Metal complexes of very bulky N,N′-diarylimidazolylidene N-heterocyclic carbene (NHC) ligands with 2,4,6-cycloalkyl substituents

Article abstract of DOI:10.1002/ejic.201402830

1,3,5-Tricycloalkylbenzene (cycloalkyl = C₅H₉, C₆H₁₁) was converted into the respective anilines (by means of nitration and reduction) and then into the corresponding diimines (with glyoxal), the cyclization of

Full text of DOI:10.1002/ejic.201402830

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DOI:10.1002/ejic.201402830
Metal Complexes of Very Bulky N,NЈ-
Diarylimidazolylidene N-Heterocyclic Carbene (NHC)
Ligands with 2,4,6-Cycloalkyl Substituents
Roman Savka[a] and Herbert Plenio*[a]
Keywords: Carbenes / Transition metals / Ligand design / NHC ligands / Steric hindrance
1,3,5-Tricycloalkylbenzene (cycloalkyl = C5H9, C6H11) was
converted into the respective anilines (by means of nitration
and reduction) and then into the corresponding diimines
(with glyoxal), the cyclization of which with (HCHO)n/ZnCl2
provided the respective 1,3-bis(2,4,6-tricyclopentylphenyl)-
imidazolium salt in modest yields. An analogous reaction
sequence that employed acenaphthene-1,2-dione instead of
glyoxal yielded the two azolium salts in good yields, which
were converted into the respective N-heterocyclic carbene
(NHC) complexes [(NHC)AgCl], [(NHC)AuCl], [(NHC)-
RhCl(cod)] (cod = cyclooctadiene), and [(NHC)RhCl(CO)2].
One of the prevalent structural motifs in NHC ligands is
the 1,3-bis(2,6-dialkylphenyl)imidazol-2-ylidene-type carb-
Introduction
N-Heterocyclic carbenes are an established class of enes.[6] Four alkyl substituents in the 2,6-position enhance
ligands, and a large number of these complexes with metals the stability of the metal complexes relative to the respective
from all over the periodic table have been synthesized.[1] complexes with sp2-CH bonds, which are much more prone
Consequently, the electronic and steric properties of NHC to CH activation than sp3-CH bonds. Another important
ligands have been modified in many ways to modulate the factor is added stability owing to steric shielding of the
properties of the respective NHC metal complexes.[2] Dif- metal center by utilizing the bulk of the 2,6-substituents.
ferent σ-donor or π-acceptor behavior of NHC ligands Modification of the four ortho-alkyl groups thus has a pro-
change the electronic properties and can lead to unexpected nounced influence on the stability of the respective metal
reactivity patterns[3] or unusual complexes.[4] Sterically complexes.[7] The two most commonly applied substituents
demanding NHC ligands lend kinetic stability to fleeting are methyl and isopropyl groups (Scheme 1, A and C) since
intermediates in catalytic transformations, thus enabling the respective anilines (2,6-dimethylaniline and 2,6-diiso-
more efficient catalysis.[5]
propylaniline) needed for the synthesis of the respective
Scheme 1. N,NЈ-Diaryl-NHC ligands with sterically demanding ortho substituents.
carbenes are commercially available at low cost. The ethyl
[a] Organometallic Chemistry, TU Darmstadt,
Alarich-Weiss-Strasse 12, 64287 Darmstadt, Germany
E-mail: plenio@tu-darmstadt.de
group (Scheme 1, B) is used less often,[8] since the respective
aniline is expensive; furthermore, the stability of the respec-
tive NHC metal complexes appears to be slightly inferior
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201402830.
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to those with A- and C-type ligands (Scheme 1).[8a] NHC
ligands with larger alkyl groups are desirable since this can
lead to more powerful NHC–metal catalysts, as was demon-
strated convincingly by Organ et al. for various Pd-cata-
lyzed cross-coupling reactions.[4b] Several different D-[9] and
Results and Discussion
Synthesis of New NHC Ligands
The synthesis of the respective azolium salts and NHC
E-type[10] NHC ligands (Scheme 1) were synthesized to pro- metal complexes is summarized in Scheme 2. Starting from
vide even more bulk next to the metal center. However, in the known 1,3,5-tricycloalkylbenzenes[13] 1a and 1b [R =
some cases the synthesis of the respective anilines requires cyclopentyl (a), cyclohexyl (b)], the nitration with HNO3
significant synthetic effort[11] and consequently the avail- leads to the 1-nitro-2,4,6-tricycloalkylbenzenes 2a and 2b.
ability of such NHC ligands is limited; cyclization of the A minor impurity (Ͻ2%) in such compounds is the 1,2,4-
linear precursors to the cyclic azolium can also be fraught substituted benzene formed as a byproduct in the 1,3,5-alk-
with problems. Originating from work of Marko et al., ylation. For most metal complexes, this impurity is not
several sterically even more demanding F-type ligands visible in the NMR spectra. Reduction of the nitro group
(Scheme 1) have been synthesized.[12]
gave the anilines 3a and 3b, which were treated with glyoxal
Consequently, we were interested in developing new pro- according to standard procedures[2b] to provide the respec-
tocols for the synthesis of NHC ligands with very bulky tive diimines in good yields. However, the cyclization of the
alkyl substituents. In the present manuscript we wish to re- sterically demanding diimines to the respective azolium
port on the synthesis of new NHC ligands with cycloalkyl salts turned out to be difficult as the standard procedure[2b]
groups in the 2,4,6-positions. The respective anilines are did not lead to the formation of the desired products. Next,
easily available on a large scale from benzene by employing the Hintermann synthesis[14] was tested for the cyclization
a classic Friedel–Crafts/nitration/reduction sequence.
reaction of diimine 4a. The desired azolium salt was ob-
Scheme 2. Synthesis of new NHC ligands and metal complexes. Reagents and reaction conditions: (a) HNO3, HOAc/Ac2O in CH2Cl2,
0 °C; (b) Zn/HCl in AcOH under reflux conditions; (c) glyoxal, HCHO in CH3OH; (d) ZnCl2, (HCHO)x, HCl in CHCl3, 60 °C; (e and
h) [AuCl(Me2S)], K2CO3, acetone; (f) acenaphthene-1,2-dione, HOAc, in CH3CN, 14 h, 95 °C; (g) CH2Cl(OEt), neat, 100 °C, 24 h; (h)
Ag2O in CH2Cl2; (j) [{RhCl(cod)}2], Na amylate in thf; and (k) CO in CH2Cl2 (yields given on reaction arrows for a-and b-type products).
