Magnetic properties of two new Fe4single-molecule magnets in the solid state and in frozen solution

Two novel tetranuclear, starshaped iron(III) clusters, [Fe4(acac)6-(Br-mp)2] and [FeIII4(acac)6(tmp)2], are described. Both have S = 5 ground states resulting from antiferromagnetic nearest-neighbour superexchange interactions, with J= - 8.2 cm-1and J=-8.5 cm-1for 1 and 2, respectively. Energy barriers for the relaxation of the magnetisation of approximately 12 cm-1were derived from AC susceptibility measurements. Magnetic resonance measurements revealed a zerofield splitting parameter D =-0.34 cm-for both complexes. AC susceptibility measurements in solution demonstrated that the complexes are reasonably stable in solution. Interestingly, the magnetisation relaxation slows down significantly in frozen solution, in contrast to what is generally observed for single-molecule magnets. This was shown to result from a large increase in τ0, the prefactor in the Arrhenius equation, with the energy barrier remaining unchanged. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim