Synthesis and characterization amino-salicylaldimine palladium-based complexes and their Suzuki coupling reaction study

Article abstract of DOI:10.1016/j.inoche.2009.06.031

A series of amino-salicylaldimine ligands, 5-R-3-morpholinomethyl-salicylaldimine (R = t-Bu, CH₃, Br) have been synthesized and structurally analyzed. These tridentate ligands add to palladium (II) to give high yields of air-stable complexes th

Full text of DOI:10.1016/j.inoche.2009.06.031

Inorganic Chemistry Communications 12 (2009) 839–841
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis and characterization amino-salicylaldimine palladium-based
complexes and their Suzuki coupling reaction study
*
Jin Cui, Mingjie Zhang , Yan Wang, Zhongbo Li
Department of Chemistry, School of Science, Tianjin University, Tianjin 300072, 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 ligands, 5-R-3-morpholinomethyl-salicylaldimine (R = t-Bu, CH3, Br)
have been synthesized and structurally analyzed. These tridentate ligands add to palladium (II) to give
high yields of air-stable complexes that are characterized by IR, 1H NMR and elemental analysis. Crystal
structure details of complex 1 are presented. These Pd (II) complexes with amino-salicylaldimine ligands
are versatile and efficient Suzuki catalysts towards cross-coupling of a variety of aryl chlorides with
phenylboronic acid using DMF as solvent.
Received 14 May 2009
Accepted 30 June 2009
Available online 3 July 2009
Keywords:
Suzuki coupling reactions
Amino-salicylaldimine
Palladium catalyst
Crystal structure
Ó 2009 Elsevier B.V. All rights reserved.
Pd-catalyzed C–C bond-formation reactions such as the Suzuki,
Heck and Sonogashira cross couplings are playing important roles
in modern organic synthesis [1]. These cross-coupling reactions are
usually performed with 1–5 mol% of Pd catalyst along with phos-
phine ligands, which show superior donor capability and stabiliza-
tion effects. However, many phosphines are air and moisture-
sensitive, toxic and therefore difficult to handle. They are also
prone to dissociation, oxidation and hydrolysis [2]. Accordingly,
an interesting current challenge is to develop Pd catalysts that
can utilize stable and easily prepared phosphine free ligands. Het-
erocyclic ligands containing nitrogen atoms are drawing a great
deal of attention in coordination chemistry and homogeneous
catalysis [3] because of the versatility of their steric and electronic
properties which can be modified by choosing appropriate ring
substituents [4]. In this regard a number of ligands including N-
heterocyclic carbenes (NHC) [5], oxazolines [6], amines [7], pyri-
dines [8], pyrazoles [9], tetrazoles [10], quinolines [11] and imida-
zoles [12] compounds have been examined for Pd catalysis
recently.
As a part of our interest in designing new, easily prepared li-
gands and studying their coordination behavior and catalytic appli-
cations, we report herein the synthesis and characterization of a
series of palladium complexes bearing 3-morpholinomethyl-sali-
cylaldimine ligands which contain different side groups and to at-
tempt to correlate the disparity of catalytic activity with these
groups. We used our catalysts in the catalytic Suzuki cross-cou-
pling of various aryl chlorides with aryl boronic acids.
