2-methoxy-4-(((5-nitropyridin-2-yl)imino)methyl)phenol Schiff base ligand and its Cu(II) and Zn(II) complexes: synthesis, characterization and biological investigations

In this current work we have prepared a Schiff base ligand, (HL) derived from 5- nitropyridine-2-amine with 4-hydroxy-3-methoxybenzaldehyde and its Cu(II), and Zn(II) in 2:1 stoichiometric ratio (2HL:M). The formation of the ligand and the metal complexes were evaluated by means of MS, FT-IR, UV-Visible, 1H-NMR, 13C-NMR and thermogravimetric methods. The free radical scavenging activity of compounds was evaluated through a sequence of in vitro assays viz., DPPH, ABTS and Superoxide where BHA was used as a positive controller. In vitro α-glucosidase inhibitory activities showed that complexes had considerable inhibitory potential when compared to the ligand.


Introduction
Compounds comprising an azomethine group (-CH ¼ N-) are imines or azomethines, which are more generally recognized as Schiff bases. Schiff bases are capable of stabilizing several metals in different oxidation states. In several areas such as biological, clinical, analytical and industrial fields [1], the complexes of Schiff bases have a wide range of useful applications. Moreover, in catalysis and organic synthesis they have essential functions [2,3].
The complexation activity metal ions (Zn, Hg, or Cd) in the existence of multidentate Schiff base ligands has been systematically deliberated in the area of coordination chemistry over the last few decades [2,3,4,5,6,7].
Considering flexible functional behaviour, interesting structural motifs [8,9,10,11] and optoelectronics properties [12,13,14], the molecular metal ensembles formed from Schiff base ligands and metal ions have recently gained a lot of attention.
The steric or electronic characteristics of ligands may be studied due to their easy preparations, wide structural variations, various denticities, and ease of tunability in ligands. Schiff bases have lately been used by a number of curious inorganic chemists in their study. Therefore, Schiff base complex formulation has been extensively studied in the current research scenario on account of the numerous modes of applications such as catalysis [15,16,17], magnetism [18,19], crystal engineering synthons and common chemistry materials [20,21,22].
Attention in Schiff base complexes has grown in tandem with the development of bioinorganic chemistry, subsequently it has become clear that numerous of these compounds might assist as models for biologically vital organisms [23,24,25,26,27]. The notable biological behaviour of acid hydrazides R-CO-NH-NH 2 , a Schiff base class, and their related arylhydrazones, R-CO-NH-NCH-R 0 , as well as their dependency on their mechanism of chelation with transition metal ions contemporary in the living system, have altogether important [28,29]. Since they have numerous applications in the different fields like industrial, antifungal, antibacterial, anticancer and herbicidal. Schiff base metal complexes have been extensively researched [30,31] in various bioorganic and bioinorganic fields.
Inorganic materials play a key role in biomedical and biological medicinal systems. Many organic chemicals used in medicine are activated or bio-transformed by metal ion metabolism, and some of them have an entirely organic mechanism of action. In the form of metal complex, several drugs have been modified based on toxicological and pharmacological properties. Schiff bases are flexible -CH ¼ N-(imine) containing compounds with an extensive biological activity and metal integration in the form of complexes exhibiting some degree of antibacterial, antifungal, antitumor and anti-inflammatory activities [32,33].
The Schiff base metal complexes of Cu(II) and Zn(II) ions played a crucial part in the growth of coordination chemistry. Transition metal complexes have sparked attention due to their DNA binding and cleavage characteristics below physiological circumstances. Current research focuses on applications of metal complexes as chemical nucleases. It has been shown that inorganic complexes as chemical nucleases are the subject of current research. It has been shown that inorganic complexes can be used as sequence specific DNA binding agents in foot printing tests, as investigative agents in pharmaceutical applications and for genomic research.

