Structural elucidation and bioassays of newly synthesized pentavalent antimony complexes

Article abstract of DOI:10.1016/j.jorganchem.2017.09.002

Antimony (V) organometallics (1–5) have been synthesized with general formula [SbR′₃(O₂CR)₂], where R' = phenyl, p-tolyl and O₂CR are substituted cinnamates. These complexes have been characterized by FT-IR analysis, multinuclear (¹H, ¹³C) NMR spectroscopy and single crystal X-ray diffraction analysis. The crystal structures of [Sb(phenyl)₃(4-ClC₆H₄C₂H₂O₂C)₂] (1) and [Sb(p-tol)₃(4-OCH₃C₆H₄C₂H₂O₂C)₂].CHCl₃ (5) displayed distorted geometry between trigonal bipyramidal and square pyramidal with monomeric structure at a five coordinated Sb center. Starting reagents and complexes were evaluated for anticancer, antileishmanial, antibacterial and alpha amylase inhibition potentials. It was observed that complexes 3, 4 and 5 showed significant (p < 0.05) antileishmanial and anticancer activities against Leishmania tropica KWH23 promastigotes and HepG2 cell lines respectively. Antibacterial activity of compound 3 was also significant against E. coli (MIC: 5.55 μg/mL), K. pneumoniae (MIC: 16.66 μg/mL), S. aureus (MIC: 5.55 μg/mL) and P. aeruginosa (MIC: 50 μg/mL). Hence, these new antimony complexes can act as good drug candidates.

Full text of DOI:10.1016/j.jorganchem.2017.09.002

Accepted Manuscript
Structural elucidation and bioassays of newly synthesized pentavalent antimony
complexes
Tuba Iftikhar, Muhammad Khawar Rauf, Sidra Sarwar, Amin Badshah, Durdana
Waseem, Muhammad Nawaz Tahir, Amjad Khan, Khalid Mohammad Khan, Gul Majid
Khan
PII:
S0022-328X(17)30513-2
10.1016/j.jorganchem.2017.09.002
JOM 20077
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 22 March 2017
Revised Date: 24 August 2017
Accepted Date: 2 September 2017
Please cite this article as: T. Iftikhar, M.K. Rauf, S. Sarwar, A. Badshah, D. Waseem, M.N. Tahir,
A. Khan, K.M. Khan, G.M. Khan, Structural elucidation and bioassays of newly synthesized
pentavalent antimony complexes, Journal of Organometallic Chemistry (2017), doi: 10.1016/
j.jorganchem.2017.09.002.
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ACCEPTED MANUSCRIPT
Structural elucidation and bioassays of newly synthesized pentavalent
antimony complexes
Tuba Iftikhar 1, Muhammad Khawar Rauf 1*, Sidra Sarwar1, Amin Badshah1*, Durdana Waseem2,
Muhammad Nawaz Tahir3, Amjad Khan2, Khalid Mohammad Khan4, Gul Majid Khan2
1Department of Chemistry, Quaid-i-Azam University, Islamabad-45320, Pakistan
2Department of Pharmacy, Faculty of Biological Sciences, Quaid-i-Azam University Islamabad-
45320, Pakistan
3Department of Physics, University of Sargodha, Pakistan
4 H.E.J. Research Institute, University of Karachi, Karachi-75270, Pakistan
*Corresponding Authors: Email: aminbadshah@yahoo.com (A. Badshah), Email:
mkhawarrauf@yahoo.co.uk (Muhammad Khawar Rauf) orcid.org/0000-0001-9994-4033
Abstract
Antimony (V) organometallics (1-5) have been synthesized with general formula
[SbR'3(O2CR)2], where R'= phenyl, p-tolyl and O2CR are substituted cinnamates. These
complexes have been characterized by FT-IR analysis, multinuclear (1H, 13C) NMR spectroscopy
and single crystal X-ray diffraction analysis. The crystal structures of [Sb(phenyl)3(4-
ClC6H4C2H2O2C)2] (1) and [Sb(p-tol)3(4-OCH3C6H4C2H2O2C)2].CHCl3 (5) displayed distorted
geometry between trigonal bipyramidal and square pyramidal with monomeric structure at a five
coordinated Sb center. Starting reagents and complexes were evaluated for anticancer,
antileishmanial, antibacterial and alpha amylase inhibition potentials. It was observed that
complexes 3, 4 and 5 showed significant (p<0.05) antileishmanial and anticancer activities
against Leishmania tropica KWH23 promastigotes and HepG2 cell lines respectively.
Antibacterial activity of compound 3 was also significant against E. coli (MIC: 5.55 µg/mL), K.
pneumoniae (MIC: 16.66 µg/mL), S. aureus (MIC: 5.55 µg/mL) and P. aeruginosa (MIC: 50
µg/mL). Hence, these new antimony complexes can act as good drug candidates.
