, 6 H, o-aryl-CH3) ppm. H, 4-pyr-H), 7.44 [d, 3J(H,H) = 2.1 Hz, 1 H, CH], 7.38 [d, 3J(H,H)
13C NMR (CD2Cl2): δ = 183.1 (NCN), 158.8 (6-pyr-C), 154.7 (2- = 2.1 Hz, 1 H, CH], 7.30 (m, 1 H, 5-pyr-H), 5.56 (s, 2 H, CH2).
pyr-C), 139.4 (p-aryl-C), 137.3 (4-pyr-C), 135.6 (i-aryl-C), 134.9 (o-
aryl-C), 129.2 (m-aryl-C), 122.9 (5-pyr-C), 122.7 (CH), 122.2 (CH),
13C NMR (CD2Cl2): δ = 182.6 (NCN), 154.8 (6-pyr-C), 149.9 (2-
pyr-C), 137.3 (3-pyr-C), 135.4 (p-aryl-C), 130.9 (CF3), 127.0 (i-aryl-
118.6 (3-pyr-C), 56.9 (CH2), 24.2 (pyr-CH3), 20.9 (p-aryl-CH3), C), 124.4 (aryl-C), 123.5 (5-pyr-C), 123.1 (CH), 122.9 (aryl-C),
17.5 (o-aryl-CH3) ppm. MS (FAB+, m/z): calcd. for C19H21N3Ag
121.5 (CH), 121.2 (4-pyr-C), 57.5 (CH2). MS (FAB+, m/z): calcd.
[M – Br]+ 398.0786; found 398.0786.
for C32H24F6N6Ag [M – AgBr2]+ 713.1018; found 713.1019.
bis({1-[2-(6-phenyl)pyridyl]methylene-3-mesityl}imidazol-2-ylidene)-
silver(I) Silver(I) Dibromide (9c): The product (0.10 g, 99%) was
obtained as a beige solid.[12] 1H NMR (CDCl3): δ = 7.97 [dd,
General Synthesis of (N-Heterocyclic Carbene)(Maleic Anhydride)–
Palladium(0) Complexes 13–16: In a typical experiment, sil-
ver(I)(NHC) complex (0.5 mmol with respect to the carbene) and
4
3
3J(H,H) = 8, J(H,H) = 1.6 Hz, 2 H, Ph], 7.77 [dd, J(H,H) = 7.6, 0.9 equiv. Pd(tBuDAB) were dissolved in dichloromethane to 0.1 m
3
3J(H,H) = 7.6 Hz, 1 H, 4-pyr-H], 7.70 [d, J(H,H) = 7.6 Hz, 1 H,
and stirred for 1 h. The reaction mixture was then filtered of a pad
of celite and concentrated to a small volume. It is then layered with
5-pyr-H], 7.5–7.4 (m, 5 H, CH, Ph, 3-pyr-H), 6.95 [d, 3J(H,H) =
1.6 Hz, 1 H, CH], 6.93 (s, 2 H, aryl-H), 5.55 (s, 2 H, CH2), 2.32 (s, pentane, allowing the product to precipitate. After decanting the
3 H, p-aryl-CH3), 1.98 (s, 6 H, o-aryl-CH3) ppm.
solution, the product is washed with pentane and dried in vacuo.
