Nickel(II) complexes of bidentate N-heterocyclic carbene/phosphine ligands: Efficient catalysts for suzuki coupling of aryl chlorides

Article abstract of DOI:10.1002/chem.200600502

Nickel(II) complexes of bidentate N-heterocyclic carbene (NHC)/phosphane ligand L were prepared and structurally characterized. Unlike palladium, which forms [PdCl₂(L)], the stable nickel product isolated is the ionic [Ni(L)₂]Cl₂. These Ni^II complexes are highly robust in air. Among different N-substituents on the ligand framework, the nickel complex of ligand L bearing N-1-naphthylmethyl groups (2 a) is a highly effective catalyst for Suzuki cross-coupling between phenylboronic acid and a range of aryl halides, including unreactive aryl chlorides. The activities of 2a are largely superior to those of other reported nickel NHC complexes and their palladium counterparts. Unlike the previously reported [NiCl₂(dppe)] (dppe = 1,2-bis(diphenylphosphino)ethane), 2a can effectively catalyze the cross-coupling reaction without the need for a catalytic amount of PPh ₃, and this suggests that the PPh₂ functionality of hybrid NHC ligand L can partially take on the role of free PPh₃. However, for unreactive aryl chlorides at low catalyst loading, the presence of PPh ₃, accelerates the reaction.

Full text of DOI:10.1002/chem.200600502

DOI: 10.1002/chem.200600502
Nickel(II) Complexes of Bidentate N-Heterocyclic Carbene/Phosphine
Ligands: Efficient Catalysts for Suzuki Coupling of Aryl Chlorides
Chun-Chin Lee, Wei-Chi Ke, Kai-Ting Chan, Chun-Liang Lai, Ching-Han Hu, and
[
a]
Hon Man Lee*
Abstract: Nickel(II) complexes of bi-
dentate N-heterocyclic carbene (NHC)/
phosphane ligand L were prepared and
structurally characterized. Unlike palla-
Suzuki cross-coupling between phenyl-
boronic acid and a range of aryl ha-
ously reported [NiCl (dppe)] (dppe=
A
H
R
U
G
2
1,2-bis(diphenylphosphino)ethane), 2a
can effectively catalyze the cross-cou-
pling reaction without the need for a
AHCTREUNG
AHCTREUNG
dium, which forms [PdCl (L)], the
superior to those of other reported
nickel NHC complexes and their palla-
dium counterparts. Unlike the previ-
catalytic amount of PPh , and this sug-
gests that the PPh2 functionality of
hybrid NHC ligand L can partially take
2
3
stable nickel product isolated is the
II
ionic [Ni(L) ]Cl . These Ni complexes
2
2
are highly robust in air. Among differ-
ent N-substituents on the ligand frame-
work, the nickel complex of ligand L
bearing N-1-naphthylmethyl groups
on the role of free PPh . However, for
unreactive aryl chlorides at low catalyst
3
Keywords: carbene ligands · cross-
loading, the presence of PPh acceler-
3
coupling
·
nickel
·
phosphane
ates the reaction.
ligands · synthetic methods
(
2a) is a highly effective catalyst for
Introduction
easily accessible N-substituted imidazoles and pyridine de-
rivatives as starting materials led to diverse polydentate li-
gands, some of which were applicable in palladium-catalyzed
Monoligated palladium complexes containing electron-rich
and bulky phosphane or N-heterocyclic carbene (NHC) li-
gands are widely used in cross-coupling reactions for the for-
mation of CꢀC, CꢀN, CꢀO, and CꢀSbonds from aryl ha-
[4,5,9]
CꢀC coupling reactions.
For example, the tridentate
pincer framework of CNC ligands forms highly robust palla-
dium complexes that show very efficient catalytic activities
[1]
[9a,f]
A
C
H
T
R
E
U
N
G
lides. Some highly effective palladium systems based on
in the Heck reaction.
However, the combination of phos-
electron-rich, bulky ligands of these two classes allowed the
use of less expensive, more widely available, but less reac-
phane and NHC functionalities has received less atten-
[11,12]
tion.
In contrast to monoligated palladium catalysts
[1]
tive aryl chlorides as substrates. In particular, NHC ligands
are attracting considerable interest because of their easy ac-
cessibility, remarkable catalytic activities, and high thermal
with electron-rich phosphane or NHC ligands, others and
we have shown that bidentate ligand L exhibits only moder-
ate activity in palladium-catalyzed CꢀC coupling reac-
[2]
[11b,12a,c]
stability imparted to the palladium catalysts. Recently, re-
search has also been devoted to the synthesis of palladium
complexes with polydentate NHC-based ligands. In particu-
lar, the combination of nitrogen functionality, such as pyri-
dine, and an NHC moiety has attracted considerable inter-
tions.
[
3–10]
est.
The straightforward synthetic procedures involving
[
a] C.-C. Lee, W.-C. Ke, K.-T. Chan, C.-L. Lai, Prof. Dr. C.-H. Hu,
Prof. Dr. H. M. Lee
Department of Chemistry
National Changhua University of Education
Changhua, Taiwan 50058
Fax : (+886)4-721-1190
E-mail: leehm@cc.ncue.edu.tw
On the other hand, the replacement of palladium by
nickel catalysts in CꢀC coupling reactions is being promot-
ed, because nickel is less expensive and can be effectively
[
13]
removed from the final products.
nickel catalysts have been shown to be more effective than
the palladium systems.
Indeed, some of the
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
[13,14]
To date, various nickel catalysts
582
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Chem. Eur. J. 2007, 13, 582 – 591

FULL PAPER
have been shown to catalyze a wide range of Suzuki cross-
[14–19]
coupling reactions.
gands are commonly employed.
et al. have shown that [NiCl2(dppe)] (dppe=1,2-bis(diphe-
Among these systems, phosphane li-
[18,19]
For example, Percec
AHCTREUNG
nylphosphino)ethane) in the presence of an excess of PPh3
is an efficient catalyst for the Suzuki cross-coupling of aryl
mesylates and chlorides with both electron-donating (deacti-
[19]
vating) and electron-withdrawing (activating) substituents.
However, only a very few reports have been made on using
nickel NHC complexes as catalysts in Suzuki coupling reac-
[
14–16]
tions.
