Computational insights into Diels-Alder reactions of paramagnetic endohedral metallofullerenes: M@C82 (M = Sc, Y, La) and La@C72

In fullerene chemistry, Diels-Alder cycloaddition is an essential reaction for exohedral modification of carbon cages. M@C2v(9)-C82 (M = Sc, Y, and La), incorporating one metal atom within the fullerene cage, are key compounds for understanding the impact of both endohedral and exohedral modifications on their electronic structures. In this work, the Diels-Alder (DA) cycloaddition of cyclopentadiene (Cp) to M@C2v(9)-C82 (M = Sc, Y, and La) and La@C2(10612)-C72 was systematically studied using density functional theory. The most reactive bonds were initially chosen for detailed mechanistic exploration, considering both concerted and stepwise mechanisms. Our findings revealed that DA cycloadditions for the three metals (Sc, Y, and La) consistently exhibit the same regioselectivity, favoring the concerted attack on the [5,6] bond. This observation is in agreement with previous experimental and theoretical studies on the regioselectivity of the Diels-Alder reaction between La@C2v(9)-C82 and Cp. In the case of La@C2(10612)-C72, the most favored pathway is the concerted attack on the [6,6] bond both kinetically and thermodynamically. In toluene and ortho-dichlorobenzene, while the energy barriers and the reaction free energies increased to different extents for most pathways, the regioselectivity largely mirrored that observed in the gas phase.

Computational Design of 3D Lantern Organic Framework

This study employed a computational approach, particularly Density Functional Theory at B3LYP-D3/6-31+G(d) level to design two new classes of three-dimensional (3D) Lantern Organic Frameworks (LOFs) materials based on trisilasumanene and porphyrin core building units. Particularly, we detail strategies for transitioning from 1D-LOF nanowires to extended 3D structures: first by connecting planar-molecule base units of trisilasumanene or porphyrin using benzene-based linkers, and then connecting silicon anchoring atoms on the bases with other bases that are vertically stacked by sp3-hydrocarbon chains. The 3D-LOF structures are designed to have different pore sizes through the use of various bases, bridges, and linkers. Comparisons of electronic properties of these 3D structures lead to one designing rule. That is, the gap between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the 3D materials depends only on its base and is nearly independent of the stack size or the length of the sp3-hydrocarbon bridges. Additionally, connecting base units with linkers also extends π-electron conjugation system leading to a reduction in HOMO-LUMO gap. For instance, linking two trisilasumanene molecules significantly narrows HOMO-LUMO gap by 1.75 eV while stacking these bases vertically and connecting them by linear pentane-based bridges yield insignificant change to the gap.

Insight into the interaction of host-guest structures for pyrrole-based metal compounds and C70

This study focuses on the recognition and isolation of fullerenes, which are crucial for further exploration of their physical and chemical properties. Our goal is to investigate the potential recognition of the D5h-C70 fullerene using crown-shaped metal compositions through density functional theory calculations. We assess the effectiveness of fullerene C70 recognition by studying the binding energy. Additionally, various analyses were conducted, including natural bond order charge analysis and reduced density gradient analysis, to understand the interaction mechanism between the host and guest molecules. These investigations provide valuable insights into the nature of the interaction and the stability of the host-guest system. To facilitate the release of the fullerene guest molecule, the vis-NIR spectra were simulated for the host-guest structures. This analysis offers guidance on the specific wavelengths that can be utilized to release the fullerene guest from the host-guest structures. Overall, this work proposes a new strategy for the effective recognition of various fullerene molecules and their subsequent release from host-guest systems. These findings could potentially be applied in assemblies involving fullerenes, advancing their practical applications.

Endohedral metallofullerene molecular nanomagnets

Magnetic lanthanide (Ln) metal complexes exhibiting magnetic bistability can behave as molecular nanomagnets, also known as single-molecule magnets (SMMs), suitable for storing magnetic information at the molecular level, thus attracting extensive interest in the quest for high-density information storage and quantum information technologies. Upon encapsulating Ln ion(s) into fullerene cages, endohedral metallofullerenes (EMFs) have been proven as a promising and versatile platform to realize chemically robust SMMs, in which the magnetic properties are able to be readily tailored by altering the configurations of the encapsulated species and the host cages. In this review, we present critical discussions on the molecular structures and magnetic characterizations of EMF-SMMs, with the focus on their peculiar molecular and electronic structures and on the intriguing molecular magnetism arising from such structural uniqueness. In this context, different families of magnetic EMFs are summarized, including mononuclear EMF-SMMs wherein single-ion anisotropy is decisive, dinuclear clusterfullerenes whose magnetism is governed by intramolecular magnetic interaction, and radical-bridged dimetallic EMFs with high-spin ground states that arise from the strong ferromagnetic coupling. We then discuss how molecular assemblies of SMMs can be constructed, in a way that the original SMM behavior is either retained or altered in a controlled manner, thanks to the chemical robustness of EMFs. Finally, on the basis of understanding the structure-magnetic property correlation, we propose design strategies for high-performance EMF-SMMs by engineering ligand fields, electronic structures, magnetic interactions, and molecular vibrations that can couple to the spin states.

