Open-Cage Fullerene as a Selective Molecular Trap for LiF/[BeF]. Angew Chem Int Ed Engl 2023;62:e202300151. [PMID: 36718977 DOI: 10.1002/anie.202300151] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The insertion of ionic compounds into open-cage fullerenes is a challenging task due to the electropositive nature of the cavity. The present work reports the preparation of an open-cage C₆₀ derivative with a hydroxy group pointing towards the centre of the cavity, which can coordinate to a metal cation, thus acting as a bait/hook to trap the metal cation such as the lithium cation in neutral LiF and the beryllium cation in the cationic [BeF]⁺ species. Other metal salts could not be inserted under similar conditions. The structure of MF in the cage was unambiguously determined by single-crystal X-ray diffraction. Owing to its tendency to undergo polycoordination, Li⁺ monomer salts have not been isolated before, despite extensive research on Li bonds. The present results provide a unique example of a Li bond.

An H2 O2 Molecule Stabilized inside Open-Cage C60 Derivatives by a Hydroxy Stopper

An H₂ O₂ molecule was isolated inside hydroxylated open-cage fullerene derivatives by mixing an H₂ O₂ solution with a precursor molecule followed by reduction of one of carbonyl groups on its orifice. Depending on the reduction site, two structural isomers for H₂ O₂ @open-fullerenes were obtained. A high encapsulation ratio of 81 % was attained at low temperature. The structures of the peroxosolvate complexes thus obtained were studied by ¹ H NMR spectroscopy, X-ray analysis, and DFT calculations, showing strong hydrogen bonding between the encapsulated H₂ O₂ and the hydroxy group located at the center of the orifice. This OH group was found to act as a kinetic stopper, and the formation of the hydrogen bonding caused thermodynamic stabilization of the H₂ O₂ molecule, both of which prevent its escape from the cage. One of the peroxosolvates was isolated by HPLC, affording H₂ O₂ @open-fullerene with 100 % encapsulation ratio, likely due to the intramolecular hydrogen-bonding interaction.

Synthesis of Open-Cage [60]Fullerenes with Five Carbonyl Groups on the Rim of the 15-Membered Orifice

A new type of open-cage [60]fullerene was prepared starting from our previously reported open-cage [60]fullerenes containing hydroxy and tert-butylperoxo groups, and an iminofuranone moiety on the rim of the orifice. The key reactions are alcoholysis with MeOH/PCl₅ and reductive aromatization with SnCl₂ or HI (aq). The resulting open-cage compound contains two ketone carbonyls, two amide carbonyls, and one ester carbonyl group. The X-ray crystal structure of the precursor compound shows that the 18-membered orifice is almost completely blocked because of the presence of the amide group directly above the orifice.

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.

Synthesis and Properties of Open Fullerenes Encapsulating Ammonia and Methane

We describe the synthesis and characterisation of open fullerene (1) and its reduced form (2) in which CH₄ and NH₃ are encapsulated, respectively. The ¹H NMR resonance of endohedral NH₃ is broadened by scalar coupling to the quadrupolar ¹⁴n nucleus, which relaxes rapidly. This broadening is absent for small satellite peaks, which are attributed to natural abundance ¹⁵N. The influence of the scalar relaxation mechanism on the linewidth of the ¹H ammonia resonance is probed by variable temperature NMR. A rotational correlation time of τ_c=1.5 ps. is determined for endohedral NH₃, and of τ_c=57±5 ps. for the open fullerene, indicating free rotation of the encapsulated molecule. IR spectroscopy of NH₃@2 at 5 K identifies three vibrations of NH₃ (ν₁, ν₃ and ν₄) redshifted in comparison with free NH₃, and temperature dependence of the IR peak intensity indicates the presence of a large number of excited translational/ rotational states. Variable temperature ¹H NMR spectra indicate that endohedral CH₄ is also able to rotate freely at 223 K, on the NMR timescale. Inelastic neutron scattering (INS) spectra of CH₄@1 show both rotational and translational modes of CH₄. Energy of the first excited rotational state (J=1) of CH₄@1 is significantly lower than that of free CH₄.