Studies on oligomeric metal complexes derived from 2,5-bis((1H-1,2,4-triazol-3-yl)carbamoyl)terephthalic acid

Abstract

Novel oligomer–metal complexes (4a–f) of the ligand 2,5-bis ((1H-1,2,4-triazol-3-yl) carbamoyl) terephthalic acid (3) were prepared using various metal salts viz. Mn(II), Fe(II), Co(II), Ni(II), Cu(II), and Zn(II) and characterized by physicochemical, thermogravimetric, and spectroscopic techniques. Electronic spectral analysis and magnetic measurement studies were taken into account for the geometry of oligomer–metal complexes. Polymeric properties like number average molecular weight () and degree of polymerization have also been carried out. Ligand (3) was synthesized using pyromellitic diahydride and 1H-1,2,4-triazol-3-amine. Ligand was duly characterized by physicochemical, spectroscopic, and thermogravimetric techniques. All novel synthesized compounds 3 and 4a–f were evaluated for their antibacterial activity against various Gram-positive and Gram-negative bacterial strains. Growth inhibition was compared with the standard drug ciprofloxacin. Antifungal activity was also carried out against different fungal strains. The antifungal drug, ketoconazole, was used as a positive control. The results showed significantly higher antibacterial and antifungal activity of oligomer–metal complexes compared to the ligand.

Keywords:

• oligomer–metal complexes
• spectral study
• thermogravimetric analysis
• antimicrobial activity

1. Introduction

In recent years, the design and construction of polymer–metal complexes, often referred to as metal-organic frameworks (MOFs) have received great attention.[1–4] Numerous polymer–metal complexes have been reported not only because of their intriguing variety of architectures and topologies [5–7] but also for their properties of high porosity and enormous internal surface areas.[8,9] These properties have potential applications in clean energy, such as storage media for methane, hydrogen, and acetylene [10,11] or as absorbents or membrane fillers in separation and purifications of chemicals.[12,13] Polymer–metal complexes can be used in catalysis, magnetism, luminosity, and chemical sensors.[14,15] The structures of polymer–metal complexes are greatly depending upon the structure of the organic ligands, the coordinative geometry of metal ions, metal–ligand ratio, and other factors.[16,17] Among various organic ligands, multicarboxylate ligands are often used to synthesized polymer–metal complexes e.g. 1,2,4,5-benzenetetracarboxylate, 3,3′,4,4′-biphenyltetracarboxylic acid,[18,19] 1,1′-biphenyl-2,3′,3,4′-tetracarboxylic acid, [20] and methylene diisophthalic acid [21] have been extensively used for the synthesis of various polymer–metal complexes. On the other hand, the use of auxiliary N-containing ligands is also an effective method for the framework formation of polymer–metal complexes owing to the fact that they can satisfy and even mediate the coordination needs of the metal center and consequently generate more meaningful architectures.[22,23] Patel and his co-workers have reported the studies on various polymer/oligomer–metal complexes.[24–27] A more detailed exploration has been undertaken by us in the study of polymer/oligomer–metal complexes. With the aim of investigating the influence of auxiliary ligands on frameworks of oligomer–metal complexes, based on bisamic acid containing carboxylic and amide group, we carried out the study for the reaction of metal (II) salts with novel bisamic acid (3). The MOF based on bisamic acid of pyromellitic dianhydride has not attracted any attention. Hence, initial work in this direction has been reported by us recently.[28–30] This prompted us to extend our work using other auxiliary ligand such as 2,5-bis((1H-1,2,4-triazol-3-yl)carbamoyl) terephthalic acid (3). Figure 1 summarize our synthetic approach to the various phases of this work, viz., (a) preparation of ligand (bisamic acid) from pyromellitic dianhydride and 1H-1,2,4-triazol-3-amine; (b) synthesis of oligomer–metal complexes using various metal (II) acetates, e.g. Mn(II), Fe(II), Co(II), Ni(II), Cu(II), and Zn(II) metal ions. Ligand when incorporated with transition metal ions would produce a broad spectrum of antimicrobial property. The details of these procedures and the results obtained are discussed below.

Figure 1. Synthetic route for the oligomer–metal complexes.