bis({1-[2-(6-mesityl)pyridyl]methylene-3-mesityl}imidazol-2-ylidene)-
silver(I) Silver(I) Dibromide (9d): The product (0.18 g, 99%) was
obtained as a beige solid.[12] 1H NMR (CDCl3): δ = 7.77 [dd,
{[1-(2-Pyridyl)methylene-3-mesityl]imidazol-2-ylidene}palladium(0)
Maleic Anhydride (13a): The product (0.27 g, 98%) was obtained as
a yellow solid. 1H NMR (CD2Cl2): δ = 8.92 [d, 3J(H,H) = 5.1 Hz, 1
H, 6-pyr-H], 7.85 [dt, 3J(H,H) = 7.2, 3J(H,H) = 1.2 Hz, 1 H, 4-pyr-
3
3
3J(H,H) = 7.6, J(H,H) = 7.6 Hz, 1 H, 4-pyr-H], 7.37 [d, J(H,H)
3
3
= 1.6 Hz, 1 H, CH], 7.34 [d, 3J(H,H) = 7.6 Hz, 1 H, 3-pyr-H], 7.18 H], 7.56 [d, J(H,H) = 7.2 Hz, 1 H, 3-pyr-H], 7.35 [dt, J(H,H) =
3
3
3
[d, J(H,H) = 7.6 Hz, 1 H, 5-pyr-H], 6.92 (s, 4 H, aryl-H), 6.88 [d,
6.0, J(H,H) = 1.2 Hz, 1 H, 5-pyr-H], 7.25 [d, J(H,H) = 2.1 Hz, 1
5560
www.eurjic.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2010, 5556–5562

Bulky Picolyl Substituted NHC Ligands and Their Pd0 Complexes
H, CH], 7.08 (s, 1 H, aryl-H), 6.91 (s, 1 H, aryl-H), 6.85 [d, 3J(H,H)
p-aryl-C, 2 i-aryl-C, 4 o-aryl-C), 128.9, 128.5, 128.2, 127.3 (4 m-
aryl-CH), 125.9 (CH), 122.7 (5-pyr-CH), 121.7 (3-pyr-CH), 120.9
(CH), 56.4 (CH2), 39.9 (alkene), 37.8 (alkene), 20.8 (2 p-aryl-CH3),
2
= 1.2 Hz, 1 H, CH], 5.41 [d, J(H,H) = 15 Hz, 1 H, CH2], 5.12 [d,
2J(H,H) = 15 Hz, 1 H, CH2], 3.30 (br. s, 1 H, ma), 3.08 (br. s, 1 H,
ma), 2.37 (s, 3 H, p-aryl-CH3), 2.05 (s, 3 H, o-aryl-CH3), 1.92 (s, 3 20.5, 20.3 (2 o-aryl-CH3), 17.7, 17.6 (2 o-aryl-CH3). MS (FAB+,
H, o-aryl-CH3). 13C NMR (CD2Cl2): δ = 188.7 (NCN), 173.6 (CO), m/z): calcd. for C27H29N3Pd [M – ma]+ 501.1396; found 501.1410.
173.5 (CO), 155.9 (6-pyr-C), 154.3 (2-pyr-CH), 139.8 (4-pyr-C),
{[1-(2-Pyridyl)methylene-3-(2,6-diisopropylphenyl)]imidazol-2-
139.1 (p-aryl-C), 137.1 (i-aryl-C), 136.1 (o-aryl-C), 135.6 (o-aryl-
ylidene}palladium(0) Maleic Anhydride (14a): The product (83 mg,
C), 129.6 (m-aryl-CH), 128.9 (m-aryl-CH), 125.4 (CH), 125.2
98%) was obtained as yellow crystals. 1H NMR (CD2Cl2): δ = 8.83
(m, 1 H, 6-pyr-H), 7.88 (m, 1 H, 4-pyr-H), 7.57 (m, 1 H, 3-pyr-H),
(CH), 123.4 (5-pyr-CH), 121.8 (3-pyr-CH), 55.9 (CH2), 41.5 (al-
kene), 40.4 (alkene), 21.4 (p-aryl-CH3), 18.1 (br., o-aryl-CH3). MS
7.48 [t, J(HH) = 7.8 Hz, 1 H, p-aryl-H], 7.39 (m, 1 H, 5-pyr-H),
(FAB+, m/z): calcd. for C22H22N3O3Pd [M + H]+ 482.0696; found
7.29 [d, 3J(HH) = 7.8 Hz, 2 H, m-aryl-H], 7.26 [d, 3J(HH) = 1.8 Hz,
482.0675.
1 H, CH], 6.95 [d, 3J(HH) = 1.8 Hz, 1 H, CH], 5.27 (br. s, 2 H,
CH2), 3.34 (br. s, 2 H, ma), 2.58–2.24 (2 br. m, 2 H, iPr), 1.28 [br.
