An efficient and recyclable water-soluble cyclopalladated complex for aqueous Suzuki reactions under aerial conditions

Article abstract of DOI:10.1016/j.jorganchem.2009.09.039

Several water-soluble cyclopalladated complexes with five- or six-membered rings have been prepared as air-stable solids from Schiff base ligands bearing an N-phenyl sulfonate groups. Cyclopalladated complexes with six-membered rings show high catalytic e

Full text of DOI:10.1016/j.jorganchem.2009.09.039

Journal of Organometallic Chemistry 695 (2010) 297–303
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
An efficient and recyclable water-soluble cyclopalladated complex for aqueous
Suzuki reactions under aerial conditions
Jin Zhou a,b, Xiaoyan Li a, Hongjian Sun a,
*
a School of Chemistry and Chemical Engineering, Shandong University, Shanda Nanlu 27, 250100 Jinan, People’s Republic of China
b School of Chemical Engineering, Shandong University of Technology, Zibo 255049, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Several water-soluble cyclopalladated complexes with five- or six-membered rings have been prepared
as air-stable solids from Schiff base ligands bearing an N-phenyl sulfonate groups. Cyclopalladated com-
plexes with six-membered rings show high catalytic efficiency for the Suzuki reactions of aryl bromides
with phenylboronic acid in aqueous solvents under mild conditions. Palladium complex 1 can be used for
five reaction cycles in high conversions for the Suzuki reactions in neat water without additives. The cat-
alytic process for the Suzuki couplings is proved by TEM analysis to proceed on Pd(0) nanoparticles. Sur-
factant-protected palladium nanoparticles present lower activities and poorer recyclability for the
coupling reactions than those generated in situ without additives.
Received 6 August 2009
Received in revised form 17 September
2009
Accepted 25 September 2009
Available online 3 October 2009
Keywords:
Cyclopalladated complex
Palladium
Ó 2009 Elsevier B.V. All rights reserved.
Suzuki reaction
Water soluble
Aqueous
1. Introduction
atom) reported as precatalysts for aqueous Suzuki reactions [22–
30]. The simple Pd-oximed complexes with a hydroxy substituent
In the past two decades, the palladium-catalyzed Suzuki reac-
tion of aryl halides with arylboronic acids has evolved into one of
the most important and powerful methods to form biaryls [1–8].
Under increasing environmental awareness, water is an attractive
alternative to traditional organic solvents because it is inexpensive,
nonflammable, nontoxic, and environmentally sustainable. Since
Casalnuovo’s initial report of TPPMS (triphenylphosphine mono-
sulfonate)/Pd(OAc)2 catalyst system for cross coupling reactions
in aqueous solvents [9], Suzuki reaction in aqueous phase includ-
ing neat water and water–organic mixed solvents has been re-
ceived much more attention [10–15].
Recently, cyclopalladated complexes have attracted much atten-
tion as exciting catalyst precursors to cross coupling reactions [16–
20]. The first water-soluble cyclopalladated imine complex reported
by Ryabov was applied as catalyst for ester hydrolysis [21]. The use
of the hydrophilic cyclopalladated complexes in water is expected to
perform as a recyclable and highly active catalyst system for the Su-
zuki reaction in aqueous solvents. There have been several examples
of hydrophilic cyclopalladated complexes or Pd pincer complexes
(these so-called pincer complexes are usually denoted as Pd-ECE,
such as Pd-NCN, Pd-PCNor Pd-SCS-pincercomplex, where E is a neu-
tral two-electron donor atom, and C represents the anionic carbon
on the aromatic rings reported by Nájera are highly active catalysts
for Suzuki couplings of aryl bromides, activated aryl chloridesor het-
eroaromatic chlorides [22,23]. Pd-NCN and Pd-PCN pincer com-
plexes reported by SanMartin and his co-workers were employed
as good catalyst for Suzuki couplings in neat water, although these
two precatalysts were not exactly water soluble [24,25]. A hydro-
philic Pd-CNC pincer complex was proved to be a highly efficient
recyclable homogeneous catalyst for Suzuki couplings, and the reac-
tion could be repeated up to five times [26]. Additionally, the poly-
mer-supported SCS-pincer complexes [27,28] and the oximed
cyclopalladated complex attached silica [29] were also used as recy-
clable heterogeneous catalysts for Suzuki couplings, and it was sug-
gested that the nanopalladium generated gradually in situ was the
active catalytic species for the reactions. Combined with a water-
soluble phosphine ligand (t-Bu-Amphos), cyclopalladated imine
complex with sulfonate groups could be efficiently used for coupling
reaction of 4-bromotoluene with phenylboronic acid in eleven reac-
tion cycles, but those Pd complexes were not efficient precatalysts
on their own [30]. As one of the most popular classes of cyclopalla-
dated derivatives, a cyclopalladated imine complex has proved to
be efficient for Suzuki reactions in aqueous solvent with TBAB (tet-
rabutylammonium bromide) additive [31]. However, no cyclopalla-
dated imine complexes have been reported as efficient and
recyclable catalytic precursors for Suzuki reaction in neat water
without supports or additives.