Open-cage fullerenes as ligands for metals

The remarkable structures of open-cage fullerenes with functionalization on the outer surface and an accessible inner void make them interesting ligands for reactions with metal complexes. The behaviors of open-cage fullerenes in reactions with various metal complexes are examined and compared to the corresponding reactions with intact fullerenes. The structural results from X-ray diffraction are emphasized. Open-cage fullerenes frequently undergo unanticipated structural changes such as carbon-carbon bond cleavage upon reactions with metal complexes. Much more remains to be learned about the possibility of inserting metal ions larger than Li+ into the interior void of these open-cage fullerenes and about the effects of redox reactions on metal complexes of open-cage fullerenes.

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.

(Ro)vibrational Spectroscopic Constants, Lifetime and QTAIM Evaluation of Fullerene Dimers Stability

The iconic caged shape of fullerenes gives rise to a series of unique chemical and physical properties; hence a deeper understanding of the attractive and repulsive forces between two buckyballs can bring detrimental information about the structural stability of such complexes, providing significant data applicable for several studies. The potential energy curves for the interaction of multiple van der Waals buckyball complexes with increasing mass were theoretically obtained within the DFT framework at ωB97xD/6-31G(d) compound model. These potential energy curves were employed to estimate the spectroscopic constants and the lifetime of the fullerene complexes with the Discrete Variable Representation and with the Dunham approaches. It was revealed that both methods are compatible in determining the rovibrational structure of the dimers and that they are genuinely stable, i.e., long-lived complexes. To further inquire into the nature of such interaction, Bader's QTAIM approach was applied. QTAIM descriptors indicate that the interactions of these closed-shell systems are dominated by weak van der Waals forces. This non-covalent interaction character was confirmed by the RDG analysis scheme. Indirectly, QTAIM also allowed us to confirm the stability of the non-covalent bonded fullerene dimers. Our lifetime calculations have shown that the studied dimers are stable for more than 1 ps, which increases accordingly with the number of carbon atoms.

Theoretical Study on the Structure and Spectral Properties of Several Classical C84 Isomers and Their Newly Synthesized Derivatives

The ground-state electronic/geometrical structures of the three classical isomers C_s(15)-C₈₄, C₂(13)-C₈₄, and C₂(8)-C₈₄ as well as the corresponding embedded derivatives U@C_s(15)-C₈₄, YCN@C₂(13)-C₈₄, and U@C₂(8)-C₈₄ have been calculated at the density functional theory (DFT) level. Then, the isomers of C₈₄ were theoretically identified by X-ray photoelectron spectroscopy (XPS) and near X-ray absorption fine-structure spectroscopy (NEXAFS). The spectral components of total spectra for carbon atoms in various local environments have been investigated. The ultraviolet-visible (UV-vis) absorption spectroscopies of U@C_s(15)-C₈₄, YCN@C₂(13)-C₈₄, and U@C₂(8)-C₈₄ have also been performed utilizing time-dependent (TD) DFT calculations. The UV-vis spectra are in good agreement with the experimental results. These spectra provide an effective method for the identification of isomers. The results of this study can offer useful data for further experimental and theoretical studies using X-ray and UV-vis spectroscopy methods on freshly synthesized fullerene isomers and their derivatives.

Metal Atoms (Li, Na, and K) Tuning the Configuration of Pyrrole for the Selective Recognition of C60

Host-guest structure assembly is significant in the recognition of molecules, and the fullerene-based host-guest structure is a convenient method to determine the structures of fullerenes of which recognition is with many difficulties in experiments. Here, with density functional theory calculations, we designed several crown-shaped pyrrole-based hosts tuned by doping metal atoms (Li, Na, and K) for the effective recognition of C₆₀ with modest interaction between the host and guest. Binding energy calculations showed an enhanced interaction of the concave-convex host-guest system with the doped metal atoms, enabling the selective recognition of C₆₀. The electrostatic interaction between the host and guest was studied by the natural bond order charge analysis, reduced density gradient, and electrostatic potential. Furthermore, the UV-vis-NIR spectra of host-guest structures were simulated to give guidance on the release of the fullerene guest. With much expectation, this work would give a new strategy to design new hosts for effectively recognizing much more fullerene molecules with modest interaction and would be useful for the assembly involving fullerenes.

Tb2O@C2(13333)-C74: A Non-Isolated Pentagon Endohedral Fullerene Containing a Nearly Linear Tb-O-Tb Unit

Terbium has been added to the list of elements that form oxide clusters inside fullerene cages. Tb₂O@C₂(13333)-C₇₄ has been isolated as a byproduct of the electric arc synthesis of the azafullerene Tb₂@C₇₉N. Cocrystallization of Tb₂O@C₂(13333)-C₇₄ with Ni(OEP) (where OEP is the dianion of octaethylporphyrin) in toluene yielded black needles of Tb₂O@C₂(13333)-C₇₄·Ni^II(OEP)·1.5C₇H₈ that have been examined by single-crystal X-ray diffraction. The resulting structure shows that a nearly linear Tb-O-Tb unit is contained in a C₂(13333)-C₇₄, which has two sites where pentagons share an edge to form pentalene units at opposite ends of the fullerene. Unlike the usual situations where metal atoms in fullerenes that do not obey the isolated pentagon rule are situated within the folds of the pentalene units, the Tb atoms in Tb₂O@C₂(13333)-C₇₄ are positioned to the side of the pentalene units and near-neighboring hexagons. The magnetic properties of Tb₂O@C₂(13333)-C₇₄ have been examined starting from the experimental geometry, using ab-initio multiconfigurational methods. The computations predict that Tb₂O@C₂(13333)-C₇₄ will show strong axiality, which would make it a single-molecule magnet with a large magnetic anisotropy barrier.

