Water-soluble palladium click chelating complex: An efficient and reusable precatalyst for Suzuki-Miyaura and Hiyama reactions in water

Article abstract of DOI:10.1002/cplu.201300067

A water-soluble ionic palladium(II) nitrogen-containing chelating complex, [palladium(II) 1-(4-N,N′,N′′-trimethylbutylammonium)-4-(2- pyridyl)-1H-1,2,3-triazole dichloride] chloride (3), was prepared through the click reaction of 1-chloro-4-bromobutane, sodium azide, and 2-ethynylpyridine, followed by the quarternization of Me₃N and subsequent reaction with [Pd(cod)Cl₂] (cod=1,5-cyclooctadiene). The catalytic performances of complex 3 were preliminarily evaluated through Suzuki-Miyaura and Hiyama cross-coupling reactions of aryl bromides; excellent catalytic activity in water was observed. TEM analysis revealed that small palladium nanoparticles (NPs) with a narrow size distribution were formed after the catalytic reaction. The NPs were stabilized by the synergetic effect of coordination and electrostatic interactions from the ionic, bidentate, nitrogen-containing ligand; no palladium black was detected after the aqueous solution of palladium NPs was stored in air for months. The use of 3 as a precursor in the formation of palladium NPs was further explored by using NaBH₄ and hydrogen as reductive reagents. The resulting NPs displayed different sizes, surface properties, and catalytic performances in the Suzuki-Miyaura cross-coupling reaction in water. Copyright

Full text of DOI:10.1002/cplu.201300067

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DOI: 10.1002/cplu.201300067
Water-Soluble Palladium Click Chelating Complex: An
Efficient and Reusable Precatalyst for Suzuki–Miyaura and
Hiyama Reactions in Water
[
a, b]
[a]
[a]
[b]
[a]
Fanzhen Kong,
Chunshan Zhou, Jinyun Wang, Zhangyu Yu,* and Ruihu Wang*
A water-soluble ionic palladium(II) nitrogen-containing chelat-
ing complex, [palladium(II) 1-(4-N,N’,N’’-trimethylbutylammoni-
um)-4-(2-pyridyl)-1H-1,2,3-triazole dichloride] chloride (3), was
prepared through the click reaction of 1-chloro-4-bromobu-
tane, sodium azide, and 2-ethynylpyridine, followed by the
a narrow size distribution were formed after the catalytic reac-
tion. The NPs were stabilized by the synergetic effect of coordi-
nation and electrostatic interactions from the ionic, bidentate,
nitrogen-containing ligand; no palladium black was detected
after the aqueous solution of palladium NPs was stored in air
for months. The use of 3 as a precursor in the formation of pal-
quarternization of Me N and subsequent reaction with [Pd-
3
(
cod)Cl ] (cod=1,5-cyclooctadiene). The catalytic performances
ladium NPs was further explored by using NaBH and hydro-
2
4
of complex 3 were preliminarily evaluated through Suzuki–
Miyaura and Hiyama cross-coupling reactions of aryl bromides;
excellent catalytic activity in water was observed. TEM analysis
revealed that small palladium nanoparticles (NPs) with
gen as reductive reagents. The resulting NPs displayed differ-
ent sizes, surface properties, and catalytic performances in the
Suzuki–Miyaura cross-coupling reaction in water.
Introduction
Palladium-catalyzed carbon–carbon cross-coupling reactions of
aryl halides are very important routes for the construction of
context, the use of readily available building units has been re-
garded as one ideal strategy.
[
1–3]
biaryl units in organic synthesis.
Palladium nanoparticles
The click reaction of organic azides and terminal alkynes has
attracted considerable attention owing to its superior reliabili-
ty, mild reaction conditions, wide functional-group tolerance,
(NPs) were usually identified as the species formed during
cross-coupling reactions when using palladium(II) complexes
as the catalytic precursors, in which organic ligands served as
[9–12]
high yield, and regioselectivity.
These features have result-
[3–7]
stabilizers to prevent the agglomeration of palladium NPs.
ed in the extensive use of 1,4-disubstituted 1,2,3-triazolyl as
a stable linkage for the connection of two chemical/biological
The steric and electronic characteristics of organic ligands may
tune the size distribution and surface state of NPs; thus, the
optimum balance between stability and activity of NPs can be
achieved through the careful choice and modification of or-
[10]
components. The importance of 1,2,3-triazolyl as a potential
[11]
[12]
group for binding metal ions and stabilizing of NPs has
been realized, leading to increased interest in the application
of catalytic reactions. Recently, we reported a series of click
[
5]
ganic ligands, which greatly enhances the scope of NPs for
different catalytic applications. Various palladium(II) complexes
[13]
ionic liquids, the steric and electronic properties of which
could be easily tuned and modified through variation of sub-
stituents at the imidazolium and/or triazolyl rings, that were
successfully used as solvents and stabilizers of palladium NPs
in carbon–carbon cross-coupling reactions. However, the syn-
thetic complexity and expensive cost limit their further devel-
opment as reaction media. From an economic and environ-
mental viewpoint, it is more practical and valuable to use
water as a reaction medium.
[
6]
[7]
supported by nitrogen-containing, phosphorus-containing,
[
8]
and carbene ligands were selected as NPs precursors, but the
preparation and modification of most of ligands required labo-
rious multistep syntheses and/or conditions that did not readi-
ly allow the incorporation of useful functional groups. There-
fore, the development of new methods for the facile synthesis
and modification of organic ligands is very desirable. In this
[
a] F. Kong, Dr. C. Zhou, J. Wang, Prof. R. Wang
State Key Laboratory of Structural Chemistry
Key Laboratory of Coal to Ethylene Glycol and Its Related Technology
Fujian Institute of Research on the Structure of Matter
Chinese Academy of Science, Fuzhou, Fujian 350002 (P.R. China)
E-mail: ruihu@fjirsm.ac.cn
Water is of great interest in organic synthesis and industrial
applications because it is inexpensive, readily available, safe,
[14–17]
and nontoxic.