* Corresponding author. Tel.: +86 0531 88361350; fax: +86 0531 88564464.
E-mail addresses: zhoujin820514@163.com (J. Zhou), hjsun@sdu.edu.cn (H. Sun).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.09.039

298
J. Zhou et al. / Journal of Organometallic Chemistry 695 (2010) 297–303
Herein, we report several water-soluble cyclopalladated imine
dium, allowing the simple cyclopalladated dimmers to be isolated
[30]. The C–H activation in 1-position of the anthracene ring was
monitored by the decrease of the 1H NMR signal intensity. Com-
plexes 1 and 2 are soluble in strongly polar solvents such as water,
methanol, dimethyl sulfoxide or others. The corresponding hydro-
phobic 3 and 4 were prepared by a similar method. IR spectrum re-
veal a red-shift of mC@N from about 1628 cmÀ1 (for Schiff base
ligands) to about 1606 cmÀ1 (for complexes). An IR absorption at
complexes and its catalytic performance toward Suzuki reactions
in aqueous phase. It was found that the chelated structure of Pd
complexes were crucial for the catalytic efficiency. The new com-
plex 1 with six-membered rings is proved to be a highly efficient
and recyclable catalyst precursor for the Suzuki reactions of aryl
bromides in neat water without additives.
1188 cmÀ1
(mS@O) is displayed for both sulfonated ligands and com-
2. Results and discussion
plexes. Two Shaughnessy-type cyclopalladated complexes (5 and
6) were also prepared.
2.1. Preparation of ligands and complexes
The detailed structure of complex 3 was determined by an X-
ray diffraction study on orange crystals obtained from a dichloro-
methane solution [32]. Fig. 1 displays an ORTEP plot of complex
3. The complex possesses a palladium dimer configuration. In the
crystal, each palladium center is in slightly distorted square-planar
coordination geometry with nitrogen and carbon donors in cis-
positions as expected.
Condensation of the sodium salt of 3,5-dialkyl sulfanilic acid (or
2,6-dialkyl aniline) with anthracene-9-carbaldehyde in anhydrous
methanol yielded the desired Schiff base ligands. Then the hydro-
philic cyclopalladated complexes (1 and 2) were prepared by a tra-
ditional C–H activation reaction of the sulfonated Schiff base
ligands with Li2PdCl4 in the presence of sodium acetate in anhy-
drous methanol (Scheme 1). It has been proved that the weakly
coordinating sulfonate anion does not strongly coordinate to palla-
2.2. Comparison of catalytic activities of cyclopalladated complexes
The coupling of 4-bromoanisole with phenylboronic acid was
selected as a model reaction (Table 1). In a typical experiment, 4-
bromoanisole, phenylboronic acid and base in a ratio of 1:1.5:2
were placed in the flask, followed by the addition of the catalyst
and the solvent. All instances were carried out under aerial condi-
tions and at appropriate temperature. Evidently, the Pd complexes
with six-membered rings (complexes 1 and 2) are more efficient
than Shaughnessy-type Pd complexes with five-membered rings
(complexes 5 and 6). Palladium black was found in the reactions
using five-membered Pd complex. The hydrophobic cyclopalladat-
ed complexes also gave high conversions in neat water without
additives with higher Pd loading of 0.05 mol% at refluxing temper-
ature (Table 1, entries 6 and 7). It appears that the steric hindrance
has little influence on catalytic activities of Schiff base ligands (Ta-
ble 1, entries 1–4). Interestingly, quantitative conversions were
achieved for coupling reactions in H2O/EtOH mixed solvent with
either 1 or 5 as catalytic precursor (Table 1, entries 12 and 13),
indicating a potential solvent effect.
