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@C1(42)-C92, Sm@Cs(24)-C92, and Sm@C3v(134)-C94

Abstract

Two isomers of Sm@C(92) and four isomers of Sm@C(94) were isolated from carbon soot obtained by electric arc vaporization of carbon rods doped with Sm(2)O(3). 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@C(92) isomers: Sm@C(92)(I), which is the more abundant isomer, is Sm@C(1)(42)-C(92), and Sm@C(92)(II) is Sm@C(s)(24)-C(92). The structure of the most abundant form of the four isomers of Sm@C(94), Sm@C(94)(I), is Sm@C(3v)(134)-C(94), which utilizes the same cage isomer as the previously known Ca@C(3v)(134)-C(94) and Tm@C(3v)(134)-C(94). All of the structurally characterized isomers obey the isolated pentagon rule. While the four Sm@C(90) and five isomers of Sm@C(84) belong to common isomerization maps that allow these isomers to be interconverted through Stone-Wales transformations, Sm@C(1)(42)-C(92) and Sm@C(s)(24)-C(92) are not related to each other by any set of Stone-Wales transformations. UV-vis-NIR spectroscopy and computational studies indicate that Sm@C(1)(42)-C(92) is more stable than Sm@C(s)(24)-C(92) but possesses a smaller HOMO-LUMO gap. While the electronic structures of these endohedrals can be formally described as Sm(2+)@C(2n)(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.