Synthesis, X-Ray Structure and In Vitro Cytotoxic Activity of a New Dinuclear Cobalt(III) Complex with Diazine Ligands

Article abstract of DOI:10.1007/s11094-018-1711-y

A new dinuclear cobalt(III) compound [Co₂ (2,2′-azinodimethyldiphenol)₃] has been synthesized, which shows better cytotoxic activity than carboplatin and exhibits potent in vitro antiproliferative activity.

Full text of DOI:10.1007/s11094-018-1711-y

DOI 10.1007/s11094-018-1711-y
Pharmaceutical Chemistry Journal, Vol. 51, No. 10, January, 2018 (Russian Original Vol. 51, No. 10, October, 2017)
SYNTHESIS, X-RAY STRUCTURE AND IN VITRO
CYTOTOXIC ACTIVITY OF A NEW DINUCLEAR
COBALT(III) COMPLEX WITH DIAZINE LIGANDS
Fan-fang Li,1 Hong-Yan Chen,1 Yi Jin,1 Qi-Hua Zhao,1 Ming-Jin Xie1,*
Original article submitted September 13, 2016.
A new dinuclear cobalt(III) compound [Co2(2,2¢-azinodimethyldiphenol)3] has been synthesized, which
shows better cytotoxic activity than carboplatin and exhibits potent in vitro antiproliferative activity.
Keywords: triple helical complexes; antitumor activity
1. INTRODUCTION
ing cellular uptake and a cycle of reduction and reoxidation.
Moreover, the discovery that certain cobalt(III) Schiff base
complexes are potent antiviral agents prompted us to initiate
an investigation of Co(III) interactions with proteins and nu-
cleic acids [12 – 14]. In addition, complexes with Schiff’
base ligands proved to be highly efficient chemo-sensitizers
in combination with cisplatin, radiation, or hyperthermia.
Their modifying effect on cancer treatment was also ob-
served in vivo on rats [15, 16]. Therefore, we tried to design
cobalt salen complexes with high cytotoxicity and selectivity
against tumor cells. Here, we report the synthesis, x-ray
Cisplatin and its analogs have been successfully used as
metal-based chemotherapeutic drugs, which is a milestone in
the application of metals in medicine [1]. Recently, the study
of helical coordination complexes or helicates has become an
area of intense research activity due to their potential appli-
cation in enantioselective processes, developing new optical
devices and magnetic materials, probing DNA structures,
and understanding helical self-organization operative in the
nature [2, 3]. On the other hand, acetylenehexacarbonyl
dicobalt complexes were identified as a new class of anti-
tumor agents [2, 3]. R. Gust and co-workers [4 – 7] demon-
strated strong dependence of the antitumor activity as well as
tumor selectivity on the kind of alkyne ligand. Hexacarbonyl
dicobalt complexes with alkyne ligands (cobalt-alkyne com-
plexes) are now accepted as a new class of cytotoxics. These
compounds produced obvious cytotoxic effects on a variety
of human tumor cell lines [4 – 7]. It was of particular impor-
tance that some of these complexes were most effective in
human breast cancer cells.
structure, and cytotoxic activity of [Co (2,2¢-azinodimethyl-
diphenol) ] (1), where the ligand acts as a bidentate group.
2
3
2. RESULTS AND DISCUSSION
2.1. Chemistry
Synthesis of complex 1 is outlined in Scheme 1. Ligand
2,2¢-azinodimethyldiphenol (L) was prepared using reported
methods [17]. Co(C H COO) × 4H O was added dropwise
2
5
2
2
to dimethylformamide solution of the ligand. The quantita-
tive reaction with ligand L yielded the final product, which
was then recrystallized from methenyl trichloride to obtain
the sample for structural characterization and biological
tests. Structural characterization was performed by elemental
analysis, FT-IR spectroscopy, and x-ray crystallography. The
data are in good agreement with the proposed structure of
complex 1.
Another very interesting class of potential cytotoxic
drugs represent cobalt salen complexes (salen is 1,6-bis(2-
hydroxyphenyl)-2,5-diazahexa-1,5-diene) mainly developed
as reversible oxygen carriers and catalysts for oxidation reac-
tions [8]. However, metal complexes can also serve as
prodrugs, primarily with a cytotoxic agent bound to the metal
ion that effectively “chaperones” the drug to the site of action
[9 – 11]. In principle, Co(III) complex provides an inert
framework for transportation of the cytotoxic mustard allow-
1
The structure of compound 1, as determined from sin-
gle-crystal x-ray diffraction studies, is illustrated in Fig. 1.
The complex is triclinic with Pî space group in the crystal
School of Chemical Science and Technology, Yunnan University,
Kunming 650091, People’s Republic of China.
*
e-mail: mjxie@ynu.edu.cn
889
0091-150X/17/5110-0889 © 2017 Springer Science+Business Media, LLC

