Unexpected Formation of Metallofulleroids from Multicomponent Reactions, with Crystallographic and Computational Studies of the Cluster Motion

Electrode-Supported Endohedral Metallofullerenes: Insights into the Confined Internal Dynamics

Endohedral metallofullerenes show great promise as molecular-scale memory units due to their robust architecture and protective capability for encapsulated atoms. However, the flat potential-energy surface within the cage often results in a severe disorder of encapsulated atoms. Here, we focused on prototypical systems involving Li@C60 on metallic surfaces, emphasizing the electrode's confinement effect on caged dynamics. We demonstrated that the varying interfacial stabilities induced by Li motion predominantly depend on the synergetic effect of van der Waals forces and covalent bonds rather than the previously assumed electrostatic interactions. We unveiled that the repulsion effect between encapsulated atom and the metal electrode primarily arises from the antibonding states between the metal states below the Fermi level and the degenerated frontier orbitals from HOMO-4 to HOMO. By manipulating orbital interactions, we observed an ordered arrangement of the encapsulated atom on Rec-Pt(111) at room temperature. Furthermore, our findings underscore the disruptive influence of electric fields on the stability of distinct Li positions, a phenomenon closely tied to the dipole moment induced by Li motion. This research provides a new perspective on the confined internal dynamics of endohedral metallofullerenes by manipulating cage-electrode interactions, contributing to precisely controlled molecular electronics.

The formation mechanism of Sc-based metallofullerenes: a molecular dynamics simulation study

The practical applications of endohedral metallofullerenes (EMFs) are mainly constrained by their low yields. Understanding the formation mechanisms is therefore crucial for developing methods for high-yield and selective synthesis. To address this, a novel force-field parameter set, "CSc.ff", was created using a single-parameter search optimization method, then molecular dynamics simulations of various systems with a carbon-to-scandium atomic ratio of 12.5 were carried out. The simulations were run under a constant atomic number, volume, and energy (NVE) ensemble. The influence of the working gas, helium, as well as temperature gradients on the formation process was examined. Our simulations reveal that the cage growth patterns of Sc-based EMFs (Sc-EMFs) closely resemble those of hollow fullerenes, evolving from free carbon atoms to chains, rings, and, ultimately, to cage-shaped clusters. Importantly, the Sc-EMFs formed in the simulation frequently exhibit structural defects or under-coordinated carbon atoms. Scandium atoms, whether at the periphery or on the surface of these cages, can be incorporated into the cages, forming Sc-EMFs. Helium was found to not only promote the formation of carbon cages but also facilitate the encapsulation of scandium atoms, playing a crucial role in the formation of cluster fullerenes. Moreover, cooling effectively inhibits the uncontrollable growth of the carbon cage and is essential for forming stable, appropriate-sized cages. This study enhances our understanding of the formation of Sc-EMFs and provides valuable insights for developing more efficient synthetic methods.

Dearomative Ring-Fused Azafulleroids and Carbazole-Derived Metallofullerenes: Reactivity Dictated by Encapsulation in a Fullerene Cage

Herein, we report divergent additions of 2,2'-diazidobiphenyls to C60 and Sc3 N@Ih -C80 . In stark contrast to that of the previously reported bis-azide additions, the unexpected cascade reaction leads to the dearomative formation of azafulleroids 2 fused with a 7-6-5-membered ring system in the case of C60 . In contrast, the corresponding reaction with Sc3 N@Ih -C80 switches to the C-H insertion pathway, thereby resulting in multiple isomers, including a carbazole-derived [6,6]-azametallofulleroid 3 and a [5,6]-azametallofulleroid 4 and an unusual 1,2,3,6-tetrahydropyrrolo[3,2-c]carbazole-derived metallofullerene 5, whose molecular structures have been unambiguously determined by single-crystal X-ray diffraction analyses. Among them, the addition type of 5 is observed for the first time in all reported additions of azides to fullerenes. Furthermore, unexpected isomerizations from 3 to 5 and from 4 to 5 have been discovered, providing the first examples of the isomerization of an azafulleroid to a carbazole-derived fullerene rather than an aziridinofullerene. In particular, the isomerism of the [5,6]-isomer 4 to the [5,6]-isomer 5 is unprecedented in fullerene chemistry, contradicting the present understanding that isomerization generally occurs between [5,6]- and [6,6]-isomers. Control experiments have been carried out to rationalize the reaction mechanism. Furthermore, representative azafulleroids have been applied in organic solar cells, thereby resulting in improved power conversion efficiencies.

Structurally Defined Water-Soluble Metallofullerene Derivatives towards Biomedical Applications

Endohedral metallofullerenes (EMFs) are excellent carriers of rare-earth element (REE) ions in biomedical applications because they preclude the release of toxic metal ions. However, existing approaches to synthesize water-soluble EMF derivatives yield mixtures that inhibit precise drug design. Here we report the synthesis of metallobuckytrio (MBT), a three-buckyball system, as a modular platform to develop structurally defined water-soluble EMF derivatives with ligands by choice. Demonstrated with PEG ligands, the resulting water-soluble MBTs show superb biocompatibility. The Gd MBTs exhibit superior T1 relaxivity than typical Gd complexes, potentially superseding current clinical MRI contrast agents in both safety and efficiency. The Lu MBTs generated reactive oxygen species upon light irradiation, showing promise as photosensitizers. With their modular nature to incorporate other ligands, we anticipate the MBT platform to open new paths towards bio-specific REE drugs.

Synthesis of [60]- and [70]Fullerene-Fused Tetrahydroquinoxaline Derivatives by Oxidative [4 + 2] Cycloaddition with Unusual Reactivity and Regioselectivity

The oxidative [4 + 2] reaction of o-phenylenediamine-derived disulfonamides with fullerene C₆₀ and C₇₀ is reported, in which electron-deficient reactants showed high reactivity. The reaction of C₇₀ exhibited unusual regioselectivity, yielding a [5,6]-adduct as the major product, which was characterized by ¹H, ¹³C NMR and single-crystal X-ray diffraction. DFT calculations revealed the reaction is an inverse-electron-demand Diels-Alder (IEDDA) reaction, and the [5,6]-adduct of C₇₀ is a kinetic product.

Multicomponent Reactions Among Alkyl Isocyanides, sp reactants, and sp2 Carbon Cages

We explored the reactivity and substrate scope of the reactions among an alkyl isocyanide, an sp-hybridized reactant (i.e. alkyne or allene), and a carbon cage, as a new approach to functionalize fullerenes and metallofullerenes. This account summarizes the key findings in our recent published work, and some original data for the reaction involving an isocyanide, allenes, and metallofullerene Lu3N@C80.

Isocyanide-Induced Annulation Leading to Cyclopento-, Methano-, and Cyclopentano-[60]Fullerene Derivatives

The three-component annulation reactions of C₆₀, alkyl isocyanide, and dimethyl acetylenedicarboxylate (DMAD) or unsymmetric alkynes are investigated to afford cyclopent-2-en-1-imino- and ketenimine methano-[60]fullerene derivatives, which, upon hydration in the presence of acid, yield the corresponding fullerene amides. Dimethyl 2,3-pentadienedioate, the allene counterpart of DMAD, and ethyl buta-2,3-dienoate undergo four-component annulation with C₆₀, alkyl isocyanide, and water under similar conditions to yield cyclopentano-[60]fullerene derivatives with similar amide groups.