Endohedral Metallofullerenes: Synthesis and Electronic Properties

Endohedral metallofullerenes are hollow carbon cages that encapsulate one or more metal atoms or clusters, combining the unique structural framework of fullerenes with the electronic complexity of metal centres. Since the first isolation of a metallofullerene in the early 1990s, synthetic advances—ranging from arc-discharge and laser-vaporisation methods to controlled chemical functionalisation—have yielded a rich variety of mono- and multi-metal systems. Key variables include cage size (from C60 to larger C2n frameworks), metal identity (transition, lanthanide or actinide), and exohedral modifications. Electronic properties are governed by charge transfer between metal and cage, metal-metal bonding interactions inside the cavity, and the frontier molecular‐orbital energy gap. Single-electron bonds between encapsulated metals give rise to novel magnetic ground states, while cage isomerism and exohedral functionalisation can tune redox potentials and spin dynamics. These systems hold promise for applications in molecular magnetism, nanoelectronics and quantum information, as well as in catalysis and energy storage where robust metal–carbon interactions are desirable.

Research from Nature Portfolio

Recent studies have explored how mixed-valence actinide–lanthanide pairs form unprecedented single-electron metal–metal bonds within fullerene cages. Structural and spectroscopic analyses demonstrate that variable cage sizes (C72–C80) can host Th–Dy and Th–Y dimetallofullerenes, revealing high-spin ground states and clear electron spin resonance signatures of the bonded metal pair. Another foundational contribution stabilised a diuranium carbide cluster inside an Ih(7)-C80 cage, unambiguously showing U=C double bonds with unusually short U–C distances and confirming formal oxidation states through X-ray and quantum-chemical studies. Complementing these efforts, functionalisation of the smallest Ih-C60 cage with perfluoroalkyl groups has yielded crystalline Gd@C60(CF3)5 and La@C60(CF3)5, demonstrating that exohedral attachment can widen the HOMO–LUMO gap by an order of magnitude and induce weak antiferromagnetic coupling at low temperature. These achievements underscore the dual role of the carbon cage as both a protective nanoconfiner and an electronic modulator of the encapsulated metal centre.

Endohedral Metallofullerenes: Synthesis and Electronic Properties publication trend

The graph below shows the total number of articles in endohedral metallofullerenes: synthesis and electronic properties across all publications each year (not limited to Nature Index journals).

Technical terms

Endohedral metallofullerene: A carbon fullerene cage encapsulating metal atom(s) or cluster(s) within its internal cavity.

Cage isomer: A structural variant of the fullerene carbon framework distinguished by different arrangements of pentagons and hexagons.

Metal–metal bond: A direct covalent or single-electron interaction between two metal centres confined inside a fullerene cage.

Charge transfer: The movement of electron density from the metal centre to the carbon cage or vice versa, often determining oxidation state.

HOMO–LUMO gap: The energy difference between the highest occupied and lowest unoccupied molecular orbitals, indicative of electronic stability and reactivity.

Exohedral functionalisation: Chemical modification of the external surface of the fullerene cage by covalent attachment of substituent groups.

References

1. Actinide-lanthanide single electron metal-metal bond formed in mixed-valence di-metallofullerenes. Nature Communications (2023).
2. A diuranium carbide cluster stabilized inside a C80 fullerene cage. Nature Communications (2018).
3. Crystalline functionalized endohedral C60 metallofullerides. Nature Communications (2018).
4. Nd─Nd Bond in Ih and D5h Cage Isomers of Nd2@C80 Stabilized by Electrophilic CF3 Addition. Advanced Science (2023).
5. Single crystal structures and theoretical calculations of uranium endohedral metallofullerenes (U@ C 2n , 2 n = 74, 82) show cage isomer dependent oxidation states for U. Chemical Science (2017).