Single-Molecule Magnet Behaviour in a Tetrathiafulvalene-Based Electroactive Antiferromagnetically Coupled Dinuclear Dysprosium(III) Complex

The reactions between the [Ln(tta)(3)]center dot 2H(2)O precursors (tta(-) = 2-thenoyltrifluoroacetonate anion) and the tetrathiafulvalene-3-pyridine-N-oxide ligands (L-1) lead to dinuclear complexes of formula [{Ln(tta)(3)(L-1)}(2)]center dot xCH(2)Cl(2) (x = 0.5 for Ln = Dy-III (1) and x = 0 for Ln = Gd-III (2)). The crystal structure reveals that two {Ln(tta)(3)} moieties are bridged by two donors through the nitroxide groups. The DyIII centre adopts a distorted square antiprismatic oxygenated polyhedron structure. The antiferromagnetic nature of the exchange interaction between the two DyIII ions has been determined by two methods: 1) an empirical method using the [Dy-(hfac)(3)(L-2)(2)] mononuclear complex as a model (3) (hfac(-) = 1,1,1,5,5,5-hexafluoroacetylacetonate anion, L-2 = tetrathiafulvaleneamido-2-pyridine-N-oxide ligand), and 2) assuming an Ising model for the Dy-III ion giving an exchange energy of -2.30 cm(-1), g = 19.2 in the temperature range of 2-10 K. The antiferromagnetic interactions have been confirmed by a quantitative determination of J for the isotropic Gd-III derivative (J = -0.031 cm(-1), g = 2.003). Compound 1 displays a slow magnetisation relaxation without applied external magnetic fields. Alternating current susceptibility shows a thermally activated behaviour with pre-exponential factors of 5.48(4) x 10(-7) s and an energy barrier of 87(1) K. The application of an external field of 1.6 kOe compensates the antiferromagnetic interactions and opens a new quantum tunnelling path.