Combinatorial Synthesis and Direct-to-Biology Antibacterial Evaluation of Ruthenium(II) and Iridium(III) N^O -Schiff−Base Complexes

Antimicrobial resistance (AMR) represents a major global health challenge, compounded by a limited antibiotic discovery pipeline and restricted chemical diversity among clinically used antibiotics. Metal-based complexes offer promising antimicrobial agents owing to their structural diversity and unique modes of action. Here, we report the synthesis and biological evaluation of two combinatorial libraries comprising 176 novel ruthenium(II) p-cymene (RuCy) and iridium(III) Cp* (IrCp) N^O-Schiff-base complexes. Syntheses used a robust three-step, room-temperature methodology, with automated LC−MS characterization enabling Direct-to-Biology screening. Against Staphylococcus aureus, 50−80% of compounds inhibited growth, whereas no significant activity was observed against Escherichia coli. Three leads exhibited MICs of 1.56−12.5 μM against Gram-positive pathogens, therapeutic indices exceeding 15, and negligible haemolysis. Mechanistic studies suggest that IrCp-B5, incorporating the ligand (E)-2-bromo-4-chloro-6-(((naphthalen-2-ylmethyl)imino)methyl)phenol, disrupts membrane associated processes, while RuCy-J3, incorporating (E)-1-((cyclohexylimino)methyl)naphthalen-2-ol, targets intracellular DNA-related pathways. This work demonstrates the utility of Direct-to-Biology approaches for accelerated antimicrobial lead identification and prioritization.