Halogen substitution for fine-tuning the energy gap and intermolecular interactions of dinuclear iron(III) half schiff base complexes: experimental and theoretical investigation

• Systematic study on the precise regulation of the iron energy gap in bimetallic iron half-ligands by halogen atoms. • Multi-scale computational study on the intermolecular interactions of halogen atoms. • Non-planar ligand energy gap constraint principle. To elucidate the modulation mechanism of halogen substitution on the electronic structure and solid-state packing of half-Salen Schiff base complexes, a combination of single-crystal X-ray diffraction, spectroscopic characterization, and density functional theory (DFT) calculations was employed. The research reveals a subtle balance: constrained by the non-planar conformation of the ligand, chloro-substitution results in minimal perturbation of the frontier molecular orbitals (FMOs), with the HOMO-LUMO gap only slightly decreasing from 2.87 eV to 2.80 eV. In stark contrast, the intermolecular interactions are significantly enhanced. Hirshfeld surface analysis and binding energy (ΔE) calculations confirmed that the newly formed C–H···Cl and halogen-π non-covalent interactions increased the pathways for intermolecular interactions, strengthening the dimer binding energy from –23.67 kcal/mol to –25.90 kcal/mol. These findings deepen the understanding of the effects of halogen substitution and provide new experimental and theoretical evidence for the regulation of band gaps and intermolecular interactions in Schiff base complexes .