Chiral, bridged bis(imidazolin-2-ylidene) complexes of palladium

Article abstract of DOI:10.1016/j.jorganchem.2007.04.049

Varieties of chiral, bridged bisimidazolium salts as well as the synthesis of palladium complexes of general formula [bridge {over(NC(H) {double bond, long} C(H)N (R^*) C, -)}₂ PdBr₂] with the corresponding chelating N-heterocyclic carbene ligands is reported. This is the first systematic study of chiral bis(imidazolin-2-ylidene)palladium(II) complexes bearing chiral groups on the endocyclic nitrogens. Structural proof of such a chiral palladium(II) complex is presented by way of an X-ray diffraction study of complex 3a.

Full text of DOI:10.1016/j.jorganchem.2007.04.049

Journal of Organometallic Chemistry 692 (2007) 4560–4568
www.elsevier.com/locate/jorganchem
q
Chiral, bridged bis(imidazolin-2-ylidene) complexes of palladium
Sabine K. Schneider, Jurgen Schwarz, Guido D. Frey, Eberhardt Herdtweck,
¨
*
Wolfgang A. Herrmann
Department Chemie, Lehrstuhl fu¨r Anorganische Chemie, Technische Universita¨t Mu¨nchen, Lichtenbergstraße 4, D-85747 Garching, Germany
Received 20 March 2007; received in revised form 13 April 2007; accepted 13 April 2007
Available online 17 May 2007
Dedicated to Prof. Dr. Gerhard Erker on the occasion of his 60th birthday.
Abstract
Varieties of chiral, bridged bisimidazolium salts as well as the synthesis of palladium complexes of general formula ½bridge
fNCðHÞ@C(H)NðRꢀÞCg2PdBr2ꢁ with the corresponding chelating N-heterocyclic carbene ligands is reported. This is the first systematic
study of chiral bis(imidazolin-2-ylidene)palladium(II) complexes bearing chiral groups on the endocyclic nitrogens. Structural proof of
such a chiral palladium(II) complex is presented by way of an X-ray diffraction study of complex 3a.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Chelating ligands; Chiral ligands; Bisimidazolium salts; Palladium
1. Introduction
stantial amount of work on the preparation of chiral
NHC ligands has been published [10], following our report
of the first chiral NHC-complex employed in homogeneous
catalysis [4]. The rigidity of chelating chiral carbene ligands
is expected to result in higher optical inductions in asym-
metric reactions relative to two non-chelating ligands. Such
ligands can be constructed, for example, by a bridged
bis(imidazolin-2-ylidene) ligand, a well known framework
for achiral NHC ligands [11–17]. The complexes with che-
lating carbene ligands are presumably highly thermal stable
because of the chelating effect.
The preparation of chelating ligands of N-heterocyclic
carbenes, in order to impart higher air- and moisture stabil-
ity to palladium centers, is receiving nowadays much atten-
tion [18,19]. We and others have published chelated
palladium NHC complexes which can be used in C–C cou-
pling reactions [13,15,20–22]. For example, some methy-
lene-bridged bis(imidazolium) salts, in combination with
Pd(OAc)2, were equally efficient as the non-bridged biscar-
bene complexes in the Suzuki coupling reaction with aryl
chlorides as substrates [23,24] and the bridged biscarbene
complexes even achieve the C–H activation of methane
[25]. In the first part of the present article, a variety of
N-Heterocyclic carbenes have become universal ligands
in organometallic and inorganic coordination chemistry
[1,2]. The bond between the N-heterocyclic carbene and
the metal centre of a complex can be best described as a
dative r-bond as the M–C distance falls comfortably in
the range of typical single M–C bond lengths (X-ray dif-
fraction) [3,4]. Metal complexes of imidazolin-2-ylidenes
have attracted considerable attention as homogeneous cat-
alysts [2]. For example, complexes of ruthenium and palla-
dium show excellent catalytic properties for olefin
metathesis and C–C coupling reactions [5]. In contrast to
the corresponding phosphine complexes, ligand dissocia-
tion is not favored in NHC complexes [6]. This eliminates
the need for an excess of ligand [7], and makes them possi-
ble candidates for asymmetric catalysis [8,9]. Thus, a sub-
q
N-Heterocyclic Carbenes, Part 52. For Part 51, see: W.A. Herrmann,
G.D. Frey, E. Herdtweck, M. Steinbeck, Adv. Synth. Catal. 349 (2007)
1677.
*
Corresponding author. Tel.: +49 89 289 13080; fax: +49 89 289 13473.
E-mail address: lit@arthur.anorg.chemie.tu.muenchen.de (W.A. Herr-
mann).
0022-328X/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.04.049

S.K. Schneider et al. / Journal of Organometallic Chemistry 692 (2007) 4560–4568
4561
H
Table 1
H
2+
Chelating imidazolium salts as carbene (NHC) precursors
Y
H
H
H
H
N
H
N
H
2+
2 X-
N
N
Y
H
H
R*
R*
N
H
N
H
2 Br-
N
N
R*
R*
Fig. 1. R*-substituted (R* = chiral alkyl group) imidazolium salts with Y-
bridging groups (Y = bridging moiety).
