Synthesis, characterisation and antibacterial activity of [(p-cym)RuX(L)]+/2+ (X = Cl, H2O; L = bpmo, bpms) complexes

Abstract

Mononuclear half-sandwiched complexes [(p-cym)RuCl(bpmo)](ClO₄) {[1](ClO₄)} and [(p-cym)RuCl(bpms)](PF₆) {[2](PF₆)} have been prepared by reacting heteroscorpionate ligands bpmo = 2-methoxyphenyl-bis(3,5-dimethylpyrazol-1-yl)methane and bpms = 2-methylthiophenyl-bis(3,5-dimethylpyrazol-1-yl)methane, respectively, with a dimeric precursor complex [(p-cym)RuCl(μ-Cl)]₂ (p-cym = 1-isopropyl-4-methylbenzene) in methanol. The corresponding aqua derivatives [(p-cym)Ru(H₂O)(bpmo)](ClO₄)₂ {[3](ClO₄)₂} and [(p-cym)Ru(H₂O)(bpms)](PF₆)₂ {[4](PF₆)₂} are obtained from {[1](ClO₄)} and {[2](PF₆)}, respectively, via Cl⁻/H₂O exchange process in the presence of appropriate equivalents of AgClO₄/AgNO₃ + KPF₆ in a methanol–water mixture. The molecular structures of the complexes {[1]Cl, [3](ClO₄)₂ and [4](PF₆)(NO₃)} are authenticated by their single crystal X-ray structures. The complexes show the expected piano-stool geometry with p-cym in the η⁶ binding mode. The aqua complexes [3](ClO₄)₂ and [4](PF₆)₂ show significantly good antibacterial activity towards E. coli (gram negative) and B. subtilis (gram positive) strains, while chloro derivatives ({[1](ClO₄)} and {[2](PF₆)} are found to be virtually inactive. The order of antibacterial activity of the complexes according to their MIC values is [1](ClO₄) (both 1000 μg mL⁻¹) < [2](PF₆) (580 μg mL⁻¹ and 750 μg mL⁻¹) < [3](ClO₄)₂ (both 100 μg mL⁻¹) < [4](PF₆)₂ (30 μg mL⁻¹ and 60 μg mL⁻¹) for E. coli and B. subtilis strains, respectively. Further, the aqua complexes [3](ClO₄)₂ and [4](PF₆)₂ show clear zones of inhibition against kanamycin, ampicillin and chloramphenicol resistant E. coli strains. The detailed mechanistic aspects of the aforesaid active aqua complexes [3](ClO₄)₂ and [4](PF₆)₂ have been explored, and it reveals that both the complexes inhibit the number of nucleoids per cell in vivo and bind to DNA in vitro. The results indeed demonstrate that both [3](ClO₄)₂ and [4](PF₆)₂ facilitate the inhibition of bacterial growth by binding to DNA.