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FEATURE ARTICLE

Cite this: DOI: 10.1039/c4cc05164e

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Recent progress in the chemistry of endohedral metallofullerenes Xing Lu,*a Lipiao Bao,a Takeshi Akasaka*ab and Shigeru Nagasec 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

Received 5th July 2014, Accepted 5th August 2014

the role that the metals play in controlling the properties of the whole EMF molecules. In this review,

DOI: 10.1039/c4cc05164e

NMR spectroscopic data but will also include computational studies which have indeed enhanced our

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understanding of the chemical properties of EMFs. Applicable materials based on EMFs are also mentioned but are not discussed in detail.

however, we will not only focus on concrete experimental results such as X-ray crystallographic and

1. Introduction New carbon allotropes such as fullerenes, carbon nanotubes and graphene have received great attention due to their unique structures and fascinating properties with huge potential applications in many fields such as electronics, materials science and biomedicine.1 In particular, fullerenes, emerged as the third but the only soluble form of element carbon with definite molecular structures, are viewed as an ideal platform for investigating the structures, properties and formation mechanisms of related carbon allotropes.2 The research on fullerenes has blossomed greatly during the past three decades. Fullerenes can undergo a variety of a

State Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology (HUST), Wuhan 430074, China. E-mail: [email protected]; Fax: +86-27-87559404; Tel: +86-27-87559404 b Life Science Center of Tsukuba Advanced Research Alliance, University of Tsukuba, Foundation for Advancement of International Science & Tokyo Gakugei University, Tsukuba, Ibaraki 305-8577, Japan. E-mail: [email protected] c Fukui Institute for Fundamental Chemistry, Kyoto University, Kyoto 606-8103, Japan

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chemical reactions, producing many novel derivatives whose structures can be accurately determined by a collection of experimental techniques.3 More interestingly, the interior of fullerene cages can host many species such as atoms, molecules and otherwise unstable clusters, forming endofullerenes.4 When metallic clusters are encapsulated, a certain number of electrons are transferred from the internal metallic cluster to the surrounding fullerene cage, resulting in a zwitterion structure. These molecules with metallic species encapsulated inside the fullerene cages are called endohedral metallofullerenes (EMFs), which show potential applications in materials science, biomedicine, organic electronics and photovoltaics.5 Although in the beginning of EMF-research, great efforts have been devoted to improving their synthesis yields and to exploring the possibility of trapped elements, recent concerns about the chemical modification of EMFs have paved a wide road toward the applications of these hybrid molecules.6 The first study of the chemical properties of EMFs was reported by Akasaka et al. in 1995 when the available amount of EMF-samples was very limited (less than milligrams).7 During the recent fifteen years, exciting results have been achieved in the synthesis and isolation processes of EMFs, and many new

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strategies have been proposed. Nowadays, it is easy to get sufficient amount of samples of some common EMFs (such as M@C82 and M3N@C80) to investigate their physico-chemical properties. As a direct result, chemical functionalization of EMFs has become an attractive topic and many interesting results have been reported.8 A general finding is that the chemical properties of EMFs are highly dependent on the metallic species encapsulated and thus are much different from that of empty fullerenes such as C60 and C70.9 In an earlier review paper published in 2011, we have systematically summarized the results of the chemical functionalization of EMFs, specializing on the effect of the encapsulated metallic species on dictating the chemical properties of the cage carbons, based on experimental results, especially the concrete X-ray results.10 During recent years, it is not surprising to see that the chemistry of EMFs has blossomed greatly and many new results that do not relate to the common EMFs have been obtained. Some of the achievements even challenge the existing knowledge of modern chemistry. To this end, we are intending here to give an updated review article concerning the recent progresses in the chemistry of EMFs. In this article, we will focus on not only the experimental results which are certainly straightforward and comprehensive, but also on the theoretical results which are really instructive and somehow predicative. Moreover, it should be noted that the interplay between experimental and theoretical studies has enhanced our knowledge of the chemical properties of EMFs.11

2. Chemical properties of mono-EMFs As the simplest examples of EMFs, mono-EMFs were identified and characterized in the beginning era of EMF research. Although the raw soot produced by the arc discharge method contains a large variety of M@C2n-type EMFs (60 o 2n o 200), M@C82 EMFs are always the most abundant species in the extract and thus they are most intensively investigated.12 In an earlier review paper published in 2011, we have systematically summarized the relevant results related to the chemical properties of M@C82-type EMFs.8 However, many new results have been achieved afterwards, and they are described here. 2.1

1,3-Dipolar reaction

Cycloaddition of 1,3-dipolar reagents such as azomethine ylide to fullerenes is an effective way to generate pyrrolidino-ring fused derivatives, which is also called Prato reaction.13 Early studies focused on the simple reactions between azomethine ylide and M@C82 (M = La, Y, Gd etc.). Due to the high reactivity of the EMFs and low regioselectivity, however, no pure isomers of the adducts have been isolated and thus the characterizations of the derivatives were rather limited.14,15 Recently, 1,3-dipolar reactions have been utilized to construct several applicable systems, such as donor–acceptor conjugates, self-assembled mono-layers (SAM), by introducing different functional groups onto the fullerene surface. Akasaka, Veciana and coworkers reported the 1,3-dipolar reaction of La@C82

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using a thioacetate-terminated aldehyde (1, Table 1).16 Instead of using N-methylglycine or N-n-octylglycine which were commonly used in previous studies, using 2-methylaminoisobutyric acid afforded a major isomer selectively, which was further isolated using HPLC and characterized by many experimental tools. Electrochemical studies revealed that the derivative maintains the intrinsic properties of the frontier orbitals of pristine La@C82. Taking advantage of the specific coordination between the thioacetate group and gold, SAMs of the derivative were prepared from a dilute solution of the EMF on the Au(111) surface. Atomic force microscopy confirmed the formation of a homogenous layer with round spheres of 0.4–1.0 nm height, suggesting the existence of fullerene cages (Fig. 1). Presence of the lanthanum element was evidenced by X-ray photoelectron spectroscopy. More directly, mass spectrometry revealed the molecular ion peak of La@C82. The electrochemical and magnetic properties of the SAM were respectively investigated using cyclic voltammetry and electron spin resonance spectrometry, which are similar to those of the corresponding EMF in solution. Such materials may be used as high-density memory devices. Akasaka, Guldi and coworkers performed the 1,3-dipolar reactions of porphyrin (Py, 2, Table 1) with C60, La@C82 and La2@C80, respectively, to synthesize the corresponding derivatives featuring a pyridyl group.17 Although it is not surprising that the reaction involving C60 and La2@C80 afforded only one isomer of the adducts because of the high cage symmetry of these two cages, the highly regioselective formation of a single adduct of La@C82Py is unexpected because in principle 35 types of nonequivalent C–C bonds exist on the La@C82 cage. These molecules were used as acceptor materials to generate coordinative electron-donor/electron-acceptor systems with zinc tetraphenylporphyrin which acted as an electron donor. Steady-state and time-resolved photophysical characterizations revealed that electron transfer governs the excited-state deactivation in all of these systems. It was also found that the binding between the fullerene-based porphyrin and Zn ion is dominated by axial coordination, whereas the influence of the orbital overlap between the curved and planar p-systems is not obvious. A pure isomer of the dichlorophenyl derivatives of the insoluble La@C72 which bears a pair of fused pentagons, namely La@C72(C6H3Cl2), whose structure and molecular orbital diagram are shown in Fig. 2a and b, was used as a starting material to react with N-metalated azomethine ylide (3).18 By using a chiral catalyst formed by the reaction between copper(II) acetate and a chiral ligand, the racemic mixture of La@C72(C6H3Cl2) afforded eight optically pure isomers of the corresponding bisadducts (Fig. 2c). This is rather surprising because there are 71 nonequivalent cage carbon atoms on the cage of La@C72(C6H3Cl2) which in principle can produce 108 possible reaction sites. After isolation using nonchiral HPLC, these isomers showed a high optical purity of 98%. Circular dichroic measurements of these isomers (Fig. 2d) revealed the strong impact of chirality of the starting enantiomer on the observed values, which is in sharp contrast to that previously observed for the Ih-symmetrical C60. It is noteworthy that the same reaction

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Table 1

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Summary of the chemical reactions that have been performed on mono-EMFs since 2011

Reaction specification

Compound

1,3-Dipolar cycloaddition reaction

La@C82

1,3-Dipolar cycloaddition reaction

Reactant

Main product

Characterizations

Ref.

