Synthesis, crystal structure, and in vitro antitumor activities of copper(II) complexes containing tetradentate pyridine-based ligands

Article abstract of DOI:10.1007/s11243-011-9483-1

Several new Cu(II) complexes of Schiff bases obtained by condensation of 2-[N-(α-picolyl)-amino]-benzophenone with different chiral amino acids were synthesized and characterized by physico-chemical and spectroscopic methods. The crystal structure of one

Full text of DOI:10.1007/s11243-011-9483-1

Transition Met Chem (2011) 36:403–407
DOI 10.1007/s11243-011-9483-1
Synthesis, crystal structure, and in vitro antitumor activities
of copper(II) complexes containing tetradentate pyridine-based
ligands
•
Xin-Tao Yang Hao Wu Shi-Jun Ma
•
•
•
Juan-Juan Hu Yong-mei Wang
Received: 13 January 2011 / Accepted: 10 March 2011 / Published online: 27 March 2011
Ó Springer Science+Business Media B.V. 2011
Abstract Several new Cu(II) complexes of Schiff bases
obtained by condensation of 2-[N-(a-picolyl)-amino]-benzo-
phenone with different chiral amino acids were synthesized
and characterized by physico-chemical and spectroscopic
methods. The crystal structure of one of the complexes was
determined using single crystal X-ray diffraction. The
ligands were coordinated to the metal atom in a tetraden-
tate manner with ONNN donor sets using the carboxyl
oxygen, azomethine nitrogen, CON-, and pyridine nitro-
gen. The cytotoxicities of the complexes were evaluated
against human cancer cells. The substituents on the aro-
matic rings strongly influenced the cytotoxicities of the
complexes. The complex with bromine substituents on the
pyridine rings showed the highest cytotoxicity. The anti-
tumor activities against tumor cell lines were assayed in
vitro, and the complexes were found to be highly effective,
with six of the nine complexes having inhibition ratios
better than that of 5-Fluorouracil. This behavior is indica-
tive of a high ability to circumvent the cellular drug
resistance mechanisms.
large number of Schiff base complexes have been used as
ligands in metal coordination chemistry. Some Schiff base
metal complexes have been found to possess biological
activity [3, 4]. Our interest in the study of Schiff bases and
their complexes containing sulfur and nitrogen donor atoms
arises from their significant anticancer activities [5–8].
Among them, amino acid (such as glycine, a-valine, b-
alanine) Schiff bases and their first-row transition metal
complexes were reported to exhibit antitumor activities [9–
16]. The clinical success of cisplatin in the treatment of
several human malignant tumors was motivated major
research efforts toward the discovery of alternative metal
complexes with potential as anticancer drugs [17, 18]. So
far, some of the most promising results have been reported
for copper, which exhibits antitumor activity even in the
form of simple copper(II) salts [19]. However, the use of
copper(II) salts as anticancer drugs presents some draw-
backs, which are mainly related to their toxicity and rela-
tively poor bioavailability. To overcome these limitations,
copper(II) complexes of multidentate chelate ligands have
been evaluated in recent years as alternatives to simple
copper salts. In this paper, we report the synthesis of several
new copper(II) complexes of various amino acid-derived
Schiff base ligands. It is well known that 5-Fluorouracil (5-
FU) remains the most commonly used anticancer drug for
the treatment of solid tumors, although objective response
rates are as low as 20%. Biological activity data showed that
the complexes obtained in this study were more cytotoxic
than 5-FU against tumor cell lines.
Introduction
Schiff bases are usually formed by the condensation of a
primary amine with an active carbonyl compound [1, 2]. A
X.-T. Yang ꢀ H. Wu ꢀ S.-J. Ma ꢀ Y. Wang (&)
Department of Chemistry, The State Key Laboratory
of Elemento-Organic Chemistry, Nankai University,
300071 Tianjin, China
e-mail: ymw@nankai.edu.cn
Experimental
J.-J. Hu
Institute of Materia Medica, Chinese Academy of Medical
Sciences, 100050 Beijing, China
All the reagents and solvents employed were of the best
grade available and used without further purification. The
123

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Transition Met Chem (2011) 36:403–407
Synthesis of 2-[N-(a-picolyl)amino]-20,5-
dichlorobenzophenone (3)
elemental analyses were carried out with a YANACO CHN
CORDER MT-3 analyzer. Melting points were determined
using an electrothermal apparatus and are uncorrected.