R'
R'
Li2PdCl4, NaOAc
N
R
N
R
MeOH
Pd
Cl
R'
R'
2
Ligand 1: R' = iPr, R = SO3Na
Ligand 2: R' = Me, R = SO3Na
Ligand 3: R' = iPr, R = H
Ligand 4: R' = Me, R = H
Complex 1 (43.5%)
Complex 2 (45.5%)
Complex 3 (62.2%)
Complex 4 (60.2%)
N
SO3Na
N
SO3Na
Pd
2
Cl
Pd
Cl
2
Today, it is now widely accepted that the cyclopalladated com-
plex operate via a traditional Pd(0)–Pd(II) catalytic cycle. These Pd
complexes are hypothesized as a source of ligand-free palladium
[33]. To know the nature of the catalytic system, TEM analysis
Complex 5
Complex 6
Scheme 1. Complexes 1–6.
Fig. 1. ORTEP drawing of 3 (drawn with 30% probability ellipsoids).

J. Zhou et al. / Journal of Organometallic Chemistry 695 (2010) 297–303
299
Table 1
for the coupling reaction. It is likely that the cyclopalladated com-
plex reacted with phenylboronic acid to give the 2-phenyl ana-
logue of the new imine ligand (Scheme 2), and released the
palladium nanoparticles [34]. The loose protection due to the weak
stabilizing effect of the new ligands towards nanoparticles could
prevent the aggregation of nanoparticles into inactive palladium
black, and makes these nanoparticles highly active towards the
substrates. The fact that the six-membered 1 is more active than
the five-membered 5 is due to the size effect of nanopalladium
[31,35].
Comparison of catalytic activities of complexes 1–6.a
Entry
Complex
[Pd] (mol%)
T (°C)
t (h)
Solventb
Yield (%)c
1
2
3
4
5
6
7
8
1
2
5
6
3
4
5
1
5
1
5
1
5
0.01
0.01
0.01
0.01
0.05
0.05
0.1
0.01
0.01
0.1
100
100
100
100
100
100
100
80
80
30
30
15
1
1
1
1
1
1
1
1
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
99
98
80d
81d
96
96
81d
92
9
1
H2O
H2O
H2O
80
10
11
12
13
15
15
10
10
86
0.1
0.05
0.05
64d
100
100
2.3. Suzuki reactions catalyzed by cyclopalladated complexes with six-
membered rings
H2O/EtOHe
H2O/EtOHe
15
a
Reaction conditions: 1 mmol of 4-bromoanisole, 1.5 mmol of phenylboronic acid,
The catalytic performance of the six-membered complexes was
studied (Table 2). All reactions proceeded smoothly under aerial
conditions. Initially, the coupling reactions were carried out in dif-
ferent solvents. Generally, complex 1 or 3 gave poorer catalytic
activities in pure organic solvents (CH3CN and EtOH), and low con-
versions were obtained for these reactions (Table 2, entries 1–4).
The reaction yields were dramatically improved when water was
introduced as a medium (Table 2, entries 5 and 6). KOH gave higher
yields than K2CO3 or NaOAc (Table 2, entries 6–8). An outstanding
coupling reaction could be carried out with quantitative conver-
sion at 15 °C with a loading of 0.05 mol% Pd in H2O/EtOH mixed
solvents (Table 2, entries 9 and 10). Interestingly, the hydrophobic
3 present higher activity in H2O/EtOH mixed solvent (Table 2, en-
tries 8–10), while the hydrophilic 1 was more efficient in neat
water (Table 2, entries 12 and 13). The hydrophilic 1 could
smoothly dissolve in either neat water or aqueous solvents, while
complex 3 is soluble in H2O/EtOH mixed solvent and insoluble in
neat water. In other words, complex 3 could not deliver high dis-
2 mol of KOH, solvent.
b
10 mL of H2O.
GC yield based on 4-bromoanisole.
Pd black was found.
H2O:EtOH = 2 mL:2 mL.
c
d
e
was used to monitor the pathway of the coupling reaction. First, a
sample was examined which was activated by refluxing the com-
bination of 1 (or 5) + phenylboronic acid + KOH in water. The
TEM analysis of 1 (Fig. 2a) showed the formation of palladium
nanoparticles with a diameter in the range 20–150 nm, while com-
plex 5 (Fig. 2b) aggregated into larger nanoparticles (20–250 nm)
with much more nanoparticles over 150 nm than complex 1. Quan-
titative conversion was achieved using the above solution of com-
plex 1 for the coupling of 4-bromoanisole and phenylboronic acid,
while 80% conversion was achieved for complex 5, suggesting that
the palladium nanoparticles might be the active catalytic species
Fig. 2. TEM micrographs of palladium nanoparticles.