2. Experimental

2.1. Materials and measurements

All common reagents and solvents were used of analytical grade and were used without further purification. Alumina-supported pre-coated silica gel 60 F254 thin layer chromatography (TLC) plates were purchased from the E. Merck (India) Limited, Mumbai and were used to check purity of compounds, and to study the progress of the reaction whereby TLC plates were illuminated under Ultraviolet light (254 nm), evaluated in I₂ vapors, and visualized by spraying with Draggendorff’s reagent. Infrared spectra (FT–IR) were obtained from KBr pellets in the range of 4000–400 cm⁻¹ with a Perkin Elmer spectrum GX spectrophotometer (FT–IR) instrument. ¹H NMR and ¹³C NMR spectra were acquired at 400 MHz on a Bruker NMR spectrometer using DMSO-d₆ (residual peak at δ ~2.5 or ~ 39.5 ppm, 300 °K) as a solvent as well as TMS an internal reference standard. Liquid chromatography – atmospheric pressure chemical ionization-mass spectrometry (LC/APCI/MS) in both positive and negative modes of operation was used for the determination of the compounds. The eluent was delivered by a liquid chromatograph model HP 1090 (Hewlett-Packard, CA). The mobile phases used for the elution of the analytes consisted of 40% acetonitrile, 60% water, and 0.3% acetic acid at a flow rate of 0.8 mL/min. This HPLC system was connected to a HP mass spectrometer, model HP1100, system equipped with an APCI probe. Micro analytical (C, N, H) data were obtained using a Perkin–Elmer 2400 CHN elemental analyzer. The solid diffuse electronic spectra were recorded on a Beckman DK–2A spectrophotometer with a solid reflectance attachment. MgO was employed as a reference. Magnetic moments [31] were determined by the Gouy method with mercury tetra thiocyanetocobaltate (II), [HgCo(NCS)₄] as calibrant (Xg = 1644 × 10⁻⁶cgs units at 20 °C), by Citizen Balance (at room temperature). Molar susceptibilities were corrected using Pascal’s Constant.[32] The thermogravimetric studies were carried out with a model Perkin Elmer thermogravimetry analyzer at a heating rate of 10 °C min⁻¹ in the temperature range 50–700 °C under nitrogen. The metal content of the oligomer–metal complexes was carried by decomposing a weighed amount of each oligomer–metal complexes with HClO₄, H₂SO₄, and HNO₃ (1:1.5:2.5) mixture followed by standard EDTA titration method.[33] Due to poor solubilty of oligomer–metal complexes in DMSO, number average molecular weight () of oligomer–metal complexes was determined by non-aqueous conductometric titration. It was carried out in pyridine solution against standard sodium methoxide in pyridine soloution as titrant. The number average molecular weight of each sample was calculated according to method reported in literature.[34] The melting point was checked by the standard open capillary method. In order to facilitate the correct structural assessment, i.e. the coordination site, we have tried a lot to generate a crystal for single crystal X ray analysis but we did not succeed. Hasanzadeh et al. [35] have reported this type of acid amide metal complex. So from the obtained data and reference article, interpretations become straightforward.

2.2. Synthesis of ligand 2,5-bis((1H-1,2,4-triazol-3-yl)carbamoyl)terephthalic acid (3)

Ligand was synthesized by adding a solution of 1H-1,2,4-triazol-3-amine (16.816 g, 0.2 mol) in 30 mL THF dropwise to a stirred solution of pyromellitic dianhydride (21.813 g, 0.1 mol) in 45 mL THF and keeping the temperature of the medium close to 70–80 °C for 2 h, thus the obtained ensuing solution was poured into ice water in which the reaction product precipitated. The final light yellow precipitates were filtered, washed with dry THF followed by ethanol, and purified by column chromatography using ethyl acetate: hexane (1:1) solvent system and dried at room temperature. ¹H NMR (DMSO-d₆, δ ppm): 12.47 (d, 2H, H₉,H₉′), 10.89 (s, 2H, 2–COOH), 9.70 (s, 2H, 2–NH–), 8.63 (s, 2H, H₁,H₁′), 7.87 (d, 2H, H₁₀,H₁₀′); ¹³C NMR (DMSO–d₆, δ ppm): 125.45, 133.63, 135.12, 142.98, 151.89, 166.80, 169.42.

