Sm@C2v(19138)-C76: A Non-IPR Cage Stabilized by a Divalent Metal Ion

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.

Functionalized Fullerenes and Their Applications in Electrochemistry, Solar Cells, and Nanoelectronics

Carbon-based nanomaterials have rapidly advanced over the last few decades. Fullerenes, carbon nanotubes, graphene and its derivatives, graphene oxide, nanodiamonds, and carbon-based quantum dots have been developed and intensively studied. Among them, fullerenes have attracted increasing research attention due to their unique chemical and physical properties, which have great potential in a wide range of applications. In this article, we offer a comprehensive review of recent progress in the synthesis and the chemical and physical properties of fullerenes and related composites. The review begins with the introduction of various methods for the synthesis of functionalized fullerenes. A discussion then follows on their chemical and physical properties. Thereafter, various intriguing applications, such as using carbon nanotubes as nanoreactors for fullerene chemical reactions, are highlighted. Finally, this review concludes with a summary of future research, major challenges to be met, and possible solutions.

Steering Lu3N clusters in C76-78 cages: cluster configuration dominated by cage transformation

While the strong interaction between the internal unit and the fullerene cage inside metallofullerenes is widely acknowledged, how the cage transformation interacts with the cluster configuration remains elusive. For this purpose, we herein synthesized three metallofullerene molecules with an easy-to-compare cluster configuration and cage arrangement, namely Lu₃N@C_s(17 490)-C₇₆, Lu₃N@C₂(22 010)-C₇₈, and Lu₃N@D_3h(5)-C₇₈. The three lutetium-based nitride clusterfullerenes (NCFs) with small C_76-78 carbon cages were synthesized by a modified arc-discharge method and their structures were unambiguously confirmed by X-ray crystallography. Notably, the cage transformation from C_s(17 490)-C₇₆ to C₂(22 010)-C₇₈via a simple C₂-unit insertion leads to a remarkable configuration change of the encapsulated Lu₃N cluster from an unusual asymmetric plane to a common symmetric one. This close correlation between the cluster configuration and cage transformation is further confirmed by the pyramidal Lu₃N cluster in Lu₃N@D_3h(5)-C₇₈ other than the symmetric planar Lu₃N unit in Lu₃N@C₂(22 010)-C₇₈, as a result of an even larger difference in the cage arrangement. Astonishingly, such a cluster shrinkage, accompanied by an increase in the cage size from C_s(17 490)-C₇₆ to D_3h(5)-C₇₈, is dramatically opposite to the cluster expansion with cage elongation found in La₂C₂- or Y₂C₂-based metallofullerenes.

An unprecedented C80 cage that violates the isolated pentagon rule

Two Lu2O@C80 isomers have been successfully isolated and unambiguously assigned as Lu2O@C1(31876)-C80 and Lu2O@C2v(5)-C80, respectively, by X-ray crystallography. Interestingly, C1(31876)-C80 is an unprecedented cage with a pair of adjacent pentagons,...

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@C_s(6)-C₈₂, U@C₂(8)-C₈₄, U@C_s(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.

Binding Sites, Vibrations and Spin-Lattice Relaxation Times in Europium(II)-Based Metallofullerene Spin Qubits

To design molecular spin qubits with enhanced quantum coherence, a control of the coupling between the local vibrations and the spin states is crucial, which could be realized in principle by engineering molecular structures via coordination chemistry. To this end, understanding the underlying structural factors that govern the spin relaxation is a central topic. Here, we report the investigation of the spin dynamics in a series of chemically designed europium(II)-based endohedral metallofullerenes (EMFs). By introducing a unique structural difference, i. e. metal-cage binding site, while keeping other molecular parameters constant between different complexes, these manifest the key role of the three low-energy metal-displacing vibrations in mediating the spin-lattice relaxation times (T1 ). The temperature dependence of T1 can thus be normalized by the frequencies of these low energy vibrations to show an unprecedentedly universal behavior for EMFs in frozen CS2 solution. Our theoretical analysis indicates that this structural difference determines not only the vibrational rigidity but also spin-vibration coupling in these EMF-based qubit candidates.