Keywords:
Pentavalent antimony complexes, Antileishmanial, Anticancer, Single crystal XRD
1

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1. Introduction
Significance of antimony in modern medicinal chemistry cannot be denied. Although many
synthetic drugs were reported in the sixteenth and seventeenth centuries but there was a perpetual
debate over antimony compounds than any of the others [1]. A group of researchers had stated
the cosmetic use of antimony in prehistoric Egypt [2]. The most substantial medicinal practice of
organoantimony (V) complexes in the nineteenth century was surely in the cure of leishmaniasis
[3]. Nowadays, the use of antimony compounds in chemotherapy has also gained importance in
the treatment of cancer. Derivatives of organoantimony were also investigated for their
cytotoxicity against cancer cell lines [4-5]. It was found that organoantimony (III) and (V)
complexes are effective against cervical carcinoma and leukemia cell lines. It has been
investigated antimony complexes can target protein tyrosine phosphatases, induce apoptosis and
increase amount of reactive O2 species in cancer cells [6]. Organoantimony (V) compounds also
exhibited high antitumor activity [7].
About twenty years ago, numerous organoantimony compounds have been successfully
synthesized and tested in preclinical assays, which was assumed to be an effort to invent more
effective and less toxic antibacterial and antileishmanial medicines. In the last few decades
organoantimony (III) complexes were replaced by less toxic organoantimony (V) complexes,
like meglumine antimoniate, to cure all types of leishmaniasis [8]. Antibacterial and antifungal
activities of various antimony complexes have also been reported [9]. Extensive studies have
been made on organoantimony (V) complexes on the basis of their various bonding modes [10-
12] and biological applications [13-15]. Derivatives of organoantimony also show important
properties such as catalyst components, biocides, antioxidants and fungicides [16-17].
The synthesis of antimony complexes of type [SbR'3(O2CR)2] and to check their leishmanicidal
properties for the development of pentavalent antimonial drugs against leishmaniasis is a part of
our ongoing project [18-19]. Five organoantimony (V) carboxylates, with variety of cinnamate
ligands were synthesized, characterized and screened for their potential as antileishmanial
(Leishmania major promastigotes), anticancer (hepG2 cell line), antibacterial and alpha amylase
enzyme inhibition. Different substituents around the antimony center may lead to more
effectiveness as compared to the already present pentavalent antimonials.
2. Material and Methods
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Aromatic aldehydes like 4-chlorobenzaldehyde, 2-nitrobenzaldehyde, 4-methoxybenzaldehyde,
bromobenzene, magnesium turnings and 4-bromotoluene were obtained from Sigma Aldrich and
used without any further purification. Toluene, chloroform, dichloromethane,
dimethylsulphoxide and diethyl ether were distilled prior to their use according to standard
method [20]. BIO COTE Model SMP10 melting point apparatus was used for determination of
melting point and values are reported without correction. The multinuclear NMR spectra i.e. (1H
and 13C) were taken on Bruker 300 MHz spectrometer with tetramethylsilane (TMS) as a
reference. Elemental analyses were performed using a LECO-932 CHNS analyzer. FT-IR data
was recorded on a Bruker TENSOR II FT-IR Spectrometer.
2.1. General procedure for synthesis
Cinnamic acid derivatives (L1-L3) were synthesized by using reported method [21a]. In two
necked round bottom flask aldehyde (0.1 mol) and malonic acid (0.1 mol) were taken in 50 mL
pyridine and refluxed for two to three hours. Dilute HCl was added drop wise as reaction mixture
was cooled down. Precipitates formed were filtered off and washed with water. Recrystallization
was done with ethanol in order to get needle like crystalline material. The acids were converted
in to their sodium salts by treating ethanolic solution with aqueous NaOH in equimolar
quantities. Sodium salts of cinnamic acid derivatives (L1-L3) were separated by slow
evaporation at room temperature in crystalline form in good yield. Organoantimony (V)
dibromide precursors were synthesized by adapting the methodology as reported in literature
[21b]. Organoantimony (V) carboxylates were synthesized according to previously described
method with certain changes in solvent and thermal conditions [22]. In a 250 mL two necked
round bottom flask, a solution of sodium salt of the respective carboxylic acids (0.01 mol) in 50
ml of dry toluene were added to organoantimony (V) halide (0.005 mol) and stirred overnight.
Thin layer chromatography (TLC; n-hexane:chloroform in ratio 40:60) was performed in order to
check the completion of reaction. Precipitates of the salt were filtered and product was obtained
from filtrate by rotary evaporation of toluene. Product was kept for crystallization in
ethanol/chloroform mixture. The synthesized complexes are shown in Scheme 1.
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Scheme 1. Synthesis of acids (L1-L3) and complexes (1-5).
2.1.1. Bis[3-(4-chlorophenyl)acrylato]tri(phenyl)antimony (V) (1)
0.9 g (4.4 mmol) of sodium salt of 3-(4-chlorophenyl) acrylic acid and 1.126 g (2.2 mmol) of
triphenylantimony (V) dibromide were added in toluene.
