MAGNETIC-PROPERTIES OF ISOSTRUCTURAL DODECANUCLEAR POLYOXOVANADATES WITH 6 AND 8 VANADIUM(IV) IONS

We investigated the magnetic properties of two polyoxovanadate anions, [V12As8O40(HCO2)]3- (I) and [V12As8O40(HCO2)]5-(II). Both I and II comprise four localized vanadium(IV) ions while two vanadium (IV) ions in I and four in II are delocalized on eight sites, the remaining ions formally being vanadium(V). The temperature dependence of the magnetic susceptibility for the two compounds was fitted with a model which takes into account an antiferromagnetic coupling between the localized vanadium(IV) ions. In Il the delocalized vanadium(IV) ions are strongly coupled to give a ground S = 0 level by exchange and delocalization effects, while in I the delocalized vanadium(IV) must be considered to be ferromagnetically coupled to the localized ones. The magnetic behavior correlates with the geometry and the topology of the clusters. EPR spectra were recorded both at X-band frequency and with a high-field spectrometer using a far-infrared laser. The spectra of I and II show opposite behaviors at low and high fields, presumably due to field-dependent relaxation effects.