Encaged Metal Research Articles

(Invited) Carbon Phase Adjustment by Multi-Configuration Ligand in Endohedral Metallofullerene Derivatives Gd@C82(morpholine)7 Under High Pressure

Metallofullerenes present an ideal research object for developing new carbon structures with improved properties under high pressure. In this presentation, high pressure investigation is realized on metallofullerene derivatives (Gd@C82(morpholine)7) by applying in situ synchrotron XRD, Raman, and IR spectroscopy, combined with high pressure density functional theory (DFT) calculations. The regulations of multi-configuration morpholine ligands avoid the direct collapse of the carbon cages commonly happened to fullerenes, making the fine structure transformation possible for the first time, including ligand configuration exchange, anisotropic shrinkage of cage, new constructed bonds, and even the encaged metal’s movement with increasing pressure. The charge-transfer principle as the beneath driving force of structural transition is found. These findings demonstrate that multi-configuration ligand maps an innovative strategy for phase transition control of metallofullerenes under high pressure, thus providing deep insight, step by step, into the phase transition mechanism of these hybrid molecular systems on molecular level.

Clathrate metal superhydrides under high-pressure conditions: enroute to room-temperature superconductivity.

Room-temperature superconductivity has been a long-held dream of mankind and a focus of considerable interest in the research field of superconductivity. Significant progress has recently been achieved in hydrogen-based superconductors found in superhydrides (hydrides with unexpectedly high hydrogen contents) that are stabilized under high-pressure conditions and are not capturable at ambient conditions. Of particular interest is the discovery of a class of best-ever-known superconductors in clathrate metal superhydrides that hold the record for high superconductivity (e.g.T c = 250-260K for LaH10) among known superconductors and have great promise to be those that realize the long-sought room-temperature superconductivity. In these peculiar clathrate superhydrides, hydrogen forms unusual 'clathrate' cages containing encaged metal atoms, of which such a kind was first reported in a calcium hexa-superhydride (CaH6) showing a measured high T c of 215K under a pressure of 170GPa. In this review, we aim to offer an overview of the current status of research progress on the clathrate metal superhydride superconductors, discuss the superconducting mechanism and highlight the key features (e.g.structure motifs, bonding features, electronic structure, etc.) that govern the high-temperature superconductivity. Future research direction along this line to find room-temperature superconductors will be discussed.

Controlling the Surface of Aluminum Nanocrystals: From Aluminum Oxide to Aluminum Fluoride.

Aluminum nanocrystals are emerging as a promising alternative to silver and gold for various applications ranging from plasmonic functionalities to photocatalysis and as energetic materials. Such nanocrystals often exhibit an inherent surface oxidation layer, as aluminum is highly reactive. Its controlled removal is challenging but required, as it can hinder the properties of the encaged metal. Herein, two wet-chemical colloidal approaches toward the surface coating of Al nanocrystals, which afford control over the surface chemistry of the nanocrystals and the oxide thickness, are presented. The first approach utilizes oleic acid as a surface ligand by its addition toward the end of the Al nanocrystals synthesis, and the second approach is the post-synthesis treatment of Al nanocrystals with NOBF4 , in a "wet" colloidal-based approach, which is found to etch and fluorinate the surface oxides. As surface chemistry is an important handle for controlling materials' properties, this research paves a path for manipulating Al nanocrystals while promoting their utilization in diverse applications.

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.

Anomalous Cis‐Conformation Regioselectivity of Heterocycle‐Fused Sc3N@D3h‐C78 Derivatives

AbstractEndohedral 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 Sc3N@D3h‐C78 monoadducts 2 a and 2 b through the 1,3‐dipolar reaction of Sc3N@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 Sc3N@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.