Yield: 1.29 g, 82%, m.p.: 219-220°, FT-IR (cm-1): C-Haromatic (3056), C-Haliphatic (2927), C=O
(1647), Sb-C (563), Sb-O (453), 1H NMR (300 MHz, CDCl3), δ(ppm): 6.29-6.35 (d, 2H, 3J=15.9
Hz, OOC-CHe=CH-Ph), 7.29-7.45 (m, 10H, Ar-Hh, h´, i, i´, j& -CH=CHf-Ph), 7.51-7.55 (m, 9H, Ar-
Hc, c´, d), 8.08-8.12 (m, 6H, Ar-Hb, b´); 13C NMR (75 MHz, CDCl3), δ(ppm): 121.4 (2Cf), 129.0
(4Ch, h´), 129.1 (4Ci, i´), 129.4 (6Cc, c´), 131.2(3Cd), 133.4 (3Cg), 133.9 (6Cb, b´), 135.5 (4Cj),138.4
(2Ca(ipso carbon)), 142.0 (4Ce), 170.6 (2C, C=O). Elemental analyses calculated (found) for
C36H27Cl2O4Sb (%): C, 60.37 (60.35); H, 3.80 (3.79).
2.1.2. Bis[3-(2-nitrophenyl)acrylato]tri(phenyl)antimony (V) (2)
0.9 g (4.1 mmol) of sodium salt of 3-(2-nitroophenyl) acrylic acid and 1.024 g (2 mmol) of
triphenylantimony (V) dibromide were added in toluene.
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Yield: 1.21 g, 82%, m.p.: 221-222°, FT-IR (cm-1): C-Haromatic (3056), C-Haliphatic (2965), C=O
1
(1643), Sb-C (552), Sb-O (457), H NMR (300 MHz, CDCl3), δ(ppm): 6.24-6.30 (d, 2H,
3J=15.6Hz, OOC-CHe=CH-Ph), 7.48-7.62 (m, 15H, Ar-Hc, c´, d, h, i, j), 7.96-8.02 (m, 4H, Ar-Hi´& -
13
CH=CHf-Ph), 8.11-8.15 (m, 6H, Ar-Hb, b´); C NMR (75 MHz, CDCl3), δ(ppm): 124.8 (3Cd),
126.5 (2Cf, -CH=CfH-Ph), 128.9 (2Ci), 129.6 (6Cc, c´ ), 129.7 (2Ci), 130.9 (2Cj), 131.5 (3Ca),
133.3 (3Ci´), 133.9 (6Cb,b´), 137.0 (2Cg), 138.0 (2Ce, -CeH=CH-Ph), 148.2 (2Ch´, C-NO2), 169.1
(2C, C=O). Elemental analyses calculated (found) for C36H27N2O8Sb (%):C, 58.64 (58.62); H,
3.69 (3.68); N, 3.80 (3.78).
2.1.3. Bis[3-(4-chlorophenyl)acrylato]tris(p-tolyl)antimony (V) (3)
0.9 g (4.4 mmol) of sodium salt of 3-(4-chlorophenyl) acrylic acid and 1.22 g (2.2 mmol) tris(p-
tolyl)antimony (V) dibromide were added in toluene.
Yield: 1.33 g, 80%, Melting point: 165-168°, FT-IR (cm-1): C-Haromatic (3022), C-Haliphatic (2921),
1
C=O (1644), Sb-C (581), Sb-O (482), H NMR (300 MHz, CDCl3), δ(ppm): 2.40 (s, 9H, CH3),
3
6.29-6.34 (d, 2H, J=15.9Hz, OOC-CHe=CH-Ph), 7.29-7.42 (m, 16H, Ar-Hc, c´, h, h´, i, i´& -
3
13
CH=CHf-Ph), 7.95-7.98 (d, 6H, Ar-Hb, b´, J=8.4Hz); C NMR (75 MHz, CDCl3), δ(ppm): 21.5
(3C, CH3), 121.9 (2Cf, -CH=CfH-Ph), 128.9 (4Ch, h´), 129.0 (4Ci, i´), 130.1 (6Cc, c´), 133.5 (2Cg),
133.7 (6Cb, b´), 134.5 (3Cd), 135.3 (2Cj), 141.4(3Ca(ipso-carbon)) 141.6 (2Ce, -CeH=CH-Ph), 170.2
(2C, C=O). Elemental analyses calculated (found) for C39H33O4Sb (%): C, 61.77 (61.75); H, 4.39
(4.36).
2.1.4. Bis[3-(2-nitrophenyl)acrylato]tris(p-tolyl)antimony (V) (4)
0.9 g (4.1 mmol) of sodium salt of 3-(2-nitrophenyl) acrylic acid and 1.108 g (2 mmol) of tris(p-
tolyl)antimony (V) dibromide were added in toluene.
