Citrate-Bound Iron Oxide Clusters in Ionic Liquid Environments: Solvation, Spin-State Energetics, and Bond Dissociation

The interaction of citrate (C6H5O7 3– ) with kite-like Fe2O3 and tetranuclear Fe4O6 clusters was investigated across seven dielectric environments using unrestricted density functional theory at the UB3LYP-D3(BJ) level with def2-TZVP and 6-31+G(d) basis sets. Implicit solvation was modeled with the SMD continuum for water and the SMD-GIL parametrization for five ionic liquids spanning dielectric constants from 11.4 to 41.0. Citrate coordinates to the Fe2O3 core through a symmetric bidentate carboxylate motif that is preserved across all environments, with Fe–O(citrate) distances of 2.07–2.20 ˚A, while the larger Fe4O6 cluster engages two carboxylate arms simultaneously through shorter monodentate contacts. The antiferromagnetic singlet (S = 0) is the ground state in every environment for the Fe2O3 – Citrate3– complex, whereas one-electron oxidation triggers a magnetic switch to the ferromagnetic dectet (S = 9/2) that is reinforced by dielectric screening. The gas-phase spin-state manifold is compressed to within 5 kcal mol−1 and fans out upon solvation, yet the singlet-to-undecet gap remains nearly constant at 3.2–4.6 kcal mol−1 because the fully ferromagnetic and antiferromagnetic states share similar charge distributions and therefore experience comparable solvation stabilization. Solvation stabilization energies of 307–389 kcal mol−1 reflect the high formal charge of the complex. ZPEcorrected coordination energies decrease from −148 kcal mol−1 in the gas phase to −23 kcal mol−1 in water, with DFT and post-Hartree–Fock methods converging in the condensed phase. Bond dissociation energy curves for the Fe–O(citrate) coordinate reveal well depths of 24–39 kcal mol−1 that decrease monotonically with increasing dielectric constant, and a comparison of single-bond and doublebond scans demonstrates that the bidentate stability originates in the cooperative action of both Fe–O contacts. These results establish the molecular-level foundation for understanding citrate-stabilized iron oxide nanofluids in ionic liquid media.