Thirty Years of Hide-and-Seek:\nCapturing Abundant\nbut Elusive M<sup>III</sup>@<i>C</i><sub>3<i>v</i></sub>(8)‑C<sub>82</sub> Isomer, and the Study of Magnetic\nAnisotropy Induced in Dy<

Our knowledge about endohedral metallofullerenes (EMFs)\nis restricted\nto the structures with sufficient kinetic stability to be extracted\nfrom the arc-discharge soot and processed by chromatographic and structural\ntechniques. For the most abundant rare-earth monometallofullerene\nM<sup>III</sup>@C<sub>82</sub>, experimental studies repeatedly demonstrated <i>C</i><sub>2<i>v</i></sub>(9) and <i>C</i><sub><i>s</i></sub>(6) carbon cage isomers, while computations\npredicted equal stability of the “missing” <i>C</i><sub>3<i>v</i></sub>(8) isomer. Here we report that this\nisomer is indeed formed but has not been recovered from soot using\nstandard protocols. Using a combination of redox extraction and subsequent\nbenzylation and trifluoromethylation with single-crystal XRD analysis\nof CF<sub>3</sub> adduct, we prove that Dy@<i>C</i><sub>3<i>v</i></sub>(8)-C<sub>82</sub> is one of the most abundantly\nproduced metallofullerenes, which was not identified in earlier studies\nbecause of the low kinetic stability. Further, using the Dy@<i>C</i><sub>3<i>v</i></sub>(8)-C<sub>82</sub>(CF<sub>3</sub>) and Dy@<i>C</i><sub>3<i>v</i></sub>(8)-C<sub>82</sub>(CH<sub>2</sub>Ph) monoadducts for the case study, we analyzed\nthe role of metal-fullerene bonding on the single-ion magnetic anisotropy\nof Dy in EMFs. The multitechnique approach, combining <i>ab initio</i> calculations, EPR spectroscopy, and SQUID magnetometry, demonstrated\nthat coordination of the Dy ion to the fullerene cage induces moderate,\nnonaxial, and very fluid magnetic anisotropy, which strongly varies\nwith small alterations in the Dy-fullerene coordination geometry.\nAs a result, Dy@<i>C</i><sub>3<i>v</i></sub>(8)-C<sub>82</sub>(CH<sub>2</sub>Ph) is a weak field-induced single-molecule\nmagnet (SMM), whose signatures of magnetic relaxation are detectable\nonly below 3 K. Our results demonstrate that metal-cage interactions\nshould have a detrimental effect on the SMM performance of EMFs. At\nthe same time, the strong variability of the magnetic anisotropy with\nmetal position suggests tunability and offers strategies for future\nprogress.