Fluoroalkylated N-heterocyclic carbene complexes of palladium

Article abstract of DOI:10.1016/S0022-328X(00)00008-5

Fluoroalkylated N-heterocyclic carbene complexes of palladium have been synthesized from imidazolium salts and Pd(OAc)₂. The analogous carbene complexes bearing long alkyl chains have also been prepared. The formation of these carbene complexes proceeds via an intermediate binuclear species, which has been isolated. Complexes such as these may find applications in catalysis in supercritical CO₂ (scCO₂).

Full text of DOI:10.1016/S0022-328X(00)00008-5

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 598 (2000) 409–416
Fluoroalkylated N-heterocyclic carbene complexes of palladium
Lijin Xu, Weiping Chen, Jamie F. Bickley, Alexander Steiner, Jianliang Xiao *
Le6erhulme Centre for Inno6ati6e Catalysis, Department of Chemistry, Uni6ersity of Li6erpool, Li6erpool L69 7ZD, UK
Received 7 October 1999; accepted 24 November 1999
Abstract
Fluoroalkylated N-heterocyclic carbene complexes of palladium have been synthesized from imidazolium salts and Pd(OAc)2.
The analogous carbene complexes bearing long alkyl chains have also been prepared. The formation of these carbene complexes
proceeds via an intermediate binuclear species, which has been isolated. Complexes such as these may find applications in catalysis
in supercritical CO2 (scCO2). © 2000 Elsevier Science S.A. All rights reserved.
Keywords: Carbenes; Fluoroalkylated carbenes; Palladium complexes; X-ray structures
major limitations of scCO2 as a reaction medium for
homogeneous catalysis is its rather low solvent strength
towards catalysts derived from common organometallic
complexes [13–15]. The solubility of a metal complex in
scCO2 can be increased by modification of ligands such
that they become ‘CO2-philic’. This may be achieved by
attaching fluoroalkyl, alkyl or silicone groups to the
ligands, which are known to be highly soluble in scCO2.
A few types of such ligands have been prepared and
employed in catalytic reactions in scCO2. Examples
include fluorinated phosphines, porphyrins, diketo-
nates, alkyl phosphines and phosphites. Fluorinated
ligands usually show significantly higher solubility in
comparison with other funtionalized ligands in scCO2
[13–15]. However, most of the fluorinated ligands that
have been reported so far, such as phosphines bearing
fluorinated ponytails [15–20], are not easily accessible
and, in the particular case of the phosphines, PꢀC bond
cleavage may occur during a reaction. It is therefore of
interest to design and synthesize new CO2-philic ligands
for the development of catalytic processes in scCO2. We
report herein on the synthesis and characterization of
fluoroalkylated and alkylated imidazol-2-ylidene com-
plexes of palladium. Palladium carbene complexes con-
taining long alkyl chains have recently been reported as
thermally stable liquid crystals while this work was in
progress [21]. Carbene complexes with other interesting
functionalties are also known, but none contain fluori-
nated substituents [1,22–25].
1. Introduction
N-Heterocyclic carbenes are a novel class of ligands
for homogeneous catalysis [1–11]. The metal complexes
of these carbenes show high thermal stability and hy-
drolytic durability because of the strong MꢀC bonds,
which also obviate the need for excess ligands in cataly-
sis. Additional advantages of these ligands over the
classic phosphines include easy preparation, easy
derivatization and low cost. The 1,3-disubstituted imi-
dazol-2-ylidene complexes of the late transition metals
have been used in a number of interesting catalytic
reactions such as the Heck and Suzuki reactions [1–
3,11], olefin metathesis [4–6], hydrogenation and hy-
drosilylation [1,10]. A significant example is seen in
some palladium ylidene complexes, which display re-
markable activity and lifetime in catalysing the Heck
olefination of aryl bromides and even the chlorides
[1,2,11]. Prompted by the novel features of these car-
bene ligands, we decided to prepare the supercritical
CO2 (scCO2) soluble derivatives in the hope that these
carbene ligands can be adapted for catalysis in scCO2.
scCO2 is an attractive alternative to conventional
organic liquids for clean synthesis [12]. One of the
* Corresponding author. Tel.: +44-151-7942979; fax: +44-151-
7943589.