H2O·HF@C70: Encapsulation Energetics and Thermodynamics

This report deals with the quantum-chemical evaluation of the energetics and thermodynamics of the simultaneous encapsulation of HF and H2O by the IPR (isolated pentagon rule) C70 fullerene cage, yielding H2O·HF@C70 species which were synthesized and characterized recently, thus further expanding the family of fullerene endohedrals with non-metallic encapsulates. The structures were optimized at the DFT (density functional theory) M06-2X/6-31++G** level. The encapsulation energetics were further refined by the advanced B2PLYPD/6-31++G** and B2PLYPD/6-311++G** methods. After enhancement of the B2PLYPD/6-311++G** encapsulation energy for the BSSE and steric corrections, the encapsulation energy gain was obtained, as 26 kcal/mol. The equilibrium encapsulation thermodynamics were described using the M06-2X/6-31++G** partition functions. The results correspond to our previous evaluations for the water dimer encapsulation by C84 cages.

The Various Packing Structures of Tb@C82 (I, II) Isomers in Their Cocrystals with Ni(OEP)

Soot-containing terbium (Tb)-embedded fullerenes were prepared by evaporation of Tb₄O₇-doped graphite rods in an electric arc discharge chamber. After 1,2,4-trichlorobenzene extraction of the soot and rotary evaporation of the extract, a solid product was obtained and then dissolved into toluene by ultrasonication. Through a three-stage high-pressure liquid chromatographic (HPLC) process, Tb@C₈₂ (I, II) isomers were isolated from the toluene solution of fullerenes and metallofullerenes. With the success of the growth of cocrystals of Tb@C₈₂ (I, II) with Ni(OEP), the molecular structures of Tb@C₈₂ (I) and Tb@C₈₂ (II) were confirmed to be Tb@C_2v(9)-C₈₂ and Tb@Cs(6)-C₈₂, respectively, based on crystallographic data from X-ray single-crystal diffraction. Moreover, it was found that Tb@C₈₂ (I, II) isomers demonstrated different packing behaviors in their cocrystals with Ni(OEP). Tb@C_2v(9)-C₈₂ forms a 1:1 cocrystal with Ni(OEP), in which Tb@C_2v(9)-C₈₂ is aligned diagonally between the Ni(OEP) bilayers to form zigzag chains. In sharp contrast, Tb@Cs(6)-C₈₂ forms a 2:2 cocrystal with Ni(OEP), in which Tb@Cs(6)-C₈₂ forms a centrosymmetric dimer that is aligned linearly with Ni(OEP) pairs to form one-dimensional structures in the a-c lattice plane. In addition, the distance of a Ni atom in Ni(OEP) to the Cs(6)-C₈₂ cage is much shorter than that to the C_2v(9)-C₈₂ one, indicative of a stronger π-π interaction between Ni(OEP) and the C₈₂ carbon cage in the cocrystal of Tb@C_S(6)-C₈₂ and Ni(OEP). Density functional theory calculations reveal that the regionally selective dimerization of Tb@C_S(6)-C₈₂ is the result of a dominant unpaired spin existing on a particular C atom of the C_S(6)-C₈₂ cage.

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@C_2n 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.

Enhanced Electronic g-Factors in Magic Number Main Group Bimetallic Nanoclusters

We report the observation of large electronic g-factors in magic number main group bimetallic nanoclusters by performing Stern-Gerlach deflection experiments at 10 K. The clusters AlPb₁₂ and InPb₁₂ exhibit values of g = 3.5-4.0, whereas GaPb₁₂ clusters surprisingly reveal a value of g < 2.0. Multireference ab initio methods are applied to unmask the origin of the g-factors and to gain insight into the electronic structure. The interplay of the pyritohedral molecular symmetry, a particularly strong spin-orbit coupling involved in the ground state, and the presence of low-lying degenerate excited states causes large positive g-factors in AlPb₁₂ and InPb₁₂. Contrarily, the spin-orbit coupling in the GaPb₁₂ ground state is completely quenched. This is due to the d-block contraction lowering the nonbonding Ga 5s orbital and consequently forming an icosahedral ground state. Thus, endohedral p-doped tetrel clusters, composed of purely main group elements, state a novel and unique class of magnetic compounds and their study contributes to a more profound understanding of the metal-metal interaction in polynuclear clusters as well as magnetism at the molecular level.

Locating the hydrogen atoms in endohedral clusterfullerenes by density functional theory

There is a unique type of endohedral clusterfullerene containing a hydrogen atom inside the carbon cage (hydrogen-containing clusterfullerenes, HCFs). Unfortunately, the precise positions of the H atoms cannot be determined by powerful single-crystal X-ray diffraction, and thus, the reported internal cluster structures of HCFs are ambiguous. In this study, HCFs were investigated using density functional theory calculations. Various internal cluster structures were obtained for Sc₄CNH@I_h(7)-C₈₀ and then carefully inspected to summarize all the favorable H locations in the HCFs. Encouragingly, following these structural characteristics, a new Sc₄C₂H@I_h(7)-C₈₀ isomer with a μ₃-H coordination to three Sc atoms was found to be 12.6 kcal mol^-1 more stable than a previously reported isomer. It also holds a much larger SOMO-LUMO gap energy (3.57 vs. 2.36 eV). Its increased stability was further understood by the formation of multicenter bonds (three-center one-electron, three-center two-electron, and even four-center two-electron bonds) and electron density topology analyses. The changed H position may lead to rather different electronic structures, bonding states, and relative stability, indicating its critical role in HCFs. The simulated infrared and Raman spectra based on the new structure also agree fairly well with the experimental observations. Our work not only successfully locates the unpredictable H atom inside HCFs but also demonstrates a practical strategy to quickly determine the internal cluster configurations for more complex clusterfullerenes.

