Two complexes of Co(II) and Pd(II) formed in reaction with a mono-oxazoline derivative. Spectroscopic characterization and cytotoxic evaluation

Article abstract of DOI:10.1016/j.molstruc.2013.02.043

Two coordination compounds obtained from 2-(4-ethoxyphenyl)-4,5-dihydro-1H- oxazole, L, [CoCl₂(L)₂] (1) and [OHCH₂CH ₂NH₃]₂·[PdCl₄] (2) were synthesized and characterized through

Full text of DOI:10.1016/j.molstruc.2013.02.043

Journal of Molecular Structure 1041 (2013) 55–60
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Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Two complexes of Co(II) and Pd(II) formed in reaction with a mono-oxazoline
derivative. Spectroscopic characterization and cytotoxic evaluation
b
b
c
Maria-Gabriela Alexandru a, , Tanja Cirkovic Velickovic , Maja Krstic , Madalina-Marina Hrubaru ,
⇑
Constantin Draghici c
a Faculty of Applied Chemistry and Materials Science, University ‘‘Politehnica’’ of Bucharest, Str. Polizu 1ꢀ7, 011061 Bucharest, Romania
b University of Belgrade, Faculty of Chemistry, Department of Biochemistry, Belgrade, Serbia
c C.D. Nenitescu Institute of Organic Chemistry, Romanian Academy, Splaiul Independentei 202B, 060023 Bucharest, Romania
h i g h l i g h t s
ꢁ
ꢁ
ꢁ
ꢁ
1-D supramolecular polymer was obtained from the reaction of CoCl2 and oxazoline, L.
The reaction between L and PdCl2, in acid solution, determined the hydrolysis of L.
The ionic salt of Pd(II) exhibits luminescent properties.
The Pd(II) compound showed the most promising cytotoxic activity of all three tested.
a r t i c l e i n f o
a b s t r a c t
Article history:
Two coordination compounds obtained from 2-(4-ethoxyphenyl)-4,5-dihydro-1H-oxazole, L, [CoCl2(L)2]
(1) and [OHACH2ACH2ANH3]2ꢂ[PdCl4] (2) were synthesized and characterized through elemental analy-
sis, spectroscopic methods (FTIR, UV–Vis, 1H NMR) and X-ray diffraction. Cytotoxicity tests on PBMC and
HeLa cells were performed in order to evaluate the potential applications of these compounds in medi-
cine. Also, compound (2) exhibits ligand assisted luminescent properties that were determined in solid
state.
Received 27 November 2012
Received in revised form 5 February 2013
Accepted 28 February 2013
Available online 7 March 2013
Keywords:
Oxazoline
Ó 2013 Elsevier B.V. All rights reserved.
Co(II) and Pd(II) complexes
Cytotoxicity on HeLa and PBMC
1. Introduction
Herein, we present the synthesis and characterization of two
coordination compounds starting from 2-(4-ethoxyphenyl)-4,5-
The oxazoline derivatives are known to exhibit antitumoral bio-
logical activity, being synthesized and studied as analogues of
Apratoxin A, a natural cytotoxic agent containing in its structure
oxazoline rings [1]. Curiously, only few coordination compounds
with mono-oxazoline derivatives are reported to date [2], more
explored being complexes with bis-oxazoline derivatives [3].
Also, complexes of Pd(II) with Schiff bases or heterocyclic
compounds proved to be serious candidates for in vivo studies as
cancer treatment drugs. A large number of scientific papers present
cytotoxic assay data for palladium complexes as alternative to cis-
platin [4]. Other transition metal complexes were investigated as
potential antitumor drugs in order to limit the negative side effects
of cisplatin analogues. Some of results published indicate that a
suitable choice could be cobalt, which is a bioelement [5].
dihydro-1H-oxazole, L and CoCl2: [CoCl2(L)2] (1) and PdCl2:
[OHACH2ACH2ANH3]2ꢂ[PdCl4] (2), respectively. Also, the results
of cytotoxicity assessments regarding the ligand and complex
compounds (1) and (2), and unusual luminescent properties for
complex (2) are discussed.
