Acta Poloniae Pharmaceutica ñ Drug Research, Vol. 75 No. 1 pp. 265ñ271, 2018
ISSN 0001-6837 Polish Pharmaceutical Society
SYNTHESIS, CHARACTERIZATION AND IN VITRO BIOLOGICAL SCREENING OF TRIPHENYLTIN DERIVATIVES OF ISONIAZID HYDRAZONE MOHSIN ABBAS KHAN1*, SAMAR AKHTAR2, KHADIJA SHAHID2, SYED AUN MUHAMMAD3, AMARA MUMTAZ4, MUHAMMAD ARFAT YAMEEN5 and NIGHAT FATIMA5 Department of Pharmacy, Islamia University of Bahawalpur, Pakistan 2 Riphah Institute of Pharmaceutical Sciences Islamabad, Pakistan 3 Institute of Molecular Biology and Biotechnology, Bahauddin Zakariya University, Multan, Pakistan 4 Department of Chemistry, 5Department of Pharmacy, COMSATS Institute of Information Technology, Abbottabad Pakistan 1
Keywords: organotin, triphenyltin chloride, isonicotinic acid hydrazide, antibacterial, antifungal
lations (10-12). In addition to their anti-tumor activities, organotin (IV) complexes with Schiffís bases present an interesting variety of structural possibilities (13). Due to their antibacterial, antifungal, and anticancer potential, organotin compounds current study is designed to synthesize organotin derivatives and their biological evaluation.
Schiffís bases are condensation products of primary amines and aldehydes or ketones (RCH=NRí) where R and Rí are the alkyl or aryl substituentís. These have biological activity such as antibiotics, antiviral and antitumor activities (1, 2). Heteronuclear Schiffís bases have been recently used in applications as magnetic materials, catalysts and in biological engineering (3). The remarkable biological activity of acid hydrazides R-CO-NH-NH2, a class of Schiffís base, their corresponding aroylhydrazones, R-CO-NHN=CH-Rí and the dependence of their mode of chelation with transition metal ions have been known for significant biological interest in the past (4, 5). Schiffís bases form a significant class of compounds in medicinal and pharmaceutical chemistry with several biological applications that include antibacterial, antifungal and antitumor activities (6, 7). Organotin (compounds based on tin with hydrocarbon substituents) and their complexes have been used as active components in a number of biocidal formulations, such as fungicides, miticides, molluscicides, marine antifouling paints, surface disinfectants and wood preservatives (8, 9). The biological importance of organotins has been supported by studies concentrating on structure-activity corre-
EXPERIMENTAL Synthesis of compounds All chemical were purchased from Sigma Aldrich and were of analytical grade. Melting points were recorded using capillary tubes on a digital Gallenkamp apparatus. FT-IR spectra were recorded on Bruker FTIR (4000-400 cm-1), 1H NMR and 13C NMR spectra were measured on Bruker 400 MHZ spectrometer using deuterated DMSO as a solvent. General procedure Synthesis of hydrazones of isonicotinic acid hydrazide Synthesis of hydrazones (1-5) was carried out by the condensation of isonicotinic acid hydrazide with equimolar amount of substituted aromatic aldehydes or ketones in the presence of acetic acid as catalyst. The reaction mixture was refluxed for 1 h.
