H2O/Olefinic-π Interaction inside a Carbon Nanocage

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.

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.

Water Adsorption on π-Surfaces of Open-Fullerenes

Understanding the water adsorptive behavior of fullerenes is of particular importance for their material application in aqueous media. The conventional fullerenols usually provide complex physical pictures of water adsorption due to their uncontrollable hydroxylation degree and substitution pattern. Herein, we focus on poorly hydroxylated fullerenes with well-defined structures. The water adsorptive behavior was examined by synchrotron IR spectroscopy and computational studies. As a result, three types of IR bands were observed for adsorbed water. The population of the three states was considerably altered by the orientational difference of the hydroxy groups. Nevertheless, water adsorption could not occur for 9-fluorenol and catechol. This indicates that the Lewis acidic fullerene π-surface plays a prominent role in water adsorption, while the rather Lewis basic π-surface of 9-fluorenol is unable to attract much water at a boundary with humid air.

Molecular CO2 Storage: State of a Single-Molecule Gas

CO2 evolution is one of the urgent global issues; meanwhile, understanding of sorptive/dynamic behavior is crucial to create next-generation encapsulant materials with stable sorbent processes. Herein, we showcase molecular CO2 storage constructed by a [60]fullerenol nanopocket. The CO2 density reaches 2.401 g/cm3 within the nanopore, showing strong intramolecular interactions, which induce nanoconfinement effects such as forbidden translation, restricted rotation, and perturbed vibration of CO2. We also disclosed an equation of state for a molecular CO2 gas, revealing a very low pressure of 3.14 rPa (1 rPa = 10-27 Pa) generated by the rotation/vibration at 300 K. Curiously enough, the CO2 capture enabled to modulate an external property of the encapulant material itself, i.e., association of the [60]fullerenol via intercage hydrogen-bonding.

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.

Strong structuring arising from weak cooperative O-H···π and C-H···O hydrogen bonding in benzene-methanol solution

Weak hydrogen bonds, such as O-H···π and C-H···O, are thought to direct biochemical assembly, molecular recognition, and chemical selectivity but are seldom observed in solution. We have used neutron diffraction combined with H/D isotopic substitution to obtain a detailed spatial and orientational picture of the structure of benzene-methanol mixtures. Our analysis reveals that methanol fully solvates and surrounds each benzene molecule. The expected O-H···π interaction is highly localised and directional, with the methanol hydroxyl bond aligned normal to the aromatic plane and the hydrogen at a distance of 2.30 Å from the ring centroid. Simultaneously, the tendency of methanol to form chain and cyclic motifs in the bulk liquid is manifest in a highly templated solvation structure in the plane of the ring. The methanol molecules surround the benzene so that the O-H bonds are coplanar with the aromatic ring while the oxygens interact with C-H groups through simultaneous bifurcated hydrogen bonds. This demonstrates that weak hydrogen bonding can modulate existing stronger interactions to give rise to highly ordered cooperative structural motifs that persist in the liquid phase.

C2-insertion into a fullerene orifice

The embedment of a C_n-unit into a carbon network constituting fullerene(s) potentially enables a cage-expansion. Herein, we report a C2-insertion into a fullerene orifice in which the mechanism was examined computationally. The C2-embedded open-[60]fullerene possesses an orifice enlarged from an octagon to a decagon, while the inner space was notably expanded as confirmed by the dynamic motion of the incarcerated H₂O molecule.

π-Extended Fullerenes with a Reactant Inside

Fullerene-graphene hybrids potentially exhibit unprecedented properties owing to interactive communication between the two units through a linkage. However, most of their discrete molecular structures have been still undisclosed thus far. With the recent rise in the awareness of facile access to molecular nanocarbon hybrids, we showcase novel π-extended fullerenes with a fused pyrazine or imidazole. Owing to the effective planar-curved π-conjugation, their absorption coefficients significantly increased in the visible region. Curiously enough, during the formation of π-extended fullerenes, an in situ generated NH₃ molecule was spontaneously encapsulated inside the fullerene cavity. The NH₃ molecule then underwent a timed orifice-expansion triggered by its sustained release. This is the first demonstration that fullerene captures a reactant inside, suggesting their potential usage for a sustained dosing and/or material delivery toward postfunctionalization of fullerene-graphene hybrids.