2. Experimental
All reagents and solvents were purchased from Sigma–Aldrich
and used without further purification. Complete medium for cell
growth and the reagents for cytotoxicity studies were purchased
from Sigma–Aldrich.
L was synthesized as described previously [6]. Yield: 75%. Anal
Found (%): 69.50 C; 6.72 H; 7.15 N. Calculated for C11H13O2N:
(%): 69.12 C; 6.80 H; 7.32 N. IR (solid, ATR, cmꢀ1): 1643, 1604,
1510, 1249, 1166, 1041; 1H NMR (DMSO-d6, dppm, J (Hz)): 7.83
(d, 8.8, 2H, H-7, H-11); 7.03 (d, 8.8, 2H, H-8, H-10); 4.52 (t, 9.4,
⇑
Corresponding author. Tel.: +40 745755730.
E-mail address: alexandru.gabriela@gmail.com (M.-G. Alexandru).
0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2013.02.043

56
M.-G. Alexandru et al. / Journal of Molecular Structure 1041 (2013) 55–60
2H, H-4); 4.11 (qv, 7, 2H, H-13); 3.97 (t, 9.4, 2H, H-5); 1.34 (t, 7, 3H,
H-14).
1–5000 lM for (1) and 0.005–50 lM for L. All experiments were
performed in triplicate.
Synthesis of 1: A solution of 0.39 g (2 mmol) L dissolved in
methanol was added to a solution of 0.24 g (1 mmol) CoCl2ꢂ6H2O
dissolved in distilled water. Blue crystals were formed after stirring
for 4 h at 60 °C. The crystals were filtered, washed with methanol
and dried in air. Yield: 52%. Anal Found (%): 51.57 C; 5.02 H; 5.51
N. Calcd. for CoC22H26N2O4Cl2 (%): 51.53 C; 5.07 H; 5.46 N. IR (solid,
ATR, cmꢀ1): 3548, 1603, 1556, 1504, 1259, 1035, 420.
2.2. Cell vitality analysis
Cytotoxicity of tested compounds was evaluated by MTT
(3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyltetrazolium bromide)
assay, as described previously [5a]. Briefly, after the incubation
period, 20 l
L of MTT solution (5 mg mLꢀ1) was added to each well.
Synthesis of 2: A solution of 0.78 g (4 mmol) L dissolved in
methanol was added to a solution of 0.35 g (2 mmol) PdCl2 dis-
solved in 50 mL HClaq. 1 mol/L. The mixture was refluxed for 10 h
and the orange precipitate formed was filtered. Orange crystals
were obtained after recrystallization from methanol. Yield: 58%.
Anal Found (%): 12.85 C; 4.23 H; 7.44 N. Calcd. for PdC4H16N2O2Cl4
(%): 12.89 C; 4.30 H; 7.52 N. IR (solid, ATR, cmꢀ1): 3074, 2971,
2919, 1605, 1508, 1040.
Samples were further incubated for 4 h at 37 °C under 6% CO2. For-
mazan (color that was formed during MTT metabolism) was resus-
pended in DMSO and the absorbance was read at 540 nm and
670 nm. The absorbance, which represents the number of viable
cells, was calculated as A540–A670. The measure of the cytotoxicity
of the compounds was an IC50 value, which was determined as a
concentration of the tested compound that inhibited cell survival
by 50%, compared with the control cells.
The 1H NMR spectrum of (2) displays proton signals that sup-
port the preservation of the structure in solution. 1H NMR
(DMSO-d6, dppm, J (Hz)): 8.10 (s, 3H, ANHþ3 ); 4.34 (t, 4.90, 2H,
ACHb2); 4.07 (cv, 7.00, 2H, ACHa2); 3.18 (t, 9.60, 1H, AOH);
b = shielded by ANH3þ and ACH2b, a = shielded by AOH and ACHa2.
The 1H NMR experiment was performed in DMSO, the same sol-
vent used for the viability experiments.