Azolium salt
R*
Y
Yield [%]
2a
2b
2c
2d
2e
2f
(R)-CH(C6H5)CH3
(R)-CH(C6H5)CH3
(R)-CH(C10H7)CH3
(R)-CH(C6H5)CH3
(R)-CH(C6H5)CH3
(R)-CH(C6H5)CH3
(R)-CH(C10H7)CH3
–CH2–
49
83
76
97
93
80
75
o-Xylene
o-Xylene
m-Xylene
p-Xylene
2,6(CH2)2(C5H3N)
2,6(CH2)2(C5H3N)
new bis(imidazolium) salts will be presented. We will focus
on the variation of the bridging group Y and the substitu-
ents R* (Fig. 1).
2. Results and discussions
2g
2.1. General synthesis of chiral imidazole derivatives
The substituted imidazole precursors are accessible via
an one-pot synthesis route according to Gridnev and Mih-
altseva (Scheme 1) [26].
The products can be purified by extraction, recrystalliza-
tion or distillation. Via this synthesis two new chirally
modified imidazoles, 1-(R)-(phenylethyl)imidazole 1a [27],
and 1-ðRÞ-ð10-naphthylethylÞimidazole 1b, could be synthe-
sized without racemization and with retention of the (R)-
configuration employing commercially available chiral
amines.
In order to synthesize a library of chirally modified
bridged imidazolium salts, we used a general synthetic pro-
cedure. Two equivalents of the imidazole were dissolved in
THF and transferred to an ACE pressure tube. One equiv-
alent of the corresponding dibromo compound was added
and the resulting mixture was heated. The purification of
the air stable hygroscopic colorless to pale brown products
was achieved by washing the obtained solids with THF
[28]. The product was dried under high vacuum. It is
remarkable that the synthesis of chiral imidazolium salts
occurs without racemization of the chiral centers; the (R)-
configuration is maintained in each case. To the best of
our knowledge these are the first enantiomerically pure chi-
ral, bridged bisimidazolium salts with the chirality intro-
duced at the N-atoms reported to date [15]. Table 1 gives
an overview of the prepared bis(1,3-azolium) salts.
straightforward procedure, avoiding the formation of free
carbenes as intermediates [13], which utilizes longer reac-
tion times and milder temperatures to increase yields of
the palladium complexes, particularly when chiral substitu-
ents are present in the bridged bis(imidazolium) salt precur-
sors. Thereon, the bis(imidazolium) salt and Pd(OAc)2
were stirred in DMSO overnight at room temperature.
After this, slow heating over a period of 6 h not reaching
more than 120 ꢁC afforded the bis(carbene)palladium(II)
complexes 3a–3f without racemization of the chiral groups
(Scheme 2). The synthesized complexes are depicted in
Fig. 2.
2.3. X-ray crystal structure of complex 3a
The molecular structure of the square-planar Pd(II)
complex 3a in the solid state is shown in Fig. 3. Selected
bond lengths and angles are denoted in the figure caption.
The crystal structure of complex 3a shows the biscarbene
ligand chelating the palladium(II) center in a cis-arrange-
ment with a boat conformation being observed for the
six-membered C3N2Pd ring. The remaining two coordina-
tion sites of the distorted square planar coordinated palla-
dium center are occupied by bromine anions.
˚
The Pd–C distances (1.974(3), 1.992(3) A) fall comfort-
ably within the range found for the dicationic and neutral
chelating bis(carbene)palladium(II) complexes ½cis-CH2
2.2. Synthesis of chiral bis(carbene)palladium(II)
complexes
The neutral bis(carbene)palladium(II) bromide com-
plexes 3a–3f were prepared via an adaptation of our
2 Br-
R*
R*
N+
N
O
N
N
N
N
H
H
6 h RT to 120 °C
PdBr2
+ Pd(OAc)2
+ 2 HOAc
Y
Y
H
O
H
O
DMSO
reflux
- 3H2O
- HCl
*R N
N
N+
N
NH4Cl
*R NH2
R*
2a-2f
Scheme 2. Synthesis of chiral bis(carbene)palladium(II) complexes.
R*
3a-3f
Scheme 1. Synthesis of substituted imidazoles.

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S.K. Schneider et al. / Journal of Organometallic Chemistry 692 (2007) 4560–4568
N
N
N
N
N
N
Pd
Br
Pd
Br
Pd
N
N
N
N
N
N
Br
Br
Br
H3C
Br
H3C
H3C
CH3
CH3
CH3
3c
3a
3b
+ Br-
N
N
N
N
N
N
Br
N
N
Pd
Br
Pd
Br
Pd
Br
N
N
N
N
N
Br
H3C
CH3
H3C
H3C
CH3
CH3
+ Diastereomer
3d
3f
3e
Fig. 2. Synthesized palladium(II) complexes and postulated structure configurations.
