N-donor functionalized N-heterocyclic carbene nickel(II) complexes in the Kumada coupling

Article abstract of DOI:10.1039/b905036a

The synthesis and characterization of novel nickel(II) complexes bearing two bidentate N-heterocyclic carbene ligands functionalized with anionic N-donor moieties are described. Two different N-donor groups are employed, namely amido and benzimidazolato moieties. The solid-state structures of three of these complexes have been determined by X-ray crystallography. The amido-functionalized low-spin, square-planar Ni(II) complexes exhibit a cis geometry around the metal centre, while the benzimidazolato-functionalized complex crystallizes as the trans isomer. The activity of these novel complexes in the Kumada cross-coupling of phenylmagnesium chloride with 4-chloroanisole and 4-fluoroanisole was investigated. One of the benzimidazolato-functionalized complexes shows the highest activity in this reaction reported to date, yielding the desired product in quantitative yields within 30 min (4-chloroanisole), or 150 min (4-fluoroanisole) with only 1 mol% catalyst.

Full text of DOI:10.1039/b905036a

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www.rsc.org/dalton | Dalton Transactions
N-donor functionalized N-heterocyclic carbene nickel(II) complexes in the
Kumada coupling†
Joris Berding,a Thomas F. van Dijkman,a Martin Lutz,b Anthony L. Spekb and Elisabeth Bouwman*a
Received 12th March 2009, Accepted 23rd April 2009
First published as an Advance Article on the web 2nd June 2009
DOI: 10.1039/b905036a
The synthesis and characterization of novel nickel(II) complexes bearing two bidentate N-heterocyclic
carbene ligands functionalized with anionic N-donor moieties are described. Two different N-donor
groups are employed, namely amido and benzimidazolato moieties. The solid-state structures of three
of these complexes have been determined by X-ray crystallography. The amido-functionalized low-spin,
square-planar Ni(II) complexes exhibit a cis geometry around the metal centre, while the
benzimidazolato-functionalized complex crystallizes as the trans isomer. The activity of these novel
complexes in the Kumada cross-coupling of phenylmagnesium chloride with 4-chloroanisole and
4-fluoroanisole was investigated. One of the benzimidazolato-functionalized complexes shows the
highest activity in this reaction reported to date, yielding the desired product in quantitative yields
within 30 min (4-chloroanisole), or 150 min (4-fluoroanisole) with only 1 mol% catalyst.
group for the same reaction.10–13 In most cases these chelating
Introduction
ligands are neutral, leading to either cationic Ni(II) complexes
The study and application of N-heterocyclic carbenes (NHCs)
or complexes with coordinated counterions, such as halides. A
have increased rapidly in recent years, most notably for their use
recent review describing donor-functionalized NHC complexes of
as spectator ligands in homogeneous catalysis.1–4 For example,
group 9 and 10 metals reveals that NHC ligands with anionic side
groups are quite uncommon.2 A number of nickel(II) complexes
of chelating ligands with N-heterocyclic carbenes and amido
functionalities have been recently reported to form active catalysts
in the Suzuki cross-coupling.14 The present paper describes the
Kumada coupling using nickel complexes of NHC ligands with
various pendant anionic N-donor moieties.
palladium NHC complexes have been used as versatile catalysts
for a number of C–C coupling reactions, such as the Heck, Stille,
Suzuki and Sonogashira reactions, which are of great importance
for organic chemistry.5 Though not as popular as their palladium
analogues, nickel-NHC complexes have also been investigated for
a number of C–C coupling reactions. One reaction in which nickel
complexes perform particularly well, compared to palladium, is
the Kumada cross-coupling of aryl halides with aryl Grignard
reagents. This reaction, discovered simultaneously by two groups
in 1972,6,7 may be an economical alternative to other cross-
coupling reactions, as it uses cheap starting materials and only
has magnesium salts as by-products, even though it lacks the
functional group tolerance.
An early report of the use of N-heterocyclic carbenes in the
nickel-catalyzed Kumada coupling from the group of Herrmann
revealed that imidazolium salts with bulky N-substituents in
combination with Ni(acac)2 generates an active catalyst for
the coupling of aryl chlorides with aryl Grignard reagents.8
Recently, we published our results in the Kumada coupling with
nickel complexes of chelating benzimidazole-based dicarbenes.9
Other groups investigated chelating ligands or ligand precursors
consisting of one (or more) NHC and one (or more) hemi-labile
Results and discussion
Synthesis of ligand precursors
An overview of the ligand precursors used in this study is
shown in Fig. 1. Ligand precursors 1 and 2 were synthesized
following an adaptation of literature procedures,15 by reacting
N-substituted (benz)imidazoles with 2-chloro-N-phenylacet-
amide. Novel ligand precursors 3 and 4 were obtained in good
yield by a facile quaternization of N-benzyl(benz)imidazole with
2-chloromethylbenzimidazole in hot 1,4-dioxane. The 1H and 13
C
NMR spectra of 1–4 show the characteristic imidazolium NCHN
resonances around 9.5 and 140 ppm, respectively. The formation
aLeiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300,
RA, Leiden, The Netherlands. E-mail: bouwman@chem.leidenuniv.nl;
Fax: (+31) 71 5274451; Tel: (+31) 71 5274550
bBijvoet Center for Biomolecular Research, Crystal and Structural Chem-
istry, Utrecht University, Padualaan 8, 3584, CH, Utrecht, The Netherlands
† Electronic supplementary information (ESI) available: Plots of the
hydrogen bonding interactions and of the superposition of the independent
complexes in the unit cell of 5a and 8, the 1H NMR spectrum of complex
7 and the COSY NMR spectrum of 8. CCDC reference numbers 723717–
723719. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/b905036a
Fig. 1 Overview of the ligand precursors used in this study.
