| Summary: | A series of aryl-functionalized alkyl dihydrazones was prepared by condensation of succinyl or adipoyl dihydrazide and selected <i>ortho</i>-hydroxybenzaldehydes (2-hydroxybenzaldehyde, 2-hydroxy-1-naphthaldehyde, 2,3-dihydroxybenzaldehyde, and 2,4-dihydroxybenzaldehyde) in solution. The obtained products were structurally characterized in the solid state by single-crystal X-ray diffraction (SC-XRD), thermal analysis (TGA-DSC), and Fourier transform infrared (FTIR) spectroscopy and in DMSO-<i>d</i><sub>6</sub> solution by nuclear magnetic resonance (NMR) techniques. Combined FTIR and crystal structure data point to a N–NH–C=O tautomeric form of the hydrazone parts as well as the enol-imino tautomeric form of the aldehyde residues and a robust <i>trans-syn</i> conformation for the structurally investigated ones. While the molecules retain the same tautomeric form in the DMSO-<i>d</i><sub>6</sub> solution, they adopt several conformations, due to rotations around C<sub>ar</sub>–C, C–N, and N–N bonds. The compounds show exceptional thermal stability, with a complex degradation pattern. Slight differences in thermal behavior correlate to alkyl chain length and aryl substituents. The in vitro cytotoxic activity of prepared dihydrazones was evaluated on THP-1 and HepG2 cell lines, while their antibacterial activity was tested against <i>Staphylococcus aureus</i>, <i>Enterococcus faecalis</i>, <i>Escherichia coli</i>, and <i>Moraxella catarrhalis</i> bacteria. All compounds proved to be non-cytotoxic, and some exhibited moderate antibacterial activity.
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