Nickel (II) Macrocyclic Complexes: Template Synthesis, Spectroscopic Investigations, Electrochemical Behavior, DFT Studies, and Biological Evaluation

Abstract

A novel series of tetraaza nickel (II) macrocyclic complexes was prepared via template condensation involving the ligand 2-[4-bromo-2-(2-oxo-1,2-diphenyl-ethylideneamino)-phenylimino]-1,2-diphenyl-ethanone (ML) and selected diamines, such as 4-chloro 1,2-phenylenediamine, 4-bromo 1,2-phenylenediamine, 1,2-diaminotoluene, 4-nitro 1,2-phenylenediamine, 1,2-diaminopropane, and ethylenediamine, in the presence of NiCl₂.6H₂O. The ligand and its resulting complexes were characterized through elemental analysis, molecular weight determination, and a range of spectral techniques, including IR, electronic spectra, ¹H NMR, mass spectrometry, X-ray powder diffraction, and DFT-assisted structural characterization. DFT (TDDFT/B3LYP/6–311g(d) method) analysis of representative Ni(II) complexes supports plausible structures proposed based on spectroscopic studies. These analyses suggest that each complex adopts a six-coordinated octahedral geometry. Their redox behavior was examined in detail using cyclic voltammetry (CV). Furthermore, the new ligand and its complexes were tested for their antimicrobial, antioxidant, antidiabetic, and antituberculosis activities. Antimicrobial tests were conducted against E. coli, B. subtilis, A. niger, and P. chrysogenum for the synthesized compounds. The antioxidant properties of the complexes assessed using the DPPH assay showed that the [Ni(C₄₁H₂₉BrCl₂N₄)] exhibited the highest antioxidant activity, with an IC₅₀ value of 89.13 mg/mL. Additionally, the ligand and its complexes were tested for antimycobacterial activity, where the [Ni(C₄₀H₂₉BrCl₃N₄)] complex demonstrated significant effectiveness in inhibiting the growth of Mycobacterium tuberculosis.