[5,6]Adduct

HPLC, MS

16

La@C82

[5,6]Adduct

HPLC, MS

17

1,3-Dipolar cycloaddition

La@C72(C6H3Cl2)

Bis-adducts HPLC, MS, UV-Vis-NIR, CD

18

Carbene addition

Sc@C82

[5,6]Adduct

HPLC, MS, electrochemistry, X-ray crystallography

23

Carbene addition

Yb@C80

[5,6]Adduct

HPLC, MS, Vis-NIR, electrochemistry, 25 X-ray crystallography

Carbene addition

Yb@C84

[5,6]Adduct

HPLC, MS, Vis-NIR, electrochemistry, 27 X-ray crystallography

Carbene addition

La@C82

[5,6]Adduct

HPLC, MS, X-ray crystallography

28

Bingel–Hirsch

Gd@C82

Monoadducts

HPLC, MS

29

Bingel–Hirsch

Gd@C82

Multiadduct

MS, FT-IR

30

Radical reaction

Gd@C82

Monoadduct

XPS, DSC, FT-IR

31

Disilylation reaction

Yb@C74,84

Monoadducts

HPLC, MS, electrochemistry

32

did not occur on other EMFs such as Sc3N@C80, La@C82, or La2@C80, due to the mismatch between the frontier orbitals of the starting reagents.

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It is noteworthy that no single crystal XRD crystallographic results of any 1,3-dipolar derivatives of mono-EMFs have been reported until now, suggesting that the pyrrolidino structure

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Fig. 1 AFM topographic image of the gold surface with covalently linked La@C82. Reproduced with permission from ref. 16, 2011 Royal Society of Chemistry.

and the attached groups are not suitable for the crystal growth for mono-EMFs.

2.2

Carbene reaction

Adamantylidene carbene (Ad, 4), which can be generated in situ by photolysis or heating of 2-adamantane-2,3-[3H]-diazirine (Table 1), is an effective probe to investigate the chemical properties of EMFs.19 4 shows a high regioselectivity toward M@C82 (M = Y, La, Ce, Pr, Gd) to produce only two monoadduct isomers.20–22 Computational studies discovered that the preferential location of the trivalent metal ion M3+ under a hexagonal carbon ring in M@C82 and its influence on the electronic nature of the nearby carbon atoms are responsible for the high chemoselectivity. Actually, only one cage carbon is sufficiently reactive toward 4, thus resulting in only two monoadduct isomers. However, a recent study found that the reaction between Sc@C82 and 4 afforded four monoadduct isomers instead of two.23 This requires that two of the cage carbons be sufficiently reactive toward 4, thus forming four isomers. Computational results supported that two out of the 24 types of the cage carbons are involved in the reaction. Further considerations of the molecular structure of pristine Sc@C82 revealed that the small radius of Sc3+ allows a close contact between the metal ion and the cage, which causes a back-donation process of the electrons from the cage to the metal ions. X-ray crystallographic results of the most abundant isomer (Fig. 3a) established that the addition took place at the [5,6]-junction close to the Sc3+ ion. From the singly occupied molecular orbital (SOMO) distribution of Sc@C82 shown in Fig. 3b, the back-donation of electrons from the cage to Sc is evident. The carbene reagent (4) was also used to functionalize divalent EMFs which have much lower production yields than trivalent EMFs.24 Yb@C80 was first involved in the reaction. Despite there being 23 types of different cage carbons available

Chem. Commun.

Fig. 2 (a) Schematic drawing and systematic atom numbering of f,s C La@C72(C6H3Cl2). (b) LUMO of La@C72(C6H3Cl2). (c) Schlegel diagram of f,sC La@C72(C6H3Cl2): the two sites for the 1,3-dipolar cycloaddition are marked in blue and red, respectively. (d) Circular dichroic spectra of the eight optically pure bisadduct isomers of La@C72(C6H3Cl2) in CS2. Reproduced with permission from ref. 18, 2011 American Chemical Society.

on Yb@C80, only one isomer was formed in the reaction, showing a surprisingly high regioselectivity.25 The single crystal X-ray structure of Yb@C80Ad, shown in Fig. 4a, demonstrated that the addition takes place at the carbon atoms close to the internal metal ion. However, observation of multiple metal sites in the functionalized cage indicated that the modification has changed the metal motion dramatically because an earlier crystallographic study revealed that the Yb2+ ion is steady in pristine [email protected] Soon after, the most abundant Yb@C84 isomer was also allowed to react with 4 and three monoadduct isomers, in a relative ratio of 1 : 6 : 1, were formed.27 Bis- and multiple adducts were detected by HPLC in the initial stage of

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Similarly, three monoadduct isomers were isolated and characterized. Theoretical results of the major isomer demonstrated that the SOMO and lowest unoccupied molecule orbital (LUMO) are mainly localized on the endohedral whereas the highest occupied molecule orbital (HOMO) is focused on the porphyrin moiety. Together with spectroscopic and electrochemical results, it was concluded that electronic interactions between the two moieties in the ground state are identifiable. In the excited state, obvious communication between the two chromophores is evidenced by the quantitative quenching of the H2Por fluorescence. 2.3

Fig. 3 X-ray structure of (a) Sc@C82Ad and (b) the SOMO diagram of Sc@C82. Reproduced with permission from ref. 23, 2013 American Chemical Society.

the reaction, showing the high affinity of Yb@C84 to 4. After isolation, the most abundant mono-adduct isomer was fully characterized by spectroscopic and crystallographic methods. In contrast to the previously reported M@C2nAd in which the addition always takes place at the cage sites close to the internal metal ion and thus the metal is always trapped inside the cavity of the bond cleavage, X-ray structure of Yb@C84Ad (Fig. 4b) demonstrated that the addition occurs at the most curved area of the cage, which is far from the internal Yb2+ ion. The above results suggested that the chemical properties of EMFs are highly sensitive to their electronic structures which are tunable by changing the internal metallic species. A diazo precursor (5) that was used to synthesize phenyl-C61 butyric acid methyl ester (PCBM) was involved in the thermal reaction with La@C82. HPLC separation gave rise to three monoadduct isomers.28 The molecular structure of the major isomer was characterized by X-ray crystallography. The addition takes place at the positions involving two cage carbons very close to the encapsulated metal ion, indicative of a carbene addition mechanism. Then the methyl group was substituted with a functional group containing a free-base porphyrin which was used to synthesize the corresponding H2Por-La@C82 hybrids via the same route.

Addition of carbanion, which can be generated, for instance, by the dehydrogenation of diethyl 2-bromomalonate (6) in the presence of a base, to fullerenes or EMFs is an effective way to generate the propanated derivatives having fulleroid structures.33 Previous reports have demonstrated that metal nitride cluster EMFs such as Y3N@C80 could readily undergo Bingel– Hirsch reaction in a manner similar to that of empty fullerenes. However, some typical mono-EMFs with an open-shell electronic configuration like La@C82 tend to form singly bonded derivatives rather than cycloadducts.34–36 The Bingel–Hirsch reaction was recently used to produce the malonic ester derivatives of Gd@C82 with DBU as the dehydrogenation base. Surprisingly, mass spectroscopic study revealed that only cycloadducts were formed in the reaction. The isolated materials gave the water-soluble sodium salt form Gd@C82[C(COONa)2] upon hydrolysis.29 Since such materials are potential candidates for high-performance magnetic resonance imaging (MRI) reagents, their aggregation behavior was studied with microscopy and dynamic light scattering (DLS) studies. A strong peak centered at 167 nm and a minor peak at 1099 nm in the DLS spectrum suggested a bimodal aggregation distribution of the watersoluble material in the solution. Soon after, the same groups obtained multiple adducts of Gd@C82[C(COOCH2CH3)2]x from the reaction between Gd@C82 and 6 using a stronger base NaH.30 After separation with column chromatography, two fractions were identified to contain hexaadducts and octa-adducts, respectively. Subsequent hydrolysis yielded the water-soluble derivatives Gd@C82[C(COOH)2]x (x = 6, 8) which were further characterized by mass spectrometry and infrared spectroscopy. The performance of these derivative as MRI contrast agents was investigated by measuring their longitudinal relaxivity. The results demonstrated that the concentration of the derivatives and the number of the appended dydrophilic groups are critical factors controlling the proton relaxivity. 2.4

Fig. 4 X-ray structures of (a) Yb@C80Ad and (b) Yb@C84Ad.