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tained in approximately 25% yield, but turned out to be evaporation of a CH2Cl2/2-propanol solution (see the crys-
highly impure and could not be purified. The reaction of tal structure). The related gold complexes [(7a)AuCl] and
4a with ClCH2OEt gave a black oil of unknown composi- [(7b)AuCl] were also synthesized directly from the respec-
tion.[15] However, a procedure developed by Marko et tive azolium salts by using and [AuCl(Me2S)] and K2CO3
al.[12a] using ZnCl2 and (HCHO)x produced the desired azo- in acetone, analogous to recent procedures from Gimeno
lium salt 5a in moderate yield (Scheme 2). The same pro- et al.[19] and Nolan et al.[20] The synthesis of the rhodium
cedure was then applied to the cyclization of diimine 4b. complexes followed established procedures.[21] To character-
The hexakis-cyclohexyl azolium salt 5b was obtained in low ize the electron-donating ability of the new carbenes, the
yield and with modest purity. It was not possible to purify redox potentials of [(7a)RhCl(cod)] (cod = cyclooctadiene)
this salt to a satisfactory level of purity.
The attempted cyclization of the respective diamines de- E1/2 = 0.78 V. This redox potential is close to that of the
rived from the reduction of 4a and 4b using HC(OEt)3 led related rhodium complexes [(SIMes)RhCl(cod)] (E1/2
and [(7b)RhCl(cod)] in CH2Cl2 were both determined to be
=
to a mixture of the respective mono- and bis-formamides 0.83 V) [SIMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-
instead of the desired azolium salts. The same problems ylidene].[21] The infrared spectroscopic determination of
were reported by Organ et al. during the attempted synthe- ν(CO) in [(7a)RhCl(CO)2] confirms similar donating prop-
sis of related azolium salts with 3-pentyl instead of the erties of NHC ligands 7a and IMes. It is evident from these
cyclohexyl or cyclopentyl groups employed by us.[9]
A
data that the influence of the strained acenaphthene back-
reasonable explanation for the poor yields of the cyclization bone on the donor properties of the respective NHC ligands
reaction is the low population of the favorable syn orienta- is not large. Compounds 7a and 7b are normal carbenes
tion of the two nitrogen groups owing to the bulky alkyl with respect to their donating properties.[15] Owing to the
substituents. For diimines with sterically highly demanding six cycloalkyl groups, the complexes reported here display
N substituents, the diimine should preferentially exist as the excellent solubility in all solvents other than highly polar
anti isomer, which is not amenable to cyclization. With a ones.
view to the modest overall yields of the azolium salts, we
decided to study the synthesis of more easily available NHC
ligands.
To enable the synthesis of NHC ligands with highly de-
Crystal Structure of the Complex
For [(7b)AgCl],[22] in the solid state the six cyclohexyl
groups form an extended array, such that the lower part of
ligand 7b can be considered to be a large, diamond-shaped
plate with dimensions of 2.1 nmϫ1.3 nm (Figures 1 and 2).
This shape renders this ligand much larger than the conven-
tional SIMes-type ligand in two dimensions.
manding N-aryl groups, it was decided to employ diimines
with an enforced syn orientation. The diimines derived from
acenaphthene-1,2-dione appear to be ideal in this respect.
The respective diimines obtained with 2,4,6-trimethylaniline
or 2,6-diisopropyl aniline were first reported by Cowley at
al.[15] and shown to form stable metal complexes.[16] The
respective Pd complexes were successfully employed in vari-
ous cross-coupling reactions.[17] The reaction of anilines 3a
and 3b with glyoxal provided the respective dimines 6a and
6b, which were cyclized to yield the azolium salts 7a·HCl
and 7b·HCl.
Synthesis of the Metal Complexes
Owing to the limited availability of NHC ligands 5a and
5b, only the respective NHC–Au complexes were prepared.
The synthesis of the respective gold complexes [(5a)AuCl]
and [(5b)AuCl] was possible, which in turn could be purified
easily by chromatography to provide the first NHC metal
Figure 1. Space-filling plot of [(7b)AgCl] viewed along the Cl–Ag–
NHC axis.
To evaluate the steric properties of the new NHC 7b
complexes with six cyclohexyl or six cyclopentyl groups more precisely, the buried volume according to Cavallo et
(Scheme 2). An NHC ligand with four cyclopentyl groups al. was calculated[23] and compared with that of the analo-
in the 2,6-positions, which is closely related to imidazolium gous Ag complex with four 2,6-iPr groups reported by
salt 5a (which has six cyclopentyl groups in the 2,4,6-posi- Cowley et al.[15,24] To our surprise, the buried volume of
tions), was first mentioned by Organ et al.,[18] but neither a carbene [(7b)AgCl], Vbur = 35%, was calculated to be
synthetic protocol nor any characterization data were smaller than that of the related iPr-substituted Ag complex
reported.
reported by Cowley with Vbur = 39 and 43% (two Vbur for
Starting from the azolium salts 7a·HCl and 7b·HCl, the two crystallographically independent molecules).[25] This re-
direct reaction with Ag2O in CH2Cl2 led to the respective sult is unexpected since (concerning the carbon atoms) an
complexes [(7a)AgCl] and [(7b)AgCl] in excellent yields. isopropyl group is a subset of a cyclohexyl group and the
Single crystals of the latter complex were grown by slow latter unit can be only slightly sterically less demanding.[26]
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reaction was allowed to stir for longer periods of time at room
temp., the yield of product dropped dramatically). The reaction
mixture was then placed in an ice bath, carefully quenched with
ice, and diluted with diethyl ether (200 mL). The layers were sepa-
rated, and the ethereal solution was washed twice with water and
brine. The organic layer was dried with MgSO4, filtered, and the
solvent was removed under vacuum. The resulting oily mixture was
filtered through a silica plug and washed with n-pentane. After re-
moving the volatiles under vacuum, 1,3,5-tricyclopentylbenzene
(22.9 g, 90% yield) or 1,3,5-tricyclohexylbenzene (26.0 g, 89.6%
yield) were obtained as colorless oils, which crystallized upon
standing in the refrigerator. NMR spectra are in accord with the
literature data.
