Open-cage fullerenes as ligands for metals

The remarkable structures of open-cage fullerenes with functionalization on the outer surface and an accessible inner void make them interesting ligands for reactions with metal complexes. The behaviors of open-cage fullerenes in reactions with various metal complexes are examined and compared to the corresponding reactions with intact fullerenes. The structural results from X-ray diffraction are emphasized. Open-cage fullerenes frequently undergo unanticipated structural changes such as carbon-carbon bond cleavage upon reactions with metal complexes. Much more remains to be learned about the possibility of inserting metal ions larger than Li+ into the interior void of these open-cage fullerenes and about the effects of redox reactions on metal complexes of open-cage fullerenes.

Synthesis of endohedral fullerenes by molecular surgery

Encapsulation of atoms or small molecules inside fullerenes provides a unique opportunity for study of the confined species in the isolated cavity, and the synthesis of closed C₆₀ or C₇₀ fullerenes with enclosed atoms or molecules has recently developed using the method of 'molecular surgery'; in which an open-cage intermediate fullerene is the host for encapsulation of a guest species, before repair of the cage opening. In this work we review the main methods for cage-opening and closure, and the achievements of molecular surgery to date.

H2O inside the fullerene C60: Inelastic neutron scattering spectrum from rigorous quantum calculations

We present a methodology that, for the first time, allows rigorous quantum calculation of the inelastic neutron scattering (INS) spectra of a triatomic molecule in a nanoscale cavity, in this case, H₂O inside the fullerene C₆₀. Both moieties are taken to be rigid. Our treatment incorporates the quantum six-dimensional translation-rotation (TR) wave functions of the encapsulated H₂O, which serve as the spatial parts of the initial and final states of the INS transitions. As a result, the simulated INS spectra reflect the coupled TR dynamics of the nanoconfined guest molecule. They also exhibit the features arising from symmetry breaking observed for solid H₂O@C₆₀ at low temperatures. Utilizing this methodology, we compute the INS spectra of H₂O@C₆₀ for two incident neutron wavelengths and compare them with the corresponding experimental spectra. Good overall agreement is found, and the calculated spectra provide valuable additional insights.

Vibronic Coupling in Spherically Encapsulated, Diatomic Molecules: Prediction of a Renner-Teller-like Effect for Endofullerenes

In the year 1933, Herzberg and Teller realized that the potential energy surface of a triatomic, linear molecule splits into two as soon as the molecule is bent. The phenomenon, later dubbed the Renner–Teller effect due to the detailed follow-up work of Renner on the subject, describes the coupling of a symmetry-reducing molecular vibration with degenerate electronic states. In this article, we show that a very similar type of nonadiabatic coupling can occur for certain translational degrees of freedom of diatomic, electronically degenerate molecules when trapped in a nearly spherical or cylindrical quantum confinement, e.g., realized through electromagnetic fields or molecular encapsulation. We illustrate this on the example of fullerene-encapsulated nitric oxide, and provide a prediction of its interesting, perturbed vibronic spectrum.

Noncovalently bound molecular complexes beyond diatom–diatom systems: full-dimensional, fully coupled quantum calculations of rovibrational states

The methodological advances made in recent years have significantly extended the range and dimensionality of noncovalently bound molecular complexes for which full-dimensional quantum calculations of their rovibrational states are feasible.

Reactivity of the superhalogen/superalkali ion encapsulating C60 fullerenes

The Diels-Alder cycloaddition reaction between 1,3-cyclohexadiene and a series of C₆₀ fullerenes with encapsulated (super)alkali/(super)halogen species (Li⁺@C₆₀, Li₂F⁺@C₆₀, Cl^-@C₆₀, and LiF₂^-@C₆₀) was explored by means of DFT calculations. The reactivity of the ion encapsulating systems was compared to that of the parent C₆₀ fullerene. Significant enhancement in reactivity was found for cation-encapsulating Li⁺/Li₂F⁺@C₆₀ complexes. The cycloadduct formed by LiF₂^-@C₆₀ was found to be the most thermodynamically favorable among the studied ones. In contrast, encapsulation of Cl^- anions disfavors the cycloaddition reaction both kinetically and thermodynamically. Higher activation energy barrier and less stability of the reaction product in the case of Cl^-@C₆₀ were associated with the higher deformation energies of the fullerene cage and the lower interaction energy between the reactants in comparison with the other studied complexes.

