Isolation and Structure Determination of a Missing Endohedral Fullerene La@C70 through In Situ Trifluoromethylation

Synthesis and Characterization of Ionic Li+ @C70 Endohedral Fullerene

Ion-endohedral-fullerene has attracted growing interest due to the unique electronic and structural characteristics arising from its distinctive ionic nature. Although there has been only one reported ion-encapsulated fullerene, Li+ @C60 , a significant number of fundamental and applied studies have been conducted, making a substantial impact not only in chemistry and physics but also across various interdisciplinary research fields. Nevertheless, studies on ion-endohedral fullerenes are still in their infancy due to the limitations in variety, and hence, it remains an open question how the size and symmetry of fullerene, as well as the motion and position of the encapsulated ion, affect their physical/chemical properties. Herein, we report the synthesis of lithium-ion-endohedral [70]fullerene (Li+ @C70 X- , X=PF6 - and TFSI- ), a novel ionic endohedral fullerene. X-ray crystallography confirmed the encapsulation of Li+ by C70 cage as well as its ion-pair structure stabilized by external TFSI- counter anion. The encapsulated Li+ drastically lowered the orbital energy of the C70 cage by Coulomb interactions but did not affect the orbital energy gap and degeneracy. DFT studies were also performed, which supported the experimentally observed electronic effects caused by the encapsulated Li+ .

A Design Criteria to Achieve Giant Ising-Type Anisotropy in CoII -Encapsulated Metallofullerenes

Discovery of permanent magnetisation in molecules just like in hard magnets decades ago led to the proposal of utilising these molecules for information storage devices and also as Q-bits in quantum computing. A significant breakthrough with a blocking temperature as high as 80 K has been recently reported for lanthanocene complexes. While enhancing the blocking temperature further remains one of the primary challenges, obtaining molecules that are suitable for the fabrication of the devices sets the bar very high in this area. Encouraged by the fact that our earlier predictions of potential single-molecule magnets (SMMs) in lanthanide-containing endohedral fullerenes have been verified, here we set out to undertake a comprehensive study on Co^II -ion-encapsulated fullerene as potential SMMs. To study this class of molecules, we have utilised an array of theoretical methods ranging from density functional to ab initio CASSCF/NEVPT2 methods for obtaining reliable estimate of zero-field splitting parameters D and E. Additionally, we have also employed, for the first time a combination of molecular dynamics based on DFT methods coupled with CASSCF/NEVPT2 methods to seek the role of conformational isomers in the relaxation of magnetisation. Particularly, we have studied, Co@C₂₈ , Co@C₃₈ and Co@C₄₈ cages and their isomers as potential target molecules that could yield substantial magnetic anisotropy. Our calculations categorically reveal a very large Ising anisotropy in this class of molecules, with Co@C₄₈ cages predicted to yield D values as high as -127 cm^-1 . Our calculations on the smaller cages reveal the free movement of Co^II ion inside the cage, leading to the likely scenario of faster relaxation of magnetisation. However, larger fullerene cages were found to solve this issue. Further models with incorporating units such as {CoOZn}, {CoScZnN} inside larger fullerenes yield axial zero-field splitting values as high as -200 cm^-1 with negligible E/D values. As these units represent a strong axiality coupled with a viable way to obtain air-stable low-coordinate Co^II complexes, this opens up a new paradigm in the search of SMMs in this class of molecules.

Salvaging Reactive Fullerenes from Soot by Exohedral Derivatization

The awesome allotropy of carbon yields innumerable topologically possible cage structures of molecular carbon. This field is also related to endohedral metallofullerenes constructed by metal-atom encapsulation. Stable and soluble empty fullerenes and endohedral metallofullerenes are available in pure form in macroscopic amounts from carbon arc production or other physical processes followed by extraction and subsequent chromatographic separation. However, many other unidentified fullerene species, which must be reactive and insoluble in their pristine forms, remain in soot. These "missing" species must have extremely small HOMO-LUMO gaps and may have unconventional cage structures. Recent progress in this field has demonstrated that reactive fullerenes can be salvaged by exohedral derivatization, which can stabilize the reactive carbon cages. This concept provides a means of preparing macroscopic amounts of unconventional fullerenes as their derivatives.

Capturing the Unconventional Metallofullerene M@C66 by Trifluoromethylation: A Theoretical Study

The endohedral metallofullerenes M@C₆₆ (M=rare-earth metal) have a unique structure that violates the well-known "isolated pentagon rule" of fullerene science. Although the synthesis of M@C₆₆ has been achieved by using the arc discharge method, the solvent extraction and purification of M@C₆₆ remain challenges because of their radical character and extremely high reactivity. In this paper, the possibility of capturing these missing metallofullerenes by exohedral functionalization of the C₆₆ cage is demonstrated theoretically. Stable trifluoromethylated derivatives of Y@C₆₆ are revealed by density functional theory calculations. Mono- or poly-trifluoromethylation of Y@C₆₆ results in a closed-shell electronic configuration and a large band gap. Thus Y@C₆₆ can be greatly stabilized through trifluoromethylation. The trifluoromethyl group prefers to be attached to the fused pentagon region to relieve local steric strain. The mechanism of isomerization of Y@C₆₆ (CF₃ )₃ is also investigated and it is found that the attached trifluoromethyl group can migrate from a carbon atom to another via a transition state.