Recent advances in endohedral metallofullerenes

Abstract

Fullerenes are a collection of closed polycyclic polymers consisting exclusively of carbon atoms. Recent remarkable advancements in the fabrication of metal-fullerene nanocatalysts and polymeric fullerene layers have significantly expanded the potential applications of fullerenes in various domains, including electrocatalysis, transistors, energy storage devices, and superconductors. Notably, the interior of fullerenes provides an optimal environment for stabilizing a diverse range of metal ions or clusters through electron transfer, resulting in the formation of a novel class of hybrid molecules referred to as endohedral metallofullerenes (EMFs). The utilization of advanced synthetic methodologies and the progress achieved in separation techniques have played a pivotal role in expanding the diversity of the encapsulated metal constituents, consequently leading to distinctive structural, electronic, and physicochemical properties of novel EMFs that surpass conventional ones. Intriguing phenomena, including regioselective dimerization between EMFs, direct metal-metal bonding, and non-classical cage preferences, have been unveiled, offering valuable insights into the coordination interactions between metallic species and carbon. Of particular importance, the recent achievements in the comprehensive characterization of EMFs based on transition metals and actinide metals have generated a particular interest in the exploration of new metal clusters possessing long-desired bonding features within the realm of coordination chemistry. These clusters exhibit a remarkable affinity for coordinating with non-metal atoms such as carbon, nitrogen, oxygen, and sulfur, thus making them highly intriguing subjects of systematic investigations focusing on their electronic structures and physicochemical properties, ultimately leading to a deeper comprehension of their unparalleled bonding characteristics. Moreover, the versatility conferred by the encapsulated species endows EMFs with multifunctional properties, thereby unveiling potential applications in various fields including biomedicine, single-molecule magnets, and electronic devices.

Keywords: Crystallography; Fullerenes; Metal-metal bonding; Metallofullerenes; Single-molecule magnet.

Graphical abstract

Fig. 1

Novel exploration in the studies of fullerenes. Left: crystal structures of quasi-hexagonal C₆₀ and quasi-tetragonal C₆₀. Right: catalytic performance of Cu/SiO₂ and C_60—Cu/SiO₂ at 1 bar, H₂/DMO = 200 (v/v, volume ratio), temperature 190 ℃ . Copyright 2022, Springer Nature. Copyright 2022, The American Association for the Advancement of Science.

Fig. 2

Straightforward purification methodology to the desired EMFs. Selective isolation of different types of EMFs from raw soot using a Cu^II-based supramolecular nanocapsule.

Fig. 3

Regioselective dimerization between EMFs. Drawings of the dimers of pristine metallofullerenes: Y@C_s(6)-C₈₂, Er@C_s(6)-C₈₂, and Ce@C_2v(9)-C₈₂. Only the major metal sites and major cage orientation are shown. The Y atoms are highlighted in red, the Er atoms are highlighted in green, and the Ce atoms are highlighted in yellow green.

Fig. 4

Metal-metal bonding in functionalized EMFs. Drawings show the single crystal X-ray examples of benzyl radical monoadducts or azafullerenes: La₂@I_h(7)-C₈₀(C₃N₃Ph₂), Dy₂@I_h(7)-C₈₀(CH₂Ph), Gd₂@I_h(7)-C₇₉N and Tb₂@I_h(7)-C₇₉N. The La atoms are highlighted in yellow, the Dy atoms are highlighted in light blue, the Gd atoms are highlighted in orange, and the Tb atoms are highlighted in purple.

Fig. 5

Metal-metal bonding in pristine EMFs. Drawings show the single crystal X-ray structures of diverse M₂@C_2n-type (M = Lu, Er, and Y) dimetallofullerenes: Lu₂@C₁(26)-C₈₈, Lu₂@C_s(8)-C₈₆, Lu₂@C_2v(9)-C₈₆, Lu₂@C_s(15)-C₈₆, Lu₂@C_2v(7)-C₈₄, Lu₂@D_2d(23)-C₈₄, Lu₂@C_s(6)-C₈₂, Lu₂@C_3v(8)-C₈₂, Lu₂@C_2v(5)-C₈₀, Lu₂@D_3h(5)-C₇₈, Lu₂@T_d(2)-C₇₆, Er₂@C_2v(9)-C₈₆, Er₂@C₁(12)-C₈₄, Er₂@C_s(6)-C₈₂, Er₂@C_3v(8)-C₈₂, Y₂@C_s(6)-C₈₂ and Y₂@C_3v(8)-C₈₂. The Lu atoms are highlighted in pink, the Er atoms are highlighted in green, and the Y atoms are highlighted in red.