Because the majority of substrates and
products in catalytic reactions are insoluble in water, the use
of water-soluble catalytic systems contributes to simplify sepa-
ration, recovery, and recycling of the catalysts. Generally, the
smooth implementation of catalytic reactions in water requires
precatalysts supported by organic ligands to be soluble in
water. Attaching ionic groups, such as sulfonate, carboxylate,
and ammonium, to hydrophobic supporting ligands is
[b] F. Kong, Prof. Z. Yu
School of Chemistry and Chemical Engineering
Qufu Normal University, Qufu, Shandong 330031 (P.R. China)
E-mail: zyu@mail.qfnu.edu.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201300067.
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[
16]
a common method to make metal complexes water soluble.
nitrogen-containing chelating ligands 1a and 1b. Subsequent
quarternization of 1b with an excess of a 30% aqueous solu-
Although a variety of palladium complexes from these water-
soluble ionic ligands were used as precatalysts for cross-cou-
pling reactions in water, there were few reports of water-solu-
tion of Me N in MeCN produced ionic chelating ligand 2. Reac-
3
tion of 2 with an equimolar amount of [Pd(cod)Cl ] in CH Cl /
2
2
2
[
17]
ble ionic ligands as stabilizers of palladium NPs. Ionic ligands
are well known to stabilize palladium NPs through the syner-
getic effect of coordination and electrostatic interactions, in
which coordination atoms bind to the surface of palladium
NPs and ionic groups point away from the surface of the NPs.
Repulsion interactions between ionic groups may prevent the
agglomeration of adjacent NPs, resulting in small sizes and
MeOH produced 3 in almost quantitative yield.
Compounds 1–3 are stable in air and can be stored for
a long period of time. Compounds 1a and 1b possess poor
solubility in water, whereas 2 and 3 are soluble in water. The
1
H NMR spectra of 1–3 exhibit a typical singlet signal for the
proton of the triazolyl ring. The resonance for the proton of
the triazolyl ring in 3 was observed at d=8.89 ppm, which was
downshifted in comparison with that in 2 (d=8.32 ppm). A
clear shift was also observed for the other signals in the
[
18]
a narrow size distribution of NPs. The synthetic pathways of
NPs and the types of reductive reagents have a strong influ-
[
19]
1
ence on the structures and catalytic activity of NPs. Recent
studies demonstrated that the use of discrete metal complexes
supported by water-soluble organic ligands was a straightfor-
ward and reliable method for the controllable synthesis of
metal NPs with a narrow size distribution, but most reports fo-
cused on water-soluble, non-ionic ligands, such as 1,3,5-triaza-
H NMR spectra of 2 and 3; this suggests coordination of the
triazolyl–pyridine rings with palladium(II) in 3. Thermogravi-
metric analysis under an atmosphere of N revealed that ther-
2
mal degradation temperature of 3 (2438C) was close to that of
2 (2318C).
It is well known that the triazolyl ring in 2 can in principle
[
7a]
[11]
7
-phosphaadamantane (PTA),
polyethylene glycol (PEG)-
bind to a metal center through N2 or N3 atom. To learn the
[12a]
[20a]
tagged compounds,
and oxime-derived palladacycles,
coordination mode of 3, the single-crystal X-ray structure of 3
was determined. As expected, the pyridyl nitrogen atom and
triazolyl N2 atom bind to palladium(II) to generate a five-mem-
bered chelating ring (Figure 1). The coordination mode of pal-
ladium(II) in 3 is very similar to that of N,N-chelating
whereas palladium NPs stabilized by water-soluble, ionic, nitro-
[
17a]
gen-containing ligands were seldom explored.
ation of our research into catalytic reactions in environmentally
As a continu-
[
13,17]
friendly reaction media,
and characterization of a water-soluble ionic click ligand, 1-
N,N’,N’’-trimethyl-(4-butyl)ammonium]-4-(2-pyridyl)-1H-1,2,3-tri-
azole chloride (2), and its palladium complex, [palladium(II) 1-
4-N,N’,N’’-trimethylbutylammonium)-4-(2-pyridyl)-1H-1,2,3-tria-
herein, we report the synthesis
[11a,17a]
ligands.
Palladium(II) is in a slightly distorted square-
[
planar geometry and is coordinated by two chloride atoms
(
zole dichloride] chloride (3). The use of 3 as a precursor in the
formation of palladium NPs was explored, and the application
of the resulting NPs as a recoverable precatalyst in Suzuki–
Miyaura and Hiyama cross-coupling reactions in air and neat
water was investigated.
Results and Discussion
The synthetic pathway for water-soluble palladium(II) complex
3
is depicted in Scheme 1. The treatment of 1-chloro-4-bromo-
butane with sodium azide in H O/MeOH at 808C overnight, fol-
2
lowed by 1,3-dipolar cycloaddition with 2-ethynylpyridine
under standard click reaction conditions gave rise to non-ionic,
Figure 1. The structure of complex 3 with thermal ellipsoids shown at the
50% probability level.
and two nitrogen atoms from
pyridyl and triazolyl rings, re-
spectively. The bond angle of
N1-Pd1-N2 is 80.23(12)8, which is
slightly lower than those of
other cis angles around the Pd1
center. The Pd1ÀN2 bond length
of 1.995(3) ꢁ is shorter than that
of the Pd1ÀN1 bond length of
2.053(3) ꢁ; this can be ascribed
to the stronger electron-donat-
ing ability of the 1,2,3-triazolyl
ring than that of the pyridyl
Scheme 1. The synthesis of water-soluble palladium complex 3. NaAsc=sodium salt of ascorbic acid, cod=1,5-cy-
clooctadiene.