({1-[2-(6-Methyl)pyridyl]methylene-3-mesityl}imidazol-2-ylidene)pal-
d, 3J(HH) = 6.6 Hz, 6 h, iPr], 1.05 [br. d, 3J(HH) = 6.9 Hz, 6 h,
ladium(0) Maleic Anhydride (13b): The product (0.13 g, 96%) was
iPr]. 13C NMR (CD2Cl2): δ = 190.1 (NCN), 172.6 (CO), 172.3
(CO), 154.1 (6-pyr-CH), 153.8 (2-pyr-C), 145.8 (o-aryl-CH), 138.7
(4-pyr-CH), 136.2 (i-aryl-C), 129.8 (p-aryl-C), 125.0 (m-aryl-C),
124.0 (CH), 123.8 (CH), 123.0 (5-pyr-CH), 121.1 (3-pyr-CH), 55.7
(CH2), 41.2 (alkene), 40.8 (alkene), 29.9 (iPr-CH), 24.6 (iPr-CH3),
23.7 (iPr-CH3). MS (FAB+, m/z): calcd. for C25H28N3O3Pd [M +
H]+ 524.1165; found 524.1174. C25H27N3O3Pd (537.12): calcd. C
54.39, H 5.19, N 8.02; found C 54.50, H 5.31, N 7.69.
obtained as a yellow solid. 1H NMR (CD2Cl2): δ = 7.72 [dd, J(HH)
3
3
= 7.5, J(HH) = 7.8 Hz, 1 H, 4-pyr-H], 7.37 [d, J(HH) = 7.5 Hz,
1 H, 3-pyr-H], 7.34 3J(HH) = 7.8 Hz, 1 H, 5-pyr-H(), 7.29 [d,
3J(HH) = 1.8 Hz, 1 H, CH], 7.09 (br. s, 1 H, aryl-H), 6.97 (br. s, 1
H, aryl-H), 6.91 [d, 3J(HH) = 1.8 Hz, 1 H, CH], 5.50 [br. d, 2J(HH)
2
= 13.5 Hz, 1 H, CH2], 5.09 [br. d, J(HH) = 13.5 Hz, 1 H, CH2],
3.36 (br. s, 1 H, ma), 3.10 (br. s, 1 H, ma), 2.81 (s, 3 H, pyr-CH3),
2.37 (s, 3 H, p-aryl-CH3), 2.15 (br. s, 3 H, o-aryl-CH3), 1.86 (br. s,
3 H, o-aryl-CH3). 13C NMR (CD2Cl2): δ = 190.0 (NCN), 174.0
(CO), 173.3 (CO), 161.8 (6-pyr-C), 154.1 (2-pyr-C), 138.8 (p-aryl-
C), 138.4 (4-pyr-CH), 136.7 (i-aryl-C), 135.9 (o-aryl-C), 135.6 (o-
aryl-C), 129.0 (m-aryl-CH), 128.7 (m-aryl-CH), 124.5 (5-pyr-CH),
122.1 (CH), 121.6 (3-pyr-CH), 121.2 (CH), 56.2 (CH2), 39.3 (al-
kene), 39.2 (alkene), 29.0 (pyr-CH3), 21.1 (p-aryl-CH3), 17.9 (o-
aryl-CH3), 17.7 (o-aryl-CH3). MS (FAB+, m/z): calcd. for
C2 3 H2 4 N3 O3 Pd [ M + H]+ 4 9 6 . 0 8 5 2 ; fo u nd 4 96 . 0 86 6.
C23H23N3O3Pd (495.08): calcd. C 55.71, H 4.68, N 8.47; found C
55.70, H 4.74, N 8.23.
