Investigation of a Homoleptic Nickel(II) Complex: Synthesis, Crystal Structure, Computational Insights, and Antimicrobial Efficacy

A homoleptic nickel-(II) complex, [Ni-(L)₂], where HL = 2-methoxy-6-[(methylimino)-methyl]-phenol, synthesized via the condensation of bis-(3-methoxysalicylaldehyde)-nickel-(II) with methylamine in methanol, was structurally characterized by spectroscopic techniques and single crystal X-ray diffraction. The peaks at 232 and 275 nm in the UV-vis spectrum correspond to the π → π* transition, while 374 nm is assigned to the n → π* transition. The characteristic FTIR bands associated with Schiff base formation were noted at 1609 cm^-1 (-CN). The ¹H NMR spectrum displayed prominent signals at 7.26 ppm (azomethine H), 6.06 ppm (aromatic H), and 3.78 ppm (methyl protons). TGA/DTA studies revealed significant thermal stability of the nickel-(II) complex up to 260 °C, with a final residue of NiO/C at 519 °C. The mass spectra showed a primary molecular ion peak at <i>m</i>/<i>z</i> 387.08, along with the fragmentation pattern. Powder X-ray diffraction confirmed the purity of the nickel-(II) complex powder. The single-crystal X-ray diffraction revealed that the complex crystallizing in a monoclinic space group <i>P</i>2₁/<i>n</i>, has a pseudo centrosymmetric square planar geometry featuring Ni-O and Ni-N bond lengths with mean values of 1.8264(18) Å and 1.91(2) Å, respectively. Density functional theory (DFT) calculations yielded a HOMO-LUMO gap of 3.53 eV, a chemical potential of -3.31 eV, and an electrophilicity index of 3.11 eV, indicating high stability and reactivity. Hirshfeld surface analysis identified H···H (53.6%) and C···H (25.6%) as the dominant contributors, while NCI plots revealed strong van der Waals and hydrogen bonding interactions. Biological studies, conducted through antimicrobial assays, demonstrate commendable biological activity at significantly low MIC/MBC/MFC values of 30 μg/mL. Molecular docking studies revealed the highest binding affinities with <i>E. coli</i> DNA gyrase B (-8.03 kcal/mol, <i>K</i> _i = 29 μM) among the bacterial targets and <i>C. tropicalis</i> phospholipid synthase (-7.20 kcal/mol, <i>K</i> _i = 97 μM) among the fungal targets. These findings suggest that the synthesized nickel-(II) complex is a promising agent for antibacterial and antifungal applications.