Synthesis, structural characterization of benzimidazole-functionalized Ni(II) and Hg(II) N-heterocyclic carbene complexes and their applications as efficient catalysts for Friedel-Crafts alkylations

Article abstract of DOI:10.1016/j.jorganchem.2011.04.039

[Ni(L₁)₂](PF₆)₂ (3a, L ₁ = 3-(1-ethyl-1H-benzimidazol-2-yl)methyl)-1-((6-methylpyridin-2-yl) methyl)imidazolyl-idene), [Ni(L₂)₂(CH₃CN)] (PF₆)₂ (3

Full text of DOI:10.1016/j.jorganchem.2011.04.039

Journal of Organometallic Chemistry 696 (2011) 2949e2957
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis, structural characterization of benzimidazole-functionalized Ni(II)
and Hg(II) N-heterocyclic carbene complexes and their applications as efficient
catalysts for FriedeleCrafts alkylations
,
a,
Guoli Huang a, Hongsheng Sun a, Xiaojie Qiu b, Yingzhong Shen b, Juli Jiang a , Leyong Wang
*
*
a Key Laboratory of Mesoscopic Chemistry, Ministry of Education and School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China
b College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210000, China
a r t i c l e i n f o
a b s t r a c t
Article history:
[Ni(L1)2](PF6)2 (3a, L1
imidazolyl-idene), [Ni(L2)2(CH3CN)](PF6)2 (3b, L2
¼
3-(1-ethyl-1H-benzimidazol-2-yl)methyl)-1-((6-methylpyridin-2-yl)methyl)
Received 19 February 2011
Received in revised form
9 April 2011
¼
3-(1-ethyl-1H-benzimidazol-2-yl)methyl)-1-((6-
methylpyridin-2-yl)-methyl)benzimidazolylidene), and [Hg(L1)2(CH3CN)2](PF6)2 (4) have been success-
fully prepared and fully characterized by NMR, ESI-MS spectroscopy, and X-ray diffraction analysis. The
nickel complexes reveal a square-planar structure with two carbene ligands and benzimidazole groups at
the cis configuration. The nickel complex 3b has been proved to be a highly efficient catalyst for Friedele
Crafts alkylation of indoles with
The crystal packing structure of 4 shows that double-stranded 1D supramolecular chains are formed by
inter-chain benzimidazole rings and pyridine rings face-to-face stacking interactions.
Ó 2011 Elsevier B.V. All rights reserved.
Accepted 13 April 2011
Keywords:
N-Heterocyclic carbene
Nickel
Mercury
FriedeleCrafts alkylation
Indole
b-nitrostyrenes at room temperature in moderate-to-excellent yields.
p-p
Catalysis
1. Introduction
functionalized NHC ligands and found to be good catalysts for CeC
coupling reactions. However, only symmetric CNC [38e45] and
NCN [46e48] type pincer ligands with one or two NHC donors were
well studied, and there are few reports concerning the preparation
and use of unsymmetric pincer ligands [37,49e51]. An unsym-
metric pincer ligand containing a carbene and side weaker ancillary
groups would be useful in homogeneous catalysis since the strong
carbene donor facilitates the electronic density of metal, and the
weaker donor groups can dynamically dissociate to generate
coordinatively unsaturated active species [52].
FriedeleCrafts alkylation is one of the most important CeC bond
formation reactions in organic synthesis, which has been received
considerable attention because of the synthetic versatility of indole
derivatives in the preparation of pharmacologically and biologically
active indole alkaloids [53]. A number of successfully catalytic
systems have been reported for FriedeleCrafts reaction by using
metal complexes [54e56]. However, to the best of our knowledge,
only a few metal NHC complexes have been well studied as cata-
lysts in this reaction [57,58]. Nickel, as a much cheaper metal and
the most promising alternative to other expensive transition
metals, has been attracted increasingly attention.
Since the first isolation of a stable, free N-heterocyclic carbene
(NHC) by Arduengo in 1991 [1], NHCs had become crucial ligands in
organometallic chemistry and homogeneous catalysis [2e5]. As an
alternative to phosphine, N-heterocyclic carbenes have excellent s-
donating properties, which can well stabilize various oxidation
states of transition metal in a catalytic cycle. The metal-NHC
complexes show outstanding catalytic activity in tremendous
organic transformation, especially in olefin metathesis [6e10], CeC
[11e15], and CeN [16,17] bond formation reactions. Moreover, NHC
ligands can be easily modified by changing the substituents on the
nitrogen atom or the backbone in order to provide diverse organ-
ometallic materials [11,18,19]. In recent years, much attention has
been attracted for the design and synthesis of carbene hybrid
ligands containing N-heterocyclic donor groups, such as pyridine
[20e25], pyrimidine [25,26], quinoline [27,28], benzimidazole
[29,30], oxazoline [31e36], and phenanthroline [37]. The transition
metal complexes have been stabilized by the weaker donor groups
We lay our interests in the design and synthesis of transition
metal complexes with stable NHC ligands, and then development of
* Corresponding authors. Tel./fax: þ86 25 83597090.
E-mail addresses: jjl@nju.edu.cn (J. Jiang), lywang@nju.edu.cn (L. Wang).