E-mail address: j.xiao@liv.ac.uk (J. Xiao)
0022-328X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(00)00008-5

410
L. Xu et al. / Journal of Organometallic Chemistry 598 (2000) 409–416
2.2. Synthesis of the palladium complexes
N-Heterocyclic carbene metal complexes can be pre-
pared by ligand substitution reactions using free carbe-
nes, which are accessible via deprotonation of the
corresponding imidazolium salts in THF or liquid am-
monia [30,31]. An easier, well-established alternative is
the direct reaction of the imidazolium salts with metal
compounds containing basic ligands [11,25,32,33]. Us-
ing this latter approach, the N,N%-disubstituted imida-
zol-2-ylidene complexes of palladium 5–8 have been
prepared. A typical synthesis consisted of heating
Pd(OAc)2 with two equivalents of an imidazolium salt
in THF for 2 h; the ylidene complexes were isolated as
crystalline solids in good yields (Scheme 2). Like many
other ylidene complexes, 5–8 are air- and water-stable
in solution or as solids [1]. The solubility of these
compounds in common organic solvents decreases in
the order 6\8\7\5, with 5 being only scarcely
soluble in solvents like THF, acetone or DMSO. The
solubility of 5 and 7 in these solvents increases with
increasing temperature.
Scheme 1. Preparation of imidazolium salts.
2. Results and discussion
2.1. Synthesis of the imidazolium salts
N-Heterocyclic carbenes are easily accessible from
imidazolium salts, which can in turn be prepared by
alkylation of imidazoles [26]. The N,N%-difluoroalkyl-
substituted imidazolium salt 1 was prepared by alkyla-
tion of imidazole with ICH2CH2C6F13 followed by
quaternization of the resultant fluoroalkylimidazole
with a second equivalent of the iodide (Scheme 1). The
product was obtained in moderate overall yield (50%)
on the basis of the iodide, the more expensive of the
two reagents. No additional base is necessary in the
first step of the reaction, as excess imidazole was used.
The N,N%-dioctylimidazolium salt 2 was prepared in a
similar manner. For the two asymmetrically substituted
salts 3 and 4, the synthesis simply involved one-step
alkylation of methylimidazole. The yields of the two
alkylated salts 2 and 4 are significantly higher than
those for the fluoroalkylated analogues 1 and 3. This
may be due to transformation under the reaction condi-
tions of the fluoroalkyl iodide into other species such as
alkenes. Attempts to increase the yields of the reaction
by using acetyl-protected imidazole or phase-transfer
catalysis met with only slight improvement [27]. Our
interest in salts 2 and 4 arises from the fact that normal
alkyl groups enhance the solubility of a ligand in scCO2
as do perfluoroalkyl substituents, albeit to a lesser
degree [14]. However, the former is far less expensive.
The intervening methylene spacers in 1 and 3 serve to
isolate the effect of the electron-withdrawing pe-
rfluoroalkyl groups from the ring nitrogen atoms. The-
oretical studies indicate that electron donation from the
nitrogen to the carbene carbon plays an important role
in stabilizing free carbenes and their metal complexes
[9,28,29].
The characterization of these complexes has been
accomplished by NMR, MS and microanalysis. The
structures of 5 and 7 have been determined by X-ray
diffraction (vide infra). The NMR spectra of 5 and 6
are straightforward to interpret and are consistent with
the presence of only one isomer in solution. For the
two asymmetrically substituted carbene complexes 7
and 8, NMR spectra suggest the presence of equilibrat-
ing rotation isomers, described as trans–anti and trans–
syn on the basis of the relative orientations of the
N-substituents. Thus, in the case of 7 dissolved in
CDCl3, the N-CH3 resonance appears initially as one
1
singlet at l 3.89 in the H-NMR spectrum. But with
time a new peak at l 3.97 develops at the expense of
the one at l 3.89, and the two peaks arrive at a 1:1
ratio after the CDCl3 solution stands overnight. Based
on the structure of 7 revealed by X-ray analysis, the
resonance at l 3.89 can be assigned to the N-CH3
groups of the trans–anti isomer, while the one at the
Scheme 2. Preparation of imidazol-2-ylidene complexes of palladium.

L. Xu et al. / Journal of Organometallic Chemistry 598 (2000) 409–416
411
n
lower field may be attributed to the trans–syn isomer.
