Reenvisioned Pyrazine–Dangling Cu(II) Complex: Advanced Synthesis, Characterization, Crystal Structure, Integrated‐DFT Assessment, Antibacterial, and In Vitro Cytotoxicity Rationalization

We report one greenish, blue‐colored Cu(II) complex, [Cu(pyz)(NO 3 ) 2 ] n 1 , using a solvent‐assisted technique with Cu(NO 3 ) 2 ·3H 2 O and pyrazine (pyz) in MeOH + DCM + ACN (1:1:1 M) (DCM = Dichloromethane, ACN = Acetonitrile). Spectroscopy, HRMS, SEM‐EDX, and XPS structurally characterize 1 . The primary research objectives focused on x‐ray structure, DFT integration using basis set B3LYP/LanL2DZ and B3LYP/6‐31+G (d, p), FMO, MEP, molecular docking, ADME/T, and biological activity exploration. X‐ray study delineated that 1 crystallizes in the orthorhombic space group Pmna , in which the crystal unit contains one Cu(II), two pyz, and two NO 3 – . The Cu(II) center adopted a distorted octahedral geometry, where H‐bonding interactions solely hold the supramolecular synthons. Hirshfeld surface accounts H···O (50.9%), N···Cu (3.2%), and H···N, H···C(π) (4.9%) contributions. TD‐DFT rationalizes the complex optical electronic transition with (%) HOMO–LUMO contributions. Additionally, 1 and pyz showed remarkable dose‐dependent antibacterial efficacy and in vitro cytotoxicity against Gram‐negative ( E. coli / P. aeruginosa) and Gram‐positive ( S. aureus / B. subtilis ) bacterial strains, including cell lines HepG2 and H9c2. The biological potency is substantiated by SAR (structure–activity relationships) and DFT outcomes. These complexes are significant to researchers because they serve as a model platform for quantum magnetism, potent biological agents, and function as electrochemical sensors in materials science.