Synthesis, Characterization and Biological Activities of Binuclear Metal Complexes of 2-Benzoylpyridine and Phenyl(Pyridin-2-yl)Methanediol Derived from 1-Phenyl-1-(Pyridin-2-yl)-N-(Pyrimidin-2-yl)Methanimine Dihydrate Schiff Base

The Schiff base, 1-phenyl-1-(pyridin-2-yl)-N-(pyrimidin-2-yl)methanimine dihydrate (L1) has been synthesized by the condensation reaction between 2-aminopyrimidine and 2-benzoylpyridine and characterized using C-NMR, H-NMR, microanalysis, FT-IR, DEPT-135, HSQC, HMBC, COZY, NOESY. The reaction of 1-phenyl-1-(pyridin-2-yl)-N-(pyrimidin-2-yl)methanimine dihydrate (L1) with salts of V(IV), Co(II) and Cu(II), however, resulted in the hydrolysis of L1 to give binuclear metal complexes of 2-benzoylpyridine (L2) and phenyl(pyridin-2-yl)methanediol (L3) which were characterized using UV-visible spectroscopy, and TGA. The single crystal x-ray structure determined for the Copper(II) complex revealed that we obtained a compound previously obtained using a different method of synthesis. The Schiff base ligand (L1) is soluble in methanol, ethanol, DMSO, acetone and DMF. Microanalysis and Spectroscopic studies indicated that binuclear metal complexes were obtained by the coordination of metal ion to 2-benzoylpyridine (L2) and phenyl(pyridin-2-yl)methanediol (L3) from the hydrolysis of L1. Spectroscopic and elemental analysis reveal the formation of square pyramidal complexes of Co(II) and Cu(II) and a square planar complex of V(IV). In vitro antibacterial and antifungal activity against three bacterial strains (Escherichia coli, Klebsiella pneumoniae and Staphylococcus aureus) and three fungal stains (Candida albicans, Candida glabrata and Candida parapsilosis) showed moderate biological activity. Antioxidant studies reveal that only the binuclear How to cite this paper: Kamga, F.A.N., Mainsah, E.N., Kuate, M. and Ndifon, P.T. (2021) Synthesis, Characterization and Biological Activities of Binuclear Metal Complexes of 2-Benzoylpyridine and Phenyl (Pyridin-2-yl)Methanediol Derived from 1Phenyl-1-(Pyridin-2-yl)-N-(Pyrimidin-2-yl ) Methanimine Dihydrate Schiff Base. Open Journal of Inorganic Chemistry, 11, 20-42. https://doi.org/10.4236/ojic.2021.111002 Received: December 7, 2020 Accepted: January 26, 2021 Published: January 29, 2021 Copyright © 2021 by author(s) and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/ Open Access F. A. N. Kamga et al. DOI: 10.4236/ojic.2021.111002 21 Open Journal of Inorganic Chemistry Co(II) and oxovanadium(II) complexes are potent to eliminate free radicals.


Introduction
Heterocyclic Schiff base ligands containing O-or N-donors and their metal complexes have been shown to exhibit interesting properties [1] [2] [3] and those containing pyrimidine and pyridine rings represent a promising class of compounds due to their inherent biological and pharmaceutical properties [2] [3] [4] [5]. Pyrimidine and pyridine based compounds exhibit a broad spectrum of biological activities [6] [7]. Pyrimidine with its interesting heterocyclic structure has extensively been used in drug design due to its potential antimicrobial, antifungal, antiviral and antitumor properties. Many ligands containing the pyridine ring and their transition metal complexes have been synthesized and tested for biological and therapeutic properties where they sometimes act as models for the active sites of biomolecules [4] [8] [9]. The importance of 2-benzoylpyridine metal complexes, for example, is due to their ability to facilitate substrate binding thus stabilizing the reactive intermediates of transition metals [4] [10].
Copper complexes have recently been the subject of intense research because of their potentials as radiopharmaceuticals for targeting hypoxic tissues [6] and as effective drugs for the treatment of refractory neuroblastoma in children [7].
Copper is an important trace element for plants and animals and is involved in mixed ligand complex formation in a number of biological processes [11]. Many cobalt(II) complexes of Schiff base ligands containing 2-benzoylpyridine behaved as a growth inhibitor for microorganisms [9]. Binuclear copper(II) complexes with monoatomic bridges, such as halides, have attracted recent attention due to their structural diversity and interesting catalytic, magnetic and biological properties [8] [12]. Depending on the nature of the co-ligands, chloride or oxygen bridged compounds, having square planar Cu(O) 2 Cu bridging loops show different molecular structures [13] [14]. The crystalline architectures of these compounds are found to be interesting due to variations in the nature of intervening intermolecular forces.
The presence of the primary amine functionality in heterocyclic compounds containing rings such as pyridine and pyrimidine has prompted researchers to embark on the synthesis of Schiff bases of pyridine derivatives and their metal complexes. Also, Schiff bases easily undergo hydrolysis which is considered to be a very significant reaction in many pharmacological and biological processes resulting in the formation of new species [15] [16]. Recently our group embarks on studies on the biological activities of complexes of heterocyclic Schiff base ligands Open Journal of Inorganic Chemistry [17]. In this paper we report on the synthesis, characterization and evaluation of the biological activities of binuclear metal complexes of 2-benzoylpyridine (L 2 ) and phenyl(pyridin-2-yl)methanediol (L 3 ) obtained from the hydrolysis of 1phenyl-1-(pyridin-2-yl)-N-(pyrimidin-2-yl)methanimine dihydrate (L 1 ) in the presence of salts of V(IV), Co(II) and Cu(II).