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[
11a]
ring.
The bond length of
Pd1ÀCl2 trans to the coordinat-
ed nitrogen atom of the triazolyl
ring (2.2922(11) ꢁ) is slightly
longer than that of Pd1ÀCl1
trans to the pyridyl nitrogen
atom (2.2767(10) ꢁ); this is con-
sistent with the enhanced trans
influence of N2 with respect to
Figure 2. The charge distributions for 1a (a) and the cations in 2 (b) and 3 (c).
[
17a]
N1.
It should be mentioned
that the triazolyl–pyridyl–PdCl2
parts are almost coplanar with a mean deviation between
them of 0.033 ꢁ. The twisting angle between the triazolyl and
pyridyl rings is 3.88. The N,N’,N’’-trimethyl-(4-butyl)ammonium
substituent is apparently twisted to minimize steric repulsion.
The butyl spacer between the ammonium group and the tria-
zolyl ring exhibits an anti–anti–anti conformation. The torsion
angle of the four carbon atoms of the butyl spacer is 3.0688.
To obtain a better understanding of the structures of the ni-
trogen-containing chelating compounds, the natural bond or-
bital (NBO) and molecular orbital analyses based on the opti-
mized structures of non-ionic 1a, cations of water-soluble
ligand 2, and palladium complex
structure of 3. As shown in Figure S1 in the Supporting Infor-
mation, the HOMO and LUMO of 1a and the cation of 2 are
shared by the triazolyl and pyridyl rings. The HOMO of the
[22]
cation of 3 predominantly localizes at the Pd and Cl centers,
whereas the LUMO is occupied by triazolyl–pyridyl–PdCl2,
owing to delocalization over PdCl and chelation of the triazol-
2
yl–pyridine unit.
Because complex 3 is soluble and stable in water, the
Suzuki–Miyaura cross-coupling reaction between aryl bromides
and aryl boronic acid was initially tested to evaluate the cata-
lytic performances of 3 in neat water. As shown in Table 1, the
3
were implemented by using
[
21]
Gaussian 03.
As shown in
[
a]
Table 1. Suzuki–Miyaura cross-coupling reaction catalyzed by 3a.
Figure 2 and Table S1 in the Sup-
porting Information, the charge
distributions were different in
the three compounds, in which
the pyridyl nitrogen atom car-
ried the most negative charge.
The pyridyl nitrogen atom (N1)
in the cation of 2 is more nega-
tive (À0.470 e) than that in 1a
1
2
[b]
Entry
R
X
R
Pd [mol%]
Time
Yield [%]
[c]
1
4-CH
4-CH CO
4-CH
4-CH
4-CH
4-CH
4-CH CO
4-NO
4-CN
4-MeO
4-CH
3-CH3
2-CH
2-thiophenyl
3-pyridyl
3
CO
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Br
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
0.01
0.01
0.01
0.01
0.001
0.0001
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0. 1
20 min
20 min
20 min
20 min
2 h
81
98
99
82
91
43
100 (96)
98 (93)
97 (93)
97
2
3
[
d]
e]
3
4
5
6
7
8
9
1
3
3
3
3
CO
CO
CO
CO
[
(
(
À0.460 e) and in the cation of 3
À0.461 e). The charges of the
24 h
3
30 min
30 min
30 min
30 min
30 min
30 min
30 min
30 min
2 h
30 min
30 min
30 min
30 min
30 min
5 h
5 h
5 h
5 h
5 h
5 h
5 h
30 min
triazolyl N2 atom are similar in
2
1
a (À0.258 e) and in the cations
0
of 2 (À0.258 e) and 3 (À0.260 e).
Furthermore, N2 carries the most
negative charge in the triazolyl
ring, indicating that, of the nitro-
gen atoms of triazolyl the ring,
N2 prefers to coordinate with
metal ions. Notably, the pyridyl
ring in the cations of 2 and 3
possess a positive charge (see
Table S2 in the Supporting Infor-
mation), whereas 1a contains
a slight negative charge. The tri-
azolyl ring carries most of the
negative charge; thus, the bind-
ing ability of the triazolyl N2
atom is stronger than that of the
pyridyl nitrogen atom. These re-
sults are in good agreement
with those from the X-ray crystal
1
1
3
89
85
84
96
12
13
3
1
1
1
4
5
6
73
4-CH
4-CH CO
3
CO
4-F
4-MeO
4-CH
2-CH
3-CH
H
H
H
2-CH
3-CH
4-CH
4-F
99 (98)
99 (98)
99 (97)
99 (95)
99 (95)
80
100 (93)
100
100 (98)
98
17
18
3
4-CH
4-CH
4-CH
4-CH
4-CN
4-NO
4-NO
4-NO
4-NO
3
3
3
3
CO
CO
CO
CO
3
3
3
1
2
2
9
0
1
22
23
0. 1
0. 1
0. 1
0. 1
0. 1
0. 1
0.01
2
24
25
26
2
2
2
3
3
3
99 (97)
100 (93)
89
27
28
4-NO2
4-CH CO
[
f]
3
H
[
a] Reaction conditions: aryl halide (1.0 mmol), phenylboronic acid (1.5 mmol), K
tetra-n-butylammonium bromide (TBAB; 1.0 mmol). [b] Yields determined by GC; yields of isolated product are
given in parentheses. [c] No TBAB was added. [d] KOH was used as a base. [e] NEt was used as a base. [f] The
3
molar ratio of Pd to 1a (as a ligand) was 1:1.
