An orifice design: water insertion into C60

GeCl2-Mediated Ring Contraction toward Endofullerenes

Germanium(II) dichloride dioxane complex (GeCl2·dioxane) is often utilized as a source of molecular germylene, i.e., GeCl2, which is known to undergo oxidative 1,4-addition to conjugated substrates. Inspired by this nature resembled to carbenes, we demonstrated a C═C bond formation from two carbonyl moieties engaged in an aromatic macrocycle. This germylene-mediated reaction enables us to realize efficient synthesis of endo[60]fullerenes through the consecutive ring contraction of open-[60]fullerenes.

Chiroptical Response of Carbon Nanocages Enhanced by Achiral Guests

Opening up [60]fullerene makes itself inherently chiral without loss of congenital π-conjugation. An immoderately large aperture on [60]fullerene, however, renders the molecule less rigid and therefore it would reduce dissymmetry factors. Herein, we examined supramolecular technique in geometrical reinforcement of chiral open-[60]fullerenes by encasing achiral guests such as Ar, CO2, and CH3CN. At a lowest-energy transition, we confirmed a guest-dependency on chiroptical responses with increasing a dissymmetry factor by nearly twice to three times. It should be noted that the guests play a negligible role in electronic structures of the carbon cages. Instead, they are engaged in preclusion of an orifice shrinking not to reduce chiroptical response.

Single-Sided Delocalized Polarization of the C60 Cage and Reduced Infrared Intensities and Dipole Moment of H2O@C60

The C60 fullerene cage can encapsulate a small molecule like water and provides room to leave the encapsulated component rather isolated, but the true nature of the intracomplex interactions should be further elucidated for better understanding and utility of this series of complexes. Here, an analysis toward this goal is conducted for H2O@C60 by infrared spectral measurements and theoretical calculations. It is shown that the response of the π electrons of the C60 cage upon encapsulating a water molecule is single-sided and delocalized in that the electron density is partially transferred from the -z side to the +z side of the cage (when the z axis is taken along the water dipole) but almost only inside the cage, explaining the significant reduction of the dipole moment and the infrared intensities. Those infrared intensities have a large temperature dependence in a way that the bands gain intensities upon lowering the temperature down to 10 K, possibly due to coupling with lattice phonons.

Direct Through-Space Substituent-π Interactions in Noncovalent Arene-Fullerene Assemblies

The arene-arene interactions between electron-rich and deficient aromatics have been less understood. Herein, we focus on a [60]fullerene π-surface as an electron-deficient aromatics. Using a 1H signal of H2O@C60 as a magnetic probe, the presence of benzene-fullerene interactions was confirmed. To investigate substituent effects on the noncovalent arene-fullerene interactions, NMR titration experiments were carried out using an open-[60]fullerene and a series of substituted benzenes, i. e., PhX (X=NO2, CN, Cl, OMe, H, CH3, and NH2), demonstrating a 1 : 2 stoichiometry with a positive correlation between stabilization energies upon the first association (ΔG1) and Hammet constants (σm). The destabilization of the self-assembled structure for X=OMe with a σ-withdrawing nature clearly showed direct through-space substituent-π interactions describable by the Wheeler-Houk model while the second association was suggested to be considerably perturbed by the secondary effects.

Open-[60]fullerenols with water adsorbed both inside and outside

The water affinity on [60]fullerenols was found to be governed by surface electrostatic potential while water aggregation is initiated by the hydroxy groups attached on the carbon surface. The molecular water adsorption at the internal sphere caused a significnat inhibition of water adsorption at the external carbon surface.

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.

Hydrogenation of cage-opened C60 derivatives mediated by frustrated Lewis pairs

Multiply-carbonylated fullerene derivatives were found to work as one component in frustrated Lewis pairs which caused an Si-H bond activation in the presence of B(C₆F₅)₃, leading to the carbonyl hydrogenation in up to 99% yield. The Lewis acid-mediated reductive arylation also took place to furnish a corresponding ketal derivative.

