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How the electronic properties and chemical reactivity of empty fullerenes can be changed upon endohedral metal-doping? How the dynamic behavior and position of encapsulated metal atoms can be contr...

Putting metal atoms or metallic clusters into fullerenes has generated a new class of hybrid molecules, defined as endohedral metallofullerenes (EMFs), possessing novel structures and fascinating properties which are different from those of empty fullerenes. In particular, it has been revealed that the chemical properties of the cage carbons of EMFs depend strongly on the nature of the internal metallic species, such as their electronic configuration, location and even motion. Since the first report describing the successful derivatization of La@C82 in 1995, great efforts have been devoted to the chemical modification of EMFs during the last two decades. Although earlier studies mainly focused on readily available species such as M@C82, M2@C80 and M3N@C80 and the related results have been systematically summarized in our previous review paper (Chem. Commun., 2011, 47, 5942–5957), recent concerns about some relatively rare EMFs have developed rapidly. Moreover, taking advantage of single crystal X-ray crystallography, we can now clearly demonstrate the mutual influences between the internal metallic species and the chemical behaviours of the surrounding cage carbons, and the addends as well. In this article, we present recent achievements in the chemical functionalization of EMFs, which were mainly published during the last four years. For consistency, we will still pay special attention to the role that the metals play in controlling the properties of the whole EMF molecules. In this review, however, we will not only focus on concrete experimental results such as X-ray crystallographic and NMR spectroscopic data but will also include computational studies which have indeed enhanced our understanding of the chemical properties of EMFs. Applicable materials based on EMFs are also mentioned but are not discussed in detail.

Regioselective cycloaddition of adamantylidene carbene to Pr@C2v(9)-C82 affords the first derivative of praseodymium-containing metallofullerenes. Single-crystal X-ray crystallographic data in combination with theoretical studies demonstrate that the addition is dictated by the single metal ion encapsulated inside the fullerene cage.

In this paper, we report on the design, redox potentials, excited state energies and radical ion pair state energies in electron donoracceptor conjugates comprising the electron-donating -extended ...

The cage-like structure of fullerenes enables a new strategy for derivatization, that is, endohedral doping with metallic species. Distinctly different from exohedral metal-fullerene complexes, endohedral metallofullerenes (EMFs) show several new features in metal–carbon interactions which result from the electron transfer from the internal metallic species to the outer carbon cage. Taking advantage of recent achievements in the X-ray crystallographic characterization of EMFs, it was revealed that the single metal cation in mono-EMFs are always apart from the cage center, coordinating with the cage from inside, dominantly via electrostatic interactions, whereas the metal–cage interactions are much weaker in di-EMFs, featuring a moving dimetallic cluster in most cases. Moreover, formation of various metallic cluster species inside the fullerene cage suggest that the strong coordination ability of rare earth metals which always requests a maximal coordinated stereo environments, is a critical factor controlling the formation of stable EMF compounds. Finally, the structural features of the fullerene cage, such as the symmetry, the shape and the size, are also found to exert fundamental influences on the metal location or motion.

Endohedral metallofullerenes (EMFs), a new class of hybrid molecules formed by encapsulation of metallic species inside fullerene cages, exhibit unique properties that differ distinctly from those of empty fullerenes because of the presence of metals and their hybridization effects via electron transfer. This critical review provides a balanced but not an exhaustive summary regarding almost all aspects of EMFs, including the history, the classification, current progress in the synthesis, extraction, isolation, and characterization of EMFs, as well as their physiochemical properties and applications in fields such as electronics, photovoltaics, biomedicine, and materials science. Emphasis is assigned to experimentally obtained results, especially the X-ray crystallographic characterizations of EMFs and their derivatives, rather than theoretical calculations, although the latter has indeed enhanced our knowledge of metal–cage interactions. Finally, perspectives related to future developments and challenges in the research of EMFs are proposed. (381 references)

In recent years, endohedral metallofullerenes have attracted tremendous interest not only in physics and chemistry, but also in interdisciplinary areas, such as materials and biological sciences. In this concept article we highlight recent results on different endohedral metallofullerenes based on lanthanides and their derivatives. The chemical and excited state reactivities of endohedral metallofullerenes are discussed for various endohedral clusters. Most important is the part that covers spectroscopic and kinetic assays of reductive and oxidative charge transfer evolving from photoexcited electron donors and electron acceptors, respectively, in a variety of electron donor-acceptor conjugates. Towards this end, we refer to the applications of endohedral metallofullerenes in photovoltaic devices that feature greater efficiency than devices fabricated with empty fullerenes. Herein, we focus mainly on results obtained in the groups of Akasaka, Echegoyen, and Guldi.

