Exceptional Chemical Properties of Sc@C2v(9)–C82 Probed with Adamantylidene Carbene

Dynamic Metastable Characteristics of Carbon Cages Embedded with Er2C2

Novel endohedral metallofullerenes (EMFs), namely, Er2C2@C2v(5)-C80, Er2C2@Cs(6)-C82, Er2C2@Cs(15)-C84, Er2C2@C2v(9)-C86, Er2C2@Cs(15)-C86, and Er2C2@Cs(32)-C88, had been experimentally synthesized, and the unique structures and many fascinating properties had also been widely explored. Nevertheless, the position of the Er atoms inside the cage shows a severe disorder within the stable EMF monomer, which is difficult to understand and explain from the experimental point of view. In this work, based on the density functional theoretical calculations, the Er2C2@Cs(6)-C82 has 73 directional isomers and 2 Er atoms that are far beyond from Er-Er single bonding and tend to be close to the cage side (marked as "shell"), and the core (Er2C2 units) takes on a butterfly shape as generally revealed. The energy difference between any two of the isomers is in the range of 0.05 to 25.6 kcal/mol, indicating a relatively easy thermodynamic transition between the isomers. The other five Er carbide cluster EMFs (Er2C2@C2v(5)-C80, Er2C2@Cs(15)-C84, Er2C2@C2v(9)-C86, Er2C2@Cs(15)-C86, and Er2C2@Cs(32)-C88) are also studied in the same way, and 30, 37, 39, and 43 most stable Er-oriented sites inside the cage, respectively, are obtained. In addition, the shape of the Er2C2 gradually changed from butterfly to linear. Moreover, the electronic structure and molecular orbital analyses show that it is easy for Er2C2@C80-88 to form a charge transfer state of [Er2C2]4+@[C80-88]4- via the dynamic core-shell coordination equilibrium. Er2C2 with a steep drop in chemical stability is restricted to forming varying degrees of metastable states in the shell, determined by the shell size, to ensure the overall stability. The lowest unoccupied molecular orbital energy level of these EMFs is increased by 0.5-1.1 eV compared with fullerenes C80-88, potentially providing favorable conditions for suitable energy level matching with EMF as an electron acceptor used in organic solar cell devices.

CH3 CN@open-C60 : An Effective Inner-Space Modification and Isotope Effect Inside a Nano-Sized Flask

Despite several small molecules being encapsulated inside cage-opened fullerene derivatives, such species have not considerably affected the structures and properties of the outer carbon cages. Herein, we achieved an effective inner-space modification for an open-cage C60 derivative by insertion of a neutral CH3 CN molecule into the cavity. The CH3 CN@open-C60 thus obtained showed an enhanced polarity, thus affording an easy separation from a mixture containing the empty cage by column chromatography on silica gel, without the preparative HPLC that was needed for previous cases. The less negative reduction potentials with respect to those of empty cage reflect the decreased energy level of the LUMO, which is supported by the DFT calculations. NMR spectroscopy, single-crystal X-ray analysis, and theoretical calculations revealed that both the presence of the encapsulated CH3 CN and cage deformation caused by the CH3 CN play an essential role in the change of the electronic properties. Furthermore, the favored binding affinity of deuterated acetonitrile CD3 CN with internal C60 surface is discussed.

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.

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@C_2v(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@C_2v(9)-C₈₂ and Th@C₂(5)-C₈₂ possess a stable closed-shell electronic structure. The computational results further confirm that Th@C_2v(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.

Synthesis and characterization of carbene derivatives of Th@C 3v(8)-C82 and U@C 2v(9)-C82: exceptional chemical properties induced by strong actinide-carbon cage interaction

