Synthesis, Nano-structural Characterization, and Spectral Investigation of a Rare Low-Spin Distorted Octahedral (D4h) Cobalt(II) Complex Derived from Coumarin

This study describes a multi-step synthetic protocol for constructing a coumarin-based scaffold, subsequently functionalized into a versatile Schiff base intermediate. This precursor served as a foundational unit for synthesizing a series of novel heterocyclic derivatives through five distinct cyclization strategies. A specific derivative was coordinated as a bidentate ligand to yield a novel Cobalt (II) complex (Q11). Comprehensive characterization via molar conductivity and magnetic susceptibility confirmed the formation of a non-electrolytic complex, while FE-SEM analysis revealed its nanostructured nature. Spectroscopic data (µeff = 1.95 B.M.) corroborated a rare low-spin, tetragonally distorted octahedral (D4h) geometry with a (t2g6eg1) geometry for the Cobalt complex. The antimicrobial screening of the synthesized heterocyclic derivatives and the Cobalt(II) complex against Gram-positive Staphylococcus aureus and Gram-negative Klebsiella pneumoniae revealed significant biological potency. Notably, compounds (Q5-Q6) exhibited larger inhibitory zones than the antibiotic control, while the nanostructured complex (Q11) demonstrated superior activity, particularly against the resistant K. pneumoniae strain (22 mm). In addition, molecular docking simulations were performed for these derivatives against bacterial DNA gyrase targets (PDB IDs: 2XCT and 4DX5). The results revealed exceptional binding affinities of -10.6 kcal/mol for Q5 and -10.2 kcal/mol for Q6, significantly outperforming the reference drugs Ciprofloxacin and Ceftriaxone. A high correlation was observed between the in silico binding energies and the in vitro results. These findings, highlighting the superior potency of the nanostructured complex in overcoming diverse membrane barriers of both Gram-positive and Gram-negative bacteria, position this series as a promising candidate for high-surface-area catalysis and advanced biomedical applications.