0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.04.039

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G. Huang et al. / Journal of Organometallic Chemistry 696 (2011) 2949e2957
efficient catalyst systems and cost-saving catalysts. In this paper we
describe the synthesis and structural characterization of nickel and
mercury complexes containing benzimidazole-functionalized NHC
ligands. These nickel-NHC complexes are highly efficient catalysts
for FriedeleCrafts alkylation of indoles with nitroalkenes under
mild conditions. Double-stranded 1D supramolecular chains are
formed by the p-p stacking interaction in the X-ray crystal structure
of mercury complex.
2. Results and discussion
2.1. Synthesis and spectroscopic characterization
Fig. 1. Molecular structure of imidazolium salt 2b ꢀ(showing thermal ellipsoids at the
30% probability level, hydrogen atoms and the PF6 anion are omitted for clarity).
The ligand precursor [HL1]PF6 (2a) and [HL2]PF6 (2b), contain-
ing benzimidazole and pyridyl group, were prepared by refluxing
2-chloromethyl-1-ethyl-benzimidazole [29,59] with N-(6-
methylpyridyl-methyl)imidazole and N-(6-methylpyridylmethyl)
benzimidazole in toluene and subsequent anion exchange reaction
with NH4PF6 in water affording high yields, 87% for 2a and 79% for
2b (Scheme 1). The 1H NMR spectrum of 2a and 2b in DMSO-d6
showed a downfield resonance signal at 9.45 and 10.11 ppm,
assignable to the acidic NCHN proton of the imidazolium ring. The
13C NMR spectra exhibit characteristic downfield resonances at
158.4 and 158.8 ppm for the NCN carbons. The positive mode of
ESI-MS give base peaks at m/z ¼ 332.3 and 382.3 assigned to the
cations of 2a and 2b respectively. The molecular structure of 2b has
been unambiguously characterized by single-crystal X-ray diffrac-
tion (Fig. 1, CCDC number: 803959), which indeed revealed the
expected structure of benzimidazole and pyridine-functionalized
imidazolium.
(J ¼ 15.6 Hz), 6.65, 6.93 ppm (J ¼ 17.1 Hz) for 3b (Fig. 2). This
illustrates clearly that the two methylene prontons are inequivalent
and the rotation of the imidazole ring is prohibited by complexa-
tion. In the 13C NMR spectra of 3a and 3b, the characteristic reso-
nances ascribed to the carbenic carbon atoms appear at 161.7 and
174.0 ppm consistent with the reported values for Ni-NHC
complexes. The 13C chemical shifts of known Ni-NHC complexes
appear in the range of 149e180 ppm, depending upon the ancillary
ligands [66e68]. The positive mode of ESI-MS spectra give no
further structural information except confirmation of the presence
of [Ni(L)2]2þ for both 3a (m/z 360.5) and 3b (m/z 382.2). Complexes
3a and 3b are highly soluble in common solvents such as CH3CN,
DMF and DMSO, and are stable toward air and moisture in its solid
state and in solution.
Although a popular route for the preparation of transition metal
complexes is the transmetalation of the NHC from a silver complex
that can be synthesized directly by reacting Ag2O with the imida-
zolium salt, since that Wanzlick and Schönherr reported the first
NHC-mercury complex, made by reaction of an imidazolium salt
with a basic mercury source, mercury(II) acetate [69], all NHC-
mercury complexes reported in the literature are prepared by this
one-pot mercuration method [70e79]. To investigate whether the
NHC precursors 2a do form stable chelating complexes with line-
arly coordinating metals, the imidazolium salt 2a was reacted with
Hg(OAc)2 in a 1:0.5 M ratio in refluxing CH3CN for 48 h (Scheme 2),
a protocol used by Wanzlick for the synthesis of the first reported
metal NHC complex [69]. The success of the formation of the Hg-
NHC complexes was confirmed by the disappearance of the acidic
NCHN proton in the 1H NMR spectrum. Additionally, in the 13C
resonance of 4 the carbenic carbon atom exhibited a shift from
As well known to all, nickel-NHC complexes can be prepared
from the reaction of imidazolium salts with Ni(OAc)2 [42,60] or
NiCl2 [61e63], or the carbeneetransfer reaction of a silver-NHC
complex with Ni(DME)Cl2 or Ni(PPh3)2Cl2 [64]. The direct reac-
tion of [HL](PF6) (2a, 2b) and Ni(OAc)2 or NiCl2 under the reported
conditions did not give the expected complexes. Nevertheless, the
deprotonation reaction of [HL](PF6) (2a, 2b) with 0.55 equiv of Ag2O
proceed smoothly in CH3CN at room temperature, and as a result,
generate a yellow solution of silver-carbene compound according
to the procedure developed by Lin et al. [65]. Subsequently by metal
exchange with 0.5 equiv of Ni(PPh3)2Cl2 in acetonitrile resulted in
nickel(II)eNHC complexes (3a, 3b) as a pale yellow solid in 61% and
86% yield over two steps. (Scheme 2). The two nickel complexes
were fully characterized by elemental analysis, NMR spectroscopy,
and X-ray crystallography. The formation of the Ni-NHC complexes
was confirmed by the absence of the 1H NMR resonance for the
acidic NCHN proton at 9.45 and 10.11 ppm, respectively, where the
resonance signals of 2a and 2b were found. In the NMR data, two
types of methylene prontons give evidently two singlets at 5.59,
5.94 ppm for 2a, and 5.97, 6.31 ppm for 2b. Nevertheless, in the 1H
NMR spectrum of 3a, 3b, with the help of H, H-COSY spectrum,
methylene prontons give four doublets at 4.45, 5.13 ppm
(J ¼ 15.6 Hz), 6.24, 6.55 ppm (J ¼ 16.2 Hz) for 3a, and 5.02, 5.73 ppm
d
158.4 ppm for 2a to 173.8 ppm, within the range reported in
the literature for other mercury-carbene carbon signals
(d
170e185 ppm) [73,75,79]. The Hg-NHC complex, as a PF6ꢀ salt, was
readily soluble in DMSO, DMF, acetone, and acetonitrile, and
insoluble in water and diethyl ether. The complex is tolerant of air
and moisture. Exposure of CD3CN solutions of this complex to air
and moisture for more than 7 days resulted in no visible change in
Scheme 1. Synthesis of ligand precursor [HL]PF6.

