MAGNETIC CHARACTERIZATION OF TETRANUCLEAR COPPER(II) AND COBALT(II) EXCHANGE-COUPLED CLUSTERS ENCAPSULATED IN HETEROPOLYOXOTUNGSTATE COMPLEXES - STUDY OF THE NATURE OF THE GROUND-STATES

This paper presents a magnetic characterization of the heteropolyanions [M4(H2O)2(PW9O34)2]10- and [M4(H2O)2(P2W15O56)2]16- (M = Cu(II), Co(II)), with emphasis on the exchange interactions. Their individual heteropolyoxometalate molecules encapsulate a rhomblike arrangement formed by four coplanar MO6 octahedra sharing edges. The magnetic susceptibility data show that the four copper ions are antiferromagnetically coupled, while in the cobalt (II) complexes the intramolecular exchange is ferromagnetic. In all cases the ground state of the M4O16 molecules has been found to be magnetic. These behaviors are discussed from isotropic (Cu4 clusters) or anisotropic (Co4 clusters) exchange models. In case of copper compounds, the presence of a triplet ground state is in agreement with the order of energy levels deduced from the analysis of the magnetic data and is confirmed from EPR and magnetization measurements. This intermediate-spin ground state is discussed in relation with spin frustration resulting from the presence of two competing copper-copper interactions in the rhomb. Finally, the nature of the ground state as well as the presence of intercluster interactions in the cobalt clusters are examined from low-temperature susceptibility and magnetization measurements and compared with the results obtained in the copper clusters.