Mass spectra were obtained using a ThermoFinnigan
TRACE-DSQ electrospray ionization (ESI) mass spec-
trometer. IR spectra were recorded as KBr disks on a
Using a procedure similar to that described elsewhere,
reaction of picolinic acid with 2-amino-20,5-dichloroben-
zophenone in CH2Cl2 afforded ligand 3. Yield: 84%, m.p
223–225 °C. C19H12Cl2N2O2: Found: C, 61.6; H, 3.3; N, 7.6.
Calcd: C, 61.5; H, 3.4; N, 7.7. 1H-NMR (CDCl3): d = 13.34
(s, 1H), 9.06 (d, J = 9.1 Hz, 1H), 8.79 (d, J = 4.6 Hz, 1H),
8.31 (d, J = 7.9 Hz, 1H), 7.92 (td, J = 7.7, 1.5 Hz, 1H),
7.61 (dd, J = 9.1, 2.5 Hz, 1H), 7.50 (m, 3H), 7.41 (dd,
J = 3.9, 1.9 Hz, 2H), 7.37 (d, J = 2.5 Hz, 1H) ppm. 13C-
NMR (CDCl3): d = 197.5, 163.9, 149.9, 148.8, 139.5,
138.3, 137.5, 135.2, 133.4, 131.7, 131.2, 130.3, 129.1, 127.7,
126.9, 126.7, 124.2, 122.8, 122.6 ppm.
1
Nicolet 560 ESP FTIR spectrometer, while H-NMR and
13C-NMR spectra were recorded on a Bruker AC-P300
instrument in CDCl3. The following abbreviations were
used to designate chemical shift mutiplicities: s = singlet, d
= doublet, t =triplet, m = multiplet, br = broad. Tumor cells
were cultured at 37 °C under a humidified atmosphere of
5% CO2 in RPMI 1640 medium supplemented with 10%
fetal serum and dispersed in replicate 96-well plates with
1 9 104 cells per well. Complexes were then added. After
48 h exposure to the toxins, cell viability was determined
by the MTT colorimetric assay by measuring the absor-
bance at 570 nm with an ELISA reader. Each test was
performed in triplicate.
5-bromo-pyridine-2-carboxylic acid (2-benzoyl-
phenyl)-amide (4)
Using a procedure similar to that described elsewhere, reac-
tion of 5-bromo-2-pyridine carboxylic acid with o-amino-
benzophenone in dichloromethane afforded ligand 4. Yield:
80%, m.p 148–150 °C. C19H13BrN2O2: Found: C, 59.6; H,
3.5; N, 7.3. Calcd: C, 59.9; H, 3.4; N, 7.4. 1H-NMR (CDCl3):
d = 12.65 (s, 1H), 8.86 (d, J = 8.4 Hz, 1H), 8.82 (d,
J = 1.8 Hz, 1H), 8.18(d, J = 8.3 Hz, 1H), 8.02(dd, J = 8.4,
2.2 Hz, 1H), 7.77 (m, 2H), 7.63 (m, 3H), 7.49 (t, J = 7.6 Hz,
2H), 7.16 (m, 1H) ppm. 13C-NMR (CDCl3): d = 198.9,
162.8, 149.9, 148.7, 140.1, 139.6, 138.7, 133.9, 133.5, 132.5,
130.1, 128.3, 124.8, 124.3, 124.0, 122.7, 121.7 ppm.
Synthesis of 2-[N-(a-picolyl)amino]-benzophenone (1)
Thionyl chloride (3.5 mL, 50 mmol) was added dropwise
to a stirred solution of picolinic acid (6.2 g, 50 mmol)
and Et3N (10.4 mL, 75 mmol) in CH2Cl2 (35 mL) at 0 °C.
The mixture was stirred for 20 min, and then a solution
of o-aminobenzophenone (9.9 g, 50 mmol) in CH2Cl2
(20 mL) was added at 0 °C. Stirring was continued for
another 2 h at room temperature, and the reaction mixture
was left standing overnight. A saturated aqueous solution
of Na2CO3 (20 mL) was then added slowly over a period
of 30 min while stirring. The organic layer was separated,
washed with water, and dried under vacuum. The crude
product was recrystallized from an acetone–benzene
mixture to afford 1. Yield: 94%, m.p 156–157 °C.