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Bingel–Hirsch reaction

Radical reaction

As have been summarized in the previous review article, radical reactions are sensitive to the electronic structures of the EMFs involved.10 For instance, open-shell EMFs such as M@C82 tend to accept an odd number of addends, forming derivatives with a closeshell configuration.37 In contrast, close-shell EMFs like Sc3N@C80 always form an even number of new bonds with the substituents.38 Recently, free-radical polymerization of poly-N-vinylcarbazone (7)

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with either C60 or Gd@C82 was performed to synthesize the corresponding polymer-grafted fullerene derivatives.31 Spectroscopic studies confirmed the covalent attachment of the fullerene species. Fluorescence spectra suggested that the covalently grafted fullerenes had certain influences on the fluorescence properties of the polymer. 2.5

Disilylation

Although this was the first reaction that has been performed to functionalize EMFs, it has not been widely studied for its low selectivity.39 Nonetheless, disilirane reagents are particularly suitable to probe the chemical reactivity of EMFs containing different cores. In a recent study, Yb@C74 and Yb@C84 were allowed to react with 1,1,2,2-tetramesityldisilirane (8) both thermally and photochemically.32 Chromatographic and spectroscopic results indicated that the thermal reactions of both EMFs were unsuccessful under normal conditions (80 1C, 24 h) and only trace of the adducts were detected at evaluated temperatures (105 1C, 24 h). This situation is similar to that of the empty fullerenes and cluster EMFs (Sc3N@C80 and Sc2C2@C82) which can be explained by their relatively negative first reduction potentials (difficult to reduce). Surprisingly, it was found that neither of the two divalent EMFs underwent photochemical reactions, which is very different from the chemical properties of empty fullerenes and other EMFs which always reacted with 8 under photoirradiation. However, when the reaction mixture was studied with laser ionization mass spectroscopy, molecular ion peak corresponding to the mono-adduct was evident. These results suggested that divalent EMFs are more inert than empty fullerenes and other EMFs both thermally and photochemically.

3. Chemical properties of di-EMFs The examples of di-EMFs are relatively rare. Although earlier studies have investigated the chemical reactions of such species as M2@C2n (M = La, Ce; 2n = 72, 78, 80) and the results have been reviewed in our previous paper,10 the achievements after 2011 were mainly from La2@C80. 3.1

Fig. 5 Theoretically optimized structures of (a) La2@C80TCAQ and (b) the X-ray structure of La2@C80TCNEO showing all possible La sites. Reproduced with permission from ref. 40, 2013 American Chemical Society and from ref. 41, 2011 American Chemical Society, respectively.

La2@C80 generally acts as an electron acceptor, but when it was conjugated with TCAQ, it acts in its singlet state as an electron donor upon photoexcitation. Covalent attachment of a pyrrolidino-ring containing a pyridyl group to La2@C80 allowed the coordinative interaction with zinc tetraphenylporphyrin to form the electron donor– acceptor hybrid. Only the [5,6]-adduct was formed in the reaction which was characterized with a variety of measurements including steady and time-resolved absorption spectroscopy. Although the hybrid has a close-shell electronic structure, it features a rather short-lived singlet excited state. In contrast, its triplet excited state was dominated on the timescale beyond 200 ps.17 In a earlier report, the regioselective cycloaddition of tetracyanoethyleneoxide (TCNEO) to La2@C80 under thermal conditions was reportedly to form only a [5,6]-adduct.41 X-ray structure of the derivative (Fig. 5b) demonstrated that the metal atoms adopt a swing dynamic motion inside the functionalized cage, which is different from the results observed in the corresponding pyrrolidino-ring fused derivatives of La2@C80. Due to the strong electron-withdrawing ability of the tetracyanotetrahydrofuran moiety, the derivative shows an enhanced electron-accepting character and thus is promising in future applications in photovoltaic devices.

1,3-Dipolar reaction

Because of the versatility of the method and the high stability of derivatives, 1,3-dipolar reaction was also utilized to construct donor– acceptor conjugates of di-EMFs. In a recent report, La2@C80 reacted with the aldehyde containing the tetracyanoanthra-pquinodimethane moiety (TCAQ, 9) in the presence of different amino acids.40 Both [5,6]- and [6,6]-adducts were formed in the reaction and were isolated with one-step HPLC. It was demonstrated that the [5,6]-adduct is more abundant and more stable than the [6,6]-adduct. The optimized structure of the [5,6]-adduct is displayed in Fig. 5a. Ground state electronic characterization demonstrated that there is no significant interaction between TCAQ and La2@C80. However, in the excited state, time-resolved transient absorption experiments confirmed the unprecedented formation of the (La2@C80) +–(TCAQ) radical ion pair state in both polar and nonpolar solvents. This is surprising because

Chem. Commun.

3.2

Carbene cycloaddition

Carbene reaction has shown some advantages such as high reactivity, high regioselectivity and high stability of the adduct and the easiness of single crystal growth.42 A recent representative work in the regioselective carbene functionalization of di-EMFs was reported by Akasaka and coworkers.43 In an initial step, La2@C80 was allowed to undergo a photochemical reaction with phenylchlorodiazirine (11) to produce only one adduct in a high yield. The structure of the isolated adduct La2@C80 was firmly determined by X-ray crystallography which shows a [6,6]-open structure with one La atom being trapped inside the cavity of the bond cleavage and the other dangling in the opposite side (Fig. 6a). Then, La2@C80(CClPh) was used as a starting reagent to react with adamantane diazirine (4) under photoirradiation. This time, however, three bis-adduct isomers

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Recent results demonstrated that many compounds that have been assigned as di-EMFs previously are actually carbide cluster EMFs, and thus their chemistry started to develop.46 4.1

Fig. 6 X-ray structures of (a) La2@C80(CClPh), (b) La2@C80(CClPh)Ad and (c) La2@C80(C3H6COOCH3Ph).

[La2@C80(CClPh)Ad] were isolated from the reaction mixture. X-ray results of the most abundant one revealed that the second addend Ad was attached onto the fullerene surface at a [6,6]bond junction on the opposite side of the first moiety (CClPh) on the cage, and thus the two La atoms are trapped inside the two cavities provided by the bond cleavage at the two ends of the cage, respectively, featuring an extremely long La–La distance (Fig. 6b). This stepwise functionalization process of La2@C80 can be viewed as a chemical reactivity relay between the cage carbons and the internal La3+ ions. As confirmed by the crystallographic results, when the first addend was attached, the two La atoms preferred a linear configuration with the spiro carbon of the addend. In the second step, the La atom, which is far from the CClPh moiety, promotes the chemical reactivity of the nearby cage carbons through electrostatic interactions. This concept can be utilized in future studies for the rational design and synthesis of molecular devices based on di-EMFs. The diazo compound (5) containing a zinc porphyrin moiety was used to react with La2@C80 to form the corresponding donor– acceptor conjugate.44 However, due to the flexibility of the zinc porphyrin moiety which hinders single crystal formation of the derivative, a diazo compound without the zinc porphyrin moiety was used to get the reference compound suitable for single crystal growth and X-ray analysis. As shown in Fig. 6c, the results demonstrated that a [6,6]-open adduct was formed in the reaction, which is consistent with previous studies (Fig. 6a and b).42,43 Electrochemical and absorption studies confirmed an evident intramolecular ground state electronic interaction in the dyad. Upon photoexcitation, the conjugate features a considerable charge transfer process by forming radical pairs whose nature depends strongly on the solvent polarity. Carbene addition of an appropriate glycosylidene-derived diazirine (12) to La2@C80 at room temperature gave rise to the corresponding La2@C80-glyco conjugate.45 Formation of the monoadduct was firmly confirmed by mass spectrometry. As to the molecular structure, NMR and absorption results revealed that two diastereomers of the [6,6]-open adduct exist in the reaction mixture. Such derivatives may find biological and pharmacological applications in future (Table 2).

