Pentacoordinate chloro-iron(III) complexes based on new asymmetric salophen-N₂O₂ ligands: Structural, magnetic, and theoretical studies

Pentacoordinate iron(III) complexes have been significant in several catalytic processes and bioinorganic applications due to their unique structural features. This study aims to elucidate the structural characteristics and magnetic behavior of these complexes for potential applications in chemical engineering, catalysis and bioinorganic chemistry. We synthesized three new pentacoordinate chloro-iron(III) complexes using asymmetric ligands derived from salophen and Jäger compounds. The new complexes C1, C2, and C3 were fully characterized using conventional spectroscopic techniques. Single crystals of all complexes were obtained, revealing a square pyramid geometry with an average τ parameter of 0.11, with the C2 complex exhibiting the highest distortion. Crystal packing analysis using Hirshfeld surfaces revealed nonclassical C[sbnd]H⋯O interactions and π–π-stacking interactions, some of which were considered to explain the weak antiferromagnetism observed in each sample by calculating the minimum Fe⋯Fe distances. To analyze the magnetic results, we employed a binuclear model that accounts for zero-field splitting (zfs) and weak antiferromagnetic via non-covalent contacts in a two-spin system with S_a = S_b = 5/2. Coupling constants ranging from −0.25 to −0.18 J/cm⁻¹ were determined for the complexes. The crystallographic and spectroscopic data were rationalized using DFT and TD-DFT calculations. Computational models for the high-spin state (S = 5/2) provided better agreement with the crystal data compared to the intermediate-spin (S = 3/2) and low-spin (S = 1/2) structures. Additionally, TD-DFT calculations enabled the assignment of the most intense absorption bands as intraligand π–π* transitions (ILCT) and ligand-to-metal charge transfer (LMCT) bands in the UV (240–380 nm) and visible (380–500 nm) regions, respectively.