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

Abstract

While the blocking barrier (Ueff) and blocking temperature (TB) for “Dysprocenium” SIMs have been increased beyond liquid N2temperature, device fabrication of these molecules remains a challenge as low-coordinate Ln3+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. Theab initioCASSCF calculations play a pivotal role in identifying target metal ions and suitable cages in this area. Encouraged by our earlier prediction on Ln2@C79N, 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 andab initiocalculations, we have studied a series of Gd2@C2n(30 ≤ 2n≤ 80), where an antiferromagneticJGd⋯Gdof −43 cm−1was found for a stable Gd2@C38-D3hcage. 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 Gd2@C60and Gd2@C80, 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 Ln2@C59N and Ln2@C79N, where strong delocalisation of the electron yields a mixed valence-like behaviour. This suggests a double-exchange (B) is operational, and CASSCF calculations yield aBvalue of 434.8 cm−1and resultantJGd-radof 869.5 cm−1for the Gd2@C59N complex. These parameters are found to be two times larger than the world-recordJreported for Gd2@C79N. Furtherab initiocalculations reveal an unprecedentedUcalof 1183 and 1501 cm−1for Dy2@C59N and Tb2@C59N, respectively. Thus, this study offers strong exchange coupling as criteria for new generation SMMs as the existing idea of enhancing the blocking barrierviacrystal field modulation has reached its saturation point.