Facile Access to Azafullerenyl Cation C₅₉N⁺ and Specific Interaction with Entrapped Molecules

The facile preparation of azafullerenyl cation C₅₉N⁺ has been achieved by the assistance of trifluoromethanesulfonic acid. The thus formed C₅₉N⁺ was quite stable in solution over 1 month and can be used as an intermediate for the electrophilic reaction. Applying this method to endohedral azafullerenes, corresponding cations (H₂@C₅₉N⁺ and H₂O@C₅₉N⁺) were prepared and the dynamic behavior of entrapped molecules was studied on the basis of ¹H NMR relaxation time measurements. The results indicated that there is strong intramolecular C₅₉N⁺···O^δ−H₂ interaction in H₂O@C₅₉N⁺, which stands in contrast to isoelectronic H₂O@C₆₀ with no electrostatic interaction. We also demonstrated that the magnetic shielding environment inside the C₅₉N⁺ cage closely resembles that for isoelectronic C₆₀

Synthesis of Open-Cage Ketolactam Derivatives of Fullerene C₆₀ Encapsulating a Hydrogen Molecule

A novel open-cage fullerene C₆₀ derivative having a bis­(hemiketal) moiety was synthesized by the reaction of C₆₀-<i>N</i>-MEM-ketolactam (MEM: 2-methoxyethoxymethyl) with <i>N</i>-methylmorpholine <i>N</i>-oxide in the presence of water. The structure was clearly determined by single crystal X-ray analysis. Further enlargement of the opening was performed by treatment with trifluoroacetic anhydride to give the tetraketo derivative having a 15-membered ring opening. For H₂-insertion into the cage, the derivative was exposed to a high pressure of H₂. After the encapsulation, the opening size was reduced to the original one while keeping the hydrogen molecule inside the cage. This compound can be a possible precursor for endohedral azafullerenes encapsulating a hydrogen molecule

Dithieno-Fused Polycyclic Aromatic Hydrocarbon with a Pyracylene Moiety: Strong Antiaromatic Contribution to the Electronic Structure

A synthetic route to dithieno-fused CP-PAHs with a pyracylene segment is reported. A combination of experimental and theoretical studies revealed a strong contribution of antiaromatic character to the electronic structure of this dithieno-fused CP-PAH. Anisotropy of current-induced density (ACID) calculations indicated a significantly increased paramagnetic ring current on the two pentagonal rings, which is more prominent than that of the dibenzo-fused analogue. Furthermore, enhanced electron affinity and a consequently decreased HOMO–LUMO gap were observed for this dithieno-fused CP-PAH

Dithieno-Fused Polycyclic Aromatic Hydrocarbon with a Pyracylene Moiety: Strong Antiaromatic Contribution to the Electronic Structure

A synthetic route to dithieno-fused CP-PAHs with a pyracylene segment is reported. A combination of experimental and theoretical studies revealed a strong contribution of antiaromatic character to the electronic structure of this dithieno-fused CP-PAH. Anisotropy of current-induced density (ACID) calculations indicated a significantly increased paramagnetic ring current on the two pentagonal rings, which is more prominent than that of the dibenzo-fused analogue. Furthermore, enhanced electron affinity and a consequently decreased HOMO–LUMO gap were observed for this dithieno-fused CP-PAH

Dithieno-Fused Polycyclic Aromatic Hydrocarbon with a Pyracylene Moiety: Strong Antiaromatic Contribution to the Electronic Structure

A synthetic route to dithieno-fused CP-PAHs with a pyracylene segment is reported. A combination of experimental and theoretical studies revealed a strong contribution of antiaromatic character to the electronic structure of this dithieno-fused CP-PAH. Anisotropy of current-induced density (ACID) calculations indicated a significantly increased paramagnetic ring current on the two pentagonal rings, which is more prominent than that of the dibenzo-fused analogue. Furthermore, enhanced electron affinity and a consequently decreased HOMO–LUMO gap were observed for this dithieno-fused CP-PAH