PhB(OH)2
KOH, H2O
R
2
Pd0
R
R
+
N
N
N
Pd
Cl
Pd
H2O
Ph
Ph
Scheme 2. The formation of palladium nanoparticles.

300
J. Zhou et al. / Journal of Organometallic Chemistry 695 (2010) 297–303
persive and active nanoparticles in neat water due to its insolubil-
ity. We suggest that the solubility of substrates and catalyst was
crucial for the reactions in neat water (Table 2, entries 12 and
13), while the substituent effect of sulfonate groups may be related
to the lower activities of hydrophilic 1 in mixed solvents (Table 2,
entries 8 and 10), demonstrating the different performance of 1
and 3. Thereby it is evident that the hydrophilic 1 is more advan-
tageous for the Suzuki reactions in neat water than the hydropho-
bic 3.
The scope of catalytic system using complex 1 was explored
with a range of aryl bromides (Table 3). Electron-deficient and
electron-rich aryl bromides could be coupled with phenylboronic
acid in excellent yields in neat water. A quantitative conversion
and excellent yield were obtained at reflux temperature for the
coupling of 4-bromoanisole and phenylboronic acid with
0.01 mol% loading of Pd (Table 3, entry 2). The coupling reactions
were carried out at different temperatures (Table 3, entries 1–5),
and a conversion of 30% could be obtained even at low temperature
of 8 °C (the temperature of lab in winter, Table 3, entry 5). A con-
version of 93% was reached after 15 h only using 10À5 mol% load-
ing of Pd, which corresponded to a TON of 9.3 Â 106 (Table 3,
entry 8). Reactions in water appear to be sensitive to steric hin-
drance of the substrate. 2-Bromotoluene gave 96% yield of coupling
product with phenylboronic acid using an increased Pd loading
(0.02 mol%) and extended reaction time, while 2,6-dimethylb-
romobenzene gave only 30% yield (Table 3, entries 10 and 12).
Table 2
Catalytic performance of complexes 1 and 3.a
Entry Complex [Pd]
(mol%)
Solventb
Base
T (°C) t (h) Conv.c Yieldd
(%)
(%)
1
2
3
4
5
6
7
8
1
1
1
3
1
1
1
1
1
3
1
1
3
1
1
0.1
CH3CN
DMA
EtOH
EtOH
K2CO3 50
K2CO3 30
K2CO3 15
K2CO3 15
5
5
Trace
30
0.05
0.05
1
0.05
0.05
0.05
0.05
0.05
0.01
0.1
10
10
3
10
15
30
H2O/CH3CN K2CO3 50
H2O/EtOH
H2O/EtOH
H2O/EtOH
H2O/EtOH
H2O/EtOH
H2O
H2O
H2O
H2O
K2CO3 15
NaOAc 15
4
45
40
4
20
KOH
KOH
KOH
KOH
KOH
KOH
KOH
15
15
15
100
30
30
30
4
10
5
1
4
55
50
99
99
96
50
9
100
100
99
55
11
10
11
12
13
14
0.1
0.1
4
10
86
82
a
Reaction conditions: 1 mmol of 4-bromoanisole, 1.5 mmol of phenylboronic acid,
2 mmol of base, 10 mL of solvent.
b
Water:organic solvent = 2 mL:2 mL if necessary.
Determined by GC, average of three runs.
Isolated yield.
c
2.4. Catalyst recycling
d
The recyclability of 1 was also investigated for Suzuki reactions
in neat water. Three catalyst precursors, including complexes 1, 5
and Li2PdCl4, were explored (Table 4). It was shown that complex
1 could be repeated up to three times in nearly quantitative yield
before the efficiency of the catalyst began to degrade, while
Li2PdCl4 or complex 5 gave dramatically decreased conversions
(75% or 50%, respectively) in the second cycle. The six-membered
cyclopalladated complexes showed better recyclability than those
five-membered complexes. Of interest is that the catalytic system
using 1 as precatalyst could be used for five cycles in over 90% yield
by extending the reaction time after the third cycle (Table 4, en-
tries 3 and 4).