Synthesis of Nitrobenzenes 2a and 2b: 1,3,5-Tricyclopentylbenzene
(14.0 g, 49.6 mmol) or 1,3,5-tricyclohexylbenzene (26.0 g,
80.2 mmol) was dissolved in CH2Cl2, Ac2O, and AcOH [for the
1,3,5-tricyclopentylbenzene reaction: CH2Cl2 (210 mL), Ac2O
(140 mL), AcOH (105 mL); for the 1,3,5-tricyclohexylbenzene reac-
tion: CH2Cl2 (340 mL), Ac2O (217 mL), AcOH (170 mL)]. The
solution was cooled to 0 °C and fuming nitric acid (20.8 mL for
1,3,5-tricyclopentylbenzene; 34 mL for 1,3,5-tricyclohexylbenzene)
was added dropwise for 1 h while keeping temperature between 0
and 5 °C. The resulting solution was stirred for an additional 5 h
at 0 °C and then warmed to room temp. Next, water (300 mL) and
CH2Cl2 (200 mL) were added. The organic layer was separated and
washed with cold aqueous NaOH (0.1 m) to remove acids as well
as unreacted Ac2O. The organic solution was then washed with
brine, dried with MgSO4, and filtered. The volatiles were removed
under reduced pressure and the residue recrystallized from hot eth-
anol to provide 1-nitro-2,4,6-tricyclopentylbenzene (14.0 g, 86%
yield) or 1-nitro-2,4,6-tricyclohexylbenzene (17.3 g, 58% yield) as
white crystals. Compound 2a: 1H NMR (300 MHz, CDCl3): δ =
7.06 (s, 2 H, HAr), 3.05–2.91 (m, 1 H, p-CH, cyclopentyl), 2.91–
2.77 (m, 2 H, o-CH, cyclopentyl), 2.15–1.48 (m, 24 H, CH2,
cyclopentyl) ppm. 13C NMR (75 MHz, CDCl3): δ = 150.12, 148.74,
Figure 2. X-ray crystal structure of complex [(7b)AgCl] (ORTEP
plot; cyclohexyl groups: light gray, other carbon atoms: dark gray,
N: blue, Ag: orange, Cl: green). Important bond lengths [pm] and
angles [°]: Ag–C 206.8(4), Ag–Cl 231.63(14); C–Ag–Cl 177.29(12).
We therefore believe that for the present ligands the static
picture of the buried volume approach is not useful.
The coordination sphere around silver behaves according
to expectations with a two-coordinate, almost linear silver
ion (Figure 2). The Ag–C(NHC) distance and the Ag–Cl
distances are also in the expected range for such com-
plexes.[27]
Conclusion
We have reported the preparation of several very bulky
NHC ligands. Based on a classic sequence of Friedel–Crafts
alkylation/nitration/Zn reduction, the new 2,4,6-cycloalk-
ylanilines (cycloalkyl = C5H9 and C6H11) were synthesized 137.10, 123.23, 46.24, 40.67, 35.05, 34.81, 25.78, 25.63 ppm.
HRMS (EI): m/z calcd. for C21H29NO2: 327.2198 [M]+; found
and converted into the respective azolium salts by utilizing
the respective diimines. This cyclization reaction tends to
be very difficult for diimines with sterically demanding N
327.2211. Compound 2b: 1H NMR (300 MHz, CDCl3): δ = 7.01
(s, 2 H, HAr), 2.58–2.44 (m, 1 H, p-CH, cyclohexyl), 2.44–2.31 (m,
2 H, o-CH, cyclohexyl), 1.96–1.16 (m, 30 H, CH2, cyclohexyl) ppm.
substituents. However, when employing the enforced syn
13C NMR (75 MHz, CDCl3): δ = 149.99, 148.91, 138.00, 123.30,
orientation in the acenaphthene-1,2-diimines, such bulky
44.92, 39.81, 34.49, 34.37, 26.93, 26.79, 26.18, 26.14 ppm. HRMS
azolium salts can be prepared easily by facilitating the criti-
cal cyclization step. Several metal complexes with Ag, Au,
and Rh were synthesized and shown to display normal elec-
tronic properties relative to the analogous complexes with
the established SIMes NHC ligand. Future studies will be
directed towards testing the properties of such ligands in
catalysis.
(EI): m/z calcd. for C24H35NO2: 369.2668 [M]+; found 369.2655.
Synthesis of Anilines 3a and 3b: 1-Nitro-2,4,6-tricyclopentylbenzene
(2a; 14.0 g, 42.8 mmol) or 1-nitro-2,4,6-tricyclohexylbenzene (2b;
17.3 g, 46.9 mmol) was dissolved in AcOH (205 mL for 2a; 235 mL
for 2b) under reflux conditions and concentrated HCl (31 mL for
2a; 39 mL for 2b) was added in one portion. Next, Zn powder
(22.4 g, 344.6 mmol for 2a; 28.0 g, 430.7 mmol for 2b) was carefully
added in several portions. The mixture was heated to reflux for 1 h
and then cooled to room temperature. Next, a cold (0 °C) 1 m solu-
tion of NaOH was added in several portions to adjust the reaction
mixture to approximately pH 9–10. The product was extracted with
diethyl ether, then the extract was washed with brine, dried with
MgSO4, and filtered. The solvent was removed under reduced pres-
Experimental Section
General Experimental: See the Supporting Information.
Synthesis of 1,3,5-Tricycloalkylbenzenes 1a and 1b: According to a
modified literature procedure,[13] AlCl3 (38.0 g, 284.8 mmol) was sure to give 1-amino-2,4,6-tricyclopentylbenzene (3a) (11.0 g, 87%
added under an argon atmosphere to a 1 L three-necked round-
bottomed flask cooled to 0 °C that contained benzene (8.0 mL,
89.7 mmol) in CH2Cl2 (60 mL). Cyclopentyl bromide (30.7 mL,
287 mmol) or cyclohexyl bromide (35.3 mL, 287 mmol) was added
dropwise, and the temperature in the flask was kept close to 0 °C.
