In the 21^st century, coordination chemistry becomes the important area of research in inorganic chemistry. It is a rapid developing branch of chemistry due to the large applications in the field of organic synthesis, asymmetric synthesis, catalysis, as bleaching agents, industries, dyes, and drugs. The metal complexes are applied in medicinal, analytical, and diagnostic purposes in the living system. Because of the magnetic and radioactive properties, they are used in optical images and Magnetic Resonance Imaging (MRI). The metal complex of Schiff bases have a broad range of biological activities including, antiviral, synergistic, antioxidant, anti-inflammatory, antifungal analgesic, antitumor, cytotoxic, antimicrobial,  antidiabetic

fertility [1,2], anti-tumor activity [3], DNA Photoclevage activity [4], etc. Popova and Berova et al. reported novel metal complex, which exhibits an excellent biological activity, the complex used to cure liver function, and maintain its level in the blood. The urine can causes the side effect, which can affect during the pregnancy, Nephritis Hepatitis, the blood Leukemia, and Anemia in children [5].

Following our research on the synthesis of novel metal complex and their Schiff bases that have been described from the Dehydroacetic acid and 2-aminopyridine, all synthesized novel transition metal complex Schiff bases were characterized by elemental analysis, (FTIR, UV-VIS, ¹HNMR, and XRD) spectroscopic techniques, as well as thermal and magnetic study

Experimental Details

All the chemicals required in intermediates were purchased from E-Merck Germany and 2-aminopyridine AR grade from Loba Chemie Pvt. Ltd. The metal salts were purchased from the E-Merck and Loba chemie Pvt. Ltd. The solvent were purified before being used for the reaction with standard procedure [6]. The Perkins Elmer CHN Analyzer was used to calculate the exact percentage of C, H, and N. The FTIR of the complexes were measured by the Perkin Elmer (1430) FTIR spectrophotometer in the range of 4000-400 cm^-1. The himandzu UV –VISIBLE Spectrophotometer UV 160 has been applied to measure electronic spectra at Research Centre Department of Chemistry Rajarshi Shahu Mahavidyalaya, Latur (MS). The ¹HNMR and ¹³CNMR were measured by using Brucker FT-500 MHz NMR Spectrophotometer in CDCl₃ by solvent TMS as a reference substance, while TGA -DTA scanning at Central Instrumentation Centre, Savitribai Phule University, Pune.The biological testing has been done at Research Centre Department of Biotechnology R. S. M. Latur.

Synthesis of schiff bases

DHA Schiff Bases are synthesized by using standard procedure [7,8] by which the equimolar solutions of Dehydroacetic acid (0.01mol) and 2-aminopyridine (0.01 mol) were dissolved in 25 mL of dry ethanol taken in a round bottomed flask. The content of flask refluxed for four to six hours on 1 RML rotamantle with magnetic stirrer. Upon cooling, the light yellow colored solid mass of separated out Schiff base was filtered, and washed with a small portion of ethanol, and then it was dried. A pure product was obtained by recrystallisation from ethanol and dried in vacuum desiccators. The purity of product was ascertained by TLC and melting point was examined. 

Figure 1.Synthesis of Schiff base

Synthesis of metal complexes

The metal complexes were synthesized by the reaction of DHA Schiff bases 0.002 mol in methanolic solution with metal chloride salt in an 2:1 molar ratio to produce the desired metal complex [8].

Figure 2.Synthesis of Metal Complexes

Results and Discussion

The Schiff bases are generally orange color and air-stable, on-hygroscopic in nature, and soluble in water and in hot alcoholic solution.

The novel metal complexes are colored in nature, and non-hygroscopic, while stable in atmospheric air. The synthesized complexes are insoluble in polar solvent like DMF, water, methanol, ethanol, and DMSO. The open capillaries technique has been used for the M.P. determination of synthesized compounds. The analytical data is represented in Table 1

Table 1.Analytical Data

¹H-NMR

The complex shows the characteristics peak for the metal complex in the ¹H-NMR. The peak signal at δ 2.2 belongs to (3H, s, C₆–CH₃) proton, while the signal at δ15.82 ppm is a free hydroxyl group (OH) proton. However, the signal at δ2.58 (3H, s, N=C-CH₃) is methyl hydrogen linked carbon azomethine for DHA moiety [7]. The remaining signal appeared around 7.4- 8.2 (4H, m) belongs to the aromatic of pyridine moiety.

IR spectra

The details of FT-IR are indicated in Table 2. Aweak band at 3419 cm^-1 appears due to intra molecularhydrogen bonding of ν (O-H----). The IR signal appears at 1688 cm^-1 assigned to ν (C=O) lactone carbonyl, 1644 cm^-1 ν (C=N) (azomethine), 1333 cm^-1 ν (C-N) aryl azomethine and 1214 cm^-1 ν (C-O) enolic [7]. The absence of peak at weak band at 3419 cm^-1 in the spectra of all metal complexes suggests the deprotonating enolic oxygen and azomethine nitrogen in coordination to the metal ion.

Table 2.Characteristic IR Frequencies (cm^-1) of the Ligand and Metal Complexes.