As shown in Fig. 3, the TEM analysis suggests that a large num-
ber of active nanoparticles exist in every run of complex 1 that ap-
pears to sustain the catalytic activity. El-Sayed and his co-workers
reported that the reactant of phenylboronic acid could act as a sta-
bilizer because the OÀ of the deprotonated phenylboronic acid
could bind to the surface of the palladium nanoparticles with ter-
minal or bridged binding mode [36,37]. As a result, the Ostwald
ripening of nanoparticles could be greatly inhibited, and the nano-
Table 3
Suzuki reactions using complex 1 in neat water.a
Entry ArX
[Pd] (mol%) T (°C) t (h) Conv. (%)b Yield (%)c
1
2
3
4
5
6
7
8
p-MeOC6H4Br 0.01
80
1
1
12
12
10
1
5
15
1
3
1.5
5
92
99
25
30
86
99
90
93
99
96
93
30
90
98
p-MeOC6H4Br 0.01
p-MeOC6H4Br 0.01
p-MeOC6H4Br 0.1
p-MeOC6H4Br 0.1
p-MeOC6H4Br 0.001
p-MeOC6H4Br 0.0001
p-MeOC6H4Br 0.00001
100
30
8
30
82
96
87
90
95
90
90
100
100
100
100
100
100
100
9
p-NO2C6H4Br
2-MeC6H4Br
4-MeC6H4Br
0.01
0.02
0.01
10
11
12
2,6-MeC6H4Br 0.1
a
Reaction conditions: 1 mmol of aryl bromides, 1.5 mmol of phenylboronic acid,
2 equiv. of KOH, 15 mL of water, palladacycle 1.
b
Determined by GC, average of three runs.
Isolated yield.
c
Table 4
Recycling of complex 1.a
Entry
Complex
[Pd]mol%
S (mmol)
Reaction yield by cycleb
1
2
3
4
5
6
7
8
1
Li2PdCl4
0.1
0.01
0.1
0.1
0.01
0.1
0.1
0.1
0.1
0.1
97
99
97
99
83
80
99
99
98
98
75
87
82
90
35
50
80
87
80
60
20
92
98
98
10
8
93
86
60
32
2c
3d
4c
5
1
1
1
5
5
1
1
1
1
62
67
85
40
99
60
40
90
99
e
e
37
20
6
7f
8f
9f
10f
TBAB (0.1)
TBAB (1.0)
PVP (0.1)
PVP (1.0)
63
49
60
40
5
a
b
c
d
e
f
Reaction conditions: 1 mmol of 4-bromoanisole, 1.5 mmol of phenylboronic acid, 2 equiv. of KOH, 20 mL of water, Pd complex, 100 °C, 1 h.
GC yield, average of three runs.
1.5 h for cycles 1–3; 2 h for cycles 4 and 5; 15 h for cycles 6–8.
1 h for cycles 1–4.
18 h.
Reaction conditions same to entry 3 except surfactant as additive.

J. Zhou et al. / Journal of Organometallic Chemistry 695 (2010) 297–303
301
Fig. 3. TEM analysis of catalyst cycles 1–4 (Fig. 3a–d).
particles do not dramatically increase in size. Then, the palladium
nanoparticles, which could be considered to be soluble in water be-
cause of the ligation of deprotonated phenylboronic acid, could be
reused for several cycles. It could be also explained that the degra-
dation of catalytic activity is due to the thermodynamically favored
Ostwald ripening process and a greater amount of phenylboronic
acid bound to the free active sites of metallic species.
catalyst precursor for Suzuki reactions of aryl bromides in neat
water. TEM analysis suggests an active species of Pd(0) nanoparti-
cles for the Suzuki reactions. Surfactants or excess of phenylboro-
nate can occupy active free sites of palladium nanoparticles and
result in lower catalytic activity.
4. Experimental
Quaternary ammonium compounds and polymers, such as
TBAB and PVP (polyvinylpyrrolidone), are often used as surfactants
and stabilizers for nanoparticles. The effects of TBAB and PVP addi-
tives were studied. In the presence of 0.1 equiv. of TBAB, the yields
were almost same as that without additives for the reaction cycles
1–4, while a lower yield (60%) was obtained for the fifth cycle (Ta-
ble 4, entry 7). With the increased amount of TBAB (1.0 equiv.), the
catalytic system began to lose activity from the third run (Table 4,
entry 8), indicating that the excess TBAB could diminish the activ-
ity of 1. Compared with TBAB, PVP additive shows much more
diminished effect on catalytic activity of 1 and resulted in lower
yields of coupling products (Table 4, entries 9 and 10). Basing on
the pathway of palladium nanoparticles, the lower catalytic activ-
ity observed could be due to the fact that there are less free metal-
lic surface sites available for the catalysis because many of the free
sites are capped by excess surfactant stabilizer.