˚
Fig. 3. ORTEP [30] style plot of compound 3a in the solid state. Thermal ellipsoids are drawn at the 50% probability level. Selected bond lengths [A] and
bond angles [ꢁ]: Pd–Br1 2.4961(4), Pd–Br2 2.4859(4), Pd–C1 1.992(3), Pd–C2 1.974(3), N1–C1 1.357(4), N1–C3 1.457(4), N1–C4 1.389(4), N3–C2 1.359(4),
N3–C3 1.454(4), N3–C7 1.390(4); Br1–Pd–Br2 93.52(1), Br1–Pd–C1 169.81(8), Br1–Pd–C2 89.59(9), Br2–Pd–C1 92.55(9), Br2–Pd–C2 175.91(8), C1–Pd–
C2 84.0(1).
2þ
fNCðHÞ@C(H)NðCH3ÞCg PdðNCCH3Þ ꢁ ½BF4ꢁꢂ (1.966(2)
(R) via anomalous X-ray dispersion (scattering factors f0
and f00), which is in accordance with the absolute configura-
tion of the primarily employed amine. In this regard the
explored crystals were unitary and their NMR spectral
data were in accordance to those of the mother liquor.
2
2
2
˚
and 1.972(3) A) [20] and ½cis-CH2fNCðHÞ@C(H)NðCH3ÞCg2
˚
PdI2ꢁ (1.990(3) and 1.997(3) A) [5i] as well as with those
reported for the related non-chelating complex [cis-{MeNC
˚
@CN(Me)C}2PdI2] (1.990(3) and 1.997(3) A) [23,29].
This structure is further proof, that the synthesis of such
chiral palladium(II) complexes is not accompanied by any
racemization. If enantiomerically pure amines [or enantio-
merically pure bis(imidazolium) salts] are employed the
absolute configuration remains the same; in this case the
absolute configuration at each chiral centre is assigned as
2.4. Spectroscopic and structural properties of chiral
bis(carbene)palladium(II) complexes
If chelating bis(imidazolin-2-ylidene)palladium(II) com-
plexes are formed, there is the possibility of formation of

S.K. Schneider et al. / Journal of Organometallic Chemistry 692 (2007) 4560–4568
4563
the cis- as well as of the trans-arrangement, depending
on the size of the bridge. In the case of non-bridged bis-
(carbene)palladium(II) complexes, in most cases the
cis-arrangement is formed unless sterically demanding imi-
dazolium salts are employed [15,31]. If bridged bis(imi-
dazolium) salts are employed as ligands, the geometry of
the bridging moiety determines the substitution pattern.
In the case of the methylene bridge, only the cis-arrange-
ment is possible which was confirmed by a single crystal
X-ray analysis of complex 3a (Fig. 3).
process between boat and chair type configurations can be
excluded in the case of 3e, because of the restrained para-
configuration within the bridge.
Because of atropisomerism arising from triple coordina-
tion of the ligand to the palladium centre, it was expected
to present four diastereomers of 3f and therefore a quadru-
ple set of NMR signals should be detected [33]. As both of
the chiral centers have (R)-configuration the molecule is
C2-symmetric, and thus the four diastereomers are reduced
to two as well as for the NMR data set. In our case only a
double data set was detected for this compound. However,
the gross structure proposed in Fig. 2 can be derived by
comparison to a related structure of an analogous complex
with achiral N-substituents, which was determined by X-
ray analysis [34–36]. Additionally all complexes 3a–3f
showed in the FAB-MS spectrum the [MꢂBr]+ fragment
as well as the [Mꢂ2Br]+ fragment.
1H and 13C NMR spectra of complex 3a are in agreement
with the assigned structure. The appearance of inequivalent
methylene proton resonances in the 1H NMR spectrum
indicate the retention of a conformationally restrained boat
shaped six-membered chelate ring for the complex, and this
was determined by the X-ray solid state structure. Such con-
formationally rigid ring systems have been noted by us and
other groups. Similar complexes for examples show a flux-
ional behavior at room temperature [15].
The ortho-xylene bridged-complexes 3b and 3c exhibit
excellent solubility in dichloromethane, chloroform as well
as acetonitrile. In contrast, the rest of the complexes 3a,
3d–3f are only soluble in highly polar solvents like DMSO
or alcohols (MeOH/EtOH) and only sparingly soluble in
CH2Cl2 and acetonitrile. Complex 3e is only soluble in
hot DMSO which hampered the acquisition of 13C NMR
data. The biscarbene complexes 3a–3f exhibit a very high
stability towards air, moisture and high temperatures, as
is typical for this class of bis(carbene)palladium(II) com-
plexes [5i,13,14,20,21,23].