The synthetic procedures of ligands 5-R-3-morpholinomethyl-
salicylaldimine (R = t-Bu, CH3, Br) and their palladium (II) com-
plexes are generally shown in Scheme 1. Ligands were prepared
in good yields (93–96%) by the condensation reaction of one equiv-
alent of the diisopropylaniline with one equivalent of 5-R-3-mor-
pholinomethyl-salicylaldehyde (R = t-Bu, CH3, Br), yielding yellow
crystal or oil products after purified through column chromatogra-
phy on Al2O3. All the ligands were well characterized and confirmed
by IR spectrometry, 1H NMR and elemental analysis [13]. The crys-
tal-state structure of the ligand 1 has been established by X-ray sin-
gle-crystal crystallography [14]. Complexes 1–3 were synthesized
in reasonable yields (89–93%) 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 diethyl ether. Complexes 1–3 are air-stable
solids, and soluble in most organic solvents, such as CH2Cl2, DMF,
CHCl3 and acetonitrile. The complexes obtained were fully charac-
terized by IR spectra, 1H NMR and elemental analysis [15]. In com-
parison with the IR spectrometry of free ligands with C@N
stretching frequencies in the range of 1619–1624 cmꢀ1, the C@N
stretching vibrations in complexes were shifted toward lower
frequencies in the range of 1613–1620 cmꢀ1 with reduced peak
intensity, which indicated an effective coordination interaction
between the imino nitrogen and the Pd center. In the 1H NMR spec-
tra, all protons of substitutes on the aryl ring of these complexes
were shifted downfield compared with those of the free ligands,
while the chemical shifts of the aromatic ring protons decreased
to a 7.02–7.33 ppm range.
The structure of 5-tert-butyl-3-morpholinomethyl-salicylaldi-
minepalladium (II) chloride was confirmed by the result of a sin-
gle-crystal X-ray structure determination (Fig. 1). Selected bond
* Corresponding author. Tel./fax: +86 02227403475.
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.2009.06.031

840
J. Cui et al. / Inorganic Chemistry Communications 12 (2009) 839–841
The dihedral angle between the two aryl rings is 65.82° and the
Cl
Cl
O
morpholine ring possesses a stable chair conformation. The geom-
etry around the palladium center can be described as a distorted
square planar, in which the palladium was coordinated with the
sp2 N2 in the phenylimino ring instead of sp3 N1 in the morpholine
ring. The distance between the nitrogen and Pd atoms, Pd1ꢁ ꢁ ꢁN2, is
2.017(3) Å, which is slightly longer than the Pd1ꢁ ꢁ ꢁO1 bond dis-
tance (1.996(2) Å). The angles of O1–Pd1–N2, N2–Pd1–Cl1, O1–
Pd1–Cl2 and Cl1–Pd1–Cl2 are 91.08(11) Å, 91.94(9) Å, 84.69(7) Å
and 92.28(4) Å respectively.
Pd
O
O
O
N
N
OH
R
N
N
OH
R
O
N
ii)
i)
L1: R=t-Bu
1: R=t-Bu
2: R=Me
3: R=Br
R
L2: R=Me
L3: R=Br
Scheme 1. Synthesis of complexes (isolated yield). Reagents and conditions: (i) 2,6-
diisopropylaniline, EtOH, reflux; (ii) PdCl2(CH3CN)2, CH3CN, reflux.
To optimize the reaction conditions, a model reaction was car-
ried out by taking 4-chlorobenzaldehyde and phenylboronic acid
in different solvents (Entries 1–4). It was found that the reactions
performed in DMF with K2CO3 at 110 °C showed the best perfor-
mance (Entry 4). Then the scope of the cross-coupling reaction
with respect to the aryl chloride component was investigated. In
the coupling reaction, rapid precipitation of Pd-black was ob-
served. All the three complexes gave the good performance with
quantitative conversion under low catalytic loading (0.5 mol%)
within 4 h. Table 1 summarized the results obtained in the
presence of palladium (II) complexes (Entries 4–21). The electron
deficient substrates such as 4-chloronitrobenzene, 4-chlorobenzal-
dehyde and 4-chloroacetophenone afforded excellent coupling
products, while the electron rich substrate 4-chlorotoluene affor-
ded low amount of coupled products at the same condition. In
the case of non-activated electron neutral substrate chloroben-
zene, the yields of the desired product were moderate. All these
results indicated that electronic effect of substituents on the aryl
chlorides had great influence on the reaction and the electron-
withdrawing substituents were more favorable for the coupling,
which was analogous to those reported for other palladium cata-
lysts [16]. The catalysts were also tolerant of sterically hindered
ortho-substituted substrate 2-chloronitrobenzene giving a good
yields of the desired coupling products in 4 h.