4.1. Materials
Methanol and dichloromethane were purified by standard
methods. The sodium salts of 3,5-dialkyl sulfanilic acid [38,39]
and the benzaldehyde imine ligands [30] were prepared by modi-
fications of literature procedures. All other reagents were used as
received from commercial sources.
4.2. Apparatus
GC yields were determined by comparison to an internal stan-
dard. 1H NMR and 13C NMR were recorded on a Bruker 300 MHz.
TEM analysis was carried on by a JEM 100-CX II.
4.3. Preparation of ligand 1
3. Conclusions
Anthracene-9-carbaldehyde (0.22 g, 1.07 mmol) and sodium
salt of 3,5-diisopropyl sulfanilic acids (0.28 g, 1.00 mmol) were dis-
solved in 5 mL of anhydrous methanol. Two drops of formic acid
were added into the solution as catalyst. The mixture was stirred
The water-soluble cyclopalladated complex 1 with six-mem-
bered rings has been proved to be a highly efficient and recyclable

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J. Zhou et al. / Journal of Organometallic Chemistry 695 (2010) 297–303
for 24 h at 40 °C. Ethyl ether (30 mL) was transferred to the reac-
tion mixture, and the crude product was isolated. Recrystallization
from methanol/ethyl ether delivered the pure product as a yellow
(2H, s, CH@N), 8.95 (2H, s, ArH), 8.20 (4H, t, ArH), 7.88 (2H, d,
ArH), 7.70 (2H, t, ArH), 7.60 (4H, t, ArH), 7.44 (4H, s, ArH), 7.27
(2H, t, ArH), 2.51 (12H, s, –CH3). 13C NMR (75.5 MHz; DMSO-d6) d
160.1, 149.2, 146.5, 141.7, 137.4, 134.6, 133.7, 131.1, 130.8,
130.5, 129.8, 129.3, 127.2, 126.0, 125.8, 125.5, 124.9, 123.5, 19.2.
Anal. Found (calc. for C46H34N2S2O6Na2Pd2Cl2): C, 48.69 (50.02);
H, 3.15 (3.10); N, 2.69 (2.54)%.
solid (0.27 g, yield in 58%). IR (KBr) 1628 cmÀ1 C@N), 1188 cmÀ1
(m
(m
S@O). 1H NMR (300 MHz; CD3OD; Me4Si) d 9.54 (1H, s, CH@N),
8.94 (2H, d, ArH), 8.73 (1H, s, ArH), 8.15 (2H, d, ArH), 7.79 (2H, s,
ArH), 7.65–7.54 (4H, m, ArH), 3.64–3.50 (2H, m, CHMe2), 1.30
(12H, d, –CH3). 13C NMR (75.5 MHz; CD3OD) d 163.3, 151.6,
140.8, 137.7, 133.19, 131.5, 131.4, 130.81, 129.0, 127.4, 125.2,
124.0, 120.8, 28.2, 27.4, 22.5, 21.4, 17.0. Anal. Found (calc. for
C27H26NSO3Na): C, 69.13 (69.36); H, 5.78 (5.61); N, 2.74 (3.00)%.
Other Schiff base ligands were prepared by a similar method.
Complex 3: orange solid (83%). IR (KBr) 1606 cmÀ1 C@N). 1H
(m
NMR (300 MHz; CDCl3; Me4Si) d 9.50 (2H, s, CH@N), 8.95 (2H, s,
ArH), 8.66 (2H, d, ArH), 7.85–7.70 (4H, m, ArH), 7.63–7.50 (8H,
m, ArH), 7.32–7.20 (6H, m, ArH), 3.62–3.47 (4H, m, –CHMe2),
1.27 (12H, d, –CH3), 1.13 (12H, d, –CH3). Anal. Found (calc. for
C54H52N2Pd2Cl2): C, 64.10 (64.04); H, 5.20 (5.18); N, 2.91 (2.77)%.
Ligand 2: yellow solid (65%). IR (KBr): 1629 cmÀ1
(mC@N),
1181 cmÀ1 S@O). 1H NMR (300 MHz; DMSO-d6; Me4Si) d 9.56
(m
Complex 4: orange solid (86%). IR (KBr) 1606 cmÀ1 C@N). 1H
(m
(1H, s, CH@N), 8.94 (2H, d, ArH), 8.85 (1H, s, ArH), 8.21 (2H, d,
ArH), 7.79 (2H, s, ArH), 7.70–7.59 (4H, m, ArH), 2.29 (6H, s, –
CH3). 13C NMR (75.5 MHz; DMSO-d6) d 163.7, 152.7, 143.9, 131.4,
131.3, 130.5, 129.5, 128.1, 126.7, 126.0, 125.7, 125.1, 19.2. Anal.
