Electron-beam-promoted fullerene dimerization in nanotubes: insights from DFT computations.

Fullerene dimerization inside a peapod is analyzed at DFT level by characterizing the stationary points and deriving the energy profile of the initial and reversible process named phase 1. We find that the barriers for the radical cation mechanism are significantly lower than those found for the neutral pathway. The peapod is mainly providing one-dimensional confinement for the reaction to take place in a more efficient way. Car-Parrinello metadynamics simulations provide hints on structures for the initial steps of the irreversible phase 2 where bond formation and breaking lead to important structural reorganizations within the coalescence process.

Actinide-lanthanide single electron metal-metal bond formed in mixed-valence di-metallofullerenes.

Understanding metal-metal bonding involving f-block elements has been a challenging goal in chemistry. Here we report a series of mixed-valence di-metallofullerenes, ThDy@C2n (2n = 72, 76, 78, and 80) and ThY@C2n (2n = 72 and 78), which feature single electron actinide-lanthanide metal-metal bonds, characterized by structural, spectroscopic and computational methods. Crystallographic characterization unambiguously confirmed that Th and Y or Dy are encapsulated inside variably sized fullerene carbon cages. The ESR study of ThY@D3h(5)-C78 shows a doublet as expected for an unpaired electron interacting with Y, and a SQUID magnetometric study of ThDy@D3h(5)-C78 reveals a high-spin ground state for the whole molecule. Theoretical studies further confirm the presence of a single-electron bonding interaction between Y or Dy and Th, due to a significant overlap between hybrid spd orbitals of the two metals.

U@Cs(4)-C82: A Different Cage Isomer with Reactivity Controlled by U-Sumanene Interaction.

Actinide endohedral metallofullerenes (EMFs) are a fullerene family that possess unique actinide-carbon cage host-guest molecular and electronic structures. In this work, a novel actinide EMF, U@Cs(4)-C82, was successfully synthesized and characterized, and its chemical reactivity was investigated. Crystallographic analysis shows that U@Cs(4)-C82, a new isomer of U@C82, has a Cs(4)-C82 cage, which has never been discovered in the form of empty or endohedral fullerenes. Its unique chemical reactivities were further revealed through the Bingel-Hirsch reaction and carbene addition reaction studies. The Bingel-Hirsch reaction of U@Cs(4)-C82 shows exceptionally high selectivity and product yield, yielding only one major addition adduct. Moreover, the addition sites for both reactions are unexpectedly located on adjacent carbon atoms far away from the actinide metal, despite the nucleophilic (Bingel-Hirsch) and electrophilic (carbene addition) nature of either reactant. Density functional theory (DFT) calculations suggest that this chemical behavior, unprecedented for EMFs, is directed by the unusually strong interaction between U and the sumanene motif of the carbon cage in U@Cs(4)-C82, which makes the energy increase when it is disrupted. This work reveals remarkable chemical properties of actinide EMFs originating from their unique electronic structures and highlights the key role of actinide-cage interactions in the determination of their chemical behaviors.

Are U-U Bonds Inside Fullerenes Really Unwilling Bonds?

Previous characterizations of diactinide endohedral metallofullerenes (EMFs) Th2@C80 and U2@C80 have shown that although the two Th3+ ions form a strong covalent bond within the carbon cage, the interaction between the U3+ ions is weaker and described as an "unwilling" bond. To evaluate the feasibility of covalent U-U bonds, which are neglected in classical actinide chemistry, we have first investigated the formation of smaller diuranium EMFs by laser ablation using mass spectrometric detection of dimetallic U2@C2n species with 2n ≥ 50. DFT, CASPT2 calculations, and MD simulations for several fullerenes of different sizes and symmetries showed that thanks to the formation of strong U(5f3)-U(5f3) triple bonds, two U3+ ions can be incarcerated inside the fullerene. The formation of U-U bonds competes with U-cage interactions that tend to separate the U ions, hindering the observation of short U-U distances in the crystalline structures of diuranium endofullerenes as in U2@C80. Smaller cages like C60 exhibit the two interactions, and a strong triple U-U bond with an effective bond order higher than 2 is observed. Although 5f-5f interactions are responsible for the covalent interactions at distances close to 2.5 Å, overlap between 7s6d orbitals is still detected above 4 Å. In general, metal ions within fullerenes should be regarded as templates in cage formation, not as statistically confined units that have little chance of being observed.

