Solvent- free synthesis, characterization, thermogravimetric and antimicrobial studies of Cu(II) complex with imine ligands

Abstract

Grinding was used as a solvent-free synthetic approach to prepare the ligand (L₁) from 2, 4-Dinitrophenylhydrazine and 3-Hydroxybenzaldehyde with N-(3-hydroxybenzaldehyde)-p-fluoroaniline ligand (L₂) and their Cu(II) complex. Melting point, solubility test, molar conductance, UV-Vis, and IR spectrophotometry were used to characterize these compounds. Thermogravimetric analysis (TGA) was used to investigate the thermal stability of the ligands in conjunction with their Cu(II) complex. Coats-Redfern's approximation methods were used to compute the energy of activation (Ea), entropy of activation (ΔS^o), free energy of activation (ΔG), enthalpy of activation (ΔH^o) and collision frequency (Z). Using scanning electron microscopy (SEM), the surface morphologies of the solid compounds were captured. The particle sizes of the ligands and the metal complex were measured using a particle size analyzer at a diffraction angle of 10.9°. The newly synthesized compounds were screened for their antimicrobial activities. Melting points of the synthesized compounds were in the temperature range of 188 - 194℃ showing that they were fairly stable; solubility in nonpolar solvents and molar conductance values (5.6 – 14.10 Ω/cm^-2) indicates that the ligands and the complex are non-electrolytic in nature. Comparing the electronic spectra of the free ligands with that of the complex revealed a shift from lower to higher frequencies which confirmed the chelation of the ligands with Cu(II) ion. The FTIR spectra showed bidentate ligands which coordinated through nitrogen atom of the imine group and the deprotonated oxygen atom of the hydroxyl group. Thermal results of the ligands revealed absence of water crystals while those of the complex showed two molecules of water. The kinetic parameters revealed that the decomposition reactions of these compounds was a first order reaction and the activation energy (kJmol^-1) were 54.05, 25.59 and 25.95 for HL₁, L₂ and Cu(II) complex respectively, implying that HL₁ require extra energy to form activated complex. The collision frequencies were 7. 06 × 10¹⁶, 4. 01 × 10⁴ and 7. 6 × 10⁴ _s^-1 for HL₁, L₂ and Cu(II) complex, which signifies extra spaces in HL₁ and Cu(II) complex than L₂. Positive values of ΔG of the compounds was an indication that the decomposition was non-spontaneous while positive ΔH^o showed that enthalpy was the driving force for the decomposition process. Negative entropy of activation implied that the degree of disorderliness of the products formed by bonds dissociation was lower than that of the reactants. The Schiff base ligands revealed weak activity against the test bacteria and inactivity against the test fungi while the metal complex significantly increased antimicrobial sensitivity against the test microbes.