G. Huang et al. / Journal of Organometallic Chemistry 696 (2011) 2949e2957
2951
Scheme 2. Synthesis of NHC complexes of NiII (3a and 3b) and HgII (4).
1H NMR spectra. Different from complexes 3a and 3b, in the 1H
NMR spectrum of 4 two kinds of methylene prontons appeared two
signals at 5.82 and 5.54 ppm similar to 2a (Fig. 2), revealing that the
two methylene prontons are equivalent and the rotation of the
imidazole ring in 4 is free at room temperature.
normal and well consistent with those of nickel(II)eNHC
complexes displaying square-planar geometry ranging
(1.835e2.036 Å) for NieC bond length and (1.891e2.336 Å) for
NieN bond length [26,30,81].
The molecular structure of 3b is depicted in Fig. 4 (CCDC
number: 803961). 3b crystallizes in monoclinic space group P21/n
and reveals a similar square-planar structure to 3a. As anticipated,
the two pyridyl groups have moved far away to avoid conflict by
placing the two carbene carbon at a mutually cis configuration. The
equatorial NieC (1.849e1.863 Å) and NieN (1.908e1.940 Å) bond
distances are quite normal and comparable to the reported values
of the Ni-NHC complexes [80,81].
2.2. X-ray structural description
Slow diffusion of diethyl ether into the acetonitrile solution of
3a yielded suitable single crystal for X-ray diffraction studies. Chen
reported the rigidity and steric bulkiness of ligand significantly
affected the structure of corresponding complex, the coordination
numbers getting higher when the ligands become bulkier [80]. The
nickel(II) ion tends to form stable square-planar complexes with
NHC ligands. The structure of 3a is shown in Fig. 3 (CCDC number:
803962). Nickel is four-coordinated by two carbon atoms from
imidazolilydenes and other two nitrogen atoms from 1-ethyl-
benzimidazole groups at the cis configuration, but two pyridyl
groups locate in the two sides of the coordination plane of nickel
without being coordinated. This is similar to the reported
benzimidazole-functionalized imidazolium or pyridine function-
alized imidazolium Ni-NHC complexes. The NieC (1.917(4) and
1.950(4) Å) and NieN bond distances (1.988(3) and 1.922(3) Å) are
Diffusion of diethyl ether into an acetonitrile solution of 4
afforded
colorless
crystals
with
the
formulation
of
[Hg(L1)2(PF6)2(CH3CN)2]. The molecular structure of 4 determined
by X-ray diffraction analysis is shown in Fig. 5 (CCDC number:
803960). The mercury atom on this occasion is disposed on
a crystallographic inversion center, and one-half of the formula unit
comprises the asymmetric unit of the structure. Mercury is located
in a linear coordination geometry surrounded by two imidazolily-
denes, but two pyridyl and 1-ethyl-benzimidazole groups are not
coordinated and are rotated an angle from the imidazolium ring to
reduce the steric interactions. The distance of Hg(1)-C(9) (2.070(6)
Fig. 2. (I) 1H NMR spectrum of 2a and [Ni(L1)2](PF6)2 (3a) illustrating the two magnetically inequivalent methylene prontons; 2a and [Hg(L1)2](PF6)2 (4) illustrating the two
magnetically equivalent methylene prontons. (II) 1H NMR spectrum of 2b and [Ni(L2)2](PF6)2 (3b) illustrating the two magnetically inequivalent methylene prontons.

2952
G. Huang et al. / Journal of Organometallic Chemistry 696 (2011) 2949e2957
Fig. 5. ORTEP view of the cationic section of 4 with 30% thermal ellipsoids and labeling
scheme. Hydrogen atoms and CH3CN molecule are omitted for clarity. Selected bond
distances (Å) and angles (ꢁ): Hg1-C9 2.070(6), C9-N4 1.334(8), C9-N3 1.344(9), C9-Hg1-
C9a 180.0(3), N4-C9-Hg1 126.6(5), N3-C9-Hg1 127.5(4), N3-C9-N4 105.5(5).