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.

Stabilities, Geometries, Electronic Structures, and Conversion Rules of Carbide Cluster Metallofullerenes

Since the discovery of the first carbide cluster metallofullerene (CCMF) Sc₂ C₂ @C₈₄ in 2001, CCMFs have attracted great concerns with variable structures and fascinating characteristics. To date, there are hundreds of studies on CCMFs. Crystallography studies on CCMFs are carried out by single-crystal X-ray diffraction. Theoretical calculations can also be used to study CCMFs in detail without being limited by low experimental yields. This review analyzes the stability of CCMFs reported previously, and indicates that the C₂ unit contributes a lot to their stability. At the same time, the relationship between the structures of inner carbide cluster and cage size is systematically discussed, and the four-electron transfer always occurs. Furthermore, the possible transformation rule between di-EMFs and CCMFs is indicated. Finally, an outlook regarding the future developments and applications of CCMFs is presented.

Exploring the Threshold between Fullerenes and Nanotubes: Characterizing Isomerically Pure, Empty-Caged, and Tubular Fullerenes D5h-C90 and D5d-C100

We report the fully fledged photophysical characterization of isomerically pure, empty-caged, tubular fullerenes D_5h-C₉₀ and D_5d-C₁₀₀ and compare their key properties. In particular, the focus was on cage sizes between 60 and 150 carbon atoms with D₃, D_3d/h, and D_5d/h symmetry. The optical band gap of D_5d-C₁₀₀ is 1.65 eV, which is larger than 1.37 eV of D_5h-C₉₀. In stark contrast to the nonluminescent D_5h-C₉₀, D_5d-C₁₀₀ luminesces at room temperature. Transient absorption spectroscopy shows that photoexcited D_5d-C₁₀₀ is subject to a slow intersystem crossing that generates a triplet excited state. In contrast, a fast, nonradiative internal conversion governs the deactivation of D_5h-C₉₀: In this case, exploring the corresponding triplet excited state required triplet-triplet sensitization experiments with anthracene. Density functional theory calculations revealed the electronic structure of the fullertubes, and calculations are consistent with our experimental findings. The calculated band gap systematically decreases with the number of carbon atoms within the D₃ and D_3d/h series. In contrast, an oscillating behavior is noted within the series of D_5d/h fullertubes. Finally, photoexcited D_5d-C₁₀₀ was found to undergo hole transfer with electron-donating triethylamines readily but not electron transfer with electron-accepting methyl viologens.

Exploring seebeck-coefficient fluctuations in endohedral-fullerene, single-molecule junctions

For the purpose of creating single-molecule junctions, which can convert a temperature difference ΔT into a voltage ΔV via the Seebeck effect, it is of interest to screen molecules for their potential to deliver high values of the Seebeck coefficient S = -ΔV/ΔT. Here we demonstrate that insight into molecular-scale thermoelectricity can be obtained by examining the widths and extreme values of Seebeck histograms. Using a combination of experimental scanning-tunnelling-microscopy-based transport measurements and density-functional-theory-based transport calculations, we study the electrical conductance and Seebeck coefficient of three endohedral metallofullerenes (EMFs) Sc₃N@C₈₀, Sc₃C₂@C₈₀, and Er₃N@C₈₀, which based on their structures, are selected to exhibit different degrees of charge inhomogeneity and geometrical disorder within a junction. We demonstrate that standard deviations in the Seebeck coefficient σ_S of EMF-based junctions are correlated with the geometric standard deviation σ and the charge inhomogeneity σ_q. We benchmark these molecules against C₆₀ and demonstrate that both σ_q, σ_S are the largest for Sc₃C₂@C₈₀, both are the smallest for C₆₀ and for the other EMFs, they follow the order Sc₃C₂@C₈₀ > Sc₃N@C₈₀ > Er₃N@C₈₀ > C₆₀. A large value of σ_S is a sign that a molecule can exhibit a wide range of Seebeck coefficients, which means that if orientations corresponding to high values can be selected and controlled, then the molecule has the potential to exhibit high-performance thermoelectricity. For the EMFs studied here, large values of σ_S are associated with distributions of Seebeck coefficients containing both positive and negative signs, which reveals that all these EMFs are bi-thermoelectric materials.

