INTRODUCTION:

Schiff bases are an important class of ligands because such ligands and their transition metal complexes have a variety of applications including biological, clinical, and analytical applications.¹The development of the field of bioinorganic chemistry has stimulated the interest in Schiff base complexes because it has been recognized that N- and S-atoms play a key role in the coordination of transition metals at the active sites of many metallobiomolecules.^2,3 The importance of metal ions in biological systems is well established.

One of the most interesting features of metal-coordinated systems is the concerted spatial arrangement of the ligands around the metal ion.^3-5 Among the various transition metal ions used in pharmacological studies, vanadium and its derivatives have been reported to display different biological effects including antitumor, antimicrobial, anti-hyperlipidemia, antihypertension, anti-obesity, enhancement of oxygen affinity of hemoglobin and myoglobin, insulin-enhancing effects, and so on.^6-8 Vanadium complexes have also been explored for lowering of glucose levels^9-12, diuretic and natriuretic effects, antitumor activity against chemical carcinogenesis in animals and malignant cell lines (in vitro). Much effort has been done for vanadyl species coordinated to organic ligands in the search of their mimetic effects in hope of developing vanadodrugs.^13-15

Vanadium coordination chemistry is a topic of interest since the discoveries of it being an essential trace element for certain organisms^16-19, a cofactor in haloperoxidases^20,21 and nitrogenases^22,23 and recently as insulin enhancing agents.^24,25 Physiologically relevant oxidation states of vanadium are traditionally thought to be V(IV), as vanadyl, and V(V), often as vanadate.²⁶The discovery of the in vivo insulin enhancing ability of oxovanadium(IV)^24,27-35 and oxovanadate (V)^36-38 complexes and the ‘benchmark’ compound bis(maltolato) oxovanadium(IV), BMOV³⁹ stimulated the search for potential vanadium compounds for the treatment of type II diabetes. Manipulation of the ligands and the various oxidation states of vanadium in an attempt to enhance the insulin mimetic behavior is a topic of recent interest.

Transition metal complexes are also of current interest because of their potential applications as probes of DNA structure, for DNA-dependent electron transfer and site-specific cleavage of nucleic acids with the aim of developing novel therapeutic and diagnostic agents.⁴⁰ With regard to DNA cleavage activity, more attention has been focused on the middle to late transition metals than the earlier members of the series. Among the latter, vanadium with its three biologically relevant oxidation states (+3, +4 and +5) has begun to be recognized as DNA binding and cleaving agent. The bleomycin-vanadyl (IV) complex⁴¹ and [VO(phen) (H₂O)]²⁺ complex⁴²,both have been reported to induce DNA cleavage activity in the presence of H₂O₂. The diperoxovanadium(V) complexes with 2,2¢-bipyridine and 1,10-phenanthroline as ligands have been shown to cleave DNA on photoirradiation.^43-45

Keeping these facts in view, new oxovanadium(IV) complexes of α-amino acid Schiff bases and 2,2¢-bipyridine ligands were prepared, characterized and investigated their potential antimicrobial activity.

MATERIALS AND METHODS:

Reagents and Chemicals:

All the reagents used were chemically pure grade. Solvents were purified and dried according to standard procedures.

Physical Measurements:

A Mettler PM-200 electronic balance was used to perform all the weighing operations. The melting point of all the synthesized metal complexes were recorded in an electro thermal melting point apparatus (model No.AZ6512). Conductivities of 1.0 × 10^-3mol/dm³ solutions of the complexes in DMF were measured at 30°C using a WPA CM 35 conductivity meter and a dip-cell with platinized electrodes. The Sherwood Scientific Magnetic Susceptibility Balance was used to measure the magnetic moment value. Infrared spectra were recorded as KBr disc in a Shimadzu FTIR-8400 (Japan) infrared spectrophotometer, at the Central Science Laboratory of Rajshahi University, from 4000−225 cm⁻¹. The absorbance of the complexes was recorded on SHIMUDZU Spectrophotometer (Model UV-1800). The ESI-MS spectra were performed using a Shimadzu‒Ge‒Ms‒Qp 100 EX mass spectrometer using the direct inlet system. Carbon, hydrogen and nitrogen analyses for the complexes were carried out by PerkinElmer 2400 organic elemental analyzer-II at Okayama University, Japan.