Effects of Solvents on Reaction Products: Synthesis of Endohedral Metallofullerene Oxazoline and Epoxide.

Herein, we performed the reactions of M3N@Ih-C80 (M = Sc and Lu) with the methanol (CH3OH) solution of TBAOH (note that both CH3O- and OH- are nucleophiles) in benzonitrile (PhCN) and dimethylformamide, respectively. It is found that OH- ions rather than CH3O- ions selectively attacked the fullerene cage to form the M3N@C80--O- intermediate. Although the fullerene cage is initially attacked by OH- in both PhCN and DMF solvents, the products are quite different. In PhCN, two isomeric Sc3N@Ih-C80 fullerooxazoline heterocyclic products (1 and 2) were synthesized. Whereas, in DMF, an epoxide of Lu3N@Ih-C80 (3) was obtained. The preference for fullerooxazoline formation over that of fullerene epoxy in PhCN is well explained by density functional theory calculations. Plausible reaction mechanisms for the formation of metallofullerene oxazoline and epoxide were proposed based on the experimental and theoretical results.

Structurally Defined Water-Soluble Metallofullerene Derivatives towards Biomedical Applications.

Endohedral metallofullerenes (EMFs) are excellent carriers of rare-earth element (REE) ions in biomedical applications because they preclude the release of toxic metal ions. However, existing approaches to synthesize water-soluble EMF derivatives yield mixtures that inhibit precise drug design. Here we report the synthesis of metallobuckytrio (MBT), a three-buckyball system, as a modular platform to develop structurally defined water-soluble EMF derivatives with ligands by choice. Demonstrated with PEG ligands, the resulting water-soluble MBTs show superb biocompatibility. The Gd MBTs exhibit superior T1 relaxivity than typical Gd complexes, potentially superseding current clinical MRI contrast agents in both safety and efficiency. The Lu MBTs generated reactive oxygen species upon light irradiation, showing promise as photosensitizers. With their modular nature to incorporate other ligands, we anticipate the MBT platform to open new paths towards bio-specific REE drugs.

Crystallographic Characterization of U@C2n (2n = 82-86): Insights about Metal-Cage Interactions for Mono-metallofullerenes.

Endohedral mono-metallofullerenes are the prototypes to understand the fundamental nature and the unique interactions between the encapsulated metals and the fullerene cages. Herein, we report the crystallographic characterizations of four new U-based mono-metallofullerenes, namely, U@Cs(6)-C82, U@C2(8)-C84, U@Cs(15)-C84, and U@C1(12)-C86, among which the chiral cages C2(8)-C84 and C1(12)-C86 have never been previously reported for either endohedral or empty fullerenes. Symmetrical patterns, such as indacene, sumanene, and phenalene, and charge transfer are found to determine the metal positions inside the fullerene cages. In addition, a new finding concerning the metal positions inside the cages reveals that the encapsulated metal ions are always located on symmetry planes of the fullerene cages, as long as the fullerene cages possess mirror planes. DFT calculations show that the metal-fullerene motif interaction determines the stability of the metal position. In fullerenes containing symmetry planes, the metal prefers to occupy a symmetrical arrangement with respect to the interacting motifs, which share one of their symmetry planes with the fullerene. In all computationally analyzed fullerenes containing at least one symmetry plane, the actinide was found to be located on the mirror plane. This finding provides new insights into the nature of metal-cage interactions and gives new guidelines for structural determinations using crystallographic and theoretical methods.

Unexpected Formation of Metallofulleroids from Multicomponent Reactions, with Crystallographic and Computational Studies of the Cluster Motion.

New multicomponent reactions involving an isocyanide, terminal or internal alkynes, and endohedral metallofullerene (EMF) Lu3 N@C80 yield metallofulleroids which are characterized by mass-spectrometry, HPLC, and multiple 1D and 2D NMR techniques. Single crystal studies revealed one ketenimine metallofulleroid has ordered Lu3 N cluster which is unusual for EMF monoadducts. Computational analysis, based on crystallographic data, confirm that the endohedral cluster motion is controlled by the position of the exohedral organic appendants. Our findings provide a new functionalization reaction for EMFs, and a potential facile approach to freeze the endohedral cluster motion at relatively high temperatures.