Notably, MacMillian et al. described a catalytic
(cod) ]/IMes·HCl (cod=cyclooctadiene,
system, based on [Ni
AHCTREUNG
2
IMes=1,3-bis(2,4,6-trimethylphenyl)imidazole carbene), ca-
pable of utilizing aryltrimethylammonium salts for Suzuki
coupling reactions. Cavell et al. also reported on the use of
nickel(II) NHC complex containing the monodentate tmiy
ligand [NiBr
A
C
H
T
R
E
U
N
G
(tmiy) (o-tolyl)] (tmiy=1,3,4,5,-tetramethylimi-
A
C
H
T
R
E
U
N
G
2
dazol-2-ylidene) in the Suzuki cross-coupling reaction. The
activity of this nickel NHC catalyst is, however, limited to
[15]
reactive aryl bromides.
Recently, we showed that for the nickel(II) complex of
the tetradentate NHC/pyridine ligand L’, [Ni(L’)]Br , the
Scheme 1. Synthesis of [Ni(L)
2
]Cl
2
. Conditions: a) For 2a and 2c only
2
NiCl /NaOAc; for 2a–2d Ni(OAc) .
2
A
H
R
U
G
2
presence of catalytic amount of PPh significantly enhances
3
its activity in Suzuki coupling of electron-deficient (reactive)
[16]
and electron-neutral aryl chlorides.
phosphane (PCy ; HPR , R=alkyl) as a coligand in palladi-
The use of a mono-
col, which is a common synthetic route for transition-metal
3
2
[
16,21]
um-catalyzed Suzuki coupling reactions has also been de-
NHC complexes from imidazolium salts.
Hence, direct
[20]
A
C
H
T
R
E
U
N
G
scribed. The necessity of PPh in our [Ni(L’)]Br system
reaction between [LH]Cl and Ni(OAc) (2:1 molar ratio) in
ACHTREUNG
3
2
2
encouraged us to investigate whether nickel(II) complexes
of a bidentate ligand with both NHC and phosphane func-
tionalities are more effective catalysts for Suzuki coupling
reactions. Therefore, we reinvestigated ligand L and pre-
pared new nickel(II) complexes. Unlike palladium, which
DMF at 508C for 3 h afforded the same complexes in pure
form. Pale yellow compounds 2a–2d are stable in air and in
solution. They have limited solubility in halogenated sol-
vents but dissolve partially in DMF or DMSO. All of them
were fully characterized by microanalysis, spectroscopic
methods, mass spectrometry, and X-ray crystallography.
Even though the silver carbene transfer reaction is a com-
monly employed method for the preparation of metal NHC
forms [PdCl (L)], the stable nickel complex of L obtained is
2
the ionic [Ni(L) ]Cl . Highly robust [Ni(L) ]Cl is indeed
2
2
2
2
much more effective than [PdCl (L)] in utilizing unreactive,
2
[22]
electron-rich aryl chlorides as substrates for Suzuki cross-
coupling, even in the absence of PPh3.
complexes, our attempts following this route were unsuc-
cessful.
1
Complexes 2a–2d exhibit H NMR resonances in the dia-
magnetic range of d=3.0–9.0 ppm reflecting the low-spin
configuration and hence square-planar coordination envi-
ronment of the nickel(II) centers. The successful deprotona-
Results
2
Synthesis and spectroscopic characterization of nickel(II)
tion of C H on the imidazolium moiety was confirmed by
complexes 2a–2d: Ligand precursors [LH]Cl (1a–1c), was
prepared according to our previous reported procedures.
the absence of a signal at dꢁ10.5 ppm. The bidentate coor-
dination of L around the nickel ion can be inferred from the
presence of four diastereotopic proton signals for the ethyl-
ene linkage in the range of d=4.0–6.1 ppm. The singlet
[11b]
Initially, complexation of [LH]Cl with nickel was preformed
following a procedure analogous to that for their palladium
[11b]
31
1
complexes,
NiCl (1:1 molar ratio) with NaOAc as base in DMSO at
that is, one-pot reaction between [LH]Cl and
P{ H} NMR resonances for 2a–2d were observed at d
ꢁ18–19 ppm, slightly upfield from those of d=20–24 ppm
for the palladium complexes. The carbenic carbon atom of
2a was observed as a triplet at d=162.8 ppm, which is in the
normal range of other nickel(II) NHC complexes reported
2
758C for 3 h. In contrast to palladium, which forms neutral
[
PdCl (L)] complexes, subsequent studies showed that the
2
final nickel complex obtained is the ionic [Ni(L) ]Cl (2a–
2
2
[7b,23]
2
d; Scheme 1). Thus, a 2:1 ratio of [LH]Cl:NiCl was re-
in the literature.
The carbenic carbon atoms of 2b–2d,
2
quired to obtain the optimum yield. However, this original
procedure was not suitable for 2b and 2d, which were pro-
duced with unsatisfactory purity. A more general and clean-
er synthetic procedure for 2a–2d is the metal acetate proto-
however, were not observed due to insufficient solubility of
the compounds. Nevertheless, the NMR data alone were not
sufficient to identify whether the nickel(II) complexes are
ionic [Ni(L) ]Cl or neutral [NiCl (L)].
2
2
2
Chem. Eur. J. 2007, 13, 582 – 591
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
583

H. M. Lee et al.
Electrospray ionization mass spectrometry (ESI-MS) stud-
ies: Compounds 2a–2b were characterized by mass spec-
trometry with “soft” electrospray ionization in DMF, which
unambiguously confirmed the ionic form [Ni(L) ]Cl . The
2
2
base peak observed in each spectrum corresponds to
+
[
MꢀCl] , for example, m/z 951.2 for 2a. Importantly, the
2+
doubly charged ion [Mꢀ2Cl] that was also observed in
each spectrum confirms the dicationic nature of these com-
plexes in solution. For example, the peak at m/z 449.3 corre-
sponds to dicationic nickel complex 2a without counterions.
Interestingly, in the spectra of 2a, 2b, and 2d, a water mole-
cule, presumably due to the wet DMF solvent used, interacts
+
with the positive ion [MꢀCl] giving rise to the peaks of
+
[
MꢀCl+H O] at m/z 933.1, 850.7, and 911.9, respectively.