2.3. Synthesis of oligomer–metal complexes (4a–f)

All oligomer–metal complexes were synthesized using equimolar amount of ligand and various metal (II) salts. A warm clear solution of ligand (5.045 g, 0.01 mol) in dimethylsulphoxide was neutralized by adding dropwise a solution of 0.1 M sodium hydroxide. The reaction medium was maintained at pH of about 7–8. A pasty mass was observed. It was diluted with water to make the solution clear. To the above solution, a solution of metal (II) acetate viz., Mn(II), Fe(II), Co(II), Ni(II), Cu(II), and Zn(II) acetate (0.01 mol) was added with constant stirring and the pH of the reaction mixture was adjusted to 6–7. The oligomer–metal complexes thus separated out in the form of a suspension was digested on a water bath for 1 h, filtered, washed, and dried in air at room temperature. These oligomer–metal complexes are insoluble in common organic solvents like methanol, ethanol, chloroform, acetone, and benzene, while partially soluble in DMSO. The results obtained were furnished in Table 1.

Table 1. Physicochemical parameters of the ligand and its oligomer–metal complexes.

Empirical formula of Compound		Empirical Weight (gm)	Color	Yield %	M.P.^a (°C)	Elemental Analysis calc. (found %)				μeff B.M.		DP
						C	H	N	M
3	L	386.28	Light yellow	65	160^b	43.53 (43.51)	2.61 (2.56)	29.01 (28.89)	–	–	–	–
	C14H10N8O6
4a	[Mn–L–(H2O)2]n	475.23	White	45	>250	35.38 (35.32)	2.55 (2.47)	23.58 (23.51)	11.56 (11.45)	5.74	2375	5
	C14H12N8O8Mn
4b	[Fe–L–(H2O)2]n	476.14	Light brown	54	>250	35.32 (35.31)	2.54 (2.60)	23.53 (23.48)	11.73 (11.69)	5.02	2860	6
	C14H12N8O8Fe
4c	[Co–L–(H2O)2]n	479.23	Light pink	55	>250	35.09 (34.97)	2.52 (2.50)	23.38 (23.33)	12.30 (12.23)	4.39	2362	5
	C14H12N8O8Co
4d	[Ni–L–(H2O)2]n	478.99	Light green	63	>250	35.11 (35.13)	2.53 (2.49)	23.39 (23.35)	12.25 (12.20)	2.93	2389	5
	C14H12N8O8Ni
4e	[Cu–L–(H2O)2]n	483.84	Green	70	>250	34.75 (34.69)	2.50 (2.44)	23.16 (23.10)	13.13 (13.03)	1.97	2853	6
	C14H12N8O8Cu
4f	[Zn–L–(H2O)2]n	485.67	White	45	>250	34.62 (34.62)	2.49 (2.43)	23.07 (22.98)	13.46 (13.43)	D	2351	5
	C14H12N8O8Zn

^a Melting points were checked by standard open capillary method and were found to be uncorrected.

^b uncorrected.

μ_eff B.M. = Magnetic moment, Mn = Number average molecular weights, DP = Degree of Polymerization, D = Diamagnetic.

2.4. Biological activity

2.4.1. Antibacterial activity (in vitro)

Compounds (3 and 4a–f) were screened for in vitro antibacterial activity against Gram-positive bacterial strains (Bacillus subtilis [BS] and Staphylococcus aureus [SA]) and Gram-negative bacterial strains (Salmonella typhimurium [ST] and Escherichia coli [EC]) utilizing the agar diffusion assay.[36,37] The wells were dug in the media with the help of a sterile metallic borer. Recommended concentration (100 μl) of the test sample (1 mg/mL in DMSO) was introduced in the respective wells. Other wells supplemented with DMSO and reference antibacterial drug, ciprofloxacin were served as negative and positive controls, respectively. The plates were incubated immediately at 37 °C for 24 h. Activity was determined by measuring the diameter of zones showing complete inhibition (mm). Growth inhibition was compared with the standard drug. In order to clarify any participating role of DMSO in the biological screening, separate studies were carried out with the solutions alone of DMSO and they showed no activity against any bacterial strains.

