Yutaka Maeda

Chemical understanding of a non-IPR metallofullerene: stabilization of encaged metals on fused-pentagon bonds in La2@C72.

Fullerenes violating the isolated pentagon rule (IPR) are only obtained in the form of their derivatives. Since the [5,5]-bond carbons are highly reactive, they are easily attacked by reagents to release the bond strains. Non-IPR endohedral metallofullerenes, however, still have unsaturated sp (2) carbons at the [5,5] bond junctions, which allow their chemical properties to be probed. In this work, La 2@C 72 was chosen as a representative non-IPR metallofullerene, since it has been experimentally proposed to have either the #10611 or #10958 non-IPR cage structure ( J. Am. Chem. Soc. 2003, 125, 7782 ), while theoretical calculations have suggested that the #10611 cage is more stable ( J. Phys. Chem. A 2006, 110, 2231 ). La 2@C 72 was modified by photolytic reaction with the carbene reagent 2-adamantane-2,3-[3H]-diazirine. Six isomers of adamantylidene monoadducts were isolated and characterized using various kinds of measurements, including high-performance liquid chromatography, matrix-assisted laser desorption ionization mass spectrometry, UV-vis-near-infrared spectroscopy, cyclic voltammetry, differential-pulse voltammetry, (13)C NMR spectroscopy, and single-crystal X-ray diffraction. Electronic spectra and electrochemical studies revealed that the essential electronic structures of La 2@C 72 are retained in the six isomers and the adamantylidene group acts as a weak electron-donating group toward La 2@C 72. X-ray structural results unambiguously elucidated that La 2@C 72 has the #10611 chiral cage (i.e., D 2 symmetry) with two pairs of fused pentagons at each pole of the cage and that the two La atoms reside close to the two fused-pentagon pairs. On the basis of these results and theoretical calculations, it is concluded that the fused-pentagon sites are very reactive toward carbene but that the carbons forming the [5,5] junctions are less reactive than the adjacent ones; this confirms that these carbons interact strongly with the encaged metals and thus are stabilized by them.

Does Gd@C82 have an anomalous endohedral structure? Synthesis and single crystal X-ray structure of the carbene adduct.

We report here the results on single crystal X-ray crystallographic analysis of the Gd@C82 carbene adduct (Gd@C82(Ad), Ad = adamantylidene). The Gd atom in Gd@C82(Ad) is located at an off-centered position near a hexagonal ring in the C2v-C82 cage, as found for M@C82 (M = Sc and La) and La@C82(Ad). Theoretical calculation also confirms the position of the Gd atom in the X-ray crystal structure.

Bis‐Carbene Adducts of Non‐IPR La2@C72: Localization of High Reactivity around Fused Pentagons and Electrochemical Properties

