Acceptor substituted N-heterocyclic carbenes and their Rh(I)complexes: Synthesis, structure and properties

Article abstract of DOI:10.1016/j.jorganchem.2008.01.039

Rhodium(I) complexes of acceptor substituted N-heterocyclic carbenes were obtained either by transmetalation from the corresponding Ag(I) complexes or by thermal decomposition of corresponding pentafluorobenzene carbene adducts. All complexes were fully c

Full text of DOI:10.1016/j.jorganchem.2008.01.039

Available online at www.sciencedirect.com
Journal of Organometallic Chemistry 693 (2008) 2079–2090
www.elsevier.com/locate/jorganchem
Acceptor substituted N-heterocyclic carbenes and their
Rh(I)complexes: Synthesis, structure and properties
a
a,
a
b
*
Agnes Bittermann , Peter Ha¨rter , Eberhardt Herdtweck , Stephan D. Hoffmann ,
Wolfgang A. Herrmann a,*
a Department of Chemistry, Chair of Inorganic Chemistry, Technische Universita¨t Mu¨nchen, Lichtenbergstraße 4, 85747 Garching, Germany
b Chair of Inorganic Chemistry with Focus on Novel Materials, Technische Universita¨t Mu¨nchen, D-85747 Garching bei Mu¨nchen, Germany
Received 3 January 2008; received in revised form 27 January 2008; accepted 27 January 2008
Available online 2 February 2008
Abstract
Rhodium(I) complexes of acceptor substituted N-heterocyclic carbenes were obtained either by transmetalation from the correspond-
ing Ag(I) complexes or by thermal decomposition of corresponding pentafluorobenzene carbene adducts. All complexes were fully
characterized by means of NMR- and mass spectroscopy. Compounds 5, 6, 7 and 11 were although characterized by single-crystal
X-ray analysis. The relative r-donor/p-acceptor strength of the NHC ligands was determined by means of IR spectroscopy. Dimerisation
behaviour of Rh carbonyl complexes was studied.
Ó 2008 Elsevier B.V. All rights reserved.
Keywords: NHC; Rhodium complexes; r-Donor/p-acceptor-properties; Dimerisation behaviour
1. Introduction
tion of the imidazolylidene ring reduces the NHC donor
strength. As a result enhanced performance in Heck type
CC-coupling reactions is observed [7].
Since the discovery of N-heterocyclic carbenes (NHCs)
¨
by Ofele [1], Wanzlick and Scho¨nherr [2] in 1968 and isola-
On the other hand NHCs which contain only one het-
eroatom can react as lewis acids and coordinate small mol-
ecules such as CO [8]. Very strong NHC donor ligands can
even activate molecules such as hydrogen or ammonia [9].
Only few references referring to 4,5-acceptor substituted
imidazol-2-ylidene ligands can be found in the literature.
Peris et al. reports synthesis and characterization of 4,5-
dichloroimidazolylidene ligands. Rhodium and iridium
complexes of these NHCs showed enhanced activity in cat-
alytic hydrosilylation of terminal acetylenes and cyclisation
of acetylenic carboxylic acids [10].
We now report a comparative study of NHC ligands 1
(Fig. 1). Interaction of the p-system of the ligand with
the metal represents an important goal of this study. Induc-
tive effects and effects achieved by modification of the
p-system will be compared in 4,5-dichloro- and 4,5-dicy-
anoimidazolylidene ligands. Benzyl residues at N-atoms
were chosen in order to offer a possible kinetic trap of inter-
mediates by weak interaction of the residue with the metal
tion of the first stable free crystalline carbene by Arduengo
III et al. [3] in 1991, NHCs gained increasing attention as a
new class of ligands used in transition metal compounds [4]
and in catalysis [5]. The most unique feature of NHCs is the
formation of strong bonds to transition metals. The reason
for such behaviour is attributed to a strong r-donor ability
of this ligand class. However, noticeable participation of
the p-system of NHCs in the bonding to the metal has
recently been proposed on the basis of quantum chemical
studies [6]. Modification of electronic properties of NHC
ligands in order to improve the performance of molecular
catalysts is one of the main areas of our research. Introduc-
tion of electron withdrawing substituents or replacement of
carbon atoms with electronegative elements in the 4,5-posi-
*
Corresponding authors. Tel.: +49 89 289 13080; fax: +49 89 289 13473
(P. Harter).
E-mail address: lit@arthur.anorg.chemie.tu-muenchen.de (P. Ha¨rter).
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.01.039

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A. Bittermann et al. / Journal of Organometallic Chemistry 693 (2008) 2079–2090
Fig. 1. Carbene ligands.
centre. During preparation of this manuscript Bielawski
and coworkers reported the synthesis of similar NHC
ligands and their Rh complexes [11].
alkylation with benzyl bromide. Subsequent benzylation of
obtained product in toluene with another equivalent of the
benzyl bromide yielded 69% of the 4,5-dichloro-1,3-diben-
zylimidazoliumsalt 3a as an air stable white powder
(Schemes 1, 2).
2. Results and discussion
Synthesis of 4,5-dicyano-1,3-dibenzylimidazol-2-ylidene
was successful through a thermal decomposition of com-
pound 4, a new member of a compound class recently used
by Waymouth and Grubbs as a novel source of N-hetero-
cyclic carbenes [12]. Corresponding compounds were syn-
thesized by the reaction of N,N0-disubstituted diamines
and pentafluorobenzaldehyde in the presence of a catalytic
amount of acetic acid.
2.1. Synthesis of ligands
4,5-Dichloro-1-benzylimidazole was obtained in
straightforward way by deprotonation of 4,5-dichloroimi-
dazole with potassium carbonate in acetonitrile and further
a
In our own synthesis of NHC–pentafluorobenzene
adducts, we started with the acyclic commercially available
diaminomaleonitrile (DAMN). DAMN was smoothly con-
densed with benzaldehyde in methanol according to litera-
ture procedures [13]. N-Benzyl derivate was obtained upon
NaBH4 reduction of the resulting Schiff base. The second
formation of an imine and further reduction leads to
N,N0-dibenzyldiaminomaleonitrile (Scheme 3). Condensa-
tion of this sec-diamine with pentafluorobenzaldehyde in
acetic acid gave 4 in high yield within 48 h (Scheme 4).
Scheme 1. Synthesis of substituted imidazole 3.
2.1.1. TG-MS/DSC studies of 4
Decomposition of 4 was investigated by thermal gravi-
metric analysis and differential scanning calorimetry. The
corresponding plots are given in Fig. 2. Ten milligrams of
the sample were heated from 35 to 700 °C at the rate of
10 K minꢀ1. At the onset temperature of 138 °C a strong
exothermic peak occurs. This is accompanied by a sharp
increase of the ion current (167 amu) as detected by mass
spectrometry (max. at 157.9 °C), which indicates the loss
of pentafluorobenzene and formation of carbene 1c.
Scheme 2. Synthesis of imidazolium salt 2a and 3a.
Scheme 3. Synthesis of N,N0-dibenzyldiaminomaleonitrile [13].

