Endohedral Metallofullerenes Research Articles

Computational insights into Diels-Alder reactions of paramagnetic endohedral metallofullerenes: M@C82 (M = Sc, Y, La) and La@C72.

In fullerene chemistry, Diels-Alder cycloaddition is an essential reaction for exohedral modification of carbon cages. M@C2v(9)-C82 (M = Sc, Y, and La), incorporating one metal atom within the fullerene cage, are key compounds for understanding the impact of both endohedral and exohedral modifications on their electronic structures. In this work, the Diels-Alder (DA) cycloaddition of cyclopentadiene (Cp) to M@C2v(9)-C82 (M = Sc, Y, and La) and La@C2(10612)-C72 was systematically studied using density functional theory. The most reactive bonds were initially chosen for detailed mechanistic exploration, considering both concerted and stepwise mechanisms. Our findings revealed that DA cycloadditions for the three metals (Sc, Y, and La) consistently exhibit the same regioselectivity, favoring the concerted attack on the [5,6] bond. This observation is in agreement with previous experimental and theoretical studies on the regioselectivity of the Diels-Alder reaction between La@C2v(9)-C82 and Cp. In the case of La@C2(10612)-C72, the most favored pathway is the concerted attack on the [6,6] bond both kinetically and thermodynamically. In toluene and ortho-dichlorobenzene, while the energy barriers and the reaction free energies increased to different extents for most pathways, the regioselectivity largely mirrored that observed in the gas phase.

Controlling the reactivity of La@C82 by reduction: reaction of the La@C82 anion with alkyl halide with high regioselectivity.

Endohedral metallofullerenes have excellent redox properties, which can be used to vary their reactivity to certain classes of molecules, such as alkyl halides. In this study, the thermal reaction of the La@C2v-C82 anion with benzyl bromide derivatives 1 at 110 °C afforded single-bonded adducts 2-5 with high regioselectivity. The products were characterized by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and visible-near infrared spectroscopy. The reaction of La@C2v-C82 with alkyl halides using the same conditions showed no consumption of La@C2v-C82, indicating that the reactivity of La@C2v-C82 toward alkyl halides was effectively increased by one-electron reduction. Single-crystal X-ray diffraction analysis of the single-bonded adduct 3a revealed the addition site of the p-methoxybenzyl group on La@C2v-C82. Theoretical calculations indicated that the addition site carbons in neutral La@C2v-C82 have high spin density, whereas those in the La@C2v-C82 anion do not have high charge densities. Thus, the reaction is believed to occur via electron transfer, followed by the radical coupling of La@C2v-C82 and benzyl radicals, rather than by bimolecular nucleophilic substitution reaction of La@C2v-C82 anion with 1.

Bandgap Engineering of Erbium-Metallofullerenes toward Switchable Photoluminescence.

Encapsulating photoluminescent lanthanide ions like erbium (Er) into fullerene cages affords photoluminescent endohedral metallofullerenes (EMFs). Few reported photoluminescent Er-EMFs are all based on encapsulation of multiple (two to three) metal atoms, whereas mono-Er-EMFs exemplified by Er@C82 are not photoluminescent due to its narrow optical bandgap. Herein, by entrapping an Er-cyanide cluster into various C82 cages to form novel Er-monometallic cyanide clusterfullerenes (CYCFs), ErCN@C82 (C2 (5), Cs (6), and C2 v (9)), the photoluminescent properties of CYCFs are investigated, and obvious near-infrared (NIR) photoluminescence only is observed for ErCN@C2 (5)-C82 . Combined with a comparative photoluminescence study of three medium-bandgap di-Er-EMFs, including Er2 @Cs (6)-C82 , Er2 O@Cs (6)-C82 , and Er2 C2 @Cs (6)-C82 , this study proposes that the optical bandgap can be used as a simple criterion for switching the photoluminescence of Er-EMFs, and the bandgap threshold is determined to be between 0.83 and 0.74eV. Furthermore, the photoluminescent patterns of these three di-Er-EMFs differ dramatically. It is found that the location of the Er atom within the same Cs (6)-C82 cage is almost fixed and independent on the endo-unit; thus the previous statement on the key role of metal position in photoluminescence of di-Er-EMFs seems erroneous, and the geometric configuration of the endo-unit, especially the bridging mode of two Er ions, is decisive instead.

