Synthesis, structure, and properties of Cd(II) complexes generated from 2-phenylquinoline derivatives

Article abstract of DOI:10.1016/j.molstruc.2014.03.052

Carboxyl functionalized 2-phenylquinoline derivatives, 2-(4-fluorophenyl) quinoline-4-carboxylic acid (HL₁) and 2-(4-methoxyphenyl)quinoline-4- carboxylic acid (HL₂) were synthesized. Further reaction of these ligands with Cd(CH

Full text of DOI:10.1016/j.molstruc.2014.03.052

Journal of Molecular Structure 1067 (2014) 220–224
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis, structure, and properties of Cd(II) complexes generated
from 2-phenylquinoline derivatives
⇑
⇑
Na Lei, Qing-Ling Ren, Ya-Ping Liu, Ji Li, Peng Cong, Jie Qin , Hai-Liang Zhu
School of Life Sciences, Shandong University of Technology, ZiBo 255049, PR China
h i g h l i g h t s
ꢀ
ꢀ
ꢀ
ꢀ
Two dinuclear Cd(II) complexes are assembled from quinoline-based carboxylic acids.
They show hydrogen-bonding driven 3D framework with dia topology.
Luminescent and antibacterial properties of all compounds were investigated.
Complexes show higher activities due to the increased lipotropism on complexation.
a r t i c l e i n f o
a b s t r a c t
Article history:
Carboxyl functionalized 2-phenylquinoline derivatives, 2-(4-fluorophenyl)quinoline-4-carboxylic acid
(HL1) and 2-(4-methoxyphenyl)quinoline-4-carboxylic acid (HL2) were synthesized. Further reaction of
these ligands with Cd(CH3COO)2 afforded new cadmium complexes, [Cd2(L1)4(CH3OH)4]Á1.5H2O (1) and
[Cd2(L2)4(CH3OH)4] (2), respectively. These complexes were characterized by elemental analysis, infrared
spectra, and single-crystal X-ray diffraction. The fluorescent behavior and antibacterial (Escherichia coli,
Pseudomonas aeruginosa, Bacillus subtilis and Staphylococcus aureus) activities of these compounds have
been studied.
Received 13 December 2013
Received in revised form 15 March 2014
Accepted 17 March 2014
Available online 28 March 2014
Keywords:
Quinoline derivative
X-ray crystal structure
Fluorescent property
Antibacterial activity
Ó 2014 Elsevier B.V. All rights reserved.
Introduction
further light on the bioactive metal complex systems, we investi-
gated the reaction of carboxyl modified quinoline derivatives with
The past decades witness numerous progress of metal com-
plexes in the field of advanced material, owing to their potential
application in catalysis, magnetic properties, gas adsorption and
separation, chemosensor, and fluorescence imaging, etc. [1–5].
Since the successful use of cisplatin and related platinum com-
plexes as anticancer agents, a renewed interest in complex-based
therapy has been raised [6–8]. This can be ascribed to the fact that,
on coordination, not only might bioactive ligands improve their
bioactivity profiles, but also inactive ligands may acquire pharma-
cological properties [8,9]. In addition, the coordination effect can
realize the slow-release of metal ions which can reduce the toxic
side effects. In these regards, exploration of metal complex is an
attractive approach to obtain drug candidates [10].
metal salts. The carboxylate ligands are of special interest because
of their various of coordination modes, such as monodentate,
bridging, chelating bidentate and bridging tridentate [14]. More-
over, they are endowed with hydrogen-bonding capabilities [15].
In this paper, 2-(4-fluorophenyl)quinoline-4-carboxylic acid (HL1)
and 2-(4-methoxyphenyl)quinoline-4-carboxylic acid (HL2) are
synthesized in high yields and fully characterized. Two Cd(II) com-
plexes [Cd2(L1)4(CH3OH)4]Á1.5H2O (1) and [(Cd2(L2)4(CH3OH)4] (2)
(Scheme 1) constructed by these two ligands and Cd(CH3COO)2,
including their synthesis, crystal structures, fluorescence and
antibacterial properties are described here.
