Nickel mixed ligand complexes against drug resistant bacteria: Synthesis, characterization, antibacterial activities and molecular docking studies

Five new nickel complexes of the type NiCl₂L₂ where (L is 4-picolylchloride (4PY), pyridin-4-ylmethyl 4-nitrobenzoate (L1), 4-(pyridine-4-ylmethoxy)aniline (L2), pyridin-4-ylmethyl 4-aminobenzoate (L3) and 4-((4-nitrophenoxy)methyl)pyridine (L4)) were synthesized, characterized, and evaluated for their ability to inhibit bacterial growth. Four of the complexes were synthesized using a green method of co-grinding using a pestle and mortar. All the complexes synthesized by the mechanochemical method were recovered at favorable yields between 70 and 78%, while Ni4PY was recovered at 47.1%. The thermal studies showed the different transformations of the complexes and their thermal stability through Differential Scanning Calorimetry (DSC), while Thermogravimetric analysis (TGA) showed the loss of different components of the complexes cementing their successful synthesis. FTIR data revealed all the significant functional groups originating from the ligands in the new complexes. The characterization results demonstrated the successful synthesis of these complexes. Electronic spectra and magnetic susceptibility studies showed all the complexes to be d⁸ tetrahedral, except NiL1 which showed a partial d⁸ octahedral geometry. To better understand the biological activities, molecular docking calculations against the PBP2a proteins from S. aureus, OmpK36 proteins from K. pneumoniae, and DHPS proteins were also carried out. According to the results, the active nickel complexes should have had better activity than the control drug AgSD, however their activity was decreased due to solubility issues.