Hiding and recovering electrons in a dimetallic endohedral fullerene: air-stable products from radical additions

Capturing Metal Fluoride inside a Carbon Cage

We report here a new type of metal fluoride cluster that can be stabilized inside fullerene via in situ fluorine encapsulation followed by exohedral trifluoromethylation, giving rise to rare-earth metal fluoride clusterfullerenes (FCFs) M2F@C80(CF3) (M = Gd and Y). The molecular structure of Gd2F@C80(CF3) was unambiguously determined by single-crystal X-ray analysis to show a μ2-fluoride-bridged Gd-F-Gd cluster with short Gd-F bonds of 2.132(7) and 2.179(7) Å. The 19F NMR spectrum of the diamagnetic Y2F@C80(CF3) confirms the existence of the endohedral F atom, which exhibits a triplet with a large 19F-89Y coupling constant of 74 Hz and a high temperature sensitivity of the 19F chemical shift of 0.057 ppm/K. Theoretical studies reveal the ionic Y-F bonding nature arising from the highest electronegativity of the F element and an electronic configuration of [Y2F]5+@[C80]5- with an open-shell carbon cage, which thus necessitates the stabilization of FCFs by exohedral trifluoromethylation.

Endohedral metallofullerene molecular nanomagnets

Magnetic lanthanide (Ln) metal complexes exhibiting magnetic bistability can behave as molecular nanomagnets, also known as single-molecule magnets (SMMs), suitable for storing magnetic information at the molecular level, thus attracting extensive interest in the quest for high-density information storage and quantum information technologies. Upon encapsulating Ln ion(s) into fullerene cages, endohedral metallofullerenes (EMFs) have been proven as a promising and versatile platform to realize chemically robust SMMs, in which the magnetic properties are able to be readily tailored by altering the configurations of the encapsulated species and the host cages. In this review, we present critical discussions on the molecular structures and magnetic characterizations of EMF-SMMs, with the focus on their peculiar molecular and electronic structures and on the intriguing molecular magnetism arising from such structural uniqueness. In this context, different families of magnetic EMFs are summarized, including mononuclear EMF-SMMs wherein single-ion anisotropy is decisive, dinuclear clusterfullerenes whose magnetism is governed by intramolecular magnetic interaction, and radical-bridged dimetallic EMFs with high-spin ground states that arise from the strong ferromagnetic coupling. We then discuss how molecular assemblies of SMMs can be constructed, in a way that the original SMM behavior is either retained or altered in a controlled manner, thanks to the chemical robustness of EMFs. Finally, on the basis of understanding the structure-magnetic property correlation, we propose design strategies for high-performance EMF-SMMs by engineering ligand fields, electronic structures, magnetic interactions, and molecular vibrations that can couple to the spin states.

Nd─Nd Bond in Ih and D5h Cage Isomers of Nd2 @C80 Stabilized by Electrophilic CF3 Addition

Synthesis of molecular compounds with metal-metal bonds between 4f elements is recognized as one of the fascinating milestones in lanthanide metallochemistry. The main focus of such studies is on heavy lanthanides due to the interest in their magnetism, while bonding between light lanthanides remains unexplored. In this work, the Nd─Nd bonding in Nd-dimetallofullerenes as a case study of metal-metal bonding between early lanthanides is demonstrated. Combined experimental and computational study proves that pristine Nd2 @C80 has an open shell structure with a single electron occupying the Nd─Nd bonding orbital. Nd2 @C80 is stabilized by a one-electron reduction and further by the electrophilic CF3 addition to [Nd2 @C80 ]- . Single-crystal X-ray diffraction reveals the formation of two Nd2 @C80 (CF3 ) isomers with D5h -C80 and Ih -C80 carbon cages, both featuring a single-electron Nd─Nd bond with the length of 3.78-3.79 Å. The mutual influence of the exohedral CF3 group and endohedral metal dimer in determining the molecular structure of the adducts is analyzed. Unlike Tb or Dy analogs, which are strong single-molecule magnets with high blocking temperature of magnetization, the slow relaxation of magnetization in Nd2 @Ih -C80 (CF3 ) is detectable via out-of-phase magnetic susceptibility only below 3 K and in the presence of magnetic field.