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Carbene cycloaddition reaction

Adamantylidine carbene (4) was first used to functionalize carbide cluster EMFs. In the reports describing the functionalization of Sc3C2@C80 and Sc2C2@C3v(8)-C82, the reaction details were not mentioned, but the authors’ purpose was to get the corresponding carbene derivatives which are particularly suitable for single crystal growth.47,48 In such a way, the carbide nature of these two compounds has been unambiguously established by single crystal X-ray crystallography for the first time. Recently, the reaction of 4 with a newly assigned carbide EMF Sc2C2@C2v(5)-C80 was systematically studied.49,50 Five regioisomers with relative abundances of 20%, 40%, 25%, 5% and 10% were isolated from the photo-irradiated reaction mixture. Thanks to the rigid structure of the admantylidene moiety, the authors have succeeded in obtaining the single crystals of the four major isomers. Fig. 7 displays their X-ray structures. Although disorder exists for both the cage and the internal metal atoms, the C2-unit remains largely unmoving. Different from previous results obtained from the reaction between 4 and mono- and di-EMFs in which the addition always takes place at the sites close to the internal metal atom(s), the addition of Ad in the current reaction tends to attack the cage carbons which are distant from the internal metal atoms. These results suggested that the chemical behaviors of CCMFs are different from those of other EMFs. In such a way, it is possible to produce a close-cage adduct instead of an open-caged one. Another interesting carbene reaction was performed on [email protected] A substituted tetrazine reagent (13), which was previously used to functionalize C60 to insert a C2-unit into the cage framework, was used to functionalize Sc3C2@C80. The reaction is highly selective by affording only one derivative. However, experimental results revealed that instead of forming the expected fourmember ring incorporated derivative, an unprecedented derivative having a bisfulleroid structure containing a doubly bridged 14-member ring was formed. Calculation results suggested that the different reactivity of the cage carbons between Sc2C2@C2v(5)-C80 and C60 should originate from the different pyramidalization degree of these cage carbons. Thus it is indicated that the chemical behaviors of EMFs are not only dependent on the internal metallic species but also on the cage structures. 4.2

1,3-Dipolar reaction

Although the 1,3-dipolar reaction has been intensively utilized to functionalize mono-EMFs, di-EMFs and nitride cluster EMFs (especially Sc3N@C80), the related studies of carbide cluster EMFs are really rare. In 2012, we reported the regioselective reaction between 3-triphenylmethyl-5-oxazolidinone (14) and Sc2C2@Cs(6)-C82 which was incorrectly assigned as Sc2@Cs(10)-C84 before

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Table 2

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Summary of the chemical reactions that have been performed on di-EMFs since 2011

Reaction specification

Compound Reactant

Main adduct

Characterizations

Prato reaction

La2@C80

[5,6]- and [6,6]monoadducts

HPLC, MALDI-TOF mass spectrometry, UV-Vis-NIR absorption spectrum, NMR, 40 CV and DPV, transient absorption spectroscopy

[5,6]-Monoadduct.

HPLC, MALDI-TOF mass spectrometry, NMR(1H NMR, 2C NMR, HMQC HMBC), 17 absorption spectra, DFT calculations, transient absorption spectroscopy

[5,6] and [6,6] monoadducts

HPLC, MALDI-TOF mass spectrometry, SC-XRD, DFT calculations, CV and DPV

41

HPLC, MALDI-TOF mass spectrometry, NMR, SC-XRD, Vis-NIR absorption spectra, CV and DPV

43

Prato reaction

La2@C80

1,3-Dipolar reaction La2@C80

La2@C80

Ref.

[6,6]-Open monoadduct

Carbene addition La2@C80(CClPh)

Bis-adduct La2@C80(CClPh)Ad

Carbene addition

La2@C80

[6,6]-Open monoadduct

HPLC, MALDI-TOF mass spectrometry, DFT calculations, CV and DPV, transient 44 absorption spectroscopy

Carbene reaction

La2@C80

[6,6]-Open monoadducts

NMR, MALDI-TOF mass spectrometry, Vis/NIR absorption spectroscopy, CV, DPV

our work.52 The reaction proceeded smoothly under heating. HPLC results revealed that only a single monoadduct isomer was formed in the reaction, together with a fraction containing multiple adducts. The XRD crystallographic study of the isolated product revealed that the addition takes place at a [6,6]-bond junction (Fig. 8a), forming a close-structure. Again, the addition takes place at the cage carbons far from either of the two scandium atoms. This addition pattern can be reasonably explained by considering LUMO distribution of the EMF (Fig. 8c). Then, the same reaction was performed on Sc2C2@C2v(9)-C82 which had been long-believed as Sc2@C2v(17)-C84.53 Three monoadduct isomers were formed this time, which were systematically characterized. Absorption spectrometric and electrochemical results demonstrated that the HOMO–LUMO bandgap values of the derivatives are all larger than that of the pristine one,

Chem. Commun.

45

indicative of enhanced stabilities of these adducts. Finally, the X-ray structure of the most abundant isomer (Fig. 8b) demonstrated that the addition also occurred at the cage positions distant from the metal atoms, and the addition pattern is readily explainable by considering the molecular orbital distribution on the cage surface (Fig. 8d). Accordingly, it is speculated that the chemical behaviors of carbide cluster EMFs are different from other types of EMFs, which are closely associated with the internal butterfly shaped clusters. 4.3

Coordination with a metal complex

The first survey into the coordination chemistry of EMFs was conducted by allowing (m-H)3Re3(CO)11(NCMe) (15) to react with Sc2C2@C3v(8)-C82 which is the most abundant carbide cluster EMF.54 The complexation is highly regioselective and

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Fig. 9 X-ray structure of (a) Sc2C2@C3v(8)-C82Re(CO)9H3 and (b) the sumanene structure.

with three pentagons and three hexagons, which ensures its reactivity with the Re3 cluster (Table 3).55 Fig. 7 X-ray structures of the four major isomers of Sc2C2@C2v(5)-C80Ad. Reproduced with permission from ref. 49, Copyright 2011 American Chemical Society.

5. Chemical properties of nitride cluster EMFs The concern about the chemical properties of nitride cluster EMFs, especially Sc3N@C80, has blossomed dramatically during recent years.56 As the most abundant class of endohedral fullerenes, the readily available amounts of samples engender systematic investigations on the chemical and electronic properties of these promising molecules. 5.1

Fig. 8 X-ray structures of the major isomers of (a) Sc2C2@Cs(6)-C82Ad, (b) Sc2C2@C2v(9)-C82Ad, and (c), (d) the corresponding LUMO diagrams of these two carbide cluster EMFs.

highly efficient. X-ray results of the isolated adduct revealed that the Re3 cluster regiospecifically added to a hexagon of the Sc2C2@C3v(8)-C82 core (Fig. 9a). Although no obvious communication between the internal scandium atoms and the exohedral Re3 cluster is evidenced, the electronic and electrochemical properties of the endohedral have been altered dramatically. This methodology was later extended to functionalize Sc2@C3v(8)-C82, Sc2C2@C2v(5)-C80, Sc2O@Cs(6)-C82, Sc3N@Ih(7)-C82, C2(17)-C86, and Cs(16)-C86. The most outstanding finding is that the sumanene structure on the fullerene framework is always necessary for the successful attachment of the Re3-cluster onto the cage. As illustrated in Fig. 9b, a sumanene is an aromatic structure with a central hexagon surrounded

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1,3-Dipolar reaction

1,3-Dipolar reaction of N-ethyl azomethine ylides (16) to Sc3N@C80 was first reported in 2005 by Echegoyen and coworkers, which afforded a [5,6]-adduct in a reasonable yield.57 In 2012, they investigated the paramagnetic properties of the mono-anion of the [5,6]-pyrrolidine-Sc3N@C80 derivative which can be generated both chemically and photochemically.58 ESR results revealed that the Sc3N cluster is fixed inside the functionalized cage, providing signals from nonequivalent Sc nuclei. Then DFT calculations provided detailed information about the cluster rotational pathways, spin density distribution, and hyperfine coupling constants. The results showed that the electronic structure and the motion of the internal unit were changed significantly by exohedral modification in response to the mutual effect between the internal cluster and the exohedral unit. The 1,3-dipolar reaction was also used to synthesize electron donor–acceptor conjugates based on nitride cluster EMFs to see the long-range charge transfer process between the two moieties.44 In a typical study, Sc3N@C80 was coupled with different organic donor moieties possessing zinc porphyrin (ZnP, 16). Two products based on different aldehydes were obtained and were systematically characterized by NMR, CV and absorption spectroscopy. In these conjugates, the center-to-center distances from

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Table 3

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Summary of the chemical reactions that have been performed on carbide cluster-EMFs since 2011

Reaction specification

Compound

Carbene reaction

Sc2C2@C2v(5)-C80

Carbene reaction

Prato reaction

Coordination

Main adduct/ addition position

Characterizations

Ref.