Stirring was continued for 2 h and the reaction mixture was then
allowed to cool to room temp. for approximately 30 min (if the
yield) as a yellow-orange oil or 1-amino-2,4,6-tricyclohexylbenzene
(3b) (13.5 g, 85% yield) as an off-white powder. These compounds
could be purified by recrystallization from hot ethanol. In the case
of 3a, its concentrated solution in hot ethanol was slowly cooled
to room temp. and then to 0 °C in the refrigerator. The ethanol
was decanted and the oily residue dried under vacuum. Compound
1
3a: H NMR (500 MHz, CDCl3): δ = 6.95 (s, 2 H, HAr), 3.66 (br.,
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2 H, NH2), 3.07–2.98 (m, 1 H, p-CH, cyclopentyl), 2.97–2.87 (m, was heated to 95 °C for 1 h, then acetic acid (7 mL) was added.
2 H, o-CH, cyclopentyl), 2.14–1.52 (m, 24 H, CH2, cyclopentyl) Heating was continued until the dione had dissolved completely.
ppm. 13C NMR (126 MHz, CDCl3): δ = 139.92, 135.84, 130.14, Aniline 3a (3.42 g, 11.53 mmol) or 3b (3.91 g, 11.53 mmol) was
122.22, 46.05, 40.56, 35.01, 32.54, 25.58, 25.38 ppm. HRMS (EI):
added to this hot solution in one portion. The resulting solution
was heated under reflux conditions overnight and cooled to room
m/z calcd. for C21H31N: 297.2457 [M]+; found 297.2420. Com-
pound 3b: 1H NMR (300 MHz, CDCl3): δ = 6.86 (s, 2 H, HAr), temperature. The mixture was cooled to –30 °C and an orange pre-
3.56 (br., 2 H, NH2), 2.59–2.33 (m, 3 H, CH, cyclohexyl), 2.05–
cipitate was collected by filtration. After washing with methanol
1.16 (m, 30 H, CH2, cyclohexyl) ppm. 13C NMR (75 MHz, CDCl3): and drying under vacuum, the imine 6a (3.52 g, 87% yield) or 6b
δ = 138.31, 138.16, 131.82, 121.82, 44.50, 39.24, 35.01, 33.23, 27.44,
27.25, 26.60, 26.43 ppm. HRMS (EI): m/z calcd. for C24H37N:
339.2926 [M]+; found 339.2905.
(2.92 g, 66% yield) was obtained as an orange powder. Compound
6a: 1H NMR (500 MHz, CDCl3): δ = 7.85 (d, J = 8.3 Hz, 2 H,
acenapht.), 7.33 (t, J = 7.7 Hz, 2 H, acenapht.), 7.12 (s, 4 H, HAr),
6.45 (d, J = 7.2 Hz, 2 H, acenapht.), 3.12–2.98 (m, 6 H, CH,
cyclopentyl), 2.22–1.24 (m, 48 H, CH2, cyclopentyl) ppm. 13C
NMR (126 MHz, CDCl3): δ = 161.49, 147.29, 141.96, 140.81,
132.68, 131.09, 129.97, 128.63, 127.98, 123.48, 122.58, 46.19, 41.21,
35.02, 34.39, 33.49, 25.75 ppm. HRMS (EI): m/z calcd. for
C54H64N2: 740.5070 [M]+; found 740.5032. Compound 6b: 1H
NMR (500 MHz, CDCl3): δ = 7.84 (d, J = 8.2 Hz, 2 H, acenapht.),
7.30 (t, J = 7.7 Hz, 2 H, acenapht.), 7.06 (s, 4 H, HAr), 6.38 (d, J
= 7.2 Hz, 2 H, acenapht.), 2.68–2.60 (m, 4 H, o-CH, cyclohexyl),
2.60–2.54 (m, 2 H, p-CH, cyclohexyl), 2.14–0.99 (m, 60 H, CH2,
cyclohexyl) ppm. 13C NMR (126 MHz, CDCl3): δ = 161.93, 145.56,
143.84, 141.06, 134.44, 131.20, 129.55, 128.73, 127.77, 123.83,
122.68, 44.69, 39.34, 35.11, 34.42, 34.27, 27.27, 27.20, 26.51,
26.38 ppm. HRMS (EI): m/z calcd. for C54H65N2: 741.5148 [M –
C6H11]+; found 741.5111.
Synthesis of Imines 4a and 4b: Aniline 3a (2.7 g, 9.09 mmol) or
aniline 3b (3.09 g, 9.09 mmol) was dissolved in hot methanol (ca.
50 mL for 3a; 150 mL for 3b). Next, a glyoxal solution (40 wt.-%,
521.3 μL, 4.54 mmol) and two drops of formic acid were added
to the hot solution, thereby resulting in the formation of yellow
precipitate within a few minutes. The reaction mixture was stirred
at room temperature overnight and then cooled in an ice bath. Af-
ter filtration, the solid yellow product was washed with cold meth-
anol and dried under vacuum. The respective imine 4a (2.26 g, 81%
yield) or 4b (2.23 g, 70% yield) was obtained as a bright-yellow
solid. The diimines could be purified by recrystallization from hot
1
ethanol. Compound 4a: H NMR (500 MHz, C6D6): δ = 8.24 (s, 2
H, NCH), 7.21 (s, 4 H, HAr), 3.25–3.16 (m, 4 H, o-CH, cy-
clopentyl), 2.99–2.91 (m, 2 H, p-CH, cyclopentyl), 2.11–1.50 (m,
48 H, CH2, cyclopentyl) ppm. 13C NMR (126 MHz, C6D6): δ =
163.86, 148.96, 142.95, 134.46, 122.85, 46.67, 41.44, 35.32, 34.66,
26.09, 25.90 ppm. HRMS (EI): m/z calcd. for C44H60N2: 616.4757
[M]+; found 616.4739. Compound 4b: 1H NMR (500 MHz, C6D6):
δ = 8.21 (s, 2 H, NCH), 7.17 (s, 4 H, HAr), 2.84 (tt, J = 11.8, 3.3 Hz,
4 H, o-CH, cyclohexyl), 2.56 (tt, J = 12.1, 3.4 Hz, 2 H, p-CH,
cyclohexyl), 2.06–1.16 (m, 60 H, CH2, cyclohexyl) ppm. 13C NMR
(126 MHz, C6D6): δ = 163.61, 147.40, 144.71, 136.12, 122.62, 45.24,
39.61, 35.28, 34.41, 27.70, 27.48, 26.77, 26.66 ppm. HRMS (EI):
m/z calcd. for C50H72N2: 700.5696 [M]+; found 700.5691.
