Structure of [60]fullerene with a mobile lithium cation inside

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+ .

Localization of nuclear wave functions of lithium in [Li+@C60]PF6-: molecular insights into two-site disorder-order transition

Lithium endohedral fullerene, Li⁺@C₆₀, is a porous system ideal for studying the quantized translational motion of the Li⁺ nucleus under subnanoscale confinement. The quantized nuclear motion strongly depends on the anharmonic and polarizable adsorbent potential within the C₆₀ cage, which can be perturbed by cage distortion and/or exterior ions. In our recent paper, H. Ando and Y. Nakao, Phys. Chem. Chem. Phys., 2021, 23, 9785-9803, we focused on a [Li⁺@C₆₀]PF₆^- salt and theoretically investigated how the Li⁺ ion in each C₆₀ cage is simultaneously localized at two equivalent disordered sites in 24 K < T ≪ 100 K. At 24 K, the salt exhibits a disorder-order transition, whereby every Li⁺ ion becomes mostly localized at one of the two disordered sites below that temperature. Herein we discuss the origin of this transition with special attention to the local structural distortion and intermolecular interactions. Using the Fourier grid Hamiltonian method and a model function that fits a post-Hartree-Fock potential energy surface, we obtained hundreds of low-energy nuclear wave functions of Li⁺ confined within the cage. The weak distortions of the C₆₀ cage and the PF₆^- coordination sphere below 24 K and concurrent inversion-symmetry breaking affect intermolecular interactions, thus making the wave functions of the nuclear ground state and several low-energy excited states localized around the experimental high-occupancy disordered site. This demonstrates that the distortions correlate closely with the two-site disorder-order transition. Finally, we reveal that two absorption peaks in the terahertz frequency range show substantial blueshifts upon cooling below 24 K, which serve as fingerprints of the transition.

Designing stable binary endohedral fullerene lattices

Nanoparticle lattices and endohedral fullerenes have both been identified as potential building blocks for future electronic, magnetic and optical devices; here it is proposed that it could be possible to combine those concepts and design stable nanoparticle lattices composed from binary collections of endohedral fullerenes. The inclusion of an atom, for example Ca or F, within a fullerene cage is known to be accompanied by a redistribution of surface charge, whereby the cage can acquire either a negative (Ca) or positive (F) charge. From calculations involving a combination of van der Waals and many-body electrostatic interactions, it is predicted that certain binary combinations, for example a metal (A) and a halogen (B), could result in the formation of stable nanoparticle lattices with the familiar AB and AB₂ stoichiometries. Much of the stability is due to Coulomb interactions, however, charge-induced and van der Waals interactions, which always enhance stability, are found to extend the range of charge on a cage over which lattices are stable. Some lattice types are shown to be three or four times more stable than an equivalent neutral C₆₀ structure. An extension of the calculations to the fabrication of structures involving endohedral C₈₄ is also discussed.

Heterospin frustration in a metal-fullerene-bonded semiconductive antiferromagnet

Lithium-ion-encapsulated fullerenes (Li⁺@C₆₀) are 3D superatoms with rich oxidative states. Here we show a conductive and magnetically frustrated metal–fullerene-bonded framework {[Cu₄(Li@C₆₀)(L)(py)₄](NTf₂)(hexane)}_n (1) (L = 1,2,4,5-tetrakis(methanesulfonamido)benzene, py = pyridine, NTf₂⁻ = bis(trifluoromethane)sulfonamide anion) prepared from redox-active dinuclear metal complex Cu₂(L)(py)₄ and lithium-ion-encapsulated fullerene salt (Li⁺@C₆₀)(NTf₂⁻). Electron donor Cu₂(L)(py)₂ bonds to acceptor Li⁺@C₆₀ via eight Cu‒C bonds. Cu–C bond formation stems from spontaneous charge transfer (CT) between Cu₂(L)(py)₄ and (Li⁺@C₆₀)(NTf₂⁻) by removing the two-terminal py molecules, yielding triplet ground state [Cu₂(L)(py)₂]⁺(Li⁺@C₆₀^•−), evidenced by absorption and electron paramagnetic resonance (EPR) spectra, magnetic properties and quantum chemical calculations. Moreover, Li⁺@C₆₀^•− radicals (S = ½) and Cu²⁺ ions (S = ½) interact antiferromagnetically in triangular spin lattices in the absence of long-range magnetic ordering to 1.8 K. The low-temperature heat capacity indicated that compound 1 is a potential candidate for an S = ½ quantum spin liquid (QSL).

Conductive and magnetically frustrated solids may enable the development of high-performance molecule-based spintronic devices. Here the authors report a conductive and magnetically frustrated metal–fullerene-bonded framework prepared from a redox-active dinuclear copper complex and lithium ion-encapsulated fullerenes.

Quantum states of the endohedral fullerene Li+@C60 surrounded by anions: energy decomposition analysis of nuclear wave functions

Lithium is the lightest metal element. To date, little is known about its quantized nuclear motion in nanoscale porous structures. Endohedral fullerene Li+@C60 is an ideal porous system for studying such a quantized motion. Recent studies suggest that the anions surrounding the C60 cage exterior and a slight cage distortion can alter the potential field in the cage interior and thus the nuclear wave function of Li+. It has yet to be clarified how the electronic state, particularly the flexible π electron cloud of the C60 cage, is associated with (de)localization of the Li+ wave function. Focusing on the [Li+@C60]PF6- crystal, we constructed a local structure model considering the PF6- coordination and the cage distortion. We developed model functions that fit the post-Hartree-Fock potential energy surface for the Li+ motion and its decomposed components, four interaction energy surfaces. The decomposition clarified the origins of the shell-like adsorbent potential and the potential wells therein. The Fourier grid Hamiltonian method allowed us to obtain low-energy Li+ wave functions. The ground state is nearly two-fold degenerate, and its wave functions are mostly localized underneath two C6 rings, near the disordered sites of Li+ in the X-ray crystal structure. By extending the energy decomposition analysis within the clamped-nuclei approximation to incorporate the delocalization of nuclear wave functions, we demonstrated that the ground state is stabilized by the polarization, dispersion, and electrostatic interactions. Beyond the common picture of Li+ moving in a classical electrostatic field, our approach will deepen the understanding of the flexible Li+ wave function confined in a polarizable porous structure by various intermolecular interactions.