2 3 2
CO (2.0 mmol), H O (3 mL),
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cross-coupling reaction between 4-bromoacetophenone and
phenylboronic acid gave 4-acetylbiphenyl in 81% yield, as de-
termined by GC, when K CO was used as a base in the pres-
bromoacetophenone and phenylboronic acid gave the target
product in 89% yield, as determined by GC (Table 1, entry 28),
which was lower than those with 2 as the supporting ligand
for palladium under identical conditions (Table 1, entry 7), indi-
cating that the water solubility of the supporting ligands had
an important effect on the catalytic activity of the cross-cou-
pling reactions. This effect was further explored by the reaction
between 4-bromoanisole and phenylboronic acid in the pres-
ence of 0.01 mol% of the precatalyst. As shown in Figure 3,
the conversion for both 3 and Pd/1a exhibited sigmoidal-
shaped curves. However, Pd/1a has a much longer induction
period and lower reaction activity than that of 3 because the
insolubility of Pd/1a inhibits the catalytic species from dispers-
ing well in water.
2
3
ence of 0.01 mol% of 3 in 20 min (Table 1, entry 1). Although
the addition of TBAB led to a 98% yield, as determined by GC,
under the same conditions (Table 1, entry 2); this was ascribed
to the fact that TBAB served as a phase-transfer catalyst to
favor better contact between the substrates and the aqueous
catalytic species and accelerate the reaction rate of the
[
20b]
Suzuki–Miyaura cross-coupling reaction.
The effect of bases
on the cross-coupling reaction was also investigated. Almost
quantitative yields were achieved in 20 min when K CO3 or
2
KOH were used as bases (Table 1, entries 2 and 3), whereas
NEt was slightly less efficient and gave 82% yield, as deter-
3
mined by GC, under the same conditions (Table 1, entry 4). In-
terestingly, when the palladium loading was decreased to
0.001 mol%, 4-acetylbiphenyl was formed in 91% yield, as de-
termined by GC, after 2 h (Table 1, entry 5). Further reducing
the palladium loading to 0.0001 mol% gave the target product
in a 43% yield after 24 h, corresponding to a turnover frequen-
À1
cy (TOF) of 18000 h (Table 1, entry 6). Aryl bromides with
electron-withdrawing groups, such as ÀCOMe, ÀCN, and ÀNO ,
2
provided the desirable products in nearly quantitative yield in
30 min (Table 1, entries 7–9). However, when aryl bromides
containing electron-donating groups, such as ÀMeO and ÀMe,
were selected as substrates under the same conditions, 97 and
8
9% yields, as determined by GC, were obtained, respectively
Table 1, entries 10 and 11). These results are consistent with
the well-established trends in palladium-catalyzed Suzuki–
(
[
12a,20]
Miyaura cross-coupling reactions.
The reactions between
Figure 3. Conversion of 4-bromoanisole as a function of time in the Suzuki–
Miyaura reaction when using 0.01 mol% of 3 or Pd/1a as a precatalyst in
the presence of K CO and TBAB in water at 1208C.
2 3
2-, 3-, or 4-bromotoluene and phenylboronic acid resulted in
similar conversion (Table 1, entries 11–13) and no clear steric
effect was observed for the substrates. Heteroaryl bromides,
such as 2-bromothiophene (Table 1, entry 14) and 3-bromopyri-
dine (Table 1, entry 15), also reacted with phenylboronic acid;
the target products were formed in 96 and 73% yield, as de-
termined by GC, after 0.5 and 2 h, respectively. The cross-cou-
pling reaction with other aryl boronic acids was also performed
Catalyst reusability is an important issue from the viewpoints
of industrial applications and green chemistry. The reaction be-
tween 4-bromoacetophenone and phenylboronic acid was se-
lected as a representative example to test the recyclability of
the water-soluble catalytic system. After the reaction was fin-
ished, the resulting product was extracted by using ethyl
ether. The residual aqueous solution was recharged with the
same substrates and one equivalent of base for the next run.
The catalytic solution containing the palladium species was
used four times; a slight loss of catalytic activity was observed
(Figure 4).
(Table 1, entries 16–20); the corresponding biaryl products
were obtained in almost quantitative yield, regardless of the
electronic and steric nature of aryl boronic acids.
Suzuki–Miyaura cross-coupling reactions of aryl chlorides
with aryl boronic acid were also performed with 3 as a precata-
lyst in water. Complete conversion was achieved in the reac-
tions of phenylboronic acid with 4-chlorobenzonitrile and 4-
chloronitrobenzene in the presence of 0.1 mol% of 3, K CO ,
The encouraging catalytic performances of 3 in the Suzuki–
Miyaura cross-coupling reaction further prompted us to inves-
tigate the Hiyama cross-coupling reaction in water. As shown
in Table 2, when the reaction of 4-bromoacetophenone and
phenyltrimethoxysilane was performed in the presence of
0.5 mol% of 3 and two equivalents of NaOH at 1208C for 6 h,
4-acetylbiphenyl was formed in 34% yield, as determined by
GC (Table 2, entry 1). However, as determined by GC, 76% yield
was achieved when using phenyltriethoxysilane instead of phe-
nyltrimethoxysilane under identical conditions (Table 2,
entry 2). The effect of the amount of arylsiloxane and base on
the Hiyama cross-coupling reaction was then investigated.
When the equivalent amount of NaOH and phenyltriethoxysi-
2
3
and TBAB in 5 h (Table 1, entries 22 and 23), but an 80% yield,
as determined by GC, was obtained when 4-chloroacetophe-
none was used as a substrate under the same conditions
(Table 1, entry 21). The cross-coupling reaction of 4-chloronitro-
benzene with various aryl boronic acids was also performed;
the corresponding biaryl products were obtained in almost
quantitative yields (Table 1, entries 24–27), in which ortho,
meta, and para substituents on the aromatic ring of boronic
acids had no clear effect on the coupling reactions.
However, when non-ionic 1a was used as a supporting
ligand for palladium(II), the cross-coupling reaction between 4-
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parent effect on the reactions; the corresponding biaryl prod-
ucts were obtained in excellent yields as determined by GC
(
Table 2, entries 7–12). Heteroaryl bromides, such as 3-bromo-
pyridine (Table 2, entry 13) and 2-bromothiophene (Table 2,
entry 14), also reacted with phenyltriethoxysilane; the resulting
products were obtained in almost quantitative yield, as deter-
mined by GC.