Materials and Methods
All reagents and solvents were obtained from commercial sources and used as received without any further purification.

Synthesis
The Schiff base ligand and its complexes were synthesized according to reported general synthetic procedures with some slight modifications [18] [19].

Synthesis of the Schiff Base (L1)
To a solution of 10 mL of methanol containing 0.95 g (10 mmol) of 2-aminopy-

Crystal Structure Determination
A suitable single crystal of C 51 H 40 Cu 2 N 7 O 12 was mounted on a Rigaku Supernova diffractometer and data were collected using Mo Kα (λ = 0.71073 Å) radiation at a temperature of 95.01 K. The structure was solved using olex2 [20] with the ShelXT [21] structure solution program using Intrinsic Phasing and refined with the olex2 refine refinement package [20], using Gauss-Newton minimisation.

Antimicrobial Study
The synthesized compounds were used for antimicrobial screening by testing against three bacterial strains (Eschericia coli, Klebsiella pneumonia and Staphylococcus aureus) and three fungal strains, (Candida albicans, Candida glabrata and Candida parapsilosi), Ceftriaxone, Rifampicin and Fluconazole were used as antibacterial and antifungal references respectively. Antimicrobial studies were carried out using the disc diffusion method based on the determination of the zone of inhibition around the paper discs [22]. In fact, 40 mg of each compound was dissolved in 1 mL of 10% DMSO making a final concentration of 40 mg/mL. Discs of wattman N˚3 filter paper of 5 mm in diameter were deposited on the surface of the seeded medium. 10 µL of products were then deposited on each corresponding disk and left for pre-diffusion for 15 minutes at room temperature before being incubated at 37˚C for 24 hours for bacteria and 48 hours for yeasts. The antimicrobial activities were assessed by measuring the diameter of the zone of inhibition of the bacterial and fungal growth around every Disc with a ruler following two axes. For every product tested, three determinations were made.

In Vitro Antioxidant Activities
The synthesized compounds were tested for in vitro antioxidant activities at 37˚C using free both the radical scavenging assay (DPPH) method and Ferric Ion Reducing Antioxidant Power Assay (FRAP) method.

DPPH Free Radical Trapping Assay Method
In this method, the antiradical activity of the ligand L I and binuclear metal complexes were tested against the stable free radical of 1, 1-diphenyl-2-picrylhydrazyl (DPPH), using the free radical scavenging assay with some modifications [23] where RSA represents the radical Scavenging Activity; Ao is the absorbance of white (DPPH + methanol) and As is the absorbance of the radical DPPH + compounds.

Ferric Ion Reducing Antioxidant Power Assay (FRAP)
In this method, the antiradical power is based on the reduction of Fe 3+ to Fe 2+ by compounds which, in the presence of 1, 10-phenanthroline, form a brown or orange-red colored complex [24]. The complex absorbs at 505 nm and the intensity of the coloration is proportional to the amount of Fe 3+ converted to Fe 2+ by the compound as shown in Scheme 2.

Synthesis and Characterization of the Schiff Base L1
The Schiff base L 1 was prepared by the condensation of equimolar amounts of The Schiff base is brownish yellow, stable at room temperature, soluble in organic solvents such as methanol, ethanol, DMSO, acetone and DMF. The 1D and 2D NMR sequences were recorded for the characterization of the molecule, in order to provide the structure of the ligand synthesized.

Infrared Study of the Schiff Base L1
The infrared spectrum of the Schiff base is presented in Figure 1.

1 H NMR Spectral Analysis of the Schiff Base L1
The 1 H-NMR spectrum of the Schiff base L 1 is presented in Figure 2 and the 1 H-NMR data of the compounds and the chemical shifts of the different types of protons are listed in Table 1.

