Stability and Electronic Properties of Mixed Rare-Earth Tri-Metallofullerenes YxDy3-x@C80 (x = 1 or 2).

Tri-metallofullerenes, specifically M3@C80 where M denotes rare-earth metal elements, are molecules that possess intriguing magnetic properties. Typically, only one metal element is involved in a given tri-metallofullerene molecule. However, mixed tri-metallofullerenes, denoted as M1xM23-x@C80 (x = 1 or 2, M1 and M2 denote different metal elements), have not been previously discovered. The investigation of such mixed tri-metallofullerenes is of interest due to the potential introduction of distinct properties resulting from the interaction between different metal atoms. This paper presents the preparation and theoretical analysis of mixed rare-earth tri-metallofullerenes, specifically YxDy3-x@C80 (x = 1 or 2). Through chemical oxidation of the arc-discharge produced soot, the formation of tri-metallofullerene cations, namely Y2Dy@C80+ and YDy2@C80+, has been observed. Density functional theory (DFT) calculations have revealed that the tri-metallofullerenes YxDy3-x@C80 (x = 1 or 2) exhibit a low oxidation potential, significantly lower than other fullerenes such as C60 and C70. This low oxidation potential can be attributed to the relatively high energy level of a singly occupied orbital. Additionally, the oxidized species demonstrate a large HOMO-LUMO gap similar to that of YxDy3-xN@C80, underscoring their high chemical stability. Theoretical investigations have uncovered the presence of a three-center two-electron metal-metal bond at the center of Y2DY@C80+ and YDy2@C80+. This unique multi-center bond assists in alleviating the electrostatic repulsion between the metal ions, thereby contributing to the overall stability of the cations. These mixed rare-earth tri-metallofullerenes hold promise as potential candidates for single-molecule magnets.

Imaging the Single-Electron Ln-Ln Bonding Orbital in a Dimetallofullerene Molecular Magnet.

Chemically robust single-molecule magnets (SMMs) with sufficiently high blocking temperatures TB are among the key building blocks for the realization of molecular spintronic or quantum computing devices. Such device applications require access to the magnetic system of a SMM molecule by means of electronic transport, which primarily depends on the interaction of magnetic orbitals with the electronic states of the metallic electrodes. Scanning tunneling microscopy in combination with ab initio calculations allows to directly address the unoccupied component of the single-electron molecular orbital that mediates the ferromagnetic exchange coupling between two 4f ions within a lanthanide endohedral dimetallofullerene deposited on a graphene surface. The single-electron metal-metal bond provides a direct access to the molecule's magnetic system in the transport experiments, paving the way for investigation and controlled manipulation of the spin system of individual dimetallofullerene SMMs, essential for molecular spintronics.

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.

Octapalladium Strings Trap C60 and C70 Fullerenes Affording Metal-Chain-Wired Bucky Balls.

Reactions of Pd8 strings supported by meso-Ph2 PCH2 P(Ph)CH2 P(Ph)CH2 PPh2 (meso-dpmppm) ligands, [Pd8 (meso-dpmppm)4 (L)2 ]4+ (L=CH3 CN (1), XylNC (2)) with C60 resulted in the exclusive formation of unprecedented metal-chain-wired C60 bucky balls, [{Pd4 (meso-dpmppm)2 (L)}2 (C60 )]4+ (L=CH3 CN (11), XylNC (12)), in which a C60 fullerene is trapped in the central Pd-Pd junction, as unambiguously established by spectroscopic, X-ray crystallographic, and theoretical techniques. The similar reaction of Pd8 strings supported by rac-dpmppm, [Pd8 (rac-dpmppm)4 (CH3 CN)2 ]4+ (3) also afforded a racemic mixture of [{Pd4 ((R*,R*)-dpmppm)2 (CH3 CN)}2 (C60 )]4+ (13) without scrambling the Pd4 fragments with (R,R)- and (S,S)-dpmppm ligands. Consequently, those of enantiopure chiral Pd8 strings, [Pd8 ((R*,R*)-dpmppm)4 (CH3 CN)2 ]4+ , certainly afforded chiral bucky balls of [{Pd4 ((R*,R*)-dpmppm)2 (CH3 CN)}2 (C60 )]4+ (13RR and 13SS ), that exhibit mirror-image circular dichroism spectra. The reactions of 1 and 2 were also applied for trapping a C70 fullerene to give 2 : 1 adducts of [{Pd4 (meso-dpmppm)2 (L)}2 (C70 )]4+ (L=CH3 CN (21), XylNC (22)). These results provide useful information for creating a platform to develop dimensionally and chirality controlled metal-carbon nanocomposite materials.

Effects of Aqueous Dispersions of C60, C70 and Gd@C82 Fullerenes on Genes Involved in Oxidative Stress and Anti-Inflammatory Pathways.