2
+
The association of water with [MꢀCl] is most probably by
hydrogen-bonding interaction with the chloride ion. Hydro-
gen-bonding interactions between water and halide anions
[24]
are commonly observed. In fact, in the solid-state struc-
ture of 2b, an incorporated water molecule forms a hydro-
gen bond with a chloride anion (vide infra). All the assign-
ments in the MSspectra show good agreement between the
observed and calculated isotopic distributions.
X-ray diffraction studies on 2a–2d: The structures deter-
mined at 150 K (Table 1) are shown in Figures 1–4. Each
II
Ni center in 2a–2d is coordinated by two bidentate L li-
gands in a distorted square-planar environment with a sum
of the four bond angles at Ni1 of 359.3 (2a), 360.2 (2b),
3
60.2 (2c), and 359.3(2d). The two carbene (phosphane)
A
H
R
U
G
moieties are coordinated trans to the two ethylene linkages
situated on one side of the coordination plane. Generally, in
Figure 1. Top: thermal ellipsoid plot of the cationic portion of 2a at the
3
5% probability level (one of the naphthyl groups is disordered and only
2
a–2d, the C-Ni-C angle is closer to linearity than the P-Ni-
P angle. For example, the C1-Ni1-C29 bond angle in 2a is
77.3(3)8, whereas the P1-Ni1-P2 angle of 164.57(8)8 devi-
the major orientation with 0.75 site of occupancy is shown). Selected
bond lengths: Ni1ꢀC1 1.914(7), Ni1ꢀC29 1.876(7), Ni1ꢀP1 2.205(2), Ni1ꢀ
1
P2 2.185(2) Š. Selected bond angles: C1-Ni1-C29 177.3(3), P1-Ni1-P2
1
64.57(8), C1-Ni1-P1 96.0(2), C1-Ni1-P2 83.7(2), C29-Ni1-P1 83.4(2),
ates significantly from the ideal 1808. The NiꢀC and NiꢀP
distances in 2a–2d are within the normal ranges. Although
there are numerous nickel NHC complexes reported in the
C29-Ni1-P2 96.2(2)8. Bottom: space-filling representation showing the
embedment of the nickel ion inside the ligand framework.
[25]
literature,
relevant examples containing both phosphane
Table 1. Crystallographic data of 2a–2d.
a2b
2
2c
2d
formula
M
crystal system
space group
a [Š]
C
56
H
50
N
4
NiP
2
Cl
2
·3CH
3
OH
C
48
H
46
N
4
NiP
2
Cl
2
·H
2
O
C
906.42
48
H
44
F
2
N
4
NiPCl
2
50 50 4 2 2 2
C H N NiO P Cl
930.49
monoclinic
C2/c
22.1786(11)
9.1773(4)
22.9439(9)
106.606(2)
4475.2(3)
150(2)
4
1066.68
monoclinic
Pc
9.7827(4)
11.5029(4)
23.8131(9)
90.796(2)
2679.42(18)
150(2)
2
888.45
monoclinic
P2 /c
11.7417(10)
18.0255(17)
21.264(2)
103.742(6)
4371.6(7)
150(2)
4
monoclinic
C2
22.0676(7)
9.0135(3)
11.8192(4)
105.591(2)
2264.41(13)
150(2)
2
1
b [Š]
c [Š]
b [8]
V [Š ]
3
T [K]
Z
ꢀ
3
1
calcd [Mgm
]
1.322
1.350
1.329
1.381
unique data
parameters
6348
688
0.0611
0.1720
9063
523
0.0600
0.1689
–
2872
276
0.0475
0.1256
0.10(2)
5341
279
0.0524
0.1606
–
[
a]
R
wR
1
[I>2s(I)]
(all data)
[
2
b]
Flack parameter
0.12(2)
2
o
2
c
2
2 2 1/2
o
[
a] R jꢀjF
1
=ꢀjjF
o
c
jj/ꢀjF
o
j. [b] wR
2
=[ꢀw(F ꢀF ) /ꢀw(F ) ]
.
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Chem. Eur. J. 2007, 13, 582 – 591

Nickel(II) Complexes of Bidentate N-Heterocyclic Carbene/Phosphine Ligands
FULL PAPER
Figure 2. Thermal ellipsoid plot of the cationic portion of 2b at the 50%
probability level. Selected bond lengths: Ni1ꢀC1 1.971(4), Ni1ꢀC25
1
.985(4), Ni1ꢀP1 2.1025(13), Ni1ꢀP2 2.1074(13) Š. Selected bond angles:
C1-Ni1-C25 179.15(18), P1-Ni1-P2 163.64(5), C1-Ni1-P1 82.82(12), C1-
Figure 4. Thermal ellipsoid plot of the cationic portion of 2d at the 35%
probability level. Selected bond lengths: Ni1
ꢀ
C1 1.901(3), Ni1ꢀP1
Ni1-P2 94.21(12), C25-Ni1-P1 97.85(13), C25-Ni1-P2 85.29(13)8.
2
1
.1931(8) Š. Selected bond angles: C1-Ni1-C1A 178.09(19), P1-Ni1-P1A
59.38(5), C1-Ni1-P1 84.44(10), C1-Ni1-P1A 95.22(10)8.
Figure 5. Plot of average NiꢀC and NiꢀP distances in 2a–2d.
Figure 3. Thermal ellipsoid plot of the cationic portion of 2c at the 35%
probability level. Selected bond lengths: Ni1ꢀC1 1.898(4), Ni1ꢀP1
and direct attachment of a bulky group on the nitrogen
atom of the imidazole ring will lead to unfavorable overlap
with the adjacent phenyl rings; therefore, the methylene
spacers appear to be crucial in directing the bulky naphthyl/
aryl moieties away from the sterically congested coordina-
tion sphere (Figure 1, bottom). In 2a and 2b, the chloride
ions form Cl···HꢀO hydrogen-bonding interactions with
2
1
.1863(9) Š. Selected bond angles: C1-Ni1-C1A 179.2(3), P1-Ni1-P1A
65.42(7), C1-Ni1-P1 97.23, C1-Ni1-P1A 82.87(11)8.
[26]
and NHC moieties are rare.
