Six-Coordinate MN₄O₂ Bis-Chelates of A Brominated Pyridyl Semicarbazone: Electronic Structure, Optical Band Gap, Magnetic Behaviour, Nanocrystalline Microstructure and Antimicrobial Activity

Abstract

The physical and biological properties of four neutral bis-chelates [M(L³)₂] (M = Cu, Co, Ni, Zn), formed by 6-bromopyridine-2-carbaldehyde semicarbazone (L³H) acting as a monoanionic N,N,O-tridentate donor, are reported. Electronic absorption spectra in DMSO show intraligand π→π* and n→π* bands together with a ligand-to-metal charge-transfer band at 386-402 nm whose energy falls in the order Zn > Ni > Co > Cu, and weak Laporte-forbidden ligand-field bands (ε = 78-128 M^-1 cm^-1) for the three open-shell complexes. The d-d maxima lie at higher energy than those reported for semicarbazone chelates with MN₂O₂ or MN₂O₄ chromophores, as the substitution of two oxygen donors by two pyridine nitrogens requires. Direct-gap Tauc analysis gives E_g = 3.69 eV for the free ligand and 2.77-3.01 eV for the complexes, a narrowing of 0.68-0.92 eV that parallels the charge-transfer energies and places the compounds in the wide-band-gap semiconductor regime. X-band ESR spectra of the copper complex (g_∥ = 2.225, g_⊥ = 2.189) indicate a d_*x*²-*y*² ground state in an environment of unusually small tetragonal anisotropy, while the observation of a well-defined anisotropic signal for the nickel complex, taken with μ_eff = 3.18 BM from vibrating-sample magnetometry, excludes square-planar geometry outright and establishes a paramagnetic, six-coordinate d⁸ centre. Room-temperature moments of 1.86 (Cu), 4.78 (Co), 3.18 (Ni) and 0.00 BM (Zn), with strictly linear M-H response and no hysteresis, confirm the +2 oxidation state throughout. All five solids are nanocrystalline and index on a monoclinic P2₁/c cell, the ligand with Z = 4 and the complexes with Z = 2; Williamson-Hall analysis gives crystallite sizes of 30.8-45.4 nm and microstrain of 0.99-1.72 × 10^-3. Thermogravimetry shows no mass loss below 232 °C, excluding coordinated and lattice water, and resolves three stages terminating in the metal(II) oxide, whose mass matches calculation to within 0.18 % and so provides a gravimetric metal determination independent of EDAX. The nickel and cobalt complexes inhibit Gram-positive and Gram-negative bacteria, fungi and mycobacteria, the nickel complex reaching an activity index of 123 % against Trichophyton rubrum and the cobalt complex 111 % against Mycobacterium bovis.