High-Field EPR Study of Resonance-Delocalized [Fe(2)(OH)(3)(tmtacn)(2)](2+).

High-frequency EPR data are reported for the Fe(II/III) valence delocalized dinuclear complex [Fe(2)(OH)(3)(tmtacn)(2)](2+). A full-matrix diagonalization approach is used to derive the spin-Hamiltonian parameters for this S(T) = (9)/(2) complex. At high fields (up to 14.5 T) and high frequencies (189-433 GHz) fine structure peaks due to resonances between the Kramers doublets (M(s) = (9)/(2), (7)/(2),.) are observed. The spacing of the fine structure reveals that the axial zero-field splitting (ZFS) parameter D is +1.08(1) cm(-)(1); a very small rhombic ZFS (|E| </= 0.01 cm(-)(1)) is suggested by line broadening of these interdoublet resonances. Simulations reveal that g is close to 2.00, and very nearly isotropic: g(x)() = g(y)() = g(z)() = 2.00(2). This complex is a model for the valence-delocalized [Fe(2)S(2)](+) pairs found in larger iron-sulfur clusters, such as the cofactors from the nitrogenase system. This work indicates that HFEPR is a viable technique for the study of high-spin centers in proteins.