2.4.2. Antifungal activity (in vitro)

Compounds (3 and 4a–f) were also examined for antifungal activity against different fungal strains, i.e. Penicillium expansum [PE], Botryodiplodia theobromae [BT], Nigrospora sp. [NS], and Trichothesium sp. [TS]. The antifungal drug, ketoconazole, was used as a positive control. Antifungal screening for compounds (3 and 4a–f) and positive control was performed at a recommended concentration. The fungal strains were grown and maintained on potato dextrose agar plates. The cultures of the fungi were purified by single spore isolation technique. Each compounds (3 and 4a–f) in DMSO solution was prepared for testing against spore germination of each fungus. The fungal culture plates were inoculated and incubated at 25 ± 2 °C for 48 h. The plates were then observed and the diameters of the zone of inhibition (in mm) were measured. The percentage inhibition for fungi was calculated after five days using the formula given below:

where X = Area of colony in control plate,

Y = Area of colony in test plate.

3. Results and discussion

3.1. Synthesis of ligand 2,5-bis((1H-1,2,4-triazol-3-yl)carbamoyl)terephthalic acid

To the best of our knowledge, ligand (3) has not been reported previously. The characterization of the reaction product provided the first unambiguous proof of the successful synthesis of 2,5-bis((1H-1,2,4-triazol-3-yl)carbamoyl)terephthalic acid. The FT–IR spectrum of ligand showed the most relevant peaks of the triazole ring and 1,2,4,5–tetra-substituted benzene ring, other than typical absorptions arising from the band at 3540 cm⁻¹ and 1709 cm⁻¹ for carboxylic acid and 3241 and 1685 cm⁻¹ for O=C–NH group.[38] In the ¹H NMR spectrum, the signal at 12.47 ppm was ascribed to the protons of the triazol rings. The singlet at 10.89 ppm was ascribed to the protons of carboxylic -OH group and a singlet at 9.70 ppm was attributed to the –NH proton of amide group, which was further confirmed by ¹³C NMR value i.e. 169.42 and 166.80 were attributed to carboxylic carbon and amide carbon, respectively. The recorded LC-MS spectrum (Figure 2) showed nine m/z peaks including molecular ion peak, which supports the structural formula of ligand as mentioned in the Figure 1. The highest m/z peak is observed at 385.96, which confirmed the molecule weight of ligand. In order to verify the attached carboxylic acid groups in ligand, thermal analysis was carried out. The ligand started to lose weight because of thermal degradation. Thermogram of product 3 indicated that the degradation occurred in two steps. First stage of degradation started from 150 to 300 °C, which might be attributed to decarboxylation of product 3. The value of wt. loss 22.6% observed during this temperature range is quite consistent with the theoretical value 22.78%. The second major stage at about 320–700 °C attributed to the monomer decomposition/pyrolysis. The 4–5% char residue remained at 700 °C. The expected structure was thus clearly verified by the spectroscopic and thermal analysis which indicated the absence of any detectable impurity, particularly of the two reagents used to prepare compound 3.

Figure 2. LCMS of 2,5-bis((1H-1,2,4-triazol-3-yl)carbamoyl)terephthalic acid.

3.2. Synthesis of oligomer–metal complexes

3.2.1. Physical properties

Elemental analysis of all oligomer–metal complexes was in good agreement with proposed structures. All oligomer–metal complexes exhibited 1:1 metal to ligand stoichiometry. The structures of oligomer–metal complexes were consistent with the FTIR, electronic spectra and TGA. The geometry of the central metal ion was confirmed by electronic spectra and magnetic susceptibility measurements. The degrees of polymerization (DP) for all oligomer–metal complexes are in the range of 5–6 (Table 1). All the data provide good evidence that the chelates are polymeric in nature. The suggested structure of the oligomer–metal complexes is shown in Figure 1.