Reductive Decarbonylation of a Cage-Opened C60 Derivative

The decarbonylation of a cage-opened C₆₀ derivative was examined by employing single-electron reductants. During the reaction, an H₂O molecule was spontaneously encapsulated inside the cage (up to 78%) through the thus-formed 14-membered-ring orifice even though the H₂O encapsulation had long been considered to require an orifice consisting of at least 16 atoms. The crystallographic analysis revealed an orifice shape closer to a circle which significantly contributes to the decreased activation barrier for the H₂O encapsulation.

Water-Mediated Thermal Rearrangement of a Cage-Opened C60 Derivative

The partial zipping of a fullerene orifice was achieved by a water-mediated thermal rearrangement at 150 °C for one day while the orifice size changed from 16- to 14-membered ring with the generation of a fused pentagon. The addition of B(C6 F5 )3 was found to facilitate the reaction likely due to the coordination to carbonyl groups on the orifice. By extending the reaction time, the decarbonylation took place to give another 14-membered-ring orifice where the Michael addition of water occurred under acidic conditions. The computational study suggested that the formation of a carboxylic acid and Fischer-type carbene plays a key role in the C-C bond cleavage/reformation processes during the rearrangement.

A Singular Molecule-to-Molecule Transformation on Video: The Bottom-Up Synthesis of Fullerene C60 from Truxene Derivative C60H30

Singular reaction events of small molecules and their dynamics remain a hardly understood territory in chemical sciences since spectroscopy relies on ensemble-averaged data, and microscopic scanning probe techniques show snapshots of frozen scenes. Herein, we report on continuous high-resolution transmission electron microscopic video imaging of the electron-beam-induced bottom-up synthesis of fullerene C₆₀ through cyclodehydrogenation of tailor-made truxene derivative 1 (C₆₀H₃₀), which was deposited on graphene as substrate. During the reaction, C₆₀H₃₀ transformed in a multistep process to fullerene C₆₀. Hereby, the precursor, metastable intermediates, and the product were identified by correlations with electron dose-corrected molecular simulations and single-molecule statistical analysis, which were substantiated with extensive density functional theory calculations. Our observations revealed that the initial cyclodehydrogenation pathway leads to thermodynamically favored intermediates through seemingly classical organic reaction mechanisms. However, dynamic interactions of the intermediates with the substrate render graphene as a non-innocent participant in the dehydrogenation process, which leads to a deviation from the classical reaction pathway. Our precise visual comprehension of the dynamic transformation implies that the outcome of electron-beam-initiated reactions can be controlled with careful molecular precursor design, which is important for the development and design of materials by electron beam lithography.

Photochemical Orifice Expansion of a Cage-Opened C60 Derivative

Upon light irradiation, a tetraketosulfoxide derivative of C₆₀ was transformed into a diketosulfide carboxylic anhydride via intermolecular nucleophilic addition of the sulfoxide moiety. The thus-formed 18-membered ring enables a spontaneous insertion of an Ar atom. In this encapsulation/release process, the phenyl ring on the orifice works as a dynamic stopper, which potentially adopts three conformations: an open form reduces distortion energy at the transition state while semiopen and closed forms reduce the orifice size.

Synthesis of a Dihydroxylated Open-Cage [70]Fullerene by a Reductive Ring-Closure Reaction

Upon heating an open-cage diketo C₇₀ derivative in carbon disulfide, the 1,2-dihydroxylated compound was unprecedentedly formed via a reductive ring-closure reaction. The crystallographic analysis of the diketo derivative revealed a benzenoid character on its orifice which suppresses the nucleophilic addition. The 1,2-dihydroxylated derivative could be transformed into a 1,4-substituted one by an acid-catalyzed reaction. The observed unique reactivity of the diketo derivative is inherently different from those of structurally related C₇₀ isomers as well as a C₆₀ analogue.

An orifice design: water insertion into C60

Using an open-cage C60 derivative possessing an orifice designed on the basis of computational studies, we have experimentally demonstrated the quantitative encapsulation of H2O as well as effective conversion into H2O@C60 in an overall yield remarkably higher than the previously reported methods by ca. 2-5 times.

Cation recognition on a fullerene-based macrocycle

Heterocyclic orifices in cage-opened fullerene derivatives are regarded as potential ligands toward metals or ions, being reminiscent of truncated fullerenes as a hypothetical class of macrocycles with spherical π-conjugation. Among a number of cage-opened examples reported thus far, the coordination ability and dynamic behavior in solution still remained unclear due to difficulties in structural determination with multiple coordination sites on the macrocycles. Herein, we present the detailed solution dynamics of a cage-opened C60 derivative bearing a diketo bis(hemiketal) moiety in the presence of alkali metal ions. The NMR spectroscopy disclosed the coordination behavior which is identified as a two-step process with a 1 : 2 stoichiometry. Upon coordination to the Li+ ion, the macrocycle largely varies its properties, i.e., increased absorption coefficients in the visible region due to weakly-allowed charge transfer transitions as well as the inner potential field from neutral to positive by the charge delocalization along with the spherical π-surface. The Li+-complexes formed in situ underwent unprecedented selective dehydroxyhydrogenation under high-pressure conditions. These findings would facilitate further studies on fullerene-based macrocycles as metal sensors, bulky ligands in organic reactions, and ion carriers in batteries and biosystems.

Wavelength-Dependent Efficiency of Sequential Photooxygenation: C=C Bond Cleavage on Open-Cage C60 Derivatives

We have studied the efficiency of the C=C double bond cleavage on open-cage fullerene C₆₀ derivatives by singlet oxygen using four different LED lamps including UV, blue, green, and red radiation. The results indicate that red radiation is the most efficient for this photochemical oxidation owing to the sufficient penetration of the longer-wavelength light. By applying this method to a 2-methoxyethoxymethyl-substituted azafulleroid derivative, we demonstrated the sequential double photooxygenation which provides a symmetric diketo imide derivative having a 15-membered-ring opening via stepwise cleavage of two C=C double bonds. Using a H₂ -encapsulating derivative, we further examined the size of the opening by comparison with the structural isomer, based on H₂ -release experiments and theoretical calculations.

First Synthesis and Characterization of CH4 @C60

The endohedral fullerene CH4 @C60 , in which each C60 fullerene cage encapsulates a single methane molecule, has been synthesized for the first time. Methane is the first organic molecule, as well as the largest, to have been encapsulated in C60 to date. The key orifice contraction step, a photochemical desulfinylation of an open fullerene, was completed, even though it is inhibited by the endohedral molecule. The crystal structure of the nickel(II) octaethylporphyrin/ benzene solvate shows no significant distortion of the carbon cage, relative to the C60 analogue, and shows the methane hydrogens as a shell of electron density around the central carbon, indicative of the quantum nature of the methane. The 1 H spin-lattice relaxation times (T1 ) for endohedral methane are similar to those observed in the gas phase, indicating that methane is freely rotating inside the C60 cage. The synthesis of CH4 @C60 opens a route to endofullerenes incorporating large guest molecules and atoms.

Germanium Fluoride Nanocages as Optically Transparent n-Type Materials and Their Endohedral Metallofullerene Derivatives

Carbon- and silicon-based n-type materials tend to suffer from instability of the corresponding radical anions. With DFT calculations, we explore a promising route to overcome such challenges with molecular nanocages which utilize the heavier element Ge. The addition of fluorine substituents creates large electron affinities in the range 2.5-5.5 eV and HOMO-LUMO gaps between 1.6 and 3.2 eV. The LUMOs envelop the surfaces of these structures, suggesting extensive delocalization of injected electrons, analogous to fullerene acceptors. Moreover, these Ge _nF _n inorganic cages are found to be transparent in the UV-visible region as probed with their excited states. Their capacitance, linear polarizabilities, and dielectric constants are computed and found to be on the same order of magnitude as saturated oligomers and some extended π-organics (azobenzenes). Furthermore, we explore fullerene-type endohedral isomers, i.e., cages with internal substituents or guest atoms, and find them to be more stable than the parent exohedral isomers by up to -206.45 kcal mol^-1. We also consider the addition of Li, He, Cs, and Bi, to probe the utility of the exo/ endo cages as host-guest systems. The endohedral He/Li@F₈@Ge₆₀F₅₂ cages are significantly more stable than their parent exohedral isomers He/Li@Ge₆₀F₅₂ by -182.46 and -49.22 kcal mol^-1, respectively. The energy of formation of endohedral He@F₈@Ge₆₀F₅₂ is exothermic by -10.4 kcal mol^-1, while Cs and Bi guests are too large to be accommodated but are stable in the exohedral parent cages. Conceivable applications of these materials include n-type semiconductors and transparent electrodes, with potential for novel energy storage modalities.