Element effects in endohedral metal-metal-bonding fullerenes M2@C82 (M = Sc, Y, La, Lu)

Endohedral metal-metal-bonding fullerenes have recently emerged, in which encapsulated metals form a metal-metal bond. However, the physical reasons why some metal elements prefer to form metal-metal bonds inside fullerene are still unclear. Herein, we reported first-principles calculations on electronic structures, bonding properties, dynamics, and thermodynamic stabilities of endohedral metallofullerenes M2@C82 (M = Sc, Y, La, Lu). Multiple bonding analysis approaches unambiguously reveal the existence of one two-center two-electron σ covalent metal-metal bond in M2@C82 (M = Sc, Y, Lu); however, the La-La bonding interaction in La2@C82 is weaker and could not be categorized as one metal-metal covalent bond. The energy decomposition analysis on bonding interactions between an encapsulated metal dimer and fullerene cages suggested that there exist two electron-sharing bonds between a metal dimer and fullerene cages. The reasons why La2 prefers to donate electrons to fullerene cages rather than form a standard σ covalent metal-metal bond are mainly attributed to two following facts: La2 has a lower ionization potential, while the hybridization of ns, (n - 1)d, and np atomic orbitals in La2 is higher. Ab initio molecular dynamic simulations reveal that the M-M bond length at room temperature follows the trend of Sc < Lu < Y. The statistical thermodynamics calculations at different temperatures reveal that the experimentally observed endohedral metal-metal-bonding fullerenes M2@C82 have high concentrations in the endohedral fullerene formation temperature range.

Synthesis and Crystallographic Characterization of a Reduced Bimetallic Yttrium ansa-Metallocene Hydride Complex, [K(crypt)][(μ-CpAn)Y(μ-H)]2 (CpAn = Me2Si[C5H3(SiMe3)-3]2), with a 3.4 Å Yttrium-Yttrium Distance

The reduction of a bimetallic yttrium ansa-metallocene hydride was examined to explore the possible formation of Y-Y bonds with 4d¹ Y(II) ions. The precursor [Cp^AnY(μ-H)(THF)]₂ (Cp^An = Me₂Si[C₅H₃(SiMe₃)-3]₂) was synthesized by hydrogenolysis of the allyl complex Cp^AnY(η³-C₃H₅)(THF), which was prepared from (C₃H₅)MgCl and [Cp^AnY(μ-Cl)]₂. Treatment of [Cp^AnY(μ-H)(THF)]₂ with excess KC₈ in the presence of one equivalent of 2.2.2-cryptand (crypt) generates an intensely colored red-brown product crystallographically identified as [K(crypt)][(μ-Cp^An)Y(μ-H)]₂. The two rings of each Cp^An ligand in the reduced anion [(μ-Cp^An)Y(μ-H)]₂^1- are attached to two yttrium centers in a "flyover" configuration. The 3.3992(6) and 3.4022(7) Å Y···Y distances between the equivalent metal centers within two crystallographically independent complexes are the shortest Y···Y distances observed to date. Ultraviolet-visible (UV-visible)/near infrared (IR) and electron paramagnetic resonance (EPR) spectroscopy support the presence of Y(II), and theoretical analysis describes the singly occupied molecular orbital (SOMO) as an Y-Y bonding orbital composed of metal 4d orbitals mixed with metallocene ligand orbitals. A dysprosium analogue, [K(18-crown-6)(THF)₂][(μ-Cp^An)Dy(μ-H)]₂, was also synthesized, crystallographically characterized, and studied by variable temperature magnetic susceptibility. The magnetic data are best modeled with the presence of one 4f⁹ Dy(III) center and one 4f⁹(5d_z²)¹ Dy(II) center with no coupling between them. CASSCF calculations are consistent with magnetic measurements supporting the absence of coupling between the Dy centers.

Regulating the electronic and spin structure of endohedral metallofullerenes: a case investigation of Sc3N@C80 and Sc3C2@C80

The electrochemical and paramagnetic properties of endohedral metallofullerenes (EMFs) have drawn extensive attention due to their huge potential in the fields of molecular devices, biomedicines, quantum information processing, etc. Exohedral modification of the fullerene carbon cage, such as in the classical Prato reaction, is an effective and facile approach to regulate the electronic structure and molecular dynamics of EMFs. In this work, novel pyrrolidine products of Sc₃N@C₈₀ and Sc₃C₂@C₈₀ were successfully synthesized via Prato reactions using L-cysteine and paraformaldehyde. Structure characterizations demonstrated that two regioisomers with a [5,6] and a [6,6] cycloaddition on the I_h-C₈₀ cage were obtained both for Sc₃N@C₈₀ and Sc₃C₂@C₈₀. Besides, the [6,6]-monoadduct of Sc₃N@C₈₀ was thermally stable while the [5,6]-monoadduct exhibited a retro-cycloaddition ability to recover the pristine Sc₃N@C₈₀. Electrochemical measurements revealed that the redox potential of Sc₃N@C₈₀ could be tuned via such exohedral modifications. Furthermore, the paramagnetic property and internal dynamics of the encapsulated Sc₃C₂ cluster of Sc₃C₂@C₈₀ can be well-regulated by controlling the spin density of the molecule. The present work could provide a new approach to regulate the electronic and/or spin structure of EMFs.

