Amino-salicylaldimine-palladium(II) complexes: New and efficient catalysts for Suzuki and Heck reactions

Article abstract of DOI:10.1016/j.inoche.2009.10.023

A series of amino-salicylaldimine-palladium(II) complexes bearing 5-methyl-3-(R-1-ylmethyl)-salicylaldimine ligands (R = morpholine, piperidine, pyrrolidine, 4-methylpiperazin, diisopropylamine) have been prepared and characterized by IR, ¹H NMR and elemental analysis. Crystal structure details of complex 2b have been confirmed by X-ray structure analysis. The obtained Pd(II) complexes were found to be effective catalysts for the Suzuki and Heck cross-coupling reactions which could be carried out in the undried solvent under air.

Full text of DOI:10.1016/j.inoche.2009.10.023

Inorganic Chemistry Communications 13 (2010) 81–85
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Amino-salicylaldimine–palladium(II) complexes: New and efficient catalysts for
Suzuki and Heck reactions
b
Jin Cui a, Mingjie Zhang a, , Yuwei Zhang
*
a Department of Chemistry, School of Science, Tianjin University, Tianjin 300072, PR China
b Department of Basic Sciences, Tianjin Institute of Urban Construction, Tianjin 300384, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of amino-salicylaldimine–palladium(II) complexes bearing 5-methyl-3-(R-1-ylmethyl)-salicyl-
aldimine ligands (R = morpholine, piperidine, pyrrolidine, 4-methylpiperazin, diisopropylamine) have
been prepared and characterized by IR, 1H NMR and elemental analysis. Crystal structure details of com-
plex 2b have been confirmed by X-ray structure analysis. The obtained Pd(II) complexes were found to be
effective catalysts for the Suzuki and Heck cross-coupling reactions which could be carried out in the
undried solvent under air.
Received 30 August 2009
Accepted 27 October 2009
Available online 30 October 2009
Keywords:
Suzuki–Miyaura
Heck–Mizoroki
Amino-salicylaldimine
Palladium
Ó 2009 Elsevier B.V. All rights reserved.
Catalysis
Palladium-catalyzed C–C bond forming reactions such as the Su-
zuki–Miyaura and Mizoroki–Heck reactions have been used exten-
sively for the synthesis of natural products, pharmaceutical
intermediates, conducting polymers, pesticides and liquid crystals
[1–4]. Phosphine is generally the ligand of choice due to its superior
donor capability and stabilization effects [5]. The major limitations
with phosphine ligands in catalytic reactions is the oxidation of
phosphines to phosphine oxides, formation of stable phosphido-
bridged catalytically inactive dimers, and also the cleavage of P–C
bond causing degradation of the ligand and thus the termination
of the catalytic cycle. These have prompted the vigorous research
on phosphine-mimics such as N-heterocyclic carbenes (NHCs) [6].
NHC complexes are largely stable, but some of them are also hand-
icapped by the lack of coordinative flexibility, which then led to the
emergence of carbine hybrids.
During the past 10 years, there has been considerable interest in
the development of new phosphorus free palladium catalysts for
higher activity, stability and substrate tolerance that allow reac-
tions to be carried out under milder reaction conditions. Schiff
bases are attractive ligands due to their facile preparation and sim-
ple synthetic modification, both electronic and steric, and have
been used to prepare a broad range of organometallic compounds
with a wide variety of applications [7]. Recently several types of
salicylaldimine ligands with pendant functionality were explored
to act as three-coordinate six-electron-donor. It also provides a
focus of our recent work in designing new, robust and easily
prepared ligands and studying their coordination behavior and cat-
alytic applications. In this paper, we report the preparation and
characterization of amino-salicylaldimine ligands 5-methyl-3-(R-
1-ylmethyl)-salicylaldimine (R = morpholine, piperidine, pyrroli-
dine, 4-methylpiperazine, diisopropylamine) and their palladium
complexes. The catalytic activities of these amino-salicylaldi-
mine–palladium complexes toward the Suzuki and Heck reactions
are investigated.
The synthetic procedures of ligands 5-methyl-3-(R-1-ylmethyl)-
salicylaldimine (R = morpholine, piperidine, pyrrolidine, 4-meth-
ylpiperazin, diisopropylamine) and their palladium(II) complexes
are generally shown in Scheme 1. Ligands 1a–e were conveniently
prepared in good yields (91–96%) by the condensation reaction of
one equivalent of the diisopropylaniline with one equivalent of
5-methyl-3-(R-1-ylmethyl)-salicylaldehyde in ethanol, yielding
yellow crystal or oil products after purified through column chro-
matography on Al2O3. All the ligands were well characterized and
confirmed by IR spectrometry, 1H NMR and elemental analysis.
The amino-salicylaldimine–Pd(II) complexes 2a–e were ob-
tained as yellow solids in reasonable yields (79–88%) by reflux of
amino-salicylaldimine ligands with PdCl2(CH3CN)2 in acetonitrile.
The resulting solutions were concentrated under vacuum, and
the formed complexes were precipitated by adding excessive
amounts of Et2O. Complexes 2a–e obtained have been confirmed
by IR spectra, 1H NMR and elemental analysis. In comparison with
the IR spectrometry of free ligands with C@N stretching frequen-
cies in the range of 1620–1622 cmꢀ1, the C@N stretching vibra-
tions in complexes were shifted toward lower frequencies in the
range of 1614–1618 cmꢀ1 with reduced peak intensity, which
* Corresponding author. Tel./fax: +86 022 27403475.
E-mail address: Mjzhangtju@163.com (M. Zhang).
1387-7003/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2009.10.023