Interestingly, even though
2
b–2d bear similar N-substituents, a plot of the average Niꢀ
C and NiꢀP distances in 2a–2d clearly reveals the signifi-
guest methanol and water molecules, respectively. The geo-
metries of the hydrogen-bonding interactions in 2a and 2b
are given in the Supporting Information.
cantly longer NiꢀC and shorter NiꢀP distances in 2b, which
reflect a stronger trans influence exerted by the phosphorus
atom than by the NHC moiety in 2b (Figure 5). In fact, the
more stable NiꢀP bonds in 2b correlate to its poorer catalyt-
Theoretical studies: Palladium forms neutral 1:1 Pd–L com-
ic activity in Suzuki coupling (vide infra). It can be envis-
aged from the space-filing representation of 2a that the
nickel ion is embedded inside the ligand framework of L,
plex [PdCl (L)], whereas for nickel ionic 1:2 Pd–L complex
2
[Ni(L) ]Cl is the stable product. We believe that the differ-
2
2
ence has thermodynamic reasons. Therefore, we undertook
Chem. Eur. J. 2007, 13, 582 – 591
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
585

H. M. Lee et al.
theoretical investigations on the relative stability of
Ni(L) ]Cl and their palladium counterparts [PdCl (L)] on a
properties (from reactive, electron-deficient aryl bromides
to unreactive, electron-rich aryl chlorides). Initially, we em-
[
2
2
2
simplified system and calculated the energy of the reaction
shown in Scheme 2 (see Computational Methods). The opti-
mized geometrical parameters of the products 5 (a model
for [PdCl (L)]) and 6 (a model for [Ni(L) ]Cl ) are in good
ployed similar catalytic conditions as reported for [NiCl -
2
[19]
[16]
AHCTREUNG
3
3
Further studies indicated that 2a is still efficient for unreac-
tive, electron-rich aryl chlorides even in the absence of
PPh3.
The different solvent and base combinations for the cou-
pling reaction were studied according to Table 2. The cou-
2
2
2
agreement with the X-ray data. For example, the computed
NiꢀC and NiꢀP distances of 6 are 1.914 and 2.233 Š, respec-
tively, which are comparable to those of the solid-state
structures of 2a, 2c, and 2d. The PdꢀC and PdꢀP distances
of 5 (2.014 and 2.274 Š, respectively) are also similar to
those obtained for [PdCl (L)] from the X-ray structure de-
2
Table 2. Effect of solvents and bases on Suzuki coupling catalyzed by
2a.
[11b]
[a]
terminations.
tion is ꢀ9.8 kcalmol , and it is ꢀ7.3 kcalmol with the 6-
11G** basis set. Hence, our computational results indeed
show that [PdCl (L)] and [Ni(L) ]Cl are energetically more
With the CPCM model, the energy of reac-
ꢀ
1
ꢀ1
3
2
2
2
Entry
Base
Solvent
Yield [%]
favorable than [NiCl (L)] and [Pd(L) ]Cl .
2
2
2
1
2
3
4
5
6
7
8
9
K
K
3
PO
PO
4
4
·H
·H
2
2
O
O
toluene
1,4-dioxane
THF
2-propanol
toluene
toluene
toluene
toluene
1,4-dioxane
100
48
63
38
76
0
82
74
11
3
Thermogravimetric analysis (TGA): The thermal stability of
a–2d was studied by TGA on amorphous powders, and the
K PO ·H O
K
K
NaOAc
KOH
Cs CO
3
3
2
4
2
PO
CO
4
·H
2
O
2
3
results suggested that the four solids are thermally stable
below 2008C. Their TGA curves are essentially similar, typi-
cally with an initial weight loss due to liberation of guest
water or solvent molecules and then a gradual weight loss
from about 200 to 5008C. For example, the TGA curve of
2
3
Cs
2
CO
3
[a] Reaction conditions: 4-chloroacetophenone (1 mmol), phenylboronic
acid (1.3 mmol), base (2.6 mmol), Ni catalyst (2 mol%), solvent (3 mL),
2
2
b shows a weight loss of 3.85% from 50 to 1308C, corre-
sponding to the release of two water molecules (calcd
.98%), and then decomposition starts to occur at an onset
h, 808C, GC yield.
3
temperature of 2378C.
pling reaction between phenylboronic acid and 4-chloroace-
tophenone over a period of 2 h was employed as standard
reaction for screening purposes. The coupling activity is
highly solvent and base dependent. The combination of
K PO ·H O/toluene affords quantitative yield (entry 1),
Suzuki cross-coupling: We have previously shown that the
precatalyst [Ni(L’)]Br is effective in Suzuki cross-cou-
2
[16]
pling. Undoubtedly, donor-atom dissociations are essential
3
4
2
II
in this bis-chelated Ni complex to generate vacant sites for
whereas no coupling product is obtained when the reaction
is conducted in toluene with NaOAc as base (entry 6). The
use of Cs CO as base in 1,4-dioxane, that is, the conditions
II
0
the incoming substrates. Also, reduction of Ni to Ni is re-
quired for an initial oxidative addition of an ArꢀX bond in
2
3
0
[11b]
the catalytic cycle. Such reduction to Ni can be mediated
used for the [PdCl (L)]-catalyzed cross-coupling reaction,
2
by bases or reactants (i.e., without the need for an external
was not applicable for the nickel system (entry 9).
[18g]
reducing agent).
The addition of a catalytic amount of re-
With the optimized solvent/base combination, 2a can in
general catalyze effectively the reaction of de-, un-, and acti-
vated aryl bromides with excellent yields; the addition of
ducing PPh should further facilitate this reduction. Since
3
the bidentate ligand L contains the phosphane functionality,
we were interested whether [Ni(L) ]Cl represents a simpli-
PPh is not necessary (Table 3). The capability of the catalyt-
2
2
3
fied catalytic system (without the necessity of PPh ) for the
ic system for the cross-coupling of the less reactive aryl
chlorides is summarized in Table 4 (entries 1–7). Quantita-
tive yields of 4-acetylbiphenyl can be obtained from 4-chlo-
3
Suzuki coupling reaction. We tested the catalytic perform-
ance of 2a–2d in the cross-coupling between phenylboronic
acid and a range of aryl halides with different electronic
A
H
R
U
G
3
2 2 2
Scheme 2. Hypothetical reaction between [Pd(L) ]Cl and [NiCl (L)].