Materials and methods
5-nitropyridine-2-amine with 4-hydroxy-3-methoxybenzaldehyde was obtained from Sigma Aldrich. All the chemicals employed for the work were attained from Merck and used as usual. The accomplishment of a reaction was observed by means of thin layer chromatography (TLC) made on pre-coated silica gel plates (Merck). Mass spectra were recorded on an Agilent technologies (HP) 5973 mass spectrometer operating at an ionization potential of 70 eV. The 1 H and 13 C-NMR spectra were recorded with a Varian 300 MHz in DMSO-d 6 as a solvent against tetramethylsilane as an internal standard. Infrared spectra were recorded on a Perki-nElmer FT-IR type 1650 spectrophotometer in the region 4000-400 cm À1 using KBr pellets. The UV-Visible spectra were recorded using UV-1800 spectrophotometer (Shimadzu). Thermogravimetric analysis (TGA) was supported out on a Universal TGA Q50 instrument at a heating rate of 2 C/min between 30 and 1000 C. The powder X-Ray diffraction pattern were recorded using Proto Manufacturing INC, XRD benchtop powder diffractometer using Cu-Kα radiation as a source (λ ¼ 0.15443nm).

Synthesis of 2-methoxy-4-(((5-nitropyridin-2-yl)imino)methyl)phenol (HL)
The Schiff base ligand (HL) was synthesized [34] by condensing of 5-nitropyridine-2-amine with 4-hydroxy-3-methoxybenzaldehyde in 1:1 M ratio using ethanol as a solvent. The reaction mixture was refluxed for 6 h. The progress of the reaction was observed by TLC. After the completion of a reaction, the solvent was evaporated under pressure by means of rotary evaporator, yellow precipitate was washed with ethanol, filtered off and product was recrystallized with warm ethanol. The reaction scheme of synthesis of ligand (HL) was shown in Figure 1.

Synthesis of metal complexes
The Cu(II) and Zn(II) Schiff base complexes were synthesized by adding an aqueous metal salt solutions of Cu(II) and Zn(II) to an ethanolic solution of ligand, HL in 2:1 (L:M) molar ratio [35]. The resulting mixtures were refluxed for 2h upon which the complexes were precipitated. The precipitated product was filtered, washed with solvent and dried. The proposed structures of complexes were shown in Figure 2.

Antioxidant assays
Three well-known methods for determining and assessing radical scavenging activities, namely DPPH free radical, ABTS cation radical, and superoxide anion radical scavenging activities, were used in this investigation [36]. Radical scavenging studies were expressed as EC 50 values. EC 50 denotes that the investigated samples scavenge 50% of free, cation, and anion radicals. As a positive control, the antioxidant butylated hydroxylanisole (BHA) was used.
The IC 50 values were calculated using a curve that plotted the % inhibition of each sample against its concentration. Each experiment was carried out in triplicate with adequate blanks in between. The IC 50 is defined as the concentration required to inhibit 50% of α -glucosidase activity under the specific test conditions.

Mass spectroscopy
For the validation of synthesised ligand, the mass spectrum of the Schiff base ligand (HL) was obtained and examined. The mass spectrum of the synthesized ligand is shown in Figure 3a. The ligand's molecular ion peak was observed at 273.1883.
The molecular ion peak for Cu and Zn complexes was observed at m/z ¼ 607.2324 and 610.3596 respectively and showed in Figure 3b, c. It indicates the co-ordination of Cu and Zn ions with the HL ligand.