6948 | Dalton Trans., 2009, 6948–6955
This journal is
The Royal Society of Chemistry 2009
©

of 1–4 was further confirmed with ESI-MS, with spectra showing
the parent peak for the respective [M - Cl]+ fragments.
compounds appear to be present as a single isomer in DMSO-
d6. The H NMR spectra of all complexes show splitting of the
1
backbone-CH2 resonances, apparently caused by the rigidity of
the structure in solution. The resonances of the benzimidazolato
group of complex 8 are split over a range of 1.5 ppm and could
be assigned with the aid of COSY NMR spectroscopy (see ESI†).
The ESI-MS spectra of the complexes show the [M + H]+ parent
peak for all complexes.
Synthesis and characterization of the nickel complexes
Treatment of ligand precursors 1–4 with potassium carbonate in
the presence of nickel(II) chloride in hot DMF yields yellow,
diamagnetic complexes 5 to 8, shown in Fig. 2. All complexes
are air- and moisture stable and could be isolated after treatment
with water in air. In principle such complexes can be obtained
as either the cis or the trans isomer. It was shown for complex
5b that the ratio of the two isomers depends on the polarity of
the solvent: more polar solvents yield a higher amount of the cis
complex, while in apolar solvents the complex isomerizes to the
trans isomer.14
Description of the structures
Single crystals suitable for X-ray crystal structure determination
were obtained from methanol (5a), by slow diffusion of hexane into
a concentrated solution of the compound in dichloromethane (6),
or by slow diffusion of diethyl ether into a concentrated solution
of the compound in chloroform (8). Molecular plots of 5a and
6 are shown in Fig. 3, selected bond lengths, angles and torsion
angles are given in Table 1.
The two independent metal entities of 5a as well as complex
6 have an approximate twofold symmetry, which is only broken
by slightly different conformations of the phenyl rings. The
Table 1 Selected bond lengths, angles, and torsion angles for 5a and 6.
[Second independent molecule of 5a in brackets]
Fig. 2 Schematic representation of the nickel complexes 5–8. Only the
trans isomer is depicted.
5a
6
Ni1–N110
Ni1–N210
Ni1–C12
Ni1–C22
C14–C15
C24–C25
1.9524(18) [1.9338(18)]
1.9511(18) [1.9582(18)]
1.849(2) [1.845(2)]
1.867(2) [1.849(2)]
1.337(4) [1.340(4)]
1.341(3) [1.349(3)]
1.926(3)
1.952(3)
1.852(4)
1.875(3)
1.387(5)
1.400(5)
In accordance with the square-planar geometry of the nickel(II)
1
complexes, the H NMR spectra of all complexes show sharp
signals in the diamagnetic region. The characteristic NCHN
signals of the precursors are absent from the NMR spectra of
these complexes, indicating carbene generation. In addition, the
NH signals are no longer present in the spectra of the complexes,
reflecting bidentate coordination of the ligand. The 1H NMR
spectrum of 7 in DMSO-d6 is indicative of a 0.7:0.3 mixture of trans
and cis isomers (see ESI†). No attempts were made to assign the
peaks to their respective isomers. In CDCl3 only one isomer of 7 is
present, presumably with the trans configuration. The other nickel
N110–Ni1–N210
N110–Ni1–C12
N210–Ni1–C22
C12–Ni1–C22
N11–C12–N13
N21–C22–N23
92.57(8) [94.12(8)]
88.44(9) [88.14(9)]
88.05(9) [87.87(8)]
91.02(10) [89.92(9)]
105.2(2) [104.98(19)]
104.74(19) [104.47(19)]
-43.82(18) [-46.37(17)]
-42.76(18) [-43.98(19)]
91.89(12)
85.60(14)
87.69(14)
94.71(15)
105.8(3)
106.3(3)
53.0(3)
N110–Ni1–C12–N13
N210–Ni1–C22–N23
42.0(3)
Fig. 3 Displacement ellipsoid plots (30% probability level) of 5a and 6 in the crystal. In 5a only one of two independent metal units is shown. In both
cases, hydrogen atoms and solvent molecules are omitted for clarity.
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 6948–6955 | 6949
©

Table 2 Selected bond lengths, angles and torsion angles for 8. [Second
nickel ions in these complexes are in a slightly distorted square-
planar geometry with cis-angles that range between 85.60(14) and
94.71(15)◦. The ligands are bound in a cis configuration. The Ni–
C and Ni–N bond distances are within the expected range.14 The
carbene (benz)imidazol-2-ylidene rings are twisted with respect to
the coordination plane by 45.08(11)–53.5(2)◦. The six-membered
chelate rings adopt a boat conformation in all cases.
The asymmetric unit of 5a contains two independent metal
complexes, four methanol and one water molecule, which are
connected by O–H ◊ ◊ ◊ O hydrogen bonds (see ESI†).
independent molecule in brackets]
Ni1–N110
Ni1–C12
C14–C15
1.8840(13) [1.8776(12)]
1.9133(19) [1.9070(15)]
1.370(3) [1.389(2)]
N110–Ni1–C12
N110–Ni1–C12i
N11–C12–N13
86.41(7) [86.59(6)]
93.59(7) [93.41(6)]
105.58(16) [106.33(13)]
42.19(14) [43.54(12)]
N110–Ni1–C12–N13
Two independent metal complexes are present in the crystal
structure of 8, which are both located on an inversion centre. The
asymmetric unit thus contains two half molecules. A molecular
plot is shown in Fig. 4 and a selection of bond lengths, angles and
torsion angles is listed in Table 2.
are slightly longer than those found in trans-[(NHC)2NiI2]
complexes, in which NHC is a monodentate benzimidazole-based
N-heterocyclic carbene.16 The Ni–N- bond lengths of compound
˚
8 of 1.8776(12)–1.8840(13) A are shorter than in other Ni
complexes with a N-deprotonated benzimidazole ligand, with
distances of 1.9004(14)17 and 2.0724(16) A.18 The carbene rings are
˚
twisted with respect to the NiC2N2 coordination plane by 45.46(9)
and 46.01(7)◦, while the N-coordinated benzimidazole rings are
twisted by 42.46(11) and 44.39(8)◦. The six-membered chelate
rings adopt boat conformations. The two independent complexes
in the asymmetric unit have roughly the same coordination
environment around the nickel centre. However, a significant
difference is observed in the orientation of the benzyl side group.