Electrophilic Trifluoromethylation of Dimetallofullerene Anions en Route to Air-Stable Single-Molecule Magnets with High Blocking Temperature of Magnetization

Lanthanide dimetallofullerenes with single-electron M-M bonds are an important class of single molecular magnets and qubit candidates, but stabilization of their unique electronic and spin structure in the form of a neutral molecule requires functionalization of the fullerene cage with a single radical group. The lack of selectivity of the currently available procedure results in a complicated and tedious separation process. Here we demonstrate that electrophilic trifluoromethylation of a mixture of metallofullerene anions with Umemoto reagent II is highly selective toward M₂@C₈₀^- (M = Tb, Y) anions, yielding M₂@C₈₀(CF₃) monoadducts as the main reaction product. Single-crystal X-ray diffraction study proved attachment of the CF₃ group to the pentagon/hexagon/hexagon junction and revealed that positions of metal atoms inside the fullerene cage in the cocrystal with NiOEP are strongly related to the position of the porphyrin moieties. Magnetic characterization of Tb₂@C₈₀(CF₃) showed that it is a robust single-molecule magnet with broad magnetic hysteresis, 100 s blocking temperature of 25 K, and the relaxation barrier of 801(4) K, corresponding to the flipping of the Tb magnetic moment in the strongly ferromagnetically coupled [Tb³⁺-e-Tb³⁺] spin system.

Temperature-dependent dynamics of endohedral fullerene Sc2@C80(CH2Ph) studied by EPR spectroscopy

Endohedral fullerenes are promising materials for the quantum information and quantum processing due to the unique properties of the electron-nuclear spin system well isolated from the environment inside the fullerene cage. The endofullerene Sc₂@C₈₀(CH₂Ph) features a strong hyperfine interaction between one electron spin 1/2 localized at the Sc₂ dimer and two equivalent ⁴⁵Sc nuclear spins 7/2, which yields 64 well resolved EPR transitions. We report a comprehensive analysis of the temperature dependence of the EPR spectrum of Sc₂@C₈₀(CH₂Ph) dissolved in d-toluene measured in a wide temperature range above and below the melting point. The nature of the electron spin coherence phase memory is investigated. The properties of all resonance lines in a liquid phase were treated within the model of the free rotational diffusion. Both, analytical expressions and numerical examination provide an excellent agreement between the experimental and simulated spectra. A detailed study of the experimental data confirms the assumption of the independent motions of the fullerene cage and the Sc₂ core. The data obtained show three regimes of molecular motion detected at different temperatures: the free rotation of both the fullerene cage and its bi-metal core, the motion of the core in the frozen fullerene cage, and, finally, a state with a fixed structure of both parts of the metallofullerene molecules. The data analysis reveals a significant nuclear quadrupole interaction playing an important role for the mixing of the different nuclear spin multiplets.

Th@D5h(6)-C80: a highly symmetric fullerene cage stabilized by a single metal ion

A novel endohedral metallofullerene (mono-EMF), Th@D5h(6)-C80, has been successfully synthesized and fully characterized by mass spectrometry, single crystal X-ray diffraction, UV-vis-NIR and Raman spectroscopy and cyclic voltammetry. Single crystal XRD analysis unambiguously assigned the fullerene cage as D5h(6)-C80, the first example in which the highly symmetric cage is stabilized by a single metal ion. The combined experimental and theoretical studies further reveal that the D5h(6)-C80 cage, known only for its stabilization by 6-electron transfer, is stabilized by the 4-electron transfer from the encapsulated Th ion for the first time.

Metal-encapsulation induces a highly regioselective Bingel-Hirsch reaction of the labile Y@Cs(6)-C82

The chemical properties of a prototypical labile mono-EMF, Y@Cs(6)-C82, have been systematically disclosed for the first time via a Bingel-Hirsch reaction. Three mono-adduct isomers, namely, 2a, 2b and 2c out of 44 possibilities for the Y@Cs(6)-C82 cage have been readily isolated, demonstrating surprisingly high regioselectivity. Crystallographic results of 2b unambiguously confirm its molecular structure with a singly bonded bromomalonate group attached onto the Cs(6)-C82 cage. Further computational results rationalize that the high regioselectivity is a consequence of the localization of high spin density and large frontier molecular orbital distribution on the corresponding carbon atoms stemming from the encapsulation of an yttrium atom into the low-symmetry Cs(6)-C82 cage with three-electron transfer from the metal to the cage.