Similar observations were made with complex 8. Rota-
tion isomers have been observed with other biscarbene
complexes of palladium, where the carbene ligands are
asymmetric and are arranged either cis or trans around
the palladium [25,34]. The trans arrangement of the
carbenes in 5–8 reflects the sterically demanding nature
of the long N-substituents. In related palladium com-
plexes with sterically less demanding N-substituents,
both cis and trans arrangements of the carbene ligands
have been reported. However, with bulkier N-sub-
stituents, the formation of the trans isomers appears to
be favoured [1,21,22,25]. The electron-withdrawing ef-
fect of the perfluoroalkyl fragments in 5 and 7 is not
completely eliminated by the methylene spacers, as can
be judged by the chemical shifts of the carbene olefinic
protons, which appear at lower fields than those of 6
olefination of 4-bromobenzaldehyde by butyl acrylate.
In DMF, the four complexes were all active, affording
the trans-cinnamate in \99% yield with 0.5 mol%
ylidene complex at 125°C for 12 h reaction time
(NaOAc as base). However, in scCO2, under otherwise
comparable conditions (200 atm CO2, NEt3 as base),
these complexes appear to be less effective, with 5
affording the cinnmate in only 6%, 6 in 10%, 7 in 7%
and 8 in 90% yield. While the lower activity of these
complexes in scCO2 may, to some degree, be related to
the much lower polarity of this medium, the reason for
the lower activity of 5 and 7 in comparison with their
non-fluorinated counterparts is not immediately clear.
Solubility does not appears to be the problem, because
a homogeneous yellow solution was formed in all the
reactions in scCO2. Olefination of iodobenzene
catalysed by palladium phosphine complexes in scCO2
has recently been reported, where again the reaction
tends to be less efficient than in dipolar solvents like
DMF [35–37].
1
and 8 in the H-NMR spectra.
The formation of the complexes 5–8 is characterized
by a colour change from dark-brown to red and finally
to orange, indicating that the reactions may proceed via
similar intermediates. Indeed, when the reaction of
Pd(OAc)2 with the salt 4 was stopped after 5 min, the
red binuclear carbene complex 9 was isolated in 90%
2.3. Structures of complexes 5 and 7
Crystal structure determinations of 5 (Fig. 1) and 7
(Fig. 2) show that both complexes have the expected
square-planar core geometry, with the palladium atoms
located at crystallographic centers of inversion. In both
complexes the carbene ligands are trans arranged and,
in 7, they are trans–anti configured. The mean planes
of the C3N2 rings of the ylidene ligands are tilted by
86.6° in 5 (80.5° in 7) with respect to the square-planar
environment of Pd1, thus minimizing steric interactions
between the N-substituents and iodides [21]. The PdꢀC
bond lengths of 2.040(8)° in 5 and 2.041(10)° in 7,
respectively, are comparable with values found in other
palladium carbene complexes (Table 1) [1,11,21,22,
24,25]. Within the solid state the fluorinated alkyl
chains of neighbouring molecules are aligned in a paral-
lel fashion. C6F13 chains in 5 are stacked intra- as well
as intermolecularly in a ‘head-to-head’ pattern resulting
in molecular stacks (Fig. 3), whereas the monosubsti-
1
yield (Scheme 3). The H- and 13C-NMR spectra of 9
resemble those of 8, except that the carbene carbon in
the former comes into resonance at a significantly
higher field (l 153 versus 168). The binuclear precursors
to 5–7 may be short lived, and attempted isolation has
not been successful. Heating 9 in the presence of 4 and
NaOAc in THF converts 9 into 8, as is expected. An
analogous intermediate binuclear species has also been
isolated in the preparation of a bisbenzimidalzol-2-yli-
dene complex of mercury [22]. Previously, closely re-
lated binuclear carbene complexes of palladium were
prepared by reacting Pd(OAc)2 with imidazolium salts
in the presence of NaI and KOtBu [25]. Clearly, Scheme
3 provides an easier route to such binuclear complexes.
As related palladium carbene complexes were shown
to be highly effective in Heck reactions [1,2], we exam-
ined in preliminary experiments complexes 5–8 for the
Scheme 3. Formation of 8 via binuclear complex 9 (R=C8H17, R%=Me).

412
L. Xu et al. / Journal of Organometallic Chemistry 598 (2000) 409–416
Fig. 1. Molecular structure of 5 in the solid state showing 50% probability thermal ellipsoids.
Fig. 2. Molecular structure of 7 in the solid state showing 50% probability thermal ellipsoids.
tuted ligands in 7 show an intermolecular ‘head-to-tail’
pattern, which results in a zigzag arrangement (Fig. 4).