82
J. Cui et al. / Inorganic Chemistry Communications 13 (2010) 81–85
Cl
Pd
Cl
N
OH
O
NH2
O
R
R
N
N
OH
R
PdCl2(CH3CN)2
CH3CN, reflux
EtOH, reflux
Me
Me
1a-e
Me
2a-e
b. R=
a. R=
N
e. R=
O
N
c. R=
N
d. R=
Me N
N
Scheme 1. Synthesis of amino-salicylaldimine ligands 1a–e and their Pd(II) complexes 2a–e.
Fig. 1. Crystal structure of complex 2b. Select bond lengths (Å) and angles (°): Pd1–Cl1 2.2939(8), Pd1–Cl2 2.3220(9), Pd1–N1 2.019(3), Pd1–O1 1.9687(19) and O1–Pd1–N1
91.03(11), O1–Pd1–Cl1 174.67(9), N1–Pd1–Cl1 94.28(8), O1–Pd1–Cl2 83.68(8), N2–Pd1–Cl2 174.66(8), Cl1–Pd1–Cl2 91.02(3).
Table 2
Effect of complexes 2a–e on Suzuki cross-coupling reaction.a
Table 1
Effect of solvents and bases on Suzuki cross-coupling reaction.a
Cat.2a-e
DMF, K2CO3
OHC
Cl
B(OH)2
CHO
Cat. 2a
Solvent, Base
OHC
Cl
B(OH)2
CHO
Entry
Catalyst
Time (h)
Yieldb (%)
Entry
Solvent
Base
Yieldb (%)
1
2
3
4
5
2a
2b
2c
2d
2e
4
4
4
4
4
96
95
94
96
92
1
2
3
4
5
6
7
8
Toluene
THF
Acetone
DCM
Methanol
DMF
DMF
DMF
DMF
DMF
DMF
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Cs2CO3
K3PO4ꢁ3H2O
Et3N
26
46
33
14
31
97
81
69
35
96c
56c
21c
a
Reaction conditions: catalyst (0.5 mol%), 4-chlorobenzaldehyde (0.5 mmol),
phenylboronic acid (0.7 mmol), K2CO3 (1.2 mmol), DMF (10 mL), 110 °C.
Conversion to the coupled product determined by GC–MS, based on 4-chloro-
benzaldehyde; average of two runs.
b
9
10
11
12
K2CO3
K2CO3
K2CO3
indicated an effective coordination interaction between the imino
nitrogen and the Pd center. In the 1H NMR spectra, the protons
of substituents on the aryl ring of these complexes were almost
shifted downfield compared with those of the free ligands, while
the aromatic ring protons exhibited either upfield shift or re-
mained unchanged in the range of 7.02–7.35 ppm.
DMF
a
Reaction conditions: catalyst (0.5 mol%), 4-chlorobenzaldehyde (0.5 mmol),
phenylboronic acid (0.7 mmol), base (1.2 mmol), and solvent (10 mL), reflux, 4 h.
Conversion to the coupled product determined by GC–MS, based on 4-chloro-
benzaldehyde; average of two runs.
b
c
Reaction temperature: 110 °C (entry 10), 80 °C (entry 11), 50 °C (entry 12).

