Synthesis, characterization, computational, catalytic and antibacterial studies of Pd(II), Ni(II) and La(III) Schiff base complexes derived from aliphatic and cyclic diamines

Schiff base metal complexes have attracted considerable interest due to their catalytic and biological application. However, studies on Pd(II), Ni(II) and La(III) complexes derived from aliphatic and cyclic Schiff bases remain underexplored, particularly in catalytic studies involving Sonogashira reaction and in antibacterial study. Moreover, there is a lack of comprehensive investigations combining these experimental studies approaches such as Density Functional Theory (DFT) and Response Surface Methodology (RSM) for reaction optimization and mechanistic insights. To address these gaps, ten (10) Schiff bases derivatives (AD1 and AD2 series) were synthesized via condensation of five salicylaldehyde derivatives (X = H, F, Cl, CH3 and OCH3 at the meta position to C=O) with two primary amines, 2,2-dimethyl-1,3-propanediamine (DMPD) and 1,2-cyclohexanediamine (CHD), in 2:1 molar ratio. These Schiff bases reacted with Pd(II), Ni(II) and La(III) salts in a 1:1 molar ratio to form 26 metal complexes. Six Schiff bases and 13 metal complexes were identified as new compounds indicated through comparison with reported structures in the Cambridge Structural Database (CSD). The compounds were characterized via elemental analysis (CHNS), melting point, FTIR, 1H and 13C NMR, UV-Visible spectroscopy, magnetic susceptibility and thermogravimetric analysis (TGA). Single crystal X-ray diffraction confirmed the structures of eight (8) Schiff bases and five (5) metal complexes. Spectral data revealed significant shift in FTIR, UV-Vis, and NMR spectra upon complexation, particularly a 3 to 36 cm-1 shift in the C=N stretching band, confirming azomethine nitrogen and phenolic oxygen coordination. Using Pd(AD1) series complexes as catalysts, the optimum conditions were determined to be KOH as the base, 2.0 mmol% catalyst, and 120 °C reaction temperature. Pd(II) complexes exhibited the highest percent conversion of iodobenzene compared to Ni(II) and La(III) complexes, with 100% selectivity for the coupling product and no detectable byproducts. Aliphatic (AD1) complexes outperforming cyclic (AD2) complexes due to greater ligand flexibility, enhancing substrate access. Under these conditions, Pd(AD1F) achieved a 100% conversion of iodobenzene in 3 hours, in agreement with the RSM calculation, except for the reaction time. The isolated yield of diphenylacetylene was 14.3% of yield, attributed to purification losses, leading to lower turnover number (TON) and turnover frequency (TOF) values with values of 7.1 and 2.4 h-1, respectively. The diphenylacetylene was characterized using CHN analysis, FTIR and 1H NMR spectroscopy. Kinetic studies showed Pd(AD1F) catalyzed reaction twice as fast as uncatalyzed one. Substitution of iodobenzene with bromobenzene resulted in a slightly reduced conversion of 92.0%. DFT studies on selected compounds, namely Pd(AD1F), Pd(AD1Me), Ni(AD1Me), AD2Cl and AD2OMe supported the experimental findings. Pd(AD1F) exhibited favourable electronic properties with high ionization potential and low electron affinity, enhancing its catalytic efficiency. Conversely, large HOMOLUMO energy gap in AD2Cl (4.615 eV) and AD2Me (4.589 eV) supports their inactive antibacterial activity. Antibacterial screening revealed Pd(AD1F) exhibited significant inhibition against Streptococcus mutans (90.87%), while Pd(AD2OMe) was active against Proteus vulgaris (93.99%). This work highlights Schiff base Pd(II) complexes, particularly Pd(AD1F), as efficient catalysts and potential antibacterial agents, with promising applications in green chemistry and medicine.