Mono-adduct

HPLC; MALDI-TOF; UV-vis-NIR; CV; DPV; NMR; X-ray crystallography

49, 50

Sc3C2@C80

Mono-adduct

HPLC; MALDI-TOF; NMR; EPR; COSY; HSQC; HMBC; ROESY; X-ray crystallography

51

Sc2C2@Cs(6)-C82 Sc2C2@C2v(9)-C82

Mono-adduct

HPLC; MALDI-TOF; UV-vis-NIR; CV; DPV; NMR; X-ray crystallography

52, 53

Sc2C2@C3v(8)-C82 Sc2C2@C2v(5)-C80

Reactant

(m-H)3Re3(CO)11(NCMe) (15)

Mono-adduct

HPLC; MALDI-TOF; NMR; UV-vis-NIR; X-ray crystallography

54, 55

the electron donor (ZnP) and the electron acceptor (Sc3N@C80) were up to 45 Å which promoted the lifetimes in the photophysical experiments. Besides, the employed solvents possessed a particular influence on the distance effects and the long-range electron transfer. 5.2

Radical reaction

For EMFs encapsulating different metallic species, the electronic configuration is a critical factor controlling the addition number in radical reactions. Meanwhile, the addition position is markedly induced by the location of the metal atoms inside the cage. Nitride cluster EMFs (e.g. Sc3N@C80) with a close-shell configuration tend to generate an even number of new bonds. In 2007, the first radical trifluoromethylation of both isomers of Sc3N@C80 using CF3I (18) yielded corresponding adducts with an even number (from 2 to 12) of CF3 groups.38 A bis-adduct isomer was isolated and confirmed by 19F NMR as a 1,4-adduct. An important finding is that CF3 radical addition could reduce the HOMO–LUMO band-gap of the EMF. Later on, a systematic structural investigation of the multiple trifluoromethylation adducts of Sc3N@C80(CF3)x (x = 2–16) by 19 F-NMR and X-ray crystallography was reported.59 The mutual influence between the addition position of the substituted groups and the orientation of the internal cluster is discovered. Recently, several Sc3N@C80(CF3)x isomers with 14 or 16 CF3 groups were synthesized and characterized. Their structures were undoubtedly confirmed by single crystal X-ray crystallography (Fig. 10). Different from the common viewpoint about the low relativity of triple-hexagon junction (THJs) carbon atoms owing to their low degree of pyramidalization, in these derivatives, however, four and eight THJ carbons are substituted. Theoretical calculations disclosed the interplay between the CF3 motifs and the cage which governed the addition pattern

Chem. Commun.

Fig. 10 X-ray structures of (a) Sc3N@Ih-C80(CF3)14 and (b) Sc3N@IhC80(CF3)16.

and then control the position and dynamic state of the internal Sc3N cluster. In a following study, the structures of Sc3N@D5hC80(CF3)18 and Sc3N@Ih-C80(CF3)14 were also determined by X-ray crystallography.60 These results were of great importance to comprehend the effects of different substitution degree on the properties of the derivatives and further design many derivatives for potential applications. 5.3

Benzyne cycloaddition

Cycloaddition of benzyne (19), generated in situ from the reaction between anthranilic acid and isoamyl nitrite, to La@C82 gave rise to multiple derivatives with an additional NO2 group.61 In contrast, the same reaction carried on Sc3N@Ih-C80 afforded both [5,6]- and [6,6]-adducts containing merely a benzene group.62 Their molecular structures were determined by single crystal X-ray diffraction (Fig. 11a and b) which shows that the cluster is fixed in both isomers but the cluster orientation relative to the cage and the addend are completely different.

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Fig. 11 X-ray structures of the benzyne monoadducts of Sc3N@C80 showing the different cluster orientation relative to the cage and the addend. (a) [5,6]-adduct and (b) [6,6]-adduct. (c) The oxidized open-cage benzyne derivative of Sc3N@C80.

Another significant finding is that both the adducts are thermally stable in contrast to the fact that the [6,6]-pyrrolidino-Sc3N@Ih-C80 converts to the [5,6]-adduct under heating. Considering this advantage, Echegoyen and coworkers investigated the electro-chemical properties of the [6,6]-monoadduct of Sc3N@C80 for the first time. Similarly, a [2+2] cycloaddition reaction of 4,5-diisopropoxybenzyne (20) with Sc3N@C80 was also reported to afford both [5,6]- and [6,6]-adducts under an argon atmosphere.63 However, the [5,6]-adduct was found to form an intriguing open-cage product upon exposure to the air. The structure of this open-cage derivative was confirmed by single crystal XRD measurement (Fig. 11c) which shows that an oxygen atom has been added to the cage and the C–C bond at the addition sites is broken. This results in a 13-member ring orifice of the cage, which is of large interest for putting different species into the cage and will stimulate great research enthusiasm on it. 5.4

Carbene addition

Electron donor–acceptor conjugates utilizing Sc3N@Ih-C80/ La2@Ih-C80 and zinc tetraphenylporphyrin were synthesized via a [1+2] cycloaddition reaction with a diazo precursor (5).44 A systematic comparison between these two isoelectronic derivatives of endohedral fullerenes was carried out. Both derivatives were identified by single crystal X-ray crystallography to be [6,6]-adducts in combination with NMR studies. Besides, the donor–acceptor conjugate base on Sc3N@C80 showed a substantial charge transfer activity and formed a radical ion pair namely (Sc3N@C80) –(ZnP) + in its excited state. NMR, electrochemical and photophysical studies combined with DFT calculations revealed the essential difference and the important role of the entrapped cluster. Recently, addition of diphenylmethane carbene containing different side groups, generated by photo-irradiation or heating of appropriate diazo precursors (21) afforded both [5,6]-open methanofullerene and [6,6]-open adduct of [email protected] This presents the first example of a [5,6]-open methanofullerene based on the Ih-C80 cage because previous reports demonstrated that cyclopropanation only occurred on the [6,6]-bond of Sc3N@C80. The X-ray structure of the [5,6]-adduct, which is shown in Fig. 12,

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Fig. 12 X-ray structure of the [5,6]-open methanofullerene derivative of Sc3N@C80.

confirmed firmly the open-cage structure from the long C–C distance at the site of addition (2.23 Å). As a result, the internal cluster is fixed with one Sc atom being trapped inside the opening of the cage. Another interesting finding is that different substituent groups at the para-position of the phenyl ring could alter the reactivity of the carbene formed and subsequently change the product ratio (Table 4). 5.5

Electrochemical synthesis

The relatively low chemical reactivity of nitride cluster EMFs stimulated new electrochemical approaches to synthesize some new derivatives. In such a way, the electrophilic addition of PhCHBr2 (22) to the Lu3N@Ih-C80 dianion was successfully achieved although the neutral Lu3N@Ih-C80 does not react with 22.65 The derivative was revealed to be a [6,6]-open adduct. However, the same reaction with Sc3N@Ih-C80 dianion failed. Then the more reactive Sc3N@Ih-C80 trianion was electrochemically generated to react with 22, affording the [6,6]-open monoadduct which was identified by various experimental characterizations.66 As suggested by theoretical calculations, the SOMO of [Sc3N@Ih-C80]3 and the HOMO of [Lu3N@Ih-C80]2 were mainly localized on the carbon cage while the HOMO of [Sc3N@Ih-C80]2 was mainly localized on the internal Sc3N unit, which provided a good explanation for the observed experimental fact of their different reaction activities with electrophiles. As such, the newly employed electrosynthetic method has many advantages over conventional methods and helps us to obtain more derivatives of fullerenes.

6. Theoretical studies of the chemical properties of EMFs Along with the progress achieved in the chemical functionalization of EMFs, theoretical considerations about the chemical reactivities of some typical EMFs were also performed. For empty fullerenes, the p-orbital axis vector (POAV) analysis plays a critical role in determining the chemical reactivity of cage carbons. This means that the carbon atoms at the more curved

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Table 4

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Summary of the chemical reactions that have been used to functionalize nitride cluster-EMFs since 2011

Reaction specification

Compound

Prato reaction

Reactant

Main adduct

Characterizations

Ref.