C19H14N2O2: Found: C, 75.6; H, 4.6; N, 9.3. Calcd: C,
Preparation of Cu(II) complex (5) of glycine Schiff
base with ligand 1
A solution of NaOH (1.6 g, 40 mmol) in methanol (40 mL)
was added to a stirred solution of ligand 1 (6.0 g, 20 mmol),
glycine(7.5 g, 100 mmol), andCuCl2ꢀ2H2O (6.8 g, 40 mmol)
in methanol (100 mL). The mixture was refluxed for 4 h,
cooled to room temperature, and a solution of ice water
(100 mL) and glacial acetic acid (10 mL) was added, then the
reaction mixture was stirred for an additional 10 min. The
solid was filtered off, washed with hexanes, and dried under
vacuum to afford the desired product 5: Yield: 85%,
m.p [270 °C. C21H15CuN3O3:Found:C, 59.9;H, 3.6;N, 9.9.
Calcd: C, 59.8; H, 3.5; N, 9.8. ESI/MS (m/z): 421.3 [M ? 1].
1
75.5; H, 4.7; N, 9.3. H-NMR (CDCl3): d = 12.72 (br, s,
1H), 8.90 (d, J = 8.3 Hz, 1H), 8.76 (ddd, J = 4.7, 1.7,
0.8 Hz, 1H), 8.29 (br, d, J = 7.8 Hz, 1H), 7.90 (td,
J = 7.7, 1.6 Hz, 1H), 7.76–7.78 (m, 2H), 7.56–7.66 (m,
3H), 7.45–7.49 (m, 3H), 7.15 (m, 1H) ppm.
Synthesis of 2-[N-(a-picolyl)amino]-acetophenone (2)
Using a procedure similar to that described elsewhere,
reaction of picolinic acid with o-amino-acetophenone in
dichloromethane afforded ligand 2. Yield: 89%, m.p
110–112 °C. C14H12N2O2: Found: C, 69.6; H, 5.0; N, 11.6.
Calcd: C, 69.9; H, 5.1; N, 11.7. 1H-NMR (CDCl3):
d = 13.55 (s, 1H), 9.03 (d, J = 8.2 Hz, 1H), 8.81 (br, s,
1H), 8.29 (d, J = 7.4 Hz, 1H), 7.97 (d, J = 7.4 Hz, 1H),
7.90 (t, J = 7.3 Hz, 1H), 7.63 (t, J = 7.3 Hz, 1H), 7.49
(br, s, 1H), 7.19 (t, J = 7.2 Hz, 1H), 2.73 (s, 3H) ppm.
Preparation of Cu(II) complex (6) of alanine Schiff base
with ligand 1
Using a procedure similar to that described elsewhere,
complex 6 was obtained. Yield: 86%, m.p [ 270 °C.
C22H17CuN3O3: Found: C, 60.8; H, 3.9; N, 9.7. Calcd: C,
60.9; H, 3.8; N, 9.8. ESI/MS (m/z): 435.4 [M ? 1].
123

Transition Met Chem (2011) 36:403–407
405
Preparation of Cu(II) complex (7) of valine Schiff base
with ligand 1
C24H20BrCuN3O3: Found: C, 53.2; H, 3.7; N, 7.8. Calcd:
C, 53.3; H, 3.8; N, 7.9. ESI/MS (m/z): 541.1 [M ? 1].
Using a procedure similar to that described elsewhere,
complex 7 was obtained. Yield: 76%, m.p [ 270 °C.
C24H21CuN3O3: Found: C, 62.3; H, 4.6; N, 9.1. Calcd: C,
62.4; H, 4.7; N, 9.2. ESI/MS (m/z): 463.4 [M ? 1].
X-ray crystal structure determination
Single crystals suitable for X-ray diffraction were obtained
from the slow evaporation of acetonitrile solutions at room
temperature. For complex 7, a single crystal of suitable size
was mounted on a Bruker SMART CCD area-detector
diffractometer with graphite monochromated Mo–Ka
Preparation of Cu(II) complex (8) of leucine Schiff base
with ligand 1
˚
Using a procedure similar to that described elsewhere,
complex 8 was obtained. Yield: 78%, m.p [ 270 °C.
