Synthesis, characterization, electrochemical properties of half-sandwich n, n’-bidentate ruthenium (ii) complexes and their antimicrobial activity

Abstract

The emergence of bacterial resistance to existing antibiotics and other drugs is a worldwide problem. New classes of antimicrobial compounds with complete new mode of action are therefore urgently needed to control the rise of the multidrug resistant pathogens. The objectiveof this study was to synthesize and characterize half-sandwich organometallic compounds of ruthenium (II) containing bipyridine and pyridine- imine ligands and to test their biological activities against one Gram positive and Gram negative bacteria. The reaction of [(η6- C6H5CH3)Ru(µ-Cl)Cl]2 dimer and the N,N′- bidentate ligands in a 1 : 2 ratio in dry acetonitrile at ambient temperatures resulted in the formation of four versatile, half-sandwich, complexes, [(η6-C6H5CH3) RuCl (N-N)]+[PF6]-, [(η6-C6H5CH3)RuCl (C5H4N-2-CH=N-X] [PF6]-, [(where (N-N) = 5,5’-dimethyl-2,2’-bipyridine , 4,4’-Di-tert-butyl-2,2’-bipyridine , 2,2’-bipyridine and X = p-fluorophenyl. The complexes were isolated as their hexafluorophosphate salts. Characterization of the complexes was accomplished using 1H NMR, (some were subjected to 13C NMR), elemental analyses, melting points determination, UV/VIS and FTIR spectroscopy which was used to confirm the formation of the imine functional group and the disappearance of the carbonyl band of the starting material containing 2-pyridinecarboxaldehyde in the formation of pyridine-imine Schiff base and also used to monitor the C=N moiety of the pyridine upon the complexation of the precursor complex and bipyridine ligands. Electrochemical properties of the complexes were determined by cyclic voltammetry. The synthesized and characterized complexes were subjected to in vitro bioassays to determine their antibacterial activity by agar disc diffusion method. They were also tested against an antimicrobial-susceptible and resistant Gram-negative Escherichia coli ATCC 11775 and Gram-positive Staphylococcus aureus ATCC 12600. Streptomycin was used as the positive control and Dimethyl sulfoxide as the negative control. Some of the synthesized mononuclear ruthenium complexes demonstrated potential antimicrobial activity against the selected bacteria with some showing better activity than well-known antibiotics such as streptomycin (S-10). The findings reported in this work including cyclic voltammetry and antimicrobial activities are reported for the first time since the synthesized ruthenium (II) bipyridine and pyridineimine Schiff base complexes containing toluene as the cyclic polyhapto aromatic ligand has never been synthesized before.