Endohedral Clusterfullerenes Research Articles

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@Ih(7)-C80 and then carefully inspected to summarize all the favorable H locations in the HCFs. Encouragingly, following these structural characteristics, a new Sc4C2H@Ih(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.

(Invited) Syntheses of Novel Dysprosium-based Metal Cyanide Clusterfullerenes by Using Solid Cyanide Sources

Metal cyanide clusterfullerenes (CYCFs) represent the only endohedral clusterfullerenes with merely one metal atom encapsulated. So far all of the reported CYCFs are synthesized by using gaseous N2 as the cyanide source, whether solid cyanide source can be applicable for syntheses of CYCFs remains an open question. In this work, we developed several solid cyanide sources, and succeeded in synthesizing novel dysprosium (Dy)-based metal cyanide clusterfullerenes in high yield. The molecular structures of three isomers of DyCN@C82 are unambiguously determined by single-crystal X-ray diffraction, indicating that the encapsulated triangular DyCN cluster is tunable due to the strong metal-cage interactions.

(Invited) Understanding Endohedral Metallofullerenes from Theoretical Calculations

Theoretical calculations based on the density functional theory (DFT) are heavily employed in the current research of endohedral metallofullerenes (EMFs), and they have greatly deepened our understandings of this kind of alluring nanomaterial. Herein, we will show a series of new computational results regarding the structural characterization, electronic properties, bonding pattern, and chemical functionalization of EMFs. The studied systems cover conventional mono-EMFs and novel members such as those containing actinide elements (Th, U). In addition, the molecular structures of several representative endohedral clusterfullerenes are discussed to provide new insight into their stability and formation mechanism. Detailed wavefunction analyses further reveal many intriguing undiscovered phenomena. Finally, our very recent works in close collaboration with experimentalists will be briefly introduced.

Capturing the Missing Carbon Cage Isomer of C84 via Mutual Stabilization of a Triangular Monometallic Cyanide Cluster.

Monometallic cyanide clusterfullerenes (CYCFs) represent a unique branch of endohedral clusterfullerenes with merely one metal atom encapsulated, offering a model system for elucidating structure-property correlation, while up to now only C82 and C76 cages have been isolated for the pristine CYCFs. C84 is one of the most abundant fullerenes and has 24 isomers obeying the isolated pentagon rule (IPR), among which 14 isomers have been already isolated, whereas the C2v(17)-C84 isomer has lower relative energy than several isolated isomers but never been found for empty and endohedral fullerenes. Herein, four novel C84-based pristine CYCFs with variable encapsulated metals and isomeric cages, including MCN@C2(13)-C84 (M = Y, Dy, Tb) and DyCN@C2v(17)-C84, have been synthesized and isolated, fulfilling the first identification of the missing C2v(17)-C84 isomer, which can be interconverted from the C2(13)-C84 isomer through two steps of Stone-Wales transformation. The molecular structures of these four C84-based CYCFs are determined unambiguously by single-crystal X-ray diffraction. Surprisingly, although the ionic radii of Y3+, Dy3+, and Tb3+ differ slightly by only 0.01 Å, such a subtle difference leads to an obvious change in the metal-cage interactions, as inferred from the distance between the metal atom and the nearest hexagon center of the C2(13)-C84 cage. On the other hand, upon altering the isomeric cage from DyCN@C2(13)-C84 to DyCN@C2v(17)-C84, the Dy-cage distance changes as well, indicating the interplay between the encapsulated DyCN cluster and the outer cage. Therefore, we demonstrate that the metal-cage interactions within CYCFs can be steered via both internal and external routes.

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@Cs(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 Cs(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-@C726- 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.

Anomalous Cis-Conformation Regioselectivity of Heterocycle-Fused Sc3 N@D3h -C78 Derivatives.