Sc3N@Ih-C80

[5,6]-Mono-adduct

HPLC, MS, NMR, HMQC, ESR, TC

58

Prato reaction

Sc3N@Ih-C80

Mono-adduct

HPLC, NMR, UV-vis-NIR, CV

44

Radical reaction

Sc3N@Ih-C80 Sc3N@D5h-C80

Multi-adducts

HPLC, MS, NMR, UV-vis-NIR, ESR, TC, X-ray crystallography

67, 68

[2+2] Cycloaddition

Sc3N@Ih-C80

[5,6]- and [6,6]mono-adducts

HPLC, MS, NMR, UV-vis-NIR, CV, X-ray crystallography

62

[2+2] Cycloaddition

Sc3N@Ih-C80

[5,6]- and [6,6]mono-adducts

HPLC, MS, NMR, UV-vis-NIR, X-ray crystallography

63

Carbene addition

Sc3N@Ih-C80

[6,6]-Mono-adduct

HPLC, MS, NMR, CV/DPV, UV-vis, TC

44

Carbene addition

Sc3N@Ih-C80

[5,6]- and [6,6]mono-adducts

HPLC, MS, NMR, CV, X-ray crystallography

64

Electrochemical synthesis

(Sc3N@Ih-C80)3 (Lu3N@Ih-C80)2

[6,6]-Mono-adduct

HPLC, MS, NMR, UV-vis-NIR, CV, TC

65, 66

area on the fullerene cage are more reactive because of their eagerness of releasing the high strains. This remains effective in regulating the chemical properties of EMFs in most cases. However, because of the presence of the metallic species and the electron transfer process, charge density on the cage carbons becomes another factor controlling the reactivity of EMF molecules, which in some cases, are more important than the POAV angles. A clear example is La@C2v(9)-C82 in which the lanthanum atom is located beneath a hexagonal ring along the symmetric

Chem. Commun.

axis accompanied by three-electron-transfer from the metal atom to the cage. Thus, the electrophilic addition prefers to take place at the cage carbons bearing high electron densities, namely these nearest to the internal metal cation, whereas nucleophilic addition occurs at the area involving these cage carbons with positive charge densities which are far from the internal metal ion (Fig. 13). In recent years, in-depth understanding of the chemical properties of cluster EMFs, from the theoretical point of view,

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Fig. 14 X-ray structure of La@C2v-C82Cp* and molecular orbital of La@C2v-C82. Fig. 13 A schematic illustration of La@C82 showing the different regions of the cage preferring different reactions, which is caused by charge density distributions on the cage surface.

has developed rapidly.69,70 The strategy is straightforward but many factors, such as the bond lengths, the influence of the metal clusters, the relative stabilities of formed adducts and the cage structures, have to be considered so as to accurately predict the results. 6.1

Diels–Alder reactions

Calculations were first conducted to predict the most reactive sites in typical nitride cluster EMFs such as M3N@C78 (M = Sc, Y) in Diels–Alder reactions with 1,3-butadiene, taking into account both the double bond character and the pyramidalization angle of the C–C bonds.71,72 The results demonstrated that the reactivity of the cage carbons is a combinational effect from both electrostatic and geometric aspects, in close relation with the internal cluster. Then the same methodology was utilized to study the different reactivity of the Ih and D5h isomers of Sc3N@C80. By considering the thermodynamic and kinetic aspects of the cycloaddition reaction, it was revealed that [5,6] bonds are more reactive than [6,6] bonds for both isomers.73 When putting different clusters inside the D5h-C80 cage, the larger and more positive Gd3N cluster significantly reduces the regioselectivity. Finally, it was confirmed that the D5h isomer is more reactive from the kinetic point of view than the Ih isomer in all cases, which is in excellent agreement with experimental results. Similarly, the reactivity of Ti2C2@D3h-C78 was also probed to see its difference from that of D3h-C78 and M3N@D3h-C78 (M = Sc and Y).74 It was not surprising to find that the free D3h-C78 cage is more reactive than the corresponding EMFs. As to the reactivity of the EMFs, the nature of the encapsulated cluster exerts marked influence on the chemical properties of the EMFs in the reaction. Aiming at the icosahedral C80 cage encapsulating different clusters such as Sc3N, Lu3N, Y3N, La2, Y3, Sc3C2 Sc4C2, Sc3CH, Sc3NC, Sc4O2 and Sc4O3, Sola and coworkers performed systematic DFT calculations of their corresponding [4+2] adducts, considering both the thermodynamic and kinetic regioselectivity in relation to the free rotation of the internal clusters.75 The results showed that encapsulation of the metallic clusters reduces the reactivity of the Ih-C80 cage. For the corresponding

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cluster EMFs, the additions prefer to occur at the [5,6]-bond although the [6,6]-adduct is favored in such EMFs with a large number of electron transfers. Diels–Alder reaction of La@C2v-C82 with cyclopentadiene was reported in 2005, which afforded only one monoadduct isomer (Fig. 14).76 The addition position and the high stereospecificity of the monoadduct of La@C2v-C82 with 1,2,3,4,5pentamethylcyclopentadiene (Cp*) were revealed in 2010 using single-crystal X-ray diffraction.77 Pericyclic reactions are quite well understood through the Woodward–Hoffmann rule and the frontier orbital theory.78,79 However, the electron-transfer catalyzed Diels–Alder reaction has also been studied and showed high reactivity via a stepwise path due to the radical character of a diene or a dienophile. The theoretical calculation revealed that the reaction proceeds in a concerted bond formation mechanism. Carbon atoms of the addition site have large a-LUMO and b-LUMO+1 coefficients. Accordingly, this is a unique example of the [4+2] cycloaddition reaction of a paramagnetic molecule via a concerted pathway.80 6.2

Bingel–Hirsch reaction

The first Bingel–Hirsch reaction was performed on La@C82. It was surprising to find that only one cycloadduct was formed while the other four were all singly bonded adducts.34 Calculations considering both the charge distribution and POAV values suggested that the positively charged areas of the fullerene cage are more reactive in this reaction (Fig. 13). Later on, theoretical considerations of the relative stability of the Bingel–Hirsch monoadducts of several typical non-IPR EMFs, namely Gd3N@Cs(51365)-C84, Y3N@C2(22010)-C78 and Sc3N@D3(6140)-C68 which have one, two, and three fusedpentagon pairs, respectively, provided interesting results that the most favorable addition always leads to the formation of an open-cage adduct but the addition never occurs at the [5,5]-bond.81 This theoretical finding is in consistence with the experimental results obtained from the reaction between La2@C72 and admantylidene carbene (4). 6.3

1,3-Dipolar reaction

The mechanistic study of the 1,3-dipolar reactions of EMFs is rare. Swart and coworkers performed DFT calculations to understand the reaction mechanism for the 1,3-dipolar reaction

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of 3-triphenylmethyl-5-oxazolidinone (14) with Sc3N@D5h-C80. From the energetic point of view, it is revealed that the [5,6]-adducts are the most stable derivatives but the pyracyclene adduct, which has been thought to be important in the 1,3-dipolar reactions, is not detected in this theoretical study. Unfortunately, no experimental results are available to verify the theoretical prediction.

7. Conclusions Here we presented a supplementary review of the chemical properties of EMFs following our previous review paper published three years ago.10 These two articles comprise a complete view of the chemistry of EMFs. We are happy to see that the recent studies have focused on the construction of applicable materials based on EMFs such as the donor–acceptor dyads, the SAMs, and the fullerene-grafted polymers, as well as the water-soluble derivatives of EMFs with potential applications in biomedicine. These achievements pave a road for EMFs towards future applications. Meanwhile, theoretical considerations aiming at the reaction mechanism of EMFs become more important. However, there are still many mysteries existing in this field. For instance, it is still not clear that the motion of the internal metallic cluster can produce what kind of new functions or applications for EMFs. Nevertheless, it becomes clearer and clearer that the chemical strategy is a necessary and efficient means to realize the future applications of EMFs and many studies have to be done in the near future.

Acknowledgements Financial support from The National Thousand Talents Program of China, NSFC (21171061, 21271067), Program for Changjiang Scholars and Innovative Research Team in University (IRT1014), KAKENHI (20108001, ‘‘pi-Space’’, 202455006, 24350019, 20036008, 20038007, 22000009), The Next Generation Super Computing Project (Nanoscience) from MEXT Japan, and The Strategic Japanese–Spanish Cooperative Program funded by JST & MICINN is gratefully acknowledged.

Notes and references 1 A. Hirsch, Nat. Mater., 2010, 9, 868–871. 2 K. M. Kadish and R. Ruoff, Fullerenes: Chemistry, Physics, and Technology, Wiley-VCH, New York, 2000. 3 A. Hirsch and M. Brettreich, Fullerenes-Chemistry and Reactions, Wiley-VCH, Weinheim, 2005. 4 A. Popov, S. Yang and L. Dunsch, Chem. Rev., 2013, 113, 5989–6113. 5 X. Lu, L. Feng, T. Akasaka and S. Nagase, Chem. Soc. Rev., 2012, 41, 7723–7760. 6 M. N. Chaur, F. Melin, A. L. Ortiz and L. Echegoyen, Angew. Chem., Int. Ed., 2009, 48, 7514–7538. 7 T. Akasaka, T. Kato, K. Kobayashi, S. Nagase, K. Yamamoto, H. Funasaka and T. Takahashi, Nature, 1995, 374, 600–601. 8 H. Maeda, T. Tsuchiya, X. Lu, Y. Takano, T. Akasaka and S. Nagase, Nanoscale, 2011, 3, 2421–2429. 9 M. Yamada, T. Akasaka and S. Nagase, Acc. Chem. Res., 2010, 43, 92–102. 10 X. Lu, T. Akasaka and S. Nagase, Chem. Commun., 2011, 47, 5942–5957.