Phosphorus ylides of cage-opened sulphide [60]fullerene derivatives

The replacement of a ketone with a sulfide moiety changes the electronic properties of cage-opened fullerene ylides, thus causing a hypsochromic shift in absorption and a cathodic shift of reduction potentials.

Inelastic Electron Transport and Ortho-Para Fluctuation of Water Molecule in H2O@C60 Single Molecule Transistors

Light molecules such as H₂O are the systems in which we can have access to quantum mechanical information on their constituent atoms. Here, we have investigated electron transport through H₂O@C₆₀ single molecule transistors (SMTs). The H₂O@C₆₀ SMTs exhibit Coulomb stability diagrams that show multiple tunneling-induced excited states below 30 meV. Furthermore, we have performed terahertz (THz) photocurrent spectroscopy on H₂O@C₆₀ SMTs and confirmed the same excitations. From comparison between experiment and theory, the excitations observed below 10 meV are identified to be the quantum rotational excitations of the water molecule. Surprisingly, the quantum rotational excitations of both para- and ortho-water molecule are observed simultaneously even for a single water molecule, indicating that the fluctuation between the ortho- and para-water states takes place in a time scale shorter than our measurement time (∼1 min), probably by the interaction between the encapsulated water molecule and conducting electrons.

Amino-Functionalized Cage-Opened C60 Derivatives

The amino-functionalized cage-opened [60]fullerene derivatives were synthesized by reactions with phenylenediamine. In this reaction, the diamine undergoes direct addition to the α,β-unsaturated carbonyl moiety. Further C-C bond scission is promoted by the intramolecular S_N2 reaction. These amino-functionalized derivatives possess high-lying highest occupied molecular orbital levels as suggested by electrochemical analyses. These compounds showed intense near-infrared absorption bands that tail to 900 nm, reflecting the optical transition with π-π* and charge transfer character.

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.

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.

Reactions of C60 with Pyridazine and Phthalazine

Cage-opened bisfulleroids are one of suitable building blocks for making a large hole on fullerenes. This work focuses on the Diels-Alder reaction of C₆₀ with azines, among synthetic methods developed thus far, to provide bisfulleroids. Surprisingly, the computational study predicted that the reaction proceeds with normal electron demand in contrast to hitherto considered inverse-electron-demand pathway. The benzoannulation to the pyridazine ring, i. e., phthalazine, resulted in the remarkably shortened reaction time due to the better interaction between the HOMO of phthalazine and the LUMO of C₆₀ as well as stronger 2,3-diaza-1,3-butadiene character in the phthalazine as confirmed crystallographically. Contrary to expectations, the benzobisfulleroid was converted into corresponding orifice-enlarged derivative via the photooxygenation slightly faster than the fulleroid derived from pyridazine.

Reactions on a 1,2-Dicarbonyl Moiety of a Fullerene Skeleton

A 1,2-dicarbonyl moiety on a cage-opened fullerene skeleton is one of suitable building blocks for the further derivatization. Herein, we discuss the chemical transformation of a 1,2-dicarbonyl compound into β-oxo-phosphorus ylide, acid anhydride, and α-methylene carbonyl derivatives. Despite possessing a sterically small methylene unit in the last one, the release of an encapsulated water molecule was significantly supressed whereas the β-oxo-phosphorus ylide bearing three bulky p-tolyl groups on the P-atom enabled the faster insertion/release dynamics, implying the flexibility of the phosphonium substituent. The replacement of the carbonyl group with phosphorus ylide and methylene units largely varied electrochemical properties of the fullerene skeleton, likely arising from the anionic charge delocalized over the entire molecule and removal of an electron-withdrawable carbonyl group, respectively.

Pressure-induced annulative orifice closure of a cage-opened C60 derivative

A cage-opened C60 derivative was found to undergo an unusual annulative orifice-closure reaction under high-pressure conditions, in which the orifice size changed from a 16- to a 13-membered ring. The structure was different from that obtained by the reaction at 1 atm. The theoretical calculations suggested that the formation of the former one is thermodynamically favored.