A superposition of the two molecules showing this difference
is given in the ESI.† One of the orientations brings the benzyl
aromatic ring in proximity of the nickel centre, resulting in an
anagostic interaction between one ortho proton and the nickel ion
◦
19
˚
(d(Ni ◊ ◊ ◊ H = 2.72 A, ∠(C–H ◊ ◊ ◊ Ni) = 151 ). Similar observa-
tions were reported recently for square-planar nickel complexes
containing phenylimidazole as a ligand.20 The other orientation
in the second independent metal complex allows a C–H ◊ ◊ ◊ p
interaction between one of the CH2-bridges as donor and the
benzyl aromatic ring as acceptor (a picture is given in the ESI†).
Catalytic studies
Complexes 5–8 were tested as catalysts in the Kumada coupling
of 4-chloroanisole (9-Cl) with phenylmagnesium chloride (10)
at room temperature (Scheme 1). The results are summarized
in Table 3. The reaction was monitored by taking samples at
regular intervals and analysis by gas chromatography. A number
of side-products was identified. Apart from the desired product
4-methoxybiphenyl (11), varying amounts of biphenyl (12), anisole
(13) and bisanisole (14) were found to be present in the reaction
mixtures.
Fig. 4 Displacement ellipsoid plot (30% probability level) of 8 in the
crystal. Only one of the independent metal complexes is shown. Hydrogen
atoms and solvent molecules are omitted for clarity. Symmetry operation
i: -x, -y, -z.
As implied by the inversion symmetry of the metal complexes,
a trans ligand geometry is present. The nickel ion has an almost
square-planar geometry, with Ni–C bond lengths of 1.9070(15)–
˚
1.9133(19) A. These distances are significantly longer than the
Initially, the catalytic reactions were performed using com-
monly used conditions, i.e. a 0.03:1:1.5 ratio of nickel complex,
Ni–C distances found for the cis complexes 5a and 6 and
Scheme 1 Products of the nickel-catalyzed Kumada cross-coupling of 4-haloanisole and phenylmagnesium chloride.
6950 | Dalton Trans., 2009, 6948–6955 This journal is The Royal Society of Chemistry 2009
©

Table 3 Nickel-catalyzed Kumada cross-coupling of 4-haloanisole and
the reaction, yielding the desired product in 85 to 90% yield.
The complexes 7 and 8 with benzimidazole-substituted ligands
are highly active, furnishing the product 11 in nearly quantitative
yields in less than 20 min. Used at a lower catalyst concentration,
compound 8 was able to couple the two reagents quantitatively
within 30 min at room temperature, corresponding to an average
turnover frequency of 200 mol. (mol cat.)-1 h-1. Moreover, even
4-fluoroanisole could be coupled efficiently and nearly quantita-
tively in 150 min at the same 1 mol% catalyst concentration.
Compared to the catalytic results obtained with nickel catalysts
with bidentate bisNHC ligands,9 the new complexes with anionic
(C,N-) ligands perform remarkably well. The best performing
bisNHC nickel catalyst found in our previous study was also able
to reach quantitative conversion of the aryl chloride to the desired
product; however, it required about 14 h to complete the reaction
(Table 3, entry 8).9
Several reports have been published on the nickel NHC
catalyzed Kumada coupling of 4-chloroanisole (9-Cl) with phenyl-
magnesium chloride.8–13,21–23 As different conditions were used
in different studies and furthermore, in most cases only the
conversion and selectivity after a fixed time are given, comparison
is difficult. The best performing nickel-NHC complexes to date
are the following. A nickel complex bearing a phosphane-NHC
(CŸP) bidentate ligand, gave 11 in 95% yield after 18 h, although
a kinetic study on one of the complexes revealed that the reaction
was actually completed within 1 h.10,21 In addition, using 0.5 mol%
Ni(IPr)(PPh3)Cl2 (IPr = 1,3-di(2,6-diisopropylphenyl)imidazole-
2-ylidene) product 11 could be obtained in 39% yield in
30 min,22 while a dinuclear nickel complex was reported to couple
4-chloroanisole with phenylmagnesium bromide in 91% yield in
12 h at the same catalyst loading.13
phenylmagnesium chloride at room temperaturea
Timeb
(min)
Yield (10-2 mmol)c
Entry
Catalyst
11
12
13
14
1
2
3
4
5
6
7
8
5a
150
75
150
20
12
30
85
87
90
99 (95)
97
99
98
99
10
13
15
3
6
3
1
3
2
0
0
0
0
0
6
4
3
0
2
0
1
0
5b
6
7
8
8d
8d,e
150
< 840
5
6
(CŸC)NiBr2
f
a Reaction conditions: 0.03 mmol cat., 1.0 mmol 4-chloroanisole, 1.5 mmol
phenylmagnesium chloride (25 wt% in THF), 1.0 mL THF, rt. b Time
needed for full consumption of 4-haloanisole. c Yields determined by GC,
average of two runs. Yields in parentheses are isolated yields. d 0.01 mmol
cat. e 4-Fluoroanisole was used a substrate. f Dibromido-1,1¢-dibenzyl-3,3¢-
(1,3-propanediyl)dibenzimidazol-2,2¢-diylidenenickel(II), see ref. 6.