Overlooked Effects of La-4f Orbitals in Endohedral Metallofullerenes

For endohedral metallofullerenes (EMFs), a central issue is how to correctly describe the intracluster and metal-cage interactions, which are critical for understanding their structures, stabilities, and various properties. In this work, density functional theory calculations were carried out for 13 La-based EMFs covering all four reported types and a rather wide cage size range (C₃₂-C₁₀₄). The results reveal that the usually core-like lanthanide 4f subshell may play a critical role in the structural characteristics, energetic stabilities, frontier orbital energy levels, metal charges, and chemical reactivities of these endofullerenes. Regardless of the encapsulated forms, the La-4f contributions to the chemical bonding and structural stability increase with the reduced cage sizes because of the gradually enhanced cage confinement. The combination of metal-to-nonmetal charge transfer and compression of the cage cavity exposes and effectively activates the otherwise chemically inert 4f orbitals. By disclosing the important role of long-neglected metal orbitals inside fullerenes, the current work not only deepens our understanding of EMFs, but also provides new insights into the chemical bondings in general confined spaces at the subnanometer scale.

Dehydrogenation of ammonia-borane to functionalize neutral and Li+-encapsulated C60, C70 and C36 fullerene cages: a DFT approach

Mechanistic investigations into the functionalization of three fullerene cages, viz. C₆₀, C₇₀, and C₃₆ through dehydrogenation of ammonia-borane (AB) have been conducted using Density Functional Theory (DFT). In this process of functionalization, different ring fusions, namely (6-6), (6-5) positions for C₆₀ and C₇₀, and an additional (5-5) for C₃₆ fullerene have been investigated. The optimized geometries of all the complexes and transition states have been characterized using the M06-2X functional in conjunction with the 6-31G(d) basis set. The effect of Li⁺-encapsulation on the energetics and activation barriers of H₂ attachment has also been examined. Although the process of functionalization of neutral fullerenes proceeds extensively through concerted pathways, a step-wise route has been observed for the encapsulated systems. NPA charge analysis and Wiberg bond index (WBI) have been used in order to detect the change in the nature of participating hydrogen atoms and validate the variation in the bond order of the C-C connectivity respectively upon hydrogenation. GCRD parameters have also been calculated to explicate the electronic properties of the hydrogenated products. The (6-6) hydrogenation is observed to be favoured thermodynamically and kinetically for both neutral and Li⁺-encapsulated C₆₀ and C₇₀, while (5-5) is found to be the most preferred site for C₃₆ systems. Our theoretical exploration suggests that the covalent functionalization of the fullerene cages can be done successfully viaAB resulting in the stabilization of these systems. In short, the present work will provide a general idea about the detailed mechanism related to the functionalization of fullerene cages, which will further motivate researchers in fullerene chemistry.

A computational study on the nature, strength and cooperativity of bonds in [M(η5–C60Me5)(CO)n] and [M(η5–Cp)(CO)n] (n = 3, M = Mn(i), Tc(i), Re(i); n = 2, M = Co(i), Rh(i), Ir(i)) complexes

It is shown that, due to cooperativity versus anticooperativity of bonds, the total interaction energy of a complex, having weaker metal–ligand bonds, can be comparable to or even larger than that of a similar complex having stronger bonds.

Carbon cage isomers and magnetic Dy⋯Dy interactions in Dy2O@C88 and Dy2C2@C88 metallofullerenes

Isomers of Dy2O@C88 and Dy2C2@C88 show a strong variation in the type and strength of Dy⋯Dy superexchange interactions and magnetization relaxation rate.

Introduction of a (Ph3P)2Pt group into the rim of an open-cage fullerene by breaking a carbon-carbon bond

Treatment of an open-cage fullerene, designated as MMK-9, with (Ph₃P)₄Pt in toluene solution at room temperature allows a (PPh₃)₂Pt unit to be incorporated into the rim of the cage so that it becomes an integral part of the carbon cage skeleton. The structure of the adduct has been determined by single crystal X-ray diffraction and reveals that the platinum atom has planar PtC₂P₂ coordination, rather than the usual η²-bonding to an intact C-C double bond of the fullerene.

Chemical shielding of H2O and HF encapsulated inside a C60 cage

Molecular surgery provides the opportunity to study relatively large molecules encapsulated within a fullerene cage. Here we determine the location of an H₂O molecule isolated within an adsorbed buckminsterfullerene cage, and compare this to the intrafullerene position of HF. Using normal incidence X-ray standing wave (NIXSW) analysis, coupled with density functional theory and molecular dynamics simulations, we show that both H₂O and HF are located at an off-centre position within the fullerene cage, caused by substantial intra-cage electrostatic fields generated by surface adsorption of the fullerene. The atomistic and electronic structure simulations also reveal significant internal rotational motion consistent with the NIXSW data. Despite this substantial intra-cage interaction, we find that neither HF or H₂O contribute to the endofullerene frontier orbitals, confirming the chemical isolation of the encapsulated molecules. We also show that our experimental NIXSW measurements and theoretical data are best described by a mixed adsorption site model.

Ground and excited electronic structures of metal encapsulated nanocages: the cases of endohedral M@C20H20 (M = K, Rb, Ca, Sr) and M@C36H36 (M = Na, K, Rb)

High-level electronic structure calculations were performed to analyze ground and excited states of neutral and cationic endohedral M@C₂₀H₂₀ (M = K, Rb, Ca, Sr) and M@C₃₆H₃₆ (M = Na, K, Rb). In their ground states, one or two electrons occupy a diffuse atomic s-type orbital, thus 1s¹ and 1s² superatomic electronic configurations are assigned for M = Na, K, Rb and M = Ca, Sr cases, respectively. These species populate 1p-, 1d-, 1f-superatomic orbitals in electronically excited states. The specific superatomic Aufbau model introduced for M@C₂₀H₂₀ (M = K, Rb) is 1s, 1p, 1d, 2s, 1f, 2p, 2d, 1g, 2f. On the other hand, excited electronic spectra of M@C₂₀H₂₀ (M = Ca, Sr) are rich in multireference characters. Excited states of bigger M@C₃₆H₃₆ molecules were investigated up to the 1d level and the transitions were found to require slightly higher energies compared to M@C₂₀H₂₀. These superatoms possess lower ionization potentials, hence can also be categorized as superalkalis.

