Ni(II), Cu(II), and Pd(II) schiff base complexes: synthesis, characterization, DFT, antibacterial study, and application as homogeneous catalysts in Stille reactions

The Stille reaction, a vital palladium-catalyzed cross-coupling method, has significantly advanced organic synthesis, yet challenges related to catalyst stability, reagent toxicity, and selectivity drive ongoing research toward improved efficiency and sustainability. This thesis presents the synthesis, characterization, catalytic evaluation in Stille reaction, DFT analysis, and antibacterial activity of Ni(II), Cu(II), and Pd(II) Schiff base complexes. Two Schiff base series, B1 and B2, were successfully prepared via condensation of ortho-vanillin and salicylaldehyde with para-substituted phenylamine derivatives in a 1:1 ratio, followed by complexation with Ni(II), Cu(II), and Pd(II) salts in a 2:1 ratio to afford 30 metal complexes. All ligands and complexes were characterized using elemental analysis, melting point, molar conductivity, magnetic susceptibility, NMR, FTIR, and UV-Visible spectroscopy. Low molar conductivity values confirmed their non-electrolytic nature. FTIR spectra showed shifts of the v(C=N) band from 1603-1618 cm-1 (ligands) to 1602-1618 cm-1 (complexes), v(C-N) from 1340-1367 cm-1 (ligands) to 1341-1394 cm-1 (complexes), and v(C-O) from 1241-1260 cm-1 (ligands) to 1211-1247 cm-1 (complexes), indicating coordination through azomethine nitrogen and phenolic oxygen. Weak M-N and M-O bands appeared at 548-703 cm-1 and 437-543 cm-1, respectively. In the 1H NMR spectra, the phenolic OH signal at 12.91-13.89 ppm disappeared upon complexation, and the HC=N proton shifted from 8.61-8.67 ppm in the ligand to 7.69-7.72 ppm in the complexes. Corresponding 13C NMR signals for the C=N carbon shifted from 160.4-161.4 ppm to 161.6-165.4 ppm, further supporting coordination. UV-Visible spectra showed π-π* and n-π* transitions at 201-316 nm and 300-430 nm for the free ligands, shifting to 208-366 nm and 334-443 nm in the complexes, consistent with metal-ligand interaction. Additionally, 11 single crystals of compounds were successfully obtained through slow evaporation, consisting of ligands (B1H, B1Me, B2H, B2Me) and complexes (CuB1Me, CuB2H, CuB2C, PdB1OMe, PdB2Me, PdB2OMe, PdB2C) and were analysed using single crystal X-ray diffraction. Six of them are new compounds, reflecting the novelty of this study. Pd(II) complexes from the B1 series were used to optimize the reaction conditions for Stille reaction by varying temperatures (30 °C, 60 °C, 80 °C, and 100 °C), bases types (KOH, triethylamine, and K2CO3), and amounts of catalysts (0.5, 1.0, 1.5, and 2.0 mmol%), with the best performance achieved using KOH, 2.0 mmol%, and 100 °C. Pd(II) Schiff base complexes have the best catalytic performance compared to Ni(II) and Cu(II) Schiff base complexes, with conversion rates of 90.8% and 50.1% after 60 minutes, respectively. PdB1OMe was the most effective catalyst, giving 100% conversion under optimized conditions afforded isolated biphenyl product of 41.9% yield, confirmed by NMR and FTIR spectroscopy. DFT calculations further supported high catalytic efficiency of PdB1OMe using B3LYP method with basis set of LANL2DZ, showing a small value of HOMO-LUMO gap (1.939 eV) and high electrophilicity (7.039 eV), consistent with strong electron-accepting ability and enhanced reactivity in catalytic activity. Furthermore, all synthesized compounds underwent antibacterial screening using the disk diffusion method against four bacterial strains, with 1% ciprofloxacin serving as positive control. The study assessed their antibacterial efficacy against Gram-positive strains; Staphylococcus aureus (S. aureus) and Bacillus subtilis (B. subtilis) and Gram-negative strains; Escherichia coli (E. coli) and Salmonella typhimurium (S. typhimurium) for both ligand series.