In summary, fluoroalkylated and alkylated imida-
zolium salts and the corresponding carbene complexes
have been prepared. In contrast to fluorophosphines,
fluorinated N-heterocyclic carbenes and their metal
complexes are easy to prepare and require no special
handling precautions, and thus provide an easy entry to
soluble catalysts for reactions in scCO2. While the
activity of the N-substituted imidazol-2-ylidene com-
plexes of palladium appears to be low in the Heck
reaction in scCO2, complexes like these may find appli-
cations in other reactions in this medium or in fluorous
phase catalysis.
spectra were recorded on a VG7070E mass spectro-
meter.
X-ray data were collected on a Stoe IPDS image
plate diffractometer using MoꢀKa radiation (u=
,
0.71073 A) and a graphite monochromator. Crystals of
5 and 7 were obtained from crystallization in acetone
and CH2Cl2–hexane, respectively. The crystals were
Table 1
,
Selected bond lengths (A) and angles (°) for 5 and 7
5
7
Bond lengths
Pd(1)ꢀC(1)
Pd(1)ꢀI(1)
C(1)ꢀN(1)
C(1)ꢀN(2)
N(1)ꢀC(2)
N(2)ꢀC(3)
C(2)ꢀC(3)
2.040(8)
2.041(10)
2.5969(8)
1.332(12)
1.345(12)
1.383(13)
1.368(13)
1.335(15)
2.5982(8)
1.339(11)
1.346(11)
1.372(12)
1.382(12)
1.330(15)
3. Experimental
The solvents were degassed and dried before use. All
the reagents were used as received. NMR spectra were
recorded on a Gemini 300 spectrometer at 300.10 (1H)
and 75.46 MHz (13C) in ppm with reference to TMS
internal standard in CDCl3, unless otherwise indicated.
Abbreviations for NMR spectral multiplicities: s=sin-
glet, t=triplet, m=multiplet, br=broad. Elemental
analyses were performed by the Microanalysis Labora-
tory, Department of Chemistry, at the University of
Liverpool, and Butterworth Laboratories Ltd. Mass
Bond angles
C(1)ꢀPd(1)ꢀI(1)
C(1)ꢀPd(1)ꢀC(1a)
I(1)ꢀPd(1)ꢀI(1a)
N(1)ꢀC(1)ꢀN(2)
C(1)ꢀN(1)ꢀC(2)
C(1)ꢀN(2)ꢀC(3)
N(1)ꢀC(2)ꢀC(3)
N(2)ꢀC(3)ꢀC(2)
91.4(2)
180
180
105.1(7)
110.5(8)
110.4(8)
107.4(9)
106.4(9)
88.9(2)
180
180
105.4(9)
110.5(9)
110.4(9)
106.5(9)
107.1(9)

L. Xu et al. / Journal of Organometallic Chemistry 598 (2000) 409–416
413
Fig. 3. Packing arrangement of 5 in the solid state viewing along the [110] vector.
Fig. 4. Packing arrangement of 5 in the solid state viewing along the [010] vector.
coated with Nujol, mounted on glass fibres and held at
200 K. The structures were solved by direct methods
and refined by full-matrix least-squares on F2 using all
data [38]. Fluoroalkyl chains of both structures show
conformational disorder. Disordered atoms were split
in two positions and refined isotropically using distance

414
L. Xu et al. / Journal of Organometallic Chemistry 598 (2000) 409–416
Table 2
and similar U restraints. All other non-hydrogen atoms
were refined anisotropically. Hydrogen atoms were
included in the calculated positions. Crystallographic
details for both structures are shown in Table 2.
Crystal data and structure refinement for 5 and 7
5
7
Empirical formula
Formula weight
Crystal system
Space group
Unit cell dimensions
C38H20F52I2N4Pd C24H18F26I2N4Pd
1880.82
Triclinic
1216.66
Monoclinic
P21/c
3.1. 1,3-Bis(1H,1H,2H,2H-perfluorooctyl)imidazolium
iodide (1)
(
P1
Imidazole (1.702 g, 0.025 mol) and 1H,1H,2H,2H-per-
fluorooctyl iodide (4.740 g, 0.010 mol) were dissolved in
30 ml of ethyl acetate. The mixture was refluxed
overnight under argon. After cooling to room tempera-
ture, the imidazolium salt formed in the reaction and
unreacted imidazole were extracted with water, and the
solvent removed in vacuo. To the residue and
1H,1H,2H,2H-perfluorooctyl iodide (4.740 g, 0.010
mol) was then added 30 ml of toluene. The mixture was
refluxed overnight under argon, during which time the
product precipitated out as a white solid. The precipi-
tate was separated, dried in vacuo, and finally crystal-
lized from CH3CN–toluene affording 1 as a white
crystalline solid (4.480 g, 50%, based on the iodide).
