MIXED-VALENT MU-OXO-BRIDGED DIIRON COMPLEXES PRODUCED BY RADIOLYTIC REDUCTION AT 77-K STUDIED BY EPR

Several dinuclear diferric complexes were prepared, bridged by only an oxo ion, by an oxo ion and additionally one or two carboxylate groups , or by two hydroxo ions. Frozen solutions of these EPR-silent diferri c complexes were radiolytically reduced by gamma-irradiation at 77 K. The reduced forms gave EPR spectra typical of mixed-valent diiron spec ies with S = 1/2. The yield was about 50% at high doses. With Fe-57 re placing Fe-56 the spectral features were broadened because of hyperfin e interaction with one or both of the Fe-57 nuclei. All ore-bridged co mplexes give mixed-valent species with unbroadened EPR spectra observa ble up to 110 K. The anisotropy is small, Delta g = g(1)-g(3) < 0.07, and g(av) = (g(1) + g(2) + g(3))/3 falls in the range 1.93-1.96. Annea ling at T greater than or equal to 115 K causes the EPR spectra to cha nge and finally become more anisotropic, 0.14 < Delta g < 0.32 and to have smaller g(av), 1.78 < g(av) < 1.89. These spectra are observable only at T less than or equal to 35 K. Radiolytic reduction of the dife rric complexes with hydroxo bridges leads directly without annealing t o this second type of mixed-valent species. The annealing-induced tran sition from a primary to a secondary mixed-valent species in the case of an ore-bridged diferric starting complex is suggested to be a struc tural relaxation including protonation of the bridge. Annealing at tem peratures above ca. 180 K and final thawing causes the mixed-valent di nuclear species to disappear, probably by disintegration to mononuclea r species. These mixed-valent species are useful models for similar sp ecies in proteins, where primary-type species may take part in transie nt electron transfer, and the secondary-type species are usually more persistent forms in redox reactions. For proteins, available data show that there is a tendency for both primary and secondary species to ha ve larger g anisotropy, Delta g, and smaller g(av), than obtained with the models. These differences suggest that the diiron centers of prot eins are affected by their specific surroundings. Our results also sho w that the nature of the bridge, oxo or hydroxo, may not always safely be determined from a measured J value. Measurement of the EPR paramet ers, Delta g, g(av) and limiting temperature of observation (relaxatio n properties) of a mixed-valent complex, or, in the case of a diferric complex, of the primary mixed-valent species induced by radiolysis at 77 K, offers an alternative empirically based method to determine the nature of the bridge.