The Unanticipated Dimerization of Ce@C2v(9)-C82upon Co-crystallization with Ni(octaethylporphyrin) and Comparison with Monomeric M@C2v(9)-C82(M = La, Sc, and Y)

Thirty Years of Hide-and-Seek: Capturing Abundant but Elusive MIII@C3v(8)-C82 Isomer, and the Study of Magnetic Anisotropy Induced in Dy3+ Ion by the Fullerene π-Ligand

Our knowledge about endohedral metallofullerenes (EMFs) is restricted to the structures with sufficient kinetic stability to be extracted from the arc-discharge soot and processed by chromatographic and structural techniques. For the most abundant rare-earth monometallofullerene MIII@C82, experimental studies repeatedly demonstrated C2v(9) and Cs(6) carbon cage isomers, while computations predicted equal stability of the "missing" C3v(8) isomer. Here we report that this isomer is indeed formed but has not been recovered from soot using standard protocols. Using a combination of redox extraction and subsequent benzylation and trifluoromethylation with single-crystal XRD analysis of CF3 adduct, we prove that Dy@C3v(8)-C82 is one of the most abundantly produced metallofullerenes, which was not identified in earlier studies because of the low kinetic stability. Further, using the Dy@C3v(8)-C82(CF3) and Dy@C3v(8)-C82(CH2Ph) monoadducts for the case study, we analyzed the role of metal-fullerene bonding on the single-ion magnetic anisotropy of Dy in EMFs. The multitechnique approach, combining ab initio calculations, EPR spectroscopy, and SQUID magnetometry, demonstrated that coordination of the Dy ion to the fullerene cage induces moderate, nonaxial, and very fluid magnetic anisotropy, which strongly varies with small alterations in the Dy-fullerene coordination geometry. As a result, Dy@C3v(8)-C82(CH2Ph) is a weak field-induced single-molecule magnet (SMM), whose signatures of magnetic relaxation are detectable only below 3 K. Our results demonstrate that metal-cage interactions should have a detrimental effect on the SMM performance of EMFs. At the same time, the strong variability of the magnetic anisotropy with metal position suggests tunability and offers strategies for future progress.

The Various Packing Structures of Tb@C82 (I, II) Isomers in Their Cocrystals with Ni(OEP)

Soot-containing terbium (Tb)-embedded fullerenes were prepared by evaporation of Tb₄O₇-doped graphite rods in an electric arc discharge chamber. After 1,2,4-trichlorobenzene extraction of the soot and rotary evaporation of the extract, a solid product was obtained and then dissolved into toluene by ultrasonication. Through a three-stage high-pressure liquid chromatographic (HPLC) process, Tb@C₈₂ (I, II) isomers were isolated from the toluene solution of fullerenes and metallofullerenes. With the success of the growth of cocrystals of Tb@C₈₂ (I, II) with Ni(OEP), the molecular structures of Tb@C₈₂ (I) and Tb@C₈₂ (II) were confirmed to be Tb@C_2v(9)-C₈₂ and Tb@Cs(6)-C₈₂, respectively, based on crystallographic data from X-ray single-crystal diffraction. Moreover, it was found that Tb@C₈₂ (I, II) isomers demonstrated different packing behaviors in their cocrystals with Ni(OEP). Tb@C_2v(9)-C₈₂ forms a 1:1 cocrystal with Ni(OEP), in which Tb@C_2v(9)-C₈₂ is aligned diagonally between the Ni(OEP) bilayers to form zigzag chains. In sharp contrast, Tb@Cs(6)-C₈₂ forms a 2:2 cocrystal with Ni(OEP), in which Tb@Cs(6)-C₈₂ forms a centrosymmetric dimer that is aligned linearly with Ni(OEP) pairs to form one-dimensional structures in the a-c lattice plane. In addition, the distance of a Ni atom in Ni(OEP) to the Cs(6)-C₈₂ cage is much shorter than that to the C_2v(9)-C₈₂ one, indicative of a stronger π-π interaction between Ni(OEP) and the C₈₂ carbon cage in the cocrystal of Tb@C_S(6)-C₈₂ and Ni(OEP). Density functional theory calculations reveal that the regionally selective dimerization of Tb@C_S(6)-C₈₂ is the result of a dominant unpaired spin existing on a particular C atom of the C_S(6)-C₈₂ cage.

