EPR spectroscopy of [Fe₂O₂(5-Et₃-TPA) ₂]³⁺: Electronic origin of the unique spin-Hamiltonian parameters of the Fe₂^III,IVO₂ diamond core

The electronic origins of the magnetic signatures of [Fe₂O ₂(5-Et₃-TPA)₂](ClO₄)₃, where 5-Et₃-TPA = tris(5-ethyl-2-pyridylmethyl)amine, were investigated by density functional calculations. These signatures consist of a near-axial EPR spectrum, anisotropic superhyperfine broadening upon ¹⁷O substitution in the Fe₂O₂ core, and an unusually large, positive zero-field splitting parameter, D = 38 ± 3 cm^-1. Density functional calculations identify the anisotropic ¹⁷O superhyperfine broadening to be due to a preponderance of oxo 2p density perpendicular to the plane of the Fe₂O₂ core in the three singly occupied molecular orbitals of the S = 3/2 ground state. The near-axial g-matrix arises from ΔS = 0 spin-orbit mixing between the singly and doubly occupied d_π orbitals of the iron d-manifold. The large D is due to ΔS = ± 1 spin-orbit mixing with low-lying d_π excited states. These experimental observables reflect the dominance of iron-oxo (rather than Fe-Fe) bonding in the Fe₂O ₂ core, and define the low-lying valence orbitals responsible for reactivity.