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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

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posted on 2012-07-04, 00:00 authored by Hongxiao Jin, Hua Yang, Meilan Yu, Ziyang Liu, Christine M. Beavers, Marilyn M. Olmstead, Alan L. Balch
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 NiII(octaethylporphyrin) reveals the identities of two of the Sm@C92 isomers: Sm@C92(I), which is the more abundant isomer, is Sm@C1(42)-C92, and Sm@C92(II) is Sm@Cs(24)-C92. The structure of the most abundant form of the four isomers of Sm@C94, Sm@C94(I), is Sm@C3v(134)-C94, which utilizes the same cage isomer as the previously known Ca@C3v(134)-C94 and Tm@C3v(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@C1(42)-C92 and Sm@Cs(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@C1(42)-C92 is more stable than Sm@Cs(24)-C92 but possesses a smaller HOMO–LUMO gap. While the electronic structures of these endohedrals can be formally described as Sm2+@C2n2–, 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.

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