C25H23CuN3O3: Found: C, 62.9; H, 4.9; N, 8.8. Calcd: C,
62.7; H, 5.0; N, 8.7. ESI/MS (m/z): 477.4 [M ? 1].
radiation (k = 0.71073 A) at 113(2) K. Corrections were
made for Lorentz and Polarization factors as well as for
absorption (numerical). Structures were solved with direct
methods using SHELXL-97 and refined by full matrix
least-squares methods on F2 with SHELXL-97. Non-
hydrogen atoms were refined with anisotropic thermal
parameters. All hydrogen atoms were geometrically fixed
and allowed to refine using a riding model. Atomic coor-
dinates, bond angles, bond lengths, and thermal parameters
Preparation of Cu(II) complex (9) of phenylalanine
Schiff base with ligand 1
Using a procedure similar to that described elsewhere,
complex 9 was obtained. Yield: 75%, m.p [ 270 °C.
C28H21CuN3O3: Found: C, 65.8; H, 4.2; N, 8.2. Calcd: C,
65.7; H, 4.3; N, 8.3. ESI/MS (m/z): 511.6 [M ? 1].
Table 1 Crystal data and structure refinement for complex 7
Complex
7
Preparation of Cu(II) complex (10) of glutamic acid
Schiff base with ligand 1
Empirical formula
Formula weight
Crystal system
C24H21N3O3Cu
462.98
Orthorhombic
P212121
Using a procedure similar to that described elsewhere,
complex 10 was obtained. Yield: 73%, m.p [ 270 °C.
C24H19CuN3O5: Found: C, 58.5; H, 3.9; N, 8.5. Calcd: C,
58.6; H, 4.0; N, 8.4. ESI/MS (m/z): 493.2 [M ? 1].
Space group
˚
Unit cell dimensions
a = 11.130 (2) A, a = 90°
b = 11.875 (2) A, b = 90°
˚
˚
c = 32.284 (7) A, c = 90°
3
˚
4,266.6 (15) A
Volume
Preparation of Cu(II) complex (11) of alanine Schiff
base with ligand 2
Z
8
Density (calculated)
Absorption coefficient
F(000)
1.442 mg/m3
1.055 mm-1
1,912
Using a procedure similar to that described elsewhere,
complex 11 was obtained. Yield: 87%, m.p [ 270 °C.
C17H15CuN3O3: Found: C, 54.8; H, 4.1; N, 11.3. Calcd: C,
54.9; H, 4.0; N, 11.4. ESI/MS (m/z): 373.4 [M ? 1].
Crystal size
0.20 9 0.18 9 0.12 mm3
T
113(2) K
˚
2.51–25.02
h Range for data collection
Index ranges
Preparation of Cu(II) complex (12) of valine Schiff
base with ligand 3
-13 B h B 10
-14 B k B 13
-32 B l B 38
Using a procedure similar to that described elsewhere,
complex 12 was obtained. Yield: 67%, m.p [ 270 °C.
C24H19Cl2CuN3O3: Found: C, 54.2; H, 3.6; N, 7.9. Calcd:
C, 54.3; H, 3.7; N, 8.0. ESI/MS (m/z): 531.3 [M ? 1].
Reflections collected
28,611
Independent reflections
Completeness to h
7,512 [R(int) = 0.0427]
99.8%
Max. and min. transmission
Data/restraints/parameters
Goodness-of-fit on F2
0.8839 and 0.8168
7512/0/563
Preparation of Cu(II) complex (13) of valine Schiff
base with ligand 4
0.989
Final R indices [I [ 2r(I)]
R indices (all data)
R1 = 0.0310, xR2 = 0.0699
R1 = 0.0338, xR2 = 0.0714
˚
0.231 and -0.291 eA
Using a procedure similar to that described elsewhere,
-3
Largest diffraction peak and hole
complex 13 was obtained. Yield: 65%, m.p [ 270 °C.
123

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Transition Met Chem (2011) 36:403–407
Scheme 1 Synthesis of
complexes
Br
O
N
O
R
O
N
O
N
N
O
N
O
N
N
N
N
Cu
Cu
O
N
O
Cu
N
N
O
O
O
Cl
Cu
O
Cl
11
12
13
associated with complex 7 have been deposited at the
Cambridge Crystallographic Data Center (CCDC code is
754970). The crystal data and structure refinement param-
eters for complex 7 are given in Table 1.
days. The solid-state structure of C24H21CuN3O3 is shown
in the Fig. 1. X-ray crystal data are summarized in Table 1;
selected bond lengths and angles are listed in Table 2.
