Th@C86, Th@C82, Th@C80, and Th@C76: role of thorium encapsulation in determining spherical aromatic and bonding properties on medium-sized endohedral metallofullerenes

Thorium encapsulated metallofullerenes (Th-EMFs) with external C-76, C-80, C-82, and C-86 cages have been synthesized, with the C-13-NMR spectrum recorded for Th@C-82. Here, we explore computationally the chemical bonding, NMR and spherical aromaticity of Th@C-82 and related thorium-encapsulated metallofullerenes. Our results show that these Th-EMFs are new examples of spherical aromatic structures, representing interesting low-symmetry exceptions to the Hirsch 2(N + 1)(2) rule of spherical aromaticity. Their electronic structures are based on pi-electron counts of 80, 84, 86, and 90, respectively, with a shell structure ranging from S(2)P(6)D(10)F(14)G(18)H(22)I(8) to S(2)P(6)D(10)F(14)G(18)H(22)I(18), where the partially filled I-shell remains as a frontier orbital. Their behavior is comparable to that of the spherical aromatic alkali-C-60(6-) phases, which in addition to the favorable endohedral Th-fullerene bonding account for their particular abundance exhibiting the ability to sustain a long-range shielding cone as a result of the favorable metal-cage bonding. This rationalization of such species as neutral spherical aromatic EMFs suggests the possibility of an extensive series of aromatic fullerenes with nuclearity larger than C-60 buckminsterfullerene as stable building blocks towards nanostructured metal-organic materials.