Synthesis of Imidazolium Chlorides 7a·HCl and 7b·HCl: Imine 4a
(2.0 g, 2.7 mmol) or imine 4b (2.0 g, 2.4 mmol) and CH2(OEt)Cl
(5.0 mL, 53.3 mmol) were added to a Schlenk flask under a nitro-
gen atmosphere. The reaction mixture was stirred at 100 °C for 24 h
and then cooled to room temperature. Diethyl ether (20 mL) was
added, and the resulting pale yellow solid was filtered, washed with
diethyl ether, hot toluene, and dried under vacuum to afford
7a·HCl (1.21 g, 57% yield) or 7b·HCl (1.54 g, 73% yield). Com-
pound 7a·HCl: 1H NMR (500 MHz, CDCl3): δ = 10.91 (s, 1 H,
NCHN), 8.03 (d, J = 8.3 Hz, 2 H, acenapht.), 7.61 (t, J = 7.7 Hz,
2 H, acenapht.), 7.28 (s, 4 H, HAr), 7.25 (d, J = 7.0 Hz, 2 H, acen-
apht.), 3.12 (p, J = 8.5 Hz, 2 H, p-CH, cyclopentyl), 2.80 (p, J =
8.0 Hz, 4 H, o-CH, cyclopentyl), 2.27–2.09 (m, 8 H, CH2, cy-
clopentyl), 1.96–1.46 (m, 36 H, CH2, cyclopentyl), 1.46–1.34 (m, 4
H, CH2, cyclopentyl) ppm. 13C NMR (126 MHz, CDCl3): δ =
150.59, 142.79, 142.69, 137.95, 130.81, 130.47, 130.19, 128.70,
128.38, 124.10, 123.48, 123.26, 46.34, 40.95, 35.37, 35.05, 34.74,
26.03, 26.00, 25.6 ppm. HRMS (EI): m/z calcd. for C55H65N2:
753.5148 [M – Cl]+; found 753.5119. Compound 7b·HCl: 1H NMR
(500 MHz, CDCl3): δ = 10.19 (s, 1 H, NCHN), 8.03 (d, J = 8.3 Hz,
2 H, acenapht.), 7.58 (t, J = 7.7 Hz, 2 H, acenapht.), 7.28 (s, 4 H,
HAr), 7.19 (d, J = 7.0 Hz, 2 H, acenapht.), 2.68–2.60 (m, 2 H, p-
CH, cyclohexyl), 2.24 (t, J = 11.7 Hz, 4 H, o-CH, cyclohexyl), 2.15
(d, J = 13.2 Hz, 4 H, CH2, cyclohexyl), 2.01 (d, J = 11.8 Hz, 4 H,
CH2, cyclohexyl), 1.91 (d, J = 12.1 Hz, 4 H, CH2, cyclohexyl), 1.80
(d, J = 12.8 Hz, 6 H, CH2, cyclohexyl), 1.66–1.15 (m, 37 H, CH2,
cyclohexyl), 0.99–0.86 (m, 5 H, CH2, cyclohexyl) ppm. 13C NMR
(126 MHz, CDCl3): δ = 151.78, 143.99, 141.45, 138.12, 130.77,
130.22, 128.29, 126.79, 124.75, 123.58, 123.14, 44.90, 39.89, 35.45,
34.45, 34.24, 26.93, 26.68, 26.45, 26.19, 25.68 ppm. MS (ESI): m/z
calcd. for C61H77N2: 837.6 [M – Cl]+; found 837.8.
Synthesis of Imidazolium Salts 5a·HCl and 5b·HCl: An HCl/ZnCl2/
(CH2O)n mixture [36% HCl, 460 μL, 5.47 mmol, 2.4 equiv; anhy-
drous ZnCl2, 372 mg, 2.73 mmol, 1.2 equiv; (CH2O)n, 82.1 mg,
2.73 mmol, 1.2 equiv.] was added dropwise to a solution of diimine
4a (1.41 g, 2.28 mmol) in CHCl3 (20 mL) at 60 °C. The reaction
was stirred at 60 °C for 1 h. The solution was cooled, diluted with
CH2Cl2 (30 mL), and washed with HCl (2 m) and brine, then dried
with anhydrous MgSO4. The solvent was evaporated under reduced
pressure, and the brownish residue was washed with Et2O. The off-
white precipitate was collected by filtration, washed a few times
with Et2O, and dried under vacuum (530 mg, 35% yield). Imid-
azolium chloride 5b·HCl (96 mg, 13% yield) was synthesized simi-
larly from diimine 4b (731 mg). Compound 5a·HCl: 1H NMR
(300 MHz, CDCl3): δ = 8.88 (t, J = 1.6 Hz, 1 H, NCHN), 8.33 (d,
J = 1.6 Hz, 2 H, NCH=CHN), 7.16 (s, 4 H, HAr), 3.10–2.95 (m, 2
H, p-CH, cyclopentyl), 2.54–2.37 (m, 4 H, o-CH, cyclopentyl),
2.19–1.39 (m, 48 H, CH2, cyclopentyl) ppm. 13C NMR (75 MHz,
CDCl3): δ = 150.77, 142.79, 136.72, 129.32, 127.96, 123.86, 46.31,
40.71, 35.73, 35.42, 34.82, 26.08, 25.97, 25.63 ppm. HRMS (EI):
m/z calcd. for C45H60N2: 628.4757 [M – HCl]+; found 628.4725.
Compound 5b·HCl: This compound contained impurities that
could not be removed. Chemical shifts for only aromatic/imid-
Synthesis of Silver Complexes [(7a)AgCl] and [(7b)AgCl]: The
Schlenk flask that contained imidazolium chloride 7a·HCl (300 mg,
0.38 mmol) or 7b·HCl (300 mg, 0.34 mmol) and Ag2O (45.2 mg,
0.19 mmol for 7a·HCl; 39.5 mg, 0.17 mmol for 7b·HCl) was evacu-
1
azolium protons are given: H NMR (500 MHz, [D6]DMSO): δ =
9.89 (s, 1 H, NCHN), 8.45 (s, 2 H, NCH=CHN), 7.36 (s, 4 H, HAr
ppm.
