Mixed-halide engineering in a homopiperazinium hybrid salt: structural disorder description, electronic structure analysis, and bioactivity evaluation

Abstract

Organic–inorganic hybrid halide materials have attracted considerable attention due to their tunable structural, optical, and biological properties. However, mixed-halide homopiperazinium-based systems remain insufficiently explored, particularly regarding the relationship between structural disorder, electronic structure, and multifunctional behavior. In this work, a new mixed-halide hybrid salt, homopiperazinium bromochloride (C₅H₁₄N₂)BrCl, was synthesized by slow evaporation and comprehensively investigated using experimental and theoretical approaches. Single-crystal X-ray diffraction analysis reveals a centrosymmetric monoclinic structure (P2₁/n) characterized by occupational disorder of Br⁻ and Cl⁻ anions over equivalent crystallographic sites. The crystal packing is stabilized by an extensive three-dimensional hydrogen-bonding network involving N–H⋯Br/Cl and C–H⋯Br/Cl interactions. Hirshfeld surface analysis confirms that halide–hydrogen contacts dominate the intermolecular interactions. Density functional theory calculations support the experimental structural model and reveal pronounced charge separation between the organic cation and mixed-halide anions. Frontier molecular orbital analysis indicates a wide HOMO–LUMO energy gap of 5.15 eV, consistent with the optical band gap determined experimentally from UV-visible diffuse reflectance and Kubelka–Munk analysis. ELF, NCI, and electrostatic potential analyses further highlight the ionic nature of the organic–inorganic interface and the role of weak non-covalent interactions in structural stabilization. Biological investigations demonstrate enhanced antibacterial, antioxidant, and antidiabetic activities of the mixed-halide salt compared with the parent homopiperazine compound. The combination of structural stability, wide band gap, strong polarity, and promising bioactivity highlights the potential of (C₅H₁₄N₂)BrCl as a multifunctional hybrid material for optoelectronic and biomedical applications.