Preparation of the oxovanadium (IV) complexes:

Oxovanadium(IV) complexes were prepared by template method in which a mixture of α-amino acids, α-alanine (ala) (0.267g, 3mmol)/DL-phenylalanine (pheala) (0.495 g, 3mmol)/leucine (leu) (0.393 g, 3mmol)/glycine (gly) (0.225g, 3mmol) / DL-methionine (met) (0.447g, 3 mmol) and NaOH (0.12g, 3mmol) in 10mL methanol were added to a methanolic solution of 3-hydorxy benzaldehyde (hb) (0.366g, 3mmol) in a round bottom flask. The resulting solution was refluxed for 1hour, followed by the addition of a methanolic solution of vanadyl sulfate (0.489g, 3mmol). A light bluish precipitate was obtained after refluxing the mixture for 1 hour. To this mixture, 2,2'-bipyridine (bpy) (0.468g, 3 mmol) taken in 10mL methanol was added. The solution on further refluxing for 1 hour gave a solid precipitate. The precipitate was filtered off on a Buchner funnel, washed with methanol and finally dried in a vacuum desiccator over anhydrous CaCl₂.⁴⁶

VOSO₄ + L + bpy → [VO(L)(bpy)]

Where, L= 3-hydroxybenzaldehyde-α-alanine (hb-ala), 3-hydroxybenzaldehyde-DL-phenylalanine (hb-pheala), 3-hydroxybenzaldehyde-leucine (hb-leu), 3-hydroxybenzaldehyde-glycine (hb-gly) and 3-hydroxybenzaldehyde-DL-methionine (hb-met) and bpy= 2,2´-bipyridine.

Oxovanadium (IV) complex of α-alanine [VO (hb-ala) (bpy)]:

Color: Dark brown, Melting point: 215 ⁰C, Yield: 0.734 g (59%), Molar conductance (Ohm^-1cm²mol^-1): 10.0, Magnetic moment (µ_eff/B.M.): 1.57, FTIR (cm^-1): 3472br, n(OH); 1619vs, n(C=O); 1544m, n(C=N); 1316m, n(C-O); 959s, n(V=O); 614m, n(V-O); 456m, n(V-N), UV-Vis. (λ_max/nm): 268−301, 364, 377. Elemental analysis for [VOC₂₀H₁₇N₃O₃]: M.W. 414.31 g/mol: Calculated: C, 57.98; H, 4.14; N, 10.14 %. Found: C, 56.85; H, 4.19; N, 10.03 %.

Oxovanadium (IV) complex of DL-phenylalanine [VO(hb-pheala) (bpy)]:

Color: Orange, Melting point: 184 ⁰C, Yield: 0.841 g (57%), Molar conductance (Ohm^-1cm²mol^-1): 19.3, Magnetic moment (µ_eff/B.M.): 1.60, FTIR (cm^-1): 3435br, n(OH); 1621vs, n(C=O); 1540s, n(C=N); 1310m, n(C-O); 942s, n(V=O); 619m, n(V-O); 445m, n(V-N), UV-Vis. (λ_max/nm): 267−302, 364, 376. Elemental analysis for [VOC₂₆H₂₁N₃O₃]: M.W. 490.40g/mol: Calculated: C, 63.68; H, 4.32; N, 8.57%. Found: C, 62.75; H, 4.13; N, 8.23 %.

Oxovanadium(IV) complex of leucine [VO(hb-leu)(bpy)]:

Color: Ash, Melting point: 226 ⁰C, Yield: 0.863 g (63%), Molar conductance (Ohm^-1cm²mol^-1): 16.1, Magnetic moment (µ_eff/B.M.): 1.55,FTIR (cm^-1): 3453w, n(OH); 1621w, n(C=O); 1530w, n(C=N); 1310w, n(C-O); 961s, n(V=O); 618m, n(V-O); 460w, n(V-N), UV-Vis. (λ_max/nm): 268–301, 364, 374. Elemental analysis for [VOC₂₃H₂₃N₃O₃]: M.W. 456.39 g/mol: Calculated: C, 60.53; H, 5.08; N, 9.21 %. Found: C, 58.93; H, 5.11; N, 9.13 %.