Effects of symmetry breaking on the translation-rotation eigenstates of H2, HF, and H2O inside the fullerene C60

Splittings of the translation-rotation (TR) eigenstates of the solid light-molecule endofullerenes M@C60 (M = H2, H2O, HF) attributed to the symmetry breaking have been observed in the infrared (IR) and inelastic neutron scattering spectra of these species in the past couple of years. In a recent paper [Felker et al., Phys. Chem. Chem. Phys., 2017, 19, 31274], we established that the electrostatic, quadrupolar interaction between the guest molecule M and the twelve nearest-neighbor C60 cages of the solid is the main source of the symmetry breaking. The splittings of the three-fold degenerate ground states of the endohedral ortho-H2, ortho-H2O and the j = 1 level of HF calculated using this model were found to be in excellent agreement with the experimental results. Utilizing the same electrostatic model, this theoretical study investigates the effects of the symmetry breaking on the excited TR eigenstates of the three species, and how they manifest in their simulated low-temperature (5-6 K) near-IR (NIR) and far-IR (FIR) spectra. The TR eigenstates are calculated variationally for both the major P and minor H crystal orientations. For the H orientation, the calculated splittings of all of the TR levels of these species are less than 0.1 cm-1. For the dominant P orientation, the splittings vary strongly depending on the character of the excitations involved. In all of the species, the splittings of the higher rotationally excited levels are comparable in magnitude to those for the j = 1 levels. For the levels corresponding to purely translational excitations, the calculated splittings are about an order of magnitude smaller than those of the purely rotational eigenstates. Based on the computed TR eigenstates, the low-temperature NIR (for M = H2) and FIR (for M = HF and H2O) spectra are simulated for both the P and H orientations, and also combined as their weighted sum (0.15H + 0.85P). The weighted sum spectra computed for M = H2 and HF match quantitatively the corresponding measured spectra, while for M = H2O, the weighted sum FIR spectrum predicts features that can potentially be observed experimentally.

Anion-encapsulating fullerenes behave as large anions: a DFT study

M06L/6-311++G(d,p)//M06L/6-31G(d,p) level density functional theory studies show that the endohedral reaction of C60 with X- (X = F, Cl, Br, OH, NH2, NO2, CN, and ClO) is exothermic by 37.8-65.2 kcal mol-1. The exothermic character of the reaction is drastically reduced in polar and nonpolar solvents due to the lack of direct solvation influence on the encapsulated anion. In all X-@C60, the occupied frontier molecular orbitals (FMOs) are located on X- while the energy levels of FMOs centered on C60 are very similar to those of the C60- radical anion. Molecular electrostatic potential (MESP) analysis of X-@C60 revealed that the negative character of the MESP minimum (Vmin) on the carbon cage increases by ∼72 fold compared to C60, which is very similar to the enhancement in the negative MESP observed on the C60- radical anion. The MESP data and quantum theory of atoms in molecules (QTAIM) analysis of charge, electron delocalization index, and Laplacian of bond critical point (bcp) support significant electron sharing from the anion to the carbon atoms of the fullerene cage, which makes the cage behave like a very large anion in a closed shell configuration. The data are also supportive of a multicenter charge-shift type of bonding interaction between the anion and the carbon cage. The anionic nature of the fullerene cage has been verified in the cases of larger systems such as Cl-@C70, Cl-@C84, and Cl-@C90. The binding of a counter cation K+ with X-@C60 is found to be highly exothermic (∼72 kcal mol-1) and very similar to the binding of K+ with the C60- radical anion (72.9 kcal mol-1), which suggests that C60 in X-@C60 behaves as a closed shell anion.

Perspective: Accurate treatment of the quantum dynamics of light molecules inside fullerene cages: Translation-rotation states, spectroscopy, and symmetry breaking

In this perspective, I review the current status of the theoretical investigations of the quantum translation-rotation (TR) dynamics and spectroscopy of light molecules encapsulated inside fullerenes, mostly C₆₀ and C₇₀. The methodologies developed in the past decade allow accurate quantum calculations of the TR eigenstates of one and two nanoconfined molecules and have led to deep insights into the nature of the underlying dynamics. Combining these bound-state methodologies with the formalism of inelastic neutron scattering (INS) has resulted in the novel and powerful approach for the quantum calculation of the INS spectra of a diatomic molecule in a nanocavity with an arbitrary geometry. These simulations have not only become indispensable for the interpretation and assignment of the experimental spectra but are also behind the surprising discovery of the INS selection rule for diatomics in near-spherical nanocavities. Promising directions for future research are discussed.