Chemical functionalization of endohedral metallofullerenes (EMFs) is essential for the application of these novel carbon materials. Actinide EMFs, a new EMF family member, have presented unique molecular and electronic structures but their chemical properties remain unexplored. Here, for the first time, we report the chemical functionalization of actinide EMFs, in which the photochemical reaction of Th@C_3v(8)-C₈₂ and U@C_2v(9)-C₈₂ with 2-adamantane-2,3′-[3H]-diazirine (AdN₂, 1) was systematically investigated. The combined HPLC and MALDI-TOF analyses show that carbene addition by photochemical reaction afforded three isomers of Th@C_3v(8)-C₈₂Ad and four isomers of U@C_2v(9)-C₈₂Ad (Ad = adamantylidene), presenting notably higher reactivity than their lanthanide analogs. Among these novel EMF derivatives, Th@C_3v(8)-C₈₂Ad(I, II, III) and U@C_2v(9)-C₈₂Ad(I, II, III) were successfully isolated and were characterized by UV-vis-NIR spectroscopy. In particular, the molecular structures of first actinide fullerene derivatives, Th@C_3v(8)-C₈₂Ad(I) and U@C_2v(9)-C₈₂Ad(I), were unambiguously determined by single crystal X-ray crystallography, both of which show a [6,6]-open cage structure. In addition, isomerization of Th@C_3v(8)-C₈₂Ad(II), Th@C_3v(8)-C₈₂Ad(III), U@C_2v(9)-C₈₂Ad(II) and U@C_2v(9)-C₈₂Ad(III) was observed at room temperature. Computational studies suggest that the attached carbon atoms on the cages of both Th@C_3v(8)-C₈₂Ad(I) and U@C_2v(9)-C₈₂Ad(I) have the largest negative charges, thus facilitating the electrophilic attack. Furthermore, it reveals that, compared to their lanthanide analogs, Th@C_3v(8)-C₈₂ and U@C_2v(9)-C₈₂ have much closer metal–cage distance, increased metal-to-cage charge transfer, and strong metal–cage interactions stemming from the significant contribution of extended Th-5f and U-5f orbitals to the occupied molecular orbitals, all of which give rise to their unusual high reactivity. This study provides first insights into the exceptional chemical properties of actinide endohedral fullerenes, which pave ways for the future functionalization and application of these novel EMF compounds.

Photochemical reaction of Th@C_3v(8)-C₈₂ and U@C_2v(9)-C₈₂ with 2-adamantane-2,3′-[3H]-diazirine (AdN₂, 1) afforded three isomers of Th@C_3v(8)-C₈₂Ad and four isomers of U@C_2v(9)-C₈₂Ad (Ad = adamantylidene), respectively.

Theoretical Insight into Thermodynamically Optimal U@C84: Three-Electron Transfer Rather Than Four-Electron Transfer

Four-electron transfer from U to the fullerene cage commonly exists in U@C_2n (2n < 82) so far, while four- and three-electron transfers, which depend on the cage isomers, simultaneously occur in U@C₈₂. Herein, detailed quantum-chemical methods combined with statistical thermodynamic analysis were applied to deeply probe into U@C₈₄, which is detected in the mass spectra without any further exploration. With triplet ground states, novel isomers including isolated-pentagon-rule U@C₂(51579)-C₈₄ and U@D₂(51573)-C₈₄ as well as nonisolated-pentagon-rule U@C_s(51365)-C₈₄ were identified as thermodynamically optimal. Surprisingly, there were unexpected three-electron transfers, which directly led to one unpaired electron on the cage, in all of the three isomers. Significant covalent interactions between the cage and U successively weakened for U@D₂(51573)-C₈₄, U@C₂(51579)-C₈₄, and U@C_s(51365)-C₈₄. Besides, the IR absorption spectra were simulated as a reference for further structural identification in the experiment. Last but not least, the potential reaction sites were predicted to facilitate further functionalization and thus achieve promising applications for U@C₈₄.

Highly regioselective complexation of tungsten with Eu@C82/Eu@C84: interplay between endohedral and exohedral metallic units induced by electron transfer

Interplay between inner and outer metallic units induced by charge transfer is observed in tungsten complexes of Eu@C₂(5)-C₈₂ and Eu@C₂(13)-C₈₄.

Interactions between the inner and outer units through a fullerene cage are of fundamental importance for the creation of molecular spintronics and machines, but the mechanism of such through-cage interplay is still unclear. In this work, we have designed and synthesized two prototypical compounds which contain only a single europium atom inside the cage and merely a tungsten atom coordinating outside to clarify the interactions between the endohedral and exohedral metallic units. They are obtained by reacting a tungsten complex W(CO)₄(Ph₂PC₂H₄PPh₂) (1) with the corresponding metallofullerenes in a highly regioselective manner (2a for Eu@C₂(5)-C₈₂ and 2b for Eu@C₂(13)-C₈₄). On the one hand, the endohedral Eu-doping has changed the LUMO distribution on C₂(5)-C₈₂/C₂(13)-C₈₄ dramatically, via electron transfer, which governs the addition pattern of the exohedral tungsten resulting in surprisingly high regioselectivity. On the other hand, the exohedral tungsten coordination with Eu@C₂(5)-C_82/Eu@C₂(13)-C₈₄ has restricted the motion of the internal europium ion to some extent by changing the electrostatic potentials, as confirmed by the X-ray results of 2a, 2b and the corresponding pristine metallofullerenes cocrystallized with Ni(OEP) (OEP is the dianion of octaethylporphyrin). We now make it clear that the interplay between the endohedral and exohedral metallic units can be realized in a single system by means of intramolecular charge transfer, which may arouse interest in the design and utilization of novel metallofullerene-based molecular devices.

Th@C1(11)-C86: an actinide encapsulated in an unexpected C86 fullerene cage

A novel actinide endohedral fullerene with an unexpected chiral cage, Th@C₁(11)-C₈₆, was synthesized and characterized.