3.2.2. Infrared spectra

IR spectral bands of the ligand and its oligomer–metal complexes suggest the formation of desired oligomer–metal complexes and support their structure. Spectral features provide valuable information regarding the nature of functional group attached to the metal atom. In order to study the bonding mode of ligand to the oligomer–metal complexes, the IR spectrum of free ligand was compared with the spectra of oligomer–metal complexes. Considerable differences to be expected were observed. The band at about 3509 cm⁻¹ for carboxylic acid in ligand was virtually disappeared from the spectra of oligomer–metal complexes (Table 2). Oligomer–metal complexes exhibit more broadened band in the region near 2970–2980 cm⁻¹ indicating the presence of coordinated water molecules.[39] The coordinated water in all the oligomeric metal (II) complexes presents different peaks at 980 cm⁻¹ (rocking) and 760 cm⁻¹ (wagging), whereas none of these vibrations appear in the spectra of uncoordinated ligands. A band at 1630 cm⁻¹ in free schiff base is due to νC–N vibration. The shifting of this group to lower frequency (~1590 cm⁻¹) in the oligomer–metal complexes when compared to free ligand suggests the coordination of metal ion through nitrogen atom of amide group,[40] it is expected that coordination of nitrogen to the metal atom would reduce the electron density in the amide link and thus lower the absorption.[41] A band at 1709 cm⁻¹ is assigned to νC=O stretching frequency in the spectrum of free schiff base which is also shifted to lower frequency at about 1690 cm⁻¹ in all the oligomer–metal complexes. This indicates the involvement of oxygen atom of hydroxy group of –COOH group in bonding with metal ions.[42] New bands, which were not present in the spectrum of ligand, appeared in the spectra of oligomer–metal complexes, e.g. presence of sharp band in the region of 525–535 cm⁻¹ can be assigned to νM–N,[42] which indicated the involvement of nitrogen in coordination. The medium intensity bands for νM–O [43] have been observed at 625–635 cm⁻¹ due to M–O coordination. The appearance of νM–N and νM–O vibrations supports the involvement of N and O atoms in complexation with metal ions under investigation. These overall data suggest that the amide-N and carboxylate-O groups are involved in coordination with the metal (II) ion in oligomer–metal complexes. These features confirmed the proposed structure of oligomer–metal complexes as shown in Figure 1.

Table 2. FT-IR frequencies and electronic spectral data of the ligand and its oligomer–metal complexes.

Compound		–COOH	–CONH	–OH	–C=O	–CONH	COO^-	C–O	M–O	M–N	Electronic spectral data
											cm^−1	Transitions
3	L	3540	3241	–	1709	1685	1468	1044	–	–	–	–
4a	[Mn–L–H2O)2]n	–	3219	2982	1690	1668	1457	1027	630	534	16,412,	6A1g → 4T1g(4G),
											17,743,	6A1g → 4T2g(4G)
											23,169	6A1g → 4A1g, 4Eg
4b	[Fe–L–(H2O)2]n	–	3222	2979	1689	1671	1452	1025	624	530	19,031	5T2g(F) → 3 Eg
											36,049	5T2g(F) → 3T1g
4c	[Co–L–(H2O)2]n	–	3215	2983	1691	1669	1445	1023	632	524	9837	4T1g(F) → 4T2g(F)
											15,544	4T1g(F) → 4A2g(F)
											22,921	4T1g(F) → 4T1g(P)
4d	[Ni–L–(H2O)2]n	–	3218	2977	1692	1667	1450	1027	637	527	9879	3A2g → 3T2g
											15,552	3A2g → 3T1g(F)
											22,967	3A2g → 3T1g(P)
4e	[Cu–L–(H2O)2]n	–	3220	2969	1693	1670	1451	1029	626	530	15,943	2T2g → 2Eg
											22,739	charge transfer
4f	[Zn–L–(H2O)2]n	–	3218	2973	1688	1663	1447	1030	635	525	–	–