Endohedral clusterfullerenes exhibit unique chemical properties due to intramolecular electron transfer of the encaged metal cluster to the outer fullerene cages. We report the synthesis of two Sc3 N@D3h -C78 monoadducts 2 a and 2 b through the 1,3-dipolar reaction of Sc3 N@D3h -C78 with carbonyl ylide bearing anomalous cis-conformation regioselectivity. The molecular structures of these monoadducts are unambiguously confirmed by single-crystal X-ray crystallography, revealing that both 2 a and 2 b have cis-conformations with the furan moiety grafted via [6,6]-closed addition patterns. Under the same conditions, the control reaction of C60 with carbonyl ylide affords two monoadducts 3 a and 3 b, which exhibit cis- and trans-conformations, respectively, with [6,6]-closed addition patterns. According to theoretical calculations, the exclusive formation of the cis-only Sc3 N@D3h -C78 monoadducts is a consequence of conjunct effects of thermodynamic stability of adducts, the reactivity of the addition site, and the cis-dipole intermediate from trans 1.

Ancient pigment to treasure: Prussian blue as a cheap solid cyanide/nitrogen dual-source affording the high-yield syntheses of pricey endohedral clusterfullerenes

Prussian blue was applied for the first time as a cheap solid cyanide/nitrogen dual-source, affording simplified yet high-yield simultaneous syntheses of novel dysprosium-based metal cyanide clusterfullerenes and metal nitride clusterfullerenes.

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@Cs(17 490)-C76 or U2O@C2v(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 Cs(17 490)- and C2v(19 138)-C76-based oxide ECFs. Moreover, U2O@C2v(19 138)-C76 is the first example of a hexavalent metal cluster within the C2v(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+@C766- 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@C2n (M = Y, Tb; 2n = 76, 82) and Sc3CN@C2n (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@C2n and Sc3CN@C2n 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.

Discovery of a Superatom inside the Fullerene Cage.

The stability of endohedral clusterfullerenes is generally understood in terms of the metal cluster shape, cage structure, and metal-cage interactions, with the electronic state of the internal cluster mostly neglected. Herein, theoretical calculations reveal that the (Ti3C3)6+ unit of recently synthesized Ti3C3@Ih(7)-C80 exhibits a superatomic state with a perfect closed-shell 1S21P61D10 electronic configuration in accordance with the famous jellium model. This "trapped superatom" features considerable aromaticity and hyperconjugation interactions never reported for other clusterfullerenes. Besides the localized two-center two-electron (2c-2e) Ti-C/C-C bonds, it also has two 3c-2e Ti-C-Ti bonds. Furthermore, the ring strain of the cyclopropane-like C3 core is effectively released upon the metal coordination. All these factors greatly stabilize the (Ti3C3)6+ cluster, showing the critical role of metal-to-cage charge transfer and cage encapsulation in enhancing the stability of this exotic metal cluster.

Pivotal Role of Nonmetal Atoms in the Stabilities, Geometries, Electronic Structures, and Isoelectronic Chemistry of Sc3X@C80 (X = C, N, and O)

The thermodynamic and dynamic stabilities of Sc3 X@C80 (X = C, N, and O) are explored via density functional theory combined with statistical thermodynamic analysis and ab initio molecular dynamics. It is the first time to comprehensively consider the effect of nonmetal atoms on trimetallic endohedral clusterfullerenes. Relative to Sc3 X@Ih (31924)-C80 (X = N and O) with general six-electron transfer, an intriguing electronic structure of unexplored Sc3 C@D5h (31923)-C80 with thermodynamic and dynamic stabilities is clearly disclosed. Natural bond orbitals and charge decomposition analysis simultaneously suggest that one unpaired electron appears on the cage for neutral Sc3 C@D5h (31923)-C80 , which could be prospectively stabilized by effective exohedral derivatization and ionization in the future. Moreover, isoelectronic endohedral clusterfullerenes, (Sc3 C@C80 )- , Sc3 N@C80 , and (Sc3 O@C80 )+ , are also uniquely taken into account. The geometries, electronic structures, reactivities, and reactive sites of isoelectronic species are examined, and it turns out that all the three isoelectronic species would rather electrophilic than nucleophilic reactions. © 2019 Wiley Periodicals, Inc.

In-Depth Theoretical Probe into Novel Mixed-Metal Uranium-Based Endohedral Clusterfullerenes Sc2UX@Ih(31924)-C80 (X = C, N).

