Synthesis, biological evaluation, and QSAR studies of organosilicon(IV) complexes derived from tridentate ONO Schiff bases of dehydroacetic acid and aromatic hydrazides

A pioneer series of organosilicon(IV) complexes with general formula R2Si(L), where R = ethyl, butyl, phenyl, and L = anion of the Schiff base ligands 4-nitro/chloro-benzoic acid [1-(4-hydroxy-6-methyl-2-oxo-2H-pyran-3-yl)ethylidene]hydrazide, and benzothiazole-2-carboxylic acid [1-(4-hydroxy-6-methyl-2-oxo-2H-pyran-3-yl)ethylidene]hydrazide)] were synthesized. These complexes were characterized by elemental analyses, molar conductance, and spectroscopic techniques (IR, H-1, C-13, and Si-29 NMR). A trigonal bipyramidal geometry around the silicon atom was proposed, where the Schiff base ligand acted as tridentate (ONO) coordinating through oxygen of hydroxyl group, azomethine nitrogen, and oxygen of keto group after enolization. The ligands and their organosilicon complexes were tested in vitro for antibacterial activity against Gram-positive bacteria viz. Staphylococcus aureus and Bacillus subtilis and Gram-negative bacterium viz. Escherichia coli and antifungal activity against Aspergillus niger and Candida albicans. Antimicrobial activity results indicated the biological activity of ligands improved on complexation with the organosilicon group and compound Ph2Si(L-2) was found to be most potent antimicrobial agent (pMIC(am) = 1.62). Quantitative structure activity relationship analysis was carried out to correlate antimicrobial activity of synthesized compounds with their molecular descriptors using the linear free energy relationship model which indicates that topological parameters, valence third order, and third-order molecular connectivity indices ((3) chi (v) and (3) chi) were most significant in explaining the antibacterial and antifungal activities of all the ligands and their organosilicon(IV) complexes.