)
Synthesis of Imines 6a and 6b: Acenaphthene-1,2-dione (1.0 g, ated and back-filled with nitrogen. CH2Cl2 (8 mL) was added with
5.49 mmol) was suspended in acetonitrile (30 mL) and the mixture
a syringe. The reaction mixture was stirred overnight at 40 °C,
Eur. J. Inorg. Chem. 0000, 0–0
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FULL PAPER
cooled to room temperature, and filtered through Celite. The fil-
trate was concentrated to half the volume and then pentane
(25 mL) was added. The yellowish precipitate was removed by fil-
tration and washed with pentane to afford complex [(7a)AgCl]
(270 mg, 79% yield) or [(7b)AgCl] (303 mg, 90% yield) as a yellow-
ish microcrystalline solid. Compound [(7a)AgCl]: 1H NMR
H, CH2, cyclohexyl), 1.98–1.90 (m, 4 H, CH2, cyclohexyl), 1.86–
1.74 (m, 6 H, CH2, cyclohexyl), 1.65–1.30 (m, 34 H, CH2, cyclo-
hexyl), 1.22 (qt, J = 13.0, 3.4 Hz, 4 H, CH2, cyclohexyl), 1.05–0.93
(m, 4 H, CH2, cyclohexyl) ppm. 13C NMR (126 MHz, CDCl3): δ
= 179.38 (Ccarbene), 149.87, 144.52, 138.53, 130.65, 130.57, 129.80,
128.44, 127.72, 125.67, 124.26, 121.59, 44.91, 39.29, 35.29, 34.59,
(500 MHz, CDCl3): δ = 7.79 (d, J = 8.3 Hz, 2 H, acenapht.), 7.42 34.49, 27.11, 26.78, 26.58, 26.37, 25.94 ppm. MS (ESI): m/z calcd.
(t, J = 7.7 Hz, 2 H, acenapht.), 7.22 (s, 4 H, HAr), 6.98 (d, J =
7.0 Hz, 2 H, acenapht.), 3.11 (p, J = 8.8 Hz, 2 H, p-CH,
cyclopentyl), 2.82 (p, J = 8.9 Hz, 4 H, o-CH, cyclopentyl), 2.26–
2.16 (m, 4 H, CH2, cyclopentyl), 2.16–2.05 (m, 4 H, CH2,
cyclopentyl), 1.94–1.31 (m, 40 H, CH2, cyclopentyl) ppm. 13C
NMR (126 MHz, CDCl3): δ = 190.14 (d, JC,Ag = 259.0 Hz), 189.99
(d, JC,Ag = 259.8 Hz), 148.85, 143.01, 139.60, 139.54, 132.87,
130.90, 129.92, 128.27, 127.84, 125.57, 123.71, 121.31, 46.36, 40.73,
35.89, 35.19, 34.71, 26.11, 26.03, 25.71 ppm. HRMS (EI): m/z
calcd. for C55H64N2: 752.5070 [M – AgCl]+; found 752.5028. Com-
for C61H76N2AuClNa: 1091.5 [M + Na]+; found 1091.5.
Preparation of [(5a)AuCl]: Compound 5a·HCl (112.5 mg,
0.169 mmol), [AuCl(Me2S)] (50 mg, 0.169 mmol), and K2CO3
(70 mg, 0.507 mmol); reaction time: 12 h; off-white complex 95 mg
1
(65% yield). H NMR (500 MHz, CDCl3): δ = 7.11 (s, 6 H, HAr
+
NCH=CHN), 3.07–2.98 (m, 2 H, p-CH, cyclopentyl), 2.63–2.51
(m, 4 H, o-CH, cyclopentyl), 2.24–2.06 (m, 8 H, CH2, cyclopentyl),
1.92–1.45 (m, 40 H, CH2, cyclopentyl) ppm. 13C NMR (126 MHz,
CDCl3): δ = 175.24 (Ccarbene), 148.68, 143.19, 133.75, 123.43, 46.39,
40.79, 35.72, 35.61, 34.75, 26.25, 26.00, 25.68 ppm. MS (ESI): m/z
calcd. for C45H60N2AuClNa: 883.4 [M + Na]+; found 883.7.
1
pound [(7b)AgCl]: H NMR (500 MHz, CDCl3): δ = 7.77 (d, J =
8.3 Hz, 2 H, acenapht.), 7.39 (dd, J = 8.3, 7.0 Hz, 2 H, acenapht.),
7.22 (s, 4 H, HAr), 6.88 (d, J = 6.9 Hz, 2 H, acenapht.), 2.63 (tt, J
= 11.7, 3.1 Hz, 2 H, p-CH, cyclohexyl), 2.42 (tt, J = 11.9, 3.2 Hz,
4 H, o-CH, cyclohexyl), 2.11–2.01 (m, 8 H, CH2, cyclohexyl), 1.97–
1.89 (m, 4 H, CH2, cyclohexyl), 1.86–1.73 (m, 6 H, CH2, cyclo-
hexyl), 1.64–1.15 (m, 38 H, CH2, cyclohexyl), 1.03–0.90 (m, 4 H,
CH2, cyclohexyl) ppm. 13C NMR (126 MHz, CDCl3): δ = 189.78
(d, JC,Ag = 257.6 Hz), 189.63 (d, JC,Ag = 257.5 Hz), 149.89, 144.49,
139.85, 139.78, 131.02, 129.91, 128.32, 127.69, 125.51, 124.31,
121.50, 44.91, 39.19, 35.34, 34.73, 34.56, 27.08, 26.75, 26.59, 26.34,
25.89 ppm. MS (ESI): m/z calcd. for C61H76N2: 836.6 [M –
AgCl]+; found 837.8.
Preparation of [(5b)AuCl]: Compound 5b·HCl (126.6 mg,
0.169 mmol), [AuCl(Me2S)] (50 mg, 0.169 mmol), and K2CO3
(70 mg, 0.507 mmol); reaction time: 12 h; off-white complex 51 mg
1
(32% yield). H NMR (500 MHz, CDCl3): δ = 7.09 (s, 4 H, HAr),
7.06 (s, 2 H, NCH=CHN), 2.55 (tt, J = 11.5, 3.3 Hz, 2 H, p-CH,
cyclohexyl), 2.24–2.10 (m, 8 H, o-CH + CH2, cyclohexyl), 2.00–
1.21 (m, 56 H, CH2, cyclohexyl) ppm. 13C NMR (126 MHz,
CDCl3): δ = 175.11 (Ccarbene), 149.69, 144.42, 123.81, 123.39, 44.87,
39.28, 36.27, 34.49, 33.74, 27.07, 27.01, 26.52, 26.32, 26.00 ppm.
MS (ESI): m/z calcd. for C51H72N2AuClNa: 967.5 [M + Na]+;
found 967.7.