Carbon phase adjustment by multi-configuration ligand in endohedral metallofullerene derivatives Gd@C82(morpholine)7 under high pressure

Abstract Metallofullerenes present an ideal research object for developing new carbon structures with improved properties under high pressure. In this study, high pressure investigation is realized on metallofullerene derivatives (Gd@C82(morpholine)7) by applying in situ synchrotron XRD, Raman, and IR spectroscopy, combined with high pressure density functional theory (DFT) calculations. The regulations of multi-configuration morpholine ligands avoid the direct collapse of the carbon cages commonly happened to fullerenes, making the fine structure transformation possible for the first time, including ligand configuration exchange, anisotropic shrinkage of cage, new constructed bonds, and even the encaged metal’s movement with increasing pressure. The charge-transfer principle as the beneath driving force of structural transition is found. These findings demonstrate that multi-configuration ligand maps an innovative strategy for phase transition control of metallofullerenes under high pressure, thus providing deep insight, step by step, into the phase transition mechanism of these hybrid molecular systems on molecular level.

Metallofullerene ions of LunC2m+ (1 ≤ n ≤ 9, 50 ≤ 2 m ≤ 198) generated by laser ablation of graphene/LuCl3

Endohedral metallofullerene (EMF) ions of LunC2m+ (n=1–9, 50≤2m≤198) were generated by laser ablation of graphene and LuCl3. Some mono-EMF ions, such as LuC66+, show noticeable relative intensities, indicating the significant stabilization of IPR-violating cages by the encaged metal atom. Among those multimetallic EMF ions, Lu3C2m+ and Lu7C2m+ have the highest and lowest signal intensities, respectively. For every increase of one Lu atom inside the fullerene, the size of the cage increases 12 carbon atoms on average. Top-down mechanism was applied to explain the formation of these large-sized, multimetallic EMF ions. However, for other lanthanide metals of Er, Tm and Yb, only very limited species of MnC2m+ (n≤4) were observed in the mass spectra. Thus whether the formation of the multimetallic EMF ions with n≥4 is performable for other lanthanide metals still needs a further study.

Generation of (M = K, Rb, Cs, 2n = 80–220) by laser ablation of graphene

RationaleThe yield of endohedral metallofullerene (EMF) ions is greatly related to the encaged metal atoms. For alkali metals, the generation of corresponding large‐sized monometallofullerene ions by traditional methods is still difficult. The aim of this work is to study the generation and the distribution of alkali‐metallofullerene ions by the method of laser ablation with graphene as the precursor.MethodsThe experiment was performed with a Fourier transform ion cyclotron (FTICR) mass spectrometer. Mass spectra were obtained by laser ablation of the mixtures of graphene and KCl, RbCl or CsCl in the positive ion mode. Collision‐induced dissociation (CID) experiments were also performed for some selected ions.ResultsEMF ions of (M = K, Rb, Cs, 2n = 80–220) can be generated by using graphene as the precursor. Other EMF ions with smaller sizes, including , were also observed under the optimized experimental conditions. CID experiments of some selected ions confirmed their endohedral structures.ConclusionsMetallofullerene ions of (M = K, Rb, Cs, 2n = 80–220) were generated by laser ablation of graphene and corresponding metal salts. Compared with previous results of alkaline earth metals, the yields of EMF ions are generally lower. However, for large‐sized EMF ions, the relative intensities of EMF ions to their corresponding fullerene ions / generally increase with the increase in cluster sizes, which is opposite to the trend observed for alkaline earth metals. Copyright © 2015 John Wiley & Sons, Ltd.

A Novel Organic Electrical Memory Device Based on Metallofullerene-Grafted Polymer

Organic memory materials have advantages in flexible, large area film-forming, and low cost compared with inorganic storage materials. Developing new materials with high read-write speed, low-power consumption, multi-value storage capacity and other excellent properties has been a subject for the study of next-generation non-volatile storage devices. Metallofullerenes could be used either as electron acceptor or as electron donor to form charge transfer compound with polymers containing electron withdrawing or electron donor compounds. We successfully used metallofullerene-based polymer to fabricate ITO/Gd@C82-PVK/Al sandwich nonvolatile memory device. The results of I-V test indicated that the new material exhibited typical bistable electrical switching and a nonvolatile rewritable memory effect. The theory calculation indicated that the encaged metal may serve as the important electron trapping center. PVK-Gd@C82 is thus an example of a new type charge-transfer complex that integrates the conventional organic and inorganic charge transfer complexes, which provides a potential method to improve the processing, morphological stability, and electrical performance of organic memory devices.