Correction: Unexpected formation of 1,2- and 1,4-bismethoxyl Sc3N@I h-C80 derivatives via regioselective anion addition: an unambiguous structural identification and mechanism study.

[This corrects the article DOI: 10.1039/D1SC01178B.].

Synthesis and Characterization of Two Isomers of Th@C82: Th@C2v(9)-C82 and Th@C2(5)-C82.

Actinide endohedral fullerenes have demonstrated remarkably different physicochemical properties compared to their lanthanide analogues. In this work, two novel isomers of Th@C82 were successfully synthesized, isolated, and fully characterized by mass spectrometry, X-ray single crystallography, UV-vis-NIR spectroscopy, Raman spectroscopy, and cyclic voltammetry. The molecular structures of the two isomers were determined unambiguously as Th@C2v(9)-C82 and Th@C2(5)-C82 by single-crystal X-ray diffraction analysis. Raman and UV-vis-NIR spectroscopies further confirm the assignment of the cage isomers. Electrochemical gaps suggest that both Th@C2v(9)-C82 and Th@C2(5)-C82 possess a stable closed-shell electronic structure. The computational results further confirm that Th@C2v(9)-C82 and Th@C2(5)-C82 exhibit a unique four-electron charge transfer from the metal to the carbon cage and are among the most abundant isomers of Th@C82.

Unexpected formation of 1,2- and 1,4-bismethoxyl Sc3N@I h-C80 derivatives via regioselective anion addition: an unambiguous structural identification and mechanism study.

An attempt to achieve heterocyclic cycloadducts of Sc3N@I h-C80 via reaction with Ph2C[double bond, length as m-dash]O, PhC[triple bond, length as m-dash]CPh or PhC[triple bond, length as m-dash]N in the presence of tetrabutylammonium hydroxide (TBAOH) stored in CH3OH led to the formation of the unexpected bismethoxyl adducts of Sc3N@I h-C80 (1 and 2). Further studies reveal that TBAOH in CH3OH can boost the CH3O- addition efficiently, regardless of the presence of other reagents. Single-crystal X-ray diffraction results firmly assign the molecular structures of 1 and 2 as respective 1,4- and 1,2-bismethoxyl adducts, and reveal unusual relationships between the internal Sc3N cluster and the addition modes, in addition to the unusual packing mode in view of the orientation of the methoxyl groups. Electrochemical results demonstrate smaller electrochemical gaps for 1 and 2, relative to that of Sc3N@I h-C80, confirming their better electroactive properties. Finally, a plausible reaction mechanism involving anion addition and a radical reaction was proposed, presenting new insights into the highly selective reactions between the methoxyl anion and metallofullerenes. 1 and 2 represent the first examples of methoxyl derivatives of metallofullerenes. This work not only presents a novel and facile strategy for the controllable synthesis of alkoxylated metallofullerene derivatives, but also provides new non-cycloadducts for the potential applications of EMFs.

Th@D5h(6)-C80: a highly symmetric fullerene cage stabilized by a single metal ion.

A novel endohedral metallofullerene (mono-EMF), Th@D5h(6)-C80, has been successfully synthesized and fully characterized by mass spectrometry, single crystal X-ray diffraction, UV-vis-NIR and Raman spectroscopy and cyclic voltammetry. Single crystal XRD analysis unambiguously assigned the fullerene cage as D5h(6)-C80, the first example in which the highly symmetric cage is stabilized by a single metal ion. The combined experimental and theoretical studies further reveal that the D5h(6)-C80 cage, known only for its stabilization by 6-electron transfer, is stabilized by the 4-electron transfer from the encapsulated Th ion for the first time.

An Ultra-Long-Lived Triplet Excited State in Water at Room Temperature: Insights on the Molecular Design of Tridecafullerenes.