It was reported that palladium NPs were usually involved in
[23]
palladium-catalyzed cross-coupling reactions. The reduction
of metal–organic complexes and subsequent stabilization by
the supporting ligands have been considered as a simple and
reliable approach for the controllable synthesis of palladium
NPs with a narrow size distribution. To identify active palladi-
um species from water-soluble complex 3, poisoning experi-
ments were originally examined in the reaction of 4-bromoani-
sole and phenylboronic acid with 0.01 mol% of 3 as a precata-
Figure 4. Reusability of 3 in the Suzuki–Miyaura reaction between 4-bromo-
acetophenone (1.0 mmol) and phenylboronic acid (1.5 mmol) in the pres-
ence of K
2 3 2
CO (2.0 mmol), complex 3 (0.01 mol%), TBAB (1.0 mmol), and H O
0
(3 mL) at 1208C for 0.5 h.
lyst. As shown in Table 3, when a drop of Hg was added to
Table 2. Hiyama cross-coupling reaction of aryl bromides with arylsilox-
anes catalyzed by 3.
Table 3. Summary of poisoning experiments for complex 3.
[
a]
Entry
Poisoning
additive
Ratio of additive
to palladium
Reaction
time [h]
Conversion
[%]
Entry
R
R’
NaOH
equiv]
Additive
Yield
[%]
[
b]
[
[a]
1
Hg
–
150
1000
1000
150
6
0.5
0.5
0.5
3
0.5
0.5
0
96
75
99
<2
90
[
[
[
c]
c]
c]
2
3
4
5
6
pyridine
pyridine
pyridine
PVPy
2
1
2
3
4
5
6
7
8
9
1
4-CH
4-CH
4-CH
4-CH
4-CH
4-Me
4-Me
3-Me
2-Me
4-MeO
4-NO
4-Cl
3-pyridyl
2-thiophenyl
3
3
3
3
3
CO
CO
CO
CO
CO
Me
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
2
2
2.5
2
5
5
5
5
5
5
–
–
–
–
–
–
34
76
93
85
99 (92)
58
95
92
91
98
[
b]
[
a] One drop of Hg was added. [b] PVPy=polyvinylpyridine.
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
0
the reaction mixture before the reaction was performed, no
catalytic activity was detected after 30 min (Table 3, entry 1);
this is ascribed to the formation of an amalgam on the surface
1
1
2
5
5
5
5
99
99
99
98
1
1
1
2
3
4
0
of the palladium NPs, resulting in Pd poisoning. However,
[
a] Reaction conditions: aryl bromide (1.0 mmol), arylsiloxane (2.0 mmol),
NaOH (2 to 5 mmol), TBAB (0 or 1.0 mmol), H O (3 mL), 6 h. [b] Yields de-
termined by GC; yields of isolated product are given in parentheses.
c] 1.5 equivalents of arylsiloxane was added.
nearly quantitative yield was obtained in the absence of mer-
cury under the same conditions (Table 3, entry 3). On the other
hand, pyridine and PVPy are known for their good binding
ability to palladium, but the insolubility of PVPy in water ena-
bles the palladium particles to be removed from water, result-
ing in an ineffective contact between the catalysts and sub-
strates, as well as extinguishing the active catalytic
2
[
lane was increased from 2 to 2.5 and from 1.5 to 2 under simi-
lar conditions the target product was generated in 93 and
[8a,23c]
species.
As expected, the use of 150 equivalents of pyri-
8
5% yield, as determined by GC, respectively (Table 2, entries 3
dine as an additive had no apparent effect on the cross-cou-
pling reaction between 4-bromoanisole and phenylboronic
acid (Table 3, entry 2). However, only a trace amount of the
product was observed when 150 equivalents PVPy were added
to the reaction mixture under the same conditions (Table 3,
entry 5). Notably, the presence of 1000 equivalents of pyridine
gave 75% yield, as determined by GC (Table 3, entry 3), but
almost quantitative yield was observed when the reaction time
was extended to 3 h (Table 3, entry 4). If the molar ratio of
ligand 2 to palladium was increased from 1 to 6, a slight de-
crease of conversion was observed (Table 3, entry 6). These re-
and 4). Almost complete conversion was observed when five
equivalents of NaOH and two equivalents of phenyltriethoxysi-
lane were used under the same conditions (Table 2, entry 5).
However, the reaction between phenyltriethoxysilane and an
aryl bromide with an electron-donating methyl group gave 4-
methylbiphenyl in 58% yield, as determined by GC, under the
same conditions (Table 2, entry 6). When one equivalent of
TBAB was added, the yield determined by GC increased to
95% (Table 2, entry 7). Moreover, the steric and electronic
properties of the substituents of the aryl bromides had no ap-
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sults further revealed the strong chelating ability of the ionic
triazolyl–pyridine ligand with palladium in water.
To find more evidence of the formation of palladium(0) NPs,
after the Suzuki–Miyaura cross-coupling reaction of 4-bromo-
anisole and phenylboronic acid with 3 as a precatalyst was fin-
ished, the crude mixture was extracted several times with ethyl
ether and the aqueous layer containing the active catalytic
species was directly analyzed by TEM. The TEM images clearly
showed the formation of palladium NPs with a mean diameter
of 3.09 nm and a standard deviation of 0.53 (Figure 5a); the
Figure 5. TEM micrographs and size distributions of palladium NPs for
a) PdÀSuzuki, b) PdÀNaBH
4
, and c) PdÀH
2
.
palladium NPs are denoted PdÀSuzuki. The value is smaller
than the size of reported palladium NPs dispersed in
[
8a,24]
water.