Endohedral metallofullerenes (EMFs) are generated by encapsulation of metal species in hollow fullerene cavities. The strong interactions between the encaged metal species and the fullerene cage lead to novel structures and properties which are not found for empty fullerenes. An intriguing aspect of EMFs is their chemical properties, which clearly differ from those of empty fullerenes due to electron transfer from the encaged metal atoms to the fullerene cage. In addition, derivatives of EMFs are more useful than the parent EMFs in many fields, and especially their adducts are generally more suitable for single-crystal Xray diffraction (XRD)measurements, since pristine EMFs are orientationally disordered in the crystal phase due to their spherical shapes. Hence, chemical modification of EMFs has blossomed in recent years. Since the first exohedral adduct of La@C82 was reported in 1995, [2] various derivatives have been synthesized by different reactions, and some of them have been isolated and structurally characterized. Most reports focused on mono-adducts of EMFs, while only a few papers deal with bis-adducts. Stevenson and co-workers isolated a bis-adduct from a Diels–Alder reaction of Gd3N@C80, but no additional structural information was presented. Feng et al. reported regioselective synthesis of a bis-adduct of La@C82 from the Bingel–Hirsch reaction. X-ray data showed that the bis-adduct tends to form a dimer in the single crystal. Cai et al. reported that Bingel–Hirsch reaction of Sc3N@C78 affords abundantly a bis-adduct, and an NMR study showed that the second reaction site is controlled by the internal metal cluster. Investigations on bis-adducts provide deeper insight into the properties and chemical reactivities of EMFs. Another fascinating aspect of EMFs is stabilization of carbon cages violating the isolated-pentagon rule (IPR) by the encaged metal species. These non-IPR EMFs are currently of considerable interest because the [5,5]-junction carbon atoms are expected to be very reactive due to the high surface curvature, so that they were previously thought to be unstable species. In the last few years, some non-IPR fullerene derivatives and EMFs have been isolated and structurally characterized. However, no investigations on the chemical behavior of non-IPR fullerenes and/or EMFs have been reported. Recently, we found that La2@C72, which has a D2-symmetric non-IPR cage with two pairs of fused pentagons, reacts readily with 2-adamantane-2,3-[3H]-diazirine (1) to generate six isomers of mono-adduct La2@C72Ad (2, Ad= adamantylidene). Experimental and theoretical results suggested that the fused-pentagon regions are more reactive than other sites of the C72 cage, and more interestingly it was found for the first time that the [5,5]-junction carbon atoms are less reactive than the adjacent ones, as they interact strongly with the encaged metal atoms and thus are stabilized. Since two pairs of fused pentagons are available in La2@C72, it is natural to elucidate the reactivity of both. We now report the synthesis, isolation, and characterization of bis-carbene adduct La2@C72Ad2 (3). The reaction of La2@C72 with 1 was induced by a highpressure mercury-arc lamp in anhydrous toluene (Scheme 1) and monitored by HPLC on a Buckyprep M column (Figure 1). Before irradiation, the retention times of 1 and La2@C72 were 3 and 15 min, respectively. After 5 min of

Isolation and characterization of two Pr@C82 isomers

Abstract The second isomer of Pr@C 82 (Pr@C 82 -II) has been isolated by a two-step HPLC method, separately from the known major isomer (Pr@C 82 -I). Visible and near-IR absorption spectra, as well as cyclic and differential pulse voltammograms show that the characteristic features of Pr@C 82 -II are significantly different from those of Pr@C 82 -I. The absorption peaks of Pr@C 82 -II have blue-shifts and the redox potentials have negative shifts, relative to those of Pr@C 82 -I. Chemical derivatization of both isomers with disilirane have also been achieved.

Spectroscopic and theoretical study of endohedral dimetallofullerene having a non-IPR fullerene cage: Ce2@C72.

The endohedral dimetallofullerene having a non-IPR fullerene cage, Ce2@C72, is spectroscopically and theoretically characterized. The (13)C NMR measurements display large temperature-dependent signals caused by paramagnetic shifts, indicating that the Ce atoms are located near the two fused pentagons in the C72 cage. Theoretical calculations are performed to clarify the metal position, which are in good agreement with the result obtained by the paramagnetic (13)C NMR analysis. Electrochemical measurements reveal that Ce2@C72 has particularly lower oxidation and higher reduction potentials than other endohedral dimetallofullerenes.

Observation of 13C NMR Chemical Shifts of Metal Carbides Encapsulated in Fullerenes: Sc2C2@C82, Sc2C2@C84, and Sc3C2@C80