Yield=1.29 g(83%). m.p=188-189°. FT-IR (cm-1): C-Haromatic (2998), C-Haliphatic (2858), C=O
1
(1638), Sb-C (583), Sb-O (481). H NMR (300 MHz, CDCl3), δ(ppm): 2.42 (s, 9H, CH3), 6.24-
3
6.29 (d, 2H, J=15.9Hz, OOC-CHe=CH-Ph), 7.25-7.64 (m, 14H, Ar-Hc, c´, h, i, i´, j), 7.95-8.02 (m,
13
8H, Ar-Hb, b´ & -CH=CHf-Ph). CNMR (75 MHz, CDCl3), δ(ppm): 21.6 (3C, CH3), 124.8(2Ci),
126.9 (2Cf, -CH=CfH-Ph), 128.9 (2Cj), 130.0 (2Ch), 130.3 (6Cc, c´), 131.0 (2Ci´), 133.3 (3Cd),
133.8 (6Cb, b´), 135.2 (2Cg), 137.7(2Ce, -CeH=CH-Ph), 141.7 (3Ca(ipso-carbon)), 148.3 (2Ch´, C-
5

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NO2), 169.0 (2C. C=O). Elemental analyses calculated (found) for C39H33N2O8Sb (%): C, 60.10
(60.07); H, 4.27 (4.26); N, 3.59 (3.57).
2.1.5. Bis[3-(4-methoxyphenyl)acrylato]tris(p-tolyl)antimony (V) chloroform solvate (5)
0.9 g (4.4 mmol) of sodium salt of 3-(4-methoxyphenyl) acrylic acid and 1.218 g (2.2 mmol) of
tris(p-tolyl)antimony (V) dibromide were added in toluene.
Yield=1.23 g(82.5%). m.p=179-180°. FT-IR (cm-1): C-Haromatic (2958), C-Haliphatic (2894), C=O
(1657), Sb-C (559), Sb-O (484), 1H NMR (300 MHz, CDCl3), δ(ppm): 2.40 (s, 9H, CH3), 3.82 (s,
3
6H, OCH3), 6.21-6.26 (d, 2H, J=15.9Hz, OOC-CHe=CH-Ph), 6.85-6.88 (d, 4H, Ar-Hh,
,
h´
3
3J=8.7Hz), 7.31-7.34 (d, 6H, Ar-Hc, c´, J=8.1Hz), 7.38-7.46 (m, 6H, Ar-Hi, i´ & -CH=CHf-Ph),
7.97-7.99 (d, 6H, Ar-Hd, d´, 3J=8.1Hz). 13C NMR (75 MHz, CDCl3), δ(ppm): 21.5 (3C, CH3), 55.3
(2C, OCH3), 114.1 (4Ch,h´), 118.9 (2Cf, -CH=CfH-Ph), 127.7 (2Cg), 129.4 (4C i, i´), 130.0 (6Cc, c´),
133.7 (6Cb, b´), 135.0 (3Cd), 141.1 (3Ca(ipso-carbon)), 142.7 (2Ce, -CeH=CH-Ph), 160.79 (2C, C-
OCH3), 170.9 (2C, C=O). Elemental analyses calculated (found) for C41H39O6Sb.CHCl3(%): C,
58.06 (58.01); H, 4.64 (4.4.63).
2.2. Single crystal X-ray diffraction (XRD) analysis
Out of all compounds synthesized, complexes 1 and 5 were crystalline and their crystal structure
was obtained by Bruker Kappa APEXII CCD diffractometer using monochromatic MoKα (λ =
0.71073Å) radiations. The temperature was maintained at 296(2) K during data collection.
Structures of complexes were elucidated using SHELX 97 [23] with structure refinement done
by SHELXL 2014 [24].
2.3. Determination of antileishmanial activity
Antileishmanial activity of complexes was investigated by in vitro analysis using 2-(4,5-
dimethyl-2-thiazolyl)-3,5-diphenyl-bromide (MTT) colorimetric assay according to predefined
protocol [25]. Complete growth medium (pH 7.4) containing Medium 199 supplemented with
,
100
and
µg/mL) penicillin G (100
fetal bovine serum (FBS; 10%) streptomycin sulphate (
IU/mL) was used to grow Leishmania tropica kwh 23 promastigotes at 24°C. Refreshed culture
of promastigotes (180 µL) at seeding density of 2 × 106 promastigotes/mL was incubated with
compounds or amphotericin B (20 µL) at a final concentration of 20 µg/mL in 96 well plate at
24°C for 72 hours. Negative controls wells contained 1% DMSO in PBS. Afterwards, culture in
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each well was incubated again for 4 h at 24°C with pre-filter sterilized 4 mg/mL MTT solution
(20 µL). Subsequently, supernatant was removed leaving formazan crystals behind, which were
dissolved in DMSO (100 µL). Absorbance was measured using microplate reader (Biotech USA,
microplate reader Elx 800) at 540 nm. Similar procedure was followed using 3-fold serial
dilutions of samples and IC50 was calculated by linear regression.
2.4. Cytotoxicity against HepG2 cancer cell line
Cytotoxicity against human hepatocellular carcinoma cell line HepG2 was determined by
sulforhodamine B (SRB) assay [26]. Cells that reached to 60-70% confluence in Dulbecco's
Modified Eagle Medium (DMEM) containing 10% FBS, 100 IU/mL penicillin, 0.25 µg/mL
100
amphotericin B and
µg/mL streptomycin sulphate were trypsinized and adjusted to seeding
density of 1×105 cells/mL. About 180 µL of cell culture was incubated with 20 µL of compounds
or doxorubicin (20 µg/mL) in CO2 (5%) incubator (Panasonic, Japan MCO-18AC-PE) at 37ºC.