A. Bittermann et al. / Journal of Organometallic Chemistry 693 (2008) 2079–2090
2081
Scheme 4. Synthesis of the NHC precursor 4.
Fig. 2. Thermogravimetric analysis (1), differential scanning calorimetry (2) and mass spectrometric ion current (3, for 167 amu) of compound 4 as a
function of the temperature.
Weight loss of 34% was observed between 138 and 180 °C.
This correlates with the theoretical weight loss of 37% of
pentafluorobenzene from 4. Because of these encouraging
results we decided to perform decomposition on prepara-
tive scale.
2.2. Synthesis of Rh(COD)(NHC)Cl
Rhodium complexes of 1a and 1b were prepared in good
yield by application of the established silver route [14–16]
Scheme 6. Synthesis of Rh(COD)(NHC)Cl complex 7.
Table 1
13C NMR chemical shifts d and Rh–Ccarbene coupling constants J of the
carbene carbon of the complexes Rh(COD)(NHC)Cl 5, 6, 7 in CDCl3
Complex
d CCarbene [ppm]
J(Rh–CCarbene) [Hz]
5
6
7
183.596
186.195
197.330
51.47
52.98
50.87
(Scheme 5). As mentioned above carbene 1c could be
generated by thermal decomposition of 4. Heating the
Scheme 5. Synthesis of Rh(COD)(NHC)Cl complex 5 and 6.

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A. Bittermann et al. / Journal of Organometallic Chemistry 693 (2008) 2079–2090
Fig. 3. ORTEP style plot of compound 5 in the solid state. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms are omitted for
clarity.
mesitylene solution of 4 in order to achieve the generation
and subsequent isolation of free carbene 1c was not suc-
cessful, showing that carbene 1c is not stable in the free
state, at least under these conditions. Performing the reac-
tion in the presence of [Rh(COD)Cl]2 under the same con-
ditions, gave nearly quantitative yield (95%) of complex 7
(Scheme 6).
the benzyl group could be detected. Signal of benzyl –CH2–
showed no splitting even upon cooling to 193 K in our low
temperature 13C NMR studies of 5 and 7. This leads to the
conclusion that the conformation of these complexes can-
not be frozen down to this temperature.
2.2.1. Discussion of crystal structures 5, 6 and 7
13C NMR data of complexes 5–7 are listed in Table 1.
Chemical shifts of the Rh–Ccarbene doublet are subse-
quently increasing from complex 5 to 7. The corresponding
resonance of 6 can be found at d = 186.2 ppm (JC–
Rh = 52.9 Hz) and in the 13C NMR spectrum of 7, carbene
C signal is at d = 197.3 ppm (JC–Rh = 50.9 Hz). Analo-
gous monocarbene chloro complexes described in the liter-
ature show 13C NMR carbene carbon signal at d = 211.0
ppm (JC–Rh = 51 Hz) for a CC-saturated NHC system,
d = 194.8 ppm (JC–Rh = 49 Hz) for a benzimidazol-2-yli-
dene system and d = 180.1 ppm (JC–Rh = 53 Hz) for a
CC-unsaturated NHC system [17]. Carbene signals of
ligands 1a–c can be found in the same region as ben-
zimidazol-2-ylidene ligands, which indicates the lack of
direct correlation between chemical shift and donor
strength.
Rh(COD)(NHC)Cl complexes bearing 4,5-acceptor
substituted imidazol-2-ylidene ligands are rarely studied
by the means of X-ray analysis. Yellow single-crystals of
5, 6 and 7 suitable for X-ray diffraction studies were
obtained by recrystallisation from dichloromethane, ace-
tone and acetonitrile solutions, respectively. The molecular
structures of 5, 6 and 7 are shown in Figs. 3–5. A selection
of characteristic bond angles and bond distances is given in
Table 2. All X-ray single-crystal diffraction studies reveal
the expected square planar arrangement of the ligands at
the metal centre. Complex 5 crystallized in a monoclinic
crystal system in the space group p21/n. Compound 6 crys-
ꢀ
tallized in a triclinic crystal system in the space group P1
and 7 crystallized in an orthorhombic crystal system in
the space group Pna21.
The Rh–carbene bond length decreases from complex 5
˚
˚
˚
The chiral nature of new rhodium complexes 5–7 was
proved by means of 1H NMR spectroscopy where two sep-
arate doublets associated with diastereotopic hydrogens of
(2.027(2) A) to 6 (2.015(5)/2.017(5) A) to 7 (1.999(2) A).
Stronger acceptor substituents at 4,5-positions of the imi-
dazol-2-ylidene lead to stronger p back-donation from

A. Bittermann et al. / Journal of Organometallic Chemistry 693 (2008) 2079–2090
2083
Fig. 5. ORTEP style plot of compound 7 in the solid state. Thermal
ellipsoids are drawn at the 50% probability level. Hydrogen atoms are
omitted for clarity.
Fig. 4. ORTEP style plot of compound 6 (molecule A) in the solid state.
Thermal ellipsoids are drawn at the 50% probability level. Hydrogen
atoms are omitted for clarity.
Table 2
˚
Selected bond distances (A) and angles (°) for 5, 6 and 7
5
6a
7
the metal to the carbene ligand resulting in shorter rho-
dium–C1 bond distance. Rh–C1 bond length in 7 is the
shortest one found in similar complexes in the literature.
Metal–carbene distances between 2.006 and 2.036 A have
been measured by other authors for comparable systems
[18,11,19].
The different trans influences of the carbene and chloride
ligands lead to different distances between the coordinated
COD carbon atoms and the Rh. As a result of the longer
distance to the metal, the C22@C23 double bond trans to
the NHC ligand is shorter than the C18@C19 bond, owing
to reduced back-donation from the metal to its p* orbital.
Rh1–Cl1
Rh1–C1
2.3754(7)
2.027(2)
2.116(3)
2.118(2)
2.214(3)
2.203(3)
1.396(4)
1.362(4)
1.358(3)
1.358(3)
1.332(3)
–
2.3733(13)/2.3822(13)
2.015(5)/2.017(5)
2.087(5)/2.091(5)
2.113(5)/2.102(5)
2.189(5)/2.212(5)
2.199(5)/2.191(5)
1.390(8)/1.387(7)
1.360(8)/1.356(8)
1.360(6)/1.344(6)
1.359(6)/1.360(6)
1.344(7)/1.337(7)
1.690(5)/1.689(5)
1.696(5)/1.692(5)
–
2.3692(6)
1.999(2)
2.122(3)
2.100(3)
2.227(3)
2.213(3)
1.394(5)
1.370(4)
1.376(3)
1.360(3)
1.362(3)
–
˚
Rh1–C18
Rh1–C19
Rh1–C22
Rh1–C23
C18–C19
C22–C23
C1–N1
C1–N2
C2–C3
C2–Cl2
C3–Cl3
–
–
–
–
–
C2–C26
C3–C27
C26–N3
C27–N4
1.419(3)
1.425(3)
1.140(4)
1.137(4)
88.64(7)
162.89(11)
158.45(10)
92.77(8)
90.34(9)
127.1(2)
128.4(2)
104.5(2)
177.3(3)
177.0(3)
–
–
–
2.3. Synthesis and analysis of Rh(CO)2(NHC)Cl
–
IR-spectroscopy is a valuable method allowing descrip-
tion of the bonding situation in metal carbonyl complexes.
The CO stretching frequency is directly proportional to the
back-donation from the metal centre to the CO ligand. r-
Donor ligands can therefore be compared to each other on
the basis of the wavenumber of the CO stretching vibration
band. A stronger r-donor is related to a lower wavenum-
ber of the carbon monoxide trans to the ligand [20].
Carbonyl substituted rhodium complexes 8, 9, 10 were
synthesized by passing carbon monoxide at room tempera-
Cl1–Rh1–C1
Cl1–Rh1–C18
Cl1–Rh1–C19
Cl1–Rh1–C22
Cl1–Rh1–C23
Rh1–C1–N1
Rh1–C1–N2
N1–C1–N2
C2–C26–N3
C3–C27–N4
a
89.13(6)
160.63(9)
160.78(6)
91.77(7)
91.69(7)
125.9(2)
129.9(2)
104.2(2)
–
87.80(14)/87.90(14)
158.77(16)/160.86(16)
162.45(16)/160.48(15)
89.89(15)/94.68(15)
92.83(15)/89.67(14)
126.6(3)/127.5(4)
129.1(4)/127.5(4)
103.9(4)/104.6(4)
–
–
–
In italic the corresponding values for the second molecule B.