BACKGROUND: Fullerenes and metallofullerenes can be considered promising nanopharmaceuticals themselves and as a basis for chemical modification. As reactive oxygen species homeostasis plays a vital role in cells, the study of their effect on genes involved in oxidative stress and anti-inflammatory responses are of particular importance. METHODS: Human fetal lung fibroblasts were incubated with aqueous dispersions of C60, C70, and Gd@C82 in concentrations of 5 nM and 1.5 µM for 1, 3, 24, and 72 h. Cell viability, intracellular ROS, NOX4, NFκB, PRAR-γ, NRF2, heme oxygenase 1, and NAD(P)H quinone dehydrogenase 1 expression have been studied. RESULTS & CONCLUSION: The aqueous dispersions of C60, C70, and Gd@C82 fullerenes are active participants in reactive oxygen species (ROS) homeostasis. Low and high concentrations of aqueous fullerene dispersions (AFD) have similar effects. C70 was the most inert substance, C60 was the most active substance. All AFDs have both "prooxidant" and "antioxidant" effects but with a different balance. Gd@C82 was a substance with more pronounced antioxidant and anti-inflammatory properties, while C70 had more pronounced "prooxidant" properties.

Endohedral Metallofullerenes: New Structures and Unseen Phenomena.

Endohedral metallofullerenes (EMFs), namely fullerenes with metallic species encapsulated inside, represent an ideal platform to investigate metal-metal or metal-carbon interactions at the sub-nanometer scale by means of single-crystal X-ray diffraction (XRD) crystallography. Herein, recent progress in the identification of new structures and unprecedented properties are discussed according to the categories of monometallofullerenes, dimetallofullerenes, carbide clusterfullerenes, and nitride clusterfullerenes. In particular, the dimerization and the cage-isomer dependent oxidation state of the inner metal atom are summarized in terms of pristine monometallofullerenes. Metal-metal bonds involving lanthanide-lanthanides or actinide-actinides are discussed based on both experimental and theoretical studies. The cluster-cage matching and/or mutual selections, as well as the rarely seen M=C double bonds, are discovered in M2 C2 @C2n , U2 C@C80 , M2 TiC@C80 , and Ti3 C3 @C80 . Subsequently, the geometries of different M3 N clusters in various cages are discussed, revealing size-matching between the internal M3 N cluster and the outer cage induced by the planarity of the cluster. Finally, an outlook regarding the future developments of the molecular structures and applications of EMFs is presented.

Preferential Formation of Mono-Metallofullerenes Governed by the Encapsulation Energy of the Metal Elements: A Case Study on Eu@C2n (2n=74-84) Revealing a General Rule.

Encapsulating one to three metal atoms or a metallic cluster inside fullerene cages affords endohedral metallofullerenes (EMFs) classified as mono-, di-, tri-, and cluster-EMFs, respectively. Although the coexistence of various EMF species in soot is common for rare-earth metals, we herein report that europium tends to prefer the formation of mono-EMFs. Mass spectroscopy reveals that mono-EMFs (Eu@C2n ) prevail in the Eu-containing soot. Theoretical calculations demonstrate that the encapsulation energy of the endohedral metal accounts for the selective formation of mono-EMFs and rationalize similar observations for EMFs containing other metals like Ca, Sr, Ba, or Yb. Consistently, all isolated Eu-EMFs are mono-EMFs, including Eu@D3h (1)-C74 , Eu@C2v (19138)-C76 , Eu@C2v (3)-C78 , Eu@C2v (3)-C80 , and Eu@D3d (19)-C84 , which are identified by crystallography. Remarkably, Eu@C2v (19138)-C76 represents the first Eu-containing EMF with a cage that violates the isolated-pentagon-rule, and Eu@C2v (3)-C78 is the first C78 -based EMF stabilized by merely one metal atom.

Substrate-Independent Magnetic Bistability in Monolayers of the Single-Molecule Magnet Dy2 ScN@C80 on Metals and Insulators.

Magnetic hysteresis is demonstrated for monolayers of the single-molecule magnet (SMM) Dy2 ScN@C80 deposited on Au(111), Ag(100), and MgO|Ag(100) surfaces by vacuum sublimation. The topography and electronic structure of Dy2 ScN@C80 adsorbed on Au(111) were studied by STM. X-ray magnetic CD studies show that the Dy2 ScN@C80 monolayers exhibit similarly broad magnetic hysteresis independent on the substrate used, but the orientation of the Dy2 ScN cluster depends strongly on the surface. DFT calculations show that the extent of the electronic interaction of the fullerene molecules with the surface is increasing dramatically from MgO to Au(111) and Ag(100). However, the charge redistribution at the fullerene-surface interface is fully absorbed by the carbon cage, leaving the state of the endohedral cluster intact. This Faraday cage effect of the fullerene preserves the magnetic bistability of fullerene-SMMs on conducting substrates and facilitates their application in molecular spintronics.

Functional Metallofullerene Materials and Their Applications in Nanomedicine, Magnetics, and Electronics.

Endohedral metallofullerenes exhibit combined properties from carbon cages as well as internal metal moieties and have great potential in a wide range of applications as molecule materials. Along with the breakthrough of mass production of metallofullerenes, their applied research has been greatly