A Palladium '(II) Complex Containing Imidazole Moieties. Synthesis and Structural Examination

Article abstract of DOI:10.3987/COM-07-S(U)44

trans-[PdCl₂(NCyIm)₂] was prepared through a route involving the corresponding silver precursor. 3-cyclohexyl-1-picolylimidazolium salt and Ag₂O give a trans-[Ag(NCylm)₂]⁺ species with the imidazole ring bound at the Nl atom, not at the C2 atom.

Full text of DOI:10.3987/COM-07-S(U)44

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HETEROCYCLES, Vol. 73, 2007, pp. 651 - 659. © The Japan Institute of Heterocyclic Chemistry
Received, 29th June, 2007, Accepted, 24th August, 2007, Published online, 28th August, 2007. COM-07-S(U)44
A PALLADIUM (II) COMPLEX CONTAINING IMIDAZOLE MOIETIES.
SYNTHESIS AND STRUCTURAL EXAMINATION
Alexandrina D. Tanase,a Eberhardt Herdtweck,a Wolfgang A. Herrmann,a,*
and Fritz E. Kühn b,*
a Department of Chemistry, Chair of Inorganic Chemistry, Technische Universität
München, Lichtenbergstraße 4, 85747, Garching, Germany (E-mail:
wolfgang.herrmann@ch.tum.de)
b Department of Chemistry, Molecular Catalysis, Technische Universität München,
Lichtenbergstraße 4, 85747, Garching, Germany, (E-mail: fritz.kuehn@ch.tum.de)
Abstract – trans-[PdCl2(NCyIm)2] was prepared through a route involving the
corresponding silver precursor. 3-cyclohexyl-1-picolylimidazolium salt and Ag2O
give a trans-[Ag(NCyIm)2]+ species with the imidazole ring bound at the N1
atom, not at the C2 atom.
Transition metal complexes bearing N-heterocyclic carbene (NHC) ligands have been known for many
years1 and have emerged as new types of ligands in homogeneous catalysis mostly during the past
decade.2,3 The combination of chelate and pincer effects of such carbenes has provided novel types of
catalysts.4 Significant innovations have been made in the synthesis and investigation of palladium (II)
complexes bearing chelating nitrogen ligands. A number of these complexes has been found to be highly
active for important organic reactions, e. g. the Heck coupling reaction or the ethylen/CO
copolymerization.5 Previous studies have been primarily devoted to cyclometallated palladium complexes
incorporating imine, amine, oxime, or oxazoline6 moieties. Several neutral and cationic palladium(II)
complexes have been reported, incorporating N-coordinated bis- and tris-imidazole chelates that were
found to be catalytically active in the Heck reaction of 4-bromoacetophenone with N-butyl acrylate.5 An
example of a mixed imidazolylidene-imidazole palladium complex and its application in the Sonogashira
coupling reaction has also been reported.7
Monomethyl and dimethyl Pd(II) complexes of bridged bisimidazole and mixed imidazole-N-heterocycle