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A. Bittermann et al. / Journal of Organometallic Chemistry 693 (2008) 2079–2090
Scheme 7. Synthesis of Rh(CO)2(NHC)Cl complexes 8–10.
Table 3
Carbonyl stretching frequencies m in KBr, 13C NMR chemical shifts d and
Rh–Ccarbene coupling constants J of the carbene carbon of the complexes
Rh(CO)2(NHC)Cl 8, 9, 10 in CDCl3
Fig. 6. IR-studies of dimerisation behaviour of 9 at 90 °C.
dene ligands show carbonyl stretching frequencies much
closer to tertiary phosphine ligands than to common
known NHCs [21,22].
Complex m(CO)sym.
m(CO)asym.
d CCarbene
[ppm]
J(Rh–CCarbene)
[Hz]
[cmꢀ1
]
[cmꢀ1
]
8
2074
2084
2088
1996
2003
2002
174.903
177.025
187.454
43.83
45.44
45.54
Strong influence of the residues R0 at N-atoms of the
NHC ligands was unexpectedly found. With R0 = Me the
CO stretching frequency is shifted by 10 cmꢀ1 to higher
wavenumbers compared to benzyl substituted ligand [11].
Because of this fact, concise conclusion about correlation
of m(CO) frequencies with the electronic properties of
NHC ligands cannot be made yet.
9
10
ture through a dichloromethane solution of COD substi-
tuted complexes 5, 6, 7 (Scheme 7). Cyclooctadiene was
quantitatively displaced within minutes by carbon monox-
ide [18]. This reaction can be visually followed by colour
change from bright to pale yellow. The CO stretching fre-
quencies were measured in KBr and are listed in Table 3.
Effects of the p-system of the NHC ligand lead to the
expected result, showing that 4,5-dicyano substituted imi-
dazol-2-ylidene has the lowest donating ability of NHC
ligands in this series, because of strong acceptor substitu-
ents in the backbone. Compared with IR data for the
related NHC complexes, in terms of the electronic ligand
properties, these carbenes seem to range between 4,5-dihy-
dro-imidazolin-2-ylidene ligands (m(CO) = 2072, 1999
cmꢀ1), triazol-2-ylidene ligands (m(CO) = 2078, 2006
cmꢀ1) and tetrazol-2-ylidene derivates (m(CO) = 2086,
2015 cmꢀ1) [17]. In contrast to Nolan and his group’s work
[21], who was performing fundamental studies on determi-
nation of electron-donating strength of various NHC
ligands previously, 4,5- acceptor substituted imidazol-2-yli-
cis-Configuration of CO ligands in these carbonyl com-
plexes was confirmed by IR and NMR spectroscopy. IR
spectra showed two strong CO stretching vibration bands
of similar intensity between 2088 and 1996 cmꢀ1. Further-
more three doublets could be detected in the 13C NMR
spectra between d = 187 and 175 ppm, two for carbonyl
and one for carbene carbons (Table 3).
All carbonyl complexes are air stable in solid state.
Moreover they are soluble in polar solvents such as dichlo-
romethane, chloroform, acetonitrile, DMSO and aromatic
solvents such as toluene, but insoluble in less polar solvents
such as n-pentane and n-hexane.
2.3.1. Dimerisation behaviour of Rh(CO)2(NHC)Cl
During stability experiments 100% conversion of 8 to
dimer 11 in dichloromethane was unexpectedly achieved.
This reaction implies spontaneous loss of one molecule of
Scheme 8. Dimerisation behaviour of Rh–NHC complexes.

A. Bittermann et al. / Journal of Organometallic Chemistry 693 (2008) 2079–2090
2085
Table 4
Selected bond distances and angles for 11
˚
Bond length (A)
Bond angles (°)
Rh–Cl
Rh–Cla
Rh–C1
Rh–C
2.3936(6)
2.4242(7)
1.983(3)
1.797(3)
1.152(4)
1.352(3)
1.354(4)
1.341(4)
Cl–Rh–Cla
Cl–Rh–C1
Cla–Rh–C1
C–Rh–C1
Cl–Rh–C
Cla–Rh–C
Rh–C1–N1
Rh–C1–N2
N1–C1–N2
84.92(2)
91.37(8)
175.09(8)
87.96(11)
174.62(9)
96.03(9)
127.7(2)
127.6(2)
104.7(2)
C–O
C1–N1
C1–N2
C2–C3
a
The symmetry operation to equivalent atom positions is (ꢀx, y,
Fig. 7. Peak area vs. time. First order reaction for 8, 9, 10.
0.5 ꢀ z).
by IR-spectroscopy utilizing CO vibration of the monomer
at 2079 cmꢀ1 for 8, 2084 cmꢀ1 for 9 and 2090 cmꢀ1 for 10,
respectively, since no overlaps with the other peaks were
detected. A gross estimation suggests a dimerisation on
the same time scale. This means that dimerisation reaction
is not the rate determining step. Fig. 6 demonstrates time
dependent IR-study of monomer–dimer behaviour on the
example of rhodium–NHC complex 9. The resulting dimers
showed CO peaks at 1959 cmꢀ1 for 11, 1968 cmꢀ1 for 12
and 1979 cmꢀ1 for 13, respectively.
A smaller increase in dimerisation rate of complex 9
compared to 8 indicates that CO elimination from 9 is con-
trolled by inductive effects. Higher elimination rate from 10
on the other hand can be attributed to mesomeric p-effects
from cyano substituents.
carbon monoxide and formation of the dimer. This conver-
sion was also repeated with complexes 8, 9 and 10 and is
shown in Scheme 8. To the best of our knowledge examples
of the formation of halogeno bridged dimers are only scar-
cely described in the literature with the exception of elegant
papers of Bielawski and Barluenga [23].
2.3.1.1. Kinetic studies. Twenty milligrams of Rh(CO)2-
(NHC)Cl complexes 8, 9, 10 were dissolved in 8 ml of
toluene and stirred in an oil bath at 90 °C under Schlenk
conditions while argon was bubbled through the reaction
solution to achieve a complete removal of evolving CO-
gas. As a result quantitative conversion to the dimers could
be reached after 45 h for complex 8, after 30 h for complex
9 and after 15 h for complex 10. The reaction was followed
Fig. 8. ORTEP style plot of compound 11 in the solid state. Thermal ellipsoids are drawn at the 50% probability level. Symmetry operation to equivalent
atom positions _a: ꢀx, y, 0.5 ꢀ z. Hydrogen atoms are omitted for clarity.