Since the first discovery of fullerenes in 1985, the insertion of one or more atoms into a hollow fullerene cage has been attempted. Furthermore, synthesis and extraction of metallofullerene, La@Cn, were reported in 1991. Recent successful isolation and purification of metallofullerenes have facilitated the investigation of their chemical properties. This mini-review presents a summary of the recent progress of chemical functionalization and supramolecular chemistry of M@C82. Selective functionalization and successful structural analysis of derivatives have revealed their chemical features arising from endohedral metal doping.

Recent breakthroughs achieved in the chemical functionalization of endohedral metallofullerenes (EMFs), especially single crystallographic X-ray characterizations of their derivatives, have presented fundamentally new insights into the structures and properties of these metal–carbon hybrid molecules, and have also brought immense potential applications. In particular, the interplay between the encapsulated metallic species and the fullerene cage has been well investigated. On one hand, the position and motion of the encapsulated metals can be effectively controlled by exohedral modification. On the other hand, the cage structures, the chemical behaviours of cage carbons and thus the chemical reactivity of the whole molecule are also apparently influenced by the electronic configuration and geometrical conformation of the internal metals via strong metal–cage interactions. In this article, we contribute a systematic review of the important chemical transformations of EMFs reported to date, including disilylation, 1,3-dipolar cycloaddition with ylides, cyclopropanation with carbenes and carbanions, cycloaddition with dienes and benzyne, radical reactions, and other miscellaneous reactions, in addition to noncovalent interactions such as supramolecular complexation. The roles that internal metals play in controlling the reactivity of cage carbons are particularly emphasized. Finally, some applicable materials based on EMFs and their derivatives are summarized and practical perspectives are proposed.

Novel C60 derivatives (III) are synthesized without causing a drastic change of the electronic properties of the fullerene cage.

The reaction of singlet oxygen with a series of 3-benzoyl-4-(methoxycarbonyl) thiazolidine derivatives and alkyl benzyl sulfides in methylene chloride in the presence of olefins has been investigated. The reaction of singlet oxygen with the sulfides caused cooxidation of olefins to the corresponding epoxides in substantial yields. The epoxidation of olefins by the active oxidizing species generated in photosensitized oxygenation of the sulfides is provided, suggesting that the new epoxidizing species is probably the S-hydroperoxysulfonium ylide intermediate derived from a persulfoxide intermediate by intramolecular α-proton abstraction

Singlet oxygenation of 2,2,3,3-tetraaryloxadisiliranes 1a-d gives the corresponding 1,2,4,3,5-trioxadisilolanes 2a-d, derivatives of a novel class of cyclic peroxides. The reaction mechanism is discussed in terms of possible peroxonium ion intermediate 6a as a nucleophilic oxidizing species.

Reaction of 1,1,2,2-tetraanisylcyclopropane ( 1a ) with both thermally and photochemically generated singlet oxygen afforded the corresponding 1,2-dioxolane 2a quantitatively. Singlet oxygenation of two stereoisomeric 1,2-dianisyl-1,2-ditolylcyclopropanes ( 1b and 1c ) gave a mixture of 1,2-dioxolanes 2b and 2c via a non-stereospecific addition in both cases.

Photosensitized and cation radical-catalysed electron-transfer oxygenations of 3-phenyl-1,1,2,2-tetramesityl-1,2-digermetene 1 affords the corresponding 1,2,3,6-dioxadigermine 2 as a dioxygen insertion product into a Ge–Ge σ bond in moderate yields together with 1,2,5-oxadigermolone 3.

Photo-induced electron transfer oxygenation of 3-phenyl-1,1,2,2-tetramesityl-1,2-disiletene ( 1 ) in acetonitrile-methylene chloride afforded the corresponding 1,2,3,6-dioxadisilin ( 2 ) in moderate yield as a dioxygen insertion product into a silicon-silicon σ-bond together with 1,2,5-oxadisilolene ( 3 ). The results are reasonably accounted for by an electron transfer from disiletene 1 to the excited singlet state of the sensitizer to form a radical ion pair followed by addition of oxygen.