586
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Chem. Eur. J. 2007, 13, 582 – 591

Nickel(II) Complexes of Bidentate N-Heterocyclic Carbene/Phosphine Ligands
FULL PAPER
[
a]
Table 3. Suzuki coupling with aryl bromides catalyzed by 2a.
with 3 mol% of the catalyst (Table 4, entries 7 and 20). A
good yield of 91% can still be achieved with 2 mol% cata-
lyst loading (Table 4, entry 13). Since [Ni(L) ]Cl contains
2
2
Entry
Cat. [%]
PPh
3
[%]
R
t [h]
Yield [%]
two bidentate ligands L, one might suspect that 2a would
give poor yields with sterically hindered substrates; howev-
er, excellent yields of coupled products can still be obtained
over 24 h with 2- and 3-chlorotoluene (Table 5, entries 2 and
[
b]
1
2
3
4
5
6
3
3
3
3
3
3
6
0
6
0
6
0
COMe
COMe
H
H
OMe
OMe
3
3
12
12
12
12
100
100
100
95
[
b]
3
). These activities reflect the fact that the true catalytic spe-
[
[
b]
b]
100
100
cies has much less steric bulk than the precatalyst.
[
(
6
a] Reaction conditions: aryl bromide (1 mmol), phenylboronic acid
1.3 mmol), K PO ·H O (2.6 mmol) as base, 2a (3 mol%), PPh (0–
mol%), toluene (3 mL), 808C, GC yield. [b] NMR yield.
3
4
2
3
[a]
Table 5. Suzuki coupling of chlorotoluenes catalyzed by 2a.
[
a]
Table 4. Suzuki coupling with aryl chlorides catalyzed by 2a.
Entry
1
Substrate
Product
Yield [%]
100 (98)
Entry
Cat. [%]
PPh
3
(%)
R
t [h]
Yield [%]
[
b]
1
2
3
4
5
6
7
8
9
3
3
3
3
3
3
3
2
2
2
2
2
2
1
1
1
1
1
1
1
6
0
6
0
6
6
0
4
0
4
0
4
0
2
2
0
2
0
2
0
COMe
COMe
Me
2
2
2
24
12
24
24
2
100 (95)
2
3
100 (67)
94 (32)
[
b]
100 (97)
100(93)
98
Me
OMe
OMe
OMe
COMe
COMe
Me
91
[
b]
100 (99)
[
b]
[a] Reaction conditions: aryl halide (1.0 mmol), phenylboronic acid
1.3 mmol), K PO ·H O (2.6 mmol), 2a (3.0 mol%), PPh (6.0 mol%),
96 (93)
100
(
3
4
2
3
3
toluene (3 mL), 808C, 24 h, NMR yield; yield without addition of PPh in
parentheses.
2
100
100
88
1
1
1
1
1
1
1
1
1
1
2
0
1
2
3
4
5
6
7
8
9
0
24
24
24
24
2
6
6
24
24
24
24
Me
OMe
OMe
COMe
COMe
COMe
Me
Me
OMe
OMe
95
91(89)
The difference in catalytic behavior with and without the
97
100
90
98
74
addition of PPh was subjected to a more detailed kinetic
study on the coupling reactions of 4-chloracetophenone and
4
3
-chlorotoluene with 3 mol% of 2a. For the reactive 4-chlo-
A
H
R
U
G
3
92
77
cause it produces no accelerating effect, as indicated by the
superposition of the two curves (Figure 6). However, for less
reactive substrates, such as 4-chlorotoluene, the reaction is
[
(
6
a] Reaction conditions: aryl chloride (1 mmol), phenylboronic acid
1.3 mmol), K PO ·H O (2.6 mmol) as base, 2a (1–3 mol%), PPh (0–
mol%), toluene (3 mL), 808C, GC yield; yields of isolated compounds
in parentheses. [b] NMR yield.
3
4
2
3
slightly accelerated in the presence of PPh (Figure 7).
3
and 2). Notably, 2a can also utilize electron-neutral and
electron-rich (unreactive) aryl chlorides as coupling partners
for the formation of unsymmetrical biphenyls (Table 4, en-
tries 3–7). For example, even without the addition of
6
mol% PPh , 4-methoxybiphenyl can still be obtained in an
3
excellent 93% yield of isolated product from the unreactive
4-chloroanisole within 24 h (Table 4, entries 6 and 7).
In the presence of PPh , the catalyst loading can be re-
3
duced to 2 or 1 mol% without significant reduction of yield.
For example, even with the unreactive 4-chloroanisole, the
yields of the products only decreased slightly from 99 to 95
to 92% with catalyst loadings of 3, 2, and 1 mol%, respec-
tively (Table 4, entries 6, 12, and 19). However, in the ab-
Figure 6. Comparison of the time/yield characteristics of 2a with and
sence of PPh , a low catalyst loading of 1 mol% furnishes
3
without the addition of PPh in the cross-coupling of phenylboronic acid
3
only 77% yield of 4-methoxybiphenyl, instead of 96% yield
and 4-chloroacetophenone.
Chem. Eur. J. 2007, 13, 582 – 591
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
587

H. M. Lee et al.
[
a]
2
Table 7. Effect of conducting Suzuki coupling under N and air.
Entry
Atmosphere
R
t [h]
Yield [%]
1
2
3
4
5
6
N
air
N
air
N
air
2
COMe
COMe
Me
Me
OMe
OMe
2
2
24
24
24
24
100
42
88
6
91
41
2
2
[
a] Reaction conditions: aryl halide (1 mmol), phenylboronic acid
1.3 mmol), K PO ·H O (2.6 mmol) as base, Ni catalyst (2 mol%), tol-
uene (3 mL), 808C, GC yield.
(
3
4
2
Figure 7. Comparison of the time/yield characteristics of 2a with and
0
the fact that both the intermediate Ni state and the free
phosphane arms are reactive towards oxygen. After irrever-
sible oxidation of phosphane to phosphane oxide by air, the
presumed recoordination of the phosphane arm to stabilize
the resting states in the catalytic cycle becomes impossible.
The catalytic performance of 2a was compared with other
nickel and palladium catalysts reported in the literature.
Nickel complexes of L or dppe are much more effective
without the addition of PPh
and 4-chlorotoluene.
3
in the cross-coupling of phenylboronic acid
We compared the catalytic activities of 2a–2d (Table 6).
Entry 1 clearly indicates that 2a containing the bulkier N-1-
naphthylmethyl groups is the most active complex. Interest-
than Ni(OAc) and NiCl2 (Table 8, entries 1–4 and 6, 7).
