Exchange interactions and electron delocalization in the mixed-valence cluster V4IVV2VO7(OC2H5)12

The mixed-valence cluster compound (V4V2O7)-V-IV-O-V(OC2H5)(12) was studied by X-band electron paramagnetic resonance (EPR) in the temperature range of 4.2-293 K. According to X-ray diffraction study, the crystal structure of the compound was described by a Mm space group at 295 K (four d(1) electrons are equally delocalized on all vanadium ions) and changed to a P2(1)/n space group on cooling the crystals to 173 K (the electrons are preferably localized on the four equatorial vanadium ions). The EPR spectra originate from the S = 1 total spin states with the fine structure averaged to a single Lorentzian line and from the S = 2 total spin states with fine structure partly averaged in the temperature range of 295-200 K and well averaged below 45-50 K. The states of S = 1 and S = 2 of comparable energy (Delta E similar to 2 cm(-1); E-s=1 < E-s=2) were shown to be the lowest ones. The V-IV <-> V-V unpaired electron transfers together with isotropic Heisenberg exchange were shown to determine the total spin states composition and the intracluster dynamics of the compound. Two types of electron transfers were assumed: the single-jump transfer leading to the averaged configurations of the (V4V2V)-V-IV <-> (V3VV)-V-IV (VVV)-V-IV type and to the splitting of the total spin states by intervals comparable in magnitude with the isotropic exchange parameter J 100 cm-1 and the double-jump transfer resulting in dynamics. Temperature dependence of the transition rates nu(tr) was observed. In the range of 295-210 K, the value of nu(tr) = (0.5-0.6) x 10(10) s(-1) is sufficient for averaging the fine structure of the S = 1 states, and below 45 K the value of nu(tr) approximate to 1.5 x 10(10) s(-1) also averages the fine structure of the S = 2 state. A change in the localization plane of the V-IV ions in the temperature range of 40-50 K was discovered.