Assessing the exchange coupling in binuclear lanthanide(III) complexes and the slow relaxation of the magnetization in the antiferromagnetically coupled Dy2 derivative

We report here the synthesis and the investigation of the magnetic properties of a series of binuclear lanthanide complexes belonging to the metallacrown family. The isostructural complexes have a core structure with the general formula [Ga(4)Ln(2)(shi(3-))(4)(Hshi(2-))(2)(H(2)shi(-))(2)(C5H5N)(4)(CH3OH)(x)(H2O)(x)]center dot xC(5)H(5)N center dot xCH(3)OH center dot xH(2)O (where H(3)shi = salicylhydroxamic acid and Ln = Gd-III 1; Tb-III 2; Dy-III 3; Er-III 4; Y-III 5; (Y0.9Dy0.1III)-Dy-III 6). Apart from the Er-containing complex, all complexes exhibit an antiferromagnetic exchange coupling leading to a diamagnetic ground state. Magnetic studies, below 2 K, on a single crystal of 3 using a micro-squid array reveal an opening of the magnetic hysteresis cycle at zero field. The dynamic susceptibility studies of 3 and of the diluted DyY 6 complexes reveal the presence of two relaxation processes for 3 that are due to the excited ferromagnetic state and to the uncoupled Dy-III ions. The antiferromagnetic coupling in 3 was shown to be mainly due to an exchange mechanism, which accounts for about 2/3 of the energy gap between the antiferro- and the ferromagnetic states. The overlap integrals between the Natural Spin Orbitals (NSOs) of the mononuclear fragments, which are related to the magnitude of the antiferromagnetic exchange, are one order of magnitude larger for the Dy-2 than for the Er-2 complex.