The elemental analysis was performed on a Perkin–Elmer Series
II CHNS/O Analyzer 2400. 1H NMR spectra were recorded on a
Gemini 300 BB operating at 300 MHz in DMSO-d6 using TMS as
internal standard. UV–Vis reflectance spectra were recorded on
Jasco V-560 spectrophotometer and IR spectra on a FT-IR Bruker
VERTEX 70 equipped with a diamond ATR. The photoluminescence
measurements were carried out using a Jasco FP6500 spectrofluo-
rimeter at room temperature.
X-ray diffraction measurements were performed on a STOE-
IPDS II diffractometer. The structures were solved (SHELXS-97)
by direct methods and refined (SHELXL-97) by fullmatrix
least-square procedures on F2 [7]. Crystallographic data for the
structural analysis have been deposited with the Cambridge Crys-
tallographic Data Centre, CCDC No. 795965 (1) and 795963 (2)
(October 2010).
3. Results and discussion
Starting from oxazoline derivative as ligand we aimed to obtain
coordination compounds of Co(II) and Pd(II) and determine their
potential antitumoral effect. Chloride salts of Co(II) and Pd(II) were
reacted in water–methanol with oxazoline derivative, L (molar
ratio = 1:2) to afford crystals of (1) and (2), respectively. The differ-
ence between the syntheses resides in the acid conditions the
Pd(II) derivative was obtained. Hydrochloric acid (1 mol/L) was
added in order to completely dissolve the chloride salt (PdCl2) in
water. The reactants and the reactions conditions for the final com-
pounds resulted in both cases are displayed in Scheme 1.
An identical structure with (2) was published in 2010 (the same
year our structure was deposited at CCDC without being aware of
the results of Darkwab et al.) [8]. The authors mentioned the
hydrolysis of the Schiff base used as ligand in different conditions
as solvent (dichloromethane), pH and different precursors. Inter-
estingly, the fragment, which underwent hydrolysis, is very similar
to our system. In our case the broken linkage OAC@N can be
explained on account of using acid medium and as a result, the pal-
ladium ion, most likely forms a very stable complex ½PdCl4ꢃ2ꢀ. Also,
the metal assisted hydrolysis of the imines in acidic aqueous solu-
tions was studied and proved for Schiff base compounds and for
imidazolines, which are very similar to the organic precursor used
in this paper [9].
2.1. Cell culture
Peripheral blood mononuclear cells (PBMCs) were separated
from whole heparinized blood of healthy volunteer. Blood was
diluted with PBS (1:1) and layered on Histopaque solution. After
centrifugation, interface cells were collected and washed three
times with phosphate buffered solution (PBS). After counting, cells
were resuspended in nutrient medium. Nutrient medium was
RPMI medium, supplemented with 10% fetal calf serum (FCS), 1%
3.1. UV–Vis spectra
The most characteristic absorption for tetrahedral Co(II) cations,
which gives the blue coloration of the cobalt complexes, is identi-
fied in the 600–700 nm region as an intense two structured band
for (1), with shoulders at 620 and 665 nm (Fig. 1). These bands re-
sult from the allowed spin transition 4T1(P) 4A2(F) specific for
tetrahedral Co(II) chromophores. Their structured pattern has re-
sulted most likely from the significant effects of the spin–orbit cou-
pling which split the 4T state into three components [10].
glutamine (200 mM), 1% b-mercaptoethanol (5 lM), 1% penicillin
(10,000 IU mLꢀ1) and 1% streptomycin (10 mg mLꢀ1). Cells were
seeded (200,000 cells per well) in 96-well plates in nutrient med-
ium with 0.5% phytohemagglutinin (PHA) (1 mg mLꢀ1) and differ-
ent concentrations of complexes. Cells were incubated for 72 h at
37 °C in a humidified atmosphere with 6% CO2.