Fig. 6

Specific non-classical cage preferences. Single crystal X-ray examples of three novel cluster metallofullerenes: Tb₃N@C₂(22,010)-C₇₈, Ho₃N@C₂(22,010)-C₇₈ and Ho₂O@C₂(13,333)-C₇₄. The Tb atoms are highlighted in purple, the Ho atoms are highlighted in claret-red, the N atoms are highlighted in blue, and the oxygen atom is highlighted in dark green. The fused-pentagons are highlighted in yellow.

Fig. 7

The macroscopic synthesis of d-block transition metal-containing EMFs. Single crystal X-ray examples of four Ti-only cluster metallofullerenes: Ti₂C₂@C_3v(8)-C₈₂, Ti₂C₂@C_s(6)-C₈₂, Ti₂C₂@D_3h(5)-C₇₈ and Ti₃C₃@I_h(7)-C₈₀. The Ti atoms are highlighted in olive green.

Fig. 8

The encapsulation of transition metals by using Group-3 metals as the core to create mixed-metal clusters. Single crystal X-ray examples of mixed-metal cluster fullerenes containing Ti or V: TiSc₂C@I_h(7)-C₈₀, TiLu₂C@I_h(7)-C₈₀, VSc₂C@I_h(7)-C₈₀, VSc₂C₂@I_h(7)-C₈₀, VSc₂N@I_h(7)-C₈₀, VSc₂N@D_5h(6)-C₈₀ and V₂ScN@I_h(7)-C₈₀. The Ti atoms are highlighted in olive green, the V atoms are highlighted in fuchsia, the Lu atoms are highlighted in pink, the Sc atoms are highlighted in brown, and the N atoms are highlighted in blue.

Fig. 9

The exploration of long-desired bonding features via the strong coordination interaction between uranium and non-metal atoms. Single crystal X-ray examples of U-containing cluster metallofullerenes: U₂C@I_h(7)-C₈₀, U₂N@I_h(7)-C₈₀, USc₂C@I_h(7)-C₈₀, U₂C₂@I_h(7)-C₈₀, U₂C₂@D_3h(5)-C₇₈, USc₂C₂@D_5h(6)-C₈₀, USc₂CN@D_5h(6)-C₈₀, UN@C₂(5)-C₈₂, UN@C_s(6)-C₈₂ and UCN@C_s(6)-C₈₂. The U atoms are highlighted in light green, the Sc atoms are highlighted in brown, and the N atoms are highlighted in blue.

Fig. 10

Biomedical applications based on water-soluble derivatives of Gd-EMFs. The very high antineoplastic efficiency of Gd@C₈₂(OH)₂₂ nanoparticles with an average size of approximately 22 nm against H₂₂ hepatoma in mice . Copyright 2005, American Chemical Society.

Fig. 11

High-performance metallofullerene single molecular magnets. The comparison of the SMM properties between Tb₂@I_h(7)-C₇₉N and Tb₂@I_h(7)-C₈₀(CH₂Ph) [129,130]. (a) The spin density distributions in Tb₂@I_h(7)-C₇₉N and Tb₂@I_h(7)-C₈₀(CH₂Ph). (b) The alignment of magnetic moments of Tb₂ dimers in Tb₂@I_h(7)-C₇₉N, as well as the magnetic hysteresis curves and blocking temperature of magnetization for Tb₂@I_h(7)-C₇₉N. (c) The alignment of magnetic moments of M₂ dimers in a series of M₂@I_h(7)-C₈₀(CH₂Ph) compounds (M = Tb, Dy, Ho and Er), as well as the magnetic hysteresis curves and blocking temperature of magnetization for Tb₂@I_h(7)-C₈₀(CH₂Ph). Copyright 2019, Springer Nature. Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA.

Fig. 12

Room-temperature logic-in-memory single-metallofullerene (Sc₂C₂@C_s(hept)-C₈₈) device. (a) Different single Sc₂C₂@C_s(hept)-C₈₈ junction configurations when states I and II are embedded in two-terminal gold electrodes. (b) Logic operations of the single-metallofullerene device. (c) The truth table of 14 fundamental Boolean logic functions implemented in the single-Sc₂C₂@C_s(hept)-C₈₈ devices. Copyright 2022, Springer Nature.