Attaining record-high magnetic exchange, magnetic anisotropy and blocking barriers in dilanthanofullerenes.

While the blocking barrier ( ) and blocking temperature ( ) for "Dysprocenium" SIMs have been increased beyond liquid N temperature, device fabrication of these molecules remains a challenge as low-coordinate Ln complexes are very unstable. Encapsulating the lanthanide ion inside a cage such as a fullerene (called endohedral metallofullerene or EMF) opens up a new avenue leading to several Ln@EMF SMMs. The CASSCF calculations play a pivotal role in identifying target metal ions and suitable cages in this area. Encouraged by our earlier prediction on Ln@CN, which was verified by experiments, here we have undertaken a search to enhance the exchange coupling in this class of molecules beyond the highest reported value. Using DFT and calculations, we have studied a series of Gd@C (30 ≤ 2 ≤ 80), where an antiferromagnetic of -43 cm was found for a stable Gd@C- cage. This extremely large and exceptionally rare 4f⋯4f interaction results from a direct overlap of 4f orbitals due to the confinement effect. In larger cages such as Gd@C and Gd@C, the formation of two centre-one-electron (2c-1e) Gd-Gd bonds is perceived. This results in a radical formation in the fullerene cage leading to its instability. To avoid this, we have studied heterofullerenes where one of the carbon atoms is replaced by a nitrogen atom. Specifically, we have studied Ln@CN and Ln@CN, where strong delocalisation of the electron yields a mixed valence-like behaviour. This suggests a double-exchange () is operational, and CASSCF calculations yield a value of 434.8 cm and resultant of 869.5 cm for the Gd@CN complex. These parameters are found to be two times larger than the world-record reported for Gd@CN. Further calculations reveal an unprecedented of 1183 and 1501 cm for Dy@CN and Tb@CN, respectively. Thus, this study offers strong exchange coupling as criteria for new generation SMMs as the existing idea of enhancing the blocking barrier crystal field modulation has reached its saturation point.