Novel Quaternary Ammonium Aldimine Derivatives Featuring 3,4,5-Trimethoxy Phenyl Fragment: Synthesis, Crystal Structure and Evaluation of Antioxidant and Antibacterial Activity

This study demonstrates the synthesis of five novel quaternary ammonium aldimines through a two-step synthetic route involving a condensation reaction between 4-pyridincarboxyaldehyde and 3,4,5-trimethoxyaniline, followed by the quaternization of the pyridine N-atom with various aromatic α-bromo ketones. The newly obtained compounds underwent characterization for both purity and molecular structure, utilizing HR-MS, 1D, and 2D NMR spectroscopy in solution, as well as a comparison between single-crystal and powder X-ray analyses in a solid state. The thermal behavior of the studied compounds was evaluated using differential scanning calorimetry (DSC). The antioxidant properties of the compounds were assessed through DPPH (2,2-diphenyl-1-picrylhydrazyl) radical scavenging and ferric-reducing antioxidant power (FRAP) assays, employing Trolox as a standard. The performed in vitro antibacterial screening indicates a selective antibacterial activity against Gram-negative <i>K. pneumoniae</i> and <i>P. aeruginosa</i>, while no such activity is detected for Gram-negative <i>E. coli</i> and Gram-positive <i>S. aureus</i>.