Unusually large hyperfine structure of the electron spin levels in an endohedral dimetallofullerene and its spin coherent properties

We report the synthesis, ESR spectroscopic and spin coherent properties of the dimetallofullerene Sc2@C80(CH2Ph). The single-electron metal-metal bond of the Sc2 dimer inside the fullerene's cage is stabilized with the electron spin density being fully localized at the metal bond. This results in an extraordinary strong hyperfine interaction of the electron spin with the 45Sc nuclear spins with a coupling constant a = 18.2 mT (∼510 MHz) and yields a fully resolved hyperfine-split ESR spectrum comprising 64 lines. The splitting is present even at low temperatures where the molecular dynamics are completely frozen. The large extent and the robustness of the hyperfine-split spectra enable us to identify and control the well-defined transitions between specific electron-nuclear quantum states. This made it possible to demonstrate in our pulse ESR study the remarkable spin coherent dynamics of Sc2@C80(CH2Ph), such as the generation of arbitrary superpositions of the spin states in a nutation experiment and the spin dephasing times above 10 μs at temperatures T < 80 K reaching the value of 17 μs at T ≤ 20 K. These observations suggest Sc2@C80(CH2Ph) as an interesting qubit candidate and motivate further synthetic efforts to obtain fullerene-based systems with superior spin properties.

Improving Fatigue Resistance of Dihydropyrene by Encapsulation within a Coordination Cage

Photochromic molecules undergo reversible isomerization upon irradiation with light at different wavelengths, a process that can alter their physical and chemical properties. For instance, dihydropyrene (DHP) is a deep-colored compound that isomerizes to light-brown cyclophanediene (CPD) upon irradiation with visible light. CPD can then isomerize back to DHP upon irradiation with UV light or thermally in the dark. Conversion between DHP and CPD is thought to proceed via a biradical intermediate; bimolecular events involving this unstable intermediate thus result in rapid decomposition and poor cycling performance. Here, we show that the reversible isomerization of DHP can be stabilized upon confinement within a Pd^II₆L₄ coordination cage. By protecting this reactive intermediate using the cage, each isomerization reaction proceeds to higher yield, which significantly decreases the fatigue experienced by the system upon repeated photocycling. Although molecular confinement is known to help stabilize reactive species, this effect is not typically employed to protect reactive intermediates and thus improve reaction yields. We envisage that performing reactions under confinement will not only improve the cyclic performance of photochromic molecules, but may also increase the amount of product obtainable from traditionally low-yielding organic reactions.

New Horizons in Chemical Functionalization of Endohedral Metallofullerenes

This overview explains some new aspects of chemical functionalization of endohedral metallofullerenes (EMFs) that have been unveiled in recent years. After differences in chemical reactivity between EMFs and the corresponding empty fullerenes are discussed, cage-opening reactions of EMFs are examined. Then, the selective bisfunctionalization of EMFs is explained. Finally, single-bonding derivatization of EMFs is addressed. The diversity and applicability of the chemical functionalization of endohedral metallofullerenes are presented to readers worldwide.

Endohedral Metallofullerenes: New Structures and Unseen Phenomena

Endohedral metallofullerenes (EMFs), namely fullerenes with metallic species encapsulated inside, represent an ideal platform to investigate metal-metal or metal-carbon interactions at the sub-nanometer scale by means of single-crystal X-ray diffraction (XRD) crystallography. Herein, recent progress in the identification of new structures and unprecedented properties are discussed according to the categories of monometallofullerenes, dimetallofullerenes, carbide clusterfullerenes, and nitride clusterfullerenes. In particular, the dimerization and the cage-isomer dependent oxidation state of the inner metal atom are summarized in terms of pristine monometallofullerenes. Metal-metal bonds involving lanthanide-lanthanides or actinide-actinides are discussed based on both experimental and theoretical studies. The cluster-cage matching and/or mutual selections, as well as the rarely seen M=C double bonds, are discovered in M₂ C₂ @C_2n , U₂ C@C₈₀ , M₂ TiC@C₈₀ , and Ti₃ C₃ @C₈₀ . Subsequently, the geometries of different M₃ N clusters in various cages are discussed, revealing size-matching between the internal M₃ N cluster and the outer cage induced by the planarity of the cluster. Finally, an outlook regarding the future developments of the molecular structures and applications of EMFs is presented.