Literature examples of related achiral ortho-xylene
bridged palladium(II) complexes also exhibit cis-geometry
which has been confirmed by X-ray analysis; therefore such
a arrangement can also be postulated for 3b and 3c, as the
spectroscopic data are in line with the literature examples
[15,31]. The NMR data of the two ortho-xylene bridged
complexes 3b and 3c are very similar to 3a, where also a
double set of signals for the carbene carbon atoms were
obtained between 161 and 164 ppm. This also proves the
postulated cis-arrangement in the complex. Our results
are in agreement with the work of RajanBabu [32], where
chiral chelating bis(carbene)palladium(II) complexes bear-
ing a chiral bridging moiety were described. In this case
the cis-complex showed a double set of NMR signals con-
trary to the trans-complex, where only a single set was
obtained, due to a mirror plane in the molecule [32]. In con-
trast to 3a–3c, complex 3d shows broadened NMR signals,
which is probably due to either rapid cis/trans isomeriza-
tion, or a dynamic ‘ring flipping’ process in which the xylene
ring may interchange between a boat and a chair type con-
figuration. Similar behavior with achiral meta-xylene
bridged palladium(II) complexes was discovered by Magill
and Cavell [31]. As low solubility of complex 3d in conven-
tional NMR solvents prevented us from carrying out low
temperature NMR studies of this complex, in this case the
broadening could not be confirmed. Repeated attempts to
crystallize 3d were invariably unsuccessful, and at present
the true geometry neither of this complex nor of meta-
xylene bridged literature examples has been established
[31]. However, given the NMR data, which also manifests
as a double set of signals, the cis-orientation for 3d is more
likely. RajanBabu noted only a single NMR data set for chi-
ral trans-complexes of this kind [32]. This is also in accor-
dance with the NMR spectra of complex 3e. Additionally
the signals are broadened like in the spectra of 3d, which
may also arise from phenomena like cis/trans isomerization
(with a strong preference of the trans-isomer) or restricted
rotation around bonds within the molecule. A ‘ring flipping’
2.5. Conclusion
A variety of chiral, bridged palladium(II) complexes of
the general formula ½bridgefNCðHÞ@C(H)NðRꢀÞCg2PdBr2ꢁ
were synthesized according to established routes. The syn-
thesis of all new ligands and catalysts presented in this work
is simple and proceeds without any racemization of the chi-
ral groups. Moreover, the complexes are exceedingly stable
towards air, moisture and heat. The majority of chiral cat-
alysts known in literature – mainly consisting of phosphine
ligands – would not bear reaction mediums under oxidizing
conditions and high temperatures. This makes these new
chiral complexes potentially suitable for asymmetric cataly-
sis under extreme reaction conditions like high tempera-
tures or reactions in oxidizing mediums. Therefore, we
expect that this class of chiral ligands and complexes will
find application in enantioselective catalytic processes and
should, individually, be considered as useful alternatives
to classical chiral bis- or trisphosphine ligands.
3. Experimental
All experiments were carried out under dry argon using
standard Schlenk techniques [37]. Solvents were dried by
standard methods and distilled under nitrogen. 1H and
13C NMR spectra were recorded on a JEOL-JMX-GX
400 MHz spectrometer at room temperature and refer-
1
enced to the residual H and 13C signals of the solvents.

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S.K. Schneider et al. / Journal of Organometallic Chemistry 692 (2007) 4560–4568
NMR multiplicities are abbreviated as follows: s = singlet,
d = doublet, t = triplet, sept. = septet, m = multiplet,
br = broad signal. Chemical shifts d are given in ppm, cou-
pling constants J are given in Hz. Elemental analyses were
carried out by the Microanalytical Laboratory at the TU
ethyl)pyridine were added. The solution was heated for
24 h at 80 ꢁC and a colorless precipitate was obtained.
The precipitate was filtered off and washed three times with
10 mL THF (2a, 2d), or recrystallized from dichlorometh-
ane/diethyl ether (2b, 2c, 2e, 2f), and dried in high vacuum
to obtain a highly hygroscopic colorless powder.
Munchen. Mass spectra were performed at the TU Mun-
¨
¨
chen Mass Spectrometry Laboratory on a Finnigan MAT
90 spectrometer using the CI or FAB technique.
3.3.1. 1,10-Di((R)-100-phenylethyl)-3,30-methylene-
diimidazolium dibromide 2a
3.1. 1-(R)-(Phenylethyl)imidazole 1a
Yield: 635 mg (49%). Mp >300 ꢁC. 1H NMR (400 MHz,
DMSO-d6): d = 10.11 (s, 2H, NCHN), 8.80 (s, 2H, NCH),
8.02 (s, 2H, NCH), 7.52–7.35 (m, 10H, CHarom), 6.82 (s,
1-(R)-phenylethylamine (12.1 g, 0.1 mol) was dissolved
in 100 mL H2O and phosphoric acid (85%) was added until
a pH of approximately 2 was reached. After addition of
3.0 g of paraformaldehyde (0.1 mol) and 11.5 mL of glyoxal
(40% in water, 0.1 mol), the reaction mixture was heated to
100 ꢁC and a saturated ammonium chloride solution (5.4 g,
0.1 mol, 20 mL H2O) was added drop wise over a period of
60 min. After stirring for another 1 h at 100 ꢁC, the reaction
mixture was cooled to 0 ꢁC and sodium hydroxide was
added until a pH higher than 12 was observed. The product
was extracted three times with 100 mL methylene chloride,
dried over MgSO4 and afterwards the solvent was removed
in vacuo. The crude product was purified by vacuum distil-
lation to obtain a colorless oil [27].