4-chloroanisole, and phenylmagnesium chloride in THF at room
temperature (Table 3, entry 1–5).8–10,21 Additionally, complex 8
was used at a lower catalyst loading of 1.0 mol% (entry 6) and it
was used to couple 4-fluoroanisole (9-F) with phenylmagnesium
chloride at the same low catalyst loading (entry 7). As an example,
a plot of the evolution of 4-chloroanisole and the products in time,
using 8 as a catalyst (1.0 mol%) is depicted in Fig. 5.
The best performing nickel NHC catalysts for the Kumada
coupling of 4-fluoroanisole (9-F) with phenylmagnesium bromide
reported to date consists of a 1:1 mixture of Ni(acac)2 and a bulky
imidazolium salt, leading to 58% conversion to 11 in 18 h with
5 mol% nickel.24 Evidently, complex 8 is an extremely efficient
catalyst, in terms of both reaction rate and selectivity.
The reactivity of complex 5b has been reported for the Suzuki
coupling of phenylboronic acid with 4-chloroanisole, amongst
others.14 This yielded 4-methoxybiphenyl in 92% yield after 18 h,
using 3 mol% of catalyst, 6 mol% of PPh3 and stoichiometric
amounts of K3PO4 as additional base, at 80 ◦C in toluene. Clearly,
to obtain the desired coupling product the Kumada coupling is
more efficient and economical.
Fig. 5 Evolution of the products in time of a typical catalytic experiment
(Table 3, entry 6): (ꢀ) 4-chloroanisole, (ꢁ) 4-methoxybiphenyl, (ꢀ)
biphenyl.
The results of the catalytic reactions are summarized in Table 3
as the time needed to quantitatively consume the 4-haloanisole
with the yields (in mmol) of the desired product and the side
products. As the Grignard reagent was used in excess, and
unreacted Grignard reagent cannot be detected on GC, the
following constraints should apply:9 The sum of the amounts of
11, 13 and twice the amount of 14 should be equal to 1 mmol,
while the amount of 12 should not exceed half the amount of
Grignard reagent that did not react to form 11, which is equal to
0.5 ¥ (1.5 mmol—the amount of 11). Both requirements are met
in all catalytic experiments.
All complexes 5–8 are catalytically active in the coupling of
4-chloroanisole with phenylmagnesium chloride. The time needed
to consume all starting aryl halide and the selectivity, however, are
strongly dependent on the ligand used. The catalysts 5 and 6 with
the amide-substituted ligands need 75 to 150 min to complete
Mechanistic considerations
As our catalysts are the only ones described for the Kumada
coupling using a ligand with an anionic pendant arm, we believe
that the relatively high rates in our catalytic runs must be due
to the presence of these anionic side groups. The commonly
accepted mechanism for the Kumada coupling consists of three
consecutive steps: (1) oxidative addition of the aryl halide to a
Ni(0) active species, (2) substitution of the halide on the nickel
ion with the aryl group of the Grignard reagent (transmetalation)
and (3) reductive elimination of the biaryl product, furnishing
the coupling product and the starting Ni(0) species.25 The Ni(0)
species is believed to be obtained from the starting Ni(II) complex
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 6948–6955 | 6951
©

Scheme 2 Proposed mechanism for the activation of the pre-catalyst and the transition state for carbon–halide bond activation.
by two consecutive transmetalation steps, followed by a reductive
elimination of biphenyl.
1H and 13C NMR spectra were recorded on a Bruker DPX-300
spectrometer and are referenced against tetramethylsilane. IR
spectra were obtained on a Perkin-Elmer Paragon 1000 FT-IR
spectrophotometer. C, H, N determinations were performed
on a Perkin-Elmer 2400 Series II analyzer. Electrospray mass
spectra were recorded on a Finnigan TSQ-quantum instrument
using an electrospray ionization technique (ESI-MS), using
water/acetonitrile solutions. GC measurements were performed
on a Varian CP-3800 gas chromatograph equipped with an
autosampler. Retention times were compared to commercially
obtained compounds. Diethyleneglycol-di-n-butyl ether was used
as an internal standard.
In the present case this implies that the two anionic N-donor
moieties must be replaced by two phenyl groups, yielding a pen-
dant anionic N-donor group, coordinated to the liberated [MgCl]+
species (Scheme 2, B). These magnesium cations may then aid in
the oxidative addition of the aryl halide, similar to a mechanism
proposed by Yoshikai et al.26 Based on a theoretical model, they
designed a phosphane ligand with a phenolate pendant arm, which
accelerated the Kumada coupling of aryl fluorides presumably by
a nickel/magnesium bimetallic cooperation. A transition state for
the oxidative addition of the C–F bond was calculated, showing
how a magnesium ion coordinated to the phenolato group assisted
in the bond activation. The activation barrier was calculated to be
only 6.4 kcal mol-1 instead of the 35.5 kcal mol-1 needed for the
activation in the absence of the magnesium centre. The high rate of
our catalytic system may therefore also be explained by bimetallic
cooperation, i.e. through a similar transition state (Scheme 2, TS).