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.

Structural, Electronic, and Nonlinear Optical Properties of C66H4 and C70Cl6 Encapsulating Li and F Atoms

Recently, nonclassical fullerene derivatives C66H4 and C70Cl6, which both contain two negatively curved moieties of heptagons, have been successfully synthesized. Inspired by these experimental achievements, the structural and electronic properties of C66H4, C70Cl6, Li@C66H4, F@C66H4, Li@C70Cl6, and F@C70Cl6 were systematical studied through density functional theory calculations in this work. Our results show that the reduction of the front molecular orbital gap of fullerene derivatives occurs with the introduction of Li and F atoms. After quantitative analysis of back-donations of charge between an encapsulated atom and an external carbon cage, it is found that C66H4 and C70Cl6 prefer to act as electron acceptors. It is interesting to note that the strong covalent nature of the interactions between a F atom and a carbon cage is observed, whereas the weak covalent and strong ionic interactions occur between a Li atom and a carbon cage. On the other hand, according to the first hyperpolarizability results, the encapsulation of the Li atom enhances the nonlinear optical response of fullerene derivatives. This work provides a strategy to improve nonlinear optical properties of C66H4 and C70Cl6, reveals the internal mechanism of the contribution from Li and F atoms to endohedral fullerene derivatives, and will contribute to the designation of endohedral fullerene derivative devices.

Small Amount Makes a Big Difference: Critical (n - 1)d Valence Orbitals of Heavy Alkaline Earth Metals inside Cage Clusters

Heavy alkaline earth metals (Aes) are usually considered to engage in chemical bonding by donating the two electrons on ns atomic orbitals (AOs). In this work, a series of typical endohedrally doped cage clusters Ae@cage (Ae = Ca, Sr, Ba; cage = C₃₂, C₇₄, C₉₄, B₄₀, Si₂₀, Sn₁₂, Au₁₆) were thoroughly investigated by means of density functional theory calculations. We found that their occupied molecular orbitals have ∼1 to 14% contributions from Ae-(n - 1)d AOs due to electron back-donation from the cage. Though the amount is small, it is hard to ignore: with the d orbitals, all these endohedral clusters exhibit obviously shortened Ae-cage distances, greatly enhanced encapsulation stabilities, changed highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gaps, and much lowered Ae valences far from ideal +2. Evidently, the valence orbitals of Ca/Sr/Ba in these systems should include both ns and (n - 1)d. By disclosing the critical role of unnoticed metal orbitals, our work provides completely new insights into the cluster field.

Discovery of Non-Isolated-Pentagon-Rule Fullerenes from Computational Characterization of U2O@C72

The reported actinide-based endohedral clusterfullerenes (ECFs) are rather scarce thus far. Though several members have been detected in mass spectra, their exact structures and properties mostly remain unclear. Herein, density functional theory calculations revealed that the U₂O@C₇₂ observed in recent experiments should be U₂O@D₂(10611)-C₇₂, U₂O@C₁(10610)-C₇₂, or U₂O@C_s(10616)-C₇₂. Featuring two pairs of fused pentagons, their outer cages all break the well-known isolated pentagon rule. U₂O@D₂(10611)-C₇₂ is the first clusterfullerene based on the D₂(10611)-C₇₂ cage, which only encapsulated dimetals (Sc₂, La₂, Ce₂, Pr₂) before. It is also the first time to reveal that C₁(10610)-C₇₂ and C_s(10616)-C₇₂ can serve as the parent cage of an endohedral fullerene. Interestingly, the three isomers could interconvert with each other via Stone-Wales transformation with one internal U atom dynamically changing its orientation according to the position of pentagon adjacencies. A common electronic structure of (U⁴⁺)₂(O)^2-@C₇₂^6- can be formally assigned to the three ECFs but with obvious covalent character for both U-O and U-C bonds. Their spatially extended U-5f orbitals substantially enhance the metal-cage interactions. Their various spectra were also simulated to assist future experiments. Moreover, our work shows that the careful choice of exchange-correlation functionals is rather critical for the structural characterization of ECFs.

Metallofullerene photoswitches driven by photoinduced fullerene-to-metal electron transfer