After 72 h of incubation, cells were fixed with 100 µL of 20% trichloroacetic acid, washed with
water and air-dried. It was stained with 0.057% SRB (0.4%) 1% in acetic acid for 30 min. Wells
were washed with 1% acetic acid, dried and SRB was solubilized in 10 mM Tris base (200 µL;
pH10). Absorbance was observed at 515 nm with a microplate reader and linear regression
analysis was used to calculate IC50 from data of 3-fold serial dilutions.
2.5. Evaluation of antibacterial activity
Antibacterial activity of compounds was investigated by disc diffusion method [27] against gram
positive and gram negative strains. Overnight refreshed cultures of Staphylococcus aureus
(ATCC-6538), Escherichia Coli (ATCC-15224), Klebsiella pneumonia (ATCC-1705) and
Pseudomonas aeruginosa (ATCC-15442) were used. Sterile filter paper discs were impregnated
with 5 µL of compounds (10 mg/mL in DMSO) or standards or DMSO. Roxithromycin and
cefixime (10 µg/disc) were used as standards. Impregnated discs were placed on sterile agar
plates swabbed with bacterial culture. These plates were incubated at 37°C for 24 h and diameter
(mm) of zone of inhibition (ZOI) was measured. Compounds showing ZOI ≥ 12 mm were
subjected to minimum inhibitory concentration (MIC) determination by microbroth dilution
method [28]. Nutrient broth seeded with bacterial inoculum at density of approximately 5 × 104
CFU/mL was incubated with 3-fold dilutions (50 µg/mL to 1.85 µg/mL) of compounds in 96-
well plate for 24 h at 37oC. Plate was visually analyzed for presence of turbidity and lowest
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concentration of compounds inhibiting the growth of bacterial strain (no turbidity) was
considered as MIC.
2.6. Evaluation of alpha amylase inhibition activity
Compounds were tested for their enzyme inhibition capacity against alpha amylase enzyme
according to previously described procedure with minor modification [29]. Sample, negative
control, positive control and blank wells were prepared in 96 well plate and observed at 540 nm
using microplate reader. Sample wells contained 15 µL of phosphate buffer (PB) (pH 6.8), 25 µL
of α-amylase enzyme (0.14 U/mL), 10 µL of compounds (4 mg/mL DMSO) and 40 µL of starch
solution (2 mg/mL in PB). Acarbose (250 µM) was used as positive control. Negative control
was prepared without compounds whereas blank was prepared without compounds and amylase
enzyme; each being replaced by equal quantities of buffer. After incubating the plate for 30 min
at 50°C, 20 µL of 1 M HCl was added in order to stop the reaction. Then 90 µL of iodine reagent
(5 mM iodine, 5 mM potassium iodide) was added to each well and activity was expressed as
percent α-amylase inhibition/mg compound and calculated from absorbance values by the
following equation:
% α-amylase inhibition = (Os - On) / (Ob - On) × 100
Where On = Absorbance of negative control, Os = Absorbance of sample and Ob = Absorbance
of blank well.
2.7. Statistical Analysis
In the present study, data was calculated as mean±SD of respective parameters. Means were
compared by ANOVA using GraphPad Prism 5 software.
3. Results and Discussion
The structures of organoantimony (V) complexes were confirmed by FT-IR, NMR spectroscopy
and single crystal X-ray diffraction.
3.1. Characterization
In FT-IR spectra of synthesized complexes, a signal at 3500-3300 cm-1 due to OH stretching is
absent which indicates the formation of Sb-O bond in the organoantimony (V) carboxylates. The
carboxylate ligands act as monodentate as the difference of ∆ν value of (νCO2 asymm) and (νCO2
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symm) is lower than 200 cm−1. After bonding to antimony, the carbonyl group (C=O) stretching
frequency show a valuable shift due to the electron resonance that induces a single bond character
to the carbonyl group and hence lowers the frequency. The characteristic IR bands of
organoantimony (V) carboxylates includes, 3056-2958 cm-1 due to aromatic (C-H), 2965-2858
aliphatic (C-H), 1657-1638 cm-1 (C=O), 1587-1580 cm-1 (C-N), 735-740 cm-1 (C-Cl), 583-552
cm-1 (Sb-C), 484-453 cm-1 (Sb-O). Signals in the lower infra-red region (583-552 cm-1 and 484-
453 cm-1) are absent in the spectra of starting carboxylates and are according to reported values
[30-31].
1H-NMR spectra of free ligands showed singlet around 12.23 ppm due to carboxylic acid
(-COOH) proton, which was not found in the spectra of the synthesized complexes indicating the
removal of acid group (-proton from carboxylic COOH) and binding of carboxylate ligand to
antimony. Intensity and multiplicity patterns were used to identify all the chemical shifts [38].
Signals due the aromatic protons were visible in the region of 7.19-8.53 ppm. The aromatic
1
signals in the H NMR spectra of the complexes varied depending on the type of cinnamic acid
used as the starting material. The splitting pattern of the synthesized phenyl and p-tolyl
derivatives of antimony complexes consisted of singlets, doublets and multiplets, which gave
information about neighboring protons. The methoxy proton appeared as a high intensity singlet
at 3.82 ppm. The signal of methyl protons observed between 2.40 to 2.68 ppm.
The 13C NMR data showed the signals due to different types of carbon atoms in the
compounds. In the spectra of all the complexes (1-5), the carbonyl carbon atoms appeared in the
range of 169.0-171.3 ppm. Aromatic carbon signals of antimony (V) complexes were assigned on
the basis of intensities of signal and comparison of chemical shift with the reported values [18,
30, 31]. Carbon atoms of the aromatic ring were visible in the region 114.1-160.1 ppm. The
methyl carbon atoms were appeared between 21.5-21.6 ppm. The methoxy carbon appeared at
55.3 ppm.
3.2. X-ray Crystallography
Crystallization of the products was done in chloroform-toluene mixture. Colorless prism-like
crystals of complexes 1 and 5 were obtained, which were analyzed by X-ray diffraction
technique. Molecular structures of these complexes are shown in Fig. 1 & 3. Crystallographic
data summery is shown in Table 1. Selected bond lengths and bond angles are given in Table 2.
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Fig. 1. Molecular structure of 1. Hydrogen atoms are represented as circles of arbitrary radii.
Fig. 2. Packing view of 1 along c-axis showing intermolecular (C-H…O) hydrogen bonding.
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Fig. 3. Molecular structure of 5. Hydrogen atoms are represented as circles of arbitrary radii.
CHCl3 solvent is also shown in lattice.
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Fig. 4. Packing view of (5) along c-axis showing intermolecular (C-H…O) hydrogen bonding.
Crystal structures of complexes (1) and (5) displayed distorted geometry between trigonal
bipyramidal and square pyramidal with monomeric structure at a five coordinated Sb center.
Two ligand molecules are covalently bonded with antimony through the oxygen atom at axial
positions, while three p-tolyl/phenyl groups are at equatorial positions. Two of the aromatic rings
(phenyl/p-tolyl) constituting a wider angle at antimony centre C19-Sb1-C25=146.60° (1) and
C21-Sb1-C28=155.67° (5) are oriented in the same plane while the third one is twisted at
somewhat right angle. The two non-bonded oxygen atoms of the carboxylate groups are also
bisecting the C-Sb-C angles around the antimony centre and causing a considerable distortion in
C-Sb-C angle i.e. 120°, typical for ideal trigonal planar geometries. At the equatorial position of
the molecule, summation of all the angles is almost 360.0°. Bond lengths of these complexes are
comparable to the respective bond lengths of [SbPh3(O2CCF3)]2O i.e. [2.100(6)–2.112(6) Å] [18,
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32] Sb–O distances are 2.1088(16) Å and 2.1298(16) Å indicating the covalent mode of bonding
between them, as their bond lengths are shorter than (3.6 Å) which is the sum total of the
antimony and oxygen atoms van der Waals radii [30-31, 33-34].
In the crystal packing of 1 (Fig. 3), the molecular structures have been stabilized by
intermolecular C(11)-H(11)···O(4) 2.640 Å and C(17)-H(17)···O(2) 2.666 Å hydrogen bonds.
These molecular structures also fashioned the centrosymmetric dimers {···H-C-C=O···
}
2
connected by C–H···O (2.640 Å). The crystal packing of 5 (Fig. 4) shows only intermolecular
interactions between the chloroform C-H and C=O.
Table 1. Crystallographic data and structure refinement for complexes 1 and 5.
1
5
Empirical formula
Formula weight
Temperature/K
Crystal system
Space group
a/Å
C36H27Cl2O4Sb
716.22
C41H39O6Sb.CHCl3
868.84
296(2)
296(2)
Triclinic
Triclinic
P-1
P-1
8.9509(10)
12.5624(15)
15.9537 (17)
109.587
10.8119(12)
12.6981(15)
14.5631(17)
85.769(5)
82.265(5)
86.316(5)
1972.8(4)
2
b/Å
c/Å
α
/°
/°
/°
β
104.397(4)
95.536(4)
1604.9(3)
2
γ
Volume/Å3
Z
ρcalcg/cm3
F(000)
1.482
1.463
720.0
884.0
Crystal size/mm3
0.40×0.36×0.32
4.784 to 54
24387
0.38×0.32×0.22
5.032 to 54
30656
2Θ range for data collection/°
Reflections collected
Independent reflections
6968 [Rint =0.0313,
8501 [Rint = 0.0272,
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Rsigma = 0.0271]
6968/0/338
Rsigma = 0.0247]
Data/restraints/parameters
Goodness-of-fit on F2
8501/0/474
1.038
1.059
Final R indexes [I>=2
σ
(I)]
R1 = 0.0229, wR2 = 0.0562
R1=0.0266, wR2 = 0.0588
R1 = 0.0258, wR2 = 0.0617
R1 = 0.0306, wR2 = 0.0649
Final R indexes [all data]
Largest diff. peak/hole / e Å-3
0.25/-0.40
0.38/-0.44
Table 2. Selected geometric parameters; bond lengths (Å) and bond angles (°) for 1 and 5.