1H-NMR (DMSO-d6): l 9.44 (s, 1H, N2CH), 7.94 (s,
,
a (A)
7.3660(11)
8.9609(14)
21.651(4)
98.74(2)
91.06(2)
96.06(2)
1403.8(4)
1
2.225
1.492
896
200(2)
0.71073
8899
4141
0.035
16.333(3)
8.994(2)
14.160(3)
90
113.68(3)
90
1905.0(7)
2
2.121
2.078
1152
200(2)
0.71073
5313
2846
0.053
5.3–48.1
88.3
0.057
0.145
1.565 and
−1.315
,
b (A)
,
c (A)
h (°)
i (°)
k (°)
V (A )
3
,
Z
Dcalc (g cm−3
)
v (MoꢀKa) (mm−1
F(000)
)
T (K)
,
u (A)
Reflections collected
Reflections independent
Rint
2q Range
3.8–48.3
91.8
0.065
0.189
1.432 and
−1.619
Completeness (%)
R1 (I\2|(I))
wR2 (all data)
Largest difference peak
and hole (e A
3
2H, NCH), 4.61 (t, JHH=5 Hz, 4H, NCH2), 2.99 ( br
t, 3JHF=19 Hz, 4H, CH2CF2). 13C{1H}-NMR
(DMSO): l 137.7 (N2CH), 123.1 (NCH), 41.8 (NCH2),
30.2 (NCH2CH2). MS (FAB): m/z 761 ([M+−I], 100).
Anal. Calc. for C19H11N2F26I (888.17): C, 25.69; H,
1.25; N, 3.15. Found: C, 26.05; H, 1.48; N, 3.35%.
−3
,
)
3.2. 1,3-Dioctylimidazolium iodide (2)
1-methylimidazole). 1H-NMR: 10.16 (s, 1H, N2CH),
7.62 (t, JHH=1.5 Hz, 1H, NCH), 7.47 (t, JHH=1.5 Hz,
3
The procedure was similar to that for 1. Starting
from imidazole (1.360 g, 0.020 mol) and octyl bromide
(1.550 g, 0.008 mol) and following quaternization with
octyl iodide (2.400 g, 0.010 mol), 2 was obtained as a
1H, NCH), 4.86 (t, JHH=7 Hz, 2H, NCH2), 4.09 (s,
3H, NCH3), 2.99 (m, 2H, CH2CF2). 13C{1H}-NMR: l
138.0 (N2CH), 123.6 (NCH), 122.9 (NCH), 42.7
(NCH2), 37.3 (NCH3), 32.0 (NCH2CH2). MS (FAB):
m/z 429 ([M+−I], 100). Anal. Calc. for
C12H10N2F13I.H2O (574.12): C, 25.10; H, 2.11; N, 4.88.
Found: C, 25.19; H, 1.81; N, 4.87%.
1
yellow oil (3.030 g, 90%, based on the bromide). H-
4
NMR: l 10.28 (s, 1H, N2CH), 7.62 (d, JHH=1.5 Hz,
3
2H, NCH), 4.39 (t, JHH=7 Hz, 4H, NCH2), 1.94 (m,
4H, NCH2CH2), 1.35–1.22 (m, 20H, CH2), 0.83 (t,
3JHH=7 Hz, 6H, CH3). 13C{1H}-NMR: l 135.8
(N2CH), 122.2 (NCH), 49.8 (NCH2), 31–22 (CH2), 13.6
(CH3). MS (FAB): m/z 293 ([M+ −I], 100). Anal.
Calc. for C19H37N2I (420.42): C, 54.28; H, 8.87; N,
6.66. Found: C, 53.78; H, 8.95; N, 6.53%.
3.4. 1-Methyl-3-octylimidazolium iodide (4)
The procedure was similar to that for 3. Starting
from 1-methylimidazole (1.650 g, 0.020 mol) and 1-
iodooctane (5.310 g, 0.022 mol), 4 was obtained as a
yellow oil (5.830 g, 90%, based on 1-methylimidazole).
