EPR Spectroscopy of [Fe2O2(5-Et3-TPA)2]3+: Electronic Origin of the Unique Spin-Hamiltonian Parameters of the Fe2III,IVO2 Diamond Core

The electronic origins of the magnetic signatures of [Fe2O2(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 (plus-minus) 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 (delta)S = 0 spin-orbit mixing between the singly and doubly occupied d(delta) orbitals of the iron d-manifold. The large D is due to (delta)S = (plus-minus)1 spin-orbit mixing with low-lying d(pi) excited states. These experimental observables reflect the dominance of iron-oxo (rather than Fe-Fe) bonding in the Fe2O2 core, and define the low-lying valence orbitals responsible for reactivity.