Quinoline derivatives are an important class of pharmaceutical
compound showing broad spectrum of bioactivities, including
antimicrobial [11], antimalarial [12] and anticancer [13]. To throw
Experimental
Materials and instruments
All the reactions were carried out under air atmosphere. All
chemicals and solvents used in the synthesis were reagent grade
without further purification.
⇑
Corresponding authors. Tel.: +86 5332780271 (J.Qin).
E-mail addresses: qinjie_nju@163.com (J. Qin), hailiang_zhu@163.com
(H.-L. Zhu).
http://dx.doi.org/10.1016/j.molstruc.2014.03.052
0022-2860/Ó 2014 Elsevier B.V. All rights reserved.

N. Lei et al. / Journal of Molecular Structure 1067 (2014) 220–224
221
Scheme 1. Structure of complexes 1 and 2.
The IR spectra were taken on a Vector22 Bruker spectrophotom-
eter (400–4000 cmÀ1
with KBr pellets. NMR spectra were
Synthesis of [Cd2(L2)4(CH3OH)4] (2)
)
measured on a Bruker AM 500 spectrometer. Elemental analyses
for C, H and N were performed on a Perkin–Elmer 240C analyzer.
Fluorescence spectra were recorded on a Hitachi F-4500 fluores-
cence spectrophotometer.
Complex 2 was obtained with similar procedure for synthesiz-
ing 1 by using HL2 instead of HL1. Yield: 10.7 mg (73% on the basis
of HL2). IR (KBr, cmÀ1): 3060, 2937, 2836, 1603, 1516, 1419, 1394,
1333, 1253, 1212, 1173, 1114, 1033, 837, 813, 762, 683, 625, 547,
420. Anal. Calcd. for C72H64N4O16Cd2: C, 58.98; H, 4.40; N, 3.82.
Found: C, 59.21; H, 4.38; N, 3.83%.
Synthesis of 2-(4-fluorophenyl)quinoline-4-carboxylic acid (HL1)
X-ray data collection and structure refinement
A mixture of isatin (1.18 g, 8.00 mmol), 4-fluoroacetophenone
(0.28 g, 2.00 mmol) and potassium hydroxide (2.24 g, 40.00 mmol)
in 2 mL of ethanol and 18 mL of water was refluxed for 12 h. Then
the orange solution was cooled to room temperature and then
poured into 20 mL of water. The solution was adjusted to pH 5 with
1 M HCl. The resulting brown precipitate was filtered, washed with
water and dried in vacuo to give the product. Yield: 0.31 g, 58%. IR
(KBr, cmÀ1): 3442, 3081, 2935, 2839, 1714, 1646, 1548, 1413, 1368,
1243, 1226, 1029, 860, 837, 806, 766, 696, 544, 516. 1H NMR
(500 MHz, DMSO, d): 8.63 (t, 1H), 8.42 (s, 1H), 8.35 (t, 1H), 8.26
(d, J = 8.5 Hz, 1H), 8.13(q, 1H), 7.84 (q, 1H), 7.67 (m, 1H), 7.39 (t,
1H), 7.11 (d, J = 8.5 Hz, 1H). Anal. Calcd for C16H10FNO2: C, 71.91;
H, 3.77; N, 5.24. Found: C, 72.05; H, 3.75; N, 5.26%.
Single crystals of all the complexes for X-ray diffraction analy-
ses with suitable dimensions were mounted on the glass rod. The
data were collected on a Bruker Smart Apex CCD diffractrometer
equipped with graphite-monochromated Mo K
a (k = 0.71073 Å)
radiation using a –2h scan mode at 273 K. The highly redundant
x
data sets were reduced using SAINT and corrected for Lorentz and
polarization effects. Absorption corrections were applied using
SADABS supplied by Bruker. The structure was solved by direct
methods and refined by full-matrix least-squares methods on F2
using SHELXTL-97. All non-hydrogen atoms were found in alter-
nating difference Fourier syntheses and least-squares refinement
cycles, refined anisotropically. All the hydrogen atoms bonded to
C atoms were generated geometrically and refined isotropically
using the riding model. While hydroxyl H atoms of methanol mol-
ecules were first found in the Fourier map and then fixed at their
ideal positions with O-H = 0.96 (2) Å and Uiso(H) = 1.5Ueq(O).