Suitably engineered molecular systems exhibiting triplet excited states with very long lifetimes are important for high-end applications in nonlinear optics, photocatalysis, or biomedicine. We report the finding of an ultra-long-lived triplet state with a mean lifetime of 93 ms in an aqueous phase at room temperature, measured for a globular tridecafullerene with a highly compact glycodendrimeric structure. A series of three tridecafullerenes bearing different glycodendrons and spacers to the C60 units have been synthesized and characterized. UV/Vis spectra and DLS experiments confirm their aggregation in water. Steady-state and time-resolved fluorescence experiments suggest a different degree of inner solvation of the multifullerenes depending on their molecular design. Efficient quenching of the triplet states by O2 but not by waterborne azide anions has been observed. Molecular modelling reveals dissimilar access of the aqueous phase to the internal structure of the tridecafullerenes, differently shielded by the glycodendrimeric shell.

A New Class of Molecular Electrocatalysts for Hydrogen Evolution: Catalytic Activity of M3N@C2n (2n = 68, 78, and 80) Fullerenes.

The electrocatalytic properties of some endohedral fullerenes for hydrogen evolution reactions (HER) were recently predicted by DFT calculations. Nonetheless, the experimental catalytic performance under realistic electrochemical environments of these 0D-nanomaterials have not been explored. Here, for the first time, we disclose the HER electrocatalytic behavior of seven M3N@2n (2n = 68, 78, and 80) fullerenes (Gd3N@Ih(7)-C80, Y3N@Ih(7)-C80, Lu3N@Ih(7)-C80, Sc3N@Ih(7)-C80, Sc3N@D5h(6)-C80, Sc3N@D3h(5)-C78, and Sc3N@D3(6140)-C68) using a combination of experimental and theoretical techniques. The non-IPR Sc3N@D3(6140)-C68 compound exhibited the best catalytic performance toward the generation of molecular hydrogen, exhibiting an onset potential of -38 mV vs RHE, a very high mass activity of 1.75 A·mg-1 at -0.4 V vs RHE, and an excellent electrochemical stability, retaining 96% of the initial current after 24 h. The superior performance was explained on the basis of the fused pentagon rings, which represent a new and promising HER catalytic motif.

Characterization of a strong covalent Th3+-Th3+ bond inside an Ih(7)-C80 fullerene cage.

The nature of the actinide-actinide bonds is of fundamental importance to understand the electronic structure of the 5f elements. It has attracted considerable theoretical attention, but little is known experimentally as the synthesis of these chemical bonds remains extremely challenging. Herein, we report a strong covalent Th-Th bond formed between two rarely accessible Th3+ ions, stabilized inside a fullerene cage nanocontainer as Th2@Ih(7)-C80. This compound is synthesized using the arc-discharge method and fully characterized using several techniques. The single-crystal X-Ray diffraction analysis determines that the two Th atoms are separated by 3.816 Å. Both experimental and quantum-chemical results show that the two Th atoms have formal charges of +3 and confirm the presence of a strong covalent Th-Th bond inside Ih(7)-C80. Moreover, density functional theory and ab initio multireference calculations suggest that the overlap between the 7s/6d hybrid thorium orbitals is so large that the bond still exists at Th-Th separations larger than 6 Å. This work demonstrates the authenticity of covalent actinide metal-metal bonds in a stable compound and deepens our fundamental understanding of f element metal bonds.

Nanoscale rotational dynamics of four independent rotators confined in crowded crystalline layers.

We report a study where Car-Parrinello molecular dynamics simulations and variable-temperature (30-300 K) 1H spin-lattice relaxation time experiments nicely complement each other to characterize the dynamics within a set of four crystalline 1,4-diethynylbicyclo[2.2.2]octane (BCO) rotors assembled in the metal-organic rotor, {Li+4(-CO2-Ph-BCO-py)4(H2O)8}·2DMF. The remarkable finding of this work is that, despite the individual rotational barriers of four rotors being indiscernible and superimposed in a broad relaxation process, we were able to unravel a strongly interrelated series of rotational motions involving disrotatory and conrotatory motions in pairs as well as rotational steps of single rotators, all three processes with similar, sizeable rotational barriers of 6 kcal mol-1. It is noteworthy that DFT molecular dynamics simulations and variable-temperature (30-300 K) proton spin-lattice relaxation time experiments deliver the same high value for the rotational barriers stressing the potential of the combined use of the two techniques in understanding rotational motion at the nanoscale.

U2N@I h(7)-C80: fullerene cage encapsulating an unsymmetrical U(iv)[double bond, length as m-dash]N[double bond, length as m-dash]U(v) cluster.