Synthesis and Characterization of a Novel Non-Isolated-Pentagon-Rule Isomer of Th@C76:Th@C1(17418)-C76

A novel Non-Isolated-Pentagon-Rule (non-IPR) isomer of thorium-based endohedral mono-metallofullerenes (mono-EMFs), Th@C1(17418)-C76, was successfully synthesized and characterized using MALDI-TOF mass spectroscopy, single-crystal X-ray diffraction, UV-vis-NIR spectroscopy, and Raman spectroscopy. The molecular structure of this non-IPR isomer was determined unambiguously as Th@C1(17418)-C76 using a single-crystal X-ray diffraction analysis. The crystallographic results further revealed that the optimal Th site resided at the intersection of two adjacent pentagons, similar to that of U@C1(17418)-C76. Additionally, the UV-vis-NIR spectra of Th@C1(17418)-C76 exhibited distinct differences compared to the previously reported U@C1(17418)-C76, highlighting the distinctive electronic structure of actinium-based endohedral metallofullerenes (EMFs). The Raman spectrum of Th@C1(17418)-C76 exhibited similarities to that previously reported for thorium-based EMFs, indicating the analogous strong metal–cage interactions of thorium-based EMFs.

(Invited) Synthesis of Ion-Endohedral Fullerenes by Plasma Implantation

Endohedral metallofullerenes (EMFs), which are the hybrid materials consisting of fullerene cage and encapsulated metal(s), are one of the well-studied nanocarbon materials during the past 3 decades.[1] As a result of significant progress in the development of synthetic procedures and purification processes, a variety of different types of EMFs, such as mono-metallofullerenes, di-metallofullerenes and metal-cluster-fullerenes with various carbon-cage sizes, are currently available. We previously reported development of an efficient method to access EMFs, so-called plasma implantation.[2] This approach allowed us to achieve scalable synthesis of lithium-ion-encapsulated [60]fullerene Li+@C60 X−, a stable ionic form of lithium encapsulated fullerene. The compound can be regarded as “ion-endohedral fullerene” consisting of encapsulated ion, neutral fullerene cage and external counter anion X−, and it should not be categorized into general metallofullerenes because of its ion-pair structure. Despite the unique properties arising from its ionic natures, no other ionic metallofullerenes have been well studied so far.[3] For systematic and deeper studies on ion-endohedral fullerenes, we focused on Li+-endohedral [70]fullerene (Li+@C70) because C70 is the second abundant and therefore widely studied fullerene next to C60. The Li+@C70 was synthesized as PF6 − and TFSI− (TFSI=bis(trifluoromethanesulfonyl)imide) salts using our improved plasma implantation method followed by oxidative purification process. 7Li NMR spectrum showed a sharp singlet signal at −32.81 ppm (vs LiCl/D2O standard) indicating that the Li+ was encapsulated in the C70 cage. 13C NMR spectrum showed 5 aromatic peaks, which evidenced the defectless C70 structure. In addition to the NMR signals originated from the Li+@C70 core, 19F and 31P NMR spectra showed clear signals assigned to each external counter anion. The structure of Li+@C70 was unambiguously confirmed by X-ray crystal structure analysis of the TFSI− salt. The crystal structure showed encapsulation of Li+ by C70 cage as well as the 1:1 ion-paired structure of the cationic Li+@C70 core and external TFSI− anion.Moreover, we also performed collision experiments using sodium plasma to synthesize sodium-ion-endohedral [60]fullerene. Laser desorption ionization (LDI) mass spectrum of the crude products showed signals at m/z= 720 and 743 assignable to the molecular ion of empty C60 and the Na+@C60, respectively. The target compound was isolated as TFSI− salt by oxidative purification process followed by electrolyte-added HPLC. There was only a mass peak at m/z= 743 after the purification, which evidenced the complete removal of empty C60. 23Na NMR spectrum showed a sharp singlet signal at −59.6 ppm (vs NaCl/D2O standard), evidenced the formation of the target Na+@C60.