Capturing the Long-Sought Small-Bandgap Endohedral Fullerene Sc3N@C82 with Low Kinetic Stability

The long-sought small-bandgap endohedral fullerene Sc3N@C82 with low kinetic stability has been successfully synthesized and isolated for the first time, for which the molecular structure has been unambiguously determined as Sc3N@C82-C2v(39718) by single crystal X-ray diffraction. The C82-C2v(39718) (or labeled as C82-C2v(9) according to the conventional numbering of the isolated pentagon rule (IPR) isomers based on the Fowler-Monolopoulos spiral algorithm) isomeric cage of Sc3N@C82 agrees well with its most stable isomer previously predicted by DFT computations and is dramatically different to those of the reported counterparts M3N@C82-Cs(39663) (M = Gd, Y) based on a non-IPR C82 isomer, revealing the strong dependence of the cage isomeric structure on the size of the encaged metal for C82-based metal nitride clusterfullerenes (NCFs).

A novel organic electrical memory device based on the metallofullerene-grafted polymer (Gd@C82-PVK)

The ITO/Gd@C82-PVK/Al sandwich nonvolatile memory device was developed based on polymer containing carbazole moieties as electron donors and Gd@C82 as electron acceptors for the first time. The results of I–V characteristic test indicated that the new material exhibited typical bistable electrical switching and a nonvolatile rewritable memory effect, with a turn-on voltage of about −1.5V and an ON/OFF-state current ratio of more than 104. We propose that such a low turn-on voltage is caused due to the encaged metal and the DFT calculation indicated that the encaged metal served as an important electron trapping center, which facilitated the arrival of turn-on voltage.

Synthesis, Isolation, and Spectroscopic Characterization of Holmium-Based Mixed-Metal Nitride Clusterfullerenes: HoxSc3−xN@C80 (x=1, 2)

The synthesis, isolation and spectroscopic characterization of holmium-based mixed metal nitride clusterfullerenes Ho(x) Sc(3-x) N@C(80) (x=1, 2) are reported. Two isomers of Ho(x) Sc(3-x) N@C(80) (x=1, 2) were synthesized by the reactive gas atmosphere method and isolated by multistep recycling HPLC. The isomeric structures of Ho(x) Sc(3-x) N@C(80) (x=1, 2) were characterized by laser-desorption time-of-flight (LD-TOF) mass spectrometry and UV/Vis/NIR, FTIR and Raman spectroscopy. A comparative study of M(x) Sc(3-x) N@C(80) (M=Gd, Dy, Lu, Ho) demonstrates the dependence of their electronic and vibrational properties on the encaged metal. Despite the distinct perturbation induced by 4f(10) electrons, we report the first paramagnetic (13) C NMR study on Ho(x) Sc(3-x) N@C(80) (I; x=1, 2) and confirm I(h) -symmetric cage structure. A (45) Sc NMR study on HoSc(2) N@C(80) (I, II) revealed a temperature-dependent chemical shift in the temperature range of 268-308 K.