Fig. 3. ORTEP view of the cationic section of 3a with 30% thermal ellipsoids and
labeling scheme. Hydrogen atoms are omitted for clarity. Selected bond distances (Å)
and angles (ꢁ): C11-Ni1 1.917(4), C31 Ni1 1.950(4), N1-Ni1 1.988(3), N6-Ni1 1.922(3).
C11-Ni1-N6 178.63(15), C31-Ni1-N1 177.91(15), C11-Ni1-C31 91.62(16), C11-Ni1-N1
90.45(15), N6-Ni1-C31 88.32(15), N6-Ni1-N1 89.61(13), N3-C11-N4 107.3(3), N8-C31-
N9 107.0(3), C7-N1-C1 104.6(3), C27-N6-C21 105.9(3).
Additionally, in the packing diagrams of 4, two neighboring 1D
chains are further connected together through C(18)eH(18B)$$$
Å) is similar to those rꢀeported elsewhere for other carbene
complexes [74]. The PF6 anion forms hydrogen bonds with
neighboring atoms from the cations (Fig. 6I) with the CeH$$$ꢀF
bonds in the range from 3.236 (F5-C11) to 3.277 (F1-C8) Å. The PF6
anions thus connect two adjacent cations and form infinite chains.
N(6) (bond length 3.296 Å) and two types of aromatic
p-p stacking
interactions (Fig. 6II). The face-to-face interactions form inter-
p-p
chain benzimidazole and pyridine rings with interplanar separa-
tions of 3.478 Å for benzimidazole rings and 3.605 Å for pyridine
rings [center-to-center separations: 3.870 Å for benzimidazole
rings and 3.750 Å for pyridine rings] [82e84]. In all, each molecule
utilizes its two benzimidazole rings and two pyridine rings to pack
with other four adjacent molecules by p-p interactions to generate
double-stranded 1D supramolecular chains along a axis (Fig. 6III).
2.3. Catalysis
After obtaining nickel complexes 3a and 3b, we then turned our
attention to FriedeleCrafts alkylation of indole with b-nitrostyrenes
to examine the catalytic efficiencies of the complexes 3a and 3b. We
chose the initial reaction of indole and 2-nitrovinylbenzene as
model reaction to optimize the reaction conditions and evaluate the
catalytic activities of these nickel complexes. The results of these
experiments were summarized in Table 1 and revealed that the
desired product was obtained in a yield of 85% when 1.0 mol % of
nickel complex 3b was used at room temperature (20 ꢁC) in CH2Cl2
for 24 h (Table 1, entry 2), but 3a was less effective compared with
3b. Presumptively, 3a was much more difficult to form the activated
1,3-metal bonding species with nitroalkenes [54]. When the catalyst
loading was increased to 2.0 mol %, the yield was not significantly
increased (entry 3). However, decrease of the catalyst loading to
0.5 mol % lowered the yields of FriedeleCrafts product to 80% (entry
4). In the absence of the catalyst, only trace product was observed.
These data show that the employment of 1.0 mol % of nickel catalyst
was competent for FriedeleCrafts reaction of indole with 2-
nitrovinylbenzene. The effect of temperature has also been investi-
gated. The reaction at 20 ꢁC proceeded smoothly and elevation of
temperature to 40 ꢁC showed no obvious improvement of the yields
(entries 6 and 7). We have tried to prolong the reaction time from 24
Fig. 4. ORTEP view of the cationic section of 3b with 30% thermal ellipsoids and
labeling scheme. Hydrogen atoms and CH3CN molecule are omitted for clarity. Selected
bond distances (Å) and angles (ꢁ): C15-Ni1 1.849(4), C39-Ni1 1.863(4), N1-Ni1 1.908(3),
N6-Ni1 1.940(3), C15-Ni1-N6 176.67(17), C39-Ni1-N1 177.27(16), C39-Ni1-N6
86.43(15), C15-Ni1-C39 94.04(18), N1-Ni1-N6 94.01(13), C15-Ni1-N1 85.68(16), N3-
C15-N4 106.5(3), N9-C39-N8 106.0(3), C7-N1-C1 107.5(3), C31-N6-C25 106.9(3).

G. Huang et al. / Journal of Organometallic Chemistry 696 (2011) 2949e2957
2953
Fig. 6. (I) PF6ꢀ as a bridge through CeH$$$F bonds (blue dot line). (II)
of 4 showing double-stranded 1D supramolecular chains, green for the molecule, red for the PF6 anion. (For interpretation of the references to colour in this figure legend, the
p$$$
p
interactions from interꢀ-chain benzimidazole and pyridine rings (dark red dot line). (III) packing diagram
reader is referred to the web version of this article.)