Synthesis of 2-(4-methoxyphenyl)quinoline-4-carboxylic acid (HL2)
Orange product HL2 was obtained with similar procedure for
synthesizing HL1 by using 4-methoxyacetophenone instead of 4-
fluoroacetophenone. Yield: 0.37 g, 66%. IR (KBr, cmÀ1): 3440,
3077, 2936, 2840, 1713, 1651, 1602, 1521, 1425, 1374, 1247,
1189, 1077, 860, 836, 812, 761, 689, 540, 519. 1H NMR
(500 MHz, DMSO, d): 8.62 (d, J = 8.5 Hz, 1H), 8.41 (s, 1H), 8.26 (d,
J = 8.5 Hz, 2H), 8.11(d, J = 8.5 Hz, 1H), 7.82 (t, 1H), 7.66 (t, 1H),
7.11 (d, J = 8.5 Hz, 2H), 3.86 (s, 3H). Anal. Calcd for C17H13NO3: C,
73.11; H, 4.69; N, 5.01. Found: C, 73.41; H, 4.67; N, 5.03%
Water
H atoms were refined with distance restraints of
O-H = 0.85 (2) Å, HÁÁÁH = 1.44 (2) Å and Uiso(H) = 1.5Ueq(O).
Bioassay conditions
The antibacterial activity of the synthesized compounds was
tested against Bacillus subtilis, Escherichia coli, Pseudomonas aeru-
ginosa and Staphylococcus aureus using MH medium (Mueller–Hin-
ton medium: casein hydrolysate 17.5 g, soluble starch 1.5 g, beef
extract 1000 mL). The IC50 (half minimum inhibitory concentra-
tions) of the test compounds were determined by a colorimetric
method using the dye MTT (3-(4,5-dimethylth-iazol-2-yl)-2,5-di-
phenyl tetrazoliumbromide). A stock solution of the synthesized
Synthesis of [Cd2(L1)4(CH3OH)4]Á1.5H2O (1)
A CH3OH solution (5 mL) of Cd(CH3COO)2Á2H2O (0.04 mmol,
10.7 mg) was added to
a CH3OH solution (10 mL) of HL1
(0.04 mmol, 10.7 mg) with stirring. The mixture was stirred for
20 min at room temperature. The resulting solution was allowed
to stand in air for three days. Colorless block-shaped crystals suit-
able for X-ray single crystal analysis were formed at the bottom of
the vessel. Yield: 9.6 mg (67% on the basis of HL1). IR (KBr, cmÀ1):
3064, 2838, 1601, 1513, 1416, 1392, 1332, 1253, 1233, 1174, 1156,
905, 840, 812, 762, 665, 624, 552, 465. Anal. Calcd. for
compound (100 lg/mL) in DMSO was prepared and graded quanti-
ties of the test compounds were incorporated in specified quantity
of sterilized liquid MH medium. A specified quantity of the med-
ium containing the compound was poured into microtitration
plates. Suspension of the microorganism was prepared to contain
approximately 105 cfu/mL and applied to microtitration plates
with serially diluted compounds in DMSO to be tested and incu-
bated at 37 °C for 24 h. After the MICs were visually determined
C68H55N4F4O13.5Cd2: C, 56.52; H, 3.83; N, 3.87. Found: C, 56.71;
H, 3.81; N, 3.89%.
on each of the microtitration plates, 50 lL of PBS (phosphate

222
N. Lei et al. / Journal of Molecular Structure 1067 (2014) 220–224
buffered saline 0.01 mol/L, pH 7.4, Na2HPO4Á12H2O 2.9 g, KH2PO4
0.2 g, NaCl 8.0 g, KCl 0.2 g, distilled water 1000 mL) containing
2 mg of MTT/mL was added to each well. Incubation was continued
at room temperature for 4–5 h. The content of each well was re-
Most of the paddle wheel carboxylate complexes found in CCDC
have a coordination number 5, while for complexes 1 and 2, two
methanol molecules are coordinated to one CdII center to achieve
the coordination number 6. As shown in Fig. 1, the complexes
are pseudo-octahedral, with the equatorial plane defined by meth-
anol O atoms (O3 and O3A) and carboxylate O atoms (O2 and O2A),
while the two axial positions come from the carboxylic O atoms
(O1 and O1A). Of all the Cd-O bond lengths, the axial Cd1-O1dis-
tance is the shortest, 2.2118 (7) Å for 1, and 2.2166 (7) Å for 2.