Found (calc. for C23H18NSO3Na): C, 66.56 (67.14); H, 4.62 (4.41);
N, 3.40 (3.58)%.
NMR (300 MHz; CDCl3; Me4Si) d 9.52 (2H, s, CH@N), 8.93 (2H, s,
ArH), 8.69 (2H, d, ArH), 7.85–7.70 (4H, m, ArH), 7.63–7.50 (8H,
m, ArH), 7.36–7.22 (6H, m, ArH), 2.48 (12H, s, –CH3). Anal. Found
(calc. for C46H36N2Pd2Cl2): C, 61.41 (61.35); H, 3.91 (4.03); N,
3.28 (3.11)%.
Complex 5: yellow solid (60%). IR (KBr) 1600 mÀ1
(mC@N),
Ligand 3: yellow solid (75%). IR (KBr) 1630 cmÀ1
(m
C@N). 1H NMR
1187 cmÀ1
(m
S@O). 1H NMR (DMSO-d6, 300 MHz) d 8.26 (2H, s,
(300 MHz; CDCl3; Me4Si) d 9.52 (1H, s, CH@N), 8.98 (2H, d, ArH),
8.62 (1H, s, ArH), 8.10 (2H, d, ArH), 7.64–7.52 (4H, m, ArH), 7.31–
7.21 (3H, m, ArH), 3.34–3.25 (2H, m, CHMe2), 1.28 (12H, d, –
CH3). Anal. Found (calc. for C27H27N): C, 88.72 (88.52); H, 7.50
(7.45); N, 3.75 (3.83)%.
CH@N), 7.83 (2H, s, ArH), 7.49–7.39 (6H, m, ArH), 7.23–7.11 (4H,
m, ArH), 3.29–3.20 (4H, m, CHMe2), 1.27 (12H, d, –CH3), 1.09
(12H, d, –CH3). 13C NMR (DMSO-d6, 75.5 MHz) d 147.2, 146.0,
142.2, 140.7, 135.1, 131.0, 130.7, 129.1, 124.4, 120.3, 28.18,
24.57, 22.95. Anal. Found (calc. for C38H42N2S2O6Na2Pd2Cl2): C,
42.51 (44.90); H, 4.51 (4.16); N, 3.17 (2.76)%.
Ligand 4: yellow solid (70%). IR (KBr) 1630 cmÀ1 C@N). 1H NMR
(m
(300 MHz; CDCl3; Me4Si) d 9.52 (1H, s, CH@N), 8.98 (2H, d, ArH),
8.62 (1H, s, ArH), 8.10 (2H, d, ArH), 7.64–7.52 (4H, m, ArH), 7.31–
7.21 (3H, m, ArH), 2.29 (6H, s, –CH3). Anal. Found (calc. for
C23H19N): C, 89.20 (89.28); H, 6.22 (6.19); N, 4.69 (4.53)%.
Complex 6: yellow solid (50%). IR (KBr) 1600 cmÀ1
(mC@N),
1188 cmÀ1 S@O). 1H NMR (300 MHz; D2O; Me4Si) d 9.97 (2H, s,
(m
CH@N), 7.99–7.68 (6H, m, ArH), 7.67–7.52 (4H, m, ArH), 6.88
(8H, d, ArH). Anal. Found (calc. for C26H18N2S2O6Na2Pd2Cl2): C,
37.56 (36.81); H, 2.58 (2.14); N, 3.79 (3.30)%.
Ligand 5: white solid (40%). IR (KBr) 1648 cmÀ1
(m
C@N),
1178 cmÀ1 S@O). 1H NMR (300 MHz; DMSO-d6; Me4Si) d 8.31
(m
(1H, s, CH@N), 7.97 (2H, d, ArH), 7.61–7.55 (3H, m, ArH), 7.40
(2H, s, ArH), 2.91–2.82 (2H, m, –CHMe2), 1.12 (12H, d, –CH3). 13C
NMR (75.5 MHz; DMSO-d6) d 163.3, 149.8, 143.9, 136.5, 136.0,
132.2, 129.5, 128.9, 120.8, 28.0, 23.6. Anal. Found (calc. for
C19H22NSO3Na): C, 61.08 (62.11); H, 6.10 (6.03); N, 3.51 (3.81)%.