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A. Bittermann et al. / Journal of Organometallic Chemistry 693 (2008) 2079–2090
Reaction order for the rate-determining CO-elimination
dures [13,24]. All other starting materials were obtained
commercially and were used as received. All syntheses,
except as noted, were performed under an atmosphere of
argon, using solvents dried on an alumina-based solvent
purification system. Mesitylene was dried over LiAlH4.
1H and 13C NMR spectra were recorded on a JEOL
JMX-GX 400 MHz spectrometer at room temperature
and referenced to the residual 1H and 13C signals of the sol-
vents. Chemical shifts are given in ppm. NMR multiplici-
ties are abbreviated as follows: s = singlet, d = doublet,
step was found by plotting the peak area vs. the time,
Fig. 7. First order reaction was found for complexes 8, 9
and 10. As a result of these measurements reaction rate
coefficient was shown to increase in the series from 8 (with
0.084 sꢀ1) to 9 (with 0.207 sꢀ1) to 10 (with 0.518 sꢀ1).
Dimerisation process is completely reversible for all
three complexes. The conversion back to the monomer
was completed after only 15 min by passing carbon monox-
ide through the toluene solution of [Rh(CO)(NHC)Cl]2.
This reaction could be followed by the colour change from
dark to pale yellow.
t = triplet,
q = quartet,
p = quintet,
sept = septet,
m = multiplet, br = broad signal. MS spectra were mea-
Reaction scheme described above is supported by these
kinetic results. Since 1c is a better p-acceptor ligand, higher
CO-elimination rate is favoured due to M–CO bond
destabilisation.
sured at the TU Munchen Mass Spectrometry Laboratory
¨
on a Finnigan MAT 90 mass spectrometer using the CI or
FAB technique. IR spectra were acquired using a Jasco
FT/IR-460 Plus spectrometer. Thermogravimetric mass
spectra (TGA) analysis measurements were conducted with
a Netzsch TG209 system; typically about 10 mg of each
sample was heated from 35 to 700 °C at 10 K minꢀ1. Ele-
mental analyses were carried out by the Microanalytical
2.4. Discussion of crystal structures 11
Crystal structure of 11 (Fig. 8) consists of discrete dinu-
clear units of the formula [Rh(CO)(C17H16N2)Cl]2. A selec-
tion of characteristic bond angles and bond distances is
given in Table 4. The complex crystallized out of dichloro-
methane in a monoclinic crystal system in the space group
C2/c. The bonding distance between Rh and C1 is
Laboratory at the TU Munchen. Melting points were mea-
¨
sured with a Buchi melting point apparatus system (Dr.
¨
Tottoli).
4.2. 4,5-Dichloro-1-benzylimidazole 3
˚
˚
1.983(3) A. The Rh–Cl distance averages 2.409 A. The
coordination around each rhodium atom is square-planar.
The dihedral angle between the two square planes of 55.92°
gives a bent configuration of the molecule in which the CO
and the carbene ligands are in a cis arrangement. Dimers
similar to our system are only rarely known. By the com-
parison of data presented in this paper with the results of
Bielawski et al. similarities in the crystal system and bond-
ing distances for [Rh(CO)(NHC)Cl]2 can be found [23].
4,5-Dichloro-1H-imidazole (0.80 g, 5.84 mmol) and
K2CO3 (1.21 g, 8.76 mmol) were stirred for 10 min in
30 ml of acetonitrile. Benzyl bromide (0.69 ml, 5.84 mmol)
was added in one portion and stirring was continued at
room temperature for further 2 days. After the solvent
was removed under reduced pressure 70 ml of water were
added. The aqueous phase was extracted with CH2Cl2
(4 ꢁ 20 ml). Organic phases were combined and dried over
sodium sulphate. 3 was obtained as orange solid after sol-
vent removal under reduced pressure (1.24 g, 93%).
3. Conclusion
Mp: 50–51 °C. 1H NMR (CDCl3): d = 7.32 (s, 1H,
NCHN), 7.24 (m, 3H, Cbenzyl), 7.08 (m, 2H, Cbenzyl), 4.95
(s, 2H, CH2). 13C NMR (CDCl3): d = 134.32 (NCHN),
134.15, 128.71, 128.17, 127.06 (Cbenzyl), 125.80, 113.15
(CCl), 49.34 (CH2). IR (KBr): mmax/cmꢀ1 = 3122 (C–Cl),
1520, 1496, 1483, 1455, 1437, 1388, 1346, 1254, 1180,
1115, 976, 808, 721, 694, 663, 638, 503. MS (CI): m/z
227.1 (100%, M+H+), 226.1 (4%, M+).
New types of 4,5-acceptor substituted NHC ligands have
been synthesized. Neutral rhodium complexes were obtained
either via in situ transmetalation from silver complexes or via
thermaldecompositionofpentafluorobenzeneadducts.Intro-
duction of acceptor substituents such as chloro- or cyano-
groups at the 4,5-positions of imidazol-2-ylidene reduces
donor strength of the ligands either due inductive (Cl) or p-
acceptor (CN) effects. This was supported by dimerisation
behaviour analysis of NHC–rhodium carbonyl complexes.
Theoretical studies of these effects are currently on investiga-
tion. Since N-substitution has a strong influence on the elec-
tronic properties, NHC ligands will be further modified in
our laboratory. We shall report on this in due time soon.
4.3. 4,5-Dichloro-1,3-dibenzylimidazoliumbromide 3a
Benzyl bromide (0.79 ml, 6.61 mmol) was added in one
portion to a stirred suspension of 4,5-dichloro-1-benzylimi-
dazole (0.50 g, 2.20 mmol) in 5 ml of toluene. The mixture
was stirred for 40 h at room temperature. Afterwards the
solvent was removed under reduced pressure. The residue
was washed with THF (4 ꢁ 5 ml) and the title compound
was isolated as a white solid (609 mg, 69%).
4. Experimental
4.1. General
1
Mp: 177–178 °C. H NMR (CDCl3): d = 11.78 (s, 1H,
1,3-Diphenylimidazolium salt, N,N0-dibenzyldiamino-
maleonitrile were prepared according to literature proce-
NCHN), 7.54 (m, 4H, Cbenzyl), 7.32 (m, 6H, Cbenzyl), 5.63
(s, 4H, CH2). 13C NMR (CDCl3): d = 138.03 (NCHN),