A
H
R
U
G
[
a]
2
Table 6. Suzuki coupling catalyzed by 2a–2d.
Table 8. Suzuki coupling catalyzed by different nickel and palladium cat-
[
a]
alysts.
Entry
Catalyst
Yield [%]
1
2
3
4
2a
2b
2c
2d
91
37
56
62
Entry
Catalyst
Cat. [%]
PPh
3
[%]
Yield [%]
1
2
3
4
5
6
7
2a
2a
1
1
1
1
1
1
1
1
1
2
0
2
0
2
2
2
0
2
97
88
90
55
26
8
1
3
22
[
a] Reaction conditions: 4-chloroanisole (1.0 mmol), phenylboronic acid
[NiCl
[NiCl
[NiCl
2
(dppe)]
2
(dppe)]
2
(dppe)]
(
1.3 mmol), K
3 4 2
PO ·H O (2.6 mmol), nickel catalyst (2.0 mol%), toluene
(
3 mL), 24 h, 808C, GC yield.
Ni(OAc)
NiCl
[PdCl
[PdCl (L)] (L=1a)
2 2
·4H O
2
ingly, the least reactive compound is 2b, which contains N-
phenyl groups and afforded only 37% yield of 4-methoxybi-
phenyl. This can be explained by the fact that the stronger
NiꢀP bonds in 2b, as shown by the solid-state structure, lead
[b]
8
9
2
(L)] (L=1a)
[
b]
2
[
a] For entries 1–7: reaction conditions: 4-chloroacetophenone (1 mmol),
phenylboronic acid (1.3 mmol), base (2.6 mmol), catalyst (1 mol%), tol-
uene (3 mL), 2 h, 808C, GC yield. [b] Cs CO (2 mmol) as base, 1,4-dix-
to inefficient phosphane-arm dissociation to generate the co-
ordinatively unsaturated active species and hence poor cata-
lytic activity of the precatalyst. The structure/activity rela-
tionship for precatalysts was also explored by Nolan et al.
2
3
oane (3 mL) as solvent; other conditions as in entries 1–7.
These complexes are also reactive than [NiCl (PPh ) ]
A
C
H
T
R
E
U
N
G
2 3 2
[27]
recently.
They reported a family of [Pd
A
H
R
N
(allyl)Cl
A
H
R
N
(IPr)]
(Table 8, entry 5). The nickel catalyst with L is much more
reactive than its palladium counterpart in catalyzing cross-
coupling reaction with the aryl chloride (Table 8, entries 8
and 9). To further differentiate the efficiency between the
(
IPr=N,N’-bis(2,6-diisopropylphenyl)imidazol-2-ylidene)
complexes in which the allyl moiety is substituted. As shown
by the X-ray structural data, substitution increases the dis-
symmetry of the allyl ligand around the Pd center, and this
allows facile activation of Pd to Pd . The ease of activation
translates into a much higher activity of, for example, [Pd-
most reactive 2a and [NiCl (dppe)], their performances in
A
H
R
U
G
2
II
0
the cross-coupling reactions were compared. For the reac-
tive 4-chloroacetophenone substrate, their activities are
comparable when a catalytic amount of PPh3 is present
(Table 9, entries 1 and 2). However, in the absence of PPh3,
2a gives a better yield of 88% instead of 55% given by
A
C
H
T
R
E
U
N
G
(cinnamyl)Cl
A
C
H
T
R
E
U
N
G
(IPr)] relative to [Pd
A
H
E
N
A
H
R
U
G
We also briefly investigated the effect of conducting the
catalytic reaction in air and compared the results with those
performed under nitrogen. Even though 2a–2d are stable in
air, Table 7 shows that when the catalytic reactions were
conducted in air instead of under nitrogen, drastic reduction
of product yields were observed. This can be rationalized by
[NiCl2
activities became more evident when the unreactive 4-chlo-
ranisole was used as substrate. While 2a afforded a 92%
yield of 4-methoxybiphenyl, only 48% yield was recorded
A
H
R
U
G
AHCTREUNG
588
www.chemeurj.org
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2007, 13, 582 – 591

Nickel(II) Complexes of Bidentate N-Heterocyclic Carbene/Phosphine Ligands
FULL PAPER
[
a]
Table 9. Suzuki coupling catalyzed by 2a and [NiCl
2
(dppe)].
phosphane arm). The scope of the nickel complexes in other
CꢀC coupling reactions is being investigated.
Entry
Catalyst
PPh
3
[%]
R
t [h]
Yield [%]
Experimental Section
1
2
3
4
5
6
7
8
2a
[NiCl
2a
[NiCl
2a
[NiCl
2a
2
2
0
0
2
2
0
0
COMe
COMe
COMe
COMe
OMe
OMe
OMe
OMe
2
2
2
97
90
88
55
92
48
77
28
2
2
2
2
(dppe)]
(dppe)]
(dppe)]
(dppe)]
General procedure: All reactions were performed under a dry nitrogen
2
atmosphere by standard Schlenk techniques. All solvents used were puri-
[
28]
fied according to standard procedures. Commercially available chemi-
24
24
24
24
cals were purchased from Aldrich or Acros. The ligand precursors 1a–1d
were synthesized according to the literature procedure. H, C{ H}, and
P{ H} NMR spectra were recorded at 300.13, 75.48, and 121.49 Hz, re-
spectively, on a Bruker AV-300 spectrometer. Chemical shifts for H and
1
13
1
3
1
1
[NiCl
1
[
a] Reaction conditions: aryl halide (1.0 mmol), phenylboronic acid
13
C spectra are reported in parts per million relative to residual signal of
(
1.3 mmol), K
3
PO
4
·H
2
O (2.6 mmol), nickel catalyst (1.0 mol%), toluene
1
13
1
CDCl
2
3
( H: d=7.24 ppm; C: d=77.0 ppm) or [D
6
]DMSO ( H: d=
(
3 mL), 808C, GC yield.