J. Cui et al. / Inorganic Chemistry Communications 13 (2010) 81–85
83
Perspective view of molecular structure of complex 2b with
atom numbering scheme is shown in Fig. 1. The yellow crystal of
2b, suitable for X-ray diffraction analysis was obtained through
slow diffusion of diethyl ether into their acetonitrile solution at
room temperature. In the molecular structure of 2b, the imino
group in the solid state is in the E conformation with the typical
imino C@N double-bond length of 1.293(4) Å. The dihedral angle
between the two aryl rings is 82.23° and the piperidine ring pos-
sesses a stable chair configuration. The geometry around the palla-
dium center can be described as a distorted square planar, in which
the palladium was coordinated with the sp2 N1 in the phenylimino
ring instead of sp3 N2 in the piperidine ring. The Pd1–O1 (1.969 Å)
bond length is slightly shorter than that found in the related
bis(phenoxyketimine) palladium complex (1.985 Å) [8] and schiff
bases of 1-hydroxy-2-acetonaphthone palladium complexes
(1.973–2.061 Å) [9]. The Pd1–N1 (2.019 Å) bond length is compara-
ble to that in schiff bases of 1-hydroxy-2-acetonaphthone palla-
dium complexes (2.010–2.019 Å), and is shorter than that in
product distribution of the coupling reaction, so the influence of
several solvents was examined (entries 1–6). DMF was found to
be much better than others (entry 6). This may be due to better sol-
ubility of the reagents and easier reduction of Pd2+ to Pd(0) and
hence facile entry to the catalytic cycle. Similarly, several bases
were employed in Suzuki reactions (entries 6–9), K2CO3 proving
superior (entry 6). Further studies indicated that the catalytic
activity could be reduced with lowering the reaction temperature
(entries 10–12).
The catalytic activities of complexes 2a–e were examined in the
Suzuki reaction of 4-chlorobenzaldehyde with phenylboronic acid
(Table 2). They generally gave good yields in DMF at 110 °C within
4 h under a low catalyst loading of 0.5 mol%. Complexes 2a and 2d
showed relatively better activities towards the coupling reaction
(entries 1 and 4).
Under the above optimized reaction condition, complex 2d was
applied to a representative range of aryl chlorides (Table 3). In the
presence of K2CO3, it was effective towards the coupling of various
aryl chlorides with phenylboronic acid in DMF at 110 °C within 4 h.
The activated electron-deficient aryl chloride, such as 4-chloroace-
tophenone, 4-chlorobenzaldehyde, and 4-chloronitrobenzene effi-
ciently coupled with phenylboronic acids to give high yields
(entries 1–3). The non-activated electron neutral substrate, such
as chlorobenzene afforded moderate amount of coupled product
at the same condition (49%, entry 4). In the case of deactivated
electron-rich substrates such as 4-chlorotoluene and 4-chlorophe-
nol (entries 5 and 6), the yields of the desired product were very
low even when prolonging the reaction time to 12 h. All these re-
sults indicated that electronic effect of substituents on the aryl
bis(phenoxyketimine)
palladium
complex
(2.033 Å)
and
bis(imino)pyridine palladium complexes (2.04–2.06 Å) [10]. The
angles of N1–Pd1–O1, N1–Pd1–Cl1, O1–Pd1–Cl2 and Cl1–Pd1–
Cl2 are 91.03(11), 94.28(8), 83.68(8) and 91.02(3), respectively.
The angles of O1–Pd1–Cl1 [174.67(9)°] and N1–Pd1–Cl2
[174.66(8)°] are both slightly less than 180 °C.
The effectiveness of amino-salicylaldimine–Pd complexes 2a–e
was first tested in the Suzuki reaction. Coupling of 4-chlorobenzal-
dehyde with phenylboronic acid in the presence of 0.5 mol% of 2a
for 4 h was chosen as a model to optimize the reaction conditions
(Table 1). Solvent usually plays a crucial role in the rate and the
Table 3
Suzuki cross-coupling of aryl chloride with phenylboronic acid.a
Cat.2d
DMF, K2CO3
R
Cl
B(OH)2
R
Entry
1
Aryl halide
product
Time (h)
4
Yieldb (%)
96
OHC
O
Cl
CHO
O
2
4
95
Cl
Cl
3
4
5
6
7
4
98
49
11
6
O2N
NO2
4
Cl
12
12
4
H3C
HO
Cl
Cl
CH3
OH
87
Cl
NO2
NO2
8
4
13
4
Cl
Cl
Cl
9
12
4
Cl
CH3
CH3
10
45
Cl
a
Reaction conditions: complex 2d (0.5 mol%), aryl chloride (0.5 mmol), phenylboronic acid (0.7 mmol), K2CO3 (1.2 mmol), DMF (10 mL), 110 °C.
Conversion to the coupled product determined by GC–MS, based on aryl chloride; average of two runs.
b