A non-isolated pentagon rule C82 cage stabilized by a stretched Sc3N cluster

A novel cluster fullerene, Sc3N@Cs(39 663)-C82, has been synthesized and characterized. Crystallograpic charaterization unambiguously determines the non-IPR cage structure of Cs(39 663)-C82. Structural analyses further reveal that the Sc3N cluster is notably stretched to facilitate the interaction with the non-IPR fullerene cage.

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.

Strain Release of Fused Pentagons in Fullerene Cages by Chemical Functionalization

According to the isolated pentagon rule (IPR), for stable fullerenes, the 12 pentagons should be isolated from one another by hexagons, otherwise the fused pentagons will result in an increase in the local steric strain of the fullerene cage. However, the successful isolation of more than 100 endohedral and exohedral fullerenes containing fused pentagons over the past 20 years has shown that strain release of fused pentagons in fullerene cages is feasible. Herein, we present a general overview on fused-pentagon-containing (i.e. non-IPR) fullerenes through an exhaustive review of all the types of fused-pentagon-containing fullerenes reported to date. We clarify how the strain of fused pentagons can be released in different manners, and provide an in-depth understanding of the role of fused pentagons in the stability, electronic properties, and chemical reactivity of fullerene cages.

Fullerenes as Nanocontainers That Stabilize Unique Actinide Species Inside: Structures, Formation, and Reactivity

Fullerene carbon cages can encapsulate a wide variety of atoms, ions, clusters, or small molecules inside, resulting in stable compounds with unusual structures and electronic properties. These compounds are collectively defined as endohedral fullerenes. The most studied endohedral fullerenes are those containing metal atoms or ions inside, and these are referred to as endohedral metallofullerenes (EMFs). For EMFs, the inner isolated space of the fullerene cages can lead to the stabilization of unique clusters, which are otherwise not synthetically accessible. This offers an excellent environment and opportunity for investigating the nature of previously unobserved metal-metal, metal-non-metal, and metal-fullerene interactions, which are of fundamental interest and importance. Up until now, most of the work in this field has been mainly focused on the rare-earth metals and related elements (groups II, III, and IV). The encapsulation of other elements of the periodic table could potentially lead to totally new structures and bonding motifs and to material properties beyond those of the existing EMFs. Actinides were originally explored as encapsulated elements in fullerenes when Smalley et al. ( Science 1992 , 257 , 1661 ) reported mass spectral evidence of actinide endohedral fullerenes back in 1992. However, the full characterization of these actinide endohedral fullerenes, including single crystal X-ray diffractometric analyses, was not reported until very recently, in 2017. In this Account, we highlight some recent advances made in the field of EMF compounds, focusing primarily on the molecular and electronic structures of novel actinide-based EMFs, new evidence for the formation mechanisms of EMFs, and the influence of the entrapped species on the reactivity and regiochemistry of EMF compounds. We recently reported that some monometallic actinide EMFs represent the first examples of tetravalent metals encapsulated inside fullerenes that exhibit considerably stronger host-guest interactions when compared to those observed for the lanthanide EMFs. These unusually strong metal-cage interactions, along with very high mobilities of the actinides inside the fullerene cages at high temperatures, result in the stabilization of unexpected non-IPR (isolated pentagon rule) fullerene cages encapsulating only one metal ion. Strikingly, such covalent stabilization factors had never been previously observed, although Sm@C_2v(19138)-C₇₆ was the first reported mono-EMF with a non-IPR cage, see details below. In addition, we showed that a long sought-after actinide-actinide bond was obtained upon encapsulation of U₂ inside an I_h(7)-C₈₀ fullerene cage. More interestingly, we demonstrated that actinide multiple bonds, which are very difficult to prepare by conventional synthetic methods, are stabilized when trapped inside fullerene cages. A totally unexpected and previously unreported uranium carbide cluster, U═C═U, was fully characterized inside an EMF, U₂C@I_h(7)-C₈₀, which, for the first time, clearly exhibits two unsupported axial U═C double bonds that are ∼2.03 Å long. We also discovered that synthetic bis-porphyrin nanocapsules exhibit exquisitely selective complexation of some of these uranium endohedral compounds, providing the basis for a nonchromatographic EMF purification method for actinide EMFs. Regarding EMF formation mechanisms, we suggested that novel carbide EMF structures, that is, Sc₂C₂@C_s(hept)-C₈₈, are likely key intermediates in a bottom-up fullerene growth process. Additionally, the structural correlation between chiral carbon cages during a bottom-up growth process was shown to be enantiomer-dependent. The influence of the encapsulated clusters on the chemical reactivity of EMFs is discussed at the end, which showed that the regioselectivities of multiple additions to the fullerene cages are remarkably controlled by the encapsulated metal clusters.