Strong intramolecular hydrogen bonding in confined amino acids

Intramolecular hydrogen bonding is evaluated in three different amino acids encapsulated in C₆₀ fullerene in the context of electron density analysis. While conventional intramolecular hydrogen bonding in isolated amino acids are dominated by electrostatic character, it is shown that strong intramolecular hydrogen bonding can be formed in confined amino acids so that in two cases covalent intramolecular hydrogen bonding is appeared in the confined species. Also, results show that zwitterionic amino acids are stable in confined state, where no implicit or explicit solvation is applied. Covalent character for intramolecular hydrogen bonding in amino acids have not yet been reported.

DCM self-trapping by the host deformation in flexible host–guest molecules

The desolvated structure can self-trap the DCM molecules to return to the 1·DCM state via ligand deformation even under weak host–guest interactions. The capture behavior of DCM is mostly due to the flexibility of the ligand.

Molecular spinning top: visualizing the dynamics of M3N@C80 with variable temperature single crystal X-ray diffraction

Variable temperature X-ray diffraction studies on two single crystals containing M3N@C80, i.e., Ho2LuN@C80·NiOEP·2(C6H6) and Lu3N@C80·NiOEP·2(C6H6), (NiOEP = Nickel octaethylporphyrin) unravelled the temperature dependent rotation of the M3N cluster and C80 cage on the static NiOEP molecule.

The thermodynamic properties and molecular dynamics of [Li+@C60](PF6-) associated with structural phase transitions

Calorimetric and terahertz-far-infrared (THz-FIR) spectroscopic and infrared (IR) spectroscopic measurements were conducted for [Li+@C60](PF6-) at temperatures between 1.8 and 395 K. [Li+@C60](PF6-) underwent a structural phase transition at around 360 K accompanied by the orientational order-disorder transition of Li+@C60 and PF6-. The transition occurred in a step-wise manner. The total transition entropy (ΔtrsS) of 40.1 ± 0.4 J K-1 mol-1 was smaller than that of the orientational order-disorder transition in a pristine C60 crystal (ΔtrsS = 45.4 ± 0.5 J K-1 mol-1). Thus, the orientational disorder of Li+@C60 in the high-temperature phase of [Li+@C60](PF6-) was much less excited than that of the pristine C60 owing to the Coulombic interactions, which stabilized the ionic crystal lattice of [Li+@C60](PF6-). At T < 100 K, upon cooling, Li+ ions were trapped in two pockets on the inner surface of C60, and no phase transition was observed. Finally, the Li+ ions achieved a complete order at 24 K through antiferroelectric transition. The ΔtrsS value of 4.6 ± 0.4 J K-1 mol-1 was slightly smaller than R ln 2 = 5.76 J K-1 mol-1 expected for the two-site order-disorder transition. The extent of the Li+ motion in the C60 cage was related to the selection rule in the THz-FIR and IR spectroscopy of the C60 internal vibrations, because a C60 cage should be polarized by the Li+ ion. It is shown that the local symmetry of the caged molecule can be modified by the rotational or hopping motion of the encaged ions.

C70 Fullerene Cage as a Novel Catalyst for Efficient Proton Transfer Reactions between Small Molecules: A Theoretical study

When acids are supplied with an excess electron (or placed in an Ar or the more polarizable N₂ matrix) in the presence of species such as NH₃, the formation of ion-pairs is a likely outcome. Using density functional theory and first-principles calculations, however, we show that, without supplying an external electron or an electric field, or introducing photo-excitation and -ionization, a single molecule of HCl or HBr in the presence of a single molecule of water inside a C₇₀ fullerene cage is susceptible to cleavage of the σ-bond of the Brønsted-Lowry acid into X⁻ and H⁺ ions, with concomitant transfer of the proton along the reaction coordinate. This leads to the formation of an X⁻···⁺HOH₂ (X = Cl, Br) conjugate acid-base ion-pair, similar to the structure in water of a Zundel ion. This process is unlikely to occur in other fullerene derivatives in the presence of H₂O without significantly affecting the geometry of the carbon cage, suggesting that the interior of C₇₀ is an ideal catalytic platform for proton transfer reactions and the design of related novel materials. By contrast, when a single molecule of HF is reacted with a single molecule of H₂O inside the C₇₀ cage, partial proton transfers from HF to H₂O is an immediate consequence, as recently observed experimentally. The geometrical, energetic, electron density, orbital, optoelectronic and vibrational characteristics supporting these observations are presented. In contrast with the views that have been advanced in several recent studies, we show that the encaged species experiences significant non-covalent interaction with the interior of the cage. We also show that the inability of current experiments to detect many infrared active vibrational bands of the endo species in these systems is likely to be a consequence of the substantial electrostatic screening effect of the cage.

Endohedral alkali cations promote charge transfer transitions in complexes of C60 with [10]cycloparaphenylenes

The endohedral alkali cations in M⁺@C₆₀⋯[10]CPP complexes boost the near infrared absorption bands associated with charge transfer from the nanoring to the fullerene.