Complex 7 crystallizes in orthorhombic chiral space group
P212121. The central copper(II) atom adopts a slightly
distorted square-planar geometry in a N3–O donor envi-
˚
ronment. Bond lengths around the Cu atom are 1.982(2) A
Results and discussion
˚
˚
to N(1), 1.921(2) A to N(2), 1.926(2) A to N(3). The
deviation of average trans angles (177.56° and 161.02°)
from 180° indicates slight tetrahedral distortion from a
basically square-planar geometry. The Cu(1)ꢀꢀꢀCu(2) dis-
When the Schiff base ligands [R = H, methyl, isopropyl,
-CH2CH(CH3)2, benzyl, or -CH2CH2COOH] were reac-
ted with CuCl2ꢀ2H2O in refluxing methanol for 4 h, the
corresponding complexes were obtained. When alanine
Schiff base with ligand 2, valine Schiff base with ligand 3
and ligand 4 were reacted with CuCl2ꢀ2H2O in refluxing
methanol for 4 h, the corresponding complexes were
obtained, respectively (Scheme 1).
˚
tance is 3.399 A in the one asymmetric unit, which makes
˚
˚
weak Cu(1)ꢀꢀꢀO(4) [2.611 A] and Cu(2)ꢀꢀꢀN(1) [3.170 A]
interactions possible. Besides, the interdimer interactions
involve C–HꢀꢀꢀO and C–HꢀꢀꢀN intermolecular contacts. The
adjacent chains are connected through C–HꢀꢀꢀO hydrogen
bonds from C–H of CH3 in valine, together with carboxylic
O(3) atom, and carboxylic O(2) atom with C–H of the
benzene ring, leading to a 3D infinite hydrogen-bond
network.
The IR spectra of copper complexes are all very similar,
and all show two strong carbonyl absorptions at 1,600–
1,700 cm-1 and a strong imine band at 1,605–1,607 cm-1
.
Crystal and molecular structure of complex 7
X-ray quality crystals of the copper(II) complex were
grown from acetonitrile at room temperature for several
˚
Table 2 Selected bond lengths (A) and angles (°) for complex 7
˚
Bond length (A)
Bond angle (°)
Cu(1)–N(2)
Cu(1)–N(3)
Cu(1)–O(2)
Cu(1)–N(1)
Cu(2)–N(6)
Cu(2)–O(5)
Cu(2)–N(5)
Cu(2)–N(4)
N(1)–C(1)
N(4)–C(25)
O(1)–C(6)
O(4)–C(30)
N(2)–C(6)
N(5)–C(30)
1.921(2)
1.926(2)
1.9267
N(2)–Cu(1)–N(3)
N(2)–Cu(1)–O(2)
N(3)–Cu(1)–O(2)
N(2)–Cu(1)–N(1)
N(3)–Cu(1)–N(1)
O(2)–Cu(1)–N(1)
N(6)–Cu(2)–O(5)
N(6)–Cu(2)–N(5)
O(5)–Cu(2)–N(5)
N(6)–Cu(2)–N(4)
O(5)–Cu(2)–N(4)
N(5)–Cu(2)–N(4)
C(20)–O(2)–Cu(1)
C(44)–O(5)–Cu(2)
94.29(9)
177.56(9)
85.81(8)
1.982(2)
1.911(2)
1.9163
84.61(9)
161.02(9)
96.08(8)
1.925(2)
1.960(2)
1.342(3)
1.339(4)
1.233(3)
1.217(3)
1.369(3)
1.367(3)
86.18(8)
95.38(9)
171.88(9)
170.54(10)
94.15(8)
85.63(9)
114.00(17)
114.23(17)
Fig. 1 ORTEP diagram of complex 7 with atom labeling. All H
atoms are omitted for clarity
123

Transition Met Chem (2011) 36:403–407
407
charge via http://www.ccdc.cam.ac.uk/data_request/cif, or
from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK, e-mail: deposit@
ccdc.cam.ac.uk.
Table 3 Percentage inhibition of tumor cell lines (A-549, HCT-8,
and Bel-7402)
Complexes
A-549
HCT-8
Bel-7402
5
79.9
77.8
75.0
12.8
75.8
7.6
80.4
74.0
76.1
17.7
55.3
9.0
74.5
81.3
65.3
9.3
6
7
8
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Acknowledgments This work was financially supported by the
National Natural Science Foundation of China (Grant No. 20472032
and 2010CB833300, China). We also thank the support of Nankai
University State Key Laboratory of Elemento-Organic Chemistry,
and Beijing Institute of Materia Medica, Chinese Academy of Med-
ical Sciences which carried out the antitumor activity measurements.
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Appendix: Supplementary data
CCDC 754970 contains the supplementary crystallographic
data for this paper. These data can be obtained free of
123