Fig. 1. Crystal structure of complex 1. Select bond lengths (Å) and angles (°): Pd1–
Cl1 2.2930(10), Pd1–Cl2 2.3115(12), Pd1–N2 2.017(3), Pd1–O1 1.996(2) and O1–
Pd1–N2 91.08(11), O1–Pd1–Cl1 176.95(7), N2–Pd1–Cl1 91.94(9), O1–Pd1–Cl2
84.69(7), N2–Pd1–Cl2 175.64(8), Cl1–Pd1–Cl2 92.28(4).
lengths and angles are listed in the caption of Fig. 1. The yellow
crystal of complex 1, suitable for X-ray diffraction analysis was
grown by the slow evaporation of dichloromethane and ethanol
mixture at room temperature. In the molecular structure of com-
plex 1, the imino group in the solid state is in the E conformation
with the typical imino C@N double-bond length of 1.296(5) Å.
In comparison with other salicylaldimine palladium catalysts,
the catalytic activities of these catalysts are superior to those found
in salicylaldimine mono- or dinuclear pd (II) complexes, whose
reactions were carried out in DMF with K2CO3 at 100 °C [17a],
Pd–salen complexes at 110 °C in DMF/H2O [17b], three pendant
benzamidine palladium complexes in toluene with K3PO4 at 80 °C
Table 1
0:5mol%½Cat:ꢂ
The Suzuki coupling reaction of aryl chlorides with phenylboronic acid.a ArCl þ PhBðOHÞ2
Ar—Ph
ƒƒƒƒƒƒ!
K2CO3 ;Solvent
Entry
ArCl
Catalyst
Solvent
Product
Yieldb(%)
1
2
3
4
5
6
7
8
4-Chlorobenzaldehyde
4-Chlorobenzaldehyde
4-Chlorobenzaldehyde
4-Chlorobenzaldehyde
4-Chloroacetophenone
4-Nitrochlorobenzene
2-Nitrochlorobenzene
Chlorobenzene
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
3
3
3
3
3
3
Acetone
THF
4-Fomylbiphenyl
4-Fomylbiphenyl
4-Fomylbiphenyl
4-Fomylbiphenyl
4-Acetylbiphenyl
4-Nitrobiphenyl
2-Nitrobiphenyl
Biphenyl
4-Methylbiphenyl
4-Fomylbiphenyl
4-Acetylbiphenyl
4-Nitrobiphenyl
2-Nitrobiphenyl
Biphenyl
4-Methylbiphenyl
4-Fomylbiphenyl
4-Acetylbiphenyl
4-Nitrobiphenyl
2-Nitrobiphenyl
Biphenyl
33c
46c
14c
97
98
99
85
46
25
99
98
99
81
52
23
99
97
99
84
49
17
CH2Cl2
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
9
4-Chlorotoluene
10
11
12
13
14
15
16
17
18
19
20
21
4-Chlorobenzaldehyde
4-Chloroacetophenone
4-Nitrochlorobenzene
2-Nitrochlorobenzene
Chlorobenzene
4-Chlorotoluene
4-Chlorobenzaldehyde
4-Chloroacetophenone
4-Nitrochlorobenzene
2-Nitrochlorobenzene
Chlorobenzene
4-Chlorotoluene
4-Methylbiphenyl
a
Reaction conditions: aryl chloride (0.5 mmol), phenylboronic acid (0.7 mmo m), K2CO3 (1.0 mmol), catalyst (0.5 mol%), DMF, 110 °C, 4 h, under air atmosphere.
Conversion to the coupled product determined by GC–MS, based on aryl chloride, average of two runs.