We report on the discovery and detailed exploration of the unconventional photo-switching mechanism in metallofullerenes, in which the energy of the photon absorbed by the carbon cage π-system is transformed to mechanical motion of the endohedral cluster accompanied by accumulation of spin density on the metal atoms. Comprehensive photophysical and electron paramagnetic resonance (EPR) studies augmented by theoretical modelling are performed to address the phenomenon of the light-induced photo-switching and triplet state spin dynamics in a series of Y _x Sc_3-x N@C₈₀ (x = 0-3) nitride clusterfullerenes. Variable temperature and time-resolved photoluminescence studies revealed a strong dependence of their photophysical properties on the number of Sc atoms in the cluster. All molecules in the series exhibit temperature-dependent luminescence assigned to the near-infrared thermally-activated delayed fluorescence (TADF) and phosphorescence. The emission wavelengths and Stokes shift increase systematically with the number of Sc atoms in the endohedral cluster, whereas the triplet state lifetime and S₁-T₁ gap decrease in this row. For Sc₃N@C₈₀, we also applied photoelectron spectroscopy to obtain the triplet state energy as well as the electron affinity. Spin distribution and dynamics in the triplet states are then studied by light-induced pulsed EPR and ENDOR spectroscopies. The spin-lattice relaxation times and triplet state lifetimes are determined from the temporal evolution of the electron spin echo after the laser pulse. Well resolved ENDOR spectra of triplets with a rich structure caused by the hyperfine and quadrupolar interactions with ¹⁴N, ⁴⁵Sc, and ⁸⁹Y nuclear spins are obtained. The systematic increase of the metal contribution to the triplet spin density from Y₃N to Sc₃N found in the ENDOR study points to a substantial fullerene-to-metal charge transfer in the excited state. These experimental results are rationalized with the help of ground-state and time-dependent DFT calculations, which revealed a substantial variation of the endohedral cluster position in the photoexcited states driven by the predisposition of Sc atoms to maximize their spin population.

Evaluation of the C60 biodistribution in mice in a micellar ExtraOx form and in an oil solution

The article is devoted to the study of the pharmacokinetics of fullerene C₆₀ in oil and micellar forms, analysis of its content in blood, liver, lungs, kidneys, heart, brain, adrenal glands, thymus, testicles, and spleen. The highest accumulation of C₆₀ was found in the liver and adrenal glands. As a result of the studies carried out, it was shown that the bioavailability of C₆₀ in the micellar form is higher than that in an oil solution.

Versatile Types of Cyclodextrin-Based Nucleic Acid Delivery Systems

Nowadays, nucleic acid therapy has become a promising way for the treatment of various malignant diseases. Cyclodextrin (CD)-based nucleic acid delivery systems have attracted widespread attention due to the favorable chemical structures and excellent biological properties of CD. Recently, a variety of CD-based nucleic acid delivery systems has been designed according to the different functions of CD for flexible gene therapies. In this review, the construction strategies and biomedical applications of CD-based nucleic acid delivery systems are mainly focused on. The review begins with an introduction to the synthesis and properties of simple CD-grafted polycations. Thereafter, CD-related supramolecular assemblies based on different guest components are discussed in detail. Finally, different CD-based organic/inorganic nanohybrids and their relevant functions are demonstrated. It is hoped that this brief review will motivate the delicate design of CD-based nucleic acid delivery systems for potential clinical applications.

Bingel-Hirsch Addition of Diethyl Bromomalonate to Ion-Encapsulated Fullerenes M@C60 (M=Ø, Li+, Na+, K+, Mg2+, Ca2+, and Cl-)

In the last 30 years, fullerene-based materials have become popular building blocks for devices with a broad range of applications. Among fullerene derivatives, endohedral metallofullerenes (EMFs, M@C_x) have been widely studied owing to their unique properties and reactivity. For real applications, fullerenes and EMFs must be exohedrally functionalized. It has been shown that encapsulated metal cations facilitate the Diels-Alder reaction in fullerenes. Herein, the Bingel-Hirsch (BH) addition of ethyl bromomalonate over a series of ion-encapsulated M@C₆₀ (M=Ø, Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, and Cl^-; Ø@C₆₀ stands for C₆₀ without any endohedral metal) is quantum mechanically explored to analyze the effect of these ions on the BH addition. The results show that the incarcerated ion has a very important effect on the kinetics and thermodynamics of this reaction. Among the systems studied, K⁺@C₆₀ is the one that leads to the fastest BH reaction, whereas the slowest reaction is given by Cl^-@C₆₀.

An orifice design: water insertion into C60

Using an open-cage C60 derivative possessing an orifice designed on the basis of computational studies, we have experimentally demonstrated the quantitative encapsulation of H2O as well as effective conversion into H2O@C60 in an overall yield remarkably higher than the previously reported methods by ca. 2-5 times.

New Horizons in Chemical Functionalization of Endohedral Metallofullerenes

This overview explains some new aspects of chemical functionalization of endohedral metallofullerenes (EMFs) that have been unveiled in recent years. After differences in chemical reactivity between EMFs and the corresponding empty fullerenes are discussed, cage-opening reactions of EMFs are examined. Then, the selective bisfunctionalization of EMFs is explained. Finally, single-bonding derivatization of EMFs is addressed. The diversity and applicability of the chemical functionalization of endohedral metallofullerenes are presented to readers worldwide.