84
J. Cui et al. / Inorganic Chemistry Communications 13 (2010) 81–85
chlorides had great influence on the reaction and the electron-
withdrawing substituents were more favorable for the coupling.
The sterically hindered ortho-substituted substrate 2-chloronitro-
benzene could also couple efficiently giving 87% yield within 4 h
(entry 7), but o-dichlorobenzene and 2-chlorotoluene gave very
between activated aryl chlorides and phenylboronic acid in DMF/
H2O (1:1) at 110 °C giving moderate yields [11]. In comparison
with other N-coordinated palladium catalysts, the catalytic activity
of this catalyst is similar to those found in 1,4-bis(2-hydroxy-3,5-
di-tert-butylbenzyl)piperazine Pd system in DMF with K2CO3 at
110 °C [12], in benzimidazolium–pyrazole–palladium(II) com-
plexes in MeOH/H2O(2:1) at room temperature [13], in diazabut-
adiene/palladium system in dioxane with Cs2O3 at 100 °C [14].
The catalytic activity of catalyst 2d was also examined in the
Heck reaction of aryl bromides with styrene or n-butyl acrylate
low yield. The Suzuki coupling was also extended to Cðsp3Þ—Cðsp2
Þ
coupling by reacting benzyl chloride with phenylboronic acid,
which afforded the corresponding coupling product in moderate
yield (45%, entry 10). The activity of this catalyst is found to be
superior to that in Pd–salen system, which catalyzes the reaction
Table 4
Heck cross-coupling reactions of aryl bromide with styrene or n-butyl acrylatea
R2
Cat.2d
Br
R2
DMF, K2CO3
R1
R1
Entry
1
Aryl bromide
Product
Time (h)
5
Yieldb (%)
94
O
O
Br
O
H
H
O
OnBu
2
3
5
5
89
92
O
H
O
Br
Br
H
O
O
OnBu
4
5
5
5
88
O
O
Br
100
O
O2N
Br
O2N
O2N
OnBu
6
7
5
96
44
O2N
Br
12
Br
O
OnBu
8
9
12
5
10
46
Br
O
N
Br
N
N
OnBu
10
11
5
6
N
Br
Br
12
42
O
OnBu
12
13
12
12
5
Br
55
O
OnBu
Br
Cl
14
24
Tracec
O
H
O
H
O
OnBu
a
b
c
Reaction conditions: complex 2d (1 mol%), aryl bromide (0.5 mmol), olefin (1.5 mmol), K2CO3 (1.5 mmol), DMF (10 mL), 110 °C.
Conversion to the coupled product determined by GC–MS, based on aryl bromide; average of two runs.
Catalyst (complex 2d) loading: 2 mol%.

J. Cui et al. / Inorganic Chemistry Communications 13 (2010) 81–85
85
under the optimized reaction conditions (K2CO3 as base, DMF as
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Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.inoche.2009.10.023.