Reflux.
b
c

J. Cui et al. / Inorganic Chemistry Communications 12 (2009) 839–841
841
(d, J = 2.5 Hz, 1H, H–Ar–CH@N), 7.320–7.325 (d, J = 2.5 Hz, 1H, H–Ar–CH@N),
7.204 (s, 3H, C@NAr–H), 3.792–3.810 (t, J = 4.5 Hz, 4H, NCH2CH2O), 3.694 (s,
2H, Ar–CH2N), 2.933–2.995 (m, 2H, Ar(CH(CH3)2)2), 2.619 (s, 4H, NCH2CH2O),
1.384 (s, 9H, ArC(CH3)3), 1.190–1.203 (d, J = 6.5 Hz, 12H, Ar(CH(CH3)2))2. Calcd
for C28H40N2O2: C 77.02%, H 9.23%, N 6.42%; Found: C 77.05%, H 9.30%, N 6.44%.
L2: Yellow oil, yield 93%. IR (KBr, cmꢀ1): 1622 (C@N). 1H NMR (500 MHz,
CDC13): d = 8.232 (s, 1H, ArCH@N), 7.549 (s, 1H, H–ArCH@N), 7.335 (s, 1H, H–
ArCH@N), 7.208 (s, 3H, C@NAr–H), 3.807(t, 4H, NCH2CH2O), 3.686 (s, 2H,
ArCH2N), 2.940–2.991 (m, 2H, Ar(CH(CH3)2)2), 2.615 (s, 4H, NCH2CH2O), 2.235
(s, 3H, H3C–Ar), 1.197–1.213 (d, 12H, Ar(CH(CH3)2)2). Calcd for C25H34N2O2: C
76.10%, H 8.69%, N 7.10%; Found: C 76.13%, H 8.65%, N 7.13%. L3: Yellow oil,
yield 96%. IR (KBr, cmꢀ1): 1624 (C@N). 1H NMR (500 MHz, CDC13): d = 8.204
(s, 1H, ArCH@N), 7.554 (s, 1H, H–ArCH@N), 7.384 (s, 1H, H–ArCH@N), 7.201 (s,
3H, C@NAr–H), 3.801 (t, J = 4.5 Hz, 4H, NCH2CH2O), 3.688 (s, 2H, ArCH2N),
2.944–2.990 (m, 2H, Ar(CH(CH3)2)2), 2.611 (s, 4H, NCH2CH2O), 1.197–1.213 (d,
12H, Ar(CH(CH3)2)2). Calcd for C24H31BrN2O2: C 62.74%, H 6.80%, N 6.10%;
Found: C 62.77%, H 6.77%, N 6.15%. The synthetic details of ligands and the
complexes are available in the supplementary materials.
[17c]. And also the activities of these catalysts are found to be sim-
ilar to that in PdCl2/bis(2-hydroxy-3,5-di-tert-butylbenzyl)pipera-
zine system, which catalysed the reaction between aryl chlorides
and phenylboronic acid in DMF with K2CO3 at 110 °C [18].
In conclusion, we have designed and synthesized a series of no-
vel air-stable amino-salicylaldimine–palladium complexes. The
molecular and crystal structure of the complex 1 was elucidated.
The complexes are found to exhibit good catalytic activity in the
Suzuki coupling reaction of activated aryl chlorides with phenylbo-
ronic acid under the atmosphere, using DMF as solvent.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.inoche.2009.06.031.
[14] Z.C. Zhu, J. Cui, M.J. Zhang, Acta Cryst. E64 (2008) o743.
[15] Characterization for complexes 1–3: 1: Yellow solid, Yield 93%. IR (KBr, cmꢀ1):
1613 (C@N). 1H NMR (500 MHz, CDC13): d = 9.867 (s, 1H, ArCH@N), 7.092–
7.335 (m, 5H, H–ArCH@N–Ar–H), 4.345–4.423 (m, 4H, NCH2CH2O), 4.057–
4.117 (m, 2H, ArCH2N), 3.625–3.651 (m, 2H, Ar(CH(CH3)2)2), 3.461–3.503 (m,
4H, NCH2CH2O), 1.438–1.493 (m, 9H, ArC(CH3)3), 1.152–1.282 (m, 12H,
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