The tetracarboxylate bridging framework can accommodate me-
tal–metal separation of up to 3.452 Å [18,19]. In complex 1, the
CdÁÁÁCd separation of 3.4399(5) Å is shorter than this maximum,
indicating of a degree of metal–metal interaction. While the corre-
sponding CdÁÁÁCd distance observed in complex 2 is longer being
3.4657(5) Å. The carboxylic ligands in complexes are twisted, as
the phenyl ring (C(1)-C(6)) rotate by about 31.67(1)° for 1,
30.40(1)° for 2, respectively, with respect to the quinoline hetero-
cycle. The coordination plane (Cd1, O1, C16, O2, Cd1A) and the
plane of quinoline heterocycle are also non-coplanar, the dihedral
angle between them amounts to 66.76(1)° for 1, 66.10(1)° for 2,
respectively.
For both of the two compounds, the uncoordinated N1 atom of
the quinoline ring serves as an acceptor to form intermolecular
hydrogen bond O3-H3AÁÁÁN1 with the coordinated methanol mole-
cule. For 1, H3A to N1 distance 1.859(2) Å, O3-H3AÁÁÁN1 angle
165.53(5ꢁ), symmetry code: Ày + 3/4, x + 1/4, z + 1/4; for 2, H3A to
N1 distance 1.859(3) Å, O3-H3AÁÁÁN1 angle 172.68(5)°, symmetry
code: Ày + 7/4, x + 1/4, z + 1/4. Therefore, each binuclear unit can
be regarded as self-complementary module, with four H-bond do-
nors (OH) and four acceptors (heteroaryl-N atoms). Aggregation of
such modules generates the non-covalent 3D framework, with
four-connected net nodes and dia topology (Fig. 2).
moved, and 100 lL of isopropanol containing 5% 1 mol/L HCl was
added to extract the dye. After 12 h of incubation at room temper-
ature, the optical density (OD) was measured with a microplate
reader at 550 nm.
Results and discussion
Synthesis and general characterization
The carboxylic functionalized quinoline ligands HL1 and HL2
were prepared via the Pfitzinger reaction [16]. They can dissolve
in strong polar solvents such as methanol and dimethyl sulfoxide
(DMSO). Characterization of the ligands has been accomplished
by IR, 1H NMR, and elemental analysis. In IR spectra, the typical
stretching band v(C = O) of carboxylic group for HL1 and HL2 occurs
at 1714 cmÀ1 and 1713 cmÀ1
stretching band v(O-H) of carboxylic group appears around
3442 cmÀ1. The separation value
v (vas(COOꢀ)–vs(COOꢀ) of the car-
boxylic based complex can be used to discriminate the coordina-
tion mode of the carboxyl group.
v < 200 cmÀ1 indicates the
bidentate mode, whereas
v > 200 cmÀ1 indicates the monoden-
, respectively. While the broad
D
D
D
tate mode. For the cadmium complexes 1 and 2, the asymmetric
stretching mode of carboxylate vas(COOꢀ) is located around
1601 cmÀ1, while the symmetric stretching mode vs(COOꢀ) is
around 1419 and 1416 cmÀ1, respectively. Therefore, the
Dv values
are 182 cmÀ1 for 1 and 185 cmÀ1 for 2, which is in good agreement
with their bridging bidentate coordination mode features shown
by the results of crystal structures [17].