13
31
.50 ppm; C: d=39.5 ppm). P NMR chemical shifts are reported rela-
3
1
3 4
tive to 85% H PO external standard ( P: d=0.0 ppm). Elemental analy-
ses and ESI mass spectrometry were performed on a Heraeus CHN-OS
Rapid Elemental Analyzer and a Finnigan/Thermo Quest MAT 95XL,
respectively, at the Instruments Center of National Chung Hsing Univer-
sity, Taiwan. GC analyses were performed on a Varian CP-3800 GC-FID
system equipped with a CP-Sil 5 CB capillary column (length: 30 mm,
ID: 0.33 mm, film thickness: 1.0 mm). Thermogravimetric analysis (TGA)
was performed on a Perkin–Elmer Pyris 6 TGA Thermogravimetric Ana-
for [NiCl (dppe)] with PPh added. In the absence of PPh ,
A
C
H
T
R
E
U
N
G
2
3
3
2
a still gives a decent yield of 77%, whereas [NiCl (dppe)]
A
H
R
U
G
2
becomes ineffective (Table 9, entries 7 and 8).
ꢀ
1
2
lyzer under flowing N gas (40 mLmin ), and the heating rate was
ꢀ
1
2
08Cmin
Preparation of 2a
Method A: mixture of 1a (0.270 g, 0.591 mmol), NiCl
.295 mmol), and NaOAc (0.0484 g, 0.591 mmol) in dried DMSO (5 mL)
.
Conclusion
A
2
(0.038 g,
II
We have prepared and characterized Ni complexes of bi-
dentate phosphane/NHC ligands L. Unlike palladium, which
0
was stirred at 758C for 3 h. After cooling, the solvent was completely re-
moved under vacuum. The residue was washed twice with water (25 mL)
and THF (25 mL). Subsequently, the yellow solid was collected on a frit
and dried under vacuum overnight. Yield: 0.19 g, 67%.
forms [PdCl (L)], the stable nickel product isolated is the
2
ionic [Ni(L) ]Cl , which is consistent with theoretical calcu-
2
2
II
0
lations. These Ni complexes 2a–2d, in contrast to Ni com-
pounds, have high stability in air, which is a desirable prop-
erty of a catalyst precursor. Notably, the novel nickel com-
plex 2a is highly effective in Suzuki cross-coupling of phe-
nylboronic acid with a range of aryl halides, including un-
reactive aryl chlorides. The activities of [Ni(L) ]Cl are
Method B: A mixture of 1a (0.900 g, 1.97 mmol) and Ni
A
H
R
U
G
2 2
(OAc) ·4H O
(
0.245 g, 0.985 mmol) in dried DMF (5 mL) was stirred at 508C for 3 h.
The subsequent workup procedure was same as that of method A. Yield:
0.65 g, 68%; m.p. 196–1988C; elemental analysis calcd (%) for
C H50Cl N P Ni: C 69.41, H 5.20, N 5.79; found: C 69.42, H 5.21, N 5.75;
56 2 4 2
1
2
ACHTREUNG
H NMR ([D
6
]DMSO): d=4.64 (d,
.98 (brs, 2H; PCH CH ), 5.31 (brs, 2H; PCH
(H,H)=13.9 Hz, 2H; NCH ), 6.46 (d, J
(H,H)=7.5 Hz, 2H; Ar-H), 7.08 (m, 4H; CH
2
J
A B
H ),
2
2
4
6
2
2
2
2
3
largely superior to those of other reported nickel NHC com-
2
.23 (d, J
A
H
R
U
G
A
H
B
ACHTREUNG
[15,16]
[11b]
plexes
NiCl2(dppe)]
and their palladium counterparts.
Unlike
3
ACHTREUNG
Ar-H), 6.86 (d,
7.27–8.19 ppm (m, 32H; Ar-H, imi-H, Ph-H);
[D
J
[19]
13
1
[
ACHTREUNG
2a can effectively catalyze cross-coupling
(
6
without the need for a catalytic amount of PPh , and this
3
1
1
ACHTREUNG
suggests that the PPh2 functionality of the hybrid NHC
2
31
1
J
A
H
R
U
G
ligand L can partially replace the role of free PPh . For un-
3
([D ]DMSO): d=19.4 ppm (s); ESI-MS (DMF): m/z (%): 951.2 (6)
6
+
+
2+
reactive aryl chlorides at low catalyst loading, the presence
[MꢀCl+H O] , 933.1 (100) [MꢀCl] , 449.3 (60) [Mꢀ2Cl]
Preparation of 2b: The compound was prepared by a similar procedure
to method B for 2a. Compound 1b (0.120 g, 0.295 mmol) and Ni-
.
2
of PPh , however, accelerates the reaction.
3
For nickel and palladium complexes containing monoden-
tate ligand L’’ (L’’=bulky phosphane and NHC), the mono-
A
H
R
U
G
2 2
·4H O (0.367 g, 0.147 mmol) were used. A pale yellow powder
0
ligated species [M
A
C
H
T
R
E
U
N
G
(L’’)] is the generally accepted active spe-
H
46
N
4
P
2
2
[1]
cies for initial oxidative addition of a CꢀX bond. For the
1
2
ACHTREUNG
6
]DMSO): d=4.18 (d,
CH ), 5.17 (brs, 2H; PCH
), 6.29 (s, 2H; imi-H), 6.67 (m,
J
formation of an active species from the coordinatively satu-
A
H
B
2
2
2
2
II
0
2
ACHTREUNG
5
4
.78 (d,
H; CH
J
A B
H
rated tetraligated Ni complexes 2a–2d, reduction to Ni
and ligand dissociation can be assumed. Since NHC ligands
1
3
1
2
NMR ([D
6
are stronger s donors than a PPh moiety, dissociation of
2
3
1
1
the phosphane arm is more likely to occur. In fact, the poor
sufficient solubility of the compound; P{ H} NMR ([D ]DMSO): d=
6
+
activities of 2b among [Ni(L) ]Cl can be correlated with its
18.8 ppm (s); ESI-MS (DMF): m/z (%): 850.7 (15) [MꢀCl+H
2
O] , 832.1
2
2
+
2+
(
100) [MꢀCl] , 399.3 (80) [Mꢀ2Cl]
.