Oxovanadium(IV) complex of glycine [VO(hb-gly)(bpy)]:

Color: Blackish red, Melting point: 253 ⁰C, Yield: 0.781 g (65%), Molar conductance (Ohm^-1cm²mol^-1): 16.1, Magnetic moment (µ_eff/B.M.): 1.54, FTIR (cm^-1): 3414br, n(OH); 1654s, n(C=O); 1536s, n(C=N); 1312m, n(C-O); 964s, n(V=O); 618s, n(V-O); 463m, n(V-N), UV-Vis. (λ_max/nm): 270−303, 364, 375. Elemental analysis for [VOC₁₉H₁₅N₃O₃]: M.W. 400.28 g/mol: Calculated: C, 57.01; H, 3.78; N, 10.50 %. Found: C, 56.79; H, 3.83; N, 9.97 %.

Oxovanadium(IV) complex of DL-methionine [VO(hb-met)(bpy)]:

Color: Brown, Melting point: 213 ⁰C, Yield: 0.896 g (63%), Molar conductance (Ohm^-1cm²mol^-1): 7.9, Magnetic moment (µ_eff/B.M.): 1.80, FTIR (cm^-1): 3413br, n(OH); 1643s, n(C=O); 1536s, n(C=N); 1312m, n(C-O); 960vs, n(V=O); 616m, n(V-O); 452m, n(V-N), UV-Vis. (λ_max/nm): 269−307, 363, 388. Elemental analysis for [VOC₂₂H₂₁N₃O₃S]: M.W. 474.43 g/mol: Calculated: C, 55.70; H, 4.46; N, 8.86 %. Found: C, 55.27; H, 4.52; N, 8.77 %.

RESULTS AND DISCUSSION:

Solubility and Melting point:

The complexes were prepared in a high yield according to the template method. All the complexes are soluble in dimethylformamide (DMF) and dimethylsulfoxide (DMSO) but insoluble in common organic solvents such as methanol, ethanol, benzene, chloroform. Melting point (184-253 ⁰C) gives an approximate idea about the nature of the complexes and can suggest whether it is covalent or ionic.⁴⁷

Elemental Analysis and Conductivity:

The molar conductance of the complexes in DMF (10^-3 M solution) was measured at 30°C. The molar conductance values (7.9 to 19.3 Ohm-1cm²mol^-¹)indicate that all the complexes are non-electrolyte in nature. Their structures have been proposed based on the elemental analysis, conductivity and magnetic susceptibility measurements, IR spectral and UV-Visible spectral analysis.⁴⁸ Also, the analytical data are in good agreement with their proposed empirical formula

Magnetic Moment and Electronic Spectra:

Magnetic susceptibility measurements have been carried out in a Sherwood Scientific magnetic susceptibility balance at room temperature (303K).⁴⁹ The observed values of effective magnetic moment (m_eff) (1.54 to 1.80 BM) at room temperature indicated that all the complexes are paramagnetic in nature. The absorption spectra of the complexes were recorded in DMSO in the wavelength of 200–800nm range. All the complexes exhibit a band at ~375 nm due to ligand-to-metal charge-transfer (LMCT, PhO^-®V) transition.⁵⁰All complexes display bands in between 267–307nm which are assignable to the π®π* transition.⁴⁶

FTIR spectral studies:

The FTIR spectral data of oxovanadium(IV) complexes show a broad band in the 3413-3472 cm^-¹ region which is possibly due to the n(O-H) bands of hydrated water molecule in the complexes.⁵¹ The complexes exhibit n(C=O) bands in between 1619-1654 cm^-¹ and n(C-O) bands in 1310-1316 cm^-¹ region which are significantly lower than the values for respective bands of uncoordinated amino acids. Further, the appearance of n(V-O) stretching modes at around 618 cm^-¹ confirms the coordination of carboxylate ion to the central metal ion.⁵²The absence of frequency at ~3600 cm^–1 for the phenolic –OH indicates the coordination of phenolic oxygen to vanadyl ion. The bands appeared at around 1540 cm^-¹ may be assigned to n(C=N) stretching frequency suggesting the coordination of the azomethine nitrogen to the VO²⁺ moiety. The coordination of azomethine nitrogen and heterocyclic nitrogen is further evident by the appearance of n(V-N) modes in the 446-463 cm^-¹ region.⁵³ The present oxovanadium(IV) complexes exhibit the n(V=O) stretching frequency in the 942-964 cm^-¹ region characteristic of metal-oxygen multiple bond, thus ruling out the possibility of polymeric nature of the complexes since the polymeric oxovanadium(IV) complexes exhibit one or more broad absorption bands below 900 cm^-¹ due to bridging vanadyl group, -V-O-V-.^54-56 Here, the complexes exhibit medium intense band at ~960 cm^-¹ indicating the monomeric nature of the complexes.⁵⁷

ESI-MS Spectral Studies:

The molecular mass of the synthesized complexes are confirmed by ESI-MS spectral analysis (Figure 1). The molecular ion peak [M⁺] of the complexes [VO(hb-gly)(bpy)], [VO(hb-ala)(bpy)], [VO(hb-leu)(bpy)], [VO(hb-pheala)(bpy)] and [VO(hb-met)(bpy)] are m/z 400, m/z 414, m/z 456, m/z 490 and m/z 474, respectively. These corresponds to the molecular formulae of the synthesized complexes, respectively.

Figure-1: ESI-MS spectra of the complexes (a)[VO(hb-gly)(bpy)], (b) [VO(hb-ala)(bpy)], (c) [VO(hb-leu)(bpy)], (d) [VO(hb-pheala)(bpy)] and (e) [VO(hb-met)(bpy)].

On the basis of the elemental analysis, conductivity measurements, magnetic moment data, spectroscopic studies and literature review the structure of the complexes (Figure-2) could be illustrated as shown below:

Figure-2: Proposed structure of the Oxovanadium(IV) complexes.

Antibacterial Activity of the Complexes:

Antibacterial activities of the complexes are investigated by using the highly adoptable disc diffusion method.^58,59 The complexes (50 µg/disc in DMSO solution) were screened for their antibacterial activity against three human pathogenic bacteria- Escherichia coli, Staphylococcus aureus and Bacillus cereus with Kanamycin (K-30) standard.^60-63 The result shows that all the complexes have moderate to strong potential antibacterial activity against all the pathogenic bacteria. The result is graphically represented in Figure-3.

Figure-3: Graphical representation of the antibacterial activity of the complexes against Escherichia coli, Staphylococcus aureus and Bacillus cereus.

CONCLUSION:

The VO²⁺ complexes of O, N, O-donor α-amino acid Schiff bases and 2,2´-bipyridine have been synthesized and characterized. The analytical data reveal that the complexes are non-electrolytic and paramagnetic in nature. The magnetic moment values of the complexes are in accordance with the d¹ electronic configuration of the V^IVO²⁺ moiety. IR spectral data indicates the coordination of tridentate amino acid Schiff base ligands to the vanadyl (VO²⁺) ion. Thus, on the basis of physical properties, elemental analysis, conductivity measurements, magnetic moment data, spectroscopic studies and literature review, the structure of the complexes (Figure-2) may be proposed as distorted octahedral geometry with VO₃N₃coordination environment. All the complexes were screened for their antibacterial activity against three human pathogenic bacteria- Escherichia coli, Staphylococcus aureus and Bacillus cereus with Kanamycin (K-30) standard. The result shows that all the complexes have moderate to strong potential antibacterial activity against all the tested organisms.

ACKNOWLEDGEMENT:

The authors are thankful to the Chairman, Department of Chemistry, University of Rajshahi, Rajshahi-6205, Bangladesh for the laboratory facilities.

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Received on 21.02.2024 Modified on 19.03.2024

Asian J. Research Chem. 2024; 17(2):97-102.

DOI: 10.52711/0974-4150.2024.00020