Endohedral Metal Atoms in Pristine and Functionalized Fullerene Cages

Fullerene, an allotropic form of carbon made up of spherical molecules formed from pentagonal and hexagonal rings, was first discovered in 1985. Because fullerenes have spacious inner cavities, atoms and clusters can be encapsulated inside the fullerene cages to form endohedral fullerenes. In particular, the unique structural and electronic properties of endohedral metallofullerenes (EMFs), where metal atoms are encapsulated within the fullerene, have attracted wide interest from physicists and chemists as well as materials scientists and biologists. The remarkable characteristics of these molecules originate in the electron transfer from the encapsulated metal atoms to the carbon cage. The positions and movements of the encapsulated metal atoms are important determinants of the chemical and physical properties of EMFs. In this Account, we specifically describe the positions and dynamic behavior of the metal atoms encapsulated in pristine and functionalized fullerene cages. First, we examined whether the metal atoms are attached rigidly to cage carbons or move around. Our systematic investigations of EMFs, including M@C(2v)-C(82), M(2)@D(2)(10611)-C(72), M(2)@D(3h)(5)-C(78), M(2)@I(h)-C(80), and M(2)@D(5h)-C(80), revealed that the metal positions and movements vary widely with different cage structures and numbers of metal atoms. Second, we wanted to understand whether we could control the positions and movements of the untouchable metal atoms in EMFs. One possible way to modulate this behavior was through attachment of a molecule to the outer surface of the cage. We developed synthetic methods to modify EMFs and have examined the metal positions and movements in the functionalized carbon cages. Remarkably, we could alter the dynamic behavior of the encaged metal atoms in M(2)@I(h)-C(80) drastically through chemical modification of the outer cage. We anticipate that the control of metal atom structures and dynamics within a cage could be valuable for designing functional molecular devices with new electronic or magnetic properties.

Mixed Metal Nitride Clusterfullerenes in Cage Isomers: LuxSc3−xN@C80 (x = 1, 2) As Compared with MxSc3−xN@C80 (M = Er, Dy, Gd, Nd)

Two isomers of lutetium−scandium mixed metal nitride clusterfullerenes (MMNCFs), LuxSc3−xN@C80(x = 1, 2), have been synthesized and isolated for the first time, and accordingly, the interplay of the cluster structure and cage isomerism is followed. The electronic and vibrational properties of LuxSc3−xN@C80 (I, II, x = 1, 2) are characterized by UV−vis−NIR and FTIR spectroscopies, revealing the effect of the composition of the encaged LuxSc3−xN cluster. A comparative study of LuxSc3−xN@C80 (I) with other MxSc3−xN@C80(I; M = Er, Dy, Gd, Nd) MMNCFs points to the dependence of their electronic and vibrational properties on the encaged metal. The cage structures of LuxSc3−xN@C80 (I, II; x = 1, 2) are determined by 13C NMR spectroscopy. The 45Sc NMR spectroscopic study is carried out to probe the dynamics of the encaged Sc atoms. Finally, DFT computations are used to address the effect of the composition of the encaged LuxSc3−xN cluster on the NMR lines and the structures of LuxSc3−xN@C80 (I, II; x = 1, 2) especially with respect to the carbon atoms.

Modulation of Structural and Electronic Properties of Fullerene and Metallofullerenes by Surface Chemical Modifications

Many applications of fullerene, metallofullerene, and carbon nanotubes request the chemical modifications. But how the modification influences the stability, structural, and electronic properties that directly relate to the practical functions of the carbon nanomaterials, this issue is discussed in this paper. The outer chemical modifications can sensitively influence the stability of final derivatives of fullerenes, depending on parameters such as the number of the modified groups, the unintended impurity groups on the cage surface, and chemical conditions in synthesizing processes. The outer chemical modification can induce alteration in electronic properties of metallofullerene. Gd@C82 is taken as a model to show how the electronic properties of the encaged metal atom are modulated by the cage surface modification. There exist sandwich-type electronic interactions along pathway: [outer modification group]-[cage surface]-[inner atom], and their synergistic effects are discussed. Beside the number of the modified groups, whose distribution patterns on the cage surface also play crucial roles in determining how the electronic interactions are modulated. Their synergistic effects dominate the electronic, optical, or magnetic properties of functionalized fullerenic nanomaterials. As the surface of nanotube is chemically more inert as compared to that of fullerene or metallofullerene, to achieve desirable modulations of nanotubes is probably more difficult. The research aimed to explore the changes in their structural or electronic property after the outer chemical functionalization was few, though this is of special significant and needs investigations because the chemical modifications are widely applying to the carbon nanotubes in applications.