3
2H, NCH2N), 5.93 (q, 2H, JHH = 7.4 Hz, NCH), 1.86 (d,
3
6H, JHH = 7.4 Hz, CH3). 13C{1H} NMR (100.5 MHz,
DMSO-d6): d = 139.5 (NCHN), 137.7, 129.4, 129.3, 127.3
(CHarom), 123.1 (NCH), 122.4 (NCH@), 59.6 (NCH), 58.6
(s, NCH2CH2N), 21.1 (NCH3). MS (FAB): m/z (%) = 437
(13, [M+]), 357 (57, [M2+]), 253 (70, [C15H17N4]+), 186 (8,
[C12H14N2]+), 173 (100, [C11H12N2]+). Anal. Calc. for
C23H26Br2N4 (518.19): C, 53.30; H, 5.06; N, 10.81. Found:
C, 53.73; H, 5.37; N, 10.62%.
3.3.2. 1,10-Di((R)-100-phenylethyl)-3,30-o-xylylene-
diimidazolium dibromide 2b
Yield: 1.26 g (83%). 1H NMR (400 MHz, CDCl3):
d = 10.39 (s, 2H, NCHN), 7.69 (s, 2H, NCH), 7.49–7.20
(m, 14H, Ar), 7.09 (s, 2H, NCH), 6.15 (d, 2H,
JHH = 16.0 Hz, CH2), 5.93 (d, 2H, JHH = 12.0 Hz, CH2),
5.82 (q, 2H, J = 8.0 Hz, CH(phenylethyl)), 1.96 (d, 6H,
3JHH = 7.6 Hz, CH3 (phenylethyl)). 13C{1H} NMR
(100.5 MHz, CDCl3): d = 137.9 (NCN), 136.1, 132.3,
130.4, 130.2, 129.5, 129.5, 127.1 (Ar), 123.1, 121.0 (NC),
60.4 (CH (phenylethyl)), 50.5 (CH2), 21.1 (CH3 (phenyl-
ethyl)). MS (FAB): m/z (%) = 527 (100, [M+]). Anal. Calc.
for C30H32Br2N4 (608.41): C, 59.22; H, 5.30; N, 9.21.
Found: C, 59.45; H, 5.39; N, 9.13%.
Yield: 5.81 g (34%); bp: 102 ꢁC/1 mbar. 1H NMR
(270 MHz, CDCl3): d = 7.52 (s, 1H, NCHN), 7.28–7.22
(m, 3H, CHarom), 7.13 (m, 2H, CHarom), 7.09 (d, 1H,
3
3JHH = 2.0 Hz, NCH@), 7.02 (d, 1H, JHH = 2.0 Hz,
3
NCH), 5.27 (q, 1H, JHH = 6.9 Hz, CH(C6H5)CH3), 1.78
3
(d, 3H, JHH = 7.2 Hz, CH3). 13C{1H} NMR (67.5 MHz,
CDCl3): d = 142.4 (NCHN), 136.5, 130.2, 129.4, 128.9,
126.1 (Ar), 118.1 (NCH@), 56.6 (CH(C6H5)CH3), 21.9
(CH3). GC–MS: m/z (%) = 173 (73, [MH+]).
3.2. 1-(R)-(10-Naphthylethyl)imidazole 1b
1-(R)-(1-Naphthylethyl)imidazole was synthesized anal-
ogously to 1a from (R)-1-naphthylethylamine (5.0 g,
29.0 mmol). The crude product was purified by extracting
with diethyl ether to obtain a yellow oil.
3.3.3. 1,10-Di((R)-100-naphthylethyl)-3,30-o-xylylene-
diimidazolium dibromide 2c
Yield: 1.35 g (76 %). Mp >300 ꢁC. 1H NMR (270 MHz,
CDCl3): d = 10.50 (s, 2H, NCHN), 8.20–6.81 (m, 22H,
CHarom (14 naphthyl, 4 o-Tol, 4 imidazole)), 6.62 (q, 2H,
3JHH = 7.5 Hz, CH (naphthylethyl)), 6.21 (d, 2H,
Yield: 2.10 g (33%). 1H NMR (270 MHz, CDCl3):
d = 7.90–7.79 (m, 2H, CHarom), 7.61 (s, 1H, NCHN),
7.55–7.20 (m, 5H, CHarom), 7.08 (s, 1H, NCH), 6.96 (s,
1H, NCH), 6.24 (q, 1H, J = 6.8 Hz, CH (naphthylethyl)),
2.00 (d, 3H, J = 7.1 Hz, CH3 (naphthylethyl)). 13C{1H}
NMR (67.5 MHz, CDCl3): d = 136.6 (NCN), 129.1, 118.6
(NC), 136.3, 134.0, 130.4, 129.3, 127.0, 126.1, 125.5,
123.4, 122.2, 120.0 (Ar), 53.2 (CH (naphthylethyl)), 22.0
(CH3 (naphthylethyl)). GC–MS: m/z = 222 [M+], 155
[M+ꢂimidazole], 128 [naphthyl+].
3
J = 13.5 Hz, CH2), 5.91 (d, 2H, JHH = 16.2 Hz, CH2),
3
2.13 (d, 6H, JHH = 7.3 Hz, CH3 (naphthylethyl)).