A. Bittermann et al. / Journal of Organometallic Chemistry 693 (2008) 2079–2090
2087
131.73, 129.61, 129.47, 128.99 (Cbenzyl), 119.36 (CCl), 52.83
(CH2). IR (KBr): mmax/cmꢀ1 = 3098, 3012, 2861 (C–Cl),
1586, 1550, 1498, 1456, 1405, 1363, 1343, 1304, 1205,
1181, 1146, 1078, 1032, 769, 731, 709, 697, 607. Anal. Calc.
for C17H15Cl2N2Br (398.12): C, 51.3; H, 3.8; N, 7.0; Cl,
17.8. Found: C, 51.24; H, 3.72; N, 7.05; Cl, 18.21%. MS
(FAB): m/z 319.1 (67%, M+ꢀBr), 317.1 (100%,
M+ꢀBrꢀ2H).
C–H) = 14.8 Hz, CH2benzyl), 5.76 (d, 2H, 2J(H–C–
H) = 14.8 Hz, CH2benzyl), 5.07 (m, 2H, codvinyl), 3.32 (m,
2H, codvinyl), 2.31 (m, 4H, codallyl), 1.88 (m, 4H, codallyl).
13C NMR (CDCl3): d = 183.60 (d, 1J(C–Rh) = 51.5 Hz,
NCN), 136.54, 128.96, 128.26, 128.18, (Cbenzyl), 120.98
(Cimidazol), 98.90, 98.83, 68.49, 68.35 (codvinyl), 54.69
(CH2benzyl), 32.94, 28.85 (codallyl). IR (KBr): mmax
/
cmꢀ1 = 2933, 2905, 2873, 2832, 1637, 1619, 1496, 1453,
1416, 1398, 1361, 1223, 1078, 1029, 960, 766, 727, 715,
699. MS (FAB): m/z 494.1 (4%, M+), 459.1 (21%,
M+ꢀCl), 386.0 (38%, M+ꢀCOD), 351.1 (100%,
M+ꢀClꢀCOD).
4.4. 4,5-Dicyano-1,3-dibenzyl-2-(pentafluorophenyl)-2,3-
dihydro-imidazole 4
In a 5 ml vial equipped with a magnetic stirrer bar pen-
tafluorobenzaldehyde (1.36 g, 6.94 mmol) was dissolved in
1 ml of acetic acid. N,N0-Dibenzyldiaminomaleonitrile
(1.00 g, 3.47 mmol) was added in one portion and the reac-
tion mixture was stirred at room temperature for 2 days.
The brown precipitate was filtered off and washed once
with 5 ml of water and three times with 3 ml of cold meth-
anol. Compound 4 was obtained as a light yellow solid
after drying under reduced pressure (1.317 g, 81%).
4.5.2. Chloro(g4-1,5-COD)(4,5-dichloro-1,3-
dibenzylimidazol-2-yliden)rhodium(I) 6
Two hundred milligrams (0.502 mmol) of 3a, 58 mg
(0.251 mmol) of silver(I) oxide and 124 mg (0.251 mmol)
of [Rh(COD)Cl]2 yielded 6 as of a yellow solid (142 mg,
100%).
1
Mp: 214 °C (decomp.). H NMR (CDCl3): d = 7.37 (m,
10H, CHbenzyl), 6.04 (d, 2H, 2J(H–C–H) = 15.6 Hz,
Mp: 155 °C (decomp.). 1H NMR (CDCl3): d = 7.22 (br,
6H, Cbenzyl), 7.04 (br, 4H, Cbenzyl), 5.79 (s, 1H, NCHN),
4.09 (d, 2H, 2J(H–C–H) = 14.8 Hz, CH2benzyl), 3.94 (d,
2H, 2J(H–C–H) = 14.4 Hz, CH2benzyl). 13C NMR (CDCl3):
d = 146.50, 143.98, 140.49, 138.49, 136.00 (m, CF), 133.22,
128.85, 128.76 (Cbenzyl), 113.88 (CCN), 112.40 (m, CCF),
110.61 (NCCN), 77.66 (NCHN), 54.19 (CH2). IR (KBr):
CH2benzyl), 5.89 (d, 2H, J(H–C–H) = 15.6 Hz, CH2benzyl),
2
5.06 (m, 2H, codvinyl), 3.07 (m, 2H, codvinyl), 2.24 (m, 2H,
codallyl), 2.01 (m, 2H, codallyl), 1.82 (m, 2H, codallyl), 1.68
(m, 2H, codallyl). 13C NMR (CDCl3): d = 186.20 (d,
1J(C–Rh) = 53.0 Hz, NCN), 135.82, 128.90, 128.01,
127.07 (Cbenzyl), 117.20 (CCl), 99.71, 99.64, 69.81, 69.67
(codvinyl), 54.00 (CH2benzyl), 32.66, 28.67 (codallyl). IR
m
max/cmꢀ1 = 2215 (CN), 1655, 1592, 1522, 1511, 1456,
(KBr):
m
max/cmꢀ1 = 2934, 2914, 2878 (C–Cl), 2830
1383, 1359, 1209, 1188, 1150, 1002, 921, 747, 703, 693.
Anal. Calc. for C25H15F5N4 (466.41): C, 64.4; H, 3.24; N,
12.0. Found: C, 64.5; H, 3.10; N, 11.6%. MS (FAB): m/z
467.1 (52%, M+H+), 466.1 (100%, M+).
(C–Cl), 1637, 1618, 1592, 1495, 1454, 1429, 1389, 1350,
1329, 1216, 968, 722, 692. Anal. Calc. for C25H26Cl3N2Rh
(563.75): C, 53.3; H, 4.6; N, 4.97. Found: C, 53.96; H, 4.83;
N, 4.81%. MS (FAB): m/z 562.0 (8%, M+), 527.1 (44%,
M+ꢀCl), 419.0 (100%, M+ꢀClꢀCOD).
4.6. Chloro(g4-1,5-COD)(4,5-dicyano-1,3-
dibenzylimidazol-2-yliden)rhodium(I) 7
4.5. General procedure for the preparation of NHC
complexes 5 and 6
To the solution of imidazolium salt (2 equiv.) in CH2Cl2
(20 ml) silver(I) oxide (1 equiv.) was added in one portion.
Resulting suspension was stirred for 3 h in the darkness,
during which time the black colour gradually diminished.
The reaction mixture was filtered through a small pad of
Celite and 1 equiv. of [Rh(COD)Cl]2 was added in one por-
tion. Almost immediately a white precipitate of silver salt
was formed. The reaction mixture was stirred for addi-
tional 18 h. The solvent was evaporated and the residue
was purified by flash chromatography on silica gel with
CH2Cl2 as eluent.
Ninety-seven milligrams (0.208 mmol) of 4 were dis-
solved in 6 ml of dry mesitylene. 51 mg (0.104 mmol) of
[Rh(COD)Cl]2 were added in one portion and the mixture
was heated to 165 °C. After 16 h the solvent was evapo-
rated and the residue was purified by flash chromatography
on silica gel with CH2Cl2 as eluent. Recrystallisation out of
1-propanole yielded 7 (54 mg, 95%) as a yellow solid.
1
Mp: 132 °C (decomp.). H NMR (CDCl3): d = 7.47 (m,
10H, CHbenzyl), 6.26 (d, 2H, 2J(H–C–H) = 15.2 Hz,
2
2
CH2benzyl), 5.89 (d, H, J(H–C–H) = 15.2 Hz, CH2benzyl),
5.24 (m, 2H, codvinyl), 3.16 (m, 2H, codvinyl), 2.35 (m, 2H,
codallyl), 2.22 (m, 2H, codallyl), 1.92 (m, 4H, codallyl). 13C
NMR (CDCl3): d = 197.33 (d, 1J(C–Rh) = 50.9 Hz,
NCN), 133.54, 129.38, 128.44 (Cbenzyl), 115.76 (CN),
106.43 (CCN), 102.32, 102.25, 70.66, 70.52 (codvinyl),
4.5.1. Chloro(g4-1,5-COD)(1,3-dibenzylimidazol-2-
yliden)rhodium(I) 5
Two hundred milligrams (0.607 mmol) of 2a, 70 mg
(0.304 mmol) of silver(I) oxide and 150 mg (0.304 mmol)
of [Rh(COD)Cl]2 yielded 5 as a yellow solid (150 mg,
100%).
55.88 (CH2benzyl), 32.73, 28.68 (codallyl). IR (KBr): mmax
/
cmꢀ1 = 2938, 2914, 2882, 2830, 2360, 2342, 2238 (CN),
1637, 1617,1497, 1459, 1440, 1400, 1332, 1214, 735, 697.
Anal. Calc. for C27H26ClN4Rh (544.85): C, 59.5; H, 4.81;
1
Mp: 208 °C (decomp.). H NMR (CDCl3): d = 7.38 (m,
10H, CHbenzyl), 6.65 (s, 2H, CHimidazol), 5.88 (d, 2H, 2J(H–

