Citrate-Bound iron oxide molecules in ionic liquid environments: Solvation, spin-state energetics, and bond dissociation

The interaction of citrate with kite-like Fe ₂ O ₃ and tetranuclear Fe ₄ O ₆ molecules was investigated across seven dielectric environments at the UB3LYP-D3(BJ) level with def2-TZVP and 6-31+G(d) basis sets. Water was described with the Solvation Model based on Density (SMD), while five ionic liquids were represented with the generalized ionic-liquid SMD parameterization (SMD-GIL) across dielectric constants from 11.40 to 41.00. For Fe ₂ O ₃- Citrate ³⁻ , citrate binds through a bidentate O6/O7 carboxylate anchor that is preserved across the dielectric series, whereas Fe ₄ O ₆ engages two carboxylate arms through shorter monodentate contacts. The protonated central hydroxyl does not coordinate Fe directly, but stabilizes the bound citrate conformation through an intramolecular O–H ⋯ O hydrogen bond. Natural bond orbital (NBO) analysis assigns this interaction mainly to LP(2) O14 → σ ^* (O18–H19) donation, with E⁽²⁾ values of 3.65–5.46 kcal mol ⁻¹ across the dielectric sweep. Hybrid explicit-continuum tests with six water molecules and explicit [CH ₃ NH ₃ ][HCOO] ion pairs retain the same inner-sphere Fe2–O6/O7 attachment, showing that first-shell hydrogen bonding and ion-pair orientation refine rather than overturn the continuum description. The antiferromagnetic singlet is the ground state of Fe ₂ O ₃- Citrate ³⁻ in all environments, while one-electron oxidation favors a ferromagnetic dectet. BSSE-estimated coordination energies decrease from $$-144.90$$ kcal mol ⁻¹ in the gas phase to $$-19.31$$ kcal mol ⁻¹ in water, confirming strong dielectric attenuation while preserving stabilizing carboxylate binding. Formal deprotonation analysis shows that hydroxyl deprotonation of free citric acid is less favorable than carboxyl deprotonation by 22.09 kcal mol ⁻¹ in SMD water. Imposed Cit ⁴⁻ binding opens a mixed alkoxide/carboxylate coordination channel, best viewed as a model for hydroxyl-deprotonating conditions rather than the dominant citrate form. Bond-dissociation scans give Fe–O(citrate) well depths of 24–39 kcal mol ⁻¹ and demonstrate cooperative bidentate stabilization.