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(py, pz) ligands have been prepared by Canty et al. and their dynamic behavior has been studied with
variable temperature NMR spectroscopy.8 Potentially hemilabile carbene ligands containing both strong
and weak donor groups have found widespread use in homogeneous catalysis. The hemilabile part in such
ligands is capable of reversible dissociation from the metal center. Such dynamic behavior is known to
produce vacant coordination sites that allow complexation of substrates during the catalytic cycle, at the
same time the strong donor moiety remains connected to the metal center.9
The synthesis and structural characterization of a Pd (II) complex 3 bearing imidazole moieties is reported
in the present work. The corresponding Pd (II) complexes of N-functionalized heterocyclic carbene ligands
5, 6 have been synthesized through a route involving carbene transfer from an Ag (I), carbene precursor.
RESULTS AND DISCUSSION
The N-functionalized imidazolium bromide salt 1 was prepared by reacting the corresponding halide with
1-cyclohexylimidazole according to Scheme 1. Attempts to prepare the free carbene by deprotonation of
the imidazolium salt failed due to the high acidity of the methylene protons in 1. The yellow solid
obtained was characterized by analytical and spectroscopic methods.
Scheme 1
X
N
MeOH
+
N
N
0 o
C
N
N
N
R
Br-
R
R= Cy; X = Br
1 (63 % yield)
While attempting to prepare a palladium (II) complex from the picolylimidazolium salt 1 by
transmetallation of the corresponding silver complex, an trans-[Ag(NCyIm)2]+ species was isolated, with
1
the imidazole rings unexpectedly bound directly to the N1 atom (Figure 1 and Scheme 2). The H NMR
data of the silver complex 2 show equivalent cyclohexyl and imidazole protons, but no signal
corresponding to the methylene linker group was detected. It was at first considered that the compound
obtained could be a result of the unreacted 1-cyclohexylimidazole present in the original ligand precursor
1, but this possibility was excluded by elemental analysis and mass spectrometry. The formation of 2 in
refluxing CH2Cl2 for 2 days, however, requires a C-N bond cleavage of the ligand precursor 1.
It was observed that under mild conditions in N-heterocyclic carbene precursors a cleavage of their C-N
bonds occurs, yielding the alkylimidazole fragments.7,10 The formation of the bisimidazole silver complex
2, is in contrast to previous studies on C-H bond activation in imidazolium systems, which have so far

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653
always, led to the usual C2 carbenes silver complexes.9,11 Occasionally, the Ag2O reagent can give
unexpected products in its reaction with an imidazolium salt, therefore quite some care needs to be taken
to verify if the desired reaction has indeed occurred.
Scheme 2. Synthesis of AgI and PdII Complexes a
Cl
-
+
X
Cy
N
Cy
N
N
Pd
Cl
i
N
iii
N
N
N
Ag
Cy
N
Cy
3 (82% yield)
2 (66% yield)
N
N
Cy
Br-
N
N
N
Cl
Cl
N
N
Pd
iii
1 (63% yield)
N
N
N
N
ii
Cy
N
N
Ag AgBr2
Cy
N
N
5 (69% yield)
iv
N
4 (60% yield)
N
N
Pd
Cl
Cl
Cy
6 (81% yield)
a Reagents: (i) Ag2O, CH2Cl2, reflux 40 oC, 2 days; (ii) Ag2O, CH2Cl2, rt, 2 h; (iii) PdCl2(CH3CN)2,
CH2Cl2, 1h; (iv) 2 PdCl2(COD), CH2Cl2, 40 min.
The silver complex 2 reacts smoothly with the Pd(II) precursor PdCl2(MeCN)2 to yield the corresponding
trans-[PdCl2(NCyIm)2] palladium complex 3. In a typical reaction the Ag complex was stirred with the
Pd precursor in CH2Cl2 solution for 1 h before being filtered to remove the precipitated AgBr. Subsequent
workup of the solution afforded the complex in 82 % yield. The crystal structure of 3 has been determined
by X-ray diffraction analysis. The ORTEP drawing of 3 is shown in (Figure 1).