13 C NMR Spectral Analysis of the Schiff Base L1
Analysis of the 13  ppm, corresponding to the carbon at C-4 and C-9 of the pyrimidine and pyridine ring. Another signal at δ = 136.48 corresponding to the C-10 carbon of the aromatic ring. All the data result of 13 C-NMR analysis is summarize in Table 1.    The analysis of all the IR and NMR data confirm the proposed structure of the Schiff base ligand (L 1 ) as shown in Scheme 3.

Synthesis and Characterization of Metal Complexes of the Schiff Base L1
The Schiff base (L 1 ) was reacted with metal salts in a 1:2 molar ratio in an at-  Table 2.

Infrared Study of the Binuclear Metal(II) Complexes
The IR spectrum of the ligand when compared to that the of M(II) complexes reveals the binding of the ligands, L 2 and L 3 to M(II) ions. In the spectra of the complexes ( Figure 6) and IR data (Table 3) Table 3.

Conductivity Measurement
The molar conductivities of the complexes were determined in methanol for

UV-Visible Spectroscopy
The electronic spectral data for the compounds are summarized in Table 4. High intensity bands appeared in the ultraviolet and near-visible regions at 382 and 460 nm in the spectrum of ligand L I and was assigned to the n  π* transition. In

Thermogravimetric Analysis of the Binuclear Copper (II) Complex
The differential and thermogravimetric analyses of the [Cu 2 (L 2 ) 2 (L 3 ) 2 )] complex was determined under an inert atmosphere in the range 0˚C and 600˚C. The thermogram for the [Cu 2 (L 2 ) 2 (L 3 ) 2 ] is represented in Figure 8.

Crystal Structure of the Binuclear Copper Complex
Suitable crystals of Cu(II) complex were obtained for single crystal x-ray structure determination. Table 5 presents the crystal data and refinement parameters.     The molecular structure of the binuclear Cu(II) complex is given in Figure 9 and the crystallographic data summarized in Table 5. The complex crystallizes in a monoclinic crystal system with two ligands in the unit cell just like the reported structure [27]. The selected bond lengths and angles are shown in Table   6. The molecular structure of the complex consists of a non-centro symmetric [Cu 2 (L 1 ) 2 (L 3 ) 2 ] 2+ cation, a water molecule, an ethanolate ion and two nitrate ions.
The two copper atoms are separated by a distance of 3.023 Å comparable to that reported for bridged alkoxo complexes. On the other hand, the bridge angles Cu (1)

Antimicrobial Activity of the Schiff Base and Binuclear Metal Complexes
The Schiff base and complexes were tested against three bacterial strains Staphylococcus aureus, Klebsiella pneumoniae and Eschericia coli and three fungal strains Candida albicans, Candida glabrata and Candida parapsilosis. Rifampicin and Fluconazole were used as the standards for bacterial and fungal studies respectively. The diameters of the zone of inhibition of the ligand and binuclear complexes were determined using disc diffusion method [23]. The diameters of zone of inhibition of the bacteria are summarized in

DPPH Free Radical Scavenging Test
The synthesized compounds were screened for their free radical scavenging activities by DPPH method using Vitamin C as a standard. Antioxidant activities of these compounds were investigated by determining the concentration of substance necessary to reduce 50% of the DPPH • radical (EC 50 values) of each compound. The results of the determination of EC 50 values of the compounds are shown in Table 8. It is evident from these results that, all the compounds do not exhibited antiradical activity even at different concentrations. This observation could be due to the absence of proton in the Schiff base structure and the deprotonation of gem-diol proton in the complexes during chelation.

Ferric Ion Reducing Antioxidant Power Assay (FRAP) Test
The reducing powers of the compounds are associated with their antiradical power. The reducing powers of the synthesized compounds were determined using the FRAP method. This technique determines the ability of the tested compounds to reduce ferric iron (Fe 3+ ) present in K 3 Fe(CN) 6 complex to ferrous iron (Fe 2+ ) [38]. Only [(VO) 2 (L 2 ) 2 ] and [Co 2 (L 2 )(L 3 ) 2 ] showed reduction capacity which are presented in Figure 10 and Figure 11. It is evident from these results that, the reducing ability of the compounds are concentration dependent [39].
From the result, we find that the reduction of iron by the FRAP method is more pronounced by [(VO 2 )(L 2 ) 2 ] and [Co 2 (L 2 )(L 3 ) 2 ] complexes with maxima at Optical Density, OD = 2.87 and 2.94. We can deduce that [(VO 2 )(L 2 ) 2 ] and [Co 2 (L 2 )(L 3 ) 2 ] complexes have the capacity to reduce iron. This ability to reduce iron is greater than that of ascorbic acid (OD = 2.12), employed as standard.
Thus, the antioxidant activity of these complexes can be attributed to the oxidation of Co(II) to Co(III) and V(IV) to V(V) during the reduction of Iron(III) to Iron(II).