Crystallographic Characterization of Ti2C2@D3h(5)-C78, Ti2C2@C3v(8)-C82, and Ti2C2@Cs(6)-C82: Identification of Unsupported Ti2C2 Cluster with Cage-Dependent Configurations

Fullerene cages are ideal hosts to encapsulate otherwise unstable metallic clusters to form endohedral metallofullerenes (EMFs). Herein, a novel Ti₂C₂ cluster with two titanium atoms bridged by a C₂-unit has been stabilized by three different fullerene cages to form Ti₂C₂@D_3h(5)-C₇₈, Ti₂C₂@C_3v(8)-C₈₂, and Ti₂C₂@C_s(6)-C₈₂, representing the first examples of unsupported titanium carbide clusters. Crystallographic results show that the configuration of the Ti₂C₂ cluster changes upon cage variation. In detail, the Ti₂C₂ cluster adopts a butterfly shape in Ti₂C₂@C_3v(8)-C₈₂ and Ti₂C₂@C_s(6)-C₈₂ with Ti-C₂-Ti dihedral angles of 156.35 and 147.52° and Ti-Ti distances of 3.633 and 3.860 Å, respectively. In sharp contrast, a stretched planar geometry of Ti₂C₂ is observed in Ti₂C₂@D_3h(5)-C₇₈, where a Ti-C₂-Ti angle of 176.87° and a long Ti-Ti distance of 5.000 Å are presented. Consistently, theoretical calculations reveal that the cluster configuration is very sensitive to the cage shape which eventually determines the electronic structures of the hybrid EMF-molecules, thus adding new insights into modern coordination chemistry.

Scandium Tetrahedron Supported by H Anion and CN Pentaanion inside Fullerene C80

Endohedral metallofullerenes have greatly expanded the range of the fullerene family due to their nesting structure and unusual encapsulated clusters protected by a fullerene cage. Herein, we report a metallofullerene Sc₄CNH@I_h-C₈₀, which has a scandium tetrahedron supported by H and CN anions inside fullerene C₈₀. Sc₄CNH@I_h-C₈₀ has a rare multilayer nesting structure, and the internal Sc₄CNH is the most complex endohedral cluster disclosed to date. Sc₄CNH@I_h-C₈₀ has so many bonding types (metal-carbide, metal-nitride, and metal-hydride), which weave a polyhedron of Sc₄CNH clusters. This work shows that the endohedral metallofullerenes have the potential to build inorganic nesting polyhedra that have distinctive architectures and unique electronic properties. Sc₄CNH@I_h-C₈₀ was synthesized by means of the arc-discharge method using scandium and graphite under the mixed atmosphere of hydrogen, nitrogen, and helium. It is the first time to disclose an unprecedented metal-hydride bond in a fullerene cage. This result shows that the endohedral fullerenes bearing hydrogen species can be synthesized by the arc-discharge technique under an atmosphere of hydrogen. This work demonstrates that a fullerene cage can be an ample carrier to encapsulate unusual cluster moieties.

U2O@C76: Non-Isolated-Pentagon-Rule Cages Prevail with the U2O Configuration Determined by Cage Shape and Dominated by Multicenter Bonds

Endohedral clusterfullerenes (ECFs) are fullerene cages with various metallic clusters trapped inside. So far, the actinide-based ECFs are rather scarce with their possible structures and chemistry remaining largely unexplored. Herein, density functional theory calculations characterized that the recently synthesized U₂O@C₇₆ could be U₂O@C_s(17 490)-C₇₆ or U₂O@C_2v(19 138)-C₇₆, whose cages have two or one pentagon adjacencies (PAs) and thus both violate the isolated pentagon rule (IPR). It is noteworthy that they are the first actinide-based ECFs bearing non-IPR outer cages. They are also the first C_s(17 490)- and C_2v(19 138)-C₇₆-based oxide ECFs. Moreover, U₂O@C_2v(19 138)-C₇₆ is the first example of a hexavalent metal cluster within the C_2v(19 138)-C₇₆ cage. Interestingly, although trapped by the two same-sized cages, the U₂O unit exhibits a bent and a perfect linear configuration, respectively, indicative of the crucial role of cage shape in steering the internal cluster configuration. Their electronic structures can be formally described as (U₂O)⁶⁺@C₇₆^6- with primary electrostatic attractions and secondary covalent interactions between cluster and cage. Significantly, bonding analyses reveal that the encaged U₂O moiety may only features two three-center, two-electron (3c-2e) U-O-U bonds with completely absent common two-center bonds.

Undiscovered Effect of C↔N Interchange Inside the Metal Carbonitride Clusterfullerenes: A Density Functional Theory Investigation

Putting different metal clusters into the fullerene cages form the so-called "endohedral clusterfullerenes" (ECFs), among which all the carbonitride ECFs feature a common NC unit coordinating with either one or three metal atoms. Unfortunately, their internal N and C atoms are difficult to be distinguished experimentally, resulting in the fact that the exact structure and bonding nature of the encased metal cluster still remain unclear thus far. In this work, density functional theory calculations were performed for several representative carbonitride ECFs: MNC@C_2n (M = Y, Tb; 2n = 76, 82) and Sc₃CN@C_2n (2n = 78, 80). For the first time, we focused on the C ↔ N interchange inside the cages and its effect on the chemical bonding of the trapped clusters. Computational results reveal that the two types of ECFs energetically favor the N and C atoms at the cluster center, respectively. The preference can be interpreted by the difference in several aspects, such as the energy of isolated clusters, charge states of (CN)^-/3-, and cluster-cage interactions, as well as hyperconjugation of the internal clusters. The detailed wave function analyses indicate that MNC@C_2n and Sc₃CN@C_2n bear a C≡N triple bond and a C═N double bond, respectively, regardless of the NC orientation. Compared with its slightly influence on the bonding patterns of the encaged MNC clusters, the C ↔ N interchange dramatically affects that of the Sc₃CN units involving two-center two-electron (2c-2e) bonds, undiscovered three-center two-electron (3c-2e), and four-center two-electron (4c-2e) bonds.