Fractionation of rare-earth metallofullerenes via reversible uptake and release from reactive silica

Abstract

Minimal research exists for non-chromatographic separations of rare-earth metallofullerenes containing di-metallic (M₂), di-metallic carbide (M₂C₂), and tri-metallic nitride (M₃N) clusters trapped inside fullerene cages. Herein, we demonstrate a non-HPLC method (i.e., SAFA, Stir and Filter Approach) for purifying Er₃N@I_h-C₈₀, a rare-earth, metallic nitride clusterfullerene. We describe a strategic method that chemically releases rare-earth metallofullerenes (e.g., M₂@C_2n, M₃N@C_2n) trapped by aminosilica during SAFA. Recovery of metallofullerenes from spent silica represents a “green approach” because the spent silica and its useful, immobilized rare-earth metallofullerenes would have been discarded as waste material. We observe selectivity during metallofullerene uptake to aminosilica and also during its release from spent silica via addition of CS₂. We describe a procedure to obtain samples enriched in M₂ and M₃N endohedrals. M₂C_2n fractions from our SAFA release process contain a wide range of higher metallofullerenes (e.g., Gd₂C₉₀–Gd₂C₁₄₀ or Er₂C₇₆–Er₂C₁₂₂). It is facile to obtain samples enriched in M₃N@C₈₂–M₃N@C₉₂. Note that unreacted M₃N@C₈₀ remains in the filtrate. The strategy for handling rare-earth metallofullerenes with different degrees of reactivity toward aminosilica is also discussed.