Gd₂@C₇₉N: Isolation, characterization, and monoadduct formation of a very stable heterofullerene with a magnetic spin state of S = 15/2

The dimetallic endohedral heterofullerene (EHF), Gd₂@C ₇₉N, was prepared and isolated in a relatively high yield when compared with the earlier reported heterofullerene, Y₂@C ₇₉N. Computational (DFT), chemical reactivity, Raman, and electrochemical studies all suggest that the purified Gd₂@C ₇₉N, with the heterofullerene cage, (C₇₉N)^5- has comparable stability with other better known isoelectronic metallofullerene (C₈₀)^6- cage species (e.g., Gd₃N@C ₈₀). These results describe an exceptionally stable paramagnetic molecule with low chemical reactivity with the unpaired electron spin density localized on the internal diatomic gadolinium cluster and not on the heterofullerene cage. EPR studies confirm that the spin state of Gd ₂@C₇₉N is characterized by a half-integer spin quantum number of S = ¹⁵/₂. The spin (S = ¹/ ₂) on the N atom of the fullerene cage and two octet spins (S = ⁷/₂) of two encapsulated gadoliniums are coupled with each other in a ferromagnetic manner with a small zero-field splitting parameter D. Because the central line of Gd₂@C₇₉N is due to the Kramer's doublet with a half-integer spin quantum number of S = ¹⁵/₂, this relatively sharp line is prominent and the anisotropic nature of the line is weak. Interestingly, in contrast with most Gd³⁺ ion environments, the central EPR line (g = 1.978) is observable even at room temperature in a toluene solution. Finally, we report the first EHF derivative, a diethyl bromomalonate monoadduct of Gd₂@C ₇₉N, which was prepared and isolated via a modified Bingel-Hirsch reaction.