NMR spectroscopy
The 1 H and 13 C-NMR spectra of the Schiff base ligand (HL) were obtained in DMSO-d 6 . Figure 4a depicts the 1 H-NMR spectrum of HL. At δ ¼ 9.67 ppm, the singlet peak resembles to azomethine group, confirming the establishment of an imine bond in a ligand. The singlet peak at δ ¼ 10.31 ppm corresponds to the hydroxy proton present in a ligand. The  aromatic protons of HL were corresponding to the peaks between δ ¼ 6.87 and 7.81 ppm. The peak at δ ¼ 3.99 ppm indicates the presence of a methoxy proton in the ligand [38].   Whereas in the 1 H NMR spectra of Cu(II) and Zn(II) metal complexes shown in Figure 4b, c, the azomethine proton shifted to lower chemical shift value owing to the coordination of metal to the azomethine nitrogen. The singlet hydroxy proton peak present in a ligand was disappears in the complexes which indicates the formation of M-O bond in the complexes.
In the 13 C-NMR, the peak seen in HL at δ ¼ 161.5 ppm supports the existence of the azomethine group, as shown in Figure 5a. Carbon of the methoxy group (-OCH 3 ) was found at δ ¼ 57.1 ppm. Between δ ¼ 113.4 and 155.1 ppm, aromatic ring carbon signals were detected. The synthesis of the reported ligand, HL, is therefore confirmed by both 1 H NMR and 13 C-NMR findings [39].     However, in 13 C-NMR of Cu(II) and Zn(II) metal complexes shown in Figure 5b, c, the azomethine carbon peak shifted to up field due to the complexation which supports the formation of metal complexes.

IR spectra
The FT-IR spectra of the synthesized ligand was showed in Figure 6a. The formation of the HL ligand and its complexes with the appropriate metals has been confirmed by the detection of -CH ¼ N-group peaks. In HL ligand, the peak was found at stretching frequency of 1669 cm À1 indicates the formation of imine bond in the ligand. However, in the case of metal complexes, this peak was displaced to lower/higher levels, owing to an increase in conjugation as showed in Figure 6b,c. As a result, as shown in Table 1, the maxima of -CH ¼ N-groups were observed at stretching frequencies of 1656cm À1 and 1675cm À1 for Cu and Zn complexes, respectively [34].
Meanwhile, the peak of metal-nitrogen (M-N) and metal-oxygen (M-O) bonds, has verified the formation of metal complexes. For Cu and Zn complexes, M-N bonds were detected at 451 and 483 cm À1 , respectively, and M-O bonds were observed at 573 and 586 cm À1 [24]. The existence of these peaks, which are completely absent in the spectrum of HL ligand as given in Table 1, further supports the formation of metal complexes. Figure 7a shows the UV-Visible spectra of the ligand in DMSO at room temperature. In the spectrum of the free Schiff base ligand, absorption bands about 324 and 359 nm were found, indicating the existence of π→π* and n→π* transitions related with benzene rings and azomethine groups, respectively. As a result of coordination to metal, the π-π* and nπ* transitions in the metal complexes were strapped to longer wavelengths disclosed in Figure 7b, c, representing the formation of Schiff base metal complexes [25]. Electronic absorption spectral data of HL ligand and its metal complexes were tabulated in Table 2.

Thermal property
The thermal properties of the complexes were considered by TGA and DTG studies. Figure 8 shows the TGA and DTG curves of Cu and Zn complexes under nitrogen atmosphere. From the TGA curves it is clear that Cu and Zn complexes undergo decomposition in three steps and leaving a residue as their respective metal oxides. The step wise decomposition of the complexes was given in a Table 3.
The findings indicated that the Cu(II) complex decomposes in three stages as shown in Figure 8a. In the first step, the temperature was raised from 25.08 C to 223.04 C, resulting in a 9.32% weight reduction, this is probable because of the loss of water molecules. The loss of pyridine moiety of a ligand in the second stage, between 223.04 C and 527.11 C, corresponds to a weight loss of 18.56%, and the Schiff base ligand (HL) in the third step, between 527.11 C and 691.77 C, corresponds to a weight loss of 28.67%, leaving 43.45% residue as CuO.
Similarly, the findings indicated that the Zn(II) complex decomposes in three stages as shown in Figure 8b. In the first stage, the temperature was raised from 29.20 C to 224.04 C, resulting in a weight loss of 10.56% due to the loss of water molecules present in a compound. The loss of pyridine moiety of a ligand in the second step, between 224.04 C and 531.71 C, corresponds to a weight loss of 34.22%, and the Schiff base ligand (HL) in the third step, between 531.71 C and 710.98 C, corresponds to a weight loss of 24.71%, leaving 27.54% residue as ZnO.