1H-NMR: l 9.96 (s, 1H, N2CH), 7.74 (t, JHH=1.8 Hz,
1H, NCH), 7.63 (t, JHH=1.8 Hz, 1H, NCH), 4.36 (t,
3JHH=7 Hz, 2H, NCH2), 4.15 (s, 3H, NCH3), 1.94 (m,
2H, NCH2CH2), 1.35–1.22 (m, 10H, CH2), 0.83 (t,
3JHH=7 Hz, 3H, CH3). 13C{1H}-NMR: l 136.5
(N2CH), 123.9 (NCH), 122.3 (NCH), 50.1 (NCH2), 37.1
(NCH3), 31–22 (CH2), 13.9 (CH3). MS (FAB): m/z 195
([M+−I], 100). Anal. Calc. for C12H23N2I (322.23): C,
44.73; H, 7.19; N, 8.69. Found: C, 44.64; H, 7.35; N
8.57%.
3.3. 1-Methyl-3-(1H,1H,2H,2H-perfluorooctyl)-
imidazolium iodide (3)
1-Methylimidazole (1.640 g, 0.020 mol) and
1H,1H,2H,2H-perfluorooctyl iodide (10.428 g, 0.022
mol) were dissolved in 30 ml of toluene. The mixture
was refluxed overnight, during which time an oily pre-
cipitate formed. The solvent was removed in vacuo, and
the precipitate crystallized from CH3CN–toluene af-
fording 3 as white crystals (7.080 g, 67%, based on

L. Xu et al. / Journal of Organometallic Chemistry 598 (2000) 409–416
415
3.5. Bis[1,3-bis(1H,1H,2H,2H-perfluorooctyl)imidazol-
2-ylidene]diiodopalladium(II) (5)
3JHH=7 Hz, NCH2), 3.95 (s, 6H, NCH3), 2.04 (br m,
4H, NCH2CH2), 1.36–1.27 (m, 20H, CH2), 0.88 (t,
3JHH=7 Hz, 6H, CH3). 13C{1H}-NMR (75 MHz): l
167.7 (N2C), 122.3 (NCH), 121.0 (NCH), 51.3 (NCH2),
38.4 (NCH3), 31–22 (CH2), 14.0 (CH3). MS (FAB):
m/z 621 ([M+−I], 2.5). Anal. Calc. for C24H44N4I2Pd
(748.87): C, 38.49; H, 5.9; N 7.5. Found: C, 38.46; H,
5.87; N, 7.44%.
Complex 5 was prepared using previously established
procedures [11,25]. In a typical experiment, the imid-
azolium salt 1 (460 mg, 0.52 mmol) and palladium
acetate (55 mg, 0.25 mmol) were dissolved in 20 ml of
THF. After refluxing for 2 h under argon, the mixture
was filtered through a pad of silica gel. The solvent was
then removed in vacuo, and the residue crystallized
from hot acetone to give 5 as orange crystals (343 mg,
3.9. Bis(1-methyl-3-octylimidazol-2-ylidene)di-
iodo-vv%-diiododipalladium(II) (9)
1
73%). H-NMR (200 MHz, acetone-d6): l 7.52 (s, 4H,
3
NCH), 4.83 (t, JHH=8 Hz, 8H, NCH2), 3.12 (br m,
Palladium acetate (100 mg, 0.45 mmol) and the imi-
dazolium salt 4 (302 mg, 0.94 mmol) were dissolved in
20 ml of THF. After stirring for 1 or 2 min at reflux,
the initially dark-brown solution turned red. The reac-
tion was stopped after 5 min. The red solution was
cooled to room temperature and filtered through a pad
of silica gel. The solvent was then removed under
vacuo. Crystallization in toluene–hexane afforded 9 as
8H, CH2CF2). MS (FAB): m/z 1753 ([M+−I], 2.5).
Anal. Calc. for C38H20N4F52I2Pd (1880.82): C, 24.27;
H, 1.07; N, 2.98. Found: C, 24.32; H, 0.89; N, 2.83%.
3.6. Bis(1,3-dioctylimidazol-2-ylidene)di-
iodopalladium(II) (6)
1
Complex 6 was prepared using the same procedure as
for 5 except that palladium acetate (87 mg, 0.39 mmol)
was reacted with the imidazolium salt 2 (340 mg, 0.81
mmol). Yellow crystals of 6 were obtained from crystal-
a red crystalline solid (224 mg, 90%). H-NMR: l 6.95
(s, 4H, NCH), 4.39 (br m, 4H, NCH2), 4.02 (s, 6H,
NCH3), 2.02 (br m, 4H, NCH2CH2), 1.44–1.31 (m,
3
20H, CH2), 0.90 (t, JHH=7 Hz, 6H, CH3). 13C{1H}-
1
lization in pentane (314.0 mg, 85%). H-NMR: l 6.87
NMR: l 153.0 (N2C), 123.6 (NCH), 122.1 (NCH), 51.7
(NCH2), 39.1 (NCH3), 31–22 (CH2), 14.1 (CH3). Anal.