* Corresponding author: e-mail:
[email protected]
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Completion of reaction was checked by TLC. Upon completion, the precipitates obtained were filtered and dried (14). Synthesis of triphenyltin complexes of hydrazones of isonicotinic acid hydrazide For the synthesis of triphenyl tin complexes, hydrazones of isonicotinic acid (1-5) (1 mM) were treated with triethylamine Et3N (1 mM) in toluene and refluxed for 3 h. Then, a solution of triphenyltin chloride (1 mM) in dry toluene was added slowly with constant stirring and the reaction mixture was further refluxed for 3 h. The solvent was evaporated using rotary apparatus. The residue was recrystallized from CHCl3 (15). Ligand 1 (Hydrazone of isoniazid and benzaldehyde) Yield 66%, m.p. 206OC, m.w. 225.25. FT-IR (KBr, cm-1): 3200 (NH), 3024 (CH), 1682 (C=O), 1625 (C=N), 1470 (CH=CH), 1148 (C-N). 1H NMR (DMSO-d6, δ, ppm): 8.70-8.73 (d, -C5H4N), 7.96 (s, =CH-), 7.90 (s -NH-CO). 7.55-7.65 (d, -C5H4N), 7.23-7.31 (m, -C6H5), 7.11-7.15 (m, -C6H5), 13C NMR (DMSO-d6, δ, ppm): 157.8 (C-6), 147.6 (C-3/4), 141.1 (C-7), 138.8 (C-1), 131.5 (C-8), 130.9 (C-11), 128.1 (C-9/13), 127.5 (C-10/12), 119.4 (C-2/5). Analysis: calcd. for C13H11N3O: C, 69.32; H, 4.92; N, 18.66%; found: C, 69.25; H, 4.96; N, 18.57%. Ligand 2 (Hydrazone of isoniazid and salicylaldehyde) Yield 52%, m.p. 240OC, m.w. 243.25. FT-IR (KBr, cm-1): 3283 (NH), 3002 (CH), 1680 (C=O), 1612 (C=N), 1489 (CH=CH), 1158 (C-N), 1272 (CO). 1H NMR (DMSO-d6, δ, ppm): 8.6-8.65 (d, -C5H4N), 8.01 (-NH-CO), 7.43-7.49 (d, -C5H4N), 7.25-7.31 (d, -C6H4OH), 7.11 (m, -C6H4OH), 7.05 (d, -C6H4OH), 5.5 (s, -OH), 13C NMR (DMSO-d6, δ, ppm): 154.9 (C-6), 153.7 (C-9), 148.4 (C-3/4), 142.1 (C-7), 137.6 (C-1), 132.7 (C-8), 131.8 (C-11), 129.2 (C-13), 120.6 (C-12), 118.8 (C-2/5), 112.3 (C10). Analysis: calcd. for C13H11N3O2: C, 64.72; H, 4.60; N, 17.42%; found: C, 64.85; H, 4.52; N, 17.31%. Ligand 3 (Hydrazone of isoniazid and cinnamonaldehyde) Yield 60%, m.p. 185OC, m.w. 251.28. FT-IR (KBr, cm-1): 3264 (NH), 3058 (CH), 1700 (C=O), 1625 (C=N), 1449 (CH=CH), 1176 (C-N). 1H NMR (DMSO-d6, δ, ppm): 8.80-8.83 (d, -C5H4N), 7.46-7.5 (d -C5H4N), 7.40 (s, =CH-), 7.25 (s, -NH-CO), 7.147.18 (d, -C6H5), 7.07-7.11 (m, -C6H5), 6.98 (m,
-C6H5), 6.31 (d, -CH=CH-), 5.21 (m, -CH=CH-),, C NMR (DMSO-d6, δ, ppm): 157.8 (C-6), 148.8 (C-3/4), 139.3 (C-1), 138.6 (C-9), 135.4 (C-7), 134.9 (C-10), 129 (C-12/14), 128.2 (C-13), 125.2 (C-11/15), 123.5 (C-8), 120.3 (C-2/5). Analysis: calcd. for C15H13N3O: C, 71.70; H, 5.21; N, 16.72%; found: C, 71.79; H, 5.13; N, 16.88%.