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Figure 1. ORTEP style reprezentation of compound 3 in the solid state. Thermal ellipsoids are drawn at
the 50 % probability level. The molecule shows a crystallographic Ci symmetry and an operator for
generating equivalent atoms of (1-x, 1-y, 1-z).
The complex consists of monomeric Pd(NCyIm)2Cl2 units. Most of the crystal structures of
corresponding dihalide-bis(1-alkylimidazole)-palladium(II) indicate that in the solid state the trans isomer
crystallizes preferentially, where as the two imidazole ligands are coordinated trans to each other at the
palladium center. The imidazole ligands were found to coordinate to the metal through the electronic
‘pair’ of the N1. The Pd atom in 3 is coordinated in a square-planar fashion to the Cl and N atoms. The
Pd–Cl and Pd–N1 bond lengths of the palladium complex 3 are slightly longer [2.3101(4) and 2.0156(12)
Å] as compared to the previous published imidazole complex [2.307(1) and 2.011(2) Å].12 The dihedral
angle between the plane of imidazole ring and the square plane around the Pd is 27.57(8)o.
To avoid the formation of the bisimidazole silver complex, the silver carbene complex 4 was prepared by
interaction of the imidazolium salt 1 with Ag2O in CH2Cl2 at room temperature for 2 h. We have been
unable to prepare the desired carbene silver complex at higher temperatures in CH2Cl2 as reported by
Danopoulos et al.11 The identity of the silver carbene complex 4 was established by a combination of
analytical and spectroscopic techniques. The formation of the carbene complex was established by the
presence of a peak at δ(13C) = 180.9 ppm, which was assigned to the 2C-imidazol-2-ylidene (carbene)
carbon, and by the absence of the downfield peak for the 2H-imidazolium proton in the 1H NMR spectra
usually observed in non-protic solvents below δ(1H) = 9 ppm. The 1H NMR spectrum of 4 shows a singlet
at δ(1H) = 5.32 ppm corresponding to the CH2 linker group. A CH2Cl2 solution of the corresponding

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silver carbene precursor 4, was added drop wise to a solution of PdCl2(CH3CN)2 to yield the palladium
complex 5 containing two carbene ligands. Both carbene ligands are coordinated to the Pd center and the
donor groups remain dangling with the N atom uncoordinated to the metal center. The complex was
obtained as a pale yellow air-stable solid and characterized by spectroscopic and analytical methods. At
room temperature the 1H NMR signals sharpen and two sets of peaks, corresponding to the cis and trans
isomers are observed.
The silver complex 4 was stirred with the Pd precursor Pd(COD)Cl2 in CH2Cl2 solution for 1 h to yield
1
the palladium complex 6, containing the donor-functionalized carbene ligand 1. The H NMR spectrum
reported here shows very broad, merged signals at room temperature. Fluxional behavior was observed at
elevated temperatures. At ambient temperature a broad singlet peak is observed for the diastereotopic
1
methylene protons in the H NMR spectrum, resulting from dynamic conformational flipping of the
six-membered chelate ring. When cooling the solution to -30 °C, two sharp doublets at δ(1H) = 5.88 and
5.03 ppm are observed with geminal coupling constants of 14.68 Hz. No additional splitting or
broadening of other signals in the spectrum is observed at this lower temperature. The thermal stabilities
of the Ag(I) and Pd(II) complexes of the bidentate type ligands 4 and 5, 6 have been investigated using
similar experimental conditions as for the bisimidazolium silver complex 2. The observed spectroscopic
signals correlate well with the previous ones. A simple explanation for these observations is that the
presence of chelating ligands either as a spectator ligand in 4, 5 or as a chelate incorporating the carbene
moiety in 6, plays an important role for the stability of the complexes already formed.
EXPERIMENTAL
All reactions were performed under a dry argon atmosphere using standard Schlenk technique. Solvents
and substrates were dried by standard procedures, distilled and kept under argon over molecular sieves.
14
The precursors 1-cyclohexylimidazole13 and Pd(MeCN)2Cl2 were prepared according to the reported
1
literature procedures. H and 13C{1H} NMR spectra were recorded on a JEOL-JMX-GX 400 MHz
1
spectrometer at room temperature. H NMR peaks are labeled as singlet (s), doublet (d) , triplet (t),
multiplet (m) or broad signals (br). Infrared spectra were recorded on a Perkin-Elmer FT-IR spectrometer
using KBr pellets as IR matrix. Mass spectrometric measurements were done on a Finnigan MAT 90
spectrometer using the CI or FAB technique. Elemental analyses were performed in the Microanalytical
Laboratory at the TU München.
3-Cyclohexyl-1-picolylimidazolium bromide (1): 2-(Bromomethyl)pyridine hydrobromide (1 g, 4
mmol) was neutralized using a saturated aqueous Na2CO3. The liberated 2-(bromomethyl)pyridine was
extracted into Et2O (3 x 10 mL) at 0 °C, dried with Mg2SO4, and filtered. 1-Cyclohexylimidazole (0.60 g,
4 mmol) in MeOH was added at 0 °C to the filtrate, the Et2O removed under reduced pressure, and the