Single-Electron Lanthanide-Lanthanide Bonds Inside Fullerenes toward Robust Redox-Active Molecular Magnets

A characteristic phenomenon of lanthanide-fullerene interactions is the transfer of metal valence electrons to the carbon cage. With early lanthanides such as La, a complete transfer of six valence electrons takes place for the metal dimers encapsulated in the fullerene cage. However, the low energy of the σ-type Ln-Ln bonding orbital in the second half of the lanthanide row limits the Ln₂ → fullerene transfer to only five electrons. One electron remains in the Ln-Ln bonding orbital, whereas the fullerene cage with a formal charge of -5 is left electron-deficient. Such Ln₂@C₈₀ molecules are unstable in the neutral form but can be stabilized by substitution of one carbon atom by nitrogen to give azafullerenes Ln₂@C₇₉N or by quenching the unpaired electron on the fullerene cage by reacting it with a chemical such as benzyl bromide, transforming one sp² carbon into an sp³ carbon and yielding the monoadduct Ln₂@C₈₀(CH₂Ph). Because of the presence of the Ln-Ln bonding molecular orbital with one electron, the Ln₂@C₇₉N and Ln₂@C₈₀(R) molecules feature a unique single-electron Ln-Ln bond and an unconventional +2.5 oxidation state of the lanthanides. In this Account, which brings together metallofullerenes, molecular magnets, and lanthanides in unconventional valence states, we review the progress in the studies of dimetallofullerenes with single-electron Ln-Ln bonds and highlight the consequences of the unpaired electron residing in the Ln-Ln bonding orbital for the magnetic interactions between Ln ions. Usually, Ln···Ln exchange coupling in polynuclear lanthanide compounds is weak because of the core nature of 4f electrons. However, when interactions between Ln centers are mediated by a radical bridge, stronger coupling may be achieved because of the diffuse nature of radical-based orbitals. Ultimately, when the role of a radical bridge is played by a single unpaired electron in the Ln-Ln bonding orbital, the strength of the exchange coupling is increased dramatically. Giant exchange coupling in endohedral Ln₂ dimers is combined with a rather strong axial ligand field exerted on the lanthanide ions by the fullerene cage and the excess electron density localized between two Ln ions. As a result, Ln₂@C₇₉N and Ln₂@C₈₀(CH₂Ph) compounds exhibit slow relaxation of magnetization and exceptionally high blocking temperatures for Ln = Dy and Tb. At low temperatures, the [Ln³⁺-e-Ln³⁺] fragment behaves as a single giant spin. Furthermore, the Ln-Ln bonding orbital in dimetallofullerenes is redox-active, which allows its population to be changed by electrochemical reactions, thus changing the magnetic properties because the change in the number of electrons residing in the Ln-Ln orbital affects the magnetic structure of the molecule.

Crystallographic and Theoretical Investigations of Er2 @C2 n (2 n=82, 84, 86): Indication of Distance-Dependent Metal-Metal Bonding Nature

Successful isolation and characterization of a series of Er-based dimetallofullerenes present valuable insights into the realm of metal-metal bonding. These species are crystallographically identified as Er₂ @C_s (6)-C₈₂ , Er₂ @C_3v (8)-C₈₂ , Er₂ @C₁ (12)-C₈₄ , and Er₂ @C_2v (9)-C₈₆ , in which the structure of the C₁ (12)-C₈₄ cage is unambiguously characterized for the first time by single-crystal X-ray diffraction. Interestingly, natural bond orbital analysis demonstrates that the two Er atoms in Er₂ @C_s (6)-C₈₂ , Er₂ @C_3v (8)-C₈₂ , and Er₂ @C_2v (9)-C₈₆ form a two-electron-two-center Er-Er bond. However, for Er₂ @C₁ (12)-C₈₄ , with the longest Er⋅⋅⋅Er distance, a one-electron-two-center Er-Er bond may exist. Thus, the difference in the Er⋅⋅⋅Er separation indicates distinct metal bonding natures, suggesting a distance-dependent bonding behavior for the internal dimetallic cluster. Additionally, electrochemical studies suggest that Er₂ @C_82-86 are good electron donors instead of electron acceptors. Hence, this finding initiates a connection between metal-metal bonding chemistry and fullerene chemistry.