Dynamic Metastable Characteristics of Carbon Cages Embedded with Er2C2.

Novel endohedral metallofullerenes (EMFs), namely, Er2C2@C2v(5)-C80, Er2C2@Cs(6)-C82, Er2C2@Cs(15)-C84, Er2C2@C2v(9)-C86, Er2C2@Cs(15)-C86, and Er2C2@Cs(32)-C88, had been experimentally synthesized, and the unique structures and many fascinating properties had also been widely explored. Nevertheless, the position of the Er atoms inside the cage shows a severe disorder within the stable EMF monomer, which is difficult to understand and explain from the experimental point of view. In this work, based on the density functional theoretical calculations, the Er2C2@Cs(6)-C82 has 73 directional isomers and 2 Er atoms that are far beyond from Er-Er single bonding and tend to be close to the cage side (marked as "shell"), and the core (Er2C2 units) takes on a butterfly shape as generally revealed. The energy difference between any two of the isomers is in the range of 0.05 to 25.6 kcal/mol, indicating a relatively easy thermodynamic transition between the isomers. The other five Er carbide cluster EMFs (Er2C2@C2v(5)-C80, Er2C2@Cs(15)-C84, Er2C2@C2v(9)-C86, Er2C2@Cs(15)-C86, and Er2C2@Cs(32)-C88) are also studied in the same way, and 30, 37, 39, and 43 most stable Er-oriented sites inside the cage, respectively, are obtained. In addition, the shape of the Er2C2 gradually changed from butterfly to linear. Moreover, the electronic structure and molecular orbital analyses show that it is easy for Er2C2@C80-88 to form a charge transfer state of [Er2C2]4+@[C80-88]4- via the dynamic core-shell coordination equilibrium. Er2C2 with a steep drop in chemical stability is restricted to forming varying degrees of metastable states in the shell, determined by the shell size, to ensure the overall stability. The lowest unoccupied molecular orbital energy level of these EMFs is increased by 0.5-1.1 eV compared with fullerenes C80-88, potentially providing favorable conditions for suitable energy level matching with EMF as an electron acceptor used in organic solar cell devices.

Recent Progress on the Functionalization of Endohedral Metallofullerenes

Functionalization of endohedral metallofullerenes (EMFs) plays an important role in exploring the reactivity of EMFs and stabilizing missing EMFs, thus conferring tunable properties and turning EMFs into applicable materials. In this review, we present exhaustive progress on the functionalization of EMFs since 2019. Classic functionalization reactions include Prato reactions, Bingel–Hirsch reactions, radical addition reactions, carbene addition reactions, and so on are summarized. And new complicated multi-component reactions and other creative reactions are presented as well. We also discuss the structural features of derivatives of EMFs and the corresponding reaction mechanisms to understand the reactivity and regioselectivity of EMFs. In the end, we make conclusions and put forward an outlook on the prospect of the functionalization of EMFs.

U@Cs(4)-C82: A Different Cage Isomer with Reactivity Controlled by U-Sumanene Interaction.