2088
A. Bittermann et al. / Journal of Organometallic Chemistry 693 (2008) 2079–2090
1
N 10.3. Found: C, 59.4; H, 5.21; N, 9.82%. MS (FAB): m/z
544.0 (18%, M+), 509.1 (57%, M+ꢀCl), 436.0 (39%
M+ꢀCOD), 401.0 (100%, M+ꢀClꢀCOD).
(CDCl3): d = 187.45 (d, J(C–Rh) = 45.5 Hz, NCN), 184.05
(d, 1J(C–Rh) = 55.6 Hz, CO), 180.93 (d, 1J(C–Rh) =
72.6 Hz, CO), 132.60, 129.91, 129.53, 128.86 (Cbenzyl),
116.08 (CCN), 106.13 (CN), 56.42 (CH2benzyl). IR (KBr):
4.7. General procedure for the preparation of dicarbonyl
complexes
m
max/cmꢀ1 = 2241 (CN), 2088 (CO), 2002 (CO), 1974,
1624, 1605, 1497, 1460, 1443, 1401, 1344, 1205, 743, 697.
Anal. Calc. for C21H14ClN4O2Rh (492.73): C, 51.20; H,
2.86; N, 11.40. Found: C, 52.10; H, 3.32; N, 11.60%. MS
(FAB): m/z 492.0 (3%, M+), 436.1 (44%, M+ꢀ2CO),
401.1 (100%, M+ꢀ2COꢀCl).
Carbon monoxide was bubbled for 45 min through a
solution of NHC rhodium complex in CH2Cl2. After the
colour had changed to pale yellow, the solvent was
removed in vacuo and the crude product was washed with
n-pentane (2 ꢁ 5 ml).
4.8. Single-crystal X-ray structure determination of
compounds 5, 6, 7, and 11
4.7.1. Chloro-dicarbonyl(1,3-dibenzylimidazol-2-
yliden)rhodium(I) 8
4.8.1. General
Two hundred and fifty milligrams (0.505 mmol) of 5
yielded 8 as a light yellow solid (207 mg, 95%). Mp:
Crystal data and details of the structure determination
are presented in Table 5. Suitable single-crystals for the
X-ray diffraction study were grown with standard cooling
techniques. Crystals were stored under perfluorinated
ether, transferred in a Lindemann capillary, fixed, and
sealed. Preliminary examination and data collection were
carried out on an area detecting system and graphite-
1
211 °C (decomp.). H NMR (CDCl3): d = 7.36 (m, 10H,
2
CHbenzyl), 6.85 (s, 2H, CHimidazol), 5.56 (d, 2H, J(H–C–
H) = 14.0 Hz, CH2benzyl), 5.47 (d, 2H, 2J(H–C–
H) = 14.4 Hz, CH2benzyl). 13C NMR (CDCl3): d = 185.44
(d, 1J(C–Rh) = 54.1 Hz, CO), 182.34 (d, 1J(C–
Rh) = 74.3 Hz, CO), 174.90 (d, 1J(C–Rh) = 43.8 Hz,
NCN), 135.54, 129.01, 128.54, 128.32 (Cbenzyl), 121.91
(Cimidazol), 55.10 (CH2benzyl). IR (KBr): mmax/cmꢀ1 = 2074
(CO), 1996 (CO), 1637, 1605, 1564, 1496, 1452, 1416,
1358, 1236, 1173, 763, 744, 727, 713, 693, 584. Anal. Calc.
for C19H16ClN2O2Rh (442.70): C, 51.54; H, 3.64; N. 6.32.
Found: C, 51.64; H, 3.74; N, 6.25%. MS (FAB): m/z
442.0 (4%, M+), 386.0 (53%, M+ꢀ2CO), 351.1 (100%,
M+ꢀ2COꢀCl).
˚
monochromated Mo Ka radiation (k = 0.71073 A). The
unit cell parameters were obtained by full-matrix least-
squares refinements during the scaling procedure. Data
collections for 6, 7, and 11 were performed at low temper-
atures (Oxford Cryosystems cooling device). Each crystal
was measured with a couple of data sets in rotation scan
modus. Intensities were integrated and the raw data were
corrected for Lorentz, polarization, and, arising from the
scaling procedure for latent decay and absorption effects.
The structures were solved by a combination of direct
methods and difference Fourier syntheses. All non-hydro-
gen atoms were refined with anisotropic displacement
parameters. All hydrogen atoms were placed in ideal posi-
tions and refined using a riding model, with methylene and
4.7.2. Chloro-dicarbonyl(4,5-dichloro-1,3-dibenzylimidazol-
2-yliden)rhodium(I) 9
One hundred and fifty milligrams (0.266 mmol) of 6
yielded 9 as a light yellow solid (125 mg, 92%). Mp:
1
˚
222 °C (decomp.). H NMR (CDCl3): d = 7.38 (m, 10H,
aromatic dC–H distances of (1.00, 0.99 or 0.97 A) and (0.95
2
˚
CHbenzyl), 5.77 (d, 2H, J(H–C–H) = 15.2 Hz, CH2benzyl),
or 0.93 A), respectively, and Uiso(H) = 1.2Ueq(C). Full-
2
5.53 (d, 2H, J(H–C–H) = 15.6 Hz, CH2benzyl). 13C NMR
matrix least-squares refinements were carried out by mini-
P
1
(CDCl3): d = 184.78 (d, J(C–Rh) = 55.1 Hz, CO), 181.41
mizing w(F2o ꢀ F2c)2 with the SHELXL-97 weighting scheme
and stopped at shift/err <0.001. The final residual electron
density maps showed no remarkable features. Neutral
atom scattering factors for all atoms and anomalous dis-
persion corrections for the non-hydrogen atoms were taken
from International Tables for Crystallography. All calcula-
tions were performed with the WINGX system, including the
programs PLATON, SHELXL-97, and SIR92 [25]. Specials:
Compound 5: (Stoe & Cie. GmbH, IPDS 2T; rotating
anode, Nonius FR591; two data sets in rotation scan
modus with Dx = 1.00°; dx = 120; T = 293 K). Compound
6: (Oxford Diffraction, XCALIBUR, j-CCD; sealed tube,
Enhance X-ray Source, Spellman, DF3; five data sets in
rotation scan modus with D//Dx = 2.00°; dx = 50;
T = 150 K). The low scattering power of the very tiny crys-
tal forced us to cut the data set at h = 23.36°. The asym-
metric unit contains two crystallographic independent
molecules A and B of the target compound 6. Compound
(d, 1J(C–Rh) = 73.7 Hz, CO), 177.03 (d, 1J(C–
Rh) = 45.4 Hz, NCN), 134.38, 128.95, 128.55, 127.64
(Cbenzyl), 118.34, 118.32 (CCl), 54.03 (CH2benzyl). IR
(KBr): mmax/cmꢀ1 = 2084 (CO), 2003 (CO), 1637, 1619,
1590, 1496, 1456, 1438, 1399, 1350, 1211, 725, 693, 584.
Anal. Calc. for C19H14Cl3N2O2Rh (511.59): C, 44.6; H,
2.76; N, 5.48. Found: C, 44.67; H, 2.89; N, 5.46%. MS
(FAB): m/z 509.9 (5%, M+), 453.9 (49%, M+ꢀ2CO),
418.9 (100%, M+ꢀ2COꢀCl).
4.7.3. Chloro-dicarbonyl(4,5-dicyano-1,3-dibenzylimidazol-
2-yliden)rhodium(I) 10
One hundred and eighteen milligrams (0.217 mmol) of 7
yielded 10 as a light yellow solid (107 mg, 100%). Mp:
1
158 °C (decomp.). H NMR (CDCl3): d = 7.46 (m, 10H,
2
CHbenzyl), 5.83 (d, 2H, J(H–C–H) = 14.8 Hz, CH2benzyl),
2
5.70 (d, 2H, J(H–C–H) = 15.2 Hz, CH2benzyl). 13C NMR

