A palladium chelating complex of ionic water-soluble nitrogen-containing ligand: The efficient precatalyst for Suzuki-Miyaura reaction in water

Article abstract of DOI:10.1039/c1gc15060j

An ammonium-functionalized water-soluble bidentate nitrogen-containing ligand (3) and its palladium chelating complex (4) have been easily prepared from the commercial available starting materials in high overall yields. The charge distributions and molecular orbital analyses of 3, 4 and non-ionic hydrophobic analogue (2a) based on the optimized structures were implemented. The crystal structure of 4 has been established by single-crystal X-ray diffraction. The water-soluble palladium complex was a much more effective catalyst than 2a in palladium-catalyzed Suzuki-Miyaura cross-coupling reaction in air and pure water. High catalytic activity of aryl chlorides and aryl bromides was observed. The mercury drop test, poison experiments and TEM analysis of the catalytic solution showed Pd(0) were involved in catalysis. The palladium nanoparticles of 3.0 ± 0.6 nm were formed after the catalytic reaction, they were stabilized by synergetic effect of coordination and electrostatic interactions from the ammonium-functionalized chelating ligand. No precipitate was observed when the aqueous nanoparticles were stored in air for months.

Full text of DOI:10.1039/c1gc15060j

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PAPER
A palladium chelating complex of ionic water-soluble nitrogen-containing
ligand: the efficient precatalyst for Suzuki–Miyaura reaction in water†
Chunshan Zhou, Jinyun Wang, Liuyi Li, Ruihu Wang* and Maochun Hong
Received 15th January 2011, Accepted 5th May 2011
DOI: 10.1039/c1gc15060j
An ammonium-functionalized water-soluble bidentate nitrogen-containing ligand (3) and its
palladium chelating complex (4) have been easily prepared from the commercial available starting
materials in high overall yields. The charge distributions and molecular orbital analyses of 3, 4
and non-ionic hydrophobic analogue (2a) based on the optimized structures were implemented.
The crystal structure of 4 has been established by single-crystal X-ray diffraction. The
water-soluble palladium complex was a much more effective catalyst than 2a in
palladium-catalyzed Suzuki–Miyaura cross-coupling reaction in air and pure water. High catalytic
activity of aryl chlorides and aryl bromides was observed. The mercury drop test, poison
experiments and TEM analysis of the catalytic solution showed Pd(0) were involved in catalysis.
The palladium nanoparticles of 3.0 0.6 nm were formed after the catalytic reaction, they were
stabilized by synergetic effect of coordination and electrostatic interactions from the
ammonium-functionalized chelating ligand. No precipitate was observed when the aqueous
nanoparticles were stored in air for months.
physicochemical properties.6–14 NPs usually show excellent cat-
Introduction
alytic activity in the cross-coupling reaction of aryl halides since
Transition metal-catalyzed cross-coupling reactions of aryl
high surface-to-volume ratio of NPs provide larger number
halides and organometallic reagents represent a powerful and
of active sites per unit area compared to their heterogeneous
popular method for the formation of carbon–carbon bonds.1–3
counterparts. In addition, their preparation could be carried
The use of aryl chlorides as substrates is highly desirable because
out in air without detrimental effects on eventual catalytic
they are readily available and inexpensive in comparison to aryl
reactions. As they are thermodynamically unstable, stabilizers
iodides and bromides. A larger number of forms of palladium
should be chosen to prevent their aggregation. Various non-
supported by organic ligands were used as precatalysts in
phosphine stabilizers of palladium NPs, such as surfactants,7
the cross-coupling reaction of aryl chlorides. In this context,
polymers,8 dentrimers,9 PEG 12–14 have been reported. In contrast
significant innovations have been made through modifications
to the heterogeneous supported metal NPs that are restricted
of traditional phosphine ligands and the discoveries of novel
by support surfaces, soluble metal NPs have much superiority,
phosphine systems,3,4 but most of the phosphine ligands are
such as controllable size, more rotational degree of freedom and
sensitive to air and moisture, also their synthesis is laborious.
more active sites accessible to reactants. Moreover, the optimum
Another concern in such reactions is the replacement of harmful
balance between activity and stability of palladium NPs can
organic solvents by environmental benign reaction media,
be easily tuned through the careful choice and modification of
especially by water.4,5 These problems have driven chemists to
explore non-phosphine alternatives in water.