({1-[2-(6-Methyl)pyridyl]methylene-3-(2,6-diisopropylphenyl)}imid-
azol-2-ylidene)palladium(0) Maleic Anhydride (14b): The product
1
(0.13 g, 98%) was obtained as a yellow solid. H NMR (CD2Cl2):
3
3
δ = 7.74 [dd, J(HH) = 7.5, J(HH) = 7.8 Hz, 1 H, 4-pyr-H], 7.47
[t, 3J(HH) = 7.8 Hz, 1 H, p-aryl-H], 7.37 [d, 3J(HH) = 7.5 Hz, 1
H, 3-pyr-H], 7.35 {d, [d, 3J(HH) = 7.8 Hz, 1 H, 5-pyr-H]}, 7.30
3
(m, 2 H, m-aryl-H), 7.25 [d, J(HH) = 2.1 Hz, 1 H, CH], 6.95 [d,
3J(HH) = 2.1 Hz, 1 H, CH], 5.50 (br. s, 1 H, CH2), 5.23 (br. s, 1
H, CH2), 3.32 (s, 2 H, ma), 2.78 (s, 3 H, pyr-CH3), 2.64 (br. m, 1
H, iPr-H), 2.37 (br. m, 1 H, iPr-H), 1.4–1.0 (br. m, 12 H, iPr-CH3).
13C NMR (CD2Cl2): δ = 191.4 (NCN), 173.4 (CO), 173.2 (CO),
162.3 (6-pyr-C), 154.0 (2-pyr-C), 146 (br., o-aryl-C), 138.3 (4-pyr-
CH), 136.3 (i-aryl-C), 129.6 (p-aryl-CH), 126.8 (m-aryl-CH), 125.2
(m-aryl-CH), 124.5 (5-pyr-CH), 123.8 (CH), 121.8 (3-pyr-CH),
120.8 (CH), 56.4 (CH2), 39.4 (alkene), 39.2 (alkene), 29.3 (iPr-CH),
28.8 (iPr-CH), 28.5 (pyr-CH3), 26.0 (iPr-CH3), 25.1 (iPr-CH3),
24.1, (iPr-CH3), 23.6 (iPr-CH3). MS (FAB+, m/z): calcd. for
C22H27N3Pd [M – ma]+ 439.1240; found 439.1251.
({1-[2-(6-Phenyl)pyridyl]methylene-3-mesityl}imidazol-2-ylidene)pal-
ladium(0) Maleic Anhydride (13c): The product (48 mg, 98%) was
obtained as a yellow solid. 1H NMR (CD2Cl2): δ = 8.09 [d, 3J(H,H)
3
3
= 4.5 Hz, 2 H, Ph], 7.78 [dd, J(H,H) = 4.5, J(H,H) = 4.5 Hz, 1
3
H, 4-pyr-H], 7.70 [d, J(H,H) = 4.5 Hz, 1 H, 3-pyr-H], 7.5–7.4 (m,
3 H, 5-pyr-H, Ph), 7.14 (s, 1 H, CH), 7.08 (s, 2 H, aryl-H), 6.90 (s,
1 H, CH), 4.9–4.6 (br. s, 2 H, CH2), 3.02 (br. s, 1 H, ma), 2.41 (s,
3 H, p-aryl-CH3), 2.11 (s, 1 H, ma), 1.92 (br. s, 6 H, o-aryl-CH3).
13C NMR (CD2Cl2): δ = 191.5 (NCN), 174.1 (CO), 157.8 (6-pyr-
C), 156.6 (2-pyr-C), 139.3 (4-pyr-CH), 139.0, 138.0, 136.9, 136.7
(p-aryl-C, p-Ph–CH, i-aryl-C, i-Ph–C), 136.2 (o-aryl-C), 135.9 (o-
aryl-C), 129.4 (m-aryl-CH), 129.2 (m-aryl-CH), 128.8 (Ph-CH),
127.0 (Ph-CH), 123.4 (5-pyr-CH), 121.3 (CH), 121.0 (3-pyr-CH),
119.1 (CH), 55.9 (CH), 39.4 (alkene), 21.0 (p-aryl-CH3), 18.0 (o-
aryl-CH3), 17.6 (o-aryl-CH3). MS (FAB+, m/z): calcd. for
C24H23N3Pd [M – ma]+ 459.0927; found 459.0930.
{[1-(2-Pyridyl)methylene-3-(4-methoxyphenyl)]imidazol-2-ylidene}-
palladium(0) Maleic Anhydride: The product could be identified in
1H NMR, although decomposition was too rapid to assign the
spectrum.