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ChemComm 11 S. Nagase, Bull. Chem. Soc. Jpn., 2014, 87, 167195. 12 Y. Chai, T. Guo, C. M. Jin, R. E. Haufler, L. P. F. Chibante, J. Fure, L. H. Wang, J. M. Alford and R. E. Smalley, J. Phys. Chem., 1991, 95, 7564–7568. 13 M. Maggini, G. Scorrano and M. Prato, J. Am. Chem. Soc., 1993, 115, 9798–9799. 14 B. P. Cao, T. Wakahara, Y. Maeda, A. H. Han, T. Akasaka, T. Kato, K. Kobayashi and S. Nagase, Chem. – Eur. J., 2004, 10, 716–720. 15 X. Lu, X. R. He, L. Feng, Z. J. Shi and Z. N. Gu, Tetrahedron, 2004, 60, 3713–3716. 16 N. Crivillers, Y. Takano, Y. Matsumoto, J. Casado-Montenegro, M. Mas-Torrent, C. Rovira, T. Akasaka and J. Veciana, Chem. Commun., 2013, 49, 8145–8147. 17 T. Tsuchiya, M. Rudolf, S. Wolfrum, S. G. Radhakrishnan, R. Aoyama, Y. Yokosawa, A. Oshima, T. Akasaka, S. Nagase and D. M. Guldi, Chem. – Eur. J., 2013, 19, 558–565. 18 K. Sawai, Y. Takano, M. Izquierdo, S. Filippone, N. Martin, Z. Slanina, N. Mizorogi, M. Waelchli, T. Tsuchiya, T. Akasaka and S. Nagase, J. Am. Chem. Soc., 2011, 133, 17746–17752. 19 Y. Maeda, Y. Matsunaga, T. Wakahara, S. Takahashi, T. Tsuchiya, M. O. Ishitsuka, T. Hasegawa, T. Akasaka, M. T. H. Liu, K. Kokura, E. Horn, K. Yoza, T. Kato, S. Okubo, K. Kobayashi, S. Nagase and K. Yamamoto, J. Am. Chem. Soc., 2004, 126, 6858–6859. 20 T. Akasaka, T. Kono, Y. Takematsu, H. Nikawa, T. Nakahodo, T. Wakahara, M. O. Ishitsuka, T. Tsuchiya, Y. Maeda, M. T. H. Liu, K. Yoza, T. Kato, K. Yamamoto, N. Mizorogi, Z. Slanina and S. Nagase, J. Am. Chem. Soc., 2008, 130, 12840–12841. 21 Y. Takano, M. Aoyagi, M. Yamada, H. Nikawa, Z. Slanina, N. Mizorogi, M. O. Ishitsuka, T. Tsuchiya, Y. Maeda, T. Akasaka, T. Kato and S. Nagase, J. Am. Chem. Soc., 2009, 131, 9340–9346. 22 X. Lu, H. Nikawa, L. Feng, T. Tsuchiya, Y. Maeda, T. Akasaka, N. Mizorogi, Z. Slanina and S. Nagase, J. Am. Chem. Soc., 2009, 131, 12066–12067. 23 M. Hachiya, H. Nikawa, N. Mizorogi, T. Tsuchiya, X. Lu and T. Akasaka, J. Am. Chem. Soc., 2012, 134, 15550–15555. 24 X. Lu, Z. Slanina, T. Akasaka, T. Tsuchiya, N. Mizorogi and S. Nagase, J. Am. Chem. Soc., 2010, 132, 5896–5905. 25 Y. P. Xie, M. Suzuki, W. Cai, N. Mizorogi, S. Nagase, T. Akasaka and X. Lu, Angew. Chem., Int. Ed., 2013, 52, 5142–5145. 26 X. Lu, Y. Lian, C. M. Beavers, N. Mizorogi, Z. Slanina, S. Nagase and T. Akasaka, J. Am. Chem. Soc., 2011, 133, 10772–10775. 27 W. Zhang, M. Suzuki, Y. P. Xie, L. P. Bao, W. T. Cai, Z. Slanina, S. Nagase, M. Xu, T. Akasaka and X. Lu, J. Am. Chem. Soc., 2013, 135, 12730–12735. 28 L. Feng, Z. Slanina, S. Sato, K. Yoza, T. Tsuchiya, N. Mizorogi, T. Akasaka, S. Nagase, N. Martin and D. Guldi, Angew. Chem., Int. Ed., 2011, 50, 5909–5912. 29 R. He, H. Zhao, J. H. Liu, Y. H. Jiao, Y. Y. Liang, X. Li and C. Y. Chen, Fullerenes, Nanotubes, Carbon Nanostruct., 2013, 21, 549–559. 30 R. He, G. Xing, X. Wang, Y. Jiao, H. Zhao, H. Yuan, S. Wang, J. Dong and H. Lei, J. Nanosci. Nanotechnol., 2013, 13, 1549–1554. 31 X. L. Ruan, D. M. Yue, S. Y. Yang, X. H. Guo, J. Q. Dong, R. L. Cui and B. Y. Sun, J. Nanosci. Nanotechnol., 2012, 12, 7233–7238. 32 J. Ruan, Y. Xie, W. Cai, Z. Slanina, N. Mizorogi, S. Nagase, T. Akasaka and X. Lu, Fullerenes, Nanotubes, Carbon Nanostruct., 2014, 22, 138–146. 33 A. Hirsch, I. Lamparth and H. R. Karfunkel, Angew. Chem., Int. Ed. Engl., 1994, 33, 437–438. 34 L. Feng, T. Nakahodo, T. Wakahara, T. Tsuchiya, Y. Maeda, T. Akasaka, T. Kato, E. Horn, K. Yoza, N. Mizorogi and S. Nagase, J. Am. Chem. Soc., 2005, 127, 17136–17137. 35 L. Feng, T. Tsuchiya, T. Wakahara, T. Nakahodo, Q. Piao, Y. Maeda, T. Akasaka, T. Kato, K. Yoza, E. Horn, N. Mizorogi and S. Nagase, J. Am. Chem. Soc., 2006, 128, 5990–5991. 36 L. Feng, T. Wakahara, T. Nakahodo, T. Tsuchiya, Q. Piao, Y. Maeda, Y. Lian, T. Akasaka, E. Horn, K. Yoza, T. Kato, N. Mizorogi and S. Nagase, Chem. – Eur. J., 2006, 12, 5578–5586. 37 I. E. Kareev, S. F. Lebedkin, V. P. Bubnov, E. B. Yagubskii, I. N. Ioffe, P. A. Khavrel, I. V. Kuvychko, S. H. Strauss and O. V. Boltalina, Angew. Chem., Int. Ed., 2005, 44, 1846–1849. 38 N. B. Shustova, A. A. Popov, M. A. Mackey, C. E. Coumbe, J. P. Phillips, S. Stevenson, S. H. Strauss and O. V. Boltalina, J. Am. Chem. Soc., 2007, 129, 11676–11677. 39 M. Yamada, L. Feng, T. Wakahara, T. Tsuchiya, Y. Maeda, Y. F. Lian, M. Kako, T. Akasaka, T. Kato, K. Kobayashi and S. Nagase, J. Phys. Chem. B, 2005, 109, 6049–6051.

