Single molecule magnet with an unpaired electron trapped between two lanthanide ions inside a fullerene

Increasing the temperature at which molecules behave as single-molecule magnets is a serious challenge in molecular magnetism. One of the ways to address this problem is to create the molecules with strongly coupled lanthanide ions. In this work, endohedral metallofullerenes Y₂@C₈₀ and Dy₂@C₈₀ are obtained in the form of air-stable benzyl monoadducts. Both feature an unpaired electron trapped between metal ions, thus forming a single-electron metal-metal bond. Giant exchange interactions between lanthanide ions and the unpaired electron result in single-molecule magnetism of Dy₂@C₈₀(CH₂Ph) with a record-high 100 s blocking temperature of 18 K. All magnetic moments in Dy₂@C₈₀(CH₂Ph) are parallel and couple ferromagnetically to form a single spin unit of 21 micro_B with a dysprosium-electron exchange constant of 32 cm^-1. The barrier of the magnetization reversal of 613 K is assigned to the state in which the spin of one Dy centre is flipped.