Fullerenes as Nanocontainers That Stabilize Unique Actinide Species Inside: Structures, Formation, and Reactivity

Fullerene carbon cages can encapsulate a wide variety of atoms, ions, clusters, or small molecules inside, resulting in stable compounds with unusual structures and electronic properties. These compounds are collectively defined as endohedral fullerenes. The most studied endohedral fullerenes are those containing metal atoms or ions inside, and these are referred to as endohedral metallofullerenes (EMFs). For EMFs, the inner isolated space of the fullerene cages can lead to the stabilization of unique clusters, which are otherwise not synthetically accessible. This offers an excellent environment and opportunity for investigating the nature of previously unobserved metal-metal, metal-non-metal, and metal-fullerene interactions, which are of fundamental interest and importance. Up until now, most of the work in this field has been mainly focused on the rare-earth metals and related elements (groups II, III, and IV). The encapsulation of other elements of the periodic table could potentially lead to totally new structures and bonding motifs and to material properties beyond those of the existing EMFs. Actinides were originally explored as encapsulated elements in fullerenes when Smalley et al. ( Science 1992 , 257 , 1661 ) reported mass spectral evidence of actinide endohedral fullerenes back in 1992. However, the full characterization of these actinide endohedral fullerenes, including single crystal X-ray diffractometric analyses, was not reported until very recently, in 2017. In this Account, we highlight some recent advances made in the field of EMF compounds, focusing primarily on the molecular and electronic structures of novel actinide-based EMFs, new evidence for the formation mechanisms of EMFs, and the influence of the entrapped species on the reactivity and regiochemistry of EMF compounds. We recently reported that some monometallic actinide EMFs represent the first examples of tetravalent metals encapsulated inside fullerenes that exhibit considerably stronger host-guest interactions when compared to those observed for the lanthanide EMFs. These unusually strong metal-cage interactions, along with very high mobilities of the actinides inside the fullerene cages at high temperatures, result in the stabilization of unexpected non-IPR (isolated pentagon rule) fullerene cages encapsulating only one metal ion. Strikingly, such covalent stabilization factors had never been previously observed, although Sm@C_2v(19138)-C₇₆ was the first reported mono-EMF with a non-IPR cage, see details below. In addition, we showed that a long sought-after actinide-actinide bond was obtained upon encapsulation of U₂ inside an I_h(7)-C₈₀ fullerene cage. More interestingly, we demonstrated that actinide multiple bonds, which are very difficult to prepare by conventional synthetic methods, are stabilized when trapped inside fullerene cages. A totally unexpected and previously unreported uranium carbide cluster, U═C═U, was fully characterized inside an EMF, U₂C@I_h(7)-C₈₀, which, for the first time, clearly exhibits two unsupported axial U═C double bonds that are ∼2.03 Å long. We also discovered that synthetic bis-porphyrin nanocapsules exhibit exquisitely selective complexation of some of these uranium endohedral compounds, providing the basis for a nonchromatographic EMF purification method for actinide EMFs. Regarding EMF formation mechanisms, we suggested that novel carbide EMF structures, that is, Sc₂C₂@C_s(hept)-C₈₈, are likely key intermediates in a bottom-up fullerene growth process. Additionally, the structural correlation between chiral carbon cages during a bottom-up growth process was shown to be enantiomer-dependent. The influence of the encapsulated clusters on the chemical reactivity of EMFs is discussed at the end, which showed that the regioselectivities of multiple additions to the fullerene cages are remarkably controlled by the encapsulated metal clusters.

Crystallographic characterization of Y2C2n (2n = 82, 88–94): direct Y–Y bonding and cage-dependent cluster evolution

The long-sought Y–Y bonding is experimentally observed in organometallic complexes for the first time by encapsulation inside the hollow cavity of C_3v(8)-C₈₂ and C_s(6)-C₈₂ fullerene cages.

Actinide-transition metal bonding in heterobimetallic uranium- and thorium-molybdenum paddlewheel complexes

We report the preparation of four heterobimetallic uranium- and thorium-molybdenum paddlewheel complexes. The characterisation data suggest the presence of Mo → An σ-interactions in all cases. These complexes represent unprecedented actinide-group 6 metal-metal bonds, where before heterobimetallic uranium-metal bonds were restricted to group 7-11 metals.

