Facile and rapid synthesis of a new series of Schiff bases: characterization and evaluation of the physicochemical properties and biological activities

Abstract

Research background on Schiff bases: these organic compounds, which are secondary aldo-/keto-imines, vary according to the nature of the electron acceptor. The nitrogen atom of imine (azomethine) functional group (CHN) binds the alkyl or aryl group and can coordinate with a wide range of metal ions, such as (Co(II), Ni(II), Cu(II), Pd(II), V(IV), Ti(IV), and U(III, IV, V)). Electron donation from the heteroatoms N, S, or O modifies the electron density of the central metal and stabilizes various oxidation states. They are used in chemotherapy (due to their biological activity), as well as in organic synthesis, pigments, dyes, polymerization, corrosion control and enzymatic catalysis. Methods employed: an ethanolic solution of 3-ethoxy-2-hydroxy benzaldehyde was condensed with a primary aromatic amine in various stochiometric molar ratios (o-phenylenediamine/2,6-diamino pyridine/cytosine) in the presence of acetic acid. Reflux heating for varying reaction times facilitated nucleophilic addition, and elimination of the water, yielding three colored Schiff bases as solid products. Conclusions: a new series of Schiff base ligands (6,6′-1,2-phenylene-bis-methaneylylidene-bis 2-ethoxyphenol; 6,6′-pyridine-2,6-diylbis-methaneylylidene-bis 2-ethoxyphenol) and 4-(3-ethoxy-2-OH-benzylidene-amino)pyrimidin-2(1H)-one) were prepared from the precursor electron acceptor (A) 3-ethoxy-2-OH-benzaldehyde and electron donors (Ds) (1,2-benzenediamine (D1); 2,6-diaminopyridine (D2); and cytosine (D3)) in molar ratios of 2 : 1 for D1 and D2 due to the presence of diamine (NH₂) functional groups and a 1 : 1 ratio for D3. The Schiff base ligands (AD1, AD3) were prepared for the first time. Samples were characterized to confirm their molecular structures using CHN elemental analysis and mass, FTIR, ¹HNMR, and ¹³CNMR spectra. The biological and the optical activities depend on the molecular structures. Cytosine, a small single-ring pyrimidine base, naturally abundant in ribonucleic acid (RNA) and deoxyribonucleic (DNA), was used as a new sustainable precursor preparing the ligand AD3, effectively reducing the amount of the donor required by half while improving the biological and optical activities. Ligand 3 exhibited moderate antibacterial activity against the most tested bacterial strains and strongly affected Gram-positive bacteria. Both ligands 1 and 3 are recommended for sunscreen lotions. ligand 3 is superior because of its absorbance at the shorter wavelength 264 nm.