1-Methyl-3-(N-phenylaminocarbonylmethyl)imidazolium chlo-
ride (1a). A mixture of 1.0 g 2-chloro-N-phenylacetamide
(5.9 mmol) and 0.49 g N-methylimidazole (6.0 mmol) in 15 mL
dry THF was stirred at 75 ◦C for 48 h. The colourless precipitate
was isolated by filtration, washed with THF and diethyl ether
and dried in vacuo. Recrystallization from methanol/diethyl ether
afforded the pure compound.
Conclusions
Yield: 1.46 g (98%). 1H NMR (300 MHz, DMSO-d6, 300 K) d
11.29 (s, 1H, NH), 9.25 (s, 1H, NCHN), 7.79 (s, 1H, NCH), 7.74
(s, 1H, NCH), 7.66 (d, J = 8 Hz, 2H, Ar-H), 7.31 (t, J = 8 Hz, 2H,
A number of nickel complexes of chelating anionic (CŸN-) ligands
have been prepared and characterized. Two new compounds are
based on known amido-functionalized N-heterocyclic carbene
ligands, two other nickel complexes are obtained with novel
anionic benzimidazole-functionalized carbene ligands. Unexpect-
edly, these compounds are highly active as homogeneous catalysts
in the Kumada coupling of phenylmagnesium chloride with
4-chloroanisole. The highest rate and selectivity were obtained
using a benzimidazole-functionalized NHC, yielding the desired
product in quantitative yields with a TOF of 200 mol (mol
cat.)-1 h-1. Moreover, coupling of 4-fluoroanisole proceeded ef-
ficiently. The high rate of the reaction is explained by a possible
nickel/magnesium bimetallic cooperation.
Ar-H), 7.06 (t, J = 8 Hz, 1H, Ar-H), 5.31 (s, 2H, CH2), 3.91 (s, 3H,
13
=
CH3). C NMR (75 MHz, DMSO-d6, 300K) d 163.7 (C O), 138.5
(Cq), 137.8 (NCHN), 128.8 (Ar), 123.8 (NCH), 123.7 (Ar), 123.0
(NCH), 119.1 (Ar), 51.2 (CH2), 35.8 (CH3). IR (neat): 2979 (m),
1690 (s), 1553 (s), 1498 (m), 1444 (m), 1312 (m), 1254 (m), 1181
(s), 1108 (w), 942 (w), 784 (w), 752 (s), 695 (m), 622 (s) cm-1. ESI-
MS: m/z 216 ([M - Cl]+, 100%). Anal. Calcd for C12H14ClN3O: C,
57.26; H, 5.61; N, 16.69. Found: C, 57.58; H, 5.75; N, 17.08.
1-Benzyl-3-(N-phenylaminocarbonylmethyl)benzimidazolium ch-
loride (2). This compound was synthesized following the pro-
cedure described for 1a, starting from 1.70 g 2-chloro-N-
phenylacetamide (10 mmol) and 2.08 g N-benzylbenzimidazole
(10 mmol).
Experimental
General
1
Yield: 2.94 g (78%). H NMR (300 MHz, DMSO-d6, 300 K):
All reactions were performed under an atmosphere of dry ar-
gon using standard Schlenk techniques. N-benzylbenzimidazole27
and 2-chloro-N-phenylacetamide15 were prepared according to
literature procedures. All other chemicals were obtained from
commercial sources and used as received. Solvents were reagent
grade and used without further purification, except for THF
and 1,4-dioxane, which were distilled from CaH2 and stored on
molecular sieves under argon. DMF was degassed prior to use.
Nickel complex 5b was prepared following a reported procedure.14
d 11.15 (s, 1H, NH), 10.05 (s, 1H, NCHN), 8.00 (m, 2H, Ar-H),
7.64-7.27 (m, 10H, Ar-H), 7.08 (m, 1H, Ar-H), 5.86 (s, 2H, CH2),
5.63 (s, 2H, CH2). 13C NMR (75 MHz, DMSO-d6, 300K): d 164.7
=
(C O), 144.9 (NCHN), 139.5 (Cq), 135.1 (Cq), 133.1 (Cq), 131.6
(Cq), 130.3 (Ar), 130.2, (Ar), 130.1 (Ar), 129.5 (2 ¥ Ar), 128.2
(Ar), 128.0 (Ar), 125.3 (Ar), 120.5 (Ar), 115.1 (Ar), 51.2 (CH2),
50.4 (CH2). IR (neat): 3023 (w), 2973 (w), 1685 (m), 1602 (m),
1560 (s), 1492 (m), 1447 (m), 1351 (m), 1311 (m), 1261 (m), 1188
(m), 949 (w), 753 (s), 692 (s), 640 (m) cm-1. ESI-MS: m/z 341
6952 | Dalton Trans., 2009, 6948–6955
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The Royal Society of Chemistry 2009
©

([M - Cl]+, 100%). Anal. Calcd for C22H20ClN3O: C, 69.93; H,
5.33; N, 11.12. Found: C, 69.77; H, 5.50; N, 11.04.
125.6 (Ar), 122.6 (NCH), 122.2 (NCH), 120.8 (Ar), 56.8 (CH2),
35.2 (CH3). IR (neat): 3152 (w), 1601 (s), 1580 (s), 1558 (s), 1486
(m), 1445 (m), 1374 (s), 1296 (m), 1235 (m), 1075 (m), 754 (s), 693
(s), 534 (m), 502 (m) cm-1. ESI-MS: m/z 487 ([M + H]+, 100%),
216 ([ligand]+). Anal. Calcd for C24H24N6NiO2·H2O: C, 57.06; H,
5.19; N, 16.63. Found: C, 56.80; H, 5.51; N, 16.39.
1-(Benzimidazol-2-ylmethyl)-3-benzylimidazolium chloride (3).