Besides the hydrogen bonding interactions, the structures of 1
and 2 are also stabilized by
the adjacent quinoline rings from different dinuclear units are
p p interactions. In the solid state,
ÁÁÁ
Crystal structures of complexes 1 and 2
approximately parallel to each other, the dihedral angle is
The solid structures of complexes 1 and 2 were determined by
single-crystal X-ray diffraction. Details of the crystal parameters,
data collection, and refinement are summarized in Table 1; se-
lected bond lengths and angles are listed in Table S1. Both of the
two complexes are crystallized in tetragonal space group I41/a.
The ORTEP drawing for them is shown in Fig. 1, the two complexes
possess very similar paddlewheel dimeric structure formed by the
bidentate coordination mode of the carboxyl groups.
1.85(1)° for 1, 1.92(1)° for 2. The related parameters about the
interactions are summarized in Table 2 (Fig. 3) following the crite-
p p
ÁÁÁ
ria of Janiak [20]. The distances between two quinoline planes
range from 3.512 to 3.525 Å, indicating strong slipped
p p stack-
ÁÁÁ
ing interactions. The C1,3 (centroid–centroid distances between
pyridine and arene) values are less than 3.8 Å in complexes 1 and
2. The values of C1,4 (centroid–centroid distances between arene
and arene) and C2,3 (centroid–centroid distances between pyridine
and pyridine) are all in the normal range found in the reported
transition-metal quinoline fragments [20]. It is noteworthy to
Table 1
point out that the displacement angles of
a in complex 1 and 2
Crystallographic data for 1 and 2.
(9.35° and 9.41°, respectively) are quite smaller than the usual val-
ues observed in literature (around 20°).
1
2
Empirical formula
Mr
Crystal system
Space group
C
68H55F4N4O13.5Cd2
C72H64N4O16Cd2
1466.07
Tetragonal
I4(1)/a
1444.96
Tetragonal
I4(1)/a
Photoluminescent properties of HL1, HL2, 1 and 2
a (Å)
b (Å)
22.4253(4)
22.4253(4)
12.0499(4)
6059.8(3)
4
22.5144(3)
22.5144(3)
12.3995(5)
6285.3(3)
4
The photoluminescent properties of ligands HL1, HL2, and their
corresponding Cd(II) complexes 1 and 2 in dimethylsulfoxide/
methanol (1:1) solution were investigated. The emission spectra
is shown in Fig. 3.
c (Å)
V (Å3)
Z
Upon excitation at 373 nm at room temperature, the carboxylic
ligands exhibit broad structureless band at about 466 nm, resulting
Total reflections
Independent reflections
Observed reflections (I > 2
32,166
3361
2644
1.584
2924
293(2)
0.785
1.044
29,621
2786
2231
1.549
2992
293(2)
0.753
1.040
from the ligand-centered (LC) p⁄
–p relaxations (Fig. 4). Referenced
r(I))
qc (g cmÀ3
F (000)
T (K)
)
to that of HL2 having electron-donating group (–OCH3), the relative
luminescence intensity of electron-withdrawing group (–F) substi-
tuted ligand HL1 is about 0.61. For complex 1 and 2, a blue shift in
the emission maxima about 24 nm compared with the free ligands
l
(Mo K
a
) (mmÀ1
)
GOF (F2)
R1a, wR2 (all data)
0.0458, 0.0748
0.0410, 0.0626
b
are observed. This is probably caused by the reduced
p–p conju-
a
R1
wR2 = [
=
R
||C| À |Fc||/
RFo|.
gated effect upon coordination to the Cd(II). In the meantime, rel-
ative to the corresponding free ligands, the fluorescence intensity
b
R
w(F2o À F2c)2/
R .
w(F2o)]1/2

N. Lei et al. / Journal of Molecular Structure 1067 (2014) 220–224
223
Fig. 1. Molecular structure of 1 (right) and 2 (left) at 50% probability displacement.
Fig. 3. Scheme for the parameters of
two
ring normal of quinoline plane and the centroid–centroid vector C1,3, C2,3
p
Á Á Á
p
interactions. d, the distance between the
p
planes; Ci,j, centroid–centroid distance of Ci and Cj; a, b, the angle between the
.