Correlation between atomic rearrangement in defective fullerenes and migration behavior of encaged metal ions

Migration behavior of ${\mathrm{Gd}}^{3+}$ and ${\mathrm{Ca}}^{2+}$ ions inside ${\mathrm{C}}_{82}$ fullerene cages with irradiation-induced atomic defects is observed using in situ high-resolution electron microscopy. The encapsulated ${\mathrm{Gd}}^{3+}$ ion can easily leave the fullerene cage through the defect, whereas the ${\mathrm{Ca}}^{2+}$ ion remains inside the defective cage. Density functional theory calculations prove that this atomic behavior closely correlates with the rearrangement of carbon atoms in defective fullerene cages. The ${\mathrm{Gd}}^{3+}$ ion disturbs the self-restoration of the defective cage and assists its polymerization, but such effects are not found for the ${\mathrm{Ca}}^{2+}$ ion.

Structure determination of a missing-caged metallofullerene: Yb@C 74 (II) and the dynamic motion of the encaged ytterbium ion

A dynamic motion of the encaged metal in the D 3h cage is proposed for Yb@C 74(II) according its NMR spectrum. In addition, the calculated NMR spectra of D 3h-C 74 and Yb@C 74 with a D 3h cage and the static metal are presented. By comparing Yb@C 74’s dynamic and static NMR spectra, the dynamic metal’s effect on the NMR behavior of the metallofullerene is discussed.

Synthesis, isolation and spectroscopic characterization of Yb-containing high metallofullerenes

Here we report the first isolation of a series of Yb-containing high metallofullerene based on C 88/C 90/C 92/C 94, along with C 88, C 90 (I, II), C 92, C 94 (I, II) and C 96. Furthermore, the electronic structures of these high fullerenes and metallofullerenes are discussed according to their absorption spectra. It is concluded that for high metallofullerenes, there are usually different isomers with different molecular symmetries while the encaged metal is different, which differs from the case of other small metallofullerenes. In addition, from the discussion of the electronic structural features of the isolated high fullerenes, one can see the special electronic structures of C 94s.

Different Extraction Behaviors between Divalent and Trivalent Endohedral Metallofullerenes

On the basis of high-yield preparation, arc-produced soot containing endohedral Y, La, Nd, Sm, Gd, Tb, Yb, Ca, and Ba fullerenes are successively extracted with carbon disulfide, N,N-dimethylformamide, pyridine, and aniline. The continuous four-step extraction procedure generates endohedral metallofullerenes (EMFs) with high content and abundant species, and novel EMFs such as Tb@C66, Yb@C60, and Tb3@C80 were extracted. This makes it possible for us to systemically study the varying extraction behaviors of divalent and trivalent metallofullerenes. Trivalent EMFs are characterized by higher content and multimetallofullerenes, whereas divalent species are characterized by lower content and solely monometallofullerenes. A positive relationship between the content and the species abundance of EMFs is proposed. The differences in extracting ability of the above solvents toward certain EMFs are summarized and explained on the basis of the number of transferred electrons from the encaged metal atom(s), the back-...

Manipulation of fullerene-induced impurity states in carbon peapods

Electronic structures of several semiconducting and metallic carbon peapods have been studied using density functional theory. We have systematically investigated the effects of two key factors, the tube diameter and the type of the encaged metal atom inside ${\mathrm{C}}_{60},$ on the energy level and the electron occupation number of ${\mathrm{C}}_{60}$-induced impurity states in semiconducting peapods. The manipulation of these impurity states controls the type of the majority carrier $(p$ or $n)$ and the carrier density of a semiconducting peapod. In addition, the possibility of superconductivity of potassium-doped peapods has been discussed.