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

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 Dy-4 complex with a rhomb-like arrangement of four Dy-III as well as two dinuclear complexes having axial chlorido ligands (Dy-2 center dot Cl and Dy-2*center dot Cl) were obtained; in the latter case, substituting Cl- by SCN- gave Dy-2 center dot SCN. Magneto-structural studies revealed that the mu-OH bridges with short Dy-O bonds dominate the magnetic anisotropy of the Dy-III ions in centrosymmetric Dy-4 to give a vortex type diamagnetic ground state. Dynamic magnetic studies of Dy-4 identified two relaxation processes under zero field, one of which is suppressed after applying a dc field. For complexes Dy-2 center dot Cl and Dy-2*center dot 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 Dy-2 center dot 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.

Automated summary

This study successfully synthesized dinuclear lanthanide complexes with different coordination geometries by using pyrazolate-based macrocyclic ligands, and revealed the importance of mu-OH bridges with short Dy-O bonds in determining the magnetic properties.