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Ligand 4 (Hydrazone of isoniazid and acetophenone) Yield 69%, m.p. 180OC, m.w. 239.27. FT-IR (KBr, cm-1): 3190 (NH), 2950 (CH), 1678 (C=O), 1615 (C=N), 1452 (CH=CH), 1169 (C-N). 1H NMR (DMSO-d6, δ, ppm): 8.78-8.8 (d, -C5H4N), 7.5-7.53 (d, -C5H4N), 7.24-7.26 (d, -C6H5), 7.1-7.13 (m, -C6H5), 6.88 (m, -C6H5), 6.82 (s, -NH-CO), 1.13 (s, -CH3). 13C NMR (DMSO-d6, δ, ppm): 161.5 (C-8), 157.2 (C-6), 148.5 (C-3/4), 138.8 (C-1), 132.6 (C9), 130.7 (C-12), 127.7 (C-10/14), 127.3 (C-11/13), 121.4 (C-2/5), 15.7 (C-7). Analysis: calcd. for C14H13N3O: C, 70.28; H, 5.48; N, 17.56%; found: C, 70.19; H, 5.59; N, 17.44%. Ligand 5 (Hydrazone of isoniazid and 2-hydroxyacetophenone) Yield 75%, m.p. 160OC, m.w. 255.27. FT-IR (KBr, cm-1): 3187 (NH), 2980 (CH), 1685 (C=O), 1646 (C=N), 1443 (CH=CH), 1141 (C-N), 1276 (C-O). 1H NMR (DMSO-d6, δ, ppm): 8.79-8.81 (d, -C5H4), 7.52-7.55 (d, -C5H4), 7.16-7.19 (d, -C6H4OH), 7.11-7.13 (m, -C6H4OH), 6.92 (m, -C6H4OH), 6.87s (-NH-CO), 4.60s (-OH), 1.16 (s, -CH3). 13C NMR (DMSO-d6, δ, ppm): 162.8 (C-8), 159.5 (C-10), 156.8 (C-6), 147.9 (C-3/4), 139.78 (C1), 129.3 (C-14), 128.6 (C-12), 120.9 (C-13), 119.8 (C-2/5), 117.9 (C-9), 113.4 (C-11), 14.8 (C-7). Analysis: calcd. for C14H13N3O2: C, 65.87; H, 5.13; N, 16.46%; found: C, 65.95; H, 5.00; N, 16.58%. Complex 1í (Triphenyltin derivative of hydrazone of isoniazid and benzaldehyde) Yield 55%, m.p. 200OC, m.w. 574.26. FT-IR (KBr, cm-1): 3059 (CH), 1668 (C=O), 1624 (C=N), 1450 (CH=CH), 1147 (C-N), 442 (Sn-N), 1H NMR (DMSO-d6, δ, ppm): 8.73-8.75 (d, -C5H4N), 8.61 (s, N=CH-), 7.96-7.98 (d, -C5H4N), 7.69-7.72 (d, CC6H5), 7.431-7.435 (m, C-C6H5), 7.25-7.3m (-C6H5). 13 C NMR (DMSO-d6, δ, ppm): 161.6 (C-6), 150.3 (C-3/5), 149.1 (C-1), 140.3 (C-7), 136.5 (C-14), 134.1 (C-8), 130.3 (C-11), 128.9 (C-9/13), 127.9 (C10/12), 127.5 (C-15/19), 127.2 (C-16/18), 127.2 (C17), 121.6 (C-2/6). Analysis: calcd. for C31H25N3OSn: C, 64.84; H, 4.39; N, 7.32%; found: C, 64.79; H, 4.24; N, 7.45%.
Synthesis, characterization and in vitro biological screening of...
Complex 2í (Triphenyltin derivative of hydrazone of isoniazid and salicylaldehyde) Yield 62%, m.p. 210OC, m.w. 590.26. FT-IR (KBr, cm-1): 3050 (CH), 1685 (C=O), 1627 (C=N), 1429 (CH=CH), 1128 (C-N), 1204 (C-O), 1H NMR (DMSO-d6, δ, ppm): 8.78-8.8 (d, -C5H4N), 8.68 (s, =CH-), 7.93-7.95 (d, -C5H4N), 7.53-7.54 (d, -C6H4OH), 7.33-7.44 (m, -C6H5), 7.29 (m, -C6H4OH), 6.92-6.95 (m, -C6H4OH), 6.9s (-OH) 13C NMR (DMSO-d6, δ, ppm): 174.2 (C-6), 156.7 (C-9), 150.4 (C-3/4), 148.8 (C-1), 140.6 (C-7), 136.1 (C-14), 131.7 (C-8), 128.8 (C-11), 128.2 (C-13), 121.5 (C10), 119.4 (C-15/19), 118.7 (C-16/18), 118.6 (C17), 118.4 (C-12), 116.4 (C-2/5). Analysis: calcd. for C31H25N3O2Sn: C, 63.08; H, 4.27; N, 7.12%; found: C, 63.0; H, 4.35; N, 7.25%. Complex 3í (Triphenyltin derivative of hydrazone of isoniazid and cinnamonaldehyde) Yield 51%, m.p. 190OC, m.w. 600.3. FT-IR (KBr, cm-1): 3040 (CH), 1677 (C=O), 1632 (C=N), 1453 (CH=CH), 1144 (C-N), 447 (Sn-N), 1H NMR (DMSO-d6, δ, ppm): 8.77-8.79 (d, -C5H4N), 8.25-