3.2.3. Magnetic moments and electronic spectral data

The information regarding geometry of the oligomer–metal complexes were obtained from their electronic spectral data and magnetic moment values (Table 2). The diffuse electronic spectrum of the [Cu–L–(H₂O)₂]_n shows two broad bands around 15,943, and 22,739 cm⁻¹ due to the 2_T2g → 2_Eg transition, while the second may be due to charge transfer, respectively. This suggests a distorted octahedral structure for the [Cu–L–(H₂O)₂]_n oligomer, which was further confirmed by its μ_eff value 1.97 B.M. The [Ni–L–(H₂O)₂]_n oligomer shows two absorption bands at 15,552 cm⁻¹, 22,967 cm⁻¹, and 9879 cm⁻¹ due to 3_A2g → 3_T1g(F), 3_A2g → 3_T1g(P), and 3_A2g → 3_T2g, respectively. The [Co–L–(H₂O)₂]_n polymer shows two absorption bands at 22,921 cm⁻¹, 15,544 cm⁻¹, and 9837 cm⁻¹ corresponding to 4_T1g(F) → 4_T1g(P), 4_T1g(F) → 4_A2g(F), and 4_T1g(F) → 4_T2g(F) transitions, respectively, which indicated an octahedral configuration for the [Ni–L–(H₂O)₂]_n and [Co–L–(H₂O)₂]_n oligomers.[44] This configuration was further confirmed by its μ_eff value 2.93 B.M. and 4.39 B.M. The spectrum of [Fe–L–(H₂O)₂]_n show bands at 36,049 cm⁻¹ and 19,073 cm⁻¹ assigned to the transitions 5_T2g(F) → 3_T1g and 5_T2g(F) → 3_Eg, and its μ_eff 5.02 B.M. suggesting octahedral configuration. The spectrum of [Mn–L–(H₂O)₂]_n show weak bands at 16,412, 17,743, and 23,169 cm⁻¹ assigned to the transitions 6_A1g → 4_T1g(4G), 6_A1g → 4_T2g(4G), and 6_A1g → 4_A1g,4_Eg, respectively, suggesting an octahedral structure for the [Mn–L–(H₂O)₂]_n oligomer.[45] This configuration was further confirmed by its μ_eff value 5.74 B.M. As the spectrum of the [Zn–L–(H₂O)₂]_n oligomer is not well interpreted, but its μ_eff value shows that it is diamagnetic as expected. Magnetic moments μ_eff of all coordination oligomer revealed that all oligomers except Zn(II) metal ion oligomer are paramagnetic, while Zn(II) metal ion oligomer is diamagnetic.

3.2.4. Thermal analysis

The thermal behavior was investigated by Perkin Elmer TGA analyzer at a heating rate of 10 °C min⁻¹ in the temperature range 50–700 °C under nitrogen, which provides much information about the coordination compounds. In all the oligomer–metal complexes, there is no weight loss observed up to 100 °C. The decomposition occurred in two steps. First step occurred between 100  and 200 °C, which might be attributed to mass loss corresponding to water molecules (Figure 3). The value of weight loss during this temperature became equal to weight loss of two water molecules. These value of wt. loss indicated that two water molecules were coordinated to the metal ion. Second step that occurred between 200  and 700 °C exhibits a mass loss corresponding to decomposition of ligand part in oligomer. The weight loss of oligomer–metal complexes was noticeable between 300 and 600  °C. The rate of degradation became maximum at a temperature between 400 and 600 °C. This may be due to accelerating by metal oxide which forms in situ. Each oligomer loses about 80% of its weight when heated up to 700 °C. On the basis of the relative decomposition (% weight loss) and the nature of thermogram, the oligomer–metal complexes may be arranged in order of their increasing stability as: Cu < Fe < Ni < Co < Zn < Mn.

Figure 3. Thermogram of ligand and its oligomer–metal complexes.

3.3. Biological activity

3.3.1. Antibacterial activity

Based on the data from the antibacterial studies against both Gram-positive and Gram-negative bacterial strains, the following observations can be made. All compounds (3 and 4a-f) exhibited antibacterial activity against both Gram-positive and Gram-negative bacterial strains with zones of inhibition (ZOI) ranging from 20 to 44 mm (Figure 4).

Figure 4. Antibacterial activity of ligand and its oligomer–metal complexes.

