Hidefumi Nikawa

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.

Structural Elucidation and Regioselective Functionalization of An Unexplored Carbide Cluster Metallofullerene Sc2C2@Cs(6)-C82

A Sc(2)C(84) isomer, previously assumed to be Sc(2)@C(84), is unambiguously identified as a new carbide cluster metallofullerene Sc(2)C(2)@C(s)(6)-C(82) using both NMR spectroscopy and X-ray crystallography. The (13)C-nuclei signal of the internal C(2)-unit was observed at 244.4 ppm with a 15% (13)C-enriched sample. Temperature-dependent dynamic motion of the internal Sc(2)C(2) cluster is also revealed with NMR spectrometry. Moreover, the chemical property of Sc(2)C(2)@C(s)(6)-C(82) is investigated for the first time using 3-triphenylmethyl-5-oxazolidinone (1) which provides a 1,3-dipolar reagent under heating. Regarding the low cage symmetry of this endohedral which contains 44 types of nonequivalent cage carbons, it is surprising to find that only one monoadduct isomer is formed in the reaction. Single-crystal X-ray results of the isolated pyrrolidino derivative Sc(2)C(2)@C(s)(6)-C(82)N(CH(2))(2)Trt (2) reveal that the addition takes place at a [6,6]-bond junction, which is far from either of the two Sc atoms. Such a highly regioselective addition pattern can be reasonably interpreted by analyzing the frontier molecular orbitals of the endohedral. Electronic and electrochemical investigations reveal that adduct 2 has a larger highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap than pristine Sc(2)C(2)@C(s)(6)-C(82); accordingly, it is more stable.

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

Missing Metallofullerene with C80 Cage

Many insoluble metallofullerenes are called missing metallofullerenes. These metallofullerenes have not yet been isolated despite their detection in raw soot using mass spectrometry. They have been anticipated to show unique structures and properties that differ considerably from those of soluble metallofullerenes. Herein, a missing metallofullerene, La@C(80), was extracted and isolated as a derivative, La@C(80)(C(6)H(3)Cl(2)). The structure of La@C(80)(C(6)H(3)Cl(2)) was determined unambiguously using single-crystal X-ray crystallographic analysis. In fact, La@C(80) has a novel C(80) cage, C(2v)-C(80) (80:3), which has not been reported. The calculation of La@C(80) with C(2v)-C(80) (80:3) cage shows that La@C(2v)-C(80) (80:3) has a radical character as well as small ionization potential (Ip) and electron affinity (Ea), which are indicative of high reactivity. On the basis of those results, La@C(2v)-C(80) (80:3) is expected to interact strongly and bond with amorphous carbon or other fullerenes in soot that is made insoluble in common organic solvents.

X-ray Structures of Sc2C2@C2n (n = 40–42): In-Depth Understanding of the Core–Shell Interplay in Carbide Cluster Metallofullerenes

X-ray analyses of the cocrystals of a series of carbide cluster metallofullerenes Sc(2)C(2)@C(2n) (n = 40-42) with cobalt(II) octaethylporphyrin present new insights into the molecular structures and cluster-cage interactions of these less-explored species. Along with the unambiguous identification of the cage structures for the three isomers of Sc(2)C(2)@C(2v)(5)-C(80), Sc(2)C(2)@C(3v)(8)-C(82), and Sc(2)C(2)@D(2d)(23)-C(84), a clear correlation between the cluster strain and cage size is observed in this series: Sc-Sc distances and dihedral angles of the bent cluster increase along with cage expansion, indicating that the bending strain within the cluster makes it pursue a planar structure to the greatest degree possible. However, the C-C distances within Sc(2)C(2) remain unchanged when the cage expands, perhaps because of the unusual bent structure of the cluster, preventing contact between the cage and the C(2) unit. Moreover, analyses revealed that larger cages provide more space for the cluster to rotate. The preferential formation of cluster endohedral metallofullerenes for scandium might be associated with its small ionic radius and the strong coordination ability as well.

A Molecular Ce2@Ih-C80 Switch—Unprecedented Oxidative Pathway in Photoinduced Charge Transfer Reactivity

We report for the first time the versatile Ce(2)@I(h)-C(80) building block toward synthesizing a novel electron donor-acceptor conjugate, Ce(2)@I(h)-C(80)-ZnP (1). A systematic investigation of the charge transfer chemistry documents a reductive charge transfer (i.e., formation of (Ce(2)@I(h)-C(80))(*-)-(ZnP)(*+)) in nonpolar media (i.e., toluene/THF), while an oxidative charge transfer (i.e., formation of (Ce(2)@I(h)-C(80))(*+)-(ZnP)(*-)) dominates in polar media (i.e., benzonitrile/DMF). Reduction of the [Ce(2)](6+) cluster, which is highly localized and collinearly arranged with respect to the quaternary bridge carbon, is sufficiently exothermic in all solvents. Notably weak is the electronic coupling between the [Ce(2)](6+) cluster and the electron-donating ZnP. The oxidation of C(80)(6-) and the simultaneous reduction of ZnP, on the other hand, necessitate solvent stabilization. In such a case, the strongly exothermic (Ce(2)@I(h)-C(80))(*-)-(ZnP)(*+) radical ion pair state formation is compensated within the framework of a nonadiabatic charge transfer by a C(80)(6-)/ZnP electronic matrix element, as the sum of good overlap and short distance, that exceeds that for [Ce(2)](6+)/ZnP.

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.