Synthèse et Caractérisation des complexes de Co(II), Ni(II) et Cu(II) avec des bases de Schiff tridentées et tétradentées: Voltammétrie cyclique et évaluation des activités antimicrobiennes et antioxydantes.

Abstract

A series of Co(II), Ni(II), and Cu(II) complexes were synthesized from two tridentate and two tetradentate Schiff bases, using ethanol and methanol as solvents. All compounds were synthesized by reflux in the temperature range of 56-70°C. The synthesized compounds, were obtained in the % yield range between 60-92%. They were characterized using methods such as: micro-elemental analysis, melting point, solubility test, molar conductivity measurement, mass spectroscopy, Nuclear Magnetic Resonance, infrared, UV/Visible spectroscopy and thermogravimetric analysis. All synthesized compounds are soluble in DMSO and DMF. The ligands melt at temperatures ranging from 120° to 160°C while the decomposition residues of the complexes melt at temperatures above 306°C (temperature limit of the apparatus used). Based on spectroscopic studies, the synthesized Schiff bases coordinates to metals via the nitrogen atoms of the azomethine, oxygen of the phenol, naphthol and sulfur atoms of the thiophenol groups, forming tridentate and tetradentate chelating agents. Molar conductivity measurement indicates a molecular nature for all complexes. A square planar geometry was considered for the [CuL1]2, [CuL2]2, [CoL3] and [CoL4] complexes while a tetrahedral geometry was considered for the Co(II) and Ni(II) complexes with the H2L1 and H2L2 ligands. On the other hand, a pyramidal geometry with a square base was considered for the Cu(II) and Ni(II) complexes with the ligands H2L3 and H2L4. The cyclic voltammetric study of the complexes indicates Cu(II)/Cu(I), Co(III)/Co(II) redox systems, Co(II)/Co(0) Ni(II)/Ni(I) and Ni(III)/Ni(II) redox systems for the complexes of tridentate Schiff bases while Cu(III)/Cu(II), Co(III)/Co(II), and Ni(III)/Ni(II) redox systems for the complexes of tetradentate Schiff bases. The study of the kinetics of the different systems showed that the regime is purely diffusional in each case and that the redox systems are mostly quasi-reversible. The Schiff bases and their complexes all have low antioxidant activities compared to those of Trolox which is the reference used. On the other hand, the Schiff bases H2L1 and H2L2 show a high antioxidant activity close to that of Trolox and which evolves in the same way. This important antioxidant activity compared to those of the complexes is certainly due to the presence of phenolic, naphtholic and thiophenolic protons which are easily given to the DPPH radical. On the other hand, all the complexes of the tetradentate Schiff bases presented significant antioxidant activities compared to that of their corresponding ligand H2L3 and H2L4. For antimicrobial activities, [CuL2]2, [CoL1]2, [CuL3], [CoL3], [CuL4], [CoL3] complexes were more active than the ligands on Escherichia coli while [CuL1]2, [CoL1]2, [NiL2]2 and [CuL3] complexes were more active than the ligands on Staphylococcus aureus. On the other hand, [CuL1]2, [NiL1]2, [NiL2]2 and [NiL3] were more active than the ligands on Proteus mirabilis. Schiff's base H2L2 and the [NiL2]2 complex performed as good fungicides with minimum inhibitory concentrations MIC ≤ 20 µg/mL and minimum fungicidal concentrations MFC ≤ 128 µg/mL. It is also important to note that Schiff base H2L2 showed appreciable activity (MIC ≤ 20 µg/mL) on Candida albicans, Candida krusei and Candida parapsilosis.