Fig. 2. The schematic representations of the uninodal 4-connected net of 1 and 2
with dia topology.
Schiff base cadmium complexes [{CdCl(HATtsc)}2(
l
-Cl)2]Á2H2O
Table 2
Parameters for the
(HATtsc = 2-acetyl-2-thiazoline thiosemicarbazone) [22], [CdBr2(3-
TTSCH)2] (3TTSCH = 3-thiophenaldehyde thiosemicarbazone) [23],
and cadmium complexes with the quinolone antibacterial agent
N-propyl-norfloxacin [24]. In the present study, the antibacterial
activities of compounds HL1, HL2, 1, 2 and salt Cd(CH3COO)2 are
presented in Table 3. The antibacterial activity was performed
against two Gram-negative bacterial strains: E. coli ATCC 35218
and P. aeruginosa ATCC 27853, two Gram-positive bacterial strains:
B. subtilis ATCC 6633 and S. aureus ATCC 6538. Known antibiotic
like streptomycin was used as positive control.
pÁ Á Áp interactions in 1 and 2.
d (Å)
C1,3 (Å)
C1,4 (Å)
C2,3 (Å)
a
(°)
b (°)
1
2
3.525
3.512
3.678
3.660
4.959
4.935
3.739
3.660
9.35
9.41
28.38
28.41
of complexes are a little weaker because of the non-coplanar
arrangement upon complexation.
As shown in Table 3, against the all tested bacteria, free ligands
HL1 or HL2 were inactive. The salt Cd(CH3COO)2 showed moderate
activities against Gram-positive bacterial strains. While the com-
plexes 1 and 2 exhibited an enhancement of antibacterial level
Antibacterial Activities of HL1, HL2, 1 and 2
Unlike Cu, Co, Ni or Zn which are essential for microorganisms
as trace nutrients, Cd is a toxic heavy metal. While the research on
the toxicology of Cd2+ has shown that little cell-killing occurred at
against B. subtilis (HMIC = 3.80 and 12.23
lg/mL, respectively)
and S. aureus (HMIC = 5.58 and 21.07 g/mL, respectively). It
l
should be noticed that complex 1 having electron-withdrawing
substituent (F) exhibited higher activities than complex 2 having
electron-donating substituent (OCH3). And the antibacterial level
of complex 1 against B. subtilis and S. aureus was close to the
concentrations 50–150 lM for polymorphonuclear cells [21,22]. In
addition, researches show that the cadmium complexes are less
toxic than free Cd2+. So several cadmium complexes have been
reported for their antibacterial activity with good results, such as

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N. Lei et al. / Journal of Molecular Structure 1067 (2014) 220–224
although the free ligands are inactive against the tested bacteria,
the two complexes have good activity against B. subtilis and S. aur-
eus. Further biological experiments in order to clarify the possible
mechanism are under investigation.
Acknowledgment
This work was supported by the National Natural Science
Foundation of China (Grant No. 21301108).
Appendix A. Supplementary material
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.molstruc.
2014.03.052.
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Fig. 4. Photoluminescence spectra of HL1, HL2, 1 and 2 in DMSO/CH3OH solution
(1:1, v/v) at room temperature (kex = 372 nm).
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Table 3
Antimicrobial activity of the tested compounds.
Compounds
Half maximal inhibitory concentrations (
l
g/mL)
Gram-positive
Gram-negative
B. subtilis
S. aureus
E. coli
P. aeruginosa
1
2
HL1
HL2
Cd(OAC)2
Streptomycin
3.80
12.23
>50
>50
37.58
3.42
5.58
21.07
>50
>50
36.43
4.62
>50
>50
>50
>50
>50
3.81
>50
>50
>50
>50
>50
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ˇ
In this paper we reported the synthesis, characterization and
photophysical studies of two new Cd(II) complexes based on car-
boxyl functionalized quinoline ligands. The X-ray crystallographic
analysis reveals that the complexes exist as centrosymmetric bicy-
clic dimers constructed by the syn-syn bidentate bridging mode of
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luminescent with maximum emission wavelength at 466 nm and
442 nm, respectively. The bioassay results demonstrated that
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