Synthesis, Characterization and Biological Activity of some Schiff Base Ligands Derived from Dimedone and some of their Metal Complexes

Abstract

This work dealt with the Synthesis, Characterization, Biological Activity of Some Schiff Base Ligands (Derived from Dimedone) and Some of their Metal Complexes. The Schiff base ligands synthesized were (3,3- dimethyl-5(phenylimino) cyclohexan-1-one [L1], [(4-chlorophenyl) imino]-3,3- dimethyl-5(phenylimino) cyclohexan-1-one [L2], and 3,3- dimethyl-5-[(4-nitrophenyl) imino]-3,3- dimethyl-5(phenylimino) cyclohexan-1-one [L3]). They were prepared by condensation of an equimolar amount of dimedone with different amines such as aniline, 4- Chloroaniline and 4-nitroaniline in ethanol. The synthesized ligands were characterized by melting point and spectroscopic techniques, including FT-IR, UV- VIS spectroscopy, NMR (1H and 13C), and mass spectroscopy. All synthesized Schiff base ligands were solid, air stable and high yield products. IR spectra of all ligands showed strong bands around 1531- 1593 cm_1 assigned to 𝜈 C=N stretching mode of the azomethine function of the ligands, and bands at 1569cm_1 ,1610cm_1,1631cm_1 corresponding to the C=O group in dimedone moiety of L1, L2, L3 respectively. Electronic spectra of Schiff bases ligands exhibited three absorption bands, the band appearing at lower energy was attributed to n → π* transition, while those appearing at higher energy were attributed to π → π* of the benzene ring and π → π* transition of the azomethine group. All the Schiff bases are known to behave as a bidentate with oxygen and nitrogen as donor atoms. The electronic impact mass spectra of L1 and L2 Schiff base ligands showed molecular ion(M+) peaks at m/z = 215 a.m.u and m/z = 249 a.m.u, corresponding to the [C14H17NO]+ and [C14H16ClNO]+, respectively, confirming the empirical formulae of the ligands, but L3 molecular ion(M+) peak was absent (NO2 appeared as free radical), but showed a series of peaks confirming the empirical formulae of the ligand. Their 1H NMR spectra exhibiting proton signals in the range 1.06-1.1 ppm was assigned to the two methyl groups, while the singlet at 2.08 – 2.51 ppm, to methylene proton neighboring of the imine, and signals at 2.37-3.36 ppm, to methylene proton neighboring carbonyl group of dimedone moiety. Triplet signals observed in the 6.64-7.30 ppm range were due to the aromatic protons of the Schiff bases. The 13C NMR spectrum showing signals at 19.02-30.20 ppm range was assigned to the two methyl groups, while the signals observed at 155.99-161.05 ppm, to imine group carbon and signals observed at 156.32-207.07ppm, to a carbon atom of the carbonyl group. Aromatic carbon exhibited signals at 124.18-128.39 ppm. Complexes of Mn(II), Fe(II), Cu(II), Pb(II) and V(V) with the above mentioned ligands were synthesized, by using 2:1 molar ratio of ligand: metal in ethanol. The complexes were characterized by melting point, UV–vis spectroscopy, FTIR, and molar conductivity studies. All the metal complexes had comparatively higher melting points than the Schiff base. In conductivity experiments, all metal chelates were shown to be non-electrolyes. On complexation, IR spectra showed that the 𝜈 C=N and 𝜈 C=O of the Schiff base ligands were shifted to lower or higher frequencies in the spectra of all the complexes indicating coordination of the azomethine nitrogen and oxygen of carbonyl moiety. The electronic spectra of all the complexes, showed that the interligand transitions were shifted indicating the coordination of the ligand to the metal ion. The biological activity of both the ligands and their complexes were screened against two Gram-positive bacteria (Staphylococcus aureus and Bacillus subtilis) and two Gram-negative bacteria (Escherichia coli and Pseudomonas aeruginosa). The anti-fungal activity was screened against Candida albicans. The anti-microbial studies suggested that some metal complexes showed higher activity against bacteria and fungi microorganisms than to that of the ligands against the same microorganisms under identical experimental conditions indicating that activity increased on chelation. Application of different metal complexes led to the changes in the activities against the microorganisms. The results show that all the metal complexes exhibited significant activity against fungi species and showed a reasonable activity against Gram-negative bacteria (Escherichia coli and Pseudomonas aeruginosa), while the growth of other organisms was moderately inhibited. For metal complexes, the anti-microbial activities increased in the order: V+5, Mn+2, Pb+2, Fe+2 and Cu+2, and the highest anti-microbial activity were observed for Cu2+ complexes.