Mononuclear Clusterfullerene Single-Molecule Magnet Containing Strained Fused-Pentagons Stabilized by a Nearly Linear Metal Cyanide Cluster

Fused-pentagons results in an increase of local steric strain according to the isolated pentagon rule (IPR), and for all reported non-IPR clusterfullerenes multiple (two or three) metals are required to stabilize the strained fused-pentagons, making it difficult to access the single-atom properties. Herein, we report the syntheses and isolations of novel non-IPR mononuclear clusterfullerenes MNC@C₇₆ (M=Tb, Y), in which one pair of strained fused-pentagon is stabilized by a mononuclear cluster. The molecular structures of MNC@C₇₆ (M=Tb, Y) were determined unambiguously by single-crystal X-ray diffraction, featuring a non-IPR C_2v (19138)-C₇₆ cage entrapping a nearly linear MNC cluster, which is remarkably different from the triangular MNC cluster within the reported analogous clusterfullerenes based on IPR-obeying C₈₂ cages. The TbNC@C₇₆ molecule is found to be a field-induced single-molecule magnet (SMM).

An atlas of endohedral Sc2S cluster fullerenes

Low-energy Sc₂S@C_n isomers are connected by an intricate web of Stone–Wales isomerization and Endo–Kroto C₂ insertions, giving clues to their formation.

The inner-induced effects of YCN in C76 on the structures and nonlinear optical properties

Very recently, an unprecedented novel monometallic cluster of fullerenes entrapping a yttrium cyanide (YCN) cluster inside a popular C82 cage YCN@Cs(6)-C82 was synthesized and characterized. Inspired by this investigation, four non-IPR YCN@C1(17459)-C76, YCN@C2v(19138)-C76, YCN@C2(17646)-C76, and YCN@C1(17894)-C76 (1, 2, 3, and 4) containing a pair of adjacent pentagons are designed to explore the encapsulated molecular effect on their interaction energies and nonlinear optical properties. The interaction energy (E int) values of 1, 2, 3, and 4 are -481.35 (1), -477.91 (2), -482.04 (3), -482.69 (4) kcal mol(-1), respectively, which shows that the E int value of 4 is the largest. Furthermore, the electron-transfer is mainly from the YCN to C76 cage. When YCN is encapsulated into C76 cage, we can find that the α0 values of the four molecules are very close, ranging from 6.50 × 10(2) to 6.65 × 10(2) au. Significantly, the first hyperpolarizabilities are in relation to the encapsulated molecular: 1.63 × 10(3) (1) > 8.03 × 10(2) (2) > 7.76 × 10(2) (4) > 4.86 × 10(2) au (3), the results show that the βtot value of 1 is the largest. Besides this, the encapsulation of the YCN to C76 cage brings some distinctive changes in its UV-vis spectra along with its other electronic properties that might be used by the experimentalists to develop the potential nonlinear optical nanomaterials based on endohedral metallofullerenes.

Endohedrally stabilized C70 isomer with fused pentagons characterized by crystallography

In this study, we report for the first time, the X-ray structure of an unconventional endohedral C₇₀ fullerene, Sc₂O@C₂(7892)-C₇₀.