Electronic structure and interaction in CH4@C60: a first-principle investigation

CH₄@C₆₀ was the first example within which an organic molecule has been embedded in C₆₀. CH₄ can rotate freely in the molecular cage, and the carbon skeleton structure of the C₆₀ has no obvious deformation. The electronic structure of CH₄@C₆₀ and interaction between C₆₀ and CH₄ were studied under quantum mechanical calculation method. The different reaction sites on C-C bonds in C₆₀ and the weak Van der Waals interaction between CH₄ and C₆₀ were shown clearly. These results and the orbital interaction between CH₄ and C₆₀ were helpful for understanding and further application of this unique biggest organic molecule CH₄ contained in C₆₀ structure so far.

Cage-Expansion of Fullerenes

Despite the first proposal on the cage inflation of fullerenes in 1991, the chemical expansion of fullerenes has been still a formidable challenge. Herein, we provide an efficient methodology to expand [60] and [70]fullerene cages by the inclusion of totally C5N unit, giving nitrogen-containing closed structures as C₆₅N and C₇₅N with double fused heptagons. This method consists of two steps commenced with the construction of an opening by the reaction with triazine as a C3N source, followed by the cage reformation using N-phenylmaleimide as a C2 source. We also synthesized endohedral cages, demonstrating that the encapsulated H₂O molecule inside the C₇₅N cage prefers the orientation which maximizes the intramolecular interaction with the carbon wall. Additionally, we revealed the existence of a through-space magnetic dipolar interaction between the encapsulated H₂ molecule and the embedded N atom.

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.

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.

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.

Boosting the performance by the water solvation shell with hydrogen bonds on protonic ionic liquids: insights into the acid catalysis of the glycosidic bond

Hydrogen-bonding (HB) of protonic ionic liquids induced by the water solvation shell is proposed to dominate in the acid catalysis of the glycosidic bond in hydrolysis.

Synthesis of a Dihydroxylated Open-Cage [70]Fullerene by a Reductive Ring-Closure Reaction

Upon heating an open-cage diketo C₇₀ derivative in carbon disulfide, the 1,2-dihydroxylated compound was unprecedentedly formed via a reductive ring-closure reaction. The crystallographic analysis of the diketo derivative revealed a benzenoid character on its orifice which suppresses the nucleophilic addition. The 1,2-dihydroxylated derivative could be transformed into a 1,4-substituted one by an acid-catalyzed reaction. The observed unique reactivity of the diketo derivative is inherently different from those of structurally related C₇₀ isomers as well as a C₆₀ analogue.

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.

Catalytic degradation of micropollutant by peroxymonosulfate activation through Fe(III)/Fe(II) cycle confined in the nanoscale interlayer of Fe(III)-saturated montmorillonite

Low cost, green, regenerable catalyst for persulfate activation is the popularly concerned topic for the degradation of persistent organic micropollutants in drinking water. In this work, natural montmorillonite (MMT) saturated with Fe(III) ions was used to activate peroxymonosulfate (PMS) for the degradation of atrazine in raw drinking water. Results showed that the adsorption of atrazine was quickly completed within 1 min and the percentage degradation was finally increased up to 94.1% in 60 min. The d₀₀₁-spacing of MMT was enlarged to 2.91 nm at the most by Fe(III) saturation. Atrazine was adsorbed into the nanoscale interlayer of Fe(III)-saturated montmorillonite (Fe-MMT), where the Fe(III)/Fe(II) cycle was sustainably realized through the accelerated transformation of electrons between Fe(III) and PMS. Meanwhile, the in-situ generated Fe(II) accelerated the decomposition of PMS to further proceed the degradation of atrazine through the oxidation of HO• and SO₄•^- radicals. This nanoconfined effect of PMS activation by Fe(III) was further confirmed through the degradation of various micropollutants in the backgrounds of river water. The selective catalytic oxidation of micropollutants through PMS activation was attributed to the 2D mesoporous structure of Fe-MMT, inhibiting the interlayer adsorption of larger molecular backgrounds (humic acids etc.). Fe(III)-saturated montmorillonite (Fe-MMT) provided a feasible and scalable method of PMS activation by Fe(III) for the degradation of micropollutants in drinking water.