Sc2C2@D3h(14246)-C74: A Missing Piece of the Clusterfullerene Puzzle

Stabilizing a non-IPR C2(13333)-C74 cage with Lu2C2/Lu2O: the importance of encaged non-metallic elements

A difference in encaged non-metallic element (i.e., C2versus O) leads to a clear change of intramolecular interactions and shifts in redox potentials of Lu2C2@C2(13333)-C74 and Lu2O@C2(13333)-C74, as a result of their distinct molecular orbital energy levels. Different from these two endoherals whose HOMOs are located on the cage, experimentally absent Lu2@C2(13333)-C74 possesses a HOMO predominantly delocalized on the internal Lu-Lu bond, accompanied by a much smaller HOMO-LUMO gap, suggesting that the presence of a non-metallic unit broadens the electrochemical gaps and consequently improves the kinetic stability. These findings shed light on the role of non-metallic moieties in clusterfullerenes, providing valuable insights into the stability and properties of metallofullerenes.

Tb2O@C2(13333)-C74: A Non-Isolated Pentagon Endohedral Fullerene Containing a Nearly Linear Tb-O-Tb Unit

Terbium has been added to the list of elements that form oxide clusters inside fullerene cages. Tb₂O@C₂(13333)-C₇₄ has been isolated as a byproduct of the electric arc synthesis of the azafullerene Tb₂@C₇₉N. Cocrystallization of Tb₂O@C₂(13333)-C₇₄ with Ni(OEP) (where OEP is the dianion of octaethylporphyrin) in toluene yielded black needles of Tb₂O@C₂(13333)-C₇₄·Ni^II(OEP)·1.5C₇H₈ that have been examined by single-crystal X-ray diffraction. The resulting structure shows that a nearly linear Tb-O-Tb unit is contained in a C₂(13333)-C₇₄, which has two sites where pentagons share an edge to form pentalene units at opposite ends of the fullerene. Unlike the usual situations where metal atoms in fullerenes that do not obey the isolated pentagon rule are situated within the folds of the pentalene units, the Tb atoms in Tb₂O@C₂(13333)-C₇₄ are positioned to the side of the pentalene units and near-neighboring hexagons. The magnetic properties of Tb₂O@C₂(13333)-C₇₄ have been examined starting from the experimental geometry, using ab-initio multiconfigurational methods. The computations predict that Tb₂O@C₂(13333)-C₇₄ will show strong axiality, which would make it a single-molecule magnet with a large magnetic anisotropy barrier.

Theoretical Insights into the Metal-Nonmetal Interaction Inside M2O@C 2v (31922)-C80 (M = Sc or Gd)

The metal-nonmetal interaction is complicated but significant in organometallic chemistry and metallic catalysis and is susceptible to the coordination surroundings. Endohedral metallofullerene is considered to be an excellent model for studying metal-nonmetal interactions with the shielding effect of fullerenes. Herein, with the detection of ScGdO@C₈₀ in a previous mass spectrum, we studied the effects of metal atoms (Sc and Gd) on the metal-nonmetal interactions of the thermodynamically stable molecules M₂O@C _2v (31922)-C₈₀ (M = Sc and Gd), where metal atoms M can be the same or different, using density functional theory calculations. The inner metal atom and the fullerene cage show mainly ionic interactions with some covalent character. The Sc atom with higher electronegativity plays a greater important role in the metal-nonmetal interactions than the Gd atom. This study would be useful for the further study of the metal-nonmetal interaction.

ThC2@C82 versus Th@C84: unexpected formation of triangular thorium carbide cluster inside fullerenes

Synthesis of the first thorium-containing clusterfullerenes, ThC₂@C _s (6)-C₈₂ and ThC₂@C₂(5)-C₈₂, is reported. These two novel actinide fullerene compounds were characterized by mass spectrometry, single-crystal X-ray diffraction crystallography, UV-vis-NIR spectroscopy, and theoretical calculations. Crystallographic studies reveal that the encapsulated ThC₂ clusters in both C _s (6)-C₈₂ and C₂(5)-C₈₂ feature a novel bonding structure with one thorium metal center connected by a C[triple bond, length as m-dash]C unit, forming an isosceles triangular configuration, which has not been hitherto observed for endohedral fullerenes or for solid phase thorium carbides. Electronic structure calculations assign a formal electronic structure of [Th⁴⁺(C₂)^2-]²⁺@[C₈₂]^2-, with pronounced donation bonding from (C₂)^2- to Th⁴⁺, secondary backbonding from the fullerene to thorium and Th-C double bond character in both compounds. This work presents a new family of endohedral fullerenes, MC₂@C_2n-2, being unexpected isomers of MC_2n , and provides broader understanding of thorium bonding.

Ho2C2 Cluster with Flexible Configurations inside a Large C2(61)-C92 Cage

Carbide clusterfullerenes (CCFs) have been of great concern due to their potential applications in materials science, in which the internal carbide cluster plays vital roles in the stability and properties of CCF. However, there still remains a debate about what configuration is ideal for the internal carbide cluster. In this work, we isolated two isomers (I and II) of Ho₂C₉₄ and studied them by means of mass spectrometry, UV-vis-NIR spectroscopy, and cyclic/differential pulse voltammetry. A combined study of single-crystal X-ray diffraction (SC-XRD) and density functional theory (DFT) computation ascertains isomer-I as Ho₂C₂@C₂(61)-C₉₂, in which the Ho₂C₂ cluster displays variable configurations from planar zigzag to folded butterfly with very small distortion energy (∼10 kJ/mol). This study hence confirms that the internal carbide cluster is intrinsically flexible over a broad geometrical range in a relatively large fullerene cage, where the nanoscale compression effect is almost negligible.