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8.27 (d, -C5H4N), 7.82 (s, =CH-), 7.64-7.66 (d, CC6H5), 7.40-7.42 (m, C-C6H5), 7.34-7.38 (m, -C6H5), 7.11 (m, C-C6H5), 6.31 (d, -CH=CH-), 5.21 (m, -CH=CH-). 13C NMR (DMSO-d6, δ, ppm): 161.4 (C6), 151.0 (C-3/4), 150.3 (C-1), 140.4 (C-7), 139.9 (C9), 136.1 (C-17/21), 135.8 (C-10), 129.0 (C-16), 128.8 (C-12/14), 128.2 (C-13), 127.2 (C-11/15), 125.4 (C-8), 122.9 (C-18/20), 122.9 (C-19), 121.5 (C2/5). Analysis: calcd. for C33H27N3OSn: C, 66.03; H, 4.53; N, 7.00%; found: C, 65.95; H, 4.65; N, 7.12%. Complex 4í (Triphenyltin derivative of hydrazone of isoniazid and acetophenone) Yield 65%, m.p. 175OC, m.w. 588.29. FT-IR (KBr, cm-1): 3047 (CH), 1679 (C=O), 1621 (C=N), 1479 (CH=CH), 1153 (C-N), 452 (Sn-N). 1H NMR (DMSO-d6, δ, ppm): 8.7-8.73 (d, C6H5), 7.87-7.89 (d, C6H5), 7.45-7.5 (d, C6H5), 7.43-7.43 (m, C6H5), 7.377.4; (m, -C6H5), 1.16 (s CH3-). 13C NMR (DMSO-d6, δ, ppm): 150.3 (C-6), (C-1), 150.1 (C-2/5), 140.2 (C-1), 136.3 (C-8), 136.1 (C-16/20), 135.8 (C-9), 134.7 (C-12), 128.5 (C-10/14), 128.3 (C-11/13), 128.2 (C-15), 122.9 (C-17/19), 121.7 (C-18) 121.1
Figure 1. Antibacterial activities of ligands (L1-L5) and complexes (C1-C5) against E. coli and S. aureus (SA)
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(C-3/4) 17.6 (C-7). Analysis: calcd. for C32H27N3OSn: C, 65.33; H, 4.63; N, 7.14%; found: C, 65.45; H, 4.55; N, 7.00%. Complex 5í (Triphenyltin derivative of hydrazone of isoniazid and 2-hydroxyacetophenone) Yield 77%, m.p. 170OC, m.w. 604.29. FT-IR (KBr, cm-1): 2985 (CH), 1725 (C=O), 1641 (C=N), 1489 (CH=CH), 1139 (C-N), 455 (Sn-N), 1H NMR (DMSO-d6, δ, ppm): 8.72-8.78 (d, -C5H4N), 7.81-7.9 (d, -C5H4N), 7.41-44 (m, -C6H5), 7.38-7.40 (m, -C6H5) 6.98 (m, -C6H5), 1.18 (s, CH3-). 13C NMR (DMSO-d6, δ, ppm): 160.5 (C-6), 156.9 (C-8), 150.1 (C-10), 149.4 (C-3/4), 141.1 (C-1), 137.8 (C-16/20), 135.9 (C-12), 129.8 (C-14), 128.7 (C-15), 128.4 (C17/19), 128.3 (C-18) 128.1 (C-2/5), 126.5 (C-11), 122.0 (C-13), 121.9 (C-9), 14.8 (C-7). Analysis: calcd. for C32H27N3O2Sn: C, 63.60; H, 4.50; N, 6.95%; found: C, 63.49; H, 4.43; N, 6.89%. Biological evaluation of synthetic compounds Antibacterial activity The antibacterial activity of triphenyltin complexes of isonicotinic acid hydrazones was tested by
using disc diffusion method against Staphylococcus aureus (ATCC 6538), Escherichia coli (ATCC25922) (16). Cefixime and DMSO were used as controls. Test bacterial strains with comparable turbidity with 0.5% McFarlandís solution were used to make lawn on nutrient agar plates. Discs of 6 mm were placed at appropriate distance. Samples and standard were then poured on respective discs. Other discs were supplemented with DMSO and reference antibacterial drugs serving as negative and positive controls, respectively. Plates were incubated at 37OC for 24 h and antimicrobial activity was expressed as the diameter of the zone of inhibition (ZOI) measured in mm. Antifungal activity The antifungal activity of synthesized triphenyltin complexes of isonicotinic acid hydrazones were tested against, Aspergillus fumigatus (FCBP66), Mucor sp., (FCBP-0300), Aspergillus niger (FCBP-0198) and Aspergillus flavus (FCBP-0064). The antifungal potential of test extracts was evaluated by disc diffusion method (16). Terbinafine was used as standards drug. Samples solutions of pure
Scheme 1. Synthesis of triphenyltin complexes of hydrazones of isonicotinic acid hydrazide
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Synthesis, characterization and in vitro biological screening of...