Actinide endohedral metallofullerenes (EMFs) are a fullerene family that possess unique actinide-carbon cage host-guest molecular and electronic structures. In this work, a novel actinide EMF, U@Cs(4)-C82, was successfully synthesized and characterized, and its chemical reactivity was investigated. Crystallographic analysis shows that U@Cs(4)-C82, a new isomer of U@C82, has a Cs(4)-C82 cage, which has never been discovered in the form of empty or endohedral fullerenes. Its unique chemical reactivities were further revealed through the Bingel-Hirsch reaction and carbene addition reaction studies. The Bingel-Hirsch reaction of U@Cs(4)-C82 shows exceptionally high selectivity and product yield, yielding only one major addition adduct. Moreover, the addition sites for both reactions are unexpectedly located on adjacent carbon atoms far away from the actinide metal, despite the nucleophilic (Bingel-Hirsch) and electrophilic (carbene addition) nature of either reactant. Density functional theory (DFT) calculations suggest that this chemical behavior, unprecedented for EMFs, is directed by the unusually strong interaction between U and the sumanene motif of the carbon cage in U@Cs(4)-C82, which makes the energy increase when it is disrupted. This work reveals remarkable chemical properties of actinide EMFs originating from their unique electronic structures and highlights the key role of actinide-cage interactions in the determination of their chemical behaviors.

TmCN@C82: Monometallic Clusterfullerene Encapsulating a Tm3+ Ion

Metal cyanide clusterfullerenes (CYCFs) are formed via the encapsulation of a single metal atom and a cyanide unit inside fullerene cages, endowing them with excellent properties in various applications. In this work, we report the synthesis, isolation, and characterizations of the first cases of thulium (Tm)-based CYCFs with the popular C82 carbon cages. The structural elucidation of the two TmCN@C82 isomers was achieved via diverse analytical techniques, including mass spectrometry, Vis-NIR spectroscopy, single-crystal X-ray crystallography, and cyclic voltammetry. The crystallographic analyses unambiguously confirmed the molecular structures of the two TmCN@C82 isomers as TmCN@Cs(6)-C82 and TmCN@C2v(9)-C82. Both TmCN clusters adopt a well-established triangular configuration, with the Tm ion located on the symmetrical plane of the carbon cages. The electronic structures of both TmCN@C82 isomers adopt a Tm3+(CN)−@(C82)2− configuration, exhibiting characteristic spectral and electrochemical properties reminiscent of divalent endohedral metallofullerenes (EMFs). Intriguingly, unlike the divalent Tm2+ ion observed in the mono-metallofullerenes Tm@C2n, a higher oxidation state of Tm3+ is identified in the monometallic TmCN cluster due to bonding with the cyanide anion. This result provides valuable insight into the essential role of the non-metallic endo-units in governing the oxidation state of the metal ion and the electronic behaviors of EMFs.

Electronic communication between transition metal nanoparticle and single atom: Endohedral metallofullerenes single-atom catalysts for oxygen reduction reaction catalysis

Civil-used proton exchange membrane fuel cell (PEMFC) may be the potential candidate for the construction of the global new energy system including the use of hydrogen energy. Nevertheless, the cathode oxygen reduction reaction (ORR) occurs sluggishly, reining the conversion efficiency of it, which hinders the further commercialization of PEMFC. Currently, the replacement of undesirable Pt catalysts by more stable and economical catalytic materials is the main topic in the field of ORR. Herein, ferromagnetic metal-based endohedral metallofullerenes (EMFs) electrocatalyst with single-atomic shell decoration (M4@MN4C60, M = Fe, Co, Ni) are theoretically constructed and evaluated by the density functional theory method. The encapsulation energy calculation reveals that the synthesis of all EMFs requires additional external energy supply, which can explain why few successfully experimental synesis of EMFs have been reported. Among all studied catalysts, the CoN4C60 stands out with the highest ORR performance comparable to Pt, and the Fe- and Ni- based EMFs shows considerable activity enhancement after the cluster encapsulation. The electronic structure calculation reveals that the encapsulated Fe4 cluster leads to an electron accumulation around the Fe atom in active center, thereby weakening the *OH binding and enhance the ORR performance.

Two Metastable Endohedral Metallofullerenes Sc2C2@C1(39656)-C82 and Sc2C2@C1(51383)-C84: Direct-C2-Insertion Products from Their Most Stable Precursors.