Magnetic measurement and electronic absorption spectra

The electronic spectra of the Cu (II) complexes in DMSO indicated the band at 11155-16450 cm^-1 for ligand D Lassigned to ²Eg­> ²T_2g transition which was a characteristic of octahedral geometry [11]. This was further supported by magnetic moment values (1.78 µ_B) within the required range for d⁹-system [12]. The electronic spectra of the Ni (II)-DL complex revealed three bands in the range of 10727(v₁), 15345(v₂), and 23730(v₃) assigned to the transition of ³A_2g(F)­>³T_2g and a charge transfer transition, respectively, suggesting the octahedral geometry of the complexes [13]. The electronic absorption spectrum of Co (II) complexes had three bands in the range of 12458 (v₁), 18565(v₂), and 27475 (v₃) which may be attributed to three spin-allowed transitions of ⁴T_1g (F) ­>⁴T_2g(F), ⁴T_1g (F) ­>⁴A_2g(F), and ⁴T_1g (F) ­>⁴T_2g(P), respectively, suggesting an octahedral geometry. The effective magnetic moment values (4.41 µ_B) were found to be well within the range, as expected for octahedral geometry [14]. The electronic spectra Mn(II) complex showed two bands at 17672 cm-¹ (v₁) and 23204 cm^-1 (v₂) assigned to the transition of ⁶A_1g­> ⁴T_1g and ⁶A_1g ­>⁴T_2g, respectively, indicating octahedral geometry. The magnetic moment value (5.05 µ_B) which was slightly lower than the spin only value expected for octahedral Mn(II) complexes [15]. This may be due to the presence of magnetic exchange and small traces of Mn(II) species. The electronic spectra of the Fe(III) complexes demonstrated three bands at 16243 (v₁), 22690 (v₂), and 29476 cm^-1 (v₃) assigned to the transitions of ⁶A_1g­>⁴T_1g(D), ⁶A_1g­>⁴T_1g,and ⁶A_1g­>⁴T_1g, respectively, and the magnetic moment values (5.6 µ_B) suggesting a high spin octahedral geometry [16, 17]. The Cd complex was diamagnetic. The magnetic and electronic spectral data was relevant for the proposed structure of complexes depicted in Table 3.

Table 3. Magnetic and electronic absorption spectral data (in DMSO) of the complexes

Thermo-analytical techniques

There was no mass loss up to 200 °C indicating the absence of lattice and coordinated water in the TG curve of Ni (II)-DL metal complex and exhibited a high initial thermal stability. The TG thermo gram showed the first step decomposition in the temperature range of 200-440 °C with mass loss of 38.00% and the corresponding exothermic peaks at 195 °C and 370 °C in the DTA curve specified the decomposition of non-coordinated part of ligand in a relatively fast but moderate oxidation process. The second step of decomposition occurred in the temperature range of 500-700 °C with a further mass loss of the next 52.08% indicating the decomposition of the coordinated part of a ligand of the complex through a slow process. The organic part decomposed in the temperature range of 500-700 °C that was also shown by broad exotherms in this range in the DTA curve. Finally, the residue of Nickel oxide remained at 720 °C [18].

XRD powder diffraction

The unit cell of Fe (III)-DLcomplex yielded the constant values of lattice as a=24.7805( ), b =7.5469( ), c=24.788( ), and unit cell volume of V =624.8765( ) ³. α=ϒ=90⁰ and β=104.32ºwere required for the sample to be crystal system monoclinic and lattice type-P was found to be satisfactory [19].

Antibacterial and antifungal activity

The antimicrobial potentiality of DHA Schiff base ligands and their metal complexes was carried out by the agar well diffusion method [20]. The minimum inhibitory concentration of DHA Schiff base ligands and their metal complexes was determined by adopting the standard procedure of the National Committee for Clinical Laboratory standard (2004) [21]. In vitro antibacterial and antifungal activity was screened by considering the zone of growth inhibition. The synthesized Schiff base L and its metal complexes were screened with their different concentrations and the standard antibiotics such as streptomycin (1 mg/mL) and griseofluvin (1 mg/mL). 

However, DHA Schiff base ligand weak antibacterial revealed a strong antifungal activity. Antibacterial activity significantly increased on coordination because it reduced the polarity of metal ion due to the partial sharing of positive charge with ligands that increase the lipophilic nature of metal ion in complex [4, 22]. This enhanced antimicrobial activity of metal complexes is due to the lipophilic nature of metal ion in complex [23]. The Antimicrobial activity data is presented in Table 4 and Figure 4.

Table 4.Antibacterial and Antifungal Activities Data

m=Zone of inhibition in mm

Figure 4. Graphical presentation of antibacterial and antifungal activities

Conclusion

Composition of Schiff base and metal complexes was confirmed by elemental analysis. The structure of Schiff base and metal complexes was confirmed by IR and ¹H-NMR study. The magnetic susceptibility measurement and electronic spectral data of the complexes suggested the octahedral geometry. Thermal study showed thermal stability, and X- Ray diffraction studies concluded that all transition metal complexes synthesized having monoclinic crystal structure and lattice type-P. The DHA Schiff base ligand exhibited a weak activity against bacteria, but it revealed a strong antifungal activity. Antibacterial activity significantly increased on coordination due to the increase in lipophilic nature of metal ion in complex. Co (II) and Cu (II) complexes exhibited the maximum zone of inhibition, while Fe (III) complex indicated the minimum zone of inhibition.

Acknowledgement

The authors would like to present their gratitude to the UGC (WRO) for the award of TRF under UGC 11^th plan period. Authors are also grateful to the Principal R.S.M., Latur BCUD, SRTM University, Nanded, and all staff members of Department of Chemistry and Analytical Chemistry.

Orcid

Dhananjay Gangadhar Palke:https://www.orcid.org/0000-0003-4994-9818

, anti-