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solution stirred for 12 h at rt. The methanol was evaporated under reduced pressure, and the formed oil
was purified by repetitive precipitation from CH2Cl2/Et2O mixtures. Evaporation of the volatiles under
reduced pressure left compound 1 as an yellow solid. The product was recrystallized from CH2Cl2/ Et2O,
collected by filtration and dried in vacuo. Yield: 0.68 g (63 %). Anal. Calcd for C15H21N3Br·3/2CH2Cl2
(449.15): C, 44.08; H, 5.34; N, 9.35. Found: C, 44.34; H, 5.10; N, 9.69. MS (FAB): m/z 242.3, [M-Br]+
(100 %); 1H NMR (CDCl3, 298 K): δ 10.72 (s, 1H, NCHN), 8.50 (d, 1H, 3JHH = 4.4 Hz, Hpy), 7.89 (d, 1H,
3JHH = 8.0 Hz, Hpy), 7.71 (t, 1H, Hpy), 7.62 (s, 2H, Him), 7.27-7.25 (m, 1H, Hpy), 5.82 (s, 2H, CH2), 4.27
(m, 1H, CHCy), 2.22 - 1.39 (m, 10H, CH2,Cy). 13C{1H} NMR (CDCl3, 298 K): δ 152.87 (Cpy), 149.88 (Cpy),
137.59 (Cpy), 136.06 (NCN), 135.22 (Cpy), 123.73 (Cpy), 122.75 (Cim), 120.69 (Cim), 60.23 (CH2), 56.84
(CHCy), 33.93 (CH2,Cy), 25.18 (CH2,Cy).
trans-[Ag(1-cyclohexylimidazole)2]Br (2): A clear solution of 1 (1.13 g, 3.5 mmol) in CH2Cl2 (30 ml)
and Ag2O (0.4 g, 1.75 mmol) was heated to 40 oC for 2 days. The solution was filtered through Celite and
the solvent removed under reduced pressure. The yellow powder obtained was washed with Et2O and
dried in vacuo. The product was recrystallized from CH2Cl2/ Et2O, collected by filtration and dried in
vacuo. Yield: 0.565 g (66 %). Anal. Calcd for C18H28N4BrAg (488.21): C, 44.28; H, 5.78; N, 11.48.
Found: C, 44.21; H, 6.16; N, 11.28. IR (KBr): 2930, 2859 (νC-H aromatic); 1500 (νC=N), 1444, 1271, 1233,
1107, 1081, 988 (νC-H aliphatic) cm-1. MS (FAB): m/z 409.1, [M-Br]+ (100 %), 257.0, [M-Br-CyIm]+ (90 %).
1H NMR (CDCl3, 298 K): 8.15 (s, 2x1H, NCHN), 7.04 (s, 2H, Him), 6.95 (s, 2H, Him), 4.02 (m, 1H,
13
CHCy), 2.12 - 1.24 (m, 10H, CH2,Cy). C{1H} NMR (CDCl3, 298 K): δ 155.29 (NCN), 135.26 (Cim),
117.13 (Cim), 57.05 (CHCy), 34.45 (CH2,Cy), 25.35 (CH2,Cy).
trans-[Pd(1-cyclohexylimidazole)2]Cl2 (3): A mixture of 2 (0.5 g, 1.0 mmol) and Pd(MeCN)2Cl2 (0.13 g,
0.5 mmol) was dissolved in CH2Cl2 (5 mL) and stirred for 1 h. The solution was filtered through Celite,
and the solvent was removed in vacuo, to give a white powder. The product was recrystallized from
CH2Cl2/ n-pentane (2 mL/10 mL), collected by filtration and dried in vacuo. Yield: 0.401 g (82 %). Anal.
Calcd for C18H28N4Cl2Pd (477.77): C, 45.25; H, 5.91; N, 11.73. Found: C, 45.09; H, 5.58; N, 11.86. IR
1
(KBr): 2930, 2859 (νC-H aromatic); 1514 (νC=N), 1453, 1351, 1262, 1121, 1050, 997 (νC-H aliphatic) cm-1. H
NMR (CDCl3, 298 K): δ 8.21 (s, 1H, NCHN), 7.35 (s, H,im), 7.26 (s, H,im), 4.0 (m, 1H, CHCy), 2.14 - 1.23
13
(m, 10H, CH2,Cy). C{1H} NMR (CDCl3, 298 K): δ 138.54 (NCN), 128.94 (Cim), 117.77 (Cim), 57.83
(CHCy), 33.77 (CH2,Cy), 33.57 (CH2b,Cy), 25.35 (CH2,Cy).
[Ag(3-cyclohexyl-1-picolylimidazolin-2-ylidene)2] [AgBr2] (4): A mixture of 1 (0.5 g, 1.5 mmol) and
Ag2O (0.18 g, 0.7 mmol) was dissolved in CH2Cl2 (30 ml) and the mixture stirred for 2 h at rt. The
solution was filtered through Celite and the solvent removed in vacuo to give an yellow solid. The
product was recrystallized from CH2Cl2/ Et2O, collected by filtration and washed with Et2O (10 ml). After
drying in vacuo a pale-yellow powder was obtained. Yield: 0.5 g, (60 %). Anal. Calcd for