Endohedral metallofullerenes (EMFs) are sub-nano carbon materials with diverse applications, yet their formation mechanism, particularly for metastable isomers, remains ambiguous. The current theoretical methods focus mainly on the most stable isomers, leading to limited predictability of metastable ones due to their low stabilities and yields. Herein, we report the successful isolation and characterization of two metastable EMFs, Sc2C2@C1(39656)-C82 and Sc2C2@C1(51383)-C84, which violate the isolated pentagon rule (IPR). These two non-IPR EMFs exhibit a rare case of planar and pennant-like Sc2C2 clusters, which can be considered hybrids of the common butterfly-shaped and linear configurations. More importantly, the theoretical results reveal that despite being metastable, these two non-IPR EMFs survived as the products from their most stable precursors, Sc2C2@C2v(5)-C80 and Sc2C2@Cs(6)-C82, via a C2 insertion during the post-formation annealing stages. We propose a systematic theoretical method for predicting metastable EMFs during the post-formation stages. The unambiguous molecular-level structural evidence, combined with the theoretical calculation results, provides valuable insights into the formation mechanisms of EMFs, shedding light on the potential of post-formation mechanisms as a promising approach for EMF synthesis.

Lu-Lu Bond in Lu2@C60 Metallofullerenes

This study on Lu2@C60 isomers provides insights into the metal–metal bond through the confinement effect of fullerene cages. Density functional theory calculations were used to study the nature of the Lu-Lu bond in two stable endohedral metallofullerenes (EMFs), Lu2@C2v_C60 and Lu2@Ih_C60, both with negative endohedral energy. These two isomers are geometrically connected through a simple Stone–Wales (SW) transformation. The electronic configuration of (Lu2)4+@C604− was also confirmed, leading to the formation of a two-center two-electron (2c–2e) Lu-Lu σ single bond. By comparing the Lu-Lu bonds in Lu2@C60 with those in acknowledged Lu2@C2n, the smaller C60 fullerene compressed the geometry of Lu2 resulting in a much shorter Lu-Lu bond length. However, the Lu-Lu bond strength is slightly weaker in Lu2@C60 than that in large fullerenes, as the Lu-Lu bond in C60 is likely a p-p σ bond with an above the 40% contribution of p orbital and a strong metal–cage interaction. Additionally, the vis-NIR spectra of Lu2@C2v_C60 and Lu2@Ih_C60 were simulated, which could provide valuable information for future experimental studies on Lu-based EMFs.

High-pressure-induced electronic and structural transition of superatoms.

High pressure has been recognized as an important tool in molecule and materials research, and thus, it is expected to be used to understand the evolution of electronic states and geometric structures in superatoms. In this work, by studying three characteristic axial compressions on a typical endohedral metallofullerene superatom U@C28 with Td symmetry, we find that the triplet ground electronic state is preserved when the compression moves along the direction that reduces the symmetry to D2d, but the electronic state of the structure compressed along the direction of symmetry reduction to C2v or Cs is transformed into a singlet. The transition is attributed to the distinction in the response of electron spin to different axial compressions, which results in a change in the electron occupation mode of the system. Furthermore, we also confirm the gradual evolution from stereo to near-plane superatoms and the connection between their electron structures. This is reflected in the fact that the electron density distributions of the superatomic molecular orbitals (SAMOs) with extension along the restricted degrees of freedom (Dz2, Fz3 SAMOs) gradually contract, and the delocalization destruction of special orbitals is associated with this freedom. In addition, Raman and ultraviolet-visible spectra show a hyperchromic effect and redshift of characteristic peaks during axial compression, which are expected to be used for fingerprinting the superatomic planarization. Therefore, our work provides new insights based on high pressure for future research toward the discovery of physical properties and applications of superatoms.