Mixed-metal uranium-based endohedral clusterfullerenes, Sc2UX@C80 (X = C, N), which were recently reported in experiments, have been investigated considering heptagon-containing isomers by density functional theory calculations in conjunction with statistical thermodynamic analysis. The triplet Sc2UC@Ih(31924)-C80 and quartet Sc2UN@Ih(31924)-C80, named after the spiral number (31924), are found to be thermodynamically stable and satisfy aromaticity rules. Furthermore, the restricted movements of the Sc2UX (X = C, N) cluster in Ih(31924)-C80 have been demonstrated via ab initio molecular dynamics simulations. The six-electron transfer from the inner cluster to the cage results in the electronic structures (Sc2UX)6+@C806- (X = C, N), which were also confirmed by natural bond orbital analysis. On the basis of the frontier molecular orbitals, the oxidation states of uranium in Sc2UC@C80 and Sc2UN@C80 are +IV and +III, respectively, with residual electrons in 5f orbitals of U. The chemical bond between U and C (N) of the inner cluster is characterized as a double bond (single bond) by an analysis of the Mayer bond orders. There are covalent interactions between the inner cluster and outer cage, which is clarified by the quantum theory of atoms in molecules. IR spectra of the optimal isomers have also been simulated, which show the clear difference between Sc2UX@C80 (X = C, N). These findings, together with simulated results, are expected to supply useful information in future experiments of mixed-metal uranium-based endohedral clusterfullerenes.

Endohedral metal-nitride cluster ordering in metallofullerene-NiII(OEP) complexes and crystals: a theoretical study.

The ordering of endohedral clusterfullerenes Sc3N@C80 and YSc2N@C80 co-crystallized with Ni(OEP) and isolated complexes with Ni(OEP) have been investigated theoretically. Having used multiple orientations of M3N clusters inside the cages with Fibonacci sampling, we describe the effect of intermolecular interactions on the orientation of the endohedral cluster.

Blending Non-Group-3 Transition Metal and Rare-Earth Metal into a C80 Fullerene Cage with D5h Symmetry.

Rare-earth metals have been mostly entrapped into fullerene cages to form endohedral clusterfullerenes, whereas non-Group-3 transition metals that can form clusterfullerenes are limited to titanium (Ti) and vanadium (V), and both are exclusively entrapped within an Ih -C80 cage. Non-Group-3 transition-metal-containing endohedral fullerenes based on a C80 cage with D5h symmetry, Vx Sc3-x N@D5h -C80 (x=1, 2), have now been synthesized, which exhibit two variable cluster compositions. The molecular structure of VSc2 N@D5h -C80 was unambiguously determined by X-ray crystallography. According to a comparative study with the reported Ti- and V-containing clusterfullerenes based on a Ih -C80 cage and the analogous D5h -C80 -based metal nitride clusterfullerenes containing rare-earth metals only, the decisive role of the non-Group-3 transition metal on the formation of the corresponding D5h -C80 -based clusterfullerenes is unraveled.

Blending Non‐Group‐3 Transition Metal and Rare‐Earth Metal into a C80 Fullerene Cage with D5h Symmetry

AbstractRare‐earth metals have been mostly entrapped into fullerene cages to form endohedral clusterfullerenes, whereas non‐Group‐3 transition metals that can form clusterfullerenes are limited to titanium (Ti) and vanadium (V), and both are exclusively entrapped within an Ih‐C80 cage. Non‐Group‐3 transition‐metal‐containing endohedral fullerenes based on a C80 cage with D5h symmetry, VxSc3−xN@D5h‐C80 (x=1, 2), have now been synthesized, which exhibit two variable cluster compositions. The molecular structure of VSc2N@D5h‐C80 was unambiguously determined by X‐ray crystallography. According to a comparative study with the reported Ti‐ and V‐containing clusterfullerenes based on a Ih‐C80 cage and the analogous D5h‐C80‐based metal nitride clusterfullerenes containing rare‐earth metals only, the decisive role of the non‐Group‐3 transition metal on the formation of the corresponding D5h‐C80‐based clusterfullerenes is unraveled.