2954
G. Huang et al. / Journal of Organometallic Chemistry 696 (2011) 2949e2957
Table 1
Table 3
Reaction of indole with 2-nitrovinylbenzene catalyzed by 3a and 3b.a
FriedeleCrafts reaction of indoles with b
-nitrostyrenes catalyzed by complex 3b.a
T (ꢁC)
Time (h)
Yieldb (%)
Entry
R1
R2
Ar
Product
Yieldb (%)
Entry
Catalyst
Catalyst amount (mol %)
1
2
3
4
5
6
7
8
H
H
H
H
H
H
H
H
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
C6H5
7a
7b
7c
7d
7e
7f
7g
7h
7i
7j
7k
7l
7m
7n
7o
7p
7q
7r
85
76
78
80
77
53
71
39
84
77
96
95
77
87
88
81
44
53
1
2
3
4
5
6
7
8
3a
3b
3b
3b
None
3b
3b
3b
3b
1
1
2
0.5
0
1
1
1
1
20
20
20
20
20
0
40
20
20
20
24
24
24
24
24
24
24
36
8
27
85
85
80
trace
38
83
86
78
39
2eCH3OC6H4
3eCH3OC6H4
4eCH3OC6H4
4eClC6H4
4eBrC6H4
Furan
4eNO2C6H4
C6H5
9
9
10c
3b
1
24
10
11
12
13
14
15
16
17
18
3eCH3OC6H4
2eCH3OC6H4
4eClC6H4
4eCH3OC6H4
Furan
4eBrC6H4
1enaphthyl
C6H5
a
Unless stated otherwise, the reaction was carried out with 1 equiv of indole and
2 equiv of 2-nitrovinylbenzene in 0.5 mL of CH2Cl2 under nitrogen atmosphere.
b
Isolated yield.
In the air.
c
CH3O
CH3O
to 36 h, but the yield was not obviously improved (entries 8 and 9).
Furthermore, only 39% yield was obtained when the reaction was
carried out in the air (entry 10), indicating that the inert atmosphere
is indispensable for the reaction.
H
Furan
a
Reaction conditions: Under nitrogen atmosphere, a mixture of 3b (1 mol %),
various indoles (0.5 mmol) and b-nitrostyrenes (1.0 mmol) was dissolved in CH2Cl2
(0.5 mL). The reaction mixture was stirred at room temperature (20 ꢁC) for 24 h. The
solvent was evaporated under vacuum and products were isolated by column
chromatography on silica gel (eluent: hexane/ethyl acetate ¼ 3/1).
Next, we used 3b as the catalyst to investigate the solvent and
additives effect of this reaction under the same conditions, and the
results of these experiments are summarized in Table 2. As shown,
it was found that CH2Cl2 was the best solvent (Table 2, entries 1e7).
Moderate yields were gained when CH3CN, THF, 2-propanol or
toluene were used as solvent and no desired product was obtained
when the reaction was conducted in DMF. It is well recognized that
FriedeleCrafts reaction was efficiently accelerated by variety of
Lewis acids [85e93]. To further improve the reactivity and selec-
tivity, the effect of various benzoic acids and amines were investi-
gated (Table 2, entries 8e15). It was found that with the addition of
b
Isolated yield.
4-methylbenzoic acid or 4-methoxyaniline, the yields could be
slightly improved under identical conditions. Further experiments
revealed that, under the same experimental conditions, aniline
lowered the yields of the FriedeleCrafts product to 77% and no
desired product was obtained when Et3N used as the additive. It
can be seen that this catalyst have good tolerance when the reac-
tion was conducted in various benzoic acids, presumably due to the
fact that benzoic acids might be able to decompose the Ni-NHC
complexes to give the catalytically active Ni(II) species. Accord-
ingly, extensive screening has shown that the optimized catalytic
reaction conditions were found to be as follows: the mixture of
indole (1.0 equiv), 2-nitrovinylbenzene (2.0 equiv) and 3b (1 mol %)
was stirred in CH2Cl2 at room temperature (20 ꢁC) for 24 h under
nitrogen atmosphere.
Table 2
Solvent and additives effect on the reaction of indole with 2-nitrovinylbenzene.a
With the optimum reaction conditions at hand, the substrate
scope of the reaction was probed (Table 3). The corresponding
indole derivatives were obtained in moderate-to-excellent yields
with up to 96%. The electronic density of the indole ring would
also play an important role in this reaction. As shown in Table 3,
the indole bearing methyl group at the 2-position, afforded the
corresponding products in excellent yields (Table 3, entries 11
and 12). In the case of 5-(methoxy)-1H-indole, the yields were
slightly lower (Table 3, entries 17 and 18), probably attributed to
the steric effect of the methoxy group. This reaction also tolerated
different substituents such as Br, Cl, NO2, methoxy groups at the
Entry
Solvent
Additive
Yieldb (%)
1
2
3
4
5
6
7
8
CH2Cl2
CHCl3
CH3CN
THF
2-Propanol
Toluene
DMF
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
None
None
None
None
None
None
None
C6H5COOH
4eCH3C6H5COOH
4eNO2C6H5COOH
4eCH3OC6H5COOH
Et3N
85
80
72
65
72
59
N.d
78
87
64
81
N.d
19
77
71
87
9
10
11
12
13
13
14
15
phenyl ring in
-nitrostyrene bearing Br or NO2 group resulted in a lower yield
(Table 3, entries 6, 8 and 15), in contrast, the -nitrostyrene
b-nitrostyrenes (Table 3, entries 2, 5, 6 and 8). The
b
Et2NH
b
C6H5NH2
4eClC6H4NH2
4eCH3OC6H4NH2
bearing methoxy group at the 2- and 4-position, afforded the
corresponding products in excellent yields (Table 3, entries 2, 4,
11 and 13). Using
b-nitrovinyl furan and b-nitrovinyl naphthyl
a
Unless stated otherwise, the reaction was carried out with 1 equiv of indole and
2 equiv of 2-nitrovinyl-benzene in the presence of 1 mol % of catalyst at room
temperature (20 ꢁC) for 24 h under nitrogen atmosphere.
derived from 2-furaldehyde and 1-naphthaldehyde, the similar
products were obtained with moderate-to-good yields (Table 3,
entries 7, 14 and 16).
b
Isolated yield.

