Single Samarium Atoms in Large Fullerene Cages. Characterization of Two Isomers of Sm@C₉₂ and Four Isomers of Sm@C₉₄ with the X-ray Crystallographic Identification of Sm@<i>C</i>₁(42)-C₉₂, Sm@<i>C</i>_<i>s</i>(24)-C₉₂, and Sm@<i>C</i>_3<i>v</i>(134)-C₉₄

Abstract

Two isomers of Sm@C₉₂ and four isomers of Sm@C₉₄ were isolated from carbon soot obtained by electric arc vaporization of carbon rods doped with Sm₂O₃. Analysis of the structures by single-crystal X-ray diffraction on cocrystals formed with Ni^II(octaethylporphyrin) reveals the identities of two of the Sm@C₉₂ isomers: Sm@C₉₂(I), which is the more abundant isomer, is Sm@<i>C</i>₁(42)-C₉₂, and Sm@C₉₂(II) is Sm@<i>C</i>_<i>s</i>(24)-C₉₂. The structure of the most abundant form of the four isomers of Sm@C₉₄, Sm@C₉₄(I), is Sm@<i>C</i>_3<i>v</i>(134)-C₉₄, which utilizes the same cage isomer as the previously known Ca@<i>C</i>_3<i>v</i>(134)-C₉₄ and Tm@<i>C</i>_3<i>v</i>(134)-C₉₄. All of the structurally characterized isomers obey the isolated pentagon rule. While the four Sm@C₉₀ and five isomers of Sm@C₈₄ belong to common isomerization maps that allow these isomers to be interconverted through Stone–Wales transformations, Sm@<i>C</i>₁(42)-C₉₂ and Sm@<i>C</i>_<i>s</i>(24)-C₉₂ are not related to each other by any set of Stone–Wales transformations. UV–vis–NIR spectroscopy and computational studies indicate that Sm@<i>C</i>₁(42)-C₉₂ is more stable than Sm@<i>C</i>_<i>s</i>(24)-C₉₂ but possesses a smaller HOMO–LUMO gap. While the electronic structures of these endohedrals can be formally described as Sm²⁺@C_2<i>n</i>^2–, the net charge transferred to the cage is less than two due to some back-donation of electrons from π orbitals of the cage to the metal ion