Super-Excimer: Anomalous Bonding in a Metastable Excited-State Dimer of Superatomic Dimers

A new type of excimer formation was reported, which stems from an unexpected discovery of a short-lived excited-state dimer of superatomic dimers. In theoretical investigation of the dimer formation, it was found that the physical adsorption states maintain the closed-shell properties of the dimeric units via van der Waals interaction, while the chemical adsorption excited state is a broken-symmetry (BS) state, having a higher energy of about 0.5 eV. Potential energy surface calculations indicate that the short-lived metastable chemical bonding state can transform into energetically lower physical adsorption states by crossing a shallow energy barrier and eventually disintegrate into two ground-state dimers. Since the basic unit is a superatomic cluster, the chemical adsorption state discovered may be called "super-excimer", which opens up a new avenue for the discovery of tailorable excimer materials.

Crystallographic Characterization of U@C2n (2n = 82-86): Insights about Metal-Cage Interactions for Mono-metallofullerenes

Endohedral mono-metallofullerenes are the prototypes to understand the fundamental nature and the unique interactions between the encapsulated metals and the fullerene cages. Herein, we report the crystallographic characterizations of four new U-based mono-metallofullerenes, namely, U@C_s(6)-C₈₂, U@C₂(8)-C₈₄, U@C_s(15)-C₈₄, and U@C₁(12)-C₈₆, among which the chiral cages C₂(8)-C₈₄ and C₁(12)-C₈₆ have never been previously reported for either endohedral or empty fullerenes. Symmetrical patterns, such as indacene, sumanene, and phenalene, and charge transfer are found to determine the metal positions inside the fullerene cages. In addition, a new finding concerning the metal positions inside the cages reveals that the encapsulated metal ions are always located on symmetry planes of the fullerene cages, as long as the fullerene cages possess mirror planes. DFT calculations show that the metal-fullerene motif interaction determines the stability of the metal position. In fullerenes containing symmetry planes, the metal prefers to occupy a symmetrical arrangement with respect to the interacting motifs, which share one of their symmetry planes with the fullerene. In all computationally analyzed fullerenes containing at least one symmetry plane, the actinide was found to be located on the mirror plane. This finding provides new insights into the nature of metal-cage interactions and gives new guidelines for structural determinations using crystallographic and theoretical methods.

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.

Molecular Structure and Magnetic and Optical Properties of Endometallonitridofullerene Sc3 N@Ih -C80 in Neutral, Radical Anion, and Dimeric Anionic Forms

A series of compounds with Sc₃ N@I_h -C₈₀ in the neutral, monomeric, and dimeric anion states have been prepared in the crystalline form and their molecular structures and optical and magnetic properties have been studied. The neutral Sc₃ N@I_h -C₈₀ ⋅3 C₆ H₄ Cl₂ (1) and (Sc₃ N@I_h -C₈₀ )₃ (TPC)₂ ⋅5 C₆ H₄ Cl₂ (2, TPC=triptycene) compounds both crystallized in a high-symmetry trigonal structure. The reduction of Sc₃ N@I_h -C₈₀ to the radical anion resulted in dimerization to form diamagnetic singly bonded (Sc₃ N@I_h -C₈₀ ^- )₂ dimers. In contrast to {[2.2.2]cryptand(Na⁺ )}₂ (Sc₃ N@I_h -C₈₀ ^- )₂ ⋅2.5 C₆ H₄ Cl₂ (3) with strongly disordered components, we synthesized new dimeric phases {[2.2.2]cryptand- (K⁺ )}₂ (Sc₃ N@I_h -C₈₀ ^- )₂ ⋅2 C₆ H₄ Cl₂ (4) and {[2.2.2]cryptand- (Cs⁺ )}₂ (Sc₃ N@I_h -C₈₀ ^- )₂ ⋅2 C₆ H₄ Cl₂ (5) in which only one major dimer orientation was found. The thermal stability of the (Sc₃ N@I_h -C₈₀ ^- )₂ dimers was studied by EPR analysis of 3 to show their dissociation in the 400-460 K range producing monomeric Sc₃ N@I_h -C₈₀ ^.- radical anions. This species shows an EPR signal with a hyperfine splitting of 5.8 mT. The energy of the intercage C-C bond was estimated to be 234±7 kJ mol^-1 , the highest value among negatively charged fullerene dimers. The EPR spectra of crystalline (Bu₃ MeP⁺ )₃ (Sc₃ N@I_h -C₈₀ ^.- )₃ ⋅C₆ H₄ Cl₂ (6) are presented for the first time. The salt shows an asymmetric EPR signal, which could be fitted by three lines. Two lines were attributed to Sc₃ N@I_h -C₈₀ ^.- . Hyperfine splitting is manifested above 180 K due to the hyperfine interaction of the electron spin with the three scandium atoms (a total of 22 lines with an average splitting of 5.32 mT are observed at 220 K). Furthermore, each of the 22 lines is additionally split into six lines with an average separation of 0.82 mT. The large splitting indicates intrinsic charge and spin density transfer from the fullerene cage to the Sc₃ N cluster. Both the monomeric and dimeric Sc₃ N@I_h -C₈₀ ^- anions show an intrinsic shift of the IR bands attributed to the Sc₃ N cluster and new bands corresponding to these species appear in the NIR range of their UV/Vis/NIR spectra, which allows these anions to be distinguished from neutral species.