General Procedure for the Synthesis of [(NHC)AuCl] Complexes: A
vial was charged with the corresponding NHC·HCl (1 equiv.),
[AuCl(Me2S)] (1 equiv.), and K2CO3 (3 equiv.). The resulting mix-
ture was suspended in acetone (1.0 mL) and stirred for the specified
time at 60 °C. Next the solvent was removed under vacuum and
CH2Cl2 (5 mL) was added. The mixture was filtered through silica,
which was washed with CH2Cl2. Most of the CH2Cl2 was evapo-
rated, and methanol (10 mL) was added to the concentrated solu-
tion. A precipitate was formed, collected by filtration, washed with
methanol, and dried under vacuum.
Synthesis of Rhodium Complexes [(7a)RhCl(cod)] and [(7b)
RhCl(cod)]: A flame-dried Schlenk flask that contained imid-
azolium chloride 7a·HCl (169.8 mg, 0.215 mmol) or 7b·HCl
(272.9 mg, 0.312 mmol) and [{RhCl(cod)}2] (50.5 mg, 0.102 mmol
for 7a·HCl; 70 mg, 0.142 mmol for 7b·HCl) was evacuated and
back-filled with nitrogen three times. THF (20 mL) and a solution
of sodium tert-pentoxide in THF (2.5 m, 103 μL, 0.215 mmol for
7a·HCl; 150 μL, 0.312 mmol for 7b·HCl) were added. The mixture
was stirred for 2 h. Next the solvent was removed under vacuum
and the residue was purified by column chromatography (silica,
cyclohexane/ethyl acetate, 30:1, v/v), which afforded, after washing
with methanol, the desired complex [(7a)RhCl(cod)] (139 mg, 68%
yield) or [(7b)RhCl(cod)] (161 mg, 52% yield) as yellow microcrys-
talline solid. Compound [(7a)RhCl(cod)]: 1H NMR (500 MHz,
CDCl3): δ = 7.64 (d, J = 8.3 Hz, 2 H, acenapht.), 7.35–7.20 [m, J
= 7.2 Hz, 2 H, 6 H (t, acenapht. + s, 4 H, HAr)], 6.73 (d, J = 6.9 Hz,
2 H, acenapht.), 4.64–4.59 (m, 2 H, Hcod), 3.87 (br., 2 H, Hcod),
3.53–3.47 (m, 2 H, Hcod), 3.16 (p, J = 8.2 Hz, 2 H, p-CH, cy-
clopentyl), 2.77 (br., 2 H, Hcod), 2.38 (br., 2 H, Hcod), 2.28–0.96 (m,
54 H, o-CH, CH2, cyclopentyl, Hcod) ppm. 13C NMR (126 MHz,
CDCl3): δ = 192.83 (d, JC,Rh = 53.0 Hz), 148.07, 144.53 (br.),
140.55, 134.42, 129.90, 129.60, 127.34, 127.22, 126.82, 123.17 (br.),
121.55, 96.31 (d, JC,Rh = 7.0 Hz, cod), 67.83 (d, JC,Rh = 14.1 Hz,
cod), 46.44, 41.04 (br.), 37.22 (br.), 34.92, 34.13 (br.), 32.83, 28.48,
26.30 (br.), 25.77 ppm. MS (ESI): m/z calcd. for C63H76N2Rh:
Preparation of [(7a)AuCl]: Compound 7a·HCl (61 mg,
0.077 mmol), [AuCl(Me2S)] (24.2 mg, 0.077 mmol), K2CO3 (32 mg,
0.231 mmol); reaction time: 2 h; yellow complex 46 mg (61% yield).
1H NMR (500 MHz, CDCl3): δ = 7.79 (d, J = 8.3 Hz, 2 H, acen-
apht.), 7.42 (t, J = 7.4 Hz, 2 H, acenapht.), 7.22 (s, 4 H, HAr), 6.94
(d, J = 7.0 Hz, 2 H, acenapht.), 3.11 (p, J = 8.6 Hz, 2 H, p-CH,
cyclopentyl), 2.85 (p, J = 8.8 Hz, 4 H, o-CH, cyclopentyl), 2.29–
2.15 (m, 8 H, CH2, cyclopentyl), 1.94–1.84 (m, 4 H, CH2,
cyclopentyl), 1.84–1.44 (m, 32 H, CH2, cyclopentyl), 1.43–1.31 (m,
4 H, CH2, cyclopentyl) ppm. 13C NMR (126 MHz, CDCl3): δ =
179.74 (Ccarbene), 148.82, 143.05, 138.35, 132.42, 130.57, 129.83,
128.40, 127.88, 125.78, 123.67, 121.41, 46.39, 40.88, 35.75, 35.15,
34.73, 26.14, 26.02, 25.73 ppm. HRMS (EI): m/z calcd. for
C55H64N2: 752.5070 [M – AuCl]+; found 752.5035.
Preparation of [(7b)AuCl]: Compound 7b·HCl (89 mg, 963.5 [M – Cl]+; found 963.7. Compound [(7b)RhCl(cod)]: 1H
0.102 mmol), [AuCl(Me2S)] (30 mg, 0.102 mmol), and K2CO3 NMR (500 MHz, CDCl3): δ = 7.59 (d, J = 8.2 Hz, 2 H, acenapht.),
(42 mg, 0.306 mmol); reaction time: 20 h; yellow complex: 79 mg
(72% yield). 1H NMR (500 MHz, CDCl3): δ = 7.77 (d, J = 8.3 Hz, 6.59 (d, J = 7.0 Hz, 2 H, acenapht.), 4.62–4.57 (m, 2 H, Hcod),
2 H, acenapht.), 7.39 (t, J = 7.7 Hz, 2 H, acenapht.), 7.21 (s, 4 H, 3.57–3.51 (m, 2 H, Hcod), 3.05 (br., 2 H, Hcod), 2.67 (tt, J = 11.9,
HAr), 6.83 (d, J = 7.0 Hz, 2 H, acenapht.), 2.63 (tt, J = 11.8, 3.5 Hz, 3.3 Hz, 2 H, p-CH, cyclohexyl), 2.58–0.75 (m, 68 H, o-CH, CH2,
2 H, p-CH, cyclohexyl), 2.43 (tt, J = 12.1, 3.4 Hz, 4 H, o-CH,
cyclohexyl, Hcod) ppm. 13C NMR (75 MHz, CDCl3): δ = 192.78
cyclohexyl), 2.25–2.16 (m, 4 H, CH2, cyclohexyl), 2.10–2.02 (m, 4 (d, JC,Rh = 54.2 Hz), 150.14, 149.24, 140.27, 133.24, 129.95, 129.44,
7.29–7.21 [m, J = 8.0 Hz, 2 H, 6 H (t, acenapht. + s, 4 H, HAr)],
Eur. J. Inorg. Chem. 0000, 0–0
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FULL PAPER
127.37, 127.00, 126.82, 123.68 (br.), 121.87, 96.27 (d, JC,Rh
=
7.5 Hz, cod), 67.80 (d, JC,Rh = 13.9 Hz, cod), 45.03, 39.78 (br.),
36.91, 34.77, 33.49, 32.81, 28.45, 27.17, 26.42, 26.25 ppm. MS
(ESI): m/z calcd. for C69H88N2Rh: 1047.6 [M – Cl]+; found 1047.7.