G. Huang et al. / Journal of Organometallic Chemistry 696 (2011) 2949e2957
2955
Table 4
Summary of crystallographic data for 2b, 3a, 3b, and 4.
2b
3a
3b
4
Empirical formula
Formula weight
Crystal system
Space group
a (Ǻ)
C24H24F6N5P
527.45
triclinic
P1
10.5936(17)
11.865(3)
11.8904(19)
62.503(2)
73.042(3)
69.818(2)
1228.5(4)
2
C40H42NiF12N10P2
1011.49
triclinic
C50H49NiF12N11P2
1152.65
monoclinic
P21/c
14.0170(17)
8.573(2)
43.1200(15)
90.00
96.700(2)
90.00
5146.2(15)
4
C44H48HgF12N12P2
1235.47
triclinic
P1
9.632(2)
10.966(2)
13.234(3)
99.125(2)
111.340(3)
90.012(2)
1283.0(5)
1
P1
13.6130(16)
14.348(2)
15.3874(11)
105.785(2)
104.851(3)
113.973(2)
2401.7(5)
2
b (Ǻ)
c (Ǻ)
a
b
g
(deg)
(deg)
(deg)
V (Ǻ3)
Z
rcalcd (g cmꢀ3
)
1.426
0.180
1.399
0.557
1.488
0.531
1.599
3.150
m
(mmꢀ1
)
F (000)
544
1036
2368
614
Reflections
Collected
Independent
Observed [I > 2
GooF
6756
4727
2889
1.018
0.0227
12597
9108
5327
1.097
0.0405
26989
10117
7635
1.062
0.0409
6901
4931
4705
1.024
0.0308
s
(I)]
Rint
Final R indices [I > 2s (I)]
R1
wR2
0.0618
0.1355
0.0566
0.1179
0.0642
0.1357
0.0513
0.1332
R indices (all data)
R1
wR2
0.0861
0.1422
0.0972
0.1257
0.0848
0.1421
0.0523
0.1336
3. Conclusion
4.2. Preparation of ligand precursors
We reported the synthesis and structural characterization of
two ligands containing benzimidazole and pyridine groups and
their complexes Ni(II) (3a and 3b) and mercury(II) (4). Complex 3b
exhibits good activities in catalyzing the FriedeleCrafts reaction of
4.2.1. Synthesis of 2-chloromethyl-1-ethylbenzimidazole (1) [29,59]
A mixture of N-ethylbenzene-1,2-diamine (6.8 g, 50 mol) and
chloroacetic acid (7.1 g, 75 mmol) was refluxed in 75 mL of 4 N HCl
for overnight. The reaction mixture was neutralized with aqueous
K2CO3 and the precipitate product was collected by filtration and
then purified by flash chromatography (ethyl acetate/petroleum
ether ¼ 1/1) to give a white powder. Yield: 8.08 g (83%). 1H NMR
indole with
good yields. In the packing diagrams of 4, four adjacent mole-
cules are packed by two types of interactions to generate
b-nitrostyrene under mild conditions in moderate-to-
p-p
double-stranded 1D supramolecular chains. Further catalytic
applications of the nickel(II) complexes containing benzimidazole-
functionalized N-heterocyclic carbenes in other reactions are in
progress.
(300 MHz, CDCl3: d 7.78e7.75 (m, 1H), 7.40e7.28 (m, 3H), 4.84 (s,
2H), 4.31 (q, J ¼ 7.2 Hz, 2H), 1.51 (t, J ¼ 7.2 Hz, 3H).
4.2.2. Synthesis of [HL1](PF6) (2a)
2-chloromethyl-1-ethylbenzimidazole (3.89 g, 20.00 mmol) was
added to a solution of N-(6-methylpyridin)imidazole (1.73 g,
10.00 mmol) in 100 mL of toluene. The mixture was refluxed for
48 h. The resulting precipitate was isolated and washed with
toluene (2 ꢂ 5 mL) and Et2O (2 ꢂ 5 mL), and was then dissolved in
water (20 mL). To the aqueous solution was added an excess of
NH4PF6 (3.26 g, 20.00 mmol), resulting in an immediate pale yellow
precipitation. The yellow solid was collected and washed with
water and Et2O and dried under vacuum. Yield: 4.15 g (87%), m.p:
4. Experimental section
4.1. General information
All reactions were performed under the nitrogen atmosphere
unless noted. The commercially available reagents and solvents
were used without further purification unless otherwise noted. N-
ethylbenzene-1,2-diamine [94], N-(6-methylpyridin)imidazole
[95], N-(6-methylpyridin)benzimidazole [95], Ni(PPh3)2Cl2 [96]
166 ꢁC 1H NMR (300 MHz, DMSO-d6):
d 9.45 (s, 1H, NCHN),
7.89e7.85 (m, 2H), 7.78 (t, J ¼ 7.6 Hz, 1H), 7.65e7.59 (m, 2H),
7.32e7.20 (m, 4H), 5.94 (s, 2H), 5.59 (s, 2H), 4.36 (q, J ¼ 7.2 Hz, 2H),
2.43 (s, 3H), 1.31 (t, J ¼ 7.2 Hz, 3H). 13C NMR (300 MHz, DMSO-d6):
and
b-nitrostyrenes [97] were prepared by using published
procedures. Column chromatography was performed with silica gel
(200e300 mesh). All yields were given as isolated yields. NMR
spectra were recorded on a Bruker DPX 300 MHz spectrometer.