Crystallographic characterization of Er2C2@C2(43)-C90, Er2C2@C2(40)-C90, Er2C2@C2(44)-C90, and Er2C2@C1(21)-C90: the role of cage-shape on cluster configuration

For endohedral metallofullerenes (EMFs), that is, fullerenes encapsulating metallic species, cage size is known to be an important factor for cluster configuration adoption; however, the impact of the cage shape on the cluster geometry fitting remains poorly understood. Herein, for the first time, four dierbium-carbide EMFs with C₉₀ cages, namely, Er₂C₂@C₂(43)-C₉₀, Er₂C₂@C₂(40)-C₉₀, Er₂C₂@C₂(44)-C₉₀, and Er₂C₂@C₁(21)-C₉₀, were successfully synthesized and fully characterized using a combination of mass spectrometry, single-crystal X-ray diffractometry, vis-NIR, Raman and photoluminescence spectroscopies, and cyclic voltammetry. In particular, the fullerene cages of C₂(43)-C₉₀ and C₂(44)-C₉₀ are crystallographically identified for the first time. Interestingly, the ErEr distance of the major sites in Er₂C₂@C₂(43)-C₉₀, Er₂C₂@C₂(40)-C₉₀, Er₂C₂@C₂(44)-C₉₀, and Er₂C₂@C₁(21)-C₉₀ is 3.927, 4.058, 4.172, and 4.651 Å, respectively, which increases gradually with an increase in the major axis of the cage. Moreover, the bond length of the inner C₂-unit decreases progressively with an increase in the ErEr distance, indicating that the inserted C₂-unit can serve as a molecular spring to support the strong metal-cage interactions within cages with the same size but different shapes. Hence, the role of cage shape on the cluster configuration is unveiled safely for the as-obtained Er₂C₂@C₉₀ isomers.

Trapping Metallic Oxide Clusters inside Fullerene Cages

The sub-nanometer sized void inside a fullerene cage permits the accommodation of a single atom, atomic cluster, or even small molecule, resulting in the formation of endohedral fullerenes. Particularly, clusterfullerenes can be formed by encapsulating multiple metallic ions in most cases along with nonmetal ions (i.e., N^3-, C₂^2-, S^2-, O^2-) inside the fullerene cage. Such an association makes clusterfullerene more functional than empty fullerenes and conventional mono-metallofullerenes. To date, a variety of clusterfullerenes have been reported, including metal nitrides, carbides, oxides, sulfides, cyanides, and so on. Among them, oxide clusterfullerenes (OCFs) can contain variable oxide clusters (i.e., M₄O₂, M₄O₃, M₃O, and M₂O; M = Sc or other metal), yielding one of the most versatile families. Thus, OCFs may provide a more convenient platform for developing new functional molecules and for studying previously less-explored topics such as formation mechanisms of clusterfullerenes. In this Account, we review recent progress in the field of OCFs, including their synthesis, isolation, and structural and electrochemical studies as well as the preliminary exploration into their potential functions and applications. Thanks to the concrete crystallographic results of an OCF series, we can track the transition of endohedral cluster and fullerene cage. It is suggested that the configuration and internal dynamics of the oxide cluster are highly dependent on not only the cage size but also cage structure. On the other hand, based on the experimental observations, two competitive transformation pathways are established for the majority of OCFs, verifying the bottom-up or top-down formation mechanism. It is also found that the redox behaviors of OCFs are more or less comparable to their isoelectronic species with common cage structure and similar cluster geometry but varied greatly with the cluster variety (i.e., Sc₂O vs Sc₄O_2-3). The mechanism behind such phenomena has been discussed. In addition, the potential of Dy-based OCFs as single molecular magnets (SMMs) is presented theoretically. Nevertheless, experimental advance remains to be achieved.

When metal clusters meet carbon cages: endohedral clusterfullerenes

Fullerenes have the characteristic of a hollow interior, and this unique feature triggers intuitive inspiration to entrap atoms, ions or clusters inside the carbon cage in the form of endohedral fullerenes. In particular, upon entrapping an otherwise unstable metal cluster into a carbon cage, the so-called endohedral clusterfullerenes fulfil the mutual stabilization of the inner metal cluster and the outer fullerene cage with a specific isomeric structure which is often unstable as an empty fullerene. A variety of metal clusters have been reported to form endohedral clusterfullerenes, including metal nitrides, carbides, oxides, sulfides, cyanides and so on, making endohedral clusterfullerenes the most variable and intriguing branch of endohedral fullerenes. In this review article, we present an exhaustive review on all types of endohedral clusterfullerenes reported to date, including their discoveries, syntheses, separations, molecular structures and properties as well as their potential applications in versatile fields such as biomedicine, energy conversion, and so on. At the end, we present an outlook on the prospect of endohedral clusterfullerenes.

Facile Access to Y2C2n (2n = 92-130) and Crystallographic Characterization of Y2C2@C1(1660)-C108: A Giant Nanocapsule with a Linear Carbide Cluster

A series of giant metallofullerenes Y₂C_2n (2n = 92-130) have been successfully obtained through the treatment of the fraction enriched by 1,2-dichlorobenzene with SnCl₄. Subsequent chromatographic separation gives a pure sample with a composition of Y₂C₁₁₀. Crystallographic results reveal that this endohedral takes the carbide form, namely Y₂C₂@C₁(1660)-C₁₀₈, representing as the largest metallofullerene that has been characterized by crystallography to date. Despite the disorder of the metal cluster, the major Y₂C₂ adopts a previously predicted linear configuration, indicating that the compression of the internal cluster by the cage is almost negligible in this giant cage. Electrochemical studies suggest that Y₂C₂@C₁(1660)-C₁₀₈ is a good electron donor instead of an electron acceptor.