Single Molecule Magnetism with Strong Magnetic Anisotropy and Enhanced Dy∙∙∙Dy Coupling in Three Isomers of Dy-Oxide Clusterfullerene Dy2O@C82

A new class of single-molecule magnets (SMMs) based on Dy-oxide clusterfullerenes is synthesized. Three isomers of Dy2O@C82 with C s(6), C 3v(8), and C 2v(9) cage symmetries are characterized by single-crystal X-ray diffraction, which shows that the endohedral Dy-(µ2-O)-Dy cluster has bent shape with very short Dy-O bonds. Dy2O@C82 isomers show SMM behavior with broad magnetic hysteresis, but the temperature and magnetization relaxation depend strongly on the fullerene cage. The short Dy-O distances and the large negative charge of the oxide ion in Dy2O@C82 result in the very strong magnetic anisotropy of Dy ions. Their magnetic moments are aligned along the Dy-O bonds and are antiferromagnetically (AFM) coupled. At low temperatures, relaxation of magnetization in Dy2O@C82 proceeds via the ferromagnetically (FM)-coupled excited state, giving Arrhenius behavior with the effective barriers equal to the AFM-FM energy difference. The AFM-FM energy differences of 5.4-12.9 cm-1 in Dy2O@C82 are considerably larger than in SMMs with {Dy2O2} bridges, and the Dy∙∙∙Dy exchange coupling in Dy2O@C82 is the strongest among all dinuclear Dy SMMs with diamagnetic bridges. Dy-oxide clusterfullerenes provide a playground for the further tuning of molecular magnetism via variation of the size and shape of the fullerene cage.

Isolation and Structural Characterization of Er@ C2 v(9)-C82 and Er@ C s(6)-C82: Regioselective Dimerization of a Pristine Endohedral Metallofullerene Induced by Cage Symmetry

Two Er@C₈₂ isomers have been isolated and unambiguously characterized as Er@ C_{2 v}(9)-C₈₂ and Er@ C _s(6)-C₈₂, respectively, by single-crystal X-ray diffraction. Er@ C _s(6)-C₈₂ is identified as a dimeric structure in the crystalline state, but dimerization does not occur for Er@ C_{2 v}(9)-C₈₂ under identical crystallization conditions, indicating a cage-symmetry-induced dimerization process. Density functional theory calculations reveal that the major unpaired spin resides on a special C atom of Er@ C _s(6)-C₈₂, which leads to regioselective dimerization. Calculations also found that the dimeric structure of Er@ C _s(6)-C₈₂·Ni(OEP) is much more stable than the two monomers, suggesting a thermodynamically favorable dimerization process. Vis-near-IR spectrometric and electrochemical results demonstrate that the electronic structure of Er@C₈₂ isomers is Er³⁺@C₈₂^3-, instead of the theoretically proposed Er²⁺@C₈₂^2-.

Th@C1(11)-C86: an actinide encapsulated in an unexpected C86 fullerene cage

A novel actinide endohedral fullerene with an unexpected chiral cage, Th@C₁(11)-C₈₆, was synthesized and characterized.

Th-Based Endohedral Metallofullerenes: Anomalous Metal Position and Significant Metal-Cage Covalent Interactions with the Involvement of Th 5f Orbitals

Endohedral metallofullerenes (EMFs) containing actinides are rather intriguing due to potential 5f-orbital participation in the metal-metal or metal-cage bonding. In this work, density functional theory calculations first characterized the structure of recently synthesized ThC₇₄ as Th@ D_{3 h}(14246)-C₇₄. We found that the thorium atom adopts an unusual off-axis position inside cage due to small metal ion size and the requirement of large coordination number, which phenomenon was further extended to other Th-based EMFs. Significantly, besides the strong metal-cage electrostatic attractions, topological and orbital analysis revealed that all the investigated Th-based EMFs exhibit obvious covalent interactions between metal and cage with substantial contribution from the Th 5f orbitals. The encapsulation by fullerenes is thus proposed as a practical pathway toward the f-orbital covalency for thorium. Interestingly, the anomalous internal position of Th led to a novel three-dimensional metal trajectory at elevated temperatures in the D_{3 h}-C₇₄ cavity, as elucidated by the static computations and molecular dynamic simulations.