For example, palladium pincer carbene compound-
[
8a]
[24]
s
and Na [PdCl ]/sodium dodecyl sulfate (SDS) generated
2 4
palladium NPs in situ with average sizes of 6–8 and 5–10 nm,
respectively, after the Suzuki–Miyaura reaction in water. Inter-
estingly, these palladium NPs are stable in aqueous solution;
no precipitate was observed for months, which contributed to
the synergetic immobilization effect of the ionic triazolyl–pyri-
dine ligand. Nitrogen atoms from the triazolyl–pyridine rings
bind to the surface of the palladium NPs, while the ionic
group points away from the surface of the NPs. The repulsion
interactions between the ionic groups prevent agglomeration
of the NPs and result in small sizes and good stability of the
NPs in water.
Figure 6. XPS analyses of Pd 3d for a) PdÀSuzuki, b) PdÀNaBH , and c) PdÀ
4
2
H .
the fully reduced palladium NPs, whereas the peaks at 336.3
and 341.5 eV indicate the presence of palladium(II) at the sur-
X-ray photoelectron spectroscopy (XPS) analysis was per-
[25]
0
II
formed to gain insight into the surface properties of PdÀ face of the palladium NPs. The molar ratio of Pd /Pd was
Suzuki. As shown in Figure 6a, the Pd 3d spectrum showed
two sets of binding energy peaks, corresponding to two pairs
of spin-orbital doublets. The peaks with binding energies at
approximately estimated based on the ratio of peak area for
0
II
[26]
Pd 3d5/2 of the Pd species to that of the Pd species. The
0
II
molar ratio of Pd /Pd at the surface of palladium NPs was
1.34.
335.3 (Pd 3d5/2) and 340.6 eV (Pd 3d3/2) can be attributed to
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For comparison, palladium NPs were also prepared by using
ionic group is located around the surface of the NPs as a result
of nitrogen atoms of the triazolyl–pyridine unit coordinating to
the surface of the palladium NPs; repulsion interactions be-
tween the ionic groups resulted in a small size and narrow size
distribution of the NPs. The structures and catalytic activities of
NPs have been influenced by the synthetic routes and types of
reducing reagents when 3 was used as a precursor of the NPs.
In summary, this study provides an efficient protocol for the
NaBH and H as reducing reagents and 3 as a precursor; the
4
2
corresponding palladium NPs were designated PdÀNaBH and
4
PdÀH , respectively. As shown in Figure 5b and c, TEM images
2
revealed that the mean diameters of PdÀNaBH and PdÀH
4
2
were (3.92Æ0.79) and (6.49Æ1.05) nm, respectively, which
were bigger than that of PdÀSuzuki. Moreover, clear aggrega-
tion was observed for PdÀH . XPS analyses showed that the
2
0
II
molar ratio of Pd to Pd at the surface of PdÀNaBH and PdÀ synthesis of water-soluble, ionic, bidentate, nitrogen-contain-
4
H was 3.91 and 1.17, respectively (Table 4, entries 2 and 3). It
ing ligands, the palladium complexes of which may serve as ef-
ficient precursors to produce palladium NPs with small sizes
and a narrow size distribution. The successful application of
these NPs in catalysis not only offers a new strategy for the
synthesis of aqueous NPs through the use of ionic, bidentate,
nitrogen-containing ligands, but also opens up a green avenue
for the development of an environmentally friendly catalytic
system.
2
[
a]
Table 4. Comparison of the performance of palladium NPs.
0
II[b]
Entry
Precatalyst
Particle size
nm]
Pd /Pd
Conversion
[%]
[
1
2
3
3
3.09Æ0.53
3.92Æ0.79
6.49Æ1.05
1.34
3.91
1.17
70
78
6
PdÀNaBH
4
PdÀH
2
[
(
a] Reaction conditions: 4-bromoanisole (1.0 mmol), phenylboronic acid
1.5 mmol), K CO (2.0 mmol), and 0.1 mol% Pd in H O (3 mL) at 1208C
Experimental Section
2
3
2
for 0.5 h. [b] Oxidation state of the surface Pd defined by XPS.
General methods
All chemicals were purchased from commercial suppliers and were
1
13
used without further purification. H and C NMR spectra were re-
corded on a Bruker AVANCE III NMR spectrometer at 400 and
was known that the properties of the palladium NPs, such as
particle size and metal oxidation state, had an important influ-
1
00 MHz, respectively, using deuterated CDCl as a locking solvent
3
[
4b,12a]
ence on the catalytic activity.
To further understand the
except where otherwise indicated. The IR spectra were recorded as
KBr pellets by using a PerkinElmer instrument. Thermogravimetric
analyses were performed on a Mettler TGA/SDTA 851e analyzer
nature of the palladium NPs, the catalytic activity of 3 and the
two palladium NPs were evaluated. As shown in Table 4, when
À1
with a heating rate of 108Cmin under a nitrogen atmosphere.
using 3 and PdÀNaBH as precatalysts, the reaction of 4-bro-
4
GC analyses were performed on a Shimadzu GC-2014 instrument
equipped with a capillary column (RTX-5, 30 mꢂ0.25 mm) using
a flame ionization detector. TEM was performed on a JEOL JEM-
moanisole and phenylboronic acid in water under identical
conditions gave 4-methoxydiphenyl in 70 and 78% yield, as
determined by GC, respectively; the slightly lower catalytic ac-
tivity in 3 was mainly ascribed to the existence of the induc-
tion period at the beginning of the reaction. However, a yield
2
010 microscope operated at 200 kV. XPS was performed on
a Thermo ESCALAB 250 spectrometer. Non-monochromatic Al Ka
radiation was used as a primary excitation source. The binding en-
ergies were referenced to the C 1s line at 284.8 eV from adventi-
tious carbon. Elemental analyses were performed on an Elementar
Vario MICRO elemental analyzer.
of 8%, as determined by GC, was achieved by using PdÀH as
2
a precatalyst. These results suggested that the synthetic meth-
ods used for palladium NPs and the types of reducing reagents
had an important effect on the size and oxidation state of the
palladium species, resulting in significant differences in the cat-
alytic performances.