Insight into the thermodynamically preferred V3N@Ih(31924)-C80 and acknowledged VxSc3-xN@Ih(31924)-C80 (x=0, 1 and 2)

Very recently, the endohedral clusterfullerenes VxSc3-xN@C80 (x = 1 and 2) have been successfully prepared. Herein, the novel entire family of VxSc3-xN@C80 (x = 0, 1, 2 and 3) were systematically investigated via density functional theory combined with statistical thermodynamics methods. The isomer of Ih(31924)-C80 was unambiguously proved as the most suitable hollow cage to entrap the clusters VxSc3-xN (x = 0, 1, 2 and 3) at the range of endohedral fullerenes form-temperature through statistical thermodynamics analysis considering the influence of entropy-enthalpy. Both the interaction between cluster, VxSc3-xN (x = 0, 1, 2 and 3), and empty cage Ih(31924)-C80 and electronic properties of VxSc3-xN@Ih(31924)-C80 (x = 0, 1, 2 and 3) were explored. IR and Raman spectra of VxSc3-xN@Ih(31924)-C80 (x = 0, 1, 2 and 3) were simulated to distinguish the different structure of these isomers. The Raman absorption spectra of the vibration of inner moieties showed a similar strength interactions between each inner moiety and Ih(31924)-C80. The possibility of existence for V3N@Ih(31924)-C80 is strongly supported by thermodynamic and chemical stability. It is believed that this work will provide valuable information to further explorations of VxSc3-xN@Ih(31924)-C80 (x = 0, 1, 2 and 3) in experiments and applications.

Steering the Geometry of Butterfly-Shaped Dimetal Carbide Cluster within a Carbon Cage via Trifluoromethylation of Y2C2@C82(6)

As one of the largest sub-branches of endohedral clusterfullerenes, dimetal carbide clusterfullerene (CCF) in the form of M2C2@C2 n is quite intriguing since an alternative structure of M2@C2 n+2 as conventional dimetallofullerene may exist as well. Herein, by using high-temperature trifluoromethylation followed by HPLC separation and single-crystal X-ray diffraction study, we report for the first time the unambiguous structural determination of yttrium (Y)-based CCF as its trifluoromethyl derivatives, Y2C2@C82(6)(CF3)16. Four isomers of Y2C2@C82(6)(CF3)16 with different addition patterns of 16 CF3 groups are successfully isolated, and two Y atoms of the butterfly-shaped Y2C2 cluster are coordinated by two cage pentagons in each isomer. The butterfly geometry of Y2C2 cluster varies significantly in the four Y2C2@C82(6)(CF3)16 isomers, with Y···Y distances ranging from 3.544 to 4.051 Å dependent on the relative positions of the two yttrium-coordinated pentagons on the carbon cage.

When metal clusters meet carbon cages: endohedral clusterfullerenes.

Fullerenes have the characteristic of a hollow interior, and this unique feature triggers intuitive inspiration to entrap atoms, ions or clusters inside the carbon cage in the form of endohedral fullerenes. In particular, upon entrapping an otherwise unstable metal cluster into a carbon cage, the so-called endohedral clusterfullerenes fulfil the mutual stabilization of the inner metal cluster and the outer fullerene cage with a specific isomeric structure which is often unstable as an empty fullerene. A variety of metal clusters have been reported to form endohedral clusterfullerenes, including metal nitrides, carbides, oxides, sulfides, cyanides and so on, making endohedral clusterfullerenes the most variable and intriguing branch of endohedral fullerenes. In this review article, we present an exhaustive review on all types of endohedral clusterfullerenes reported to date, including their discoveries, syntheses, separations, molecular structures and properties as well as their potential applications in versatile fields such as biomedicine, energy conversion, and so on. At the end, we present an outlook on the prospect of endohedral clusterfullerenes.

ChemInform Abstract: Recent Progress in the Synthesis of Regio‐Isomerically Pure Bis‐adducts of Empty and Endohedral Fullerenes

AbstractReview: 69 refs.

Recent progress in the synthesis of regio‐isomerically pure bis‐adducts of empty and endohedral fullerenes

Bis‐functionalized fullerenes have resulted in very effective acceptors in organic photovoltaic solar cells. Here we present a short overview of specific and recently reported pure bis‐adduct regioisomers of C60 using short tethers, a few recent examples of bis‐adducts of C70 and finally examples of bis‐adducts of endohedral cluster fullerenes. We also discuss the unexpected reactivity differences of empty fullerenes (C60 and C70) compared with those of trimetallic nitride cluster endohedral fullerenes (M3N@Ih‐C80, M = Sc, Lu) towards tethered‐bis‐addition reactions. Finally, we describe the remarkable endohedral‐cluster regiochemical control of multiple additions. Copyright © 2016 John Wiley & Sons, Ltd.