A. Bittermann et al. / Journal of Organometallic Chemistry 693 (2008) 2079–2090
2089
Table 5
Crystallographic data for compounds 5, 6, 7, and 11
5
6
7
11
Formula
Fw
C25H28ClN2Rh
494.85
C25H26Cl3N2Rh
563.74
C27H26ClN4Rh
544.88
C36H32Cl2N4O2Rh2
829.38
Colour/habit
Crystal dimensions (mm)
Crystal system
Space group
Yellow/prism
0.18 ꢁ 0.20 ꢁ 0.53
Monoclinic
P21/n (no. 14)
12.1509(5)
10.8551(6)
17.2545(7)
90
101.815(3)
90
2227.64(18)
4
Yellow/fragment
0.02 ꢁ 0.08 ꢁ 0.10
Triclinic
Yellow/fragment
0.20 ꢁ 0.30 ꢁ 0.40
Orthorhombic
Pna21 (no. 33)
13.8499(1)
17.8600(1)
9.7770(1)
90
Orange/needle
0.05 ꢁ 0.25 ꢁ 1.02
Monoclinic
C2/c (no. 15)
25.1555(3)
9.7908(1)
13.5338(2)
90
95.387(1)
90
3318.55(7)
4
ꢀ
P1 (no. 2)
˚
a (A)
6.6565(4)
14.765(1)
24.472(2)
98.875(6)
94.681(5)
98.282(5)
2338.3(3)
4
˚
b (A)
˚
c (A)
a (°)
b (°)
c (°)
90
90
3
˚
V (A )
2418.43(3)
4
Z
T (K)
293
1.475
0.900
1016
150
1.601
1.089
1144
150
1.497
0.839
1112
153
1.660
1.195
1664
Dcalc (g cmꢀ3
)
l (mmꢀ1
F(000)
)
h Range (°)
Index ranges (h,k,l)
3.88–25.25
14, 13, 20
27660
4016/0.033
3372
4016/0/262
0.0230/0.0579
0.0297/0.0591
0.978
3.04–23.36
7, 16, 27
23652
6768/0.045
4797
6768/0/559
0.0365/0.0681
0.0609/0.0797
1.095
2.79–27.85
18, 23, 12
54826
5756/0.016
5496
5756/1/298
0.0234/0.0553
0.0255/0.0578
1.127
2.74–25.35
30, 11, 16
29441
3029/0.019
2726
3029/0/208
0.0240/0.0554
0.0293/0.0608
1.212
Number of reflections collected
Number of independent reflections/Rint
Number of observed reflections [Io > 2r(Io)]
Number of data/restraints/parameters
R1/wR2 [Io > 2r(Io)]a
R1/wR2 (all data)a
GOF (on F2)a
ꢀ3
˚
Largest difference in peak and hole (e A
)
+0.32/ꢀ0.38
+0.57/ꢀ0.45
+0.60/ꢀ0.55
.
+0.43/ꢀ0.62
P
P
P
P
a
R1 = (kFoj ꢀ jFck)/RjFoj; wR2 = { [w(F2o ꢀ Fc2)2]/ [w(F2o)2]}1/2; GOF = { [w(Fo2 ꢀ Fc2)2]/(n ꢀ p)}1/2
[2] H.W. Wanzlick, H.J. Scho¨nherr, Angew. Chem., Int. Ed. 7 (1968)
141.
[3] (a) A.J. Arduengo III, R.L. Harlow, M. Kline, J. Am. Chem. Soc.
113 (1991) 361;
(b) A.J. Arduengo III, H.V.R. Dias, R.L. Harlow, M. Kline, J. Am.
Chem. Soc. 114 (1992) 5530.
[4] (a) W.A. Herrmann, C. Ko¨cher, Angew. Chem., Int. Ed. 36 (1997)
2162;
7: (Oxford Diffraction, XCALIBUR, j-CCD; sealed tube,
Enhance X-ray Source, Spellman, DF3; nine data sets in
rotation scan modus with D//Dx = 1.00°; dx = 50;
T = 150 K). The correct enantiomer is proved by Flack’s
parameter e = 0.03(2). Compound 11: (Oxford Diffraction,
XCALIBUR, j-CCD; sealed tube, Enhance X-ray Source,
Spellman, DF3; nine data sets in rotation scan modus with
D//Dx = 1.00°; dx = 50; T = 153 K).
(b) E.A.B. Kantchev, C.J. O’Brian, M.G. Organ, Angew. Chem., Int.
Ed. 46 (2007) 2768;
(c) T. Weskamp, V.P.W. Bo¨hm, W.A. Herrmann, J. Organomet.
Chem. 600 (2000) 12.
[5] (a) W.A. Herrman, Angew. Chem., Int. Ed. 41 (2002) 1290;
(b) S.P. Nolan (Ed.), N-Heterocyclic Carbenes in Synthesis, Wiley-
VCH, 2006, ISBN 3-527-31400-8.
Acknowledgement
We gratefully acknowledge financial support from the
BMBF (Bundesministerium fur Bildung und Forschung).
¨
[6] S.K. Schneider, P. Roembke, G.R. Julius, C. Loschen, H.G.
Raubenheimer, G. Frenking, W.A. Herrmann, Eur. J. Inorg. Chem.
15 (2005) 2973.
[7] (a) Q. Yao, M. Zabawa, J. Woo, C. Zheng, J. Am. Chem. Soc. 129
(2007) 3088;
Appendix A. Supplementary material
CCDC 676401, 676402, 676403 and 676404 contain the
supplementary crystallographic data for 5, 6, 7, and 11.
These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif. Supplementary data associated
with this article can be found, in the online version, at
doi:10.1016/j.jorganchem.2008.01.039.
(b) S.G. Fiddy, J. Evans, T. Neisius, M.A. Newton, N. Tsoureas,
A.A.D. Tulloch, A.A. Danopoulos, Chem. Eur. J. 13 (2007) 3652;
(c) M.-T. Lee, H.M. Lee, C.-H. Hu, Organometallics 26 (2007) 1317;
(d) K. Hara, Y. Kanamori, M. Sawamura, Bull. Chem. Soc. Jpn. 79
(2006) 1781.
[8] V. Lavallo, Y. Canac, B. Donnadieu, W.W. Schoeller, G. Bertrand,
Angew. Chem. 118 (2006) 3568.
[9] G.D. Frey, V. Lavallo, B. Donnadieu, W.W. Schoeller, G. Bertrand,
Science 316 (2007) 439.
´
´
[10] (a) M. Viciano, E. Mas-Marza, M. Sanau, E. Peris, Organometallics
References
25 (2006) 3063;
´
´
´
(b) R. Corberan, J. Ramırez, M. Poyatos, E. Peris, E. Fernandez,
¨
[1] K.J. Ofele, J. Organomet. Chem. 12 (1968) P42.
Tetrahedron: Asymmetry 17 (2006) 1759;