Bonding inside and outside Fullerene Cages

Concrete crystallographic results of endohedral metallofullerenes (EMFs) disclose that the bonding within the metallic clusters and the interactions between the metal ions and the cage carbon atoms, which are closely associated with the coordination ability of the metal ions, play essential roles in determining the stability, the molecular structure, and the chemical behavior of the hybrid EMF molecules, in addition to the previously recognized charge transfer from metal to cage. For the carbide cluster metallofullerenes, a "size effect" between the encapsulated metallic cluster and the fullerene cage has been suggested. Thus, through the geometric effect, a series of giant fullerenes (C₉₀-C₁₀₄) have been stabilized by encapsulating a large La₂C₂ cluster, which adopts different configurations in accordance with cage size and shape. Interestingly, the crystallographic analysis of La₂C₂@ D₅(450)-C₁₀₀ has led to the direct observation of the axial compression of short carbon nanotubes caused by the internal stress. Additionally, the defective C₂(816)-C₁₀₄ cage is viewed to be a precursor that can transform into the other three ideal tubular fullerene cages, presenting crystallographic evidence for the top-down formation mechanism of fullerenes. Structural characterization of Y₂C₂@C₁₀₈ confirms a linear carbide cluster inside the large cage, indicative of a geometric effect of cage size on the bonding behavior of the internal cluster. Apart from the carbide realm, direct metal-metal bonding is observed between the two seemingly repulsive Lu²⁺ ions in Lu₂@C_82-86, adding new insights into current coordination chemistry. Meanwhile, the bonding state between the metal ions inside the cage (e.g., in La₂@ I _h(7)-C₈₀) and even the configuration of the internal metallic cluster (e.g., in Sc₃C₂@ I _h(7)-C₈₀) can be readily controlled by exohedral radical addition, illuminating their future applications as single molecule magnets and in electronics. In addition, observation of the unexpected dimerization between two paramagnetic Y@ C_s(6)-C₈₂ molecules suggests a spin-induced bonding behavior, which depends closely on the cage geometry. In contrast, synergistic effect of both electronic and geometric parameters has led to the formation of the unprecedented [6,6,6]-Lewis acid-base adduct of Sc₃N@ I _h(7)-C₈₀. However, introduction of an oxygen atom gives rise to the corresponding normal carbene adducts for both Sc₃N@ I _h(7)-C₈₀ and Lu₃N@ I _h(7)-C₈₀, presenting an unexpected way of steric hindrance release. Remarkably, the Lewis acid-base complexation is demonstrated to be a facile way toward isomerically pure metallofullerene derivatives with surprisingly high regioselectivity and quantitative conversion yield for Sc₂C₂@ C_{3 v}(8)-C₈₂. This Account aims to give an advanced summary of the recent achievements in research of EMFs, focusing mainly on the interplay between the internal metallic species and the surrounding cages through bond formation or cleavage. Perspectives suggesting the future developments of EMFs are also given in the last section.

Redox-active metal-metal bonds between lanthanides in dimetallofullerenes

The empty space inside a fullerene cage can be filled with a variety of species, including metal dimers. Encapsulation of Sc₂, Y₂, or lanthanide dimers leads to dimetallofullerenes featuring metal-metal bonding molecular orbital. Such an orbital can be either HOMO or LUMO of the dimetallofullerene molecule. In certain cases, single-occupied metal-metal bonding orbital can be also stabilized. This review is focused on redox processes involving variation of the electron population of metal-metal bonding orbitals in dimetallofullerenes.

Confining the spin between two metal atoms within the carbon cage: redox-active metal-metal bonds in dimetallofullerenes and their stable cation radicals

Lanthanide-lanthanide bonds are exceptionally rare, and dimetallofullerenes provide a unique possibility to stabilize and study these unusual bonding patterns. The presence of metal-metal bonds and consequences thereof for the electronic properties of M2@C82 (M = Sc, Er, Lu) are addressed by electrochemistry, electron paramagnetic resonance, SQUID magnetometry and other spectroscopic techniques. A simplified non-chromatographic separation procedure is developed for the isolation of Er2@C82 (Cs(6) and C3v(8) cage isomers) and Sc2@C82 (C3v(8) isomer) from fullerene mixtures. Sulfide clusterfullerenes Er2S@C82 with Cs(6) and C3v(8) fullerene cages are synthesized for the first time. The metal-metal bonding orbital of the spd hybrid character in M2@C82 is shown to be the highest occupied molecular orbital, which undergoes reversible single-electron oxidation with a metal-dependent oxidation potential. Sulfide clusterfullerenes with a fullerene-based HOMO have more positive oxidation potentials. The metal-based oxidation of Sc2@C82-C3v is confirmed by the EPR spectrum of the cation radical [Sc2@C82-C3v]+ generated by chemical oxidation in solution. The spectrum exhibits an exceptionally large a(45Sc) hyperfine coupling constant of 199.2 G, indicating a substantial 4s contribution to the metal-metal bonding orbital. The cationic salt [Er2@C82-C3v]+SbCl6- is prepared, and its magnetization behavior is compared to that of pristine Er2@C82-C3v and Er2S@C82-C3v. The formation of the single-electron Er-Er bond in the cation dramatically changes the coupling between magnetic moments of Er ions.

