Transformation of doped graphite into cluster-encapsulated fullerene cages

Unveiling fullerene formation and interconversion through molecular dynamics simulations with deep neural network potentials

Utilizing deep neural network potentials within molecular dynamics simulations, this research uncovers insights into fullerene formation and interconversion, particularly during the cooling stage of the annealing process. Our deep learning-enhanced approach effectively models the generation of fullerenes from C2 units in carbon vapor, highlighting the critical role of carbon density in structuring outcomes in a primary iron-carbon system. This study provides differences in molecular dynamics simulations for fullerene generation and also demonstrates the potential of deep learning in investigating complex carbon structures, paving the way for further investigations into the fullerene family.

Boron-Doped Endohedral Metallofullerenes: Synthesis and Computational Analysis of a Family of Heteroatom-Doped Molecular Carbons

Gas-phase synthesis and detection of boron-doped nitride clusterfullerenes and a large variety of monometallofullerenes have been achieved using a pulsed laser vaporization cluster source. Density functional theory (DFT) calculations show that the electronic structures of boron-doped endohedral metallofullerenes differ from those of the pristine all-carbon cages due to the lack of one electron upon boron substitution. For monometallofullerenes, this is likely the main reason for the somewhat different abundance distribution observed for boron-doped with respect to all-carbon cages. Moreover, the three carbon atoms directly bonded to B show the most negative charges in the cage, and consequently, metal atoms are primarily placed nearby boron.

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@C_2v(5)-C₈₀ and Sc₂C₂@C_s(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.

Are U-U Bonds Inside Fullerenes Really Unwilling Bonds?

Previous characterizations of diactinide endohedral metallofullerenes (EMFs) Th₂@C₈₀ and U₂@C₈₀ have shown that although the two Th³⁺ ions form a strong covalent bond within the carbon cage, the interaction between the U³⁺ ions is weaker and described as an "unwilling" bond. To evaluate the feasibility of covalent U-U bonds, which are neglected in classical actinide chemistry, we have first investigated the formation of smaller diuranium EMFs by laser ablation using mass spectrometric detection of dimetallic U₂@C_2n species with 2n ≥ 50. DFT, CASPT2 calculations, and MD simulations for several fullerenes of different sizes and symmetries showed that thanks to the formation of strong U(5f³)-U(5f³) triple bonds, two U³⁺ ions can be incarcerated inside the fullerene. The formation of U-U bonds competes with U-cage interactions that tend to separate the U ions, hindering the observation of short U-U distances in the crystalline structures of diuranium endofullerenes as in U₂@C₈₀. Smaller cages like C₆₀ exhibit the two interactions, and a strong triple U-U bond with an effective bond order higher than 2 is observed. Although 5f-5f interactions are responsible for the covalent interactions at distances close to 2.5 Å, overlap between 7s6d orbitals is still detected above 4 Å. In general, metal ions within fullerenes should be regarded as templates in cage formation, not as statistically confined units that have little chance of being observed.

C2-insertion into a fullerene orifice

The embedment of a C_n-unit into a carbon network constituting fullerene(s) potentially enables a cage-expansion. Herein, we report a C2-insertion into a fullerene orifice in which the mechanism was examined computationally. The C2-embedded open-[60]fullerene possesses an orifice enlarged from an octagon to a decagon, while the inner space was notably expanded as confirmed by the dynamic motion of the incarcerated H₂O molecule.

Self-driven carbon atom implantation into fullerene embedding metal-carbon cluster

Hundreds of members have been synthesized and versatile applications have been promised for endofullerenes (EFs) in the past 30 y. However, the formation mechanism of EFs is still a long-standing puzzle to chemists, especially the mechanism of embedding clusters into charged carbon cages. Here, based on synthesis and structures of two representative vanadium-scandium-carbido/carbide EFs, VSc₂C@I_h (7)-C₈₀ and VSc₂C₂@I_h (7)-C₈₀, a reasonable mechanism-C₁ implantation (a carbon atom is implanted into carbon cage)-is proposed to interpret the evolution from VSc₂C carbido to VSc₂C₂ carbide cluster. Supported by theoretical calculations together with crystallographic characterization, the single electron on vanadium (V) in VSc₂C@I_h (7)-C₈₀ is proved to facilitate the C₁ implantation. While the V=C double bond is identified for VSc₂C@I_h (7)-C₈₀, after C₁ implantation the distance between V and C atoms in VSc₂C₂@I_h (7)-C₈₀ falls into the range of single bond lengths as previously shown in typical V-based organometallic complexes. This work exemplifies in situ self-driven implantation of an outer carbon atom into a charged carbon cage, which is different from previous heterogeneous implantation of nonmetal atoms (Group-V or -VIII atoms) driven by high-energy ion bombardment or high-pressure offline, and the proposed C₁ implantation mechanism represents a heretofore unknown metal-carbon cluster encapsulation mechanism and can be the fundamental basis for EF family genesis.

Atomically defined angstrom-scale all-carbon junctions

Full-carbon electronics at the scale of several angstroms is an expeimental challenge, which could be overcome by exploiting the versatility of carbon allotropes. Here, we investigate charge transport through graphene/single-fullerene/graphene hybrid junctions using a single-molecule manipulation technique. Such sub-nanoscale electronic junctions can be tuned by band gap engineering as exemplified by various pristine fullerenes such as C₆₀, C₇₀, C₇₆ and C₉₀. In addition, we demonstrate further control of charge transport by breaking the conjugation of their π systems which lowers their conductance, and via heteroatom doping of fullerene, which introduces transport resonances and increase their conductance. Supported by our combined density functional theory (DFT) calculations, a promising future of tunable full-carbon electronics based on numerous sub-nanoscale fullerenes in the large family of carbon allotropes is anticipated.

All-carbon electronics holds promise beyond the conventional silicon-based electronics, but it remains challenging to manufacture them with well-defined structures thus tunability. Tan et al. control charge transport in single-molecule junctions using different fullerenes between graphene electrodes.

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.