Syntheses, characterization and bioactivity of half-sandwich complexes of ruthenium, rhodium, and iridium containing coumarin pyrazole benzhydrazone ligands

A series of twenty-four new ruthenium(II), rhodium(III) and iridium(III) complexes featuring eight novel coumarin-pyrazole benzhydrazone chelating ligands having the general formula [(p-cymene/Cp*)M(kappa 2 (N boolean AND O)-L)Cl] (where, L = coumarin-pyrazole benzhydrazone substituted derivatives, L1-L8 (Scheme 1), M =Ru(II), Rh(III) and Ir(III)) were synthesized and their deoxyribonucleic acid (DNA) interactions, antibacterial, antioxidant and cytotoxicity activities were investigated. All ligands and complexes were characterized by FT-IR, NMR, UV-Vis and ESI-MS. The molecular structures of complexes 1c, 2b, 2c, 3a, 3c, 5c, 6b, 6c and 8b (Scheme 2) were obtained by single-crystal X-ray diffraction study. In most of the complexes, neutral complexes were obtained, except 2c, 6b and 8b formed cationic complexes. The metal complexes were assessed for their antimicrobial activity against both Gram-negative bacteria (Escherichia coli and Pseudomonas aeruginosa) and Gram-positive bacteria (Staphylococcus aureus and Bacillus thuringiensis). Additionally, antioxidant studies indicated a moderate ability of the compounds to scavenge radicals. Fluorescence spectral titration experiments were carried out to investigate the interactions between DNA and metal complexes, unveiling a modest binding affinity. Among all the complexes, complex 6b exhibits higher binding affinity, as determined by the binding constants (Kb) of 2.67 mu M. Further, the cytotoxicity of some ligands and their corresponding complexes has been unveiled with the aid of a Trypan blue assay against Dalton's lymphoma (DL) cell line and compared with the standard drug mitomycin. Complex 6a exhibits maximum cytotoxicity against the tested cancer cell line compared to the other complexes, although its potency is less than that of mitomycin.