Powder XRD studies
The powder XRD patterns of Complexes were recorded in the range of 2Ɵ ¼ 10-70 and shown in Figure 9a, b.

Antioxidant activity
The free radical scavenging activities of test compounds were assessed using a series of in vitro procedures, including DPPH, ABTS, and Superoxide, with BHA as a positive control. When expressed as EC 50 values, Table 4 gives that the compounds revealed lesser (p < 0.05) radical scavenging measurements than the standard (positive control) (mg of tests per ml). In each test shown in this study, complexes outperformed ligands, with activity in the sequence BHA > complex > ligand. The HL-Cu complex exhibited higher free radical scavenging activity in three antioxidant assays. The HL-Cu combination possesses   Figure 11. Inhibitory activities of ligand and its metal complexes against α-amylase and α-glucosidase enzymes.
substantial antioxidant abilities and is statistically equivalent to the positive control (p < 0.05), affording to the findings. The antioxidant activity of the Schiff base ligand and its Cu(II) and Zn(II) complexes are revealed in Figure 10.

Inhibitory effects on yeast α-glucosidase and α-amylase
In vitro α-glucosidase inhibitory experiments revealed that the complex was more effective at inhibiting the enzyme than its ligand. The IC 50 values ranged from 2.14 mg/mL to 1.08 mg/mL. Acarbose has an IC 50 value of 0.70 mg/mL under the same conditions. In terms of IC 50 values, it is obvious that the HL-Cu complex tested strongly inhibited yeast α -glucosidase and was statistically similar to acarbose (p < 0.05) and higher than ligand ( Table 5). The inhibition is in the order: Acarbose > complex > ligand.
Besides, similar studies were performed to measure whether ligand and complex also inhibited α-amylase, another key carbohydrate hydrolysing enzyme. The 50% inhibition of α-amylase by the test compounds is detailed in Table 5.
The ligand and complex were also tested to see if they inhibited α -amylase, another essential carbohydrate hydrolysing enzyme. Table 5 provides the 50% inhibition of α-amylase by the test samples. The HL-Cu combination (IC 50 : 1.55 mg/mL) displayed the highest inhibitory activity when compared to its ligand (IC 50 : 3.08 mg/mL). Compounds have a stronger (p < 0.05) α-amylase inhibitory action (based on IC 50 values) than acarbose (IC 50 :0.50 mg/mL). The inhibitory actions of Schiff base ligand and its Cu(II) and Zn(II) complexes are shown in Figure 11.

Conclusion
In summary, in this present work we have synthesized 2-methoxy-4-(((5-nitropyridin-2-yl)imino)methyl)phenol (HL) and its complexes Cu(II) and Zn(II) metal complexes. Different spectral analysis was used to describe the reported compounds. The ability of test materials to scavenge free radicals was determined using a number of in vitro tests, including DPPH, ABTS, and Superoxide, with BHA serving as a positive control. Cu(II) complex has a high antioxidant ability and has a considerably lower (p0.05) antioxidant activity than the positive control. The ligand and its complexes were shown to have effective inhibitory capability against α -glucosidase enzyme. When compared to other complexes, the Cu(II) complex (IC 50 : 108 mg/mL) had the maximum inhibitory activity, while the ligand (IC 50 : 2.14 mg/mL) had the lowest inhibitory impact.

Author contribution statement
Deepika P: Conceived and designed the experiments; Performed the experiments; Wrote the paper.
Vinusha H. M: Contributed reagents, materials, analysis tools or data; Wrote the paper.
Prithvi S Shirahatti: Contributed reagents, materials, analysis tools or data.

Funding statement
This work was supported by TEQIP-III, NPIU, Sri Jayachamarajendra College of Engineering and JSS Science & Technology University, Mysuru.

Data availability statement
Data will be made available on request.