X-ray crystallography
Data collection for 3 was performed on a Rigaku AFC7R instrument
equipped with a graphite-monochromated MoKa radiation (l=
0.71073 ꢁ). The intensity data set was collected by using an w scan
technique and corrected for Lp effects. The primitive structures
Conclusion
By using the increasingly popular click reaction, a water-solu-
ble, ionic, triazolyl–pyridine ligand was easily prepared. The
crystal structure and theoretical calculation of its palladium(II)
complex revealed that palladium(II) was coordinated by the
pyridyl nitrogen atom and a triazolyl nitrogen atom to form
a stable, five-membered chelating ring, in which the electron-
donating ability of 1,2,3-triazolyl ring was stronger than that of
the pyridyl ring. Excellent catalytic activity for the palladium-
catalyzed Suzuki–Miyaura cross-coupling reaction of aryl bro-
mides and aryl chlorides and the Hiyama cross-coupling reac-
tion of aryl bromides was observed in neat water when the
water-soluble palladium complex was used as a precatalyst.
Palladium NPs were formed after the catalytic reactions, in
which the ionic ligand served as a stabilizer of the NPs. The
2
were solved with direct methods and refined on F with full-matrix
least-squares methods by using the SHELXS-97 and SHELXL-97 pro-
[27]
grams, respectively. All non-hydrogen atoms were refined aniso-
tropically. The positions of hydrogen atoms were generated geo-
metrically (CÀH bond fixed at 0.96 ꢁ) and allowed to ride on their
parent carbon atoms before the final cycle of refinement.
CCDC 910130 contains 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.
Theoretical study
Calculations were performed by using the Gaussian 03 (Revision
[21]
D.02) suite of programs.
The geometric optimization of the
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structures and frequency analyses were performed by using the
B3-LYP method with effective core potentials (ECPs). The all-elec-
tron 6-31+G** basis set was used on C, N, Cl, and H atoms, where-
as Lanl2dz, developed by Hay and Wadt, was employed to describe
the Pd atom. The vibration frequencies were used to character-
ize the optimized structures for which the energy was at a mini-
mum without imaginary frequencies on the potential energy sur-
face. Scaling factors were neglected in the harmonic vibration fre-
quencies.
tion was evaporated under reduced pressure and the residue was
washed with ethyl ether to give a yellow solid in quantitative yield.
1
H NMR (D O): d=8.89 (s, 1H), 8.36 (d, J=5.2, 1H), 8.02 (t, J=8.0,
2
1H), 7.94 (d, J=8.0, 1H), 7.33 (d, J=6.12, 1H), 4.64 (t, J=6.68, 2H),
3.41 (t, J=8.4, 2H), 3.12 (s, 9H), 2.09 (t, J=7.16, 2H), 1.92 ppm (d,
[28]
13
J=6.8, 2H); C NMR (D O): d=149.5, 148.2, 141.5, 126.0, 125.7,
2
122.7, 65.5, 58.9, 52.9, 52.1, 25.6, 19.6 ppm; IR (KBr): n˜ =3435 (s),
2953 (w), 2863 (w), 2353 (m), 1605 (m), 1476 (m), 1224 (w), 1051
À1
(w), 967 (w), 906 (w), 782 cm (m); elemental analysis calcd (%) for
C H C N Pd·2H O (509.2): C 33.02, H 5.15, N 13.75; found: C
14
22 l3
5
2
3
3.24, H 4.97, N 13.34. Single crystals suitable for X-ray diffraction
Synthesis of 1a
were obtained through slow evaporation of a solution of 3 in
MeOH at room temperature.
1
-Bromobutane (0.80 g, 6.0 mmol) was added to a solution of NaN3
(
0.32 g, 5.0 mmol) in H O (10 mL) and MeOH (10 mL); the mixture
2
was stirred at 808C overnight. 2-Ethynylpyridine (0.50 g, 4.8 mmol),
the sodium salt of l-ascorbic acid (0.50 g, 2.2 mmol), CuSO ·5H O
4
2
Preparation of PdÀNaBH4
(
0.20 g, 0.8 mmol), and isobutanol (10 mL) were added to the solu-
tion at 08C. After the reaction mixture was warmed to 258C and
stirred overnight, the resulting mixture was extracted with ethyl
acetate (3ꢂ10 mL). The combined organic layer was washed with
H O and brine, and dried over anhydrous Na SO . The solvent was
A fresh aqueous solution of NaBH
added to a stirring solution of 3 (5.1 mg, 0.01 mmol) in H
at 258C. The solution immediately turned dark brown and was
stirred at 258C for 40 min; the resulting mixture was used for the
Suzuki–Miyaura cross-coupling reaction without further purifica-
tion.
4
(0.1m, 1.0 mL) was rapidly
O (5 mL)
2
2
2
4
removed in vacuo and the residues were purified by flash column
chromatography on silica gel to afford the product as a brown oil
1
(
(
0.77 g, 80%). H NMR: d=8.55 (d, J=4.16, 1H), 8.17 (s, 1H), 8.14
d, J=3.2, 1H), 7.78–7.73 (m, 1H), 7.22–7.19 (m, 1H), 4.40 (t, J=
7
7
1
.12, 2H), 1.95–1.90 (m, 2H), 1.41–1.34 (m, 2H), 0.94 ppm (t, J=
.32, 3H); C NMR: d=150.3, 149.3, 148.3, 137.0, 122.8, 121.8,
20.2, 50.2, 32.2, 19.2, 13.4 ppm.