Probing the interaction between the encapsulated water molecule and the fullerene cages in H2O@C60- and H2O@C59N. Chem Sci 2018;9:5666-5671. [PMID: 30062000 PMCID: PMC6050629 DOI: 10.1039/c8sc01031e]

We report a high-resolution photoelectron imaging study of cryogenically-cooled H2O@C60- and H2O@C59N- endohedral fullerene anions. The electron affinity (EA) of H2O@C60 is measured to be 2.6923 ± 0.0008 eV, which is 0.0088 eV higher than the EA of C60, while the EA of H2O@C59N is measured to be 3.0058 eV ± 0.0007 eV, which is 0.0092 eV lower than the EA of C59N. The opposite shifts are found to be due to the different electrostatic interactions between the encapsulated water molecule and the fullerene cages in the two systems. There is a net coulombic attraction between the guest and host in H2O@C60-, but a repulsive interaction in H2O@C59N-. We have also observed low-frequency features in the photoelectron spectra tentatively attributed to the hindered rotational excitations of the encapsulated H2O molecule, providing further insights into the guest-host interactions in H2O@C60- and H2O@C59N-.

Synthesis and self-sensitized photo-oxidation of 2-fulleropyrrolines by palladium(ii)-catalyzed heteroannulation of [60]fullerene with benzoyl hydrazone esters

A Pd(OAc)₂-catalyzed N-heteroannulation reaction was exploited to synthesize N-unsubstituted 2-fulleropyrrolines and the photo-oxidation of 2-fulleropyrrolines was first investigated.

Structural modification of open-cage fullerene C60 derivatives having a small molecule inside their cavities

We studied the chemical modifications of open-cage fullerene C60 derivatives encapsulating a small molecule such as H2 and H2O. In these reactions, the entrapped molecules can be used as a probe for monitoring the reactions. The solubilizing group (methoxyethyl group) was successfully introduced on the bis(hemiketal) moiety in the open-cage C60 derivative. Further specific reactions were also investigated for 2-methoxyethoxymethyl (MEM)-substituted C60 derivatives.

Regioselective Diels–Alder reaction to open-cage ketolactam derivatives of C60

Open-cage ketolactam fullerenes reacted with dienes on the rim of the orifice both regio- and endo-selectively, which were confirmed by 2D INADEQUATE ¹³C NMR of ¹³C enriched material/HMBC spectra as well as the theoretical calculations.

Explaining the symmetry breaking observed in the endofullerenes H2@C60, HF@C60, and H2O@C60

Symmetry breaking has been recently observed in the endofullerenes M@C₆₀ (M = H₂, HF, H₂O), manifesting in the splittings of the three-fold degenerate ground states of the endohedral ortho-H₂, ortho-H₂O and the j = 1 level of HF.

Iridium-catalyzed reductive carbon-carbon bond cleavage reaction on a curved pyridylcorannulene skeleton

The cleavage of CC bonds in π-conjugated systems is an important method for controlling their shape and coplanarity. An efficient way for the cleavage of an aromatic CC bond in a typical buckybowl corannulene skeleton is reported. The reaction of 2-pyridylcorannulene with a catalytic amount of IrCl3 ⋅n H2 O in ethylene glycol at 250 °C resulted in a structural transformation from the curved corannulene skeleton to a strain-free flat benzo[ghi]fluoranthene skeleton through a site-selective CC cleavage reaction. This cleavage reaction was found to be driven by both the coordination of the 2-pyridyl substituent to iridium and the relief of strain in the curved corannulene skeleton. This finding should facilitate the design of carbon nanomaterials based on CC bond cleavage reactions.

Regioselective cage opening of La2 @D2 (10611)-C72 with 5,6-diphenyl-3-(2-pyridyl)-1,2,4-triazine

The thermal reaction of the endohedral metallofullerene La2 @D2 (10611)-C72 , which contains two pentalene units at opposite ends of the cage, with 5,6-diphenyl-3-(2-pyridyl)-1,2,4-triazine proceeded selectively to afford only two bisfulleroid isomers. The molecular structure of one isomer was determined using single-crystal X-ray crystallography. The results suggest that the [4+2] cycloaddition was initiated in a highly regioselective manner at the C-C bond connecting two pentagon rings of C72 . Subsequent intramolecular electrocyclization followed by cycloreversion resulted in the formation of an open-cage derivative having three seven-membered ring orifices on the cage and a significantly elongated cage geometry. The reduction potentials of the open-cage derivatives were similar to those of La2 @D2 -C72 whereas the oxidation potentials were shifted more negative than those of La2 @D2 -C72 . These results point out that further oxidation could occur easily in the derivatives.