The UV–Vis spectrum of (2) (Fig. 1) shows three bands at
Human cervix adenocarcinoma cells (HeLa) were grown in
RPMI medium supplemented with 10% fetal calf serum (FCS), 1%
glutamine (200 mM), and 1% mixture of antibiotics (penicillin
10,000 IU mLꢀ1 and streptomycin 10 mg mLꢀ1). After trypsiniza-
tion, cells were seeded (10.000 cells per well) in 96-well plates
for 24 h at 37 °C in a humidified atmosphere with 6% CO2. After this
period, different dilutions of complex compounds in complete
medium were sterilized by filtration, added to the wells and incu-
bated for 24 h under identical conditions. Only medium was added
to the cells in the control wells. The compounds were added to the
320 nm assigned to 1Eg 1A1g electronic transition, at 488 nm
1
generated by B1g 1A1g transition and the band situated at
1
643 nm, by A2g 1A1g electronic transition [10].
3.2. IR spectra
The FTIR spectrum for (1) shows a strong band at 1602 cmꢀ1
assigned to imine stretching vibrations ( C@N). The comparison be-
,
m
tween the FTIR spectra of (1) and of the free ligand L allows some
wells at different concentrations in the range 0.001–10
l
M for (2),
preliminary conclusions regarding the possible ligand–metal ion

M.-G. Alexandru et al. / Journal of Molecular Structure 1041 (2013) 55–60
57
[CoCl2(L)2] (1)
-CH2-NH3]2 [PdCl4] (2)
[OH-CH 2
Scheme 1. Scheme of the obtaining reactions for: (1) and (2).
Fig. 1. Left: UV–Vis reflectance spectra of L, [CoCl2(L2)2] (1) Right: UV–Vis reflectance spectra of [OHACH2ACH2ANH3]2ꢂ[PdCl4] (2).
Fig. 2. Left: assymmetric unit of [CoCl2(L)2]. Right: packing diagram.
interactions. Thus, shifting of
m
(C@N) from 1643 cmꢀ1 (L) to lower
a supramolecular 1-D coordination polymer. The crystallographic
data and other pertinent information are collected in Table 1.
Cobalt atom has a distorted tetrahedral geometry with coordi-
nated chloride atoms and two ligand molecules coordinating
through N atoms. Asymmetric unit and packing diagram for (1)
are shown in Fig. 2.
frequencies, at 1602 cmꢀ1 (1) sustains the formation of coordina-
tion bond between Co(II) and N(oxazoline).
The structure of compound (2) is also supported by the IR spec-
trum, which displays bands at 1605, 1451 and 3074 cmꢀ1 charac-
teristic for the asymmetric, symmetric stretching vibration and
bending of NHþ3 group, respectively.
The values of the bond lengths, which involve the Co(II) ion and
generate 0 its distorted tetrahedral geometry are: Co(1)AN(1)
0
3.3. X-ray crystal structure
2.037(3) ÅA Co(1)ACl(1) 2.2419(12) ÅA and for the angles (°)
Cl(1)ACo(1)ACl(1): 110.49(7), N(1)ACo(1)AN(1): 104.10(18),
N(1)ACo(1)ACl(1): 104.28(10) (Table 2). The ligand is not coplanar,
the dihedral angle between phenyl plane and oxazoline plane
being equal with 24.693(2)°. A similar compound was obtained
by Gossage et al. using ZnCl2 and oxazoline derivative 1:2 M ratio
[2a]. The Zn(II) complex crystallizes in triclinic P ꢀ 1 space group
X-ray structure was determined for complex compound (1),
[CoCl2(L)2], which crystallizes in the centrosymmetric space group
Pbna. The structural investigations reveals the formation of supra-
molecular chains assembled from mononuclear [CoCl2(L)2] moie-
ties (Fig. 2). Thus, the coordination compound can be considered

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M.-G. Alexandru et al. / Journal of Molecular Structure 1041 (2013) 55–60
Table 1
Crystal data and structure refinement for (1) and (2).