A mixture of 2.37 g N-benzylimidazole (15.0 mmol) and 2.50 g
2-chloromethylbenzimidazole (15.0 mmol) in 20 mL dry 1,4-
dioxane was stirred at 100 ◦C for 48 h. The reaction mixture was
cooled and the off-white precipitate that had formed was collected
by filtration, washed with THF and diethyl ether and dried in
vacuo. The compound was further purified by recrystallization
from MeOH/diethyl ether.
Yield: 4.13 g (85%). 1H NMR (300 MHz, D2O, 300 K): d 9.63
(s, 1H, NCHN), 7.97 (s, 1H, CHIm), 7.90 (s, 1H, CHIm), 7.58 (m,
2H, CHBim), 7.50–7.35 (m, 5H, Ar-H), 7.24 (m, 2H, CHBim), 5.85
(s, 2H, CH2), 5.52 (s, 2H, CH2). 13C NMR (75 MHz, D2O, 300
Bis(1-benzyl-3-(N-phenylamidocarbonylmethyl)benzimidazol-2-
ylidene)nickel(II) (6). This compound was synthesized accord-
ing to the procedure given for 5a, starting from 1.13 g ben-
zimidazolium salt 2 (3.0 mmol), 1.23 g potassium carbonate
(9.0 mmol) and 0.19 g dry NiCl2 (1.5 mmol) in 40 mL DMF.
The complex was purified by repeated recrystallization from
dichloromethane/diethyl ether.
Yield: 0.78 g (70%). 1H NMR (300 MHz, DMSO-d6, 300 K) d
7.94 (d, J = 8 Hz, 2H, Ar-H), 7.53 (d, J = 8 Hz, 2H, Ar-H), 7.40
(t, J = 8 Hz, 2H, Ar-H), 7.29 (t, J = 8 Hz, 2H, Ar-H), 7.21 (m,
6H, Ar-H), 6.95-6.70 (m, 14H, Ar-H), 5.61 (m, 4H, CH2), 4.85 (d,
J = 16 Hz, 2H, CH2), 4.58 (d, J = 16 Hz, 2H, CH2). 13C NMR
=
K): d 147.8 (N C(CH2)N), 137.3 (NCHN), 134.7 (2 ¥ Cq), 129.0
(Ar), 128.7 (Ar), 128.4 (Ar), 123.7 (NCH), 122.6 (Ar), 52.0 (CH2),
46.2 (CH2). IR (neat): 3050 (w), 2459 (w), 1559 (m) 1426 (m), 1331
(w), 1270 (w), 1218 (w), 1161 (m), 1012 (w), 839 (w), 797 (m), 741
(s), 700 (s), 635 (m), 618 (m) cm-1. ESI-MS: m/z 289 ([M - Cl]+,
100%). Anal. Calcd for C18H16ClN4·2H2O: C, 60.08; H, 5.60; N,
15.57. Found: C, 59.95; H, 5.51; N, 15.55.
=
(75 MHz, DMSO-d6, 300K) d 179.5 (Ni–C), 166.4 (C O), 146.4
(Cq), 136.2 (Cq), 134.5 (Cq), 133.8 (Cq), 128.7 (Ar), 127.7 (Ar),
126.2 (Ar), 126.1 (Ar), 125.7 (Ar), 123.8 (Ar), 123.4 (Ar), 121.2
(Ar), 111.4 (Ar), 110.6 (Ar), 53.9 (CH2), 49.8 (CH2). IR (neat):
3131 (w), 1603 (s), 1581 (s), 1570 (s), 1486 (m), 1446 (s), 1362 (s),
1220 (m), 1078 (m), 1026 (m), 970 (w), 754 (m), 739 (s), 722 (m),
693 (s) cm-1. ESI-MS: m/z 739 ([M + H]+, 100%). Anal. Calcd
for C44H36N6NiO2·1.5H2O: C, 68.94; H, 5.13; N, 10.96. Found: C,
69.09; H, 5.08; N, 10.94.
1-(Benzimidazol-2-ylmethyl)-3-benzylbenzimidazolium chloride
(4). This ligand precursor was prepared following the pro-
cedure given for imidazolium salt 3, starting from 2.08 g
N-benzylbenzimidazole (10 mmol) and 1.67 g 2-chloromethyl-
benzimidazole (10 mmol) in 15 mL dry 1,4-dioxane.
Yield: 1.95 g (52%). 1H NMR (300 MHz, D2O, 300 K): d 9.79
(s, 1H, NCHN), 7.92 (m, 1H, Ar-H), 7.70 (m, 5H, Ar-H), 7.54 (m,
7H, Ar-H), 6.36 (s, 2H, CH2), 5.80 (s, 2H, CH2). The NH was not
observed. 13C NMR (75 MHz, D2O, 300 K): d 146.3 (NC(CH2)N,
143.6 (NCHN), 133.7 (Cq), 133.3 (Cq), 132.2 (2 ¥ Cq), 130.5 (Ar),
129.7 (Ar), 129.1 (Ar), 128.8 (Ar), 127.2 (Ar), 115.6 (Ar), 115.3
(2 ¥ Ar), 113.7 (Ar), 52.2 (CH2), 43.9 (CH2). IR (neat): 2969 (w),
2485 (w), 1617 (w), 1555 (m), 1458 (w), 1428 (w), 1376 (w), 1220
(w), 1181 (m), 1027 (w), 880 (m), 754 (s), 710 (s), 619 (m), 597
(m) cm-1. ESI-MS: m/z 339 ([M - Cl]+, 100%). Anal. Calcd for
C22H19ClN4·1.8H2O: C, 64.88; H, 5.59; N, 13.76. Found: C, 64.86;
H, 5.58; N, 13.95.