{[1-(2-Pyridyl)methylene-3-(4-trifluoromethylphenyl)]imidazol-2-
ylidene}palladium(0) Maleic Anhydride: The product (18 mg, 26%)
was obtained as a yellow solid which decomposed over the course
1
of several hours. H NMR (CD2Cl2): δ = 8.88 (m, 1 H, 6-pyr-H),
7.92–7.81 (m, 6 H, aryl-H, CH, 4-pyr-H), 7.55 (m, 1 H, 3-pyr-H),
7.40 (m, 1 H, 5-pyr-H), 7.29 [d, J(HH) = 2.1 Hz, 1 H, CH], 5.40–
5.16 (2 br. d, 2 H, CH2), 3.80–3.40 (2 br. d, 2 H, ma).
({1-[2-(6-Mesityl)pyridyl]methylene-3-mesityl}imidazol-2-ylidene)pal-
ladium(0) Maleic Anhydride (13d): The product (83 mg, 95%) was
obtained as an ochre solid. 1H NMR (CD2Cl2): δ = 7.90 [dd,
3
3
3
3J(H,H) = 7.8, J(H,H) = 7.8 Hz, 1 H, 4-pyr-H], 7.54 [d, J(H,H)
X-ray Data Collection, Reduction, and Refinement: A suitable single
= 7.8 Hz, 1 H, 3-pyr-H], 7.35 [d, J(H,H) = 7.8 Hz, 1 H, 5-pyr-H], crystal of 13b was coated with Paratone-N oil, mounted using a
3
3
7.30 [d, J(H,H) = 2.1 Hz, 1 H, CH], 7.06 (br. s, 2 H, pyr-aryl-H),
glass fibre pin and frozen in the cold nitrogen stream of the goni-
ometer. X-ray diffraction data was collected on a Bruker AXS
APEX CCD diffractometer equipped with a rotation anode at
153(2) K using graphite-monochromated Mo-Kα radiation (λ =
0.71073 Å) with a scan width of 0.3° and exposure time of 15 s.
The generator setting was 50 kV and 160 mA. Diffraction data was
collected over the full sphere and the frames were integrated using
3
6.95 (br. s, 1 H, aryl-H), 6.89 [d, J(H,H) = 2.1 Hz, 1 H, CH], 6.87
(br. s, 1 H, aryl-H), 5.66 [d, 2J(H,H) = 14.4 Hz, 1 H, CH2], 5.12
2
[d, J(H,H) = 14.4 Hz, 1 H, CH2], 2.83 (br. s, 1 H, ma), 2.36 (s, 3
H, p-aryl-CH3), 2.32 (s, 3 H, p-aryl-CH3), 2.06 (s, 3 H, o-aryl-CH3),
1.99 (s, 3 H, o-aryl-CH3), 1.89 (s, 3 H, o-aryl-CH3), 1.86 (s, 3 H,
o-aryl-CH3), 1.61 (br. s, 1 H, ma). 13C NMR (CD2Cl2): δ = 190.4
(NCN), 173.9 (CO), 172.6 (CO), 163.7 (6-pyr-C), 154.6 (2-pyr-C), the Bruker SMART[22] software package using the narrow frame
139.1 (4-pyr-CH), 138.4, 138.2, 138.0, 136.6, 136.4, 136.0, 134.6 (2
algorithm. Data were corrected for absorption effects using the SA-
Eur. J. Inorg. Chem. 2010, 5556–5562
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5561

C. J. Elsevier et al.
FULL PAPER
[7] S. Warsink, R. M. Drost, M. Lutz, A. L. Spek, C. J. Elsevier,
Organometallics 2010, 29, 3109–3116.
DABS routine (empirical multi-scan method). Atomic scattering
factors for non-hydrogen elements were taken from the literature
tabulations.[23] The structure solution was found by using direct
methods as implemented in the SHELXS-97 package[24] and was
refined with SHELXL-97[25] 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 iso-
tropically. 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. 13b
[8] D. S. McGuiness, K. S. Cavell, Organometallics 2000, 19, 741–
748; A. A. D. Tulloch, A. A. Danopoulos, S. Kleinhenz, M. E.