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Synthesis of Carbonyl Complexes [(7a)RhCl(CO)2] and [(7b)
RhCl(CO)2]: Compound [(7a)RhCl(cod)] (100 mg, 0.096 mmol) or
[(7b)RhCl(cod)] (108.5 mg, 0.096 mmol) was dissolved in CH2Cl2
(7 mL), and CO was bubbled through this solution for 20 min. The
solvent was evaporated under vacuum, and the residue washed with
methanol to obtain the yellow complex [(7a)RhCl(CO)2] (86 mg,
90% yield) or [(7a)RhCl(CO)2] (90 mg, 87% yield). Compound
[(7a)RhCl(cod)]: 1H NMR (500 MHz, CDCl3): δ = 7.71 (d, J =
8.3 Hz, 2 H, acenapht.), 7.34 (t, J = 7.6 Hz, 2 H, acenapht.), 7.25
(s, 4 H, HAr), 6.72 (d, J = 7.0 Hz, 2 H, acenapht.), 3.21–3.09 (m, 6
H, CH, cyclopentyl), 2.28–1.09 (m, 60 H, CH, cyclopentyl) ppm.
13C NMR (126 MHz, CDCl3): δ = 185.18 (d, JC,Rh = 54.0 Hz,
[2]
[3]
[4]
Ccarbene), 184.25 (d, JC,Rh = 45.7 Hz, CO), 183.19 (d, JC,Rh
=
74.0 Hz, CO), 148.65, 143.58, 140.84, 133.26, 130.06, 129.79,
127.97, 127.48, 126.28, 123.45, 121.89, 46.38, 41.15, 36.98, 34.77,
33.69, 25.99, 25.93, 25.77 ppm. The attempted determination of
mass spectra with complex [(7a)RhCl(CO)2] was unsuccessful
owing to the instability of this complex. Compound [(7b)RhCl-
(CO)2]: 1H NMR (500 MHz, CDCl3): δ = 7.68 (d, J = 8.3 Hz, 2 H,
acenapht.), 7.31 (dd, J = 8.3, 7.0 Hz, 2 H, acenapht.), 7.25 (s, 4 H,
HAr), 6.66 (d, J = 7.0 Hz, 2 H, acenapht.), 2.69–2.59 (m, 6 H, CH,
cyclohexyl), 2.24–2.18 (m, 4 H, CH2, cyclohexyl), 2.10–2.03 (m, 4
H, CH2, cyclohexyl), 1.97–1.91 (m, 4 H, CH2, cyclohexyl), 1.86–
1.80 (m, 2 H, CH2, cyclohexyl), 1.79–1.72 (m, 4 H, CH2, cyclo-
hexyl), 1.68–1.44 (m, 24 H, CH2, cyclohexyl), 1.41–1.31 (m, 6 H,
CH2, cyclohexyl), 1.24–1.07 (m, 8 H, CH2, cyclohexyl), 0.95 (qt, J
= 12.1, 3.2 Hz, 4 H, CH2, cyclohexyl) ppm. 13C NMR (126 MHz,
CD2Cl2): δ = 185.74 (d, JC,Rh = 53.7 Hz, Ccarbene), 184.96 (d, JC,Rh
= 45.9 Hz, CO), 183.51 (d, JC,Rh = 73.6 Hz, CO), 150.61, 145.73,
141.11, 132.17, 130.52, 130.26, 128.54, 127.88, 126.52, 124.65,
122.71, 45.42, 39.93, 37.14, 35.09, 33.59, 27.62, 27.56, 26.86, 26.78,
26.50 ppm. The attempted determination of mass spectra with
complex [(7b)RhCl(CO)2] was unsuccessful owing to the instability
of this complex.
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Cyclic Voltammetry: Cyclic voltammograms were recorded in dry
CH2Cl2 under an argon atmosphere at ambient temperature. A
three-electrode configuration was employed. The working electrode
was a Pt disk (diameter 1 mm) sealed in soft glass with a Pt wire
as a counter electrode. The pseudoreference electrode was an Ag
wire. Potentials were calibrated internally against the formal poten-
tial of octamethylferrocene [E1/2 = –0.01 V (CH2Cl2)]. NBu4PF6
(0.1 molL–1) was used as a supporting electrolyte.
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Supporting Information (see footnote on the first page of this arti-
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Acknowledgments
This work was supported by the Deutsche Forschungsgemeinschaft
(DFG) (DFG) (grant number Pl 178/13-2). Sabine Foro is thanked
for the solution and the refinement of the crystal structure of [(7b)
AgCl].
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FULL PAPER
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Received: September 3, 2014
Published Online: ᭿
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Date: 24-11-14 11:14:59
Pages: 9
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FULL PAPER
N-Heterocyclic Carbene Complexes
R. Savka, H. Plenio* ........................ 1–9
Sterically very demanding N-heterocyclic
carbenes and metal complexes thereof were
synthesized by means of a classic sequence
of Friedel–Crafts alkylation, nitration, and
nitro-to-amine reduction to lead to the re-
spective anilines, which were then con-
verted into the respective azolium salts
through established synthetic routes.
Metal Complexes of Very Bulky N,NЈ-Di-
arylimidazolylidene N-Heterocyclic Carb-
ene (NHC) Ligands with 2,4,6-Cycloalkyl
Substituents
Keywords: Carbenes / Transition metals /
Ligand design / NHC ligands / Steric hin-
drance
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim