Solution Dynamics of Covalent Open-[60]Fullerene Dimers

The translational diffusivity of covalent open-[60]fullerene dimers in an organic solvent was found to be well describable by a prolate ellipsoid model while a monomeric open-[60]fullerene behaves like a sphere model. The water association dynamics were examined for two open-[60]fullerene dimers, showing a higher water affinity for the sp3-linked dimer relative to sp2-linked dimer owing to an effective orbital-orbital overlap identified by π(fullerene)→σ*(H2O) interactions as suggested by theoretical calculations.

Open-Cage Fullerene as a Macrocyclic Ligand for Na, Pt, and Rh Metal Complexes

An open-cage [60]fullerene derivative was prepared through Malaprade oxidation of a vicinal triol moiety as the key step. Above the 17-membered orifice, there is one carboxyl group. Three ketone carbonyl groups and one lactone carbonyl group are located on the rim of the orifice. The carboxylic and carbonyl oxygen atoms around the orifice act as strong polydentate ligands for a sodium ion. These oxygen atoms also react with [Rh(CO)2Cl]2 to form various isomeric rhodium complexes with comparable stability. The fullerene C═C bond on the rim of the orifice forms a stable platinum complex when treated with Pt(PPh3)4. Single crystal X-ray diffraction data reveal that one of the carboxylic oxygen atoms above the orifice forms a H-bond with the water molecule trapped in the cage.

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.

Open-Cage Fullerene as Molecular Container for F- , Cl- , Br- and I. Angew Chem Int Ed Engl 2022;61:e202212090. [PMID: 36316627 DOI: 10.1002/anie.202212090]

A 19-membered open-cage fullerene derivative was prepared from C₆₀ in 7 steps and 5.5 % yield through the peroxide-mediate pathway. There are four carbonyl groups, an ether oxygen and a quinoxaline moiety on the rim of the orifice. A chloride anion could be inserted into its cavity by heating with hydrochloric acid at 60 °C for 4 h. Encapsulation of fluoride, bromide and iodide anions was also achieved at slightly more forcing conditions, 90 °C for 14 h. Single crystal X-ray structures of the sodium salt of the chloride and the bromide encapsulated derivatives were obtained, which showed the halide anion in the center of the cavity and two sodium cations connecting two cages through coordination to the oxygen atoms on the rim of the orifices. The halide encapsulation ratio is quantitative in the isolated products.

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.

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.

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.

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.

Molecular Containers Derived from [60]Fullerene through Peroxide Chemistry

Molecular containers can keep guest molecules in a confined space that is completely separated from the solution. They have wide potential applications, including selective trapping of reactive intermediates, catalysis within the cavity, and molecular delivery. Numerous molecular containers have been prepared through covalent bonds, metal-ligand interactions and H-bonding or hydrophobic interactions. Fullerenes are all-carbon molecules with a spherical structure. Partial opening of the cage structure results in open-cage fullerenes, which can serve as molecular containers for various small molecules and atoms. Compared with classical molecular containers, open-cage fullerenes exhibit some unusual phenomena because of the unique structure of the fullerene cage. The synthesis of an open-cage fullerene with a large enough orifice as a molecular container requires consecutive cleavage of multiple fullerene skeleton bonds within a local area on the cage surface. In spite of the difficulty, remarkable progress has been achieved. Several reactions have been reported to cleave fullerene C-C bonds selectively to form open-cage fullerenes, some of which have been successfully used as molecular containers for molecules such as H₂O. The size and shape of the orifice play a key role in the encapsulation of the guest molecule. To date the focus in this area has been the preparation of open-cage fullerenes and encapsulation of small molecules. Little information has been reported about the functional properties of these host-guest systems. Potential applications of these systems need to be explored. This Account mainly presents our results on the encapsulation of small molecules in open-cage fullerenes prepared in my group. The preparation of our open-cage fullerenes is based on fullerene-mixed peroxides, which are briefly mentioned herein. The encapsulation and release of a single molecule of water is discussed in detail. Quantitative water encapsulation was achieved by heating the open-cage fullerene in a homogeneous CDCl₃/H₂O/EtOH mixture at 80 °C for 18 h. The kinetics of the water release process was studied by blackbody IR radiation-induced dissociation (BIRD) and theoretical calculations. The trapped water could also be released by H-bonding with HF. To control the encapsulation and release processes, we prepared open-cage fullerenes with a switchable stopper on the rim of the orifice. Besides H₂O, encapsulations of H₂, HF, CO, O₂, and H₂O₂ were also achieved by using different open-cage fullerenes. The encapsulation of CO is quite unusual in that the trapped CO is derived from a fullerene skeleton carbon that was pushed into the cavity by oxidation under ambient conditions at room temperature. The trapped O₂/H₂O₂ could be released slowly under mild conditions, and these systems are now being studied as a new type of oxygen-releasing materials for biomedical research. The present results demonstrate that open-cage fullerenes are suitable molecular containers for small molecules. Our future work will focus on optimizing the conditions for the preparation of open-cage fullerenes and applications of open-cage fullerenes in areas such as oxygen delivery for photodynamic therapy.