For the first time, an actinide nitride clusterfullerene, U2N@I h(7)-C80, is synthesized and fully characterized by X-ray single crystallography and multiple spectroscopic methods. U2N@I h(7)-C80 is by far the first endohedral fullerene that violates the well-established tri-metallic nitride template for nitride clusterfullerenes. The novel U[double bond, length as m-dash]N[double bond, length as m-dash]U cluster features two U[double bond, length as m-dash]N bonds with uneven bond distances of 2.058(3) Å and 1.943(3) Å, leading to a rare unsymmetrical structure for the dinuclear nitride motif. The combined experimental and theoretical investigations suggest that the two uranium ions show different oxidation states of +4 and +5. Quantum-chemical investigation further reveals that the f1/f2 population dominantly induces a distortion of the U[double bond, length as m-dash]N[double bond, length as m-dash]U cluster, which leads to the unsymmetrical structure. A comparative study of U2X@C80 (X = C, N and O) reveals that the U-X interaction in U[double bond, length as m-dash]X[double bond, length as m-dash]U clusters can hardly be seen as being formed by classical multiple bonds, but is more like an anionic central ion X q- with biased overlaps with the two metal ions, which decrease as the electronegativity of X increases. This study not only demonstrates the unique bonding variety of actinide clusters stabilized by fullerene cages, showing different bonding from that observed for the lanthanide analogs, it also reveals the electronic structure of the U[double bond, length as m-dash]X[double bond, length as m-dash]U clusters (X = C, N and O), which are of fundamental significance to understanding these actinide bonding motifs.

α-DTC70 Fullerene Performs Significantly Better than ß-DTC70 as Electron Transporting Material in Perovskite Solar Cells.

In this work, two new C70 isomers, α and ß bis(2-(thiophen-2-yl)ethyl)-C70-fullerene mono-adducts (DTC70), were synthesized, characterized and used as electron transporting materials (ETMs) in perovskite solar cells (PSCs). Our results show that the α isomer improves both the J sc and FF values of the devices, when compared to the results for the ß-isomer and to those for phenyl-C70-butyric acid methyl ester (PC71BM), used as control. Devices based on α-DTC70 achieved a power conversion efficiency (PCE) of 15.9%, which is higher than that observed with PC71BM (15.1%).

Chemical Reactions of Cationic Metallofullerenes: An Alternative Route for Exohedral Functionalization.

The chemistry of cationic forms of clusterfullerenes remain less explored than that of the corresponding neutral or anionic species. In the present work, M3 N@Ih -C80 (M=Sc or Lu) cations were generated by both electrochemical and chemical oxidation methods. The as-obtained cations successfully underwent the typical Bingel-Hirsch reaction that fails with neutral Sc3 N@Ih -C80 . Two isomeric Sc3 N@Ih -C80 cation derivatives, [5,6]-open and [6,6]-open adducts, were synthesized, and the former has never been prepared by means of a Bingel-Hirsch reaction with neutral clusterfullerenes. In the case of the Lu3 N@Ih -C80 cation, however, only a [6,6]-open adduct was obtained. Density functional theory (DFT) calculations indicated that the oxidized M3 N@Ih -C80 was much more reactive than the neutral compound upon addition of the diethyl bromomalonate anion. The Bingel-Hirsch reaction of M3 N@Ih -C80 cations occurred by means of an unusual outer-sphere single-electron transfer (SET) process from the diethyl bromomalonate anion to the stable intermediate [M3 N@C80 (C2 H5 COO)2 CBr]. . Remarkably, the diethyl bromomalonate anion was found to act as both a nucleophile and an electron donor.

Highly selective encapsulation and purification of U-based C78-EMFs within a supramolecular nanocapsule.

The ability of the tetragonal prismatic nanocapsule 1·(BArF)8 to selectively encapsulate U-based C78 EMFs from a soot mixture is reported, showing enhanced affinity for C78-based EMFs over C80-based EMFs. Molecular recognition driven by the electrostatic interactions between the host and guest is at the basis of the high selectivity observed for ellipsoidal C78-based EMFs compared to spherical C80-based EMFs. In addition, DFT analysis points towards an enhanced breathing adaptability of nanocapsule 1·(BArF)8 to C78-based EMFs to further explain the selectivity observed when the host is used in the solid phase.