Tuning the separation and coupling of corannulene trianion-radicals through sizable alkali metal belts

Downsizing of alkali metal belts sandwiched between triply-reduced corannulene decks allows for the fine-tune separation and magnetic coupling of C₂₀H₁₀˙^3– radicals.

The first heterobimetallic sandwich-type aggregate formed by bowl-shaped corannulene trianion-radicals, C₂₀H₁₀˙^3–, has been synthesized using mixed-metal reduction of C₂₀H₁₀. The product was crystallographically characterized to reveal the self-assembly of [Cs⁺//(C₂₀H₁₀^3–)/4K⁺/(C₂₀H₁₀^3–)//Cs⁺], in which two triply-charged corannulene decks encapsulate a rectangle of four potassium ions (the K···K separations are 4.212(4) and 5.185(4) Å), with the exterior concave bowl cavities being selectively filled by one cesium ion each. In order to provide insights into the geometrical features and electronic structure of this novel mixed-metal organometallic self-assembly, an in-depth theoretical investigation has been carried out. Specifically, the influence of internal metal binding on the geometry and magnetic coupling of C₂₀H₁₀˙^3– radicals is investigated for Group 1 metals. This study reveals that replacement of the sandwiched potassium ions with larger (Cs) and smaller (Li) ions allows variation of the size of the encapsulated metal belts, and thus enables tuning of the coupling of C₂₀H₁₀˙^3– radicals.

A crystalline anionic complex of scandium nitride endometallofullerene: experimental observation of single-bonded (Sc3N@Ih-C80(-))2 dimers

Reduction of scandium nitride clusterfullerene, Sc3N@Ih-C80, by sodium fluorenone ketyl in the presence of cryptand[2,2,2] allows the crystallization of the {cryptand[2,2,2](Na(+))}2(Sc3N@Ih-C80(-))2·2.5C6H4Cl2 (1) salt. The Sc3N@Ih-C80˙(-) radical anions are dimerized to form single-bonded (Sc3N@Ih-C80(-))2 dimers.

Crystallographic Evidence for Direct Metal-Metal Bonding in a Stable Open-Shell La2 @Ih -C80 Derivative

Endohedral metallofullerenes (EMFs) have novel structures and properties that are closely associated with the internal metallic species. Benzyl radical additions have been previously shown to form closed-shell adducts by attaching an odd number of addends to open-shell EMFs (such as Sc3 C2 @Ih -C80 ) whereas an even number of groups are added to closed-shell EMFs (for example Sc3 N@Ih -C80 ). Herein we report that benzyl radical addition to the closed-shell La2 @Ih -C80 forms a stable, open-shell monoadduct instead of the anticipated closed-shell bisadduct. Single-crystal X-ray diffraction results show the formation of a stable radical species. In this species, the La-La distance is comparable to the theoretical value of a La-La covalent bond and is shorter than reported values for other La2 @Ih -C80 derivatives, providing unambiguous evidence for the formation of direct La-La bond.

Probing the Dipolar Coupling in a Heterospin Endohedral Fullerene-Phthalocyanine Dyad

Paramagnetic endohedral fullerenes and phthalocyanine (Pc) complexes are promising building blocks for molecular quantum information processing, for which tunable dipolar coupling is required. We have linked these two spin qubit candidates together and characterized the resulting electron paramagnetic resonance properties, including the spin dipolar coupling between the fullerene spin and the copper spin. Having interpreted the distance-dependent coupling strength quantitatively and further discussed the antiferromagnetic aggregation effect of the CuPc moieties, we demonstrate two ways of tuning the dipolar coupling in such dyad systems: changing the spacer group and adjusting the solution concentration.

Supramolecular trap for a transient corannulene trianion

The first structural characterization of the transient triply-reduced state of corannulene (C₂₀H₁₀) is accomplished. The X-ray crystallographic study reveals that the C₂₀H₁₀˙^3- trianions, generated by corannulene reduction with metallic cesium, form a novel type of supramolecular sandwich-type assembly, [Cs⁺//(C₂₀H₁₀^3-)/4Cs⁺/(C₂₀H₁₀^3-)//Cs⁺]. In the product, two triply-charged corannulene decks encapsulate a rectangle of four cesium ions with the external concave bowl cavities being filled by one cesium ion each. The structural investigation is augmented by in-depth theoretical calculations to provide insights into the geometrical features and electronic structure of this unique organometallic self-assembly.