Facile Access to Y2C2n (2n = 92-130) and Crystallographic Characterization of Y2C2@C1(1660)-C108: A Giant Nanocapsule with a Linear Carbide Cluster

A series of giant metallofullerenes Y₂C_2n (2n = 92-130) have been successfully obtained through the treatment of the fraction enriched by 1,2-dichlorobenzene with SnCl₄. Subsequent chromatographic separation gives a pure sample with a composition of Y₂C₁₁₀. Crystallographic results reveal that this endohedral takes the carbide form, namely Y₂C₂@C₁(1660)-C₁₀₈, representing as the largest metallofullerene that has been characterized by crystallography to date. Despite the disorder of the metal cluster, the major Y₂C₂ adopts a previously predicted linear configuration, indicating that the compression of the internal cluster by the cage is almost negligible in this giant cage. Electrochemical studies suggest that Y₂C₂@C₁(1660)-C₁₀₈ is a good electron donor instead of an electron acceptor.

Fused-Pentagon-Configuration-Dependent Electron Transfer of Monotitanium-Encapsulated Fullerenes

We introduce monotitanium-based endohedral metallofullerenes (EMFs) using density functional theory calculations. Isomeric C₆₄ fullerenes are initially employed as hosts, and Ti@C₆₄ species show novel features on the electronic structures. Energetically, the preference of titanium residing on triple-fused-pentagon subunits is proposed in theory. More importantly, different from current knowledge on mono-EMFs, electron transfer between titanium and carbon cages is not unified but is essentially dependent on the pentagon distribution of the binding sites, giving rise to variations of the cationic titanium of Ti@C₆₄. Such selective electron-transfer character is extended to the study of the encapsulation of other neighboring metal atoms (i.e., calcium and scandium). Because of their different capabilities to accept d electrons, fullerene cages with distinct fused-pentagon motifs show selective metal encapsulation characters. In addition, some other fullerenes (C₄₄-C₄₈ and C₈₂) are selected as hosts to study the electron-transfer behavior of titanium in smaller fullerenes and larger systems without pentagon adjacency.

Regioselective synthesis and molecular structure of the first derivative of praseodymium-containing metallofullerenes

A single praseodymium atom plays a critical role in controlling the chemical properties of the cage carbons of C₈₂.

Carbide cluster metallofullerenes: structure, properties, and possible origin

Endohedral metallofullerenes (EMFs) are hybrid molecules with different metallic species encapsulated inside the fullerene cages. In addition to conventional EMFs that contain only metal ions, researchers have constructed novel compounds that encapsulate metallic clusters of nitride, carbide, oxide, cyanide, and sulfide. Among these structures, carbide cluster metallofullerenes (CCMFs) are unique because their synthesis requires only graphite and the metal source. As a result the molecular structures of CCMFs are particularly difficult to characterize. Two carbon atoms are encapsulated inside the cage, but they do not participate in constructing the cage framework. Recent X-ray crystallographic studies of EMFs have allowed researchers to unambiguously identify CCMFs (MxC₂@C2n). Previously most of these structures had been described as conventional EMFs Mx@C2n+2. Most of these species are scandium-containing compounds such as Sc3C₂@Ih(7)-C₈₀ [not Sc₃@C3v(7)-C₈₂], Sc₂C₂@C2v(5)-C₈₀ [not Sc₂@C₈₂], Sc₂C₂@Cs(6)-C₈₂ [not Sc₂@Cs(10)-C₈₄], Sc₂C₂@C2v(9)-C₈₂ [not Sc₂@C2v(17)-C₈₄], Sc₂C₂@C3v(8)-C₈₂ [not Sc₂@D2d(23)-C₈₄], and Sc₂C₂@D2d(23)-C₈₄ [not Sc₂@C₈₆]. Additional examples of CCMFs include Gd₂C₂@D₃(85)-C₉₂, Sc₂C₂@C2v(6073)-C₆₈, Ti₂C₂@D3h(5)-C₇₈, M₂C₂@C3v(8)-C₈₂, M₂C₂@Cs(6)-C₈₂ (M = Y, Er, etc.), Y₂C₂@C₈₄, Y₂C₂@D₃(85)-C₉₂, Y₂C₂@D₅(450)-C₁₀₀, and Lu₃C₂@D₂(35)-C₈₈. The existence of so many CCMF species reminds us that the symbol '@' (which denotes the encapsulation status of EMFs) should be used with caution with species whose molecular structures have not been determined unambiguously. This Account presents a detailed summary of all aspects of CCMFs, including historically erroneous assignments and corrected structural characterizations, along with their intrinsic properties such as electrochemical and chemical properties. We emphasize structural issues, features that are fundamental for understanding their intrinsic properties. Finally, we discuss the formation mechanism and possible origin of cluster EMFs, not just CCMFs.