CoC22H26N2O4Cl2
PdC4H16N2O2Cl4
Formula weight
512.28
372.39
T (K)
293(2)
297(2)
Wavelength (Å)
0.71073
0.71073
Crystal system, space group
Orthorhombic, Pbna
Monoclinic, P2(1)/c
a (Å)
b (Å)
c (Å)
9.8981(11)
14.3013(16)
16.7171(17)
9.0019(16)
8.1940(14)
8.6774(15)
a
(°)
90
90
b (°)
90
103.709(3)
c
(°)
90
90
Z, V, (Å3)
4, 2366.4(4)
1.438
0.981
1060
2, 621.82(19)
1.989
2.326
368
Calculated density (g/cm3)
Absorption coefficient (mmꢀ1
F(000)
)
Crystal dimensions, mm
Theta range for data collection, (°)
Limiting indices hkl
Reflections collected/unique
Refinement method
0.3 ꢄ 0.25 ꢄ 0.12
0.60 ꢄ 0.25 ꢄ 0.15
2.39–27.44
2.33–25.01
ꢀ12 6 h 6 12, ꢀ18 6 k 6 18, ꢀ21 6 l 6 21
12,239/2693 [R(int) = 0.0698]
Full-matrix least-squares on F2
2693/0/141
ꢀ10 6 h 6 10, ꢀ9 6 k 6 9, ꢀ10 6 l 6 10
5763/1097 [R(int) = 0.0400]
Date/constrains/parameters
1097/0/64
1.197
Goodness of fit on F2
1.062
Final R indices [I > 2
R indices
r
(I)]
R1 = 0.1103, wR2 = 0.1218
R1 = 0.0682, wR2 = 0.1097
R1 = 0.0220, wR2 = 0.0622
R1 = 0.0230, wR2 = 0.0628
Table 2
Selected bond lengths (Å) and angles [°] for (1).
molecules in a pseudo-chain configuration and thus the dimen-
sionality of the molecule is pseudo- extended from discrete 0-D
molecules to 1-D supramolecular polymer, Fig. 3.
Cl1ACo1
N1ACo1
C1AN1
C2AO1
C3AN1
C3AO1
C4AC5
C5AC6
2.2418(11)
2.037(3)
1.481(5)
1.452(5)
1.289(5)
1.346(4)
1.402(5)
1.371(6)
N1ACo1ACl1
N1ACo1ACl1
Cl1ACo1ACl1
N1ACo1AN1
C3AN1AC1
104.26(9)
117.05(10)
110.51(7)
104.08(17)
107.9(3)
The values of the centroid–centroid distances and of the angle
between the normal to the ring and the centroid–centroid vector
0
for the oxazoline and phenyl rings are 3.96(3) ÅA and 24.046°,
respectively, suggesting a weak
p–p stacking type interaction
C3AN1ACo1
N1AC3AO1
131.7(3)
115.6(3)
(Fig. 4).
The structure of (2) was discussed in [8]. 3-D supramolecular
network formed through H-bonds, is shown in Fig. 5, along with
the atom numbering scheme of the asymmetric unit.
Table 3
Geometrical parameters (Å, °) associated to the hydrogen bonds in (1).
3.4. Luminescence properties
Donor—HꢂꢂꢂAcceptor
C(1)–H(1B)ꢂ ꢂ ꢂCl(1)
C(6)–H(6)ꢂ ꢂ ꢂCl(1)a
DAH
Hꢂ ꢂ ꢂA
2.85(2)
2.83(3)
Dꢂ ꢂ ꢂA
3.42(2)
3.68(4)
DAHꢂ ꢂ ꢂA
117(6)
154(6)
The luminescent properties of (2) were determined in the solid
state at room temperature (Fig. 6). The UV–Vis spectrum shows a
strong, broad absorption band at 320 nm, which represented the
excitation wavelength. Complex (2) exhibits strong violet–blue
luminescence at 425 nm and a weak green–yellow luminescence
at 560 nm upon excitation at 320 nm.
The fluorescence of Pd(II) ion in complex (2) is enhanced by the
protonated ethanolamine [11a], which establishes hydrogen bonds
with acceptor chlorine atoms, and therefore increases the rigidity
of the Pd(II) cation and reduces the loss of energy by thermal vibra-
tions. Fluorescent emission at room temperature or low
0.97(1)
0.93(1)
a = ꢀ1+x, 1/2ꢀy, 1ꢀz.
and angles (o) NAZnAN: 109.09(11) and ClAZnACl: 114.26(4) are
higher compared to Co (II) complex (1).