890
Fan-fang Li et al.
Scheme 1. Synthesis of complex 1
Fig. 1. Structure of molecule 1 with the atomic labelling scheme.
Key bond lengths (Å) and angles (°) include the following:
Co(1)-O(2) 1.875(6), Co(1)-O(1) 1.900(6), Co(1)-N(1)1.905(7),
Co(1)-N(2) 1.919(7), Co(2)-O(5)1.882(6), Co(2)-N(3) 1.916(7);
O(2)-Co(1)-O(3) 86.6(3), O(2)-Co(1)-N(1) 92.5(3),
N(4)-Co(2)-O(4) 89.1(3).
complex. The octahedral coordination polyhedron is slightly
distorted, where all angles around the cobalt center deviate
significantly from 90°. The average Co–N distance is
1.913 Å and Co–O distance is 1.886 Å, while the N–Co–N
and O–Co–O angles are 86.6° and 92.5°, respectively. The
CoIII–N bond lengths are in good agreement with the re-
ported values for pyridine amide complexes of trivalent co-
balt [18 – 21]. The values of Co-O and Co-N bond lengths in
the ligand equatorial plane are 1.881(4) and 1.892(1) Å, re-
spectively. The distances between the cobalt atom and two
axial nitrogen donor atoms differ only slightly (1.960(6) and
1.951(6) Å) and compare well with the Co-N distances found
in cobalt-ammine complexes. The O-Co-N angles in the
equatorial plane are larger than 90° (94.06°), and the O-Co-O
and N-Co-N angles smaller than 90° (84.3 and 87.6°, respec-
tively) [22 – 25]. Though several reports of biimidazole co-
balt complexes exhibiting hydrogen bonding in their crystal
packing are available [26, 27], crystal packing in the present
case does not reveal the presence of any hydrogen bonded
bridges.
system and three ligands L coordinated to two cobalt ions in
grossly octahedral geometry. In essence, in both complexes
three pyridine nitrogen and three deprotonated amide nitro-
gen atoms of L provide six coordination sites around a
pseudo-octahedral Co(III) center. Crystal structure analysis
confirms the expected octahedral coordination geometry of
the complex. The tetradentate Schiff base ligand (ONNO
ligand) coordinates the cobalt atom in a planar fashion with
two ammonia molecules occupying axial positions in the
Complex 1 is luminescent in the solid state at room tem-
perature (Fig. 2), displaying a high-energy structured emis-
sion at 618 nm.
2.2. In vitro Cytotoxic Activity
In vitro cytotoxic activities of complex 1 and carboplatin
as a control were assessed using the standard MTT assay
[28, 29] using human tumor cell lines. The results are sum-
marized in Table 1, where GI is the drug concentration that
50
inhibits the growth of cancer cells by 50%. The values of
GI showed that complex 1 had cytotoxic activity compara-
50
ble to that of carboplatin against HepG2 and NCI-H460 can-
cer cells. However, for the selected human hepatocarcinoma
cell line, it was about 5 times less cytotoxic than carboplatin
Fig. 2. Excitation and emission spectra of complex 1 in solid state
at room temperature.

