Theoretical study of polyoxometalates with interest in molecular magnetism. Estudio teórico de polioxometalatos de interés en magnetismo molecular

Resumen

The general objective of this thesis was to use polyoxometalates as model systems to study different problems in molecular magnetism, including the interaction between localized spins and delocalized electrons and the low-temperature magnetization dynamics of single lanthanide ions. The experimental work was mainly devoted to reproduce the synthesis of known molecules to allow their detailed magnetic characterization, but some new structures were also obtained. The theoretical work extended well beyond the description of the experimental results, and included calculations from first principles with predictive value. What follows is a brief summary of the three very distinct parts of the work: a new structure of an antiferromagnetically coupled trinuclear Mn(II) moiety, the magnetic characterization of a polyoxometalate analog of the phthalocyanine single-lanthanide-ion molecular magnets, and an experimental and theoretical characterization of a mixed-valence system where delocalized electrons mediate a magnetic interaction between distant localized spins. During this thesis, we have synthesized a new trisubstituted silicowolframate {((B-ß-SiW9O32(OH)2)(b-SiW8O28(OH)3)Mn3(H2O)Mn}7-. The corresponding potassium/sodium salt Na8K4[{((B-ß-SiW9O32(OH)2)(ß-SiW8O28(OH)3)Mn3(H2O)Mn}2Mn(H2O)2].xH2O has been structurally characterized by IR spectroscopy and single crystal X-ray diffraction. Susceptometry measurements show that Mn(II) ions in this system have antiferromagnetic exchange (J=-1.33 cm-1, g=2.03). In the second part, and stimulated by the bis(pthalocyaninato)lanthanide complexes which behave as single molecule magnets, we studied the magnetic properties of polyoxometalates encapsulating single lanthanide ions with very similar coordination geometries. The series [LnIII(( ß2-SiW11O39)2]13- (Ln=Dy, Ho, Er, and Tm) and [LnIII(W5O18)2]9- (Ln=Tb, Dy, Ho and Er) were synthesized and characterized. The ac magnetic measurements resulted in interesting low temperature magnetization dynamics for some of the compounds. The susceptometry and magnetization measurements were collectively fit to avoid overparametrization problems, and this allowed the determination of their electronic structures. At last, we have reproduced the synthesis and obtained for the first time the magnetic characterization of [PMo12O40(VO)2][(C2H5)3NH]5. At T below 50K, an S=1 ground state was found, which might be due to a ferromagnetic interaction between the vanadyl groups, mediated by the delocalized electrons in the Keggin core. To study this assumption, high accuracy ab initio methods were employed to estimate microscopic parameters, followed by the exact diagonalization of a 14-center t-J-¿-V model Hamiltonian. We extended the theoretical study to different electronic populations to get a deeper insight of the mechanisms responsible for the magnetic behavior. Finally, we used the calculated microscopic parameters to obtain magneto-structural relationships for oxo-bridged Mo-Mo and Mo-V entities