This paper presents the synthesis and characterization of Schiff base ligands (L) derived from NH-pyrazole-indole and their ruthenium(II) complexes of the general formula [Ru(p-cymene)(L)Cl₂]. The structural characterization of the ligands and ruthenium complexes was performed using a range of analytical methods, including infrared spectroscopy (IR), Raman spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, ultraviolet-visible (UV-Vis) spectroscopy, elemental analysis, mass spectrometry(ESI-MS), density functional theory (DFT) calculations, and single-crystal X-ray diffraction crystallography for two of the four synthesized compounds. Schiff base ligands (L) derived from NH-pyrazole-indole and their ruthenium(II) complexes were evaluated against antibiotic-resistant bacteria. The effect of substituting the ligand with methoxy groups on biological activity was assessed. The antibacterial activity, Minimal Inhibitory Concentration (MIC), and Minimal Bactericidal Concentration (MBC) of the free ligands and the ruthenium compounds were measured against six bacterial isolates (Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, Escherichia coli, Shigella dysenteriae) and two control strains (Enterococcus faecalis ATCC 29212 and Enterobacter cloacae ATCC 13047) from the ESKAPE group of highly antibiotic-resistant pathogens. The free ligands were inactive, whereas all four ruthenium complexes displayed notable antibacterial activity. In particular, compounds Ru3 and Ru4 exhibited higher activity against Staphylococcus aureus than the Trimethoprim control, with MIC values of 15.60 µg/mL, compared with an MIC > 64 µg/mL for Trimethoprim. The activity of those two complexes was similar to that of Gentamicin against this strain. Molecular docking calculations suggested a possible mechanism of action through binding of the complexes to the active site of S. aureus PBP2a.

This work reports the isolation and structural characterization of 1-(2-aminophenyl)-3-(4-pyridyl)-3-hydroxy-1-propanone (1), a β-hydroxyketone intermediate that crystallized unexpectedly during the base-catalyzed aldol condensation of 2-aminoacetophenone with pyridine-4-carbaldehyde, a reaction intended to afford the corresponding pyridyl chalcone (2). The formation of (1) highlights the sensitivity of Claisen–Schmidt reactions to the electronic and steric features of the substrates and to the applied reaction conditions. Single-crystal X-ray diffraction unambiguously confirmed the molecular structure of (1), revealing a hydrogen-bonding network involving the amino, carbonyl, and β-hydroxyl functionalities. These interactions contribute to the solid-state stabilization of the β-hydroxyketone and hinder its dehydration to chalcone (2). The present results provide experimental insight into the mechanistic landscape of aldol condensations and emphasize the relevance of isolable intermediates as structurally defined precursors for further synthetic transformations.