1
Sb1-C19
Sb1-C25
Sb1-C31
Sb1-O3
Sb1-O1
O1-C1
2.1142(18)
2.1187(18)
2.1237(18)
2.1194(13)
2.1257(14)
1.301(3)
C19-Sb1-C25
C19-Sb1-C31
C25-Sb1-C31
C19-Sb1-O3
C25-Sb1-O3
O3-Sb1-C31
C19-Sb1-O1
C25-Sb1-O1
C31-Sb1-O1
146.58(8)
107.18(7)
106.24(7)
91.46(6)
90.02(6)
88.54(7)
89.17(6)
91.25(6)
88.15(7)
O2-C1
1.221(3)
O3-C10
O4-C10
1.318(2)
1.231(2)
5
Sb1-C28
Sb1-C21
Sb1-C35
Sb1-O1
Sb1-O4
O1-C1
2.1129(19)
2.119(2)
C28-Sb1-C21
C28-Sb1-C35
C21-Sb1-C35
C28-Sb1-O1
C21-Sb1-O1
C35-Sb1-O1
C28-Sb1-O4
155.67(8)
103.53(8)
100.80(8)
90.42(6)
90.27(7)
88.24(7)
89.39(6)
2.1215(19)
2.1429(14)
2.1517(14)
1.292(2)
O2-C1
1.232(3)
14

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O4-C11
O5-C11
1.304(3)
1.226(3)
C21-Sb1-O4
C35-Sb1-O4
91.01(7)
89.20(7
3.3. Biological Activities
Results of bioassays showed that free acids (L1-L3) displayed negligible activities, but
pentavalent antimony complexes (1-5) of these acids showed considerable activities.
3.3.1. Antileishmanial and antibacterial activities
Promastigotes stage is preferred for in vitro evaluation of leishmanicidal properties of
compounds, as it is easy to culture and screening is done in a short time period [35]. Results of
antileishmanial activity of acids precursors (L1-L3) and corresponding antimony (V) complexes
(1–5) are shown in Table 3. In this work, MTT colorimetric assay was performed in order to
check the antileishmanial properties of the compounds. The mitochondrial dehydrogenase
enzyme of living cells causes the cleavage of the pale yellow tetrazolium ring of MTT into
purple-blue formazan crystals, which are impermeable to cell membrane so, the number of viable
cells can be reflected in this way. The results of the acid precursors (L1-L3) that are not bonded
to antimony were poor relative to antimony (V) complexes, which revealed very good activities.
The complexes 3, 4 and 5 showed slightly (p>0.05) different results from amphotericin B
indicating that their efficacy can be compared to it. The results depicted that the structure of the
compound has strong influence on the activity as the complexes containing methyl (CH3) group
at para position exhibited excellent antileishmanial activity than the others. Since, the charge on
metal decreases on complexation with donor groups so liphophilicity of compounds increases;
thus, causing destruction of lipid membrane of microbe [36]. It is proposed that this activity
might be due to inhibition of kinase enzymes or any other pathway, which interfere the growth of
promastigotes. Advanced studies are in progress to find out the mechanism of action of these
synthesized compounds. Since, these compounds are highly active against leishmaniasis;
therefore, there is possibility to develop new and more effective drug for the treatment of
leishmaniasis.
Antibacterial activity was performed against four strains of bacteria (Escherichia coli, P.
aeruginosa, S. aureus and K. pneumoniae). Our compounds showed variable activity to different
bacterial strains with significant antibacterial activity produced by compound 3 against all test
15

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strains. Acid precursors (L1-L3) did not display any considerable activity against bacterial
strains. Complexes with methyl groups showed better activity that may be due to enhanced
membrane permeability. Antibacterial effect of compounds might have been triggered by
interaction with cell wall, enzymes or proteins of bacteria.
3.3.2. Cytotoxicity against cancer cell line
Phenyl and tolyl antimony (V) complexes showed significant cytotoxicity against HepG2 cell
line. Table 3 shows the results for anticancer activity of starting acids (L1-L3) and synthesized
antimony (V) complexes (1-5). Free acids depicted no activity, phenyl derivatives of antimony
(V) 1 and 2 showed moderate activities, whereas p-tolyl derivatives of antimony (V) 3, 4 and 5
exhibited excellent activity. Increased number of methyl groups in complexes 3, 4 and 5
enhanced the cell membrane permeability of the compounds; hence, causing more cytotoxicity.
Our results are consistent with previous studies where different derivatives of antimony (V)
possess anticancer potential [37]. This indicates significant potential of antimony carboxylates as
drug candidates to treat cancer.