Intra and intermolecular hydrogen bonds are established
between C(1)–H(1B)ꢂ ꢂ ꢂCl(1) and C(6)AH(6)ꢂ ꢂ ꢂCl(1), respectively
(Table 3). The intermolecular hydrogen bonds are keeping the
Fig. 3. Pseudo-chain configuration of (1).

M.-G. Alexandru et al. / Journal of Molecular Structure 1041 (2013) 55–60
59
Fig. 4. Representation of weak
p–
p
stacking involving the oxazoline and phenyl rings.
Table 4
IC50 values for L, (1) and (2).
IC50
L
(lM)
HeLa
PBMC
70
50
[CoCl2(L)2] (1)
[OHACH2ACH2ANH3]2ꢂ[PdCl4] (2)
104
0.44
176
4.4
were tested for their antiproliferative activity in vitro against
human cervical cancer cell line, HeLa. All the compounds were also
screened for cytotoxic activity against PHA stimulated normal hu-
man peripheral blood mononuclear cells (PBMCs) in order to assess
the effect on healthy cells and assess the selectivity of the obtained
cytotoxic effects in tumor cells. The results of cytotoxic activity
in vitro are expressed as IC50 – the concentration of compound
(in
lM) that decreased vitality of the tumor cells by 50% as com-
pared to control untreated cells. The IC50 values for the ligand L,
as well as for complex (1) are presented in Table 4.
Fig. 5. Packing of (2) showing 3-D supramolecular assembly.
L showed a good cytotoxic activity in both HeLa and PBMC of all
the tested compounds. The cytotoxic activity slightly decreased
when L forms coordination compound with Co(II), (1), although
better selectivity for tumor cells than for the normal cells was
achieved. Thus, complex (1) exhibited moderate antiproliferative
activity toward HeLa cells while showing better selectivity for tu-
mor cells than for the normal cells in comparison with ligand L.
Complex (2) exhibits the highest cytotoxic activity and also the
best selectivity in relation to L and 1. The IC50 on PBMC is ten times
higher than on HeLa cells which emphases the high selectivity of
the compound- see Table 4. Therefore, the results of the viability
assessments are encouraging, although it is difficult to discuss
the relation structure–activity given the hydrolysis of the ligand
during the synthesis. Interestingly, the complex compound K2-
[PdCl4], having a different counterion has no cytotoxic activity, as
shown by the IC50 value [4a]. Therefore, by changing the cation,
from inorganic K+ to organic ethanolamine HOACH2ACH2ANHþ,
3
the cytotoxicity of the complex compound against HeLa cells in-
creases significantly.
Fig. 6. Fluorescence spectrum of (2).
temperature (77 K) among Pd(II) complexes was rarely reported,
mostly cyclopalladated compounds being studied for applications
in OLED. When reacted with a fluorescent ligand, Pd(II) ion usually
quenches the fluorescence of the resulted complex compound
[11b,4c,11c–e].
4. Conclusions
Two coordination compounds obtained by reacting Co(II) and
Pd(II) with 2-(4-ethoxyphenyl)-4,5-dihydro-1H-oxazole (L) were
characterized through XRD on single crystals, elemental analysis,
UV–Vis and FTIR spectroscopy. X-ray crystal diffraction of Co(II)
complex with oxazoline derivative shows an interesting 1-D supra-
molecular polymer structure constructed from [CoCl2(L)2] units
connected through hydrogen bonds. On the other hand, the Pd(II)
cation was not coordinated by the oxazoline derivative and instead
3.5. Viability tests
The organic ligand, oxazoline derivative, L, as well as the coor-
dination compound with Co(II), (1) and Pd(II) complex salt (2),

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M.-G. Alexandru et al. / Journal of Molecular Structure 1041 (2013) 55–60
2ꢀ
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This work was supported by Project Number 172024 of the
Ministry of Education, Science and Technological Development of
the Republic of Serbia.
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