Transition metal nanoparticles (NPs) have appeared as one of
ligands, which enhances the scope of NPs for different catalytic
applications. However, most of them used organic solvents
and ionic liquids as reaction media, the water-soluble NPs in
the most promising subjects in catalysis thanks to their unique
pure water are very limited,7b,11 especially for the cross-coupling
reaction of aryl chlorides.
In our previous study, we have prepared a series of pyrazolyl
State Key Laboratory of Structural Chemistry, Fujian Institute of
and 2-pyridyl-functionalized 2-methylimidazolium-based ILs,14
Research on the Structure of Matter, Chinese Academy of Science,
which possessed high suitability as both ligands and solvents
in palladium-catalyzed cross-coupling reactions. The catalytic
system has demonstrated high catalytic activity because the
palladium NPs were stabilized by synergetic effect of coordi-
nation and electrostatic interactions from the functionalized
Fuzhou, Fujian, 350002, China. E-mail: ruihu@fjirsm.ac.cn
† Electronic supplementary information (ESI) available: Charge dis-
tribution, HOMOs and LUMOs, IR spectra and calculation data for
complexes 2–4 and characterization data for coupling products. CCDC
reference number 806910. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c1gc15060j
2100 | Green Chem., 2011, 13, 2100–2106
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ILs.13a From an economic and environmental viewpoint, it will
be more practical and valuable to using pure water as reaction
media instead of ILs. It was known that the implementation of
the catalytic reactions in water requires water-soluble ligands
as catalysts. Very recently, water-soluble non-ionic palladium
NPs stabilized by PEG-anchoring ligands were used as catalysts
of the aerobic oxidation of alcohol11a as well as cross-coupling
reactions of aryl iodides and bromides.11b Many papers have
been contributed to non-phosphine water-soluble compounds
by attaching ionic groups, such as sulfonate,15 carboxylate16
and ammonium17 to hydrophobic ones, they were used as
ligands of precatalysts for the cross-coupling reaction in pure
water. However, there were few previous reports of the ionic
water-soluble compounds as ligands to stabilize palladium
NPs in cross-coupling reaction in pure water. Inspired by
the synthetic strategies of NPs co-stabilizers in ILs through
attaching pendant imidazolium tag to coordination group to
increase catalyst solubility in ILs,13a,13g,14 herein, we report the
synthesis and characterization of a water–soluble nitrogen-
containing ligand, 1-[N,N¢,N¢¢-trimethyl-(4-butyl)ammonium]-
2-(2-pyridyl)imidazole chloride and its palladium chelating
complex, their charge distributions and molecular orbital anal-
yses based on the optimized structures were investigated. The
bidentate ligand proved to be efficient for the stabilization of
palladium NPs in pure water. High catalytic activity for Suzuki–
Miyaura reaction of aryl chlorides and aryl bromides in air and
pure water was observed.
It was known that the active palladium NPs are usually
involved in palladium-catalyzed cross-coupling reactions.7b,13a
We are interested in 4 as a catalyst precursor or a mere
reservoir of Pd(0) in catalytic system because the chelating
palladium complexes are usually robust and stable. To find the
real catalytic active species, Suzuki–Miyaura reaction between 4-
bromoacetophenone and phenylboronic acid was selected using
4 as a precatalyst at 120 ◦C in pure water. After 1 h, the
crude mixture was extracted several times with Et2O and the
aqueous layer containing the catalytically active species was
directly employed to prepare the TEM samples. It should be
pointed out that Et3N was employed as a base instead of
commonly-used alkali carbonate or hydroxide in order to avoid
interferences caused by the presence of alkali ions in the aqueous
layer.16a Energy dispersion spectrometry indicated the presence
of palladium (Fig. S1†). TEM pictures showed the size of
palladium NPs is 3.0 0.6 nm (Fig. 1a). The value is smaller than
7b,15b
the size of the reported palladium NPs dispersed in water
and ionic liquids 13a,13f For example, water-soluble palladium NPs
with a size of 6–8 nm were generated in Suzuki–Miyaura reaction
catalyzed by Na2PdCl4/SDS in water.7b Interestingly, surface
lattice can be clearly observed (Fig. 1c), the interplanar spacing
value of 0.23 nm from the 1D fringes of the lattice, matched
well with the (111) lattice spacing of face-centered cubic (fcc)
palladium metal.18 The Pd(111) facets are generally regarded as
the best surface in catalysis.19 These palladium NPs are stable in
aqueous solution and no precipitate was observed for months.
