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
posted on 2012-07-04, 00:00authored byHongxiao 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.