Photoxygenation of digermirane 1a and azadigermiridine 1b using tetraphenylporphine as sensitizer affords the corresponding peroxides, 1,2,3,5-dioxazadigermolane 2a and 1,2,4,3,5-dioxazadigermoridine 2b, respectively in good yields; 2a is structurally characterized by X-ray analysis.

Recent works on photosensitized oxygenation of organosulfur compounds are reviewed. In singlet oxygen oxidation of the designed sulfur compounds, the active oxidizing species much stronger than singlet oxygen itself and primary intermediates such as persulfoxide and thiadioxirane are formed as secondary intermediates, in which new types of active oxidizing species such as peroxysulfenic acid, S-hydroperoxysulfonium ylide, S-hydroperoxysulfurane and bis (alkanesulfenyl) peroxide are reported. These active oxidizing species can be used for an effective epoxidation of olefins. The current research interests in photosensitized oxygenation of organoselenium and organophosphorus compounds are also presented.

The ESR-active lanthanofullerene La C[sub 82] has been isolated from various hollow fullerenes by an efficient single-stage chromatographic separation process. Pyrenylethyl-silica was used as a stationary phase with toluene as an eluent. Lanthanum-containing fullerenes were prepared by the arc heating of metal-impregnated graphite rods in a He atmosphere. Fast atom bombardment mass analysis of the isolated sample confirmed the isolation of La C[sub 82]. ESR absorption spectra of the present sample prove the separation of a La C[sub 82] isomer with a 1.15-G hyperfine splitting. The UV-vis-near-IR absorption spectra of the sample reveal near-IR absorption peaks which are characteristic of the fullerene radical anions. 40 refs., 4 figs.

A comparative study using cyclic, differential pulse, and Osteryoung square wave voltammetry is presented on organofullerenes derivatized with oxygen-, carbon-, and silicon-containing groups. Electron affinities of the organofullerenes increase with increasing electronegativities of the attached atoms. C[sub 60]O is a stronger electron acceptor than C[sub 60], although its electroreductions are CV irreversible. In the carbon-derivatized C[sub 60], hybridization of the attached carbon atoms and electron-withdrawing groups also affect their reduction potentials. As previously described for C[sub 60], no CV reversible electrooxidation waves were observed for all organofullerenes studied. Electron-donating groups, such as alkyl and silyl, significantly lower the oxidation potentials of the organofullerenes. The AM1 molecular orbital calculations were performed on C[sub 60] and selected organofullerenes. The first and second reduction potentials correlate well with the LUMO energy levels, while the third reduction potentials correlate better with the LUMO+1 energy levels. The peak oxidation potentials also show a good linear correlation with the HOMO energy levels. 23 refs., 5 figs., 1 tab.

Electron-transfer photooxygenation of oxadisiliranes afforded the corresponding 1,2,4,3,5-trioxadisilolanes as dioxygen insertion products in reasonable yields. Stereochemical aspects of the oxygenation suggested formation of a peroxidic open intermediate.

The synthesis and characterization of various endohedral metallofullerenes and their derivatives are described. The encapsulated metal atoms' positions and movements were determined using NMR study, X-ray crystallographic analysis, and theoretical calcula- tions. The results of electrochemical investigations clarified the relation between the elec- tronic properties and the π-electron systems of the fullerene endohedrals.

Addition reaction of an endohedral metallofullerene La@C 82 with digermirane has been investigated by means of the mass and EPR measurements. It is found that photochemical and thermal reactions of La@C 82 with digermirane afford an exohedral adduct of the endohedral metallofullerene. This is in sharp contrast with the fact that empty fullerenes react only photochemically. The remarkable thermal reactivity of La@C 82 is ascribed to the unique electronic properties due to endohedral metal doping.

Endohedral metallofullerenes (fullerenes containing metals inside the carbon cage) have long attracted special interest as new spherical molecules with novel properties. Recent important progress has been marked by successful isolation and purification in macroscopic quantities. With the availability of such purified samples, it has been possible to investigate the structures, electronic properties, and chemical reactivities. Very recent progress in the theoretical and experimental studies of endohedral metallofullerenes is summarized to show what are known or still unknown for the future material, catalytic, and biological applications.

This paper describes the convenient method of isolating endohedral metallo- fullerenes by means of selective reduction from carbon soot extracts. Successful isolation in large amount by utilizing this method allows examination of the selective chemical modifi- cation of monometallofullerenes. Furthermore, isolation of missing metallofullerenes as their derivatives is also mentioned.