Calc. for C24H44N4I4Pd2 (1109.10): C, 25.99; H, 4.00; N
5.05. Found: C, 26.49; H, 4.02; N, 4.86%.
3
(s, 4H, NCH), 4.35 (t, JHH=8 Hz, 8H, NCH2), 2.04
(m, 8H, NCH2CH2), 1.39–1.25 (m, 40H, CH2), 0.88 (t,
3JHH=7 Hz, 12H, CH3). 13C{1H}-NMR: l 167.1
(N2C), 120.9 (NCH), 51.5 (NCH2), 31–22 (CH2), 14.0
(CH3). MS (FAB): m/z 817 ([M+−I], 1.3). Anal. Calc.
for C38H72N4I2Pd (945.24): C, 48.29; H, 7.68; N 5.93.
Found: C, 48.07; H, 7.69; N, 5.82%.
4. Supplementary material
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC 135358 for compound 5 and
CCDC 135359 for compound 7. Copies of this informa-
tion may be obtained free of charge from: The Direc-
tor, CCDC, 12 Union Road, Cambridge, CB2 1EZ,
UK (Fax: +44-1223-336033; e-mail: deposit@
ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
3.7. Bis[1-methyl-3-(1H,1H,2H,2H-perfluorooctyl)-
imidazol-2-ylidene]diiodopalladium(II) (7)
Complex 7 was prepared using the same procedure as
for 5 except that palladium acetate (206 mg, 0.92 mmol)
was reacted with the imidazolium salt 3 (1020 mg, 1.93
mmol). Orange crystals of 7 were obtained from crys-
1
tallization in CH2Cl2–hexane (851 mg, 76%). H-NMR
(200 MHz, CDCl3): l 6.93 (s, 4H, NCH), 4.67 (m, 4H,
NCH2), 3.89 (s, 6H, NCH3), 2.94 (br m, 4H, CH2CF2).
13C{1H}-NMR: l 168.8 (N2C), 123.6 (NCH), 122.4
(NCH). MS (FAB): m/z 1089 ([M+−I], 4.2). Anal.
Calc. for C24H18N4F26I2Pd (1216.66): C, 23.69; H, 1.49;
N 4.61. Found: C, 23.67; H, 1.26; N, 4.55%.
Acknowledgements
We are indebted to the University of Liverpool
Graduates Association (Hong Kong) and EPSRC for
postdoctoral research fellowships (L.X. and W.C.).
3.8. Bis(1-methyl-3-octylimidazol-2-ylidene)-
diiodopalladium(II) (8)
References
Compound 8 was prepared using the same procedure
as for 5, but starting from palladium acetate (187 mg,
0.83 mmol) and the imidazolium salt 4 (563 mg, 1.75
mmol). Yellow crystals of 8 were obtained from crystal-
lization in CH2Cl2–pentane (559 mg, 90%). H-NMR
(200 MHz, CDCl3): l 6.86 (s, 4H, NCH), 4.32 (t, 4H,
[1] W.A. Herrmann, C. Ko¨cher, Angew. Chem. Int. Ed. Engl. 36
(1997) 2162 and references therein.
[2] W.A. Herrmann, V.P.W. Bo¨hm, C.P. Reisinger, J. Organomet.
Chem. 576 (1999) 23.
[3] C. Zhang, J. Huang, M.L. Trudell, S.P. Nolan, J. Org. Chem. 64
(1999) 3804.
1

416
L. Xu et al. / Journal of Organometallic Chemistry 598 (2000) 409–416
[4] T. Weskamp, F.J. Kohl, W. Hieringer, D. Gleich, W.A. Her-
rmann, Angew. Chem. Int. Ed. Engl. 38 (1999) 2416.
[21] C.K. Lee, J.C.C. Chen, K.M. Lee, C.M. Liu, I.J.B. Lin, Chem.
Mater. 11 (1999) 1237.
[5] M. Scholl, T.M. Trnka, J.P. Morgan, R.H. Grubbs, Tetrahedron
Lett. 40 (1999) 2247.
[6] J. Huang, E.D. Stevens, S.P. Nolan, J.L. Petersen, J. Am. Chem.
Soc. 121 (1999) 2674.