13C{1H} NMR (67.5 MHz, CDCl3): d = 136.6, (NCN),
134.0, 133.2, 132.3, 132.0, 131.1, 130.3, 129.3, 127.9,
126.6, 125.6, 124.7, 122.4, 122.3 (Ar), 122.9, 121.1 (NC),
56.5 (CH (naphthylethyl)), 50.8 (CH2), 21.4 (CH3 (naph-
thylethyl)). MS (FAB): m/z (%) = 627 (100, [M+]). Anal.
Calc. for C38H36Br2N4 (708.53): C, 64.42; H, 5.12; N,
7.91. Found: C, 64.53; H, 5.17; N, 7.85%.
3.3. General synthesis of chiral bisimidazolium salts
3.3.4. 1,10-Di((R)-100-phenylethyl)-3,30-m-xylylene-
diimidazolium dibromide 2d
Yield: 1.48 g (97%). H NMR (400 MHz, DMSO-d6):
d = 9.67 (s, 2H, NCHN), 7.87–7.41 (m, 14H, CHarom (10
(R)-Imidazole (5.0 mmol) was dissolved in 5 mL THF in
an ACE pressure tube and 2.5 mmol of dibromomethane,
a,a0-dibromo-o-, -m-, -p-xylylene or 2,6-di(bromom-
1

S.K. Schneider et al. / Journal of Organometallic Chemistry 692 (2007) 4560–4568
4565
phenyl + 4 imidazole)), 5.45 (m, 4H, CH2), 5.81 (q, 2H,
3JHH = 8.0 Hz, CH (phenylethyl)), 1.88 (d, 6H,
3JHH = 7.6 Hz, CH3 (phenylethyl)). 13C{1H} NMR
(100.5 MHz, DMSO-d6): d = 139.3 (NCN), 136.0, 135.9,
130.4, 129.7, 129.4, 129.3, 128.5, 127.2 (Ar), 123.4, 122.1
(NC), 59.2 (CH (phenylethyl)), 52.2 (CH2), 20.9 (CH3
(phenylethyl)). MS (FAB): m/z (%) = 527 (100, [M+]).
Anal. Calc. for C30H32Br2N4 (608.41): C, 59.22; H, 5.30;
N, 9.21. Found: C, 59.34; H, 5.41; N, 9.11%.
(m, 10H, Ar), 6.92 (s, 2H, NCH), 6.79 (s, 2H, NCH),
6.35 (br, 2H, NCH2N), 5.91 (br, 2H, NCH), 2.06 (br,
6H, CH3). 13C{1H} NMR (100.5 MHz, DMSO-d6):
d = 172.2 (NCN), 140.2, 137.3, 129.5, 129.1, 128.8, 128.2,
127.3, 126.7 (Ar), 123.6, 123.2, 122.8, 122.6 (NCH), 63.2
(NCH2), 58.9, 57.0 (NCH), 21.3, 21.0 (CH3). MS (FAB):
m/z (%) = 623 (36, [MH+]), 543 (100, [M+]), 461 (45,
[M2+]). Anal. Calc. for C23H24Br2N4Pd (622.68): C,
44.36; H, 3.88; N, 9.00. Found: C, 44.36; H, 3.96; N, 8.95%.
3.3.5. 1,10-Di((R)-100-phenylethyl)-3,30-p-xylylene-
diimidazolium dibromide 2e
3.4.2. Dibromo{1,10-di[(R)-100-phenylethyl]-3,30-o-
xylylenediimidazolin-2,20-diylidene}palladium(II) 3b
Yield: 281 mg (88%). 1H NMR (400 MHz, CDCl3):
d = 7.71–7.05 (m, 19H, Ar (14 · phenyl + 4 · NCH,
1
Yield: 1.41 g (93%). Mp >300 ꢁC. H NMR (400 MHz,
DMSO-d6): d = 9.79 (s, 2H, NCHN), 7.94 (s, 2H, NCH),
7.90 (s, 2H, NCH), 7.53–7.30 (m, 14H, CHarom), 5.86 (m,
2H, CH (phenylethyl)), 5.49 (s, 4H, CH2), 1.88 (d, 6H,
3JHH = 6.0 Hz, CH3 (phenylethyl)). 13C{1H} NMR
(100.5 MHz, DMSO-d6): d = 140.0 (NCN), 136.2, 135.9,
129.7, 129.4, 127.2, 123.5 (Ar), 123.3, 122.2 (NC), 59.3
(CH (phenylethyl)), 51.9 (CH2), 21.0 (CH3 (phenylethyl)).
MS (FAB): m/z (%) = 527 (100, [M+]). Anal. Calc. for
C30H32Br2N4 (608.41): C, 59.22; H, 5.30; N, 9.21. Found:
C, 59.24; H, 5.27; N, 9.17%.
3
1 · CH2)), 6.95 (d, 1H, JHH = 14.0 Hz, CH2), 6.80 (q,
1H,
3JHH = 6.0 Hz,
CH(PhEt)),
5.99
(q,
1H,
3
3JHH = 6.0 Hz, CH(PhEt)), 5.04 (d, 1H, JHH = 15.2 Hz,
3
CH2), 4.98 (d, 1H, JHH = 14.8 Hz, CH2), 1.92 (d, 3H,
3JHH = 6.8 Hz, CH3(PhEt)), 0.61 (d, 3H, JHH = 9.6 Hz,
3
CH3(PhEt)). 13C{1H} NMR (100.5 MHz, CDCl3):
d = 163.8 (NCN), 161.1 (NCN), 141.0, 139.3, 135.4,
134.9, 132.1, 131.8, 130.0, 129.9, 129.1, 28.4, 128.1, 127.8,
127.4, 125.9 (Ar), 122.7, 121.8, 120.7, 119.7 (NC), 67.8,
60.6, 59.2 (CH(PhEt)), 51.3 (CH2), 25.6, 24.8 (CH3(PhEt)).