Synthesis, X-Ray Structure and In Vitro Cytotoxic Activity
891
on the basis of GI values. The concentrations required for
50
50% inhibition (GI ) of these tumor cells are less than
50
10 mM. More importantly, complex 1 is more active than
carboplatin in human hepatocarcinoma cell line tested. Data
are presented as GI values with their 95% confidence inter-
50
vals (CI 95%) obtained by nonlinear regression.
Experimental procedure. Potential cytotoxicity was eval-
uated against an in vitro panel of HepG2 tumor cells. The
compound was predissolved in DMSO (20 mM) and diluted
with cell culture medium to six required concentrations (20,
10, 2, 1, 0.2, and 0.1 mM). The content of DMSO at the final
concentration did not exceed 0.1%. At this concentration,
DMSO was found to be nontoxic to the cells tested. All cells
were cultured in RPMI 1640 supplemented with 10% fetal
bovine serum, 100 IU/mL penicilin and 100 mg/ mL strepto-
mycin, at 37° in a humidified atmosphere with 5% CO . The
2
cells were seeded at a density of 5 ´ 104 cells per well in
96-well microplates. After 24 h incubation, the cells were
treated with a serial concentration of the test compound. The
cells were exposed to drugs for 72 h. Cell growth was moni-
tored using Sulforhodamine B (SRB) assay. The optical den-
sity (OD) was read at 490 nm. All cytotoxicity tests were
performed two times in quadruplicate.
Fig. 3. Cleavage of pBR322 DNA (0.4 mg) by complex 1 at various
concentrations in the absence of reducing agents, for an incubation
time of 3 h in a buffer containing 10 mM Tris-EDTA/1mM
Na2EDTA (pH 7.5) at 37°. Line 1: DNA control; lines 2 – 6: 1080
(1000 – 1120), 810 (700 – 880), 540 (420 – 600), 270 (100 – 380),
and 135 (100 – 180) mM. (Data in 95% confidence intervals (CIs)
for concentration).
2.3. Acute Toxicity Studies in Mice
The toxicity was preliminarily evaluated by lethal doses
of the compounds in mice [30]. As is seen from Table 2,
LD (dose that causes 10% death of animals) and LD
10
50
(dose that causes 50% death) of complex 1 were found to be
189.4 and 218.6 mg/kg, respectively, which are much larger
than the values for carboplatin (LD = 121.3 mg/kg; LD
As the concentration of 1 is increased, form II plasmid DNA
is gradually converted into form I. These results indicate that
the cobalt(III) complex 1 plays a role in DNA cleavage, and
the degradation activity of the complex is dependent on con-
centration.
=
50
10
142.0 mg/kg), indicating that complex 1 is less toxic than
carboplatin in mice (healthy ICR mice of both sex weighting
18 – 22 g, were divided into 5 groups of 10 animals matched
for weight and size), following intravenous administration.
The concentration-dependent DNA cleavage by complex
1 was studied. Figure 3 shows the results of gel-electropho-
retic separations of plasmid pBR322 DNA induced by in-
creasing concentrations of complex 1 in the absence of exter-
nal agents at pH 7.5 (10 mM Tris-HCl/NaCl buffer) and 37°.
Mechanistic studies of the antitumour activity of com-
plex 1 are in progress.
ACKNOWLEDGEMENTS
We thank the National Natural Science Foundation of
China (grant Nos. 21161022 and 21061016) and the Yunnan
TABLE 1. Cytotoxic Activity of Complex 1 and Carboplatin
against HepG2 Tumor Cells
TABLE 2. Acute Toxicity of Complex 1 and Carboplatin in Mice
GI50
Max Concentration
Min Response
(%)
Lethal dose [mg/kg]
Object
(mg/mL)
(mg/mL)
LD10
LD50
Complex 1
0.35
44.52
10.00
Complex 1
189.4
218.6
(0.01 – 0.5)
(30.00 – 50.0)
(170.0 – 210.8)
(200.0 – 230.4)
Carboplatin
1.82
2.79
10.00
Carboplatin
121.3
142.0
(1.00 – 3.0)
(1.00 – 4.0)
(110.0 – 128.6)
(120.0 – 150.8)
HepG2 = Human hepatocarcinoma cell line. Data are presented as
Data are presented as GI values and 95% confidence intervals
50
GI values and 95% confidence intervals (CIs) for HepG2 tumor
50
cells.
(CIs) for LD and LD .
10
50

892
Fan-fang Li et al.
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