Bis(1-(benzimidazolato-2-ylmethyl)-3-benzylimidazol-2-ylidene)-
nickel(II) (7). This complex was obtained following the
procedure given for complex 5a, starting from 0.97 g imidazolium
salt 3 (3.0 mmol), 1.23 g potassium carbonate (9.0 mmol) and
0.19 g dry NiCl2 (1.5 mmol) in 40 mL DMF. The complex was
purified by recrystallization from dichloromethane/diethyl ether.
Yield: 0.74 g (78%). 1H NMR (300 MHz, CDCl3, 300 K): d 7.68
(d, J = 8 Hz, 2H, Ar-H), 7.45–7.05 (m, 12H, Ar-H), 6.93 (t, J =
8 Hz, 2H, Ar-H), 6.78 (m, 3H, Ar-H), 6.42 (m, 3H, Ar-H), 5.66
(d, J = 15 Hz, 2H, CH2), 5.19 (d, J = 15 Hz, 2H, CH2), 3.93 (d,
J = 16 Hz, 2H, CH2), 3.80 (d, J = 16 Hz, 2H, CH2). 13C NMR
(75 MHz, CDCl3, 300K): d 171.2 (Ni–C), 155.5 (Cq), 146.2 (Cq),
144.1 (Cq), 136.3 (Cq), 128.8 (Ar), 127.9 (Ar), 127.3 (Ar), 121.6
(Ar), 121.1 (Ar), 120.2 (Im–C), 120.0 (Im–C), 118.8 (Ar), 113.4
(Ar), 52.5 (CH2), 50.7 (CH2). IR (neat): 3060 (w), 1605 (w), 1446
(m), 1393 (m), 1304 (w), 1270 (m), 1338 (w), 1158 (w), 857 (w), 741
(s), 726 (s), 702 (s) cm-1. ESI-MS: m/z 633 ([M + H]+, 100%). Anal.
Calcd for C36H30N8Ni·H2O: C, 66.38; H, 4.95; N, 17.20. Found:
C, 66.43; H, 5.23; N, 17.43.
Bis(1-methyl-3-(N -phenylamidocarbonylmethyl)imidazol-2-
ylidene)nickel(II) (5a). Based on the procedure given by Liao
et al.,14 a mixture of 0.38 g imidazolium salt 1a (1.5 mmol), 97 mg
dry NiCl2 (0.75 mmol) and 0.62 g potassium carbonate (4.5 mmol)
were heated in DMF at 130 ◦C for 16 h. After cooling, the yellow
solution was filtered and the filtrate was evaporated to dryness
in vacuo. The remaining solid was dissolved in dichloromethane
and washed with water and brine. After drying with magnesium
sulfate, the solvent was reduced in volume to 10 mL. Addition
of diethyl ether yielded a bright yellow precipitate which was
collected by filtration, washed with diethyl ether and dried in
vacuo. The complex was purified by repeated recrystallization from
methanol/hexane.
Yield: 0.27 g (73%). 1H NMR (300 MHz, DMSO-d6, 300K) d
7.52 (d, J = 1.5 Hz, 2H, NCH), 7.25 (d, J = 7.5 Hz, 4H, Ar-H),
7.21 (d, J = 1.5 Hz, 2H, NCH), 6.91 (t, J = 7.5 Hz, 4H, Ar-H), 6.75
(t, J = 7.5 Hz, 2H, Ar-H), 5.71 (d, J = 14.5 Hz, 2H, CH2), 4.31
(d, J = 14.5 Hz, 2H, CH2), 3.13 (s, 6H, CH3). 13C NMR (75 MHz,
DMSO-d6, 300K) d 166.8 (Cq), 165.5 (Cq), 147.2 (Cq), 126.5 (Ar),
Bis(1-(benzimidazolato-2-ylmethyl)-3-benzylbenzimidazol-2-
ylidene)nickel(II) (8). This compound was synthesized following
the procedure given for complex 5a, starting from 1.13 g benzim-
idazolium salt 4 (3.0 mmol), 0.19 g dry NiCl2 (1.5 mmol), 1.23 g
potassium carbonate (9.0 mmol) in 40 mL DMF. Recrystallization
from chloroform/diethyl ether yielded the pure compound.
Yield: 0.67 g (61%). 1H NMR (300 MHz, CDCl3, 300 K): d 7.55
(d, 2H, J = 8 Hz, Ar-HBim), 7.43 (d, 2H, J = 8 Hz, Ar-HBim-), 7.29
(t, 2H, J = 8 Hz, Ar-HBim), 7.25–7.05 (m, 14H, Ar-H), 6.75 (dt,
2H, J = 1 Hz, J = 8 Hz, Ar-HBim-), 6.42 (dt, 2H, J = 1 Hz, J =
8 Hz, Ar-HBim-), 6.08 (d, 2H, J = 8 Hz, Ar-HBim-), 5.92 (d, 2H,
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The Royal Society of Chemistry 2009
Dalton Trans., 2009, 6948–6955 | 6953
©

Table 4 Details of the X-ray crystal structure determinations
5a
6
8
Formula
FW
C24H24N6NiO2·2CH3OH·0.5H2O
560.29
C44H36N6NiO2 + disordered solvent
C44H34N8Ni + disordered solvent
739.50a
733.50a
Crystal colour
crystal size [mm3]
Crystal system
Space group
Yellow
Yellow
Yellow
0.60 ¥ 0.24 ¥ 0.09
Monoclinic
P21/c (no. 14)
20.7321(3)
8.5908(1)
33.5771(6)
—
117.8263(8)
—
5288.74(14)
8
1.407
0.65
41023/11971
0.78
multi-scan
0.84–0.94
684/0
0.30 ¥ 0.30 ¥ 0.20
monoclinic
P21/c (no. 14)
11.3697(2)
16.9361(4)
24.0007(6)
—
0.30 ¥ 0.30 ¥ 0.20
triclinic
¯
P1 (no. 2)
˚
a [A]
12.5841(1)
13.4533(2)
15.7310(2)
109.7758(8)
103.2347(6)
95.7404(5)
2393.36(5)
2
˚
b [A]
˚
c [A]
a [◦]
b [◦]
g [◦]
115.0097(13)
—
3
˚
V [A ]
Z
4188.20(16)
4
Dx [g/cm3]
1.173a
1.018a
0.65
-1
˚
(sin q/l)max [A ]
refl. meas./unique
m [mm-1]
0.60
33596/7579
44462/10861
0.50a
0.44a
abs. corr.