2090
A. Bittermann et al. / Journal of Organometallic Chemistry 693 (2008) 2079–2090
´
(c) V. Lillo, J. Mata, J. Ramırez, E. Peris, E. Fernandez, Organo-
metallics 25 (2006) 5829;
[20] K. Denk, P. Sirsch, W.A. Herrmann, J. Organomet. Chem. 649
(2002) 219.
´
´
(d) C. Vicent, M. Viciano, E. Mas-Marza, M. Sanau, E. Peris,
Organometallics 25 (2006) 3713;
[21] R.A. Kelly, H. Clavier, S. Giudice, N.M. Scott, E.D. Stevens, J.
Bordner, I. Samardjiev, C.D. Hoff, L. Cavallo, S.P. Nolan, Organo-
metallics 27 (2008) 202.
´
´
(e) R. Corberan, M. Sanau, E. Peris, J. Am. Chem. Soc. 128(2006)3974.
[11] D.M. Khramov, V.M. Lynch, C.W. Bielawski, Organometallics 26
(2007) 6042.
[12] (a) G.W. Nyce, S. Csihony, R.M. Waymouth, J.L. Hedrick, Chem.
Eur. J. 10 (2004) 4073;
[22] (a) W.A. Herrmann, D. Baskakov, E. Herdtweck, S.D. Hoffmann,
T. Bunlaksananusorn, F. Rampf, L. Rodefeld, Organometallics 25
(2006) 2449;
(b) G.D. Frey, C.F. Rentzsch, D. von Preysing, T. Scherg, M.
(b) A.P. Blum, T. Ritter, R.H. Grubbs, Organometallics26(2007)2122.
[13] R.W. Begland, D.R. Hartter, F.N. Jones, D.J. Sam, W.A. Sheppard,
O.W. Webster, F.J. Weigert, J. Org. Chem. 39 (1974) 2341.
[14] H.M.J. Wang, I.J.B. Lin, Organometallics 17 (1998) 972.
[15] (a) A.R. Chianese, X. Li, M.C. Janzen, J.W. Faller, R.H. Crabtree,
Organometallics 22 (2003) 1663;
Muhlhofer, E. Herdtweck, W.A. Herrmann, J. Organomet. Chem.
¨
691 (2006) 5725.
[23] (a) M.D. Sanderson, J.W. Kaupain, C.W. Bielawski, J. Am. Chem.
Soc. 128 (2006) 16514;
´
´
(b) J. Barluenga, R. Vicente, l.A. Lopez, M. Tomas, J. Organomet.
Chem. 691 (2006) 5642.
(b) B. Bildstein, M. Malaun, H. Kopacka, K. Wurst, M. Mitterbo¨ck,
K.H. Ongania, G. Opromolla, P. Zanello, Organometallics 18 (1999)
4325;
(c) C.K. Lee, K.M. Lee, I.J.B. Lin, Organometallics 21 (2002) 10;
(d) K.M. Lee, H.M.J. Wang, I.J.B. Lin, J. Chem. Soc., Dalton Trans.
(2002) 2852.
[24] O.V. Starikova, G.V. Dolgushin, L.I. Larina, P.E. Ushakov, T.N.
Komarova, V.A. Lopyrev, Russ. J. Org. Chem. 39 (2003) 1467.
[25] (a) CRYSALIS Data Collection Software and Data Processing Software
for Oxford Diffraction Xcalibur devices, Version 1.171, Oxford
Diffraction Ltd., Oxfordshire, United Kingdom, 2005;
(b) Data Collection Software and Data Processing Software for Stoe
IPDS 2T diffractometer, X-AREA, Version 1.26, Stoe & Cie, Darmstadt,
Germany, 2004;
[16] (a) D. Baskakov, W.A. Herrmann, E. Herdtweck, S.D. Hoffmann,
Organometallics 26 (2007) 626;
(b) A. Kascatan-Nebioglu, M.J. Panter, J.C. Garrison, C.A. Tessier,
W.J. Youngs, Organometallics 23 (2004) 1928.
[17] W.A. Herrmann, J. Schutz, G.D. Frey, E. Herdtweck, Organometal-
¨
lics 25 (2006) 2437.
[18] W.A. Herrmann, M. Elison, F. Fischer, C. Ko¨cher, G.R.J. Artus,
Chem. Eur. J. 2 (1996) 772.
(c) Data Processing Software for Stoe IPDS 2T diffractometer, XRED,
XSHAPE, Version 1.26, Stoe & Cie, Darmstadt, Germany, 2004;
(d) A. Altomare, G. Cascarano, C. Giacovazzo, A. Guagliardi,
M.C. Burla, G. Polidori, M. Camalli, J. Appl. Crystallogr. 27
(1994) 435;
(e)A.J.C. Wilson (Ed.), International Tables for Crystallography,
vol. C, Kluwer Academic Publishers, Dordrecht, The Nether-
lands, 1992, Tables 6.1.1.4, 4.2.6.8, and 4.2.4.2;
[19] (a) K.H. Park, S.Y. Kim, S.U. Son, Y.K. Chung, Eur. J. Org. Chem.
(2003) 4341;
(b) H. Seo, B.Y. Kim, J.H. Lee, H.J. Park, S.U. Son, Y.K. Chung,
Organometallics 22 (2003) 4783;
(f) G.M. Sheldrick, SHELXL-97, Universita¨t Go¨ttingen, Go¨ttingen,
Germany, 1998;
(c) M.V. Baker, S.K. Brayshaw, B.W. Skelton, A.H. White, Inorg.
Chim. Acta 357 (2004) 2841;
(d) S. Saravanakumar, A.I. Oprea, M.K. Kindermann, P.G. Jones,
J. Heinicke, Chem. Eur. J. 12 (2006) 3143.
(g) A.L. Spek, PLATON, A Multipurpose Crystallographic Tool,
Utrecht University, Utrecht, The Netherlands, 2001;
(h) L.J. Farrugia, J. Appl. Crystallogr. 32 (1999) 837.