[22] B. Bildstein, M. Malaun, H. Kopacka, K.-H. Ongania, K.
Wurst, J. Organomet. Chem. 552 (1998) 45.
[23] A. Wacker, H. Pritzkow, W. Siebert, Eur. J. Inorg. Chem. (1998)
843.
[7] M.G. Gardiner, W.A. Herrmann, C.-P. Reisinger, J. Schwarz,
M. Spiegler, J. Organomet. Chem. 572 (1999) 239.
[8] D.S. McGuinness, K.J. Cavell, B.W. Skelton, A.H. White,
Organometallics 18 (1999) 1596.
[24] W.A. Herrmann, L.J. Gooßen, M. Spiegler, J. Organomet.
Chem. 547 (1997) 357.
[25] D. Enders, H. Gielen, G. Raabe, J. Runsink, J.H. Teles, Chem.
Ber. 129 (1996) 1483.
[9] K. Albert, P. Gisdakis, N. Ro¨sch, Organometallics 17 (1998)
1608.
[10] W.A. Herrmann, L.J. Gooßen, C. Ko¨cher, G.R.J. Artus, Angew.
Chem. Int. Ed. Engl. 35 (1996) 2805.
[11] W.A. Herrmann, M. Elison, J. Fischer, C. Ko¨cher, G.R.J. Artus,
Angew. Chem. Int. Ed. Engl. 34 (1995) 2371.
[12] R. Noyori, Chem. Rev. 99 (1999) 353.
[13] P.G. Jessop, T. Ikariya, R. Noyori, Chem. Rev. 99 (1999)
475.
[26] M. Begtrup, P. Larsen, Acta Chem. Scand. 44 (1990) 1050.
[27] T. Kamijo, R. Yamamoto, H. Harada, K. Iizuka, Chem. Pharm.
Bull. 31 (1983) 1213.
[28] C. Heinemann, T. Mu¨ller, Y. Apeloig, H. Schwarz, J. Am.
Chem. Soc. 118 (1996) 2023.
[29] C. Boehme, G. Frenking, J. Am. Chem. Soc. 118 (1996) 2039.
[30] A.J. Arduengo III, R.L. Harlow, M. Kline, J. Am. Chem. Soc.
113 (1991) 361.
[31] W.A. Herrmann, C. Ko¨cher, L.J. Gooßen, G.R.J. Artus, Chem.
Eur. J. 2 (1996) 1627 and references therein.
[14] N.C. Smart, T. Carleson, T. Kast, A.A. Clifford, M.D. Burford,
C.M. Wai, Talanta 44 (1997) 137.
8
[32] K. Ofele, J. Organomet. Chem. 12 (1968) 42.
[15] S. Kainz, D. Koch, W. Baumann, W. Leitner, Angew. Chem.
Int. Ed. Engl. 36 (1997) 1628.
[33] H.-W. Wanzlick, H.-J. Scho¨nherr, Angew. Chem. Int. Ed. Engl.
7 (1968) 141.
8
[16] D. Koch, W. Leitner, J. Am. Chem. Soc. 120 (1998) 13398.
[17] S. Kainz, Z. Luo, D.P. Curran, W. Leitner, Synthesis (1998)
1425.
[18] L.J. Alvey, D. Rutherford, J.J.J. Juliette, J.A. Gladysz, J. Org.
Chem. 63 (1998) 6302.
[19] P. Bhattacharyya, D. Gudmunsen, E.G. Hope, R.D.W. Kem-
mitt, D.R. Paige, A.M. Stuart, J. Chem. Soc. Perkin Trans. 1
(1997) 3609.
[34] W.A. Herrmann, J. Fischer, K. Ofele, G.R.J. Artus, J.
Organomet. Chem. 530 (1997) 259.
[35] M.A. Carroll, A.B. Holmes, Chem. Commun. (Cambridge)
(1998) 1395.
[36] D.K. Morita, D.R. Pesiri, S.A. David, W.H. Glaze, W. Tumas,
Chem. Commun. (Cambridge) (1998) 1397.
[37] N. Shezad, R.S. Oakes, A.A. Clifford, C.M. Rayner, Tetrahe-
dron Lett. 40 (1999) 2221.
[20] B. Betzemeier, P. Knochel, Angew. Chem. Int. Ed. Engl. 36
(1997) 2623.
[38] G.M. Sheldrick, Crystal Structure Determination Program, Uni-
versity of Go¨ttingen, Germany, 1997.
.