multi-scan
0.75–0.90
478/0
0.0608/0.1601
0.0893/0.1730
0.0672
multi-scan
0.76–0.92
481/0
0.0404/0.1009
0.0534/0.1050
0.0514
abs. corr. range
param./restraints
R1/wR2 [I>2s(I)]
R1/wR2 [all refl.]
R(int)
0.0420/0.1007
0.0661/0.1144
0.0549
1.065
-0.43/0.90
S
1.059
-0.43/1.03
1.061
-0.39/0.33
3
˚
Res. density [e/A ]
a Derived values do not contain the contribution of the disordered solvent.
J = 14 Hz, NCH2CN2), 5.81 (d, 2H, J = 14 Hz, NCH2CN2), 5.28
(d, 2H, J = 16 Hz, CH2Ph), 4.79 (d, 2H, J = 16 Hz, CH2Ph).
13C NMR (75 MHz, CDCl3, 300K): d 181.2 (Ni-C), 153.5 (Cq
NiNC=N(CH2)), 143.6 (Cq benzyl), 134.1 (3*Cq benzimidazole),
134.0 (Cq benzimidazole), 129.0 (Ar benzyl), 128.4 (Ar), 125.7 (Ar
benzyl), 124.4 (Ar), 123.8 (Ar), 120.1 (Ar), 119.8 (Ar), 117.6 (Ar),
114.6 (Ar), 110.5 (2*Ar), 50.7 (CH2), 47.5 (CH2). IR (neat): 3059
(w), 1606 (w), 1476 (w), 1446 (m), 1394 (m), 1340 (w), 1310 (w),
1269 (m), 1211 (w), 1182 (w), 735 (s), 702 (m) cm-1. ESI-MS: m/z
733 ([M + H]+, 100%). Anal. Calcd for C44H34N8Ni·0.25CHCl3:
C, 69.62; H, 4.52; N, 14.68. Found: C, 69.44; H, 4.86; N, 15.00.
X-Ray crystal structure determinations
X-Ray reflections were measured with Mo-Ka radiation (l =
˚
0.71073 A) on a Nonius KappaCCD diffractometer with rotating
anode at a temperature of 150 K. The intensities were integrated
using HKL2000.28 Absorption correction, scaling and merging
was performed with SORTAV.29 The structures were solved
with Direct Methods (program SIR-9730 for 5a and 6, program
SHELXS-9731 for 8). Refinement was performed with SHELXL-
9731 against F2 of all reflections. Non hydrogen atoms were refined
with anisotropic displacement parameters. In 5a all hydrogen
atoms were located in difference Fourier maps. O–H hydrogen
atoms were kept fixed at their located position; C–H hydrogen
atoms were refined with a riding model. In 6 and 8, all hydrogen
atoms were introduced in calculated positions and refined with
a riding model. Geometry calculations and checking for higher
symmetry was performed with the PLATON program.32 Further
details are given in Table 4.
General procedure for the Kumada coupling
At room temperature, 0.03 mmol nickel complex was dis-
solved/suspended in 1.0 mL THF, followed by the addition of
1.0 mmol of the 4-haloanisole. The reaction was started with the
addition of 1.5 mmol phenylmagnesium chloride (25 wt% in THF,
0.78 mL) and monitored by taking samples at regular intervals
until GC analysis showed full consumption of the 4-haloanisole.
All catalytic reactions were performed in duplicate and were found
to give consistent results.
To isolate the desired coupling product, water was added to
the reaction mixture, followed by extraction into ethyl acetate
(3 ¥ 20 mL). The organic fractions were combined, dried with
magnesium sulfate and evaporated to dryness. Purification by col-
umn chromatography on silica gel (95:5 hexane/dichloromethane)
Crystal structures 6 and 8 contain large solvent accessible voids
3
3
˚
˚
(880 A /unit cell in 6, 852 A /unit cell in 8) filled with disordered
solvent molecules. Their contribution to the structure factors was
secured by back-Fourier transformation using the SQUEEZE
routine of PLATON,32 resulting in 199 electrons/unit cell for 6
and 367 electrons/unit cell for 8.
Acknowledgements
yielded 4-methoxybiphenyl as a colourless solid. 1H NMR and 13
C
We thank Prof. J. Reedijk for continuous support of our work and
valuable discussions. This work has been supported financially
by the National Graduate Research School Combination NRSC-
Catalysis, a joint activity of the graduate research schools NIOK,
NMR spectra were in agreement with the proposed structure,12
and the GC retention time corresponded to that of a commercial
reference sample.
6954 | Dalton Trans., 2009, 6948–6955
This journal is
The Royal Society of Chemistry 2009
©

HRSMC and PTN. This work was supported in part (M. L., A.
L. S.) by the Council for the Chemical Sciences of the Netherlands
Organization for Scientific Research (CW-NWO).
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This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 6948–6955 | 6955
©