Sc2@C66 revisited: an endohedral fullerene with scandium ions nestled within two unsaturated linear triquinanes.

The geometries of fullerenes are governed by the isolated pentagon rule (IPR), which states that stable fullerenes have each of their 12 pentagons surrounded by five hexagons. At the dawn of fullerene science, it was widely believed that the IPR would also be applicable for endohedral fullerenes. In 2000, that idea was altered by the discovery of the first non-IPR fullerenes, Sc2@C66 and Sc3N@C68. The structural data for Sc2@C66 were interpreted to indicate the presence of a pair of doubly fused pentagons. However, that structure has remained a long-standing mystery, since it is thermodynamically unfavorable. Here, we demonstrate definitively that Sc2@C66 does not have the structure that was long believed to be but a brand new type. 2D NMR spectroscopic and single-crystal X-ray analyses disclose that Sc2@C66 has a C2v(4059)-C66 cage containing two sets of unsaturated linear triquinanes (ULTs), in which three pentagons abut one another and two scandium ions are located within the folds of each of the ULT units.

Unexpected formation of a Sc3C2@C80 bisfulleroid derivative.

The reaction of tetrazine 1 with Sc(3)C(2)@C(80) exclusively affords the open-cage derivative 2 instead of the expected C(2)-inserted derivative 3 bearing a four-membered ring, as previously obtained for C(60). The structure of 2 has been firmly established by NMR spectroscopy and theoretical calculations. EPR spectroscopy shows that a single Sc atom of the Sc(3)C(2) cluster gets located within the bulge created by the bridging addend, which is a first step toward release of the internal metal atoms.

Enantioselective synthesis of endohedral metallofullerenes.

Endohedral metallofullerenes are promising materials in biomedical and material sciences. In particular, they are of interest as agents for magnetic resonance imaging (MRI), photovoltaic devices, and semimetallic components. The synthesis of chiral endofullerenes represents one step further in the potential use of these carbon allotropes; however, this step has not been addressed so far. In this regard, enantiopure endofullerenes are expected to open new avenues in fields in which chirality is a key issue. Here, the synthesis and characterization of the first chiral endohedral metallofullerenes, namely, chiral bis-adducts of La@C(72), are reported. Eight optically active isomers were obtained by enantioselective 1,3-dipolar cycloaddition of a N-metalated azomethine ylide onto a non-isolated-pentagon rule metallofullerene derivative, La@C(72)(C(6)H(3)Cl(2)), catalyzed by a copper chiral complex. The chiral bis-adducts of La@C(72), isolated by nonchiral HPLC, showed optical purities as high as 98% as revealed by the remarkable positive or negative Cotton effects observed in the circular dichroic spectra.

Construction and performance of an NMR tube with a sample cavity formed within magnetic susceptibility-matched glass.

We describe the construction and performance of an NMR tube with a magnetic susceptibility matched sample cavity that confines the solution within the detection zone in the axial direction and in a quasi-rectangular region in the radial direction. The slot-like sample cavity provides both good sample volume efficiency and tolerance to sensitivity loss in the sample space. The signal-to-noise ratio per unit volume of the constructed tube was 2.2 times higher than that of a cylindrical tube of 5mm outer diameter with a sample containing 300 mM NaCl at a static magnetic field of 14.1T. Even the overall signal-to-noise ratio of the slot tube was 35% higher than that of the conventional 5mm tube for a sample containing 300 mM NaCl. Similar improvements over existing sample tube geometries were obtained at 950 MHz. Moreover the temperature rise resulting from RF heating was found to be significantly lower for the slot tube even when compared to 3 and 4mm outer diameter cylindrical tubes as measured in a 5mm cryoprobe. A further advantage of this type of tube is that a sample cavity of any desired size and shape can be formed within a cylindrical tube for use in a single cryogenic probe.

Analysis of lanthanide-induced NMR shifts of the Ce@C82 anion.

The mapping of bond connectivity in the carbon cage of [Ce@C82]- and full assignment of the NMR lines were successfully achieved by means of 2D INADEQUATE NMR measurement. Paramagnetic NMR analysis shows that the Ce atom in [Ce@C82]- is located at an off-centered position adjacent to a hexagonal ring along the C2 axis of the C2v-C82 cage.

Characterization of Ce@C82 and its anion.

Ce@C(82) is isolated by high-performance liquid chromatography (HPLC) and the cage symmetry is determined as C(2)(v)() by measuring the (13)C NMR spectra of its anion. The (13)C NMR peaks of [Ce@C(82)](-) show temperature-dependent shifts ascribed to the f electron remaining on the Ce atom. Both Ce@C(82) and [Ce@C(82)](-) are silent in electron spin resonance spectroscopy (ESR) because of the highly anisotropic g matrix as well as of the fast relaxation process originating from the orbital angular momentum of the f electron. This is the complementary relationship to the observation of the paramagnetic shift in (13)C NMR. [Ce@C(82)](-) has lower stability in air than [La@C(82)](-).