Dy(BC₄Ph₅)(C₅ⁱPr₅): A Heteroleptic Sandwich Complex with Strong Axiality Giving Rise to Magnetic Blocking at 65 K

Dysprosium sandwich complexes incorporating sterically encumbered cyclic ligands represent a superior class of mononuclear single-molecule magnets. Here, we report the synthesis of the heteroleptic complex Dy(BC₄Ph₅)(C₅ⁱPr₅), incorporating one borolide and one cyclopentadienide ligand. Strong axial coordination by both ligands engenders high-temperature single-molecule magnet behavior, with a relaxation barrier of U_eff = 1536(15) cm^-1 and a blocking temperature of T_b = 65 K. These values are similar to those of related bis(borolide) complexes yet considerably larger than in a related bis(cyclopentadienide) species, [Dy(C₅ⁱPr₅)₂]⁺. To understand this trend, we carried out single-crystal X-ray diffraction and computational analyses, which revealed reduced ligand-ligand repulsion in Dy(BC₄Ph₅)(C₅ⁱPr₅) versus a related bis(borolide) complex, but with a weaker axial field owing to the cyclopentadienide ligand. This was further understood through calculations of the effective ligand charges across the series. In all, Dy(BC₄Ph₅)(C₅ⁱPr₅) operates at one of the highest temperatures among single-molecule magnets, significantly higher than [Dy(C₅ⁱPr₅)₂]⁺. Although offsetting axial field strength and ligand-ligand repulsions limit Dy(BC₄Ph₅)(C₅ⁱPr₅) from surpassing the best single-molecule magnets, these results demonstrate that high-temperature magnetism can be modulated by incorporating ligands with different charges and axiality and suggest that heteroleptic complexes with carefully chosen ligands could operate at still higher temperatures.