Designing, spectroscopic characterization, biological screening and antioxidant activity of mononuclear transition metal complexes of bidentate Schiff base hydrazones

Abstract

In this article, a series of coordination complexes of Co(II), Ni(II), Cu(II) and Zn(II) metals were prepared by four novel Schiff base ligands (E)-N’-(4-(prop-2-yn-1-yloxy)benzylidene)benzohydrazide (HL1), (E)-4-chloro-N’-(4-(prop-2-yn-1-yloxy)benzylidene)benzohydrazide (HL2), (E)-N’-((2-(benzyloxy)naphthalen-1-yl)methylene)benzohydrazide (HL3), (E)-N’-((2-(benzyloxy)naphthalen-1-yl)methylene)-4-chlorobenzohydrazide (HL4) obtained by the condensation reaction of benzoic acid hydrazide/4-chloro benzoic acid hydrazide with oxy derivatives of 2-hydroxy-1-napthaldehyde/4-hydroxy benzaldehyde. The proposed structure of the complexes have been established by spectroscopic and analytical techniques like C, H, N and metal analysis, FT-IR, 1H and 13C NMR, mass, UV-Vis, EPR, fluorescence, molar conductance, magnetic susceptibility and TGA which concluded that Schiff bases act as bidentate NO donor ligands coordinating through nitrogen atom of azomethine linkage and uninegative oxygen atom of carbonyl group in enolic form and resulting in complexes of the type [M(L1–4)·(CH3COO)·3H2O] in 1:1 molar ratio. The synthesized compounds were screened for their in vitro antimicrobial activities against two gram +ve bacteria i.e. S. aureus and S. gordonii; two gram -ve bacteria i.e. E. coli and P. aeruginosa and two fungal strains i.e. A. niger and C. albicans and found that metal chelates were more noxious than their parent Schiff bases. The copper(II) complex 19 was found to be the highest noxious antimicrobial active compound. The compounds were also assessed for their antioxidant activity and found that the complexes are more potent and show excellent efficiency in decolorizing the DPPH purple colored solution than their free ligands. The complexes 6, 7, 10, 11, 14, 15, 18, 19 were found to be excellent antioxidants and copper(II) complexes were the most potent antioxidant compounds with lowest IC50 values (2.04–2.56 µM)) among all the tested compounds. Total 20 compounds of Schiff bases transition metal complexes were synthesized and characterized. The copper complex (19) was found to be highest antimicrobial against Streptococcus gordonii, Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, Aspergillus niger, Candida albicans. The copper(II) complex (11) exhibits outstanding and highest antioxidant activity (IC50 = 2.04 µM).