Endohedral metallofullerenes have attracted special interest as promising spherical molecules for material and catalytic applications, because of their unique properties that are not expected from empty fullerenes. The successful isolation and purification of endohedral metallofullerenes in macroscopic quantities have made it possible to investigate the structures as well as the electronic properties and reactivities through a close interplay between theory and experiment. Since the first isolation and characterization of a metal carbide encapsulated metallofullerene (Sc2C2@C84) by MEM (maximum entropy method)/Rietveld analysis of synchrotron powder diffraction data, much attention has been paid to encapsulation of metal carbides. Recently, we found by C NMR spectroscopy and X-ray single-crystal structure analyses that Sc3C82 and Sc2C84 have the structures Sc3C2@ C80(Ih) [5] and Sc2C2@C82(C3v), [6,7] encapsulating metal carbides, although it had long been believed from the MEM/Rietveld analyses that they have the conventional structures Sc3@C82 [8] and Sc2@C84 [9] , respectively. The encapsulation of metal carbides has been confirmed by the recent improved MEM/ Rietveld analyses of Y2C2@C82(C3v) [10] as well as Sc2C2@ C82(C3v) [11] and Sc3C2@C80(Ih). [12] However, attempts to detect the C NMR chemical shifts of the C2 units of metal carbides encapsulated in the fullerenes have not been yet successful. This unsuccessful detection has been explained in term of the spin-rotation interaction, because the C2 unit may rotate rapidly inside carbon cages. It is an important task to observe the C NMR chemical shift of the C2 unit in an attempt to provide insight into its electronic and magnetic properties. We present herein the first detection of the C NMR chemical shifts of the C2 unit in Sc2C2@C82(C3v), Sc2C2@ C84(D2d), and [Sc3C2@C80(Ih)] by using C-enriched samples. We also successfully assigned all C NMR chemical shifts of the cage carbon atoms for the three compounds by 2D INADEQUATE (incredible natural abundance double quantum transfer experiment) NMR measurements. The C NMR spectrum of Sc2C2@C82(C3v) (Figure 1a) shows 17 chemical shifts of the C82 cage at d = 134.6– 151.9 ppm, as reported previously. To map the bond connectivity in the carbon cage and assign the C NMR

Anisotropic Magnetic Behavior of Anionic Ce@C82 Carbene Adducts

Derivatives of Ce@C(82)(C(2v)) have been synthesized and fully characterized, and their anisotropic magnetism has been observed as paramagnetic shifts in NMR measurements. Carbene addition by photochemical reaction afforded two isomers of Ce@C(82)(C(2v))Ad (Ad = adamantylidene), 2a and 2b, demonstrating high regioselectivity. The two isomers were characterized using MALDI-TOF mass spectrometry, vis-NIR absorption spectroscopy, (1)H and (13)C NMR spectroscopy, and electrochemistry. The structure of the minor isomer (2b) was elucidated by single-crystal X-ray structural analysis. (13)C and (1)H NMR measurements revealed the characteristic anisotropic interaction between the f electron on the Ce atom and nuclear spins of the carbon atoms of the cage and the protons of the Ad group, respectively.

Addition of adamantylidene to La2@C78: isolation and single-crystal X-ray structural determination of the monoadducts.

Thermal and photochemical reactions of La2@C78 with 2-admantane-2,3-[3H]-diazirine are investigated. Four isomers of the monoadduct (La2@C78Ad) synthesized by the photoreaction are isolated by HPLC and characterized by mass, UV-vis-NIR absorption, cyclic voltammogram and differential pulse voltammogram spectroscopy, proton and 13C NMR spectroscopic analysis, single-crystal X-ray diffraction analysis, and theoretical approaches. The addition reactions occur at both the [5,6] and [6,6] positions. X-ray and theoretical studies indicate that one of the monoadduct isomers has an open structure with two La atoms on the C3 axis of the D3h cage of La2@C78.

Location of the yttrium atom in Y@C82 and its influence on the reactivity of cage carbons.

The first single-crystallographic results of Y@C(82) unambiguously disclosed that the yttrium atom is located under a hexagonal ring along the C(2) axis, which makes only 1 out of 24 nonequivalent carbons of the C(2v)-C(82) sufficiently reactive toward the electrophile adamantylidene carbene, affording only two monoadduct isomers.

La2@C80: is the circular motion of two La atoms controllable by exohedral addition?

Abstract Density functional calculations are carried out for the La2@C80 derivatives to investigate whether the motion of metals encapsulated inside fullerenes is controlled by exohedral addition. It is shown that the three-dimensional random motion of two La atoms in La2@C80 can be restricted to the circular motion in a plane by attaching electron-donating molecules on the outer surface of the C80 cage. It is expected that the two-dimensional circular motion will help to induce a unique electronic or magnetic field.