d
158.4, 152.7, 147.6, 142.0, 137.9, 137.8, 135.0, 123.8, 123.2, 122.9,
122.1, 119.6, 119.2, 110.5, 53.4, 45.2, 38.2, 23.9, 15.0. Anal. Calcd for
C20H22F6N5P: C, 50.32; H, 4.65; N, 14.67. Found: C, 50.12; H, 4.63; N,
14.72. ESI-MS m/z (%) 332.33 (100) [M-PF6]þ.
Chemical shifts (d) are reported in ppm downfield from tetrame-
thylsilane. Coupling constants (J values) are reported in Hertz. ESI-
MS spectra were measured on Finnigan Mat TSQ 7000 instruments.
Microanalyses were obtained on PerkineElmer 240 instruments,
and melting points (mp) were determined with a digital electro-
thermal apparatus without further correction.
4.2.3. Synthesis of [HL2](PF6) (2b)
This compound was prepared following the procedure described
for 2a by starting from N-(6-methylpyridin)benzimidazole (2.23 g,

2956
G. Huang et al. / Journal of Organometallic Chemistry 696 (2011) 2949e2957
10.00 mmol). Yield: 3.30 g (79%), pale yellow powder, m.p: 199 ꢁC 1H
NMR (300 MHz, DMSO-d6): 10.11, (s,1H, NCHN), 8.09e8.02 (m, 2H),
125.2, 124.6, 124.5, 124.3, 123.5, 121.6, 118.3, 111.3, 56.6, 46.5, 39.4,
22.7, 15.1. Anal. Calcd for C40H42F12HgN10P2: C, 41.65; H, 3.67; N,
12.14. Found: C, 39.65; H, 3.81; N,11.59. ESI-MS m/z (%) 433.33 (100)
d
7.79 (t, J ¼ 7.8 Hz,1H), 7.69e7.65 (m, 3H), 7.53 (d, J ¼ 7.9 Hz,1H), 7.45
(d, J ¼ 7.6 Hz,1H), 7.32e7.17 (m, 3H), 6.31 (s, 2H), 5.97 (s, 2H), 4.46 (q,
J ¼ 7.2 Hz, 2H), 2.37 (s, 3H),1.38 (t, J ¼ 7.2 Hz, 3H).13C NMR (300 MHz,
[M-2PF6]2þ
.
DMSO-d6): d 158.4, 152.1, 147.2, 144.0, 141.8, 137.8, 135.1, 131.6, 131.0,
4.4. Typical procedure for FriedeleCrafts reaction of indole with 2-
126.9, 123.2, 122.8, 122.0, 119.8, 119.1, 114.2, 114.0, 110.5, 51.1, 43.3,
38.4, 23.9, 15.0. Anal. Calcd for C24H24F6N5P: C, 54.65; H, 4.59; N,
13.28. Found: C, 54.82; H, 4.57; N,13.23. ESI-MS m/z (%) 382.33 (100)
[M-PF6]þ.
nitrovinylbenzene
In a dried Schlenk tube, a mixture of Ni-NHCs (1.0 mol %), indole
(0.088 g, 0.50 mmol) and 2-nitrovinylbenzene (0.149 g, 1.00 mmol)
was dissolved in CH2Cl2 (0.5 mL) under nitrogen atmosphere. The
reaction mixture was stirred at room temperature (20 ꢁC) for 24 h.
The solvent was evaporated under vacuum and products were
isolated by column chromatography on silica gel (eluent: hexane/
ethyl acetate ¼ 3/1).
4.3. Preparation of complexes
4.3.1. Synthesis of [Ni(L1)2](PF6)2 (3a)
To a solution of 2a (0.239 g, 0.50 mmol) in CH3CN (10 mL) was
added Ag2O (0.064 g, 0.28 mmol), and the mixture was stirred at
room temperature for 10 h. To the filtrate was added Ni(PPh3)2Cl2
(0.163 g, 0.25 mmol). After stirring overnight, the mixture was
filtered through Celite, and the filtrate was then concentrated to ca.
2 mL. Addition of Et2O (20 mL) to the filtrate afforded a yellow
precipitate immediately. The precipitate was collected and redis-
solved in CH3CN and diluted with Et2O (20 mL). This procedure was
repeated three times to remove the PPh3. Finally, the precipitate
was washed with Et2O (20 mL) to afford a light yellow powder.
