Controlling Magnetic Anisotropy of Endohedral Lanthanide Ions by Carbene Addition: Paramagnetic NMR, Lanthanide Luminescence, and Single-Molecule Magnetism in Adamantylidene Adducts of MSc₂N@C₈₀ (M = Nd, Dy)

Metallofullerenes with endohedral lanthanides have emerged as a versatile class of single-molecule magnets owing to strong single-ion magnetic anisotropy, which can be realized in the interior of the fullerene cage. Since exohedral chemical modification of fullerenes is often used to adjust their properties and processability for prospective practical applications, it is necessary to understand how it can affect their magnetic properties. In this work, we studied how a popular [2 + 1] cycloaddition reaction, photochemical addition of adamantylidene (Ad), affects single-ion magnetic anisotropy and single-molecule magnetism of MSc₂N@C₈₀ (M = Nd, Dy). For each lanthanide, the reaction yielded [5,6]-open and [6,6]-open isomers of the monoadduct MSc₂N@C₈₀(Ad). Paramagnetic ¹H NMR was demonstrated that the Ad-addition site in [5,6] isomers is predominantly coordinated by Sc, whereas both Sc and lanthanide coordination coexist in [6,6] isomers. <i>Ab initio</i> calculations and Nd-based photoluminescence showed that the [5,6] isomer has enhanced ligand-field splitting, whereas coordination of the lanthanide to the Ad-addition site in the [6,6] isomer reduces magnetic axiality and ligand-field splitting. For DySc₂N@C₈₀(Ad), Dy-Ad coordination leads to a noticeable reduction in the blocking temperature of magnetization, whereas Dy coordination to the unfunctionalized fragments of the fullerene cage improves the SMM performance in comparison to the unfunctionalized DySc₂N@C₈₀. Thus, carbene addition can enhance or deteriorate SMM properties depending on the regioisomerism and the lanthanide-cage coordination geometry in MSc₂N@C₈₀(Ad) adducts. These results demonstrate that chemical derivatization of EMFs can become a useful tool for improving their magnetic properties, but will require careful evaluation of different factors for each reaction type.