Light, M. B. Hursthouse, G. Eastham, Organometallics 2001,
20, 2027–2031; S. Winston, N. Stylianides, A. A. D. Tulloch,
J. A. Wright, A. A. Danopoulos, Polyhedron 2004, 23, 2813–
2820; X. Wang, S. Liu, G.-X. Jin, Organometallics 2004, 23,
6002–6007; M. Yigit, B. Yigit, I. Ozdemir, E. Cetinkaya, B.
Cetinkaya, Appl. Organomet. Chem. 2006, 20, 322–327; F.
Zeng, Z. Yu, J. Org. Chem. 2006, 71, 5274–5281; L. Mercs, G.
Labat, A. Neels, A. Ehlers, M. Albrecht, Organometallics 2006,
25, 5648–5656; Z. Xi, X. Zhang, W. Chen, S. Fu, D. Wang,
Organometallics 2007, 26, 6636–6642; Z. Xi, B. Liu, W. Chen,
J. Org. Chem. 2008, 73, 3954–3957; X. Zhang, Z. Xi, A. Liu,
W. Chen, Organometallics 2008, 27, 4401–4406; X. Zhang, B.
Liu, A. Liu, W. Xie, W. Chen, Organometallics 2009, 28, 1336–
1349; Z. Xi, B. Liu, C. Lu, W. Chen, Dalton Trans. 2009, 7008–
7014; B. Liu, B. Liu, Y. Zhou, W. Chen, Organometallics 2010,
29, 1457–1464.
¯
(C23H23N3O3Pd): FW = 495.84, monoclinic, space group P1, Z =
[9] A. A. D. Tulloch, A. A. Danopoulos, R. P. Tooze, S. M. Caffer-
key, S. Kleinhenz, M. B. Hursthouse, Chem. Commun. 2000,
1247–1248; A. A. D. Tulloch, S. Winston, A. A. Danopoulos,
G. Eastham, M. B. Hursthouse, Dalton Trans. 2003, 699–708;
X. Wang, S. Liu, L.-H. Weng, G.-X. Jin, Organometallics 2006,
25, 3565–3569.
2, a = 10.4832(3), b = 10.7456(3), c = 10.8393(3), α = 63.8842(4) β
= 78.1875(4), β = 71.1702(4), V = 1034.87(5), F(000) = 504, T =
153(2), μ = 0.927, 10634 reflections collected, 4929 reflections
unique (Rint = 0.0166), 4608 reflections observed [F Ͼ 2σ(F)]. The
final was R1 = 0.0231 and wR2 = 0.0608 (all data).
[10] A. A. D. Tulloch, A. A. Danopoulos, S. Winston, S. Kleinhenz,
G. Eastham, J. Chem. Soc., Dalton Trans. 2000, 4499–4506.
[11] P. L. Chiu, C.-L. Lai, C.-F. Chang, C.-H. Hu, H. M. Lee, Orga-
nometallics 2005, 24, 6169–6178; M. C. Jahnke, T. Pape, F. E.
Hahn, Eur. J. Inorg. Chem. 2009, 2009, 1960–1969.
[12] C.-Y. Wang, Y.-H. Liu, S.-M. Peng, S.-T. Liu, J. Organomet.
Chem. 2006, 691, 4012–4020.
[13] C.-Y. Wang, C.-F. Fu, Y.-H. Liu, S.-M. Peng, S.-T. Liu, Inorg.
Chem. 2007, 46, 5779–5786.
[14] C.-Y. Wang, Y.-H. Liu, S.-M. Peng, J.-T. Chen, S.-T. Liu, J.
Organomet. Chem. 2007, 692, 3976–3983.
[15] J. Liu, J. Chen, J. Zhao, Y. Zhao, L. Li, H. Zhang, Synthesis
2003, 2003, 2661–2666.
[16] R. Zhu, L. Xing, X. Wang, C. Cheng, D. Su, Y. Hu, Adv. Synth.
Catal. 2008, 350, 1253–1257.