1
3
Preparation of PdÀH2
A solution of 3 (5.1 mg, 0.01 mmol) in H O (6 mL) was placed in
2
a 50 mL autoclave and reduced by H (0.3 MPa) at 258C for 1 h.
2
After the autoclave was cooled, a black solution was obtained and
used directly for the Suzuki–Miyaura cross-coupling reaction.
Synthesis of 1b
A similar procedure to that for 1a was used, except that 1-bromo-
butane was replaced with 1-bromo-4-chlorobutane to give the
1
product as a white solid (72%). H NMR: d=8.58 (d, J=3.60, 1H),
General procedure for the Suzuki–Miyaura cross-coupling
reaction
8
6
2
1
.19 (d, J=7.92, 1H), 8.17 (s, 1H), 7.78 (t, J=7.52, 1H), 7.24 (t, J=
.20, 1H), 4.48 (t, J=6.88, 2H), 3.58 (t, J=6.28, 2H), 2.18–2.10 (m,
13
H), 1.86–1.79 ppm (m, 2H); C NMR: d=150.2, 149.4, 148.5,
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),
37.0, 122.9, 121.9, 120.3, 49.6, 43.9, 29.2, 27.5 ppm.
TBAB (1.0 mmol), and H O (3 mL). An aqueous solution of the ap-
2
propriate amount of 3 was added to the mixture, which was stirred
in a preheated oil bath (1208C). After an appropriate time, the mix-
ture was cooled with ice-cold water and the product was extracted
with hexane or ethyl acetate (3ꢂ5 mL). The combined organic
Preparation of 2
Compound 1b (0.36 g, 1.5 mmol) in acetonitrile (10 mL) was added
to a 30% aqueous solution of Me N (2 mL); the mixture was
3
heated at reflux overnight. The clear solution was then evaporated
under reduced pressure. The residue was washed several times
layer was dried over anhydrous Na SO and the solvent was re-
2 4
moved 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 compari-
son with spectroscopic data reported in the literature.
with Et O and dried in vacuo to give a white solid in a quantitative
2
1
yield. H NMR (D O): d=8.47 (s, 1H), 8.32 (s, 1H), 7.91–7.85 (m,
2
2
3
H), 7.37 (t, J=5.4, 1H), 4.53 (t, J=6.80, 2H), 3.33 (t, J=8.48, 2H),
.06 (s, 9H), 2.03–1.96 (m, 2H), 1.82–1.73 ppm (m, 2H); C NMR
13
(
D O): d=149.0, 148.0, 147.0, 138.5, 124.0, 123.6, 120.9, 65.6, 52.8,
2
4
9.7, 26.1, 19.5 ppm; IR (KBr): n˜ =3410 (s), 3053 (m), 2964 (w), 1643
Reuse of the catalytic aqueous solution in the Suzuki–
Miyaura cross-coupling reaction
(
(
m), 1481 (m), 1392 (w), 1297 (w), 1151 (w), 1051 (w), 972 (w), 922
w), 788 (m), 660 cm
À1
(w); elemental analysis calcd (%) for
C H ClN ·H O (313.8): C 53.58, H 7.71, N 22.32; found: C 53.85, H
1
4
22
5
2
A mixture of 4-bromoacetophenone (1.0 mmol), phenylboronic
7
.74, N 22.17.
acid (1.5 mmol), 3 (0.01 mol%), K CO3 (2.0 mmol), and TBAB
2
(
1.0 mmol) in H O (3.0 mL) was stirred at 1208C for 0.5 h. After
2
cooling with ice-cold water, the crude product was extracted with
diethyl ether (3ꢂ5 mL) and the conversion was determined by GC
analyses. The aqueous phase containing the catalytic species was
evaporated under reduced pressure to remove possible residual di-
ethyl ether. The aqueous catalytic system was recharged with the
same substrates and 1.0 mmol base for the next run.
Preparation of 3
A mixture of 2 (31.2 mg, 0.1 mmol) and [Pd(cod)Cl ] (28.5 mg,
2
0
.1 mmol) in CH Cl (10 mL) was stirred for 30 min to form a yellow
2 2
precipitate. After MeOH (3 mL) was added, the mixture became
clear and was stirred for an additional 30 min. The resulting solu-
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General procedure for the Hiyama cross-coupling reaction
Poteau, K. Philippot, B. Chaudret, Angew. Chem. 2011, 123, 12286–
12290; Angew. Chem. Int. Ed. 2011, 50, 12080–12084.
A 25 mL reactor equipped with a condenser was charged with aryl
halide (1.0 mmol), arylsiloxane (1.5 or 2.0 mmol), 3 (0.5 mol%),
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2
ous solution was stirred in a preheated oil bath (1208C) for 6 h.
After the mixture was cooled with ice-cold water to room tempera-
ture, the product was extracted with diethyl ether (3ꢂ5 mL). The
combined organic layer was dried over anhydrous Na SO and the
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[
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click chemistry
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nanoparticles
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2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPlusChem 0000, 00, 1 – 11
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These are not the final page numbers!

CHEMPLUSCHEM
FULL PAPERS
www.chempluschem.org
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Received: February 25, 2013
Revised: March 22, 2013
Published online on && &&, 0000
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2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers!

FULL PAPERS
F. Kong, C. Zhou, J. Wang, Z. Yu,*
R. Wang*
&& – &&
Water-Soluble Palladium Click
Chelating Complex: An Efficient and
Reusable Precatalyst for Suzuki–
Miyaura and Hiyama Reactions in
Water
In complete control: A water-soluble
palladium(II) click chelating complex
serves as an efficient precursor for the
formation of palladium nanoparticles
(NPs; see picture). Excellent catalytic
performances are achieved in Suzuki–
Miyaura and Hiyama cross-coupling re-
actions in water.
ꢀ
2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPlusChem 0000, 00, 1 – 11
&11
&
ÞÞ
These are not the final page numbers!