MS (FAB): m/z = 712 [M+], 632 [M+ꢂBr], 551 [M+ꢂ2Br].
Anal. Calc. for C30H30N4Br2Pd (712.82): C, 50.55; H, 4.24;
N, 7.86. Found: C, 50.14; H, 4.14; N, 8.00%.
3.3.6. 1,10-Di((R)-100-phenylethyl)-2,6-lutidine-
diimidazolium dibromide 2f
Yield: 1.22 g (80%). 1H NMR (400 MHz, CDCl3):
d = 10.86 (s, 2H, NCHN), 8.02–7.00 (m, 17H, CHarom (10
3
phenyl + 3 pyridyl + 4 imidazole)), 6.06 (q, 2H, JHH
=
3.4.3. Dibromo{1,10-di[(R)-100-naphthylethyl]-3,30-o-
xylylenediimidazolin-2,20-diylidene}palladium(II) 3c
Yield: 273 mg (75%). 1H NMR (400 MHz, CDCl3):
d = 8.10–7.08 (m, 22H, Ar (18 · naphthyl + 4 · NCH)),
5.6 Hz, CH (phenylethyl)), 5.79 (d, 2H, J = 14.8 Hz,
CH2), 5.67 (d, 2H, J = 14.0 Hz, CH2), 1.99 (d, 6H, 3JHH
=
6.0 Hz, CH3 (phenylethyl)). 13C{1H} NMR (100.5 MHz,
CD3CN): d = 153.3 (lutidineNC), 138.9 (NCN), 138.9,
136.3, 129.2, 129.1, 127.0, 123.0 (Ar), 123.6, 122.3 (NC),
59.5 (CH (phenylethyl)), 53.4 (CH2), 20.3 (CH3 (phenyl-
ethyl)). MS (FAB): m/z (%) = 528 (100, [M+]). Anal. Calc.
for C29H31Br2N5 (609.40): C, 57.16; H, 5.13; N, 11.49.
Found: C, 57.25; H, 5.23; N, 11.53%.
3
3
7.20 (d, 1H, JHH = 16.4 Hz, CH2), 6.68 (d, 1H, JHH
=
16.3 Hz, CH2), 5.59 (m, 3H, (2 · CH2 + CH(PhEt))), 5.29
3
(q, 1H, JHH = 7.0 Hz, CH(naphthEt)), 2.01 (d, 3H,
3
3JHH = 7.2 Hz, CH3(naphthEt)), 0.91 (d, 3H, JHH
=
7.0 Hz, CH3(napthEt)). 13C{1H} NMR (100.5 MHz,
CDCl3): d = 163.9 (NCN), 161.6 (NCN), 136.9, 136.5,
135.2, 134.9, 134.3, 134.0, 133.6, 133.4, 133.2, 132.0,
130.4, 130.2, 129.9, 129.1, 128.9, 128.5, 128.3, 127.8,
127.3, 126.4, 126.3, 125.5, 125.3, 124.8, 123.9, 123.6 (Ar),
123.2, 122.6, 121.9, 121.1 (NC), 58.7, 57.3 (CH(naphthEt)),
56.4, 56.2 (CH2), 25.0, 20.8 (CH3(naphthEt)). MS (FAB):
m/z = 731 [M+ꢂBr]. Anal. Calc. for C38H34N4Br2Pd
(812.93): C, 56.14; H, 4.22; N, 6.89. Found: C, 56.27; H,
4.28; N, 7.19%.
3.4. General procedure for chiral bisimidazolin-2-ylidene
palladium(II) complexes
0.445 mmol of the chiral bisimidazolium salt and
Pd(OAc)2 (100 mg, 0.445 mmol) were dissolved in 5 mL
DMSO and stirred at room temperature for 20 h, after-
wards the mixture was heated for 3 h at 50 ꢁC and later
1 h at 100 ꢁC. The volatile compounds were removed in
vacuo and the precipitate was washed twice with 5 mL
THF. The complexes were recrystallized by slow condensa-
tion of diethyl ether/n-pentane (3b, 3c) or ethanol/metha-
nol (3a, 3d, 3e, 3f) into a saturated DCM (3b, 3c) or
DMSO (3a, 3d, 3e, 3f) solution.
