Naomi Mizorogi

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.

Sc2C2@C80 Rather than Sc2@C82: Templated Formation of Unexpected C2v(5)-C80 and Temperature-Dependent Dynamic Motion of Internal Sc2C2 Cluster

Unambiguous X-ray crystallographic results of the carbene adduct of Sc(2)C(82) reveal a new carbide cluster metallofullerene with the unexpected C(2v)(5)-C(80) cage, that is, Sc(2)C(2)@C(2v)(5)-C(80). More interestingly, DFT calculations and NMR results disclose that the dynamic motion of the internal Sc(2)C(2) cluster depends strongly on temperature. At 293 K, the cluster is fixed inside the cage with two nonequivalent Sc atoms on the mirror plane, thereby leading to C(s) symmetry of the whole molecule. However, when the temperature increases to 413 K, the (13)C and (45)Sc NMR spectra show that the cluster rotates rapidly inside the C(2v)(5)-C(80) cage, featuring two equivalent Sc atoms and weaker metal-cage interactions.

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.

Partial Charge Transfer in the Shortest Possible Metallofullerene Peapod, La@C82⊂[11]Cycloparaphenylene

[11]Cycloparaphenylene ([11]CPP) selectively encapsulates La@C82 to form the shortest possible metallofullerene-carbon nanotube (CNT) peapod, La@C82 ⊂[11]CPP, in solution and in the solid state. Complexation in solution was affected by the polarity of the solvent and was 16 times stronger in the polar solvent nitrobenzene than in the nonpolar solvent 1,2-dichlorobenzene. Electrochemical analysis revealed that the redox potentials of La@C82 were negatively shifted upon complexation from free La@C82 . Furthermore, the shifts in the redox potentials increased with polarity of the solvent. These results are consistent with formation of a polar complex, (La@C82 )(δ-) ⊂[11]CPP(δ+) , by partial electron transfer from [11]CPP to La@C82 . This is the first observation of such an electronic interaction between a fullerene pea and CPP pod. Theoretical calculations also supported partial charge transfer (0.07) from [11]CPP to La@C82 . The structure of the complex was unambiguously determined by X-ray crystallographic analysis, which showed the La atom inside the C82 near the periphery of the [11]CPP. The dipole moment of La@C82 was projected toward the CPP pea, nearly perpendicular to the CPP axis. The position of the La atom and the direction of the dipole moment in La@C82 ⊂[11]CPP were significantly different from those observed in La@C82 ⊂CNT, thus indicating a difference in orientation of the fullerene peas between fullerene-CPP and fullerene-CNT peapods. These results highlight the importance of pea-pea interactions in determining the orientation of the metallofullerene in metallofullerene-CNT peapods.

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.

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