Co(I)-Mediated Removal of Addends on the C₆₀ Cage and Formation of the Monovalent Cobalt Complex CpCo(CO)(η²‑C₆₀)

The removal of addends on the fullerene C₆₀ cage plays an important role in the final stage for synthesizing endohedral fullerenes by the molecular surgery method. We developed a cobalt-mediated reaction to regenerate C₆₀ from <i>N</i>-substituted C₆₀ derivatives (aziridinofullerene and azafulleroid). In these reactions, we found the formation of a green monovalent-cobalt complex of C₆₀, and its structure was unambiguously determined by X-ray analysis. The characteristic electronic structure of this cobalt complex was studied by IR and UV–vis absorption spectroscopy and electrochemical analyses

Palladium-Catalyzed Cyclization: Regioselectivity and Structure of Arene-Fused C₆₀ Derivatives

The palladium-catalyzed cyclization on the fullerene C₆₀ cage has been achieved using several aryl halides and C₆₀. This reaction was found to be accelerated by the addition of pivalic acid, which can be rationally explained by the computational study based on the concerted metalation–deprotonation mechanism. We also demonstrated the regioselective π-functionalization using prefunctionalized designed molecules possessing the same substructure on the C₆₀ cage. The single crystal X-ray analysis and electrostatic potential map revealed that the orientation of entrapped H₂O inside the naphthalene-fused open-cage C₆₀ derivative is electrostatically demanded due to the naphthalene-fusion and construction of the opening

Synthesis and Properties of Endohedral Aza[60]fullerenes: H₂O@C₅₉N and H₂@C₅₉N as Their Dimers and Monomers

The macroscopic-scale syntheses of the first endohedral aza[60]­fullerenes X@C₅₉N (X = H₂O, H₂) were achieved in two different ways: (1) synthesis from endohedral fullerene H₂O@C₆₀ as a starting material and (2) molecular surgical synthesis from a C₅₉N precursor having a considerably small opening. In the neutral state of H₂O@C₅₉N, we expected the H-bonding interaction or repulsive N–O interaction between entrapped H₂O and a nitrogen atom on the C₅₉N cage. However, an attractive electrostatic N–O interaction was suggested from the results of variable temperature NMR, nuclear magnetic relaxation times (<i>T</i>₁, <i>T</i>₂), and density functional theory (DFT) calculations. Upon the reaction with acetone via cationic intermediate C₅₉N⁺, we found a difference in reaction rates between H₂O@C₅₉N and H₂@C₅₉N dimers (observed reaction rates: <i>k</i>′(H₂O)/<i>k</i>′(H₂) = 1.74 ± 0.16). The DFT calculations showed thermal stabilization of C₅₉N⁺ by entrapped H₂O through the electrostatic interaction

Orientation of a Water Molecule: Effects on Electronic Nature of the C₅₉N Cage

A hydrogen-bonding network is a key impelling force for an assembly in bulk water. The fullerene cage can incarcerate a water molecule without hydrogen-bonding. Herein, we focused on spin system H₂O@C₅₉N^·. The ¹H NMR relaxation time of entrapped H₂O was significantly reduced by the paramagnetic effect. Interestingly, the electron affinity and ionization energy were suggested to vary depending on the orientation of entrapped H₂O owing to the degree of the partial charge transfer from entrapped H₂O to C₅₉N^·

Co(I)-Mediated Removal of Addends on the C₆₀ Cage and Formation of the Monovalent Cobalt Complex CpCo(CO)(η²‑C₆₀)

The removal of addends on the fullerene C₆₀ cage plays an important role in the final stage for synthesizing endohedral fullerenes by the molecular surgery method. We developed a cobalt-mediated reaction to regenerate C₆₀ from <i>N</i>-substituted C₆₀ derivatives (aziridinofullerene and azafulleroid). In these reactions, we found the formation of a green monovalent-cobalt complex of C₆₀, and its structure was unambiguously determined by X-ray analysis. The characteristic electronic structure of this cobalt complex was studied by IR and UV–vis absorption spectroscopy and electrochemical analyses