3.4.4. Dibromo{1,10-di[(R)-100-phenylethyl]-3,30-m-
xylylenediimidazolin-2,20-diylidene}palladium(II) 3d
1
Yield: 234 mg (73%). H NMR (400 MHz, DMSO-d6):
d = 7.70–7.00 (m, 18H, ArH (14 · ArH + 4 · NCH)),
6.81 (br, 1H, CH(PhEt)), 5.90–5.10 (m, 5H,
CH(PhEt) + CH2), 1.80 (m, 6H, CH3(PhEt)). 13C{1H}
NMR (100.5 MHz, DMSO-d6): d = 168.9 (NCN), 168.7
(NCN), 140.9, 140.3, 136.1, 135.0, 129.6, 129.3, 129.0,
128.9, 128.5, 128.4, 128.1, 127.9, 127.1, 126.8 (Ar), 122.1,
120.6, 120.3, 119.5 (NC), 59.4, 58.8 (CH(PhEt)), 58.9,
3.4.1. Dibromo{1,10-di[(R)-100-phenylethyl]-3,30-
methylenediimidazolin-2,20-diylidene}palladium(II) 3a
20
Yield: 235 mg (85%). Mp 299 ꢁC. ½aꢁD ¼ þ182ꢃ
1
(DMSO). H NMR (400 MHz, DMSO-d6): d = 7.63–7.33

4566
S.K. Schneider et al. / Journal of Organometallic Chemistry 692 (2007) 4560–4568
53.6 (CH2), 20.6, 20.2 (CH3(PhEt)). MS (FAB): m/z = 551
[M+ꢂ2Br]. Anal. Calc. for C30H30N4Br2Pd (712.82): C,
50.55; H, 4.24; N, 7.86. Found: C, 51.23; H, 5.01; N, 7.21%.
to refine freely. Full-matrix least-squares refinements were
carried out by minimizing
P
2
wðF 2o ꢂ F 2cÞ and converged
with R1 = 0.0271 [Io > 2r(Io)], wR2 = 0.0670 [all data],
GOF = 1.038, and shift/error <0.001. The choice of the cor-
rect enantiomer is proved by Flack’s parameter
e = 0.015(6). The final difference Fourier map shows no
3.4.5. Dibromo{1,10-di[(R)-100-phenylethyl]-3,30-p-
xylylenediimidazolin-2,20diylidene}palladium(II) 3e
1
striking features (Demin/max = +0.40/ꢂ0.77 e Aꢂ3) [30].
˚
Yield: 291 mg (91%). H NMR (400 MHz, DMSO-d6):
d = 7.95–7.03 (m, 18H, 14 · ArH + 4 · NCH), 5.78 (m,
2H, CH(PhEt)), 5.40 (m, 4H, CH2), 1.85 (br s, CH3(PhEt)).
13C{1H} NMR (100.5 MHz, DMSO-d6): d = carbene car-
bon was not detected, 136.1, 129.6, 129.3, 129.0, 128.8,
128.1, 127.1 (Ar), 123.5, 122.2 (NC), 59.4 (CH(PhEt)),
53.7 (CH2), 21.0 (CH3(PhEt)). MS (FAB): m/z = 712
[M+], 632 [M+ꢂBr], 551 [M+ꢂ2Br]. Anal. Calc. for
C30H30N4Br2Pd (712.81): C, 50.55; H, 4.24; N, 7.86.
Found: C, 49.90; H, 4.22; N, 7.86%.
4. Supplementary material
CCDC 640944 contains the supplementary crystallo-
graphic data for 3a. The data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.
html, or from the Cambridge Crystallographic Data Cen-
tre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
(+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
3.4.6. Dibromo{1,10-di[(R)-100-phenylethyl]-2,6-lutidine-
Acknowledgements
diimidazolin-2,20diylidene}palladium(II) 3f
1
Yield: 238 mg (75%). H NMR (400 MHz, DMSO-d6):
We are grateful to Dr. Jo¨rg Eppinger, Dipl.-Chem. To-
bias Scherg and Dr. Karl Ofele for suggestions and helpful
¨
d = 7.94–6.97 (m, 17H, 13 · ArH + 4 · NCH), 5.40 (br,
1H, CH2), 6.59 (m, 1H, CH(PhEt)), 5.87 (br, 1H, CH2),
5.61 (br, 1H, CH2), 5.57 (m, 1H, CH(PhEt)), 5.32 (br,
1H, CH2), 1.68 (br, 6H, CH3(PhEt)). 13C{1H} NMR
(100.5 MHz, DMSO-d6): d = carbene carbon was not
detected, 156.1, 155.9 (lutidineNC), 140.3, 139.4, 139.0,
136.8, 129.5, 129.3, 129.0, 128.7, 128.2, 127.0 (Ar), 124.2,
121.9 (NC), 59.4, 53.8 (CH(PhEt)), 55.9, 52.3 (CH2),
21.2, 20.4 (CH3(PhEt)). MS (FAB): m/z = 633 [M+ꢂBr].
Anal. Calc. for C29H29N5Br2Pd (713.80): C, 48.80; H,
4.10; N, 9.81. Found: C, 48.00; H, 3.64; N, 10.24%.
discussions. This work was generously supported by the
Fonds der Chemischen Industrie (Ph.D fellowship for
S.K.S. and J.S.). G.D.F. gratefully acknowledges the finan-
cial support of the Alexander von Humboldt Foundation
(Feodor Lynen research fellowship). We also thank Degus-
sa AG for a generous gift of palladium(II) acetate.
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