Recent Advances in Functionalized Fullerenes in Perovskite Solar Cells†

Comprehensive SummaryEfficient charge transport and defect passivation are essential for high efficiency of organic–inorganic hybrid perovskite solar cells (PSCs). Functionalized fullerenes featuring high electron affinity and mobility as well as small reorganization energy have been extensively applied in PSCs toward facilitated electron transport and passivated trap states, leading to improvements of both device efficiency and stability. Herein, we summarize the recent advances, especially in the last three years, in applications of functionalized fullerenes including fullerene derivatives and endohedral metallofullerenes in PSCs. Their functions in trap state passivation, electron transport promotion, crystalline modulation, water/oxygen erosion inhibition, and so on, are discussed in details. In particular, we emphasize novel functions of fullerenes beyond trap state passivation, as well as the synergy of multifunction of fullerenes in improving PSC device performance and stability. Finally, we present an outlook on designing novel multifunctionalized fullerenes toward highly efficient and stable PSC devices.

Influence of Charge Transfer on Thermoelectric Properties of Endohedral Metallofullerene (EMF) Complexes

A considerable potential advantage of manufacturing electric and thermoelectric devices using endohedral metallofullerenes (EMFs) is their ability to accommodate metallic moieties inside their cavities. Published experimental and theoretical works have explained the usefulness of this resilience feature for improving the electrical conductance and thermopower. Through thorough theoretical investigations of three EMF complexes employing three different metallic moieties involving Sc3C2, Sc3N, and Er3N and their configurations on a gold (111) surface, this research demonstrates that the thermoelectric properties of these molecular complexes can be tuned by taking advantage of the charge transfer from metallic moieties to Ih-C80 cages. Mulliken, Hirshfeld, and Voronoi simulations articulate that the charge migrates from metallic moieties to cages; however, the amount of the transferred charge depends on the nature of the moiety within the complex.

20-State Molecular Switch in a Li@C60 Complex.

A substantial potential advantage of industrial electric and thermoelectric devices utilizing endohedral metallofullerenes (EMFs) is their ability to accommodate metallic moieties inside their empty cavities. Experimental and theoretical studies have elucidated the merit of this extraordinary feature with respect to developing electrical conductance and thermopower. Published research studies have demonstrated multiple state molecular switches initiated with 4, 6, and 14 distinguished switching states. Through comprehensive theoretical investigations involving electronic structure and electric transport, we report 20 molecular switching states that can be statistically recognized employing the endohedral fullerene Li@C60 complex. We propose a switching technique that counts on the location of the alkali metal that encapsulates inside a fullerene cage. The 20 switching states correspond to the 20 hexagonal rings that the Li cation energetically prefers to reside close to. We demonstrate that the multiswitching feature of such molecular complexes can be controlled by taking advantage of the off-center displacement and charge transfer from the alkali metal to the C60 cage. The most energetically favorable optimization suggests 1.2-1.4 Å off-center displacement, and Mulliken, Hirshfeld, and Voronoi simulations articulate that the charge migrates from the Li cation to C60 fullerene; however, the amount of the charge transferred depends on the nature and location of the cation within the complex. We believe that the proposed work suggests a relevant step toward the practical application of molecular switches in organic materials.

Advances in Regioselective Functionalization of Endohedral Metallofullerenes†

Comprehensive SummaryEndohedral metallofullerenes (EMFs) are a family of molecules with metal clusters or ions encapsulated inside a fullerene cage. Their unique structures provide a powerful interface for organic chemists to manipulate metal elements. Over a decade ago, many reactions on empty fullerenes were adopted for EMFs, resulting in a series of reactions that are well‐established and reviewed in the literature. In recent years, new topics have emerged in EMF chemistry. First, new addition reactions were developed, notably some that are not based on known empty‐fullerene reactions. Second, considerable progress has been made on regioselective multi‐additions on EMFs. Finally, EMFs have been covalently conjugated with fullerene C60 to form functional materials. This Recent Advances article will focus on these new developments and discuss their implication for future directions of EMFs.

Rational Design, Stabilities and Nonlinear Optical Properties of Non-Conventional Transition Metalides; New Entry into Nonlinear Optical Materials.

