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 oxo-bridged binuclear iron(III) complexes [Fe2L2O]+[I3]−X (I–VIII) and [Fe2L2(OH)2]+ (BF4)− (IX), where L2− = some substituted tetraisothiosemicarbazide ligands, X = iodine or iodinechloroform solvates, are investigated over the temperature range 4.2–300 K using EPR, Mossbauer and magnetic susceptibility measurements. The results of the investigation show that oxidation of the μ-oxo [Fe(III)2L2O] dimers gives the mixed-valence Fe(III)Fe(IV) dimers. The 57Fe Mossbauer spectra of these compounds in the temperature range 300-80 K exhibit a single quadrupole doublet which indicates delocalization of the ‘extra’ electron between iron sites, at least within the Mossbauer time scale of about 10−7 s. The presence of the antiferromagnetic exchange interaction in these dimers is evidenced by variable-temperature (4–300K) magnetic susceptibility data. These data are least-squares fit to the theoretical equation for a spin Hamiltonian which includes the isotropic exchange interaction between two iron centers (JS1S2,J ∼ 30–50 cm−1), the axial zero-field splitting of the iron centers with the parameter D ∼ 10 cm−1, and the interdimer exchange interaction (|z′Jinter| ∼ 1–9 cm−1). The ground state with S = 12 for all dimers is proved by the value of the magnetic moment at 4.2 K. Practically symmetric signals with g values ∼ 2.0 are observed in X- and Q-band EPR spectra at room temperature. Analysis of the temperature dependence of the spectra detected in the temperature range 100–300 K for I–VIII and 300-40 K for IX compounds showed that the spectra are due to transitions within the excited spin states with the total quantum number S > 12. The linewidth increases as the S value decreases and the signal due to S = 12 is not detected. It is proposed that the observed peculiarities of the EPR spectra are connected with the intramolecular electron transfer which is accompanied by some fluctuations of the dimer structure. The absence of the signals at low temperature indicates that the period of this fluctuation coincides with the EPR time scale (10−10 s).