Ligand 2,5-bis((1H-1,2,4-triazol-3-yl)carbamoyl)terephthalic acid (ZOI_[BS]= 22 mm, ZOI_[SA]= 20 mm, ZOI_[ST]= 21 mm, ZOI_[EC]= 23 mm) was found less active than its oligomer–metal complexes. Among the analogs 4a–f, compound 4e (ZOI_[BS]= 41 mm, ZOI_[SA]= 40 mm, ZOI_[ST]= 42 mm, ZOI_[EC]= 44 mm) was identified as a potent antibacterial agent against all Gram-positive and Gram-negative bacterial strains. Compound 4b (ZOI_[BS]= 35 mm, ZOI_[SA]= 37 mm, ZOI_[ST]= 38 mm, ZOI_[EC]= 36 mm) also had good antibacterial activity against bacterial strains. Compounds 4a, 4c, 4d, and 4f exhibited moderate antibacterial activity. Compounds 3 and 4a–f exhibited less antibacterial activity as compared to standard antibiotic drug, ciprofloxacin (ZOI_[BS]= 45 mm, ZOI_[SA]= 46 mm, ZOI_[ST]= 45 mm, ZOI_[EC]= 47 mm).

A comparative study of the growth inhibition zone values of schiff base and its oligomer–metal complexes indicated that the oligomer–metal complexes exhibited higher antibacterial activity than free schiff base (Figure 5). Such increased activity of the oligomeric metal complexes can be explained on the basis of Overtone’s concept and Tweedy’s chelation theory.[46] According to Overtone’s concept of cell permeability, the lipid membrane that surrounds the cell favors the passage of only lipid soluble materials due to which liposolubility is considered to be an important factor that controls the antimicrobial activity. Chelation reduces the polarity [47,48] of the metal ion mainly because of the partial sharing of its positive charge with the donor groups and possibly the π-electron delocalization within the whole chelate ring system thus formed during coordination. This process of chelation thus increases the lipophilic nature of the central metal atom, which in turn favors its permeation through the lipoid layer of the membrane. This in turn is responsible for increasing the hydrophobic character and liposolubility of the molecule in crossing cell membrane of the micro-organism, and hence enhances the biological utilization ratio and activity of the testing drug/compound. The biological activity of compounds also depends on the nature of the ligand, concentration, lipophilicity, nature of metal ion, coordinating sites, and geometry of the complex.

Figure 5. Comparative antibacterial activity.

3.3.2. Antifungal activity

Based on the screening data from the antifungal studies, the following observations can be made. All compounds (3 and 4a–f) exhibited antifungal activity against different fungal strains (Figure 6). Ligand 2,5-bis((1H-1,2,4-triazol-3-yl)carbamoyl)terephthalic acid (ZOI_[PE]= 43 mm, ZOI_[BT]= 39 mm, ZOI_[NS]= 45 mm, ZOI_[TS]= 41 mm) was found less active than its oligomer–metal complexes (Figure 7). Compound 4e (ZOI_[PE]= 40 mm, ZOI_[BT]= 37 mm, ZOI_[NS]= 42 mm, ZOI_[TS]= 38 mm) was identified as a more active against all fungal strains. Compound 4b (ZOI_[PE]= 37 mm, ZOI_[BT]= 33 mm, ZOI_[NS]= 36 mm, ZOI_[TS]= 36 mm) also had good antifungal activity against fungal strains. Compounds 4a, 4c, 4d, and 4f exhibited moderate antifungal activity. Compounds 3 and 4a–f exhibited less antifungal activity as compare to standard antibiotic drug, ketoconazole (ZOI_[PE]= 43 mm, ZOI_[BT]= 39 mm, ZOI_[NS]= 45 mm, ZOI_[TS]= 41 mm).

Figure 6. Antifungal activity of ligand and its oligomer–metal complexes.

Figure 7. Comparative antifungal activity.

4. Conclusions

New ligand and its oligomer–metal complexes were prepared in good amount of yield and were duly characterized. In the oligomer–metal complexes, ligand coordinates with one central metal atom at four coordination sites along with two water molecules. The structure of ligand and its oligomer–metal complexes is consistent with the elemental, spectral, and thermal analysis. The geometry of the central metal ion was confirmed by electronic spectra and magnetic susceptibility measurements. All the data provide good evidence that the chelate are polymeric in nature. All these oligomers do not melt up to 400 °C. The oligomers have moderate thermal stability and shows good biological activity.

Acknowledgments

Author is greatly thankful to the Head, Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar for providing practical facility. Author is also expressing his sincere thanks to the Principal, Government Science College-Gandhinagar for giving necessary facility for this work.