[17] H. M. J. Wang, I. J. B. Lin, Organometallics 1998, 17, 972–975;
J. M. Hayes, M. Viciano, E. Peris, G. Ujaque, A. Lledós, Orga-
nometallics 2007, 26, 6170–6183.
CCDC-784029 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.
General Experimental Procedure for Catalytic Transfer Semi-Hy-
drogenation of Phenyl Propyne Using Formic Acid as Hydrogen Do-
nor: A solution of 150 mm 1-phenyl-1-propyne, 750 mm triethylam-
monium formate and 150 mm p-xylene in 12 mL acetonitrile was
heated to reflux. At t = 0, a solution of 1 mol-% catalyst in 2 mL
acetonitrile was added to the reaction mixture. Periodically, sam-
ples were taken over a period of 24 h. Reaction mixtures were ana-
lyzed by GC to determine conversions. Reactions were quenched
by dilution with EtOH.
[1] W. A. Herrmann, C. Köcher, Angew. Chem. Int. Ed. Engl. 1997,
36, 2162–2187; D. Bourissou, O. Guerret, F. P. Gabbai, G. Ber-
trand, Chem. Rev. 2000, 100, 39–91; W. A. Herrmann, Angew.
[18] J. C. Garrison, W. J. Youngs, Chem. Rev. 2005, 105, 3978–4008;
I. J. B. Lin, C. S. Vasam, Coord. Chem. Rev. 2007, 251, 642–
670.
Chem. Int. Ed. 2002, 41, 1290–1309; E. A. B. Kantchev, C. J. [19] K. J. Cavell, D. J. Stufkens, K. Vrieze, Inorg. Chim. Acta 1980,
O’Brien, M. G. Organ, Angew. Chem. Int. Ed. 2007, 46, 2768–
2813; F. E. Hahn, M. C. Jahnke, Angew. Chem. Int. Ed. 2008,
47, 3122–3172; H. Jacobsen, A. Correa, A. Poater, C. Costabile,
L. Cavallo, Coord. Chem. Rev. 2009, 253, 687–703.
47, 67–76.
[20] P. Hauwert, G. Maestri, J. W. Sprengers, M. Catellani, C. J. El-
sevier, Angew. Chem. Int. Ed. 2008, 47, 3223–3226.
[21] D. D. Perrin, L. F. Armagero, Purification of Laboratory Chem-
icals, Pergamon Press, Oxford, 1998.
[22] Bruker AXS, 2000, p. program package.
[23] D. T. Cromer, J. T. Waber, Int. Tables X-ray Crystallogr. 1974,
4, 71–147.
[2] E. Peris, R. H. Crabtree, Coord. Chem. Rev. 2004, 248, 2239–
2246; M. Poyatos, J. A. Mata, E. Peris, Chem. Rev. 2009, 109,
3677–3707.
[3] H. M. Lee, C.-C. Lee, P.-Y. Cheng, Curr. Org. Chem. 2007, 11,
1491–1524; A. T. Normand, K. J. Cavell, Eur. J. Inorg. Chem. [24] G. M. Sheldrick, Sect. A: Found. Crystallogr. 1990, 46, 467–
2008, 2008, 2781–2800.
473.
[4] P. Braunstein, F. Naud, Angew. Chem. Int. Ed. 2001, 40, 680–
699.
[5] S. Warsink, P. Hauwert, M. A. Siegler, A. L. Spek, C. J. Elsev-
ier, Appl. Organomet. Chem. 2009, 23, 225–228.
[6] S. Warsink, S. Y. d. Boer, L. M. Jongens, C.-F. Fu, S.-T. Liu,
J.-T. Chen, M. Lutz, A. L. Spek, C. J. Elsevier, Dalton Trans.
2009, 7080–7086.
[25] G. M. Sheldrick, in SHELXL-97, University of Göttingen,
Germany, 1997
[26] Reference added in proof (October 22, 2010): S. Warsink, I.-
H. Chang, J. J. Weigand, P. Hauwert, J.-T. Chen, C. J. Elsevier,
Organometallics 2010, 29, 4555–4561.
Received: July 14, 2010
Published Online: October 26, 2010
5562
www.eurjic.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2010, 5556–5562