However, when the cross-coupling reaction was performed in
the presence of non-ionic 2a and PdCl2 under the identical
conditions, a palladium black was observed after reaction. TEM
pictures revealed the obvious aggregation of palladium NPs,
their size is bigger than that from 4 (Fig. 1b). This showed
that palladium NPs are not well dispersed into water and
coordination interaction is not enough to stabilize palladium
NPs in water. While the NPs from 4 are water-soluble, they were
stabilized through the nitrogen-containing group coordinating
Results and discussion
The synthetic pathway for 3 and 4 was depicted in Scheme 1.
2-(2-Pyridyl)imidazole (1) was easily prepared via the reac-
tion of 2-pyridylcarboxyaldehyde, glyoxal and concentrated
aqueous NH3,14a subsequent alkylation reaction with 1-bromo-
4-chlorobutane in DMF and 35% aqueous NaOH solution
gave rise to 1-(4-chlorobutyl)-(2-pyridyl)imidazole (2). The
quarternization of 2 with excess of 30% aqueous Me3N solu-
tion produced 1-[N,N¢,N¢¢-trimethyl-(4-butyl)ammonium]-2-(2-
pyridyl)imidazole chloride (3). Treatment of 3 with equimolar of
Pd(cod)Cl2 in CH2Cl2 gave rise to [palladium(II) 1-(4-N,N¢,N¢¢-
trimethylbutylammonium)-2-(2-pyridyl) imidazole dichloride]
chloride (4).
Scheme 1
Both 3 and 4 are stable and hyperscopic in air. The ther-
mogravimetric analysis under N2 atmosphere revealed that
the thermal degradation temperature of 4 (261 ◦C) is higher
than that of 3 (216 ◦C), which may be ascribed to chelating
coordination of 3 to Pd(II). 3 and 4 are soluble in water while
non-ionic 2a has poor solubility in water.
Fig. 1 (a) TEM image of palladium NPs from 4; (b) TEM image of Pd
NPs from Pd/2a, the histogram illustrating the particle size distribution
(inset, top right); (c) HR-TEM image of Pd NPs from 4; (d) HR-TEM
image of Pd NPs from Pd/2a.
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to NPs surface and ammonium group pointing away from the
NPs surface (Scheme 2), thereby repelling proximal neighbors.
The latter also resulted in even distribution and small size of
NPs.
water-soluble palladium NPs, which resulted in Pd(0) poison.
However, a quantitative yield was obtained in the absence of
mercury and PPh3 under the same conditions (entry 3). When
the molar ratio of 3 to PdCl2 was increased from 1 to 4, GC yield
decreased sharply to 7%, which results from the excess of ligands
coordinating to the surface of Pd(0) species, and inhibiting the
accessible palladium active sites to substrates (entry 4). These
results indicated the catalytic participation of Pd(0) and 3 as the
stabilizer of NPs in pure water.
The catalytic performance of the system was tested using
Suzuki–Miyaura cross-coupling reactions between aryl bro-
mides and aryl boronic acid in air and pure water. The effect
of varying aryl bromides was initially investigated by using
phenylboronic acid as a substrate (entries 5–11). The aryl
bromides bearing electron-withdrawing groups, such as MeCO,
CN and NO2, provided the desirable products in quantitative
yields (entries 3, 5, 6), however, a moderate yield was observed
for aryl bromides with electron-donating groups, such as MeO
and Me (entries 7, 8). These results are consistent with the
well-established trend in palladium-catalyzed Suzuki-Miyaura
reaction.14,16a,17b The heteroaromatic 3-bromopyridine also re-
acted with phenylboronic acid and produced 3-phenylpyridine
in a satisfactory yield (entry 11). However, the coupling reaction
between 2-bromotoluene and phenylboronic acid gave a lower
yield than that of 3- or 4-bromotoluene, which is probably
due to the steric effect of the methyl group located at the
ortho-position (entries 8–10). The cross-coupling reactions of 4-
bromoacetophenone with other activated and deactivated aryl
boronic acids were also performed, the corresponding biaryl
products were obtained in excellent yields (entries 12–17), in
which the ortho-, meta- and para-substituents in the aromatic
ring of boronic acids have no obvious effect on the coupling
reactions (entries 12–14). Interestingly, when the palladium
loading was reduced to 0.01 mol%, the coupling reaction
between 4-bromoacetophenone and 4-fluorophenylboronic acid
for 3 h gave a quantitative yield (entry 17). The reaction of 4-
bromoacetophenone and phenylboronic acid was completed in
1 h with 0.001 mol% palladium loading (entry 18), TOF was
up to 99 000 h-1. While a 52% GC yield was obtained employing
non-ionic 2a as a ligand under the identical conditions (entry 20),
indicating the solubility of supporting ligands have an important
effect on the activity of the coupling reaction.