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ChemComm 40 Y. Takano, S. Obuchi, N. Mizorogi, R. Garcia, M. A. Herranz, M. Rudolf, D. M. Guldi, N. Martin, S. Nagase and T. Akasaka, J. Am. Chem. Soc., 2012, 134, 19401–19408. 41 M. Yamada, M. Minowa, S. Sato, Z. Slanina, T. Tsuchiya, Y. Maeda, S. Nagase and T. Akasaka, J. Am. Chem. Soc., 2011, 133, 3796–3799. 42 M. Yamada, C. Someya, T. Wakahara, T. Tsuchiya, Y. Maeda, T. Akasaka, K. Yoza, E. Horn, M. T. H. Liu, N. Mizorogi and S. Nagase, J. Am. Chem. Soc., 2008, 130, 1171–1176. 43 M. O. Ishitsuka, S. Sano, H. Enoki, S. Sato, H. Nikawa, T. Tsuchiya, Z. Slanina, N. Mizorogi, M. T. H. Liu, T. Akasaka and S. Nagase, J. Am. Chem. Soc., 2011, 133, 7128–7134. 44 L. Feng, S. G. Radhakrishnan, N. Mizorogi, Z. Slanina, H. Nikawa, T. Tsuchiya, T. Akasaka, S. Nagase, N. Martin and D. M. Guldi, J. Am. Chem. Soc., 2011, 133, 7608–7618. 45 M. Yamada, C. I. Someya, T. Nakahodo, Y. Maeda, T. Tsuchiya and T. Akasaka, Molecules, 2011, 16, 9495–9504. 46 X. Lu, T. Akasaka and S. Nagase, Acc. Chem. Res., 2013, 46, 1627–1635. 47 Y. Iiduka, T. Wakahara, T. Nakahodo, T. Tsuchiya, A. Sakuraba, Y. Maeda, T. Akasaka, K. Yoza, E. Horn, T. Kato, M. T. H. Liu, N. Mizorogi, K. Kobayashi and S. Nagase, J. Am. Chem. Soc., 2005, 127, 12500–12501. 48 Y. Iiduka, T. Wakahara, K. Nakajima, T. Nakahodo, T. Tsuchiya, Y. Maeda, T. Akasaka, K. Yoza, M. T. H. Liu, N. Mizorogi and S. Nagase, Angew. Chem., Int. Ed., 2007, 46, 5562–5564. 49 H. Kurihara, X. Lu, Y. Iiduka, H. Nikawa, N. Mizorogi, Z. Slanina, T. Tsuchiya, S. Nagase and T. Akasaka, J. Am. Chem. Soc., 2012, 134, 3139–3144. 50 H. Kurihara, X. Lu, Y. Iiduka, N. Mizorogi, Z. Slanina, T. Tsuchiya, T. Akasaka and S. Nagase, J. Am. Chem. Soc., 2011, 133, 2382–2385. 51 H. Kurihara, Y. Iiduka, Y. Rubin, M. Waelchli, N. Mizorogi, Z. Slanina, T. Tsuchiya, S. Nagase and T. Akasaka, J. Am. Chem. Soc., 2012, 134, 4092–4095. 52 X. Lu, K. Nakajima, Y. Iiduka, H. Nikawa, N. Mizorogi, Z. Slanina, T. Tsuchiya, S. Nagase and T. Akasaka, J. Am. Chem. Soc., 2011, 133, 19553–19558. 53 X. Lu, K. Nakajima, Y. Iiduka, H. Nikawa, T. Tsuchiya, N. Mizorogi, Z. Slanina, S. Nagase and T. Akasaka, Angew. Chem., Int. Ed., 2012, 51, 5889–5892. 54 C. H. Chen, W. Y. Yeh, Y. H. Liu and G. H. lee, Angew. Chem., Int. Ed., 2012, 51, 13046–13049. 55 C.-H. Chen, D.-Y. Lin and W.-Y. Yeh, Chem. – Eur. J., 2014, 20, 5768–5775. 56 S. Stevenson, P. W. Fowler, T. Heine, J. C. Duchamp, G. Rice, T. Glass, K. Harich, E. Hajdu, R. Bible and H. C. Dorn, Nature, 2000, 408, 427–428. 57 C. M. Cardona, A. Kitaygorodskiy, A. Ortiz, M. A. Herranz and L. Echegoyen, J. Org. Chem., 2005, 70, 5092–5097. 58 B. Elliott, A. D. Pykhova, J. Rivera, C. M. Cardona, L. Dunsch, A. A. Popov and L. Echegoyen, J. Phys. Chem. C, 2013, 117, 2344–2348. 59 N. B. Shustova, D. V. Peryshkov, I. V. Kuvychko, Y. S. Chen, M. A. Mackey, C. Coumbe, D. Heaps, B. S. Confait, T. Heine,

This journal is © The Royal Society of Chemistry 2014

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J. D. Phillips, S. Stevenson, L. Dunsch, A. Popov, S. H. Strauss and O. V. Boltalina, J. Am. Chem. Soc., 2011, 133, 2672–2690. S. Yang, C. Chen, M. Jiao, N. B. Tamm, M. A. Lanskikh, E. Kemnitz and S. I. Troyanov, Inorg. Chem., 2011, 50, 3766–3771. X. Lu, H. Nikawa, T. Tsuchiya, T. Akasaka, M. Toki, H. Sawa, N. Mizorogi and S. Nagase, Angew. Chem., Int. Ed., 2010, 49, 594–597. F. F. Li, J. R. Pinzon, B. Q. Mercado, M. M. Olmstead, A. L. Balch and L. Echegoyen, J. Am. Chem. Soc., 2011, 133, 1563–1571. G.-W. Wang, T.-X. Liu, M. Jiao, N. Wang, S.-E. Zhu, C. Chen, S. Yang, F. L. Bowles, C. M. Beavers, M. M. Olmstead, B. Q. Mercado and A. L. Balch, Angew. Chem., Int. Ed., 2011, 50, 4658–4662. ´n, M. M. Olmstead, A. L. Balch and M. Izquierdo, M. R. Cero L. Echegoyen, Angew. Chem., Int. Ed., 2013, 125, 12042–12046. F. F. Li, A. Rodriguez-Fortea, J. M. Poblet and L. Echegoyen, J. Am. Chem. Soc., 2011, 133, 2760–2765. F. F. Li, A. Rodriguez-Fortea, P. Peng, G. A. C. Chavez, J. M. Poblet and L. Echegoyen, J. Am. Chem. Soc., 2012, 134, 7480–7487. N. B. Shustova, Y. S. Chen, M. A. Mackey, C. E. Coumbe, J. P. Phillips, S. Stevenson, A. A. Popov, O. V. Boltalina and S. H. Strauss, J. Am. Chem. Soc., 2009, 131, 17630–17637. S. F. Yang, C. B. Chen, M. Z. Jiao, N. B. Tamm, M. A. Lanskikh, E. Kemnitz and S. I. Troyanov, Inorg. Chem., 2011, 50, 3766–3771. A. Rodriguez-Fortea, N. Alegret, A. L. Balch and J. M. Poblet, Nat. Chem., 2010, 2, 955–961. A. Rodriguez-Fortea, A. L. Balch and J. M. Poblet, Chem. Soc. Rev., 2011, 40, 3551–3563. S. Osuna, M. Swart, J. M. Campanera, J. M. Poblet and M. Sola, J. Am. Chem. Soc., 2008, 130, 6206–6214. S. Osuna, M. Swart and M. Sola, J. Am. Chem. Soc., 2009, 131, 129–139. S. Osuna, R. Valencia, A. Rodriguez-Fortea, M. Swart, M. Sola and J. M. Poblet, Chem. – Eur. J., 2012, 18, 8944–8956. M. Garcia-Borras, S. Osuna, J. M. Luis, M. Swart and M. Sola, Chem. – Eur. J., 2012, 18, 7141–7154. M. Garcia-Borras, S. Osuna, J. M. Luis, M. Swart and M. Sola, Chem. – Eur. J., 2013, 19, 14931–14940. Y. Maeda, J. Miyashita, T. Hasegawa, T. Wakahara, T. Tsuchiya, T. Nakahodo, T. Akasaka, N. Mizorogi, K. Kobayashi, S. Nagase, T. Kato, N. Ban, H. Nakajima and Y. Watanabe, J. Am. Chem. Soc., 2005, 127, 12190–12191. Y. Maeda, S. Sato, K. Inada, H. Nikawa, M. Yamada, N. Mizorogi, T. Hasegawa, T. Tsuchiya, T. Akasaka, T. Kato, Z. Slanina and S. Nagase, Chem. – Eur. J., 2010, 16, 2193–2197. R. B. Woodward and R. Hoffmann, Angew. Chem., Int. Ed. Engl., 1969, 8, 781–853. K. Fukui, T. Yonezawa and H. Shingu, J. Chem. Phys., 1952, 20, 722–725; K. Fukui, Acc. Chem. Res., 1971, 4, 57–64. S. Sato, Y. Maeda, J.-D. Guo, M. Yamada, N. Mizorogi, S. Nagase and T. Akasaka, J. Am. Chem. Soc., 2013, 135, 5582–5587. M. Garcia-Borras, S. Osuna, M. Swart, J. M. Luis, L. Echegoyen and M. Sola, Chem. Commun., 2013, 49, 8767–8769.

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