A series of13C-enriched monoytterbium endohedral metallofullerenes (EMFs)—Yb@C2n (n = 40, 41, 42)—was synthesized and isolated. Their cage structures were systematically determined for the first time using computational and experimental 13C NMR studies. The results revealed that all isomers adopt cage structures conforming to the isolated pentagon rule. In detail, Yb@C80 possesses the C2v(3) cage; Yb@C82(I, II, III) bear Cs(6), C2(5), and C2v(9) cages, respectively; and Yb@C84(II, III, IV) have C2(13), C1(12), and C2(11) cage structures, respectively. This is the first report describing C2(13)-C84 and C1(12)-C84 cage structures. It is noteworthy that the cage structures found for mono-EMFs generally differ from either the corresponding empty fullerenes or the related EMFs encapsulating more than one metal atom, indicating that the metal atom inside the fullerene cage plays an important role in determining the EMF structure. On the basis of the fact that the structure of Yb@C2(13)-C84 resembles that of Yb@...

Treatment of baker's yeast with phenacyl chloride in an aqueous–organic solvent has been proven to be an effective method of inhibiting the enzymes that afford (S)-enantiomers of α-hydroxy esters in the reduction of α-keto esters. The procedure is effective for the whole-cell system to produce the (R)-product with high chemical yield and high enantiomeric excess.

The photochemical cycloaddition of C 7 8 (C 2 v +D 3 ) with disilirane 1 only gave the adducts of C 7 8 (C 2 v ). The D 3 isomer of C 7 8 did give no adduct with 1. The two mono-adducts were firstly isolated by a multi-stage HPLC, and characterized on the basis of the spectroscopic data. The electrochemical properties were also investigated by means of differential pulse voltammetry.

This account describes the redox behaviors of endohedral metallofullerenes having remarkable electron-accepting and -donating abilities, which accompany the transformation between their paramagneti...

The photochemical reaction of C 60 with selenium-containing bicyclooctanes affords for the first time the selenylfullerenes. The derivatives were characterized by mass, UV–vis absorption, NMR spectroscopy, and X-ray crystallographic analysis.

Benzo[1,2,3]trichalcogenoles with two bromine atoms on the benzene ring, 5,6-dibromo-4,7-diethylbenzo[1,2,3]trichalcogenoles (1a) and (1b) (chalcogen: 1a = S; 1b = Se), were first prepared by treating 2,3,5,6-tetrabromo-1,4-diethylbenzene (TBDEB) with elemental sulfur or amorphous selenium in DBU at 140 degrees C (for 1a) and 100 degrees C (for 1b) for 24 h. The structures of 1a and 1b were verified by NMR spectroscopy, mass spectrometry, and elemental analysis. X-ray crystallographic analysis ultimately showed that the substitution reactions of TBDEB proceeded at the two adjacent bromine atoms. To apply 1a and 1b to construction of phthalocyanine derivatives with sulfur or selenium functional groups, 4,5-bis(benzylchalcogeno)-3,6-diethylphthalonitriles (5a) and (5b) as key intermediates were prepared by way of introduction of alkyl groups (2-cyanoethyl or 4-nitrophenethyl groups) on two chalcogen atoms, substitution of two bromine atoms with nitrile groups, and subsequent exchange of alkyl groups with benzyl groups. Compound 5a was treated with lithium in n-pentanol at 100 degrees C for 1 h to produce 2,3,9,10,16,17,23,24-octakis(benzylthio)-1,4,8,11,15,18,22,25-octaethylphthalocyanine (6a). A similar treatment of 5b in n-hexanol at 100 degrees C for 2 h gave phthalocyanine 6b. The structures of 6a and 6b were determined by (1)H NMR spectroscopy and MALDI-TOFMS. X-ray crystallographic analysis of 6a was also performed. The Q-band absorptions (lambda(max)) for 6a and 6b in UV-vis spectra were observed at 755 nm (log epsilon = 5.1) and 757 nm (log epsilon = 5.1), respectively, and their electrochemical properties were verified by cyclic voltammetry in dichloromethane with Ag/AgNO(3) as a reference electrode. Compounds 6a and 6b were further treated with lithium in THF/NH(3) at -78 degrees C and then with dibutyltin dichloride to produce phthalocyanine derivatives 8a and 8b with four dichalcogenastannole rings by way of octachalcogenate phthalocyanines 7a and 7b.