4.5. General procedure for Suzuki reactions
To a 50 mL round-bottomed flask equipped with condenser
were placed Pd complex, aryl bromide (1.0 mmol), base
(2.0 mmol), phenylboronic acid (1.5 mmol), and solvent (5 mL).
The reaction mixture was stirred and heated at reaction tempera-
ture for a given period of time. Upon cooling the reaction mixture
was extracted with three portions of CH2Cl2 (5 mL). The extract
was dried by Na2SO4 and concentrated to yield solid material or
oil. Reaction conversions were determined by GC analysis using
decane as an internal standard. The crude materials were flash
chromatographed on a short silica gel column and eluted with a
mixture of ethyl acetate and petrol ether. The purified products
were identified by 1H NMR spectra.
Ligand 6: white solid (42%). IR (KBr); 1647 cmÀ1
1188 cmÀ1
(m
C@N),
(m
S@O). 1H NMR (300 MHz; D2O; Me4Si) d 9.90 (1H, s,
CH@N), 7.93 (2H, d, ArH), 7.79–7.68 (1H, m, ArH), 7.67–7.50 (4H,
m, ArH), 6.84 (2H, d, ArH). 13C NMR (75.5 MHz; DMSO-d6): d
163.9, 151.4, 145.9, 136.0, 131.5, 128.8, 128.7, 126.6, 120.2. Anal.
Found (calc. for C13H10NSO3Na): C, 56.52 (55.12); H, 3.70 (3.56);
N, 5.31 (4.94)%.
4.4. General procedure for preparation of cyclopalladated complexes
To a round-bottomed flask with a stir bar was placed Li2PdCl4
(130 mg, 0.5 mmol), Schiff base ligands (0.5 mmol) and sodium
acetate (41 mg, 0.5 mmol) under nitrogen. Anhydrous methanol
(3 mL) was added, and the resulting mixture was stirred at room
temperature for 2 days. The reaction mixture was filtered off. The
hydrophilic complexes were isolated as orange or yellow solids
which were recrystallized from methanol/ethyl ether solution.
4.6. General procedure for catalyst recycling trials
A 50 mL round-bottomed flask was charged with Pd complex
(0.1 mol% Pd), KOH (112 mg, 2 mmol), 4-bromoanisole (187 mg,
1 mmol) and phenylboronic acid (180 mg, 1.5 mmol). A certain
amount of TBAB or PVP (PVP k17 (C10H9NO)n = 8000) was added
to the samples. Deionized water (20 mL) was added, and the result-
ing mixture was stirred at 100 °C for a given period of time. After
cooling to room temperature, ethyl ether (15 mL) was added and
the mixture was stirred for 1 min. The upper layer was separated
and dried by Na2SO4. The reaction yields were determined by GC
analysis. After removing the volatiles under vacuum, the flask
was charged with 4-bromoanisole (1 mmol), phenylboronic acid
(1.5 mmol) and KOH (2 mmol). Each time, after cooling and extrac-
tion, the reagents and base were added and the reaction was
repeated.
Complex 1: orange solid (75%). IR (KBr) 1606 cmÀ1
(mC@N),
1188 cmÀ1 S@O). 1H NMR (CD3OD, 300 MHz) d 9.13 (2H, s, CH@N),
(m
8.83 (2H, s, ArH), 8.36 (2H, d, ArH), 8.17 (2H, d, ArH), 7.85 (6H, t,
ArH), 7.68 (2H, t, ArH), 7.59–7.50 (4H, m, ArH), 7.34 (2H, s, ArH),
3.72–3.57 (4H, m, ÀCHMe2), 1.54 (12H, d, ÀCH3), 1.16 (12H, d,
ÀCH3). 13C NMR (300 MHz; CD3OD) d 160.6, 148.2, 144.8, 140.6,
138.5, 131.22, 130.7, 129.6, 128.9, 126.4, 125.2, 121.8, 121.3,
28.5, 23.8, 22.2. Anal. Found (calc. for C54H50N2S2O6Na2Pd2Cl2): C,
52.56 (53.30); H, 4.34 (4.14); N, 2.18 (2.30)%.
Complex 2: orange solid (70%). IR (KBr) 1606 cmÀ1
1187 cmÀ1 S@O). 1H NMR (300 MHz; DMSO-d6; Me4Si) d 9.15
(mC@N),
(m

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This work was supported by NSFC No. 20372042, Shandong Sci-
entific Plan 032090105 and the Science Foundation of Shandong
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