Electronic and nonlinear optical properties of endohedral metallofullerenes are presented. The endohedral metallofullerenes contain transition metal encapsulated in inorganic fullerenes X12Y12 (X = B, Al & Y = N, P). The endohedral metallofullerenes (endo-TM@X12Y12) possess quite interesting geometric and electronic properties, which are the function of the nature of the atom and the size of fullerene. NBO charge and frontier molecular orbital analyses reveal that the transition metal encapsulated Al12N12 fullerenes (endo-TM@Al12N12) are true metalides when the transition metals are Ni, Cu and Zn. Endo-Cr@Al12N12 and endo-Co@Al12N12 are at the borderline between metalides and electrides with predominantly electride characteristics. The other members of the series are excess electron systems, which offer interesting electronic and nonlinear optical properties. The diversity of nature possessed by endo-TM@Al12N12 is not prevalent for other fullerenes. Endo-TM@Al12P12 are true metalides when the transition metals are (Cr-Zn). HOMO-LUMO gaps (EH-L) are reduced significantly for these endohedral metallofullerenes, with a maximum percent decrease in EH-L of up to 70%. Many complexes show odd-even oscillating behavior for EH-L and dipole moments. Odd electron species contain large dipole moments and small EH-L, whereas even electron systems have the opposite behavior. Despite the decrease in EH-L, these systems show high kinetic and thermodynamic stabilities. The encapsulation of transition metals is a highly exergonic process. These endo-TM@X12Y12 possess remarkable nonlinear optical response in which the first hyperpolarizability reaches up to 2.79 × 105 au for endo-V@Al12N12. This study helps in the comparative analysis of the potential nonlinear optical responses of electrides, metalides and other excess electron systems. In general, the potential nonlinear optical response of electrides is higher than metalides but lower than those of simple excess electron compounds. The higher non-linear optical response and interesting electronic characteristics of endo-TM@Al12N12 complexes may be promising contenders for potential NLO applications.

Effects of Solvents on Reaction Products: Synthesis of Endohedral Metallofullerene Oxazoline and Epoxide

Herein, we performed the reactions of M3N@Ih-C80 (M = Sc and Lu) with the methanol (CH3OH) solution of TBAOH (note that both CH3O- and OH- are nucleophiles) in benzonitrile (PhCN) and dimethylformamide, respectively. It is found that OH- ions rather than CH3O- ions selectively attacked the fullerene cage to form the M3N@C80--O- intermediate. Although the fullerene cage is initially attacked by OH- in both PhCN and DMF solvents, the products are quite different. In PhCN, two isomeric Sc3N@Ih-C80 fullerooxazoline heterocyclic products (1 and 2) were synthesized. Whereas, in DMF, an epoxide of Lu3N@Ih-C80 (3) was obtained. The preference for fullerooxazoline formation over that of fullerene epoxy in PhCN is well explained by density functional theory calculations. Plausible reaction mechanisms for the formation of metallofullerene oxazoline and epoxide were proposed based on the experimental and theoretical results.

Smallest Endohedral Metallofullerenes [Mg@C20]n (n = 4, 2, 0, −2, and −4): Endo-Ionic Interaction in Superatoms

This work reports a series of endohedral metallofullerene superatoms [Mg@C20]n, where n = 4, 2, 0, -2, and -4. It was found that Mg transfers virtually all of its 3s electrons to the C20 shell, resulting in the ionic states of Mg2+@[C20]n-2. Detailed calculations revealed that the superatomic electronic configuration of these clusters is 1S21P61D101F4-n. The first nine superatomic molecular orbitals (SAMOs), 1S21P61D10, housed with 18 electrons, are largely based on [C20]n-2 with small contributions from magnesium, while the outmost SAMOs, 1F4-n, with 4 - n extra electrons, reside solely on [C20]n-2. The interaction between the Mg2+ ion and [C20]n-2 was found to be predominately ionic in character. Furthermore, ultraviolet-visible spectra provide a theoretical basis for fingerprinting these clusters. It is hoped that this work will encourage the synthetic pursuit of these smallest superatomic systems.