Macrocycle based dinuclear dysprosium(iii) single molecule magnets with local D5h coordination geometry

Abstract

Targeted approaches for manipulating the coordination geometry of lanthanide ions are a promising way to synthesize high-performance single-molecule magnets (SMMs), but most of the successful examples reported to date focus on mononuclear complexes. Herein, we describe a strategy to assemble dinuclear SMMs with Dy^III ions in approximate D_5h coordination geometry based on pyrazolate-based macrocyclic ligands with two binding sites. A Dy₄ complex with a rhomb-like arrangement of four Dy^III as well as two dinuclear complexes having axial chlorido ligands (Dy₂·Cl and Dy₂^*·Cl) were obtained; in the latter case, substituting Cl⁻ by SCN⁻ gave Dy₂·SCN. Magneto-structural studies revealed that the μ-OH bridges with short Dy–O bonds dominate the magnetic anisotropy of the Dy^III ions in centrosymmetric Dy₄ to give a vortex type diamagnetic ground state. Dynamic magnetic studies of Dy₄ identified two relaxation processes under zero field, one of which is suppressed after applying a dc field. For complexes Dy₂·Cl and Dy₂^*·Cl, the Dy^III ions feature almost perfect D_5h environment, but both complexes only behave as field-induced SMMs (U_eff = 19 and 25 K) due to the weak axial Cl⁻ donors. In Dy₂·SCN additional MeOH coordination leads to a distorted D_2d geometry of the Dy^III ions, yet SMMs properties at zero field are observed due to the relatively strong axial ligand field provided by SCN⁻ (U_eff = 43 K). Further elaboration of preorganizing macrocyclic ligands appears to be a promising strategy for imposing a desired coordination geometry with parallel orientation of the anisotropy axes of proximate Dy^III ions in a targeted approach.