Synthesis and Characterisation of 2-Acetylbenzothiophene-Based Semicarbazone Ligands

Two semicarbazone ligands, 2-acetylbenzothiophene-3-semicarbazone (Hbtsc) and 2-acetylbenzothiophene-4-phenyl-3-semicarbazone (Hbtpsc), were synthesised via condensation reactions of semicarbazide derivatives with 2-acetylbenzothiophene. The 2-acetylbenzothiophene scaffold was chosen because of its heteroaromatic core, which is rich in electrons and is known to improve conjugation and bioactivity. Because of their biological significance and adaptable coordination behaviour, semicarbazones are the subject of extensive research. FT-IR, UV-Vis, ¹H and ¹³C NMR spectroscopy, elemental analysis, and single crystal X-ray diffraction (for b) were all used to completely characterise the ligands. Spectroscopic data confirmed the keto tautomeric form of both ligands in the solid state. IR spectra showed characteristic ν(C=N) and ν(C=O) stretching vibrations around the range of 1587-1536 cm⁻¹ and 1753-1682 cm⁻¹, respectively. Signals at δ around 10 ppm and 6–8 ppm in the ¹H NMR spectra were attributed to N–H and aromatic protons, which is in line with the keto tautomer. Electronic spectra displayed broad absorptions at 315–318 nm due to n→π* transitions. Single crystal structural analysis of b revealed a triclinic unit cell with P1 space group and two independent molecules (Z′ = 2) in the asymmetric unit. The molecules adopt an E-configuration across the C=N bond, and C=O bond lengths of 1.235 and 1.221 Å, confirming the presence of the keto form. The structure was stabilised by intramolecular hydrogen bonding and weak C–H···π interactions, which contribute to the formation of a stable zigzag packing arrangement. These structural insights provide a foundation for potential future applications in coordination chemistry and materials science.