MIXED-VALENCE FE(III)-FE(IV) DIMERS - PREPARATION AND PHYSICAL-PROPERTIES OF OXIDATION-PRODUCTS OF OXO-BRIDGED BINUCLEAR IRON(III) COMPLEXES

Polycrystalline samples of the oxidation products of nine ore-bridged binuclear iron(III) complexes [Fe(2)L(2)O](+)[I-3](-)X (I-VIII) and [F e(2)L(2)(OH)(2)]+(BF4)(-) (IX), where L(2-)=some substituted tetraisot hiosemicarbazide ligands, X=iodine or iodinechloroform solvates, are i nvestigated over the temperature range 4.2-300 K using EPR, Mossbauer and magnetic susceptibility measurements. The results of the investiga tion show that oxidation of the mu-oxo [Fe(III)(2)L(2)O] dimers gives the mixed-valence Fe(III)-Fe(IV) dimers. The Fe-57 Mossbauer spectra o f these compounds in the temperature range 300-80 K exhibit a single q uadrupole doublet which indicates delocalization of the 'extra' electr on between iron sites, at least within the Mossbauer time scale of abo ut 10(-7) s. The presence of the antiferromagnetic exchange interactio n in these dimers is evidenced by variable-temperature (4-300K) magnet ic susceptibility data. These data are least-squares fit to the theore tical equation for a spin Hamiltonian which includes the isotropic exc hange interaction between two iron centers (JS(1)S(2), J similar to 30 -50 cm(-1)), the axial zero-field splitting of the iron centers with t he parameter D similar to 10cm(-1), and the interdimer exchange intera ction (\z'J(inter)\similar to 1-9 cm(-1)). The ground state with S=1/2 for all dimers is proved by the value of the magnetic moment at 4.2 K . Practically symmetric signals with g values similar to 2.0 are obser ved in X- and Q-band EPR spectra at room temperature. Analysis of the temperature dependence of the spectra detected in the temperature rang e 300-100 K for I-VIII and 300-40 K for IX compounds showed that the s pectra are due to transitions within the excited spin states with the total quantum number S>1/2. The linewidth increases as the S value dec reases and the signal due to S=1/2 is not detected. It is proposed tha t the observed peculiarities of the EPR spectra are connected with the intramolecular electron transfer which is accompanied by some fluctua tions of the dimer structure. The absence of the signals at low temper ature indicates that the period of this fluctuation coincides with the EPR time scale (10(-10)s).