Molecular Fe2O3 Ion-Pair Adducts in Ionic-Liquid Environments: A Computational Study

Abstract

Ionic environments modify the redox and magnetic behavior of iron oxides through dielectric polarization and specific first-shell interactions. Dispersion-corrected, spin-unrestricted density functional theory combined with the solvation model based on density for generic ionic liquids was used to examine molecular Fe₂O₃ adducts. Three room-temperature ionic liquids were represented by one explicit ion pair each, while [CH₃NH₃][BF₄] adducts containing one to four pairs provided a loading model for this high-melting salt. Alternative neutral arrangements were optimized for every composition, and calculations without explicit ions provided a reference. The selected adducts are antiferromagnetic singlets when neutral, antiferromagnetic doublets after reduction, and ferromagnetic detects after oxidation. In the single ion pair systems, reduction localizes charge at one Fe center and contracts the di-μ-oxo core, whereas oxidation expands the Fe─O─Fe hinges and gives magnetic exchange constants of +140 to +216 cm⁻¹. Increasing [CH₃NH₃][BF₄] loading shifts the +1/0 and 0/−1 potentials to more positive values by 0.60 and 0.56 V, respectively. Thus, anion coordination, cation-mediated contacts, proton relocation, and crowding modulate the intrinsic Fe₂O₃ response beyond dielectric polarization alone.