Table 1. Antibacterial activity of hydrazones and their metal complexes.
Zone of inhibition (mm)
Hydrazone and their triphenyltin derivatives
S. aureus
E. coli
Ligand 1
9.2 ± 0.35
-
Ligand 2
10.0 ± 0.21
8.7 ± 0.50
Ligand 3
-
7.6 ± 0.45
Ligand 4
9.1 ± 0.25
7.5 ± 1.45
Ligand 5
-
8.2 ± 0.15
Complex 1'
7.8 ± 0.43
14.5 ± 0.32
Complex 2'
10.1 ± 0.51
7.1 ± 1.2
Complex 3'
7.9 ± 0.49
9.3 ± 0.2
Complex 4'
8.4 ± 1.25
-
Complex 5'
7.6 ± 0.46
10.2 ± 0.89
Cefixime
21.6 ± 0.4
22.5 ± 0.32
DMSO
-
-
Values (mean ± SD) are average of three samples of compound, analyzed individually in triplicate.
Table 2. Antifungal activity of a hydrazone and their metal complexes.
Zone of inhibition Sample
A. flavus
A. niger
A. fumigatus
Mucor
Ligand 1
-
7.6 ± 0.2
-
8.4 ± 0.25
Ligand 2
10.2 ± 1.15
7.5 ± 0.26
11.3 ± 1.12
7.3 ± 1.20
Ligand 3
7.8 ± 0.46
9.5 ± 0.26
10.5 ± 0.43
-
Ligand 4
-
-
7.4 ± 0.26
10.6 ± 0.36
Ligand 5
8.5 ± 0.3
10.6 ± 0.2
8.7 ± 0.3
9.4 ± 0.25
Complex 1'
7.7 ± 0.40
14.2 ± 0.2
8.5 ± 0.2
7.2 ± 0.21
Complex 2'
8.7 ± 0.30
11.5 ± 0.26
7.8 ± 0.2
8.3 ± 0.26
Complex 3'
9.5 ± 0.30
7.7 ± 1.40
-
9.2 ± 0.1
Complex 4'
7.6 ± 0.3
7.9 ± 0.40
9.7 ± 0.3
-
Complex 5'
14.1 ± 0.2
11.3 ± 0.15
11.5 ± 0.36
15.3 ± 1.45
Terbinafine
28.1 ± 0.21
33.2 ± 0.15
30.1 ± 0.21
33.3 ± 0.21
DMSO
-
-
-
-
Values (mean ± SD) are average of three samples of compound, analyzed individually in triplicate.
compounds were prepared in sterile DMSO. Each test isolate (an aliquot of spore suspension at 1 ◊ 108 spores/mL) was spread evenly on the plates by sterile glass rod and discs of 6 mm were placed at appropriate distance. Samples and standard were then poured on respective discs. Plates were then incubated at 28OC for 24-48 h. Antifungal activity was expressed as diameter of zone of inhibition and measured in millimeter (mm).
