Low-dimensional polynuclear complexes and high-spin molecules: towards molecular magnets

Abstract

Coordination chemistry allows the synthesis of new molecules which show cooperative magnetic phenomena, behaving like magnets. Two main ways of synthesizing these systems have been developed: a) low-dimensional complexes (1-D or 2-D) which are linked in the crystal net and so give ferromagnetic long-range order and b) isolated molecules with high-spin ground state and strong anisotropy, which have a long relaxation time. Two different series of molecular magnets were prepared and characterized, following these approaches: a) The pseudohalide ligands (azido, thiocyanato and selenocyanato) are good bridging ligands which give easily tunable magnetic interactions between paramagnetic metal ions like CuII, NiII or MnII. With CuII and NiII these compounds are rarely studied outside the field of molecular magnetism and they do not give rise to molecular magnets. Nevertheless they allow magneto-structural correlations to be studied in greater depth. In contrast, MnII ion with azido and pyridine-derivative ligands easily gives one or two-dimensional systems with long-range order at low temperature, owing to canting phenomena which show cooperative ferromagnetism. They are, thus, molecular magnets. b) New oxo-bridged manganese carboxylate chemistry was also developed. We focused our attention on dodecamanganese(III,IV) of general formula [Mn12O12(2-Xbenzoato)16(H2O)4] (X = F, Cl, Br, Me). These new complexes were compared with their acetato and benzoato analogues. They are all superparamagnetic molecules, with high-spin ground state (close to ST = 10) and strong anisotropy, which gives a hysteresis loop and blocking temperature close to 4K. The most particular feature is the presense of a non-zero out-ofphase (imaginary) component in their ac susceptibility performed in zero field. The maximum in this imaginary component is frequencydependent, indicating superparamagnetic behavior. Thus, these Mn12 complexes have a property rarely associated with small molecular species: th