A Stable, Soluble, and Crystalline Supramolecular System with a Triplet Ground State

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.

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.

Nonclassical Abramov Products Formed on Orifices of Cage-Opened C60 Derivatives

By nucleophilic addition of phosphite P(OMe)₃ to a cage-opened C₆₀ derivative, α-hydrophosphate and enol phosphate were obtained as kinetic and thermodynamic products, respectively. Different from classical Abramov products bearing a phosphorus-carbon bond, these products have a phosphorus-oxygen bond. The observed anomaly originates from the fully conjugated π system, which significantly stabilizes zwitterionic intermediates bearing a phosphorus-oxygen bond. The thus formed enol phosphate was found to exhibit an intense absorption band that extended to 730 nm, reflecting the intramolecular charge-transfer transitions. We also report domino phosphorylation reactions, which gave a cage-opened C₆₀ derivative bearing a direct P-C bond.

A Solid-State Intramolecular Wittig Reaction Enables Efficient Synthesis of Endofullerenes Including Ne@C60 , 3 He@C60 , and HD@C60

An open‐cage fullerene incorporating phosphorous ylid and carbonyl group moieties on the rim of the orifice can be filled with gases (H₂, He, Ne) in the solid state, and the cage opening then contracted in situ by raising the temperature to complete an intramolecular Wittig reaction, trapping the atom or molecule inside. Known transformations complete conversion of the product fullerene to C₆₀ containing the endohedral species. As well as providing an improved synthesis of large quantities of ⁴He@C₆₀, H₂@C₆₀, and D₂@C₆₀, the method allows the efficient incorporation of expensive gases such as HD and ³He, to prepare HD@C₆₀ and ³He@C₆₀. The method also enables the first synthesis of Ne@C₆₀ by molecular surgery, and its characterization by crystallography and ¹³C NMR spectroscopy.

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 ¹H 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.