stronger NiꢀP bond compared to the other complexes, as
evidenced from their solid-state structures. Even though the
catalyst precursors are highly stable in air, the sensitivity of
the catalytic systems towards air is also in line with the in-
Preparation of 2c: The compound was prepared by following similar pro-
cedures to methods A and B for 2a. Method A: Compound 1c (0.310 g,
0
.729 mmol), NiCl
2
(0.473 g, 0.364 mmol), and NaOAc (0.598 g,
0
.729 mmol) were used. A pale yellow solid was obtained. Yield: 0.178 g,
0
volvement of air-sensitive intermediates (Ni state, free
54%. Method B: Compound 1c (0.200 g, 0.470 mmol) and Ni-
Chem. Eur. J. 2007, 13, 582 – 591
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
589

H. M. Lee et al.
A
C
H
T
R
E
U
N
G
(OAc)
2
·4H
2
O (0.585 g, 0.235 mmol) were used. A pale yellow solid was
Computational methods: Gradient-corrected density functional theory
obtained. Yield: 0.12 g, 57%; m.p. 197–2008C; elemental analysis calcd
(DFT) was used with the hybrid B3LYP functional, which consists of
[
31]
(
4
%) for C48
.93, N 6.18; H NMR ([D
H
44
1
F
2
N
4
P
2
Cl
2
Ni: C 63.70, H 4.90, N 6.19; found: C 63.67, H
three-parameter mixing of the nonlocal exchange functional of Becke
2
[32]
]DMSO): d=4.02 (d,
CH
J
A
H
R
N
and correlation functional of Lee, Yang, and Parr. The quality of basis
functions is essential, especially for the metals, in the computations. For
Ni and nonmetal atoms, 6-31G* basis sets were used in geometry optimi-
zations, and subsequent single-point calculations used the 6-311G** basis
6
NCH
A
H
B
), 4.90 (brs, 2H; PCH
2
2
), 5.18 (brs, 2H; PCH
2
2
2
J
A
C
H
T
R
E
U
N
G
(H,H)=13.5 Hz, 2H; NCH
A
H
B
), 6.28 (s, 2H; imi-H), 6.67 (m, 4H;
3
CH
8
2
2
-Ar), 7.01 (m, 4H; Ar-H), 7.09 (t, JHF =9.0 Hz, 4H; m-Ar-H), 7.31–
[
33]
1
3
1
set. For Pd, we used the basis set of Hay and Wadt (including effective
core potential) LAN(L) DZ, augmented with set of functions
Solvation effects of DMSO
dielectric constant e=46.7) were evaluated using the conductor polariza-
.04 ppm (m, 22H; Ar-H, imi-H, Ph-H); C{ H} NMR ([D
5.8, 47.6, 52.7, 116.0 (J(C,F)=21.5 Hz), 116.2, 123.4, 125.6, 129.5 (J-
(C,F)=
45.3 Hz); the carbenic carbon atom was not observed due to insufficient
6
]DMSO): d=
2
a
AHCTREUNG
[
34]
(
3s3p2d3f) suggested by Langhoff et al.
A
C
H
T
R
E
U
N
G
(C,F)=25.1 Hz), 130.5, 131.2, 131.3, 132.7, 134.3, 162.4 ppm (J
ACHTREUNG
(
2
[
35]
3
1
1
ble continuum model (CPCM) of Barone and Cossi In this model, the
solvation free energy is accounted for. The molecular free energy of the
solute embedded in a continuum medium is computed with this method,
in which the solute is polarized by the solvent, and the solute–solvent dis-
persion–repulsion energy is evaluated. It has been shown that the CPCM
model provides predictions for the free energy of hydration that are com-
parable with experimental results. All calculations were carried out by
solubility of the compound. P{ H} NMR ([D
ESI-MS (DMF): m/z (%): 869.2 (100) [MꢀCl] , 417.3 (55) [Mꢀ2Cl]
6
]DMSO): d=17.8 ppm (s);
+
2+
.
Preparation of 2d: The compound was prepared by following a similar
procedure to method B for 2a. Compound 1d (0.240 g, 0.549 mmol) and
Ni
was obtained. Yield: 0.12 g, 47%; m.p. 194–1968C; elemental analysis
calcd (%) for C50 Cl Ni: C 64.64, H 5.43, N 6.03; found: C
4.60, H 5.41, N 6.00; H NMR ([D
A
C
H
T
R
E
U
N
G
2 2
(OAc) ·4H O (0.683 g, 0.274 mmol) were used. A pale yellow solid
H
50
N
4
1
O
2
P
2
2
[36]
using the Gaussian03 program suite, RevisionC.02.
6
4
5
6
6
]DMSO): d=3.72 (s, 6H; OCH
(H,H)=13.9 Hz, 2H; NCH ), 4.86 (brs, 2H; PCH CH
(H,H)=13.9 Hz, 2H; NCH
3
2
),
),
),
2
.09 (d,
.23 (brs, 2H; PCH
.17 (s, 2H; imi-H), 6.58 (m, 4H; CH
J
A
C
H
T
R
E
U
N
G
A
H
B
2
2
2
CH
2
), 5.68 (d,
J
A
H
R
U
G
A
H
B
2
-Ar), 6.83–8.02 ppm (m, 30H; Ar-
1
3
1
1
Acknowledgements
H, imi-H, Ph-H); C{ H} NMR ([D
6
]DMSO): d=25.8 ( J
A
H
R
U
G
1
1
7.8 Hz), 47.5, 53.2, 55.6, 114.6, 123.0, 125.3, 125.5, 129.5, 130.0, 130.7,
31.1, 132.5, 132.9, 134.3, 159.8 ppm; the carbenic carbon atom was not
We are grateful to the National Science Council of Taiwan for financial
support of this work. We also thank the National Center for High-per-
formance Computing for computing time and facilities.
3
1
1
observed due to insufficient solubility of the compound; P{ H} NMR
[D ]DMSO): d=19.4 ppm (s); ESI-MS (DMF): m/z (%): 911.9
MꢀCl+H
Suzuki coupling reactions: In a typical reaction, a mixture of aryl halide
1.0 mmol), phenyboronic acid (1.3 mmol), base (2.6 mmol), catalyst (1–
mol%), and triphenylphosphane (0–6 mol%) in toluene (3 mL) was
(
[
6
+
+
2+
2
O] , 895.1 [MꢀCl] , 429.6 (10) [Mꢀ2Cl]
.
[
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diethyl ether, and dried with anhydrous MgSO . The solution was then
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d are listed in Table 1 and selected bond lengths and angles are given in
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4
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Received: April 8, 2006
Published online: September 12, 2006
Chem. Eur. J. 2007, 13, 582 – 591
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
591