This effect was further investigated by the kinetic studies
of cross-coupling reaction between 4-bromoacetophenone and
phenylboronic acid in the presence of 0.001 mol% precatalyst
and K2CO3 at 120 ◦C in air and water. As shown in Fig. 2,
the kinetic curves for both Pd/3 and Pd/2a exhibit sigmoidal-
shaped kinetics.20 However, Pd/2a has a much longer induction
period and a lower reaction activity than that of Pd/3 since its
insolubility inhibits the catalyst from well dispersing in water.
The superior catalytic results of aryl bromides encouraged us to
explore Suzuki–Miyaura reactions between aryl chlorides and
aryl boronic acid. However, no desirable product was detected
for reaction between 4-chloroacetophenone and phenylboronic
acid in the presence of 0.1 mol% Pd/3 and K2CO3 at 120 ◦C
for 0.5 h. When TBAB was employed as an additive under
the same conditions, an 87% GC yield was obtained (Table 2,
entry 1), this is ascribed to the following three reasons: 1) the
addition of TBAB leads to formation of Bu4NPhB(OH)3;21 2)
Scheme 2 The schematic representation of palladium NPs stabilized
by synergetic interaction of coordination and electrostatic interaction
in 3.
The mercury drop test and poison experiments were explored
to further detect the catalytically active species, the Suzuki–
Miyaura reaction between 4-bromoacetophenone and phenyl-
boronic acid in pure water was chosen as a model reaction using
3/PdCl2 as a precatalyst and K2CO3 as a base. As shown in
Table 1, when a drop of Hg(0) and 0.5 equiv of PPh3 were added
to the reaction mixture, respectively. No product (entry 1) and
a 3% GC yield (entry 2) were detected after 1 h, respectively.
This suggested the amalgamation and phosphine-coordinated
palladium species were formed, respectively, on the surface of
Table 1 Suzuki–Miyaura coupling reaction between aryl bromide and
phenylboronic acida
Entry
R1
R2
Pd (mol%) Time (h)
Yield (%)b
1c
2d
3
4-MeCO
4-MeCO
4-MeCO
4-MeCO
4-CN
4-NO2
4-MeO
4-CH3
H
H
H
H
H
H
H
H
H
H
H
2-CH3
3-CH3
4-CH3
4-MeO
4-F
4-F
H
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
1
1
1
1
1
1
3
5
5
5
6
1
1
1
1
1
3
1
1
0
3
100(97)
7
100(99)
100(91)
82
77
41
68
89
100
100(93)
100(95)
100(92)
100(92)
100 (93)
100
4e
5
6
7
8
9
10
11f
12
13
14
15
16
17
18
19g
2-CH3
3-CH3
—
4-MeCO
4-MeCO
4-MeCO
4-MeCO
4-MeCO
4-MeCO
4-MeCO
4-MeCO
0.01
0.001
0.001
H
52
a Reaction conditions: aryl bromide (1.0 mmol), phenylboronic acid
(1.5 mmol), K2CO3 (2.0 mmol), H2O (3 mL), 120 ◦C. b GC yields, isolated
yields were given in parentheses. c One drop of Hg was added. d 0.5 equiv
of PPh3 was added (per metal atom). e The molar ratio of Pd to 3 is 1 : 4.
f 3-Bromopyridine. g 2a as a ligand with the molar ratio of Pd to 2a is
1 : 1.
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Table 2 Suzuki–Miyaura Reaction between aryl chloride and aryl
boronic acida
Pd
Time Yield
Entry R1
R2
Base
Additive (mol%) (h) (%)b
1
2
3
4-CH3CO H
4-CH3CO H
4-CH3CO H
4-CH3CO H
4-CH3CO H
4-CH3CO H
4-CH3CO H
4-CH3CO H
4-CH3CO 4-MeO KOH
4-CH3CO 4-F KOH
4-CH3CO 3-CH3 KOH
4-CH3CO 2-CH3 KOH
K2CO3
KOH
Cs2CO3 TBAB
NEt3 TBAB
Bu4NOH —
KOH
KOH
KOH
TBAB
TBAB
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.01
0.01
0.01
0.01
0.01
0.1
0.5 87
0.5 100(91)
0.5 69
4
0.5
4
5c
6
0.5 68
0.5 73
0.5 23
SDS
7
8
9
SDBS
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
2
2
2
2
2
95
74
89
97(91)
95
Fig. 2 Conversion of 4-bromoacetophenone as a function of time for
Suzuki–Miyaura reaction in water with 0.001 mol% Pd/3 and Pd/2a.
10
11
12
13d
14d
TBAB acts as a phase-transfer catalyst to favor a better contact
between the substrate and the aqueous catalytic phase;22 3)
TBAB can further stabilize palladium NPs through electrostatic
interaction.22,23 The effect of other bases and additives on the
cross-coupling reaction was also investigated. It was found that
KOH was the most efficient and gave a nearly quantitative yield
(entry 2), K2CO3 and Cs2CO3 were slightly less efficient (entries
1, 3), Et3N gave rise to only trace amount of 1-(biphenyl-4-
yl)ethanone (entry 4). It should be noted that 25% aqueous
Bu4NOH was also employed successfully as both a base and
a reaction medium in the absence of TBAB, a 68% GC yield
was obtained (entry 5). It was reported that amphiphiles can
cause acceleration or inhibition of a given chemical reaction in
aqueous solution due to formation of micelles,24 two surfactants,
sodium dodecyl sulfate (SDS) and sodium dodecylbenzene
sulfonate (SDBS) were used instead of TBAB in the catalytic
system. They enhanced the reaction rate, but were less effective
than TBAB (entries 6 and 7). Interestingly, when the palladium
loading was decreased from 0.1 mol% to 0.01 mol%, a 95%
GC yield was achieved in 2 h (entry 8). 4-Chloroacetophenone
also reacted with various substituted phenylboronic acids and
the corresponding biaryls were obtained in good yields in
2 h with TOF ranging from 3700 to 4850 h-1 (entries 9–12).
The cross-coupling reaction between 4-chloronitrobenzene and
phenylboronic acid was also investigated. When K2CO3 or KOH
was used as a base, quantitative yields were achieved in 0.5 h in
the presence of 0.1 mol% catalyst and 0.5 equiv TBAB (entries
13, 14). In contrast, a 65% GC yield was obtained when non-
ionic 2a was employed as a ligand, (entry 15), which was much
lower than those from 3 under identical conditions (entry 14).
Decreasing the catalyst loading to 0.01 mol%, 63% and 82%
GC yields were got after 1 h, respectively, under the similar
conditions (entries 16, 17). When 1.0 equivalent amount of
TBAB was added and K2CO3 was used as a base, a 98% GC
yield was generated (entry 18). Further reducing the catalyst
loading to 0.001 mol%, a 66% GC yield was afforded under
the same conditions with TOF of 22 000 h-1 (entry 19). No
obvious acceleration of reaction rate was observed when 2.0
equivalent amount of TBAB was used (entry 20). Reactions
of 4-chloronitrobenzene with various phenylboronic acids were
completed after 1 h using 0.01mol% catalyst in the presence
of K2CO3 and 1.0 equivalent amount of TBAB except 2-
4-NO2
4-NO2
H
H
H
H
H
H
H
H
KOH
0.5 99
0.5 99(97)
0.5 65
K2CO3
K2CO3
KOH
K2CO3
K2CO3
K2CO3
K2CO3
0.1
0.1
15d,e 4-NO2
16d
17d
18
4-NO2
4-NO2
4-NO2
4-NO2
4-NO2
4-NO2
4-NO2
4-NO2
4-NO2
4-CN
0.01
0.01
0.01
0.001
0.001
0.01
0.01
0.01
0.01
0.1
1
1
1
3
3
1
1
1
1
1
1
3
63
82
98
66
63
99(87)
87
99(98)
99(97)
98
19
20f
21
4-MeO K2CO3
2-CH3 K2CO3
3-CH3 K2CO3
4-F
H
22
23
24
K2CO3
K2CO3
K2CO3
K2CO3
25
26g
27h
4-COOH
—
H
H
0.1
0.1
99
58
a Reaction conditions: aryl chloride (1.0 mmol), phenylboronic acid
(1.5 mmol), base (2.0 mmol), TBAB (1.0 mmol), H2O (3.0 mL). b GC
yields, isolated yields are given in parentheses. c 25% aqueous Bu4NOH
solution (2.1 mL) as solvent and base. d 0.5 mmol TBAB was used. e 2a
as a ligand with the molar ratio of Pd to 2a is 1 : 1. f 2.0 mmol TBAB
was used. g 3.0 mmol K2CO3 was added. h 3-Chloropyridine.
methylphenylboronic acid gave an 87% GC yield owing to steric
reason (entries 21–24). The reaction of 4-chlorobenzonitrile
and 4-chlorobenzoic acid with phenylboronic acid gave almost
quantitative yields in the presence of 0.1 mol% palladium and
K2CO3 in 1 h (entries 25, 26). Heteroaryl chloride, such as 3-
chloropyridine was coupled with phenylboronic acid to gave a
58% yield after 3 h (entry 27).
In order to learn more about the coordination mode and
conformation of the water-soluble palladium complex, the single
crystal X-ray structure of 4 was determined. As shown in Fig. 3,
the coordination mode of palladium to 3 is very similar to
Fig. 3 Crystal structure of 4 with thermal ellipsoids at 30% probability.
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that of 2,2¢-bipyridine derivatives.25 The palladium(II) center is
in a distorted square-planar geometry and is coordinated by
two Cl anions and one pyridyl and one imidazolyl nitrogen
atoms from 3. The deviation of palladium(II) from square
plane is 0.0156 A. The chelating N(2)–Pd(1)–N(1) bite angle
of 80.05(10) is slightly shorter than other angles around Pd(1)
a much more effective ligand than its non-ionic analogue.
TEM analysis, mercury drop test and poison experiments of
the catalytic solution showed Pd(0) were involved in catalysis.
The ionic ligand served as an efficient protective agent for the
stabilization of palladium NPs in pure water. The NPs were
synergetically stabilized through the nitrogen-containing group
coordinating to the palladium NPs surface and ammonium
group pointing away from the surface of the NPs. The latter
resulted in small size and even distribution of NPs. In summary,
this study has widened the application of ionic ligands as
stabilizers of NPs in pure water. This provided further incentive
for its deployment on a larger scale for industrial application and
opened a new avenue to development of environment friendly
catalytic system. Further efforts to explore the relationship of
structure and properties and to extend the application of the
catalytic system are in progress in our laboratory.
˚
centers. ◦The cis-angles are in the range from 80.05(10) to
˚
96.22(7) . The Pd(1)–N(2) bond distance of 1.982(2) A is
˚
shorter than that of Pd(1)–N(1) bond distance of 2.048(2) A,
this is ascribed to the strong electron-donating property of
the imidazolyl ring in comparison with pyridyl ring. Similar
trends are known for metal complexes of other azole ligands
containing 2-pyridyl groups.26 The bond distance of Pd(1)¢Cl(1)
˚
trans to the imidazolyl nitrogen atom[2.2994(10)) A] is slightly
longer than that of Pd(1)–Cl(2) trans to pyridyl nitrogen atom
˚
[2.2911(10) A], which is consistent with the enhanced trans
influence of N2 with respect to N1.27 3 serves as a chelating ligand
through its pyridyl and imidazolyl nitrogen atoms coordinating
to palladium(II), which results in a five-membered metallocycle.
The pyridyl–PdCl2–imidazolyl parts are almost co-planar with
Experimental
General
˚
the mean deviation between them being 0.032 A. The twisting
2-(2-Pyridyl)imidazole and 1-butyl-2-(2-pyridyl)imidazole were
synthesized according to the literature method.14a Other chem-
icals were purchased from commercial suppliers and were used
without further purification. All 1H and 13C NMR spectra
were recorded on a Bruker AVANCE III NMR spectrometer
at 400 and 100 MHz, respectively, using deuterated CDCl3 as a
locking solvent except where otherwise indicated. The IR spectra
were recorded with KBr pellets using Perkin–Elmer Instrument.
Thermogravi-metric analyses were performed on a◦ Mettler
TGA/SDTA 851e analyzer with a heating rate of 10 C min-1
under nitrogen atmosphere. Gas chromatography (GC) analyses
were performed on a Shimadzu GC-2014 equipped with a
capillary column (RTX-5, 30 m ¥ 0.25 mm) using a flame
ionization detector. Transmission Electron Microscopy (TEM)
was performed on a JEOL JEM-2010 microscope and was
operated at 200 kV. Elemental analyses were performed on an
Elementar Vario MICRO Elemental analyzer.
angle between imidazolyl and pyridyl rings is 5.2◦, The N,N¢,N¢¢-
trimethyl-(4-butyl)ammonium substituent is apparently twisted
in order to minimize the steric repulsion.
In order to obtain a better understanding of non-ionic 2a, the
cations of water-soluble 3 and 4, their charge distributions and
the molecular orbital analyses based on the optimized structures
were implemented. As shown in Fig. S2†, the negative charge of
the chelating nitrogen atoms from pyridyl and imidazolyl rings in
2a are closed to each other. An obvious effect of ammonium on
charge distribution of aromatic rings in 3 is observed irrespective
of charge delocalization over them in comparison to 2a. The
pyridyl nitrogen atom (N1) in 3 is more negative (-0.498 e) than
that in 2a (-0.482 e), while imidazolyl nitrogen atom (N2) in
3 is less negative (-0.476 e) than its analogue in 2a (-0.486 e).
The negative charge of pyridyl and imidazolyl nitrogen atoms
in 4 are -0.465 and -0.494e, respectively, which are much
different from those in 3. The highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO)
further revealed that aromatic rings in 2a and the cation of
3 contribute to the HOMO, while their LUMO are occupied
by aromatic framework and ammonium group, respectively
(Fig. S3 and S4†). The HOMO in 4 consists primarily of the
contributions from PdCl2 group, while a delocalization over
PdCl2 and chelating aromatic framework were observed in
LUMO of 4 (Fig. S5†).
X-Ray Crystallography
Data collection for 4 was performed on MM007 CCD diffrac-
tometer using graphite monochromated Mo-Ka radiation (l =
˚
0.71073 A). Intensity data set was collected using a w scan tech-
nique and corrected for Lp effects. The primitive structures were
solved with direct methods and refined on F2 with full matrix
least-squares methods using the SHELXS-97 and SHELXL-
97 programs, respectively.28 The hydrogen atoms were generated
geometrically; non-hydrogen atoms were refined anisotropically.
CCDC-806910.
Conclusion
An ammonium-functionalized water-soluble bidentate
nitrogen-containing compound and its palladium chelating
complex have been easily prepared from the commercial
available starting materials in a high overall yield. Its charge
distributions and the molecular orbital analyses based on
the optimized structures were implemented and were much
different from that of its non-ionic hydrophobic analogue. The
high catalytic activity for palladium-catalyzed Suzuki–Miyaura
cross-coupling reactions of aryl chlorides and aryl bromides
in air and pure water was observed. The ionic compound was
Theoretical study
Calculations were carried out using the Gaussian 03 (Revision
D.02) suite of programs.29 The geometric optimization of the
structures and frequency analyses were carried out using B3-
LYP method with effective core potentials (ECPs). All-electron
6-31+G** basis set was used on C, N, Cl and H atoms while
Lanl2dz developed by Hay and Wadt was employed to descript
the palladium atom.30 The vibration frequencies were used to
2104 | Green Chem., 2011, 13, 2100–2106
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characterize the optimized structures where the energy is minima
without imaginary frequencies on the potential energy surface.
The scaling factors are neglected in the harmonic vibration
frequencies.
C 37.08, H 5.22, N 10.99. Crystals of 4 were obtained by slow
evaporation of ligand 3 and Pd(cod)Cl2 in CH2Cl2 at 25 ◦C.
Preparation of stock solution of 4
3 (29.5 mg, 0.1 mmol) and PdCl2 (17.7 mg, 0.1 mmol) was added
into d◦istilled water (5 mL). After the mixture was stirred for 0.5 h
at 25 C, the concentrated NH3·H2O (15 mL) was injected and
the reaction mixture was further stirred until the solution turned
to colorless. The resulting solution was transferred into a 10 mL
volumetric flask and then the volume was determined as 10 mL
with diluted water. This stock solution has a concentration of
CPd = 0.01 mol L-1.