Yield: 61%. X-ray quality crystals were obtained by slow diffusion of
Et2O into the CH3CN solution of the complex. m.p: >300 ꢁC 1H NMR
4.5. X-ray structural determination
Data were collected using a Bruker Smart Apex II CCD with
graphite monochromated Mo K
a
radiation (
l
¼ 0.71073 Å) at 291(2)
K using the -scan technique. The data were collected using the
u
SMART program, which also corrected the intensities for Lorentz
and polarization effects [98]. An empirical absorption correction
was applied using the SADABS program [99]. The structures were
solved by direct methods using the program SHELXTL and all
non-hydrogen atoms were refined anisotropically on F2 by the full-
matrix least-squares technique using the SHELXTL crystallo-
graphic software package [100]. The hydrogen atom positions
were generated using a riding model. Crystallographic data and
pertinent refinement parameters for 2b, 3a, 3b and 4 are presented
in Table 4.
(300 MHz, DMSO-d6):
d
7.74 (s, 2H), 7.67 (d, J ¼ 8.1 Hz, 2H), 7.60 (t,
J ¼ 7.5 Hz, 2H), 7.34 (s, 2H), 7.18 (d, J ¼ 7.8 Hz, 2H), 7.11 (t, J ¼ 7.5 Hz,
2H), 6.99 (d, J ¼ 7.5 Hz, 2H), 6.68 (t, J ¼ 7.8 Hz, 2H), 6.55 (d,
J ¼ 16.2 Hz, 2H), 6.24 (d, J ¼ 16.2 Hz, 2H), 6.19 (d, J ¼ 8.4 Hz, 2H), 5.13
(d, J ¼ 15.6 Hz, 2H), 4.60 (q, J ¼ 13.8 Hz, 2H), 4.45 (d, J ¼ 7.0 Hz, 2H),
2.43 (s, 6H), 1.45 (t, J ¼ 7.0 Hz, 6H). 13C NMR (300 MHz, CD3CN):
d
161.7, 159.6, 155.1, 150.1, 139.5, 138.4, 134.2, 125.2, 125.1, 124.7,
Acknowledgements
124.2, 123.7, 119.8, 117.5, 112.5, 56.2, 46.2, 41.1, 24.6, 15.6. Anal. Calcd
for C40H42F12N10NiP2: C, 47.50; H, 4.19; N, 13.85. Found: C, 47.34; H,
The authors express their thanks to the support from the
Natural Science Foundation of China (No. 20932004, 21072093),
the National Basic Research Program of China (2011CB808600), the
Program for New Century Excellent Talents in University (NCET-07-
0425) and The Doctoral Fund of Ministry of Education of China
(20090091110017). The authors acknowledge Yangfan Guan for
discussing the crystal structure.
4.17; N, 13.90. ESI-MS m/z (%) 360.50 (100) [M-2PF6]2þ
.
4.3.2. Synthesis of [Ni(L2)2(CH3CN)](PF6)2 (3b)
The compound was prepared according to the same procedure
as for 3a starting from 2b (0.264 g, 0.5 mmol), Ag2O (0.064 g,
0.28 mmol), and Ni(PPh3)2Cl2 (0.163 g, 0.25 mmol). Yield: 0.191 g
(86%), yellow powder, m.p: >300 ꢁC 1H NMR (300 MHz, DMSO-d6:
d
8.25 (d, J ¼ 8.4 Hz, 2H), 7.67 (d, J ¼ 8.4 Hz, 2H), 7.54e7.47 (m, 4H),
Appendix. Supplementary material
7.35e7.28 (m, 4H), 7.12 (t, J ¼ 7.8 Hz, 2H), 6.95e6.85 (m, 6H), 6.71 (t,
J ¼ 8.4 Hz, 2H), 6.65 (d, J ¼ 17.1 Hz, 2H), 6.33 (d, J ¼ 8.4 Hz, 2H), 5.73
(d, J ¼ 16.5 Hz, 2H), 5.02 (d, J ¼ 16.5 Hz, 2H), 4.74 (q, J ¼ 7.0 Hz, 4H),
2.26 (s, 6H), 1.46 (t, J ¼ 7.0 Hz, 6H). 13C NMR (300 MHz, CD3CN):
CCDC 803959, 803962, 803961 and 803960 contain the
supplementary crystallographic data for this paper. These data can
be obtained free of charge from the Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
d
174.0, 158.7, 153.3, 149.6, 138.9, 137.6, 134.8, 134.6, 133.7, 125.2,
124.8, 123.8, 123.1, 118.5, 117.1, 112.2, 111.9, 111.2, 53.1, 43.0, 40.6,
23.8, 15.2. Anal. Calcd for C48H46F12N10NiP2: C, 51.86; H, 4.17; N,
12.60. Found: C, 51.65; H, 4.18; N, 12.56. ESI-MS m/z (%) 410.83 (100)
Appendix. Supplementary data
[M-2PF6]2þ
.
Supplementary data related to this article can be found online at
doi:10.1016/j.jorganchem.2011.04.039.
4.3.3. Synthesis of [Hg(L1)2(CH3CN)2](PF6)2 (4)
A CH3CN solution (40 mL) of 2a (0.191 g, 0.40 mmol) and
Hg(AcO)2 (0.196 g, 0.20 mmol) was refluxed under N2 for 48 h. The
solution was evaporated to dryness, and the residue was washed
with water and diethyl ether. Recrystallization from CH3CN and
diethyl ether produced a yellow solid, which was filtered and dried
under vacuum. Yield: 0.205 g (89%), m.p: >300 ꢁC 1H NMR
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