RESULTS AND DISCUSSION The biochemistry and pharmacology of organotin based compounds are subjects of intense current interest. During last few years, a tremendous effort has been directed toward the synthesis of these compounds. Synthesis of organotin complexes in research area is of increased interest for inorganic, pharmaceutical, and medicinal chemistry as an
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approach to the development of new drugs (17). The increasing interest in the chemistry of organotin (IV) compounds has led to the extended studies on their reactions with different biomolecules (18). In current study, hydrazones of isonicotinic acid hydrazide and their metal complexes have been synthesized and evaluated chemically as well as biologically. The elemental analyses agree well with the proposed formula of the hydrazones of isonicotinic acid hydrazide and their metal complexes (organotin complexes) were prepared by the reaction of the Schiffís bases with the triphenyltin chloride in dry toluene medium. All the tin derivatives were stable in air and soluble in most common organic solvents (19). All hydrazones triphenyltin complexes have sharp melting points and were characterized by FTIR and multinuclear NMR spectroscopic techniques. IR spectra of hydrazones and their metal complexes were recorded in the range of 4000-400 cm-1 (20). FT-IR data showing the characteristic bands are in accordance with the structure of complexes. The complex formation of tin with the ligand is confirmed by the absence of a broad band in the range of 400-500 cm-1 due to ν (N-Sn) (18) and appearance of abosorption band at 3400-3200 cm-1 for the NH group (21). 1 H NMR spectral data of all the triphenyltin derivatives of hydrazones and all the protons present in the compounds have been identified by position and number. 13C NMR data gives information about the carbons of the phenyl groups attached to tin were observed (21) and have two signals that have close chemical shift to each other and are therefore difficult to assign to a particular carbon so they are reported. Synthesized compounds were also evaluated for their biological potential. All the compounds were tested against bacterial and fungal strains. Results of antibacterial assay (Figure 1) showed that activity of complexes is moderately high as compared to free ligands (Table 1) (22). Ligand 1 showed ZOI of 9.2 ± 0.35 mm against S. aureus and was inactive against E. coli. Complex of ligand 1 showed ZOI of 14.5 ± 0.32 mm against E. coli. Similarly increase in activity was observed in case of complex 5 against E.coli with ZOI of 10.2 ± 0.89 mm as compared to ligand 5. All synthesized compounds were also tested for their antifungal potential (Table 2) (23). Results showed that ligand 2 exhibit noticeable activities against A. flavus and A. fumigatus with ZOI of 10.2 ± 1.15 and 11.3 ± 1.12 mm, respectively. While complex 2 was found active only against A. niger (ZOI; 11.5 ± 0.26 mm). It was also observed that complex 5 showed enhanced activity against all fungal strains with highest value
of ZOI of 15.3 ± 1.45 mm against Mucor sp., as compared to free ligand 5. These findings are correlated with previous reports that organotin complexes possessed higher antimicrobial activities than free ligands (24, 25). Inhibitory action of organotin compounds is mainly due to their ability to interact with proteins and DNA (19). CONCLUSIONS In current study hydrazones of isonicotinic acid hydrazide and their metal complexes have been synthesized and evaluated chemically as well as biologically. So it is inferred from this work that compounds of both series possess antimicrobial potential and could be further studied through in vivo assays. The method used can be further modified to obtain more potent compounds by following virtual studies in the future. REFERENCES 1. Thangadurai T.D., Gowri M., Natarajan K.: Synth. React. Inorg. Met. Org. Chem. 32, 329 (2002). 2. Jarrahpour A., Khalili D., De Clercq E., Salmi C., Brunel J.M.: Molecules 12,1720 (2007). 3. Singh H.L., Singh J., Mukherjee A.: Bioinorg. Chem. App. 2013, 425832 (2013). 4. Zhang N., Fan Y., Zhang Z., Zuo J., Zhang P. et al.: Inorg. Chem. Commun. 22, 68 (2012). 5. Abu-Dief A.M., Ibrahim M.A., Mohamed A.: Beni-Suef University J. Basic Appl. Sci. 4, 119 (2015). 6. Pannerselvam P., Nair R.R., Vijayalakshmi G., Subramanian E.H., Sridhar S.K.: Eur. J. Med. Chem. 40, 225 (2005). 7. Ghammamy S., Sedaghatb S.: Middle-East J. Sci. Res. 3, 1213 (2013). 8. Sridhar S.K., Saravan M., Ramesh A.: Eur. J. Med. Chem. 36, 615 (2001). 9. Shah S.S., Ashfaq M., Waseem A., Ahmed M.M., Najam T. et al.: Mini Rev. Med. Chem. 15, 406 (2015). 10. Pandeya S.N., Sriram D., Nath G., Declercq E.: Eur. J. Pharmacol. 9, 25 (1999). 11. Mladenova R., Ignatova M., Manolova N., Petrova T., Rashkov I.: Eur. Polym. J. 28, 989 (2002). 12. Singh R., Kaushik N. K.: Spectrochim. Acta A 71, 669 (2008). 13. Prasad K.S., Kumar L.S., Prasad M., Revanasiddappa H.D.: Bioinorg. Chem. Appl. 2010 854514 (2010).
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