Synthesis of 1-(4-chlorobutyl)-2-(2-pyridyl)imidazole(2b)
A mixture of 2-(2-pyridyl)imidazole (1.45 g, 10 mmol) and 35%
aqueo◦us NaOH (2.0 mL) in DMF (10 mL) was stirred for 1 h
at 25 C. 1-Bromo-4-chlorobutane (13 mmol) was then added
slowly, and the mixture was stirred overnight at 25 ◦C. The
resulting solution was poured into H2O (10 mL), and extracted
with chloroform (3 ¥ 10 mL). The combined organic layers were
washed with H2O (3 ¥ 10 mL), brine (10 mL) and dried over
Na2SO4. After evaporating the solvent under reduced pressure,
the residue was purified by flash chromatography on silica gel
to give a pale yellow oil. Yield: 2.1 g (90%). 1H NMR: d 8.55 (s,
1H), 8.17 (d, J = 8.0 Hz, 1H), 7.75–7.72 (m, 1H), 7.20 (d, J =
4.4 Hz, 1H), 7.11 (s, 1H), 7.00 (s, 1H), 4.66–4.62 (t, J = 8.0 Hz,
2H), 3.54–3.50 (m, 2H), 1.97 (t, J = 8.0 Hz, 2H), 1.80 (d, J =
8.0 Hz, 2H).
General procedure for the Suzuki–Miyaura coupling reaction
A 25 mL reactor equipped with a condenser was charged
with aryl halide (1.0 mmol), phenylboronic acid (1.5 mmol),
base (2.0 mmol) and H2O (3.0 mL). The aqueous solution of
appropriate amount of 4 was added to the mixture, and then was
stirred in a preheated oil bath (120 ◦C). After an appropriate
time, the mixture was cooled with icy water, the product
was extracted with hexane or ethyl acetate (3 ¥ 5 mL). The
combined organic layer was dried over Na2SO4 and the solvent
was removed under reduced pressure. The crude products were
purified by flash column chromatography on silica gel to afford
the desired product. The identity of the products was confirmed
by comparison with literature spectroscopic data.
Preparation of 1-[N,N¢,N¢¢-trimethyl-(4-butyl)ammonium]-2-(2-
pyrydyl)imidazole chloride (3)
1-(4-Chlorobutyl)-2-(2-pyridyl)imidazole (0.35 g, 1.5 mmol) in
acetonitrile (10 mL) was added 30% aqueous Me3N solution
(20 mL), the mixture was refluxed overnight. The clear solution
was then evaporated under reduced pressure. The residue was
washed several times with Et2O and dried in vacuo to give a white
solid in a quantitative yield. 1H NMR (D2O): d 8.56 (s, 1H), 7.91
(t, J = 7.7 Hz, 1H), 7.72 (d, J = 7.2 Hz, 1H), 7.44 (t, J = 5.0 Hz,
1H), 7.26 (s, 1H), 7.08 (s, 1H), 4.32 (t, J = 6.7 Hz, 2H), 3.07(t,
J = 8.0 Hz, 2H), 2.89(s, 9H), 1.62 (t, J = 7.0 Hz, 2H), 1.48 (d,
J = 6.8 Hz, 2H); 13C NMR: d 149.0, 148.3, 145.2, 138.5, 127.6,
124.5, 124.3, 123.4, 65.7, 52.7, 46.3, 26.9, 19.4. IR (KBr): 3418
(s), 3008 (w), 2942 (w), 1638 (w), 1590 (m), 1489 (m), 1465 (m),
1317 (w), 1215 (w), 1144 (w), 1091 (w), 965 (w), 900 (w), 793 (w),
739 (w), 709 (w), 614 (w) cm-1; Anal. Calcd for C15H23N4Cl·H2O
(312.84): C 57.59, H 8.05, N 17.91. Found C 57.41, H 8.36, N
17.75.
Acknowledgements
This work was financially supported by 973 Program
(2010CB933501, 2011CBA00502), NSFC (20971122) and “One
Hundred Talent Project” from Chinese Academy of Sciences.
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©