Table 3. Anticancer and antileishmanial activities of the ligands (L1-L3) and their complexes (1-
5).
Anticancer activity
% inhibition
Antileishmanial activity
Compounds
IC50
% Mortality
(20 µg/mL)
IC50
µg/mL
µ
g/mL
(20 µg/mL)
11.6 ± 0.75
9.6± 0.34
7.5 ± 0.01
48.5± 1.20
44.9± 1.15
99.5 ± 1.82*
99.6 ± 2.67*
98.5 ± 0.05*
NA
L1
-
-
-
-
-
-
0.0 ± 0.0
0.0 ± 0.0
L2
-
L3
-
0.0 ± 0.0
48.1 ± 1.57
1
-
35.6 ± 1.15
96.7 ± 1.75*
98.2 ± 1.83*
2
-
6.19
1.58
2.05
0.03
NA
0.01
3
4.61
4
100.00 ± 2.69* 5.36
5
Doxorubicin
100 ± 0.01*
0.05
NA
Amphotericin B NA
100 ± 0.01*
Data is mean±SD; n=3; * means p<0.05, - means no activity; Na = not applied.
16

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3.3.3. Alpha amylase inhibition activity
Results for enzyme inhibition assay are shown in Table 4. Maximum inhibition by our
compounds was 13.51 % with 200 ( g/mL), which is negligible relative to standard acarbose
µ
(94.6%). Alpha amylase is carbohydrate degrading enzyme that plays important role in
maintaining plasma glucose concentration [38]. However, our results are not significant in this
aspect.
Table 4. Antibacterial and alpha amylase inhibition activities of the ligands (L1-L3) and their
complexes (1-5).
Antibacterial activity ZOI (mm) 50 µg/disc
α-amylase
Compound
inhibition (%)
MIC
MIC
MIC
MIC
K.
P.
S. aureus
E. coli
200 µg /ml
µg/ml
µgml
µg/ml
µg/ml
pneumoniae
aeruginosa
8 ± 1.24
11 ± 1.97
8 ± 1.24
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
0.72 ± 0.005
1.68 ± 0.006
1.97 ± 0.007
4.8 ± 0.015
7.6 ± 0.079
9.42 ± 0.96
L1
L2
L3
1
-
-
-
13 ± 0.15* 1.85
-
-
-
-
-
-
-
-
-
8 ± 0.07
-
-
-
-
2
15 ± 0.17* 5.55
19 ± 0.71* 5.55
13± 0.75*
16.66
16 ± 0.75*
50
3
-
-
-
-
-
-
21 ± 0.93*
50
-
16 ±0.75*
15 ± 0.83*
50
5.63 ± 0.045
13.51 ± 0.75
4
5.55
-
16.66
5
13 ± 0.5*
Cefixime
(10 µg/disc)
Roxithromycin
(10 µg/disc)
Acarbose
(250 µM)
1.24*
1.11
NA
NA
NA
NA
NA
NA
3.33
NA
29 ±1.23*
3.33
NA
NA
NA
NA
NA
30 ± 0.25* 0.33
32 ± 0.75*
NA
NA
94.6 ± 1.84
NA
NA
NA
NA
(IC50:4.5 µg/mL)
NA = not applied, (-) indicates no activity, ZOI = zone of inhibition, MIC = minimum inhibitory concentration.
4. Conclusions
Organoantimony (V) complexes of cinnamic acid derivatives were successfully synthesized and
confirmed by FT-IR, NMR and XRD analysis. Crystal structures of complexes (1) and (5)
revealed that these complexes exhibited distorted trigonal bipyramidal geometry. In solid state,
these complexes are monomeric with Sb (V). Two carboxylate ligands attached to antimony has
17

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unidentate binding mode, which is also confirmed by FT-IR data. Compounds possessed
significant antileishmanial and anticancer activities that were comparable with standards
amphotericin B and doxorubicin respectively. Result showed high potential para substitution of
methyl in tolyl derivatives. Hence, pentavalent antimony complexes, particularly p-tolyl
derivatives seem to be worthy candidates for further investigations for therapy of leishmaniasis
and cancer.
Acknowledgements
Authors are highly thankful to Quaid-i-Azam University, Islamabad and the Higher Education
Commission of Pakistan (Research Project no. 20-1826/NRPU/R&D) for financial support to
carry out this work.
Supplementary material
The supplementary crystallographic data for this paper can be obtained from The Cambridge
Crystallographic Data Centre by www.ccdc.cam.ac.uk/data_request/cif. CCDC numbers of the
crystals are 1501847 and 1501848.
Conflict of interest
There is no conflict of interest in this study.
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Highlights
►
►
►
►
The antimonials were investigated by CHN, FT-IR, NMR & single crystal XRD analysis
The pentavalent antimonials (1-5) exhibit a multitude of medicinal properties
Leishmanicidal activity assessed against the Promastigotes stage of Leishmania parasite
The IC50 values were comparable with the standard drugs used
►p-Tolyl antimonials have shown significant antileishmanial and anticancer activities