Dysprosium Compounds with Hula-Hoop-like Geometries: The Influence of Magnetic Anisotropy and Magnetic Interactions on Magnetic Relaxation.

Single-ion anisotropy is one of the crucial properties for mononuclear and even polynuclear single-molecule magnets (SMMs), which can be enhanced by the judicious choice of the coordination geometry around the metal centers. Meanwhile, magnetic interactions also play a significant role in high-performance polynuclear SMMs, especially the dinuclear SMMs. For exploring the influence of those two factors on the magnetic properties, we report a novel series of lanthanide complexes, [Dy(L)(HL)(THF)] (1), [Dy(PyCO)(CFSO)(HO)]·CHCN (2), and [Dy(PyCO)(PhCOO)(MeOH)]·MeOH (3), with hula-hoop-like geometries around the Dy ions. All three complexes display slow relaxation of magnetization under a zero applied direct-current field with anisotropy barriers of 169 and 51 K for 1 and 3, respectively, while the slow relaxation of magnetization of complex 2 may mainly result from Raman relaxation. Besides, complex 1 demonstrates butterfly-type hysteresis below 4 K, and complex 2 shows no opening of the hysteresis loop with an inflection of around 0.25 T. Although complexes 2 and 3 have similar structures, the different coordinate anions induce distinct magnetic interaction states, antiferromagnetic and ferromagnetic for 2 and 3, respectively. Ab initio calculations reveal that the better SMM behavior of complex 1 should be ascribed to stronger single-ion anisotropy compared with complexes 2 and 3. The small value of the dipolar interaction results in an overall antiferromagnetic interaction for complex 2, while the large value of the dipolar interaction causes an overall ferromagnetic interaction for complex 3, where the dipolar interactions are ferromagnetic for both complexes.