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C31H43N6Br2Ag2 (871.00): C, 42.54; H, 4.95; N, 9.60. Found: C, 42.46; H, 5.01; N, 9.64. MS (FAB): m/z
606.2, [M-AgBr2]+ (10%), 242.3, [ylidene]+ (100 %). 1H NMR (CDCl3, 298 K): 8.55 (dd, 1H, Hpy,6), 7.69
(dt, 1H, Hpy,4), 7.28 (d, 1H, Hpy,3,5), 7.23 and 7.14 (d, 2x1H, Him), 5.32 (s, 2H, CH2), 4.26 (m, 1H, CHCy),
2.03 - 1.23 (m, 10H, CH2,Cy). 13C{1H} NMR (CDCl3, 298 K): δ 180.9 (NCN), 155.51 (Cpy), 149.52 (Cpy),
137.21 (Cpy), 123.25 (Cpy), 122.64 (Cim), 121.60 (Cim), 118.22 (Cpy), 61.72 (CH2), 56.68 (CHCy), 34.65
(CH2,Cy), 25.41 (CH2,Cy).
[Pd(3-cyclohexyl- 1-picolylimidazolin-2-ylidene)2]Cl2 (5): A mixture of Pd(MeCN)2Cl2 (21.9 mg, 0.77
mmol) and 4 (0.37 g, 0.42 mmol) was dissolved in CH2Cl2 (50 mL). The mixture was stirred for 1 h, and
then filtered through Celite to remove precipitated silver bromide. The solvent was removed in vacuo
until ca. 5 mL remained, and n-hexane (10 mL) was added to it. The product was recrystallized from
CH2Cl2/ Et2O (5 mL/10 mL) followed by two washings with n-hexane (10 mL) and dried in vacuo to
+
afford an yellow powder. Yield: 0.193 g (69 %). MS (FAB): m/z 593.2, [M - 2Cl] (20%); 441.3, [M -
2Cl - CyIm] + (40 %); 242.3, [ylidene] + (100 %). 1H NMR (DMSO-d6, two isomers in a 1:2 ratio (a:b)):
δ 8.52 (m, 2H, Hpy,6b), 8.34 (m, 2H, Hpy,6a), 8.0-7.7 (m, 2H, Hpy,4a+b), 7.5-7.1 (m, 8H, Hpy,3a+b, Hpy,5a+b), 6.6
(HCCHa+b), 5.72 (s, 2H, NCH2a), 5.51 (s, 2H, NCH2b), 4.13 (m, 2H, NCHCy,a+b), 1.92-1.15 (m, 20H,
13
NCH2Cy,a+b). C{1H} NMR (DMSO-d6): δ 180.2 (NCN), 161.9, 148.5, 137.9 (Cpy), 129.1, 122.7, 120.9,
117.8 (Cpy, NCCN), 57.5 (NCH2), 33.6(CHCy), 25.3 (CH2,Cy).
[Pd(3-cyclohexyl- 1-picolylimidazolin-2-ylidene)]Cl2 (6): A CH2Cl2 solution of the corresponding silver
carbene complex 4 (0.3 g, 0.34 mmol) was added dropwise to a solution of Pd(COD)Cl2 (0.19 g, 0.66
mmol) and stirred at rt for 40 min, than it was filtered through Celite to remove precipitated silver
bromide. The remaining solid residue was washed with Et2O and dried in vacuo. The product was
recrystallized from CH2Cl2/ Et2O (5 mL/10 mL) followed by washing twice with n-hexane 10 mL and
dried in vacuo to afford an pale-yellow powder. Yield: 0.105 g (81 %). Anal. Calcd for C15H20Cl2N3Pd ·
1
1/2 CH2Cl2 (461.63): C, 40.29; H, 4.54; N, 9.09. Found: C, 40.20; H, 4.67; N, 9.05. H NMR (CD2Cl2,
-30 °C): δ 9.26 (br d, 1Hpy), 7.88 (dt, 1Hpy), 7.48 (d, 2Hpy), 7.36 (m, 1Hpy), 7.12 (d, 1H, HCCH), 6.88 (d,
1H, HCCH), 5.88 (d, 1H, NCHH), 5.03 (d, 1H, NCHH), 3.92 (m, 1H, CHCy), 2.54 - 1.12 (m, 10H,
13
CH2,Cy). C{1H} NMR (CD2Cl2): δ 182.8 (NCN), 156.7, 155.6, 133.8 (Cpy), 124.8, 123.7, 121.9, 112.2
(Cpy, NCCN), 56.7 (NCH2), 34.2(CHCy), 31.3(CH2,Cy), 25.5 (CH2,Cy).
Single crystal X-ray structure determination of compound 3. C18H28Cl2N4Pd, Mr = 477.76, yellow
fragment (0.56 × 0.58 × 0.63 mm3), monoclinic, P21/n (No.: 14), a = 7.5022(2), b = 9.8085(2), c =
13.6610(3) Å, β = 97.114(2)°, V = 997.51(4) Å3, Z = 2, dcalc = 1.591 gcm-3, F000 = 488, µ = 1.207 mm-1.
Preliminary examination and data collection were carried out on an area diffraction system (Xcalibur™3,
κ-CCD, OXFORD DIFFRACTIONS) and graphite monochromated MoKα radiation (λ = 0.71073 Å). Data
collections were performed at 153 K. Raw data were corrected for Lorentz, polarization, and, arising from

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the scaling procedure, for latent decay and absorption effects. The structure was solved by a combination
of direct methods and difference Fourier syntheses. All non-hydrogen atoms were refined with
anisotropic displacement parameters. All hydrogen atoms were found and refined with individual
isotropic displacement parameters. Full-matrix least-squares refinements were carried out by minimizing
2
Σw(Fo -Fc2)2 with the SHELXL-97 weighting scheme and converged with R1 = 0.0184 [2200 intensities,
Io>2σ(Io)], wR2 = 0.0443 [2362 intensities, all data], GOF = 1.105, and shift/error < 0.001. The final
difference-Fourier map shows no striking features (∆emin/max = +0.59/-0.41 eÅ-3). Small extinction effects
were corrected with the SHELXL-97 procedure [ε = 0.0275(9)].15
ACKNOWLEDGEMENTS
We are grateful to Dr. Karl Öfele for suggestions and fruitful discussions. This work was generously
supported by NanoCat, an International Graduate Program within the Elitenetzwerk Bayern (doctoral
fellowship for A.D.T.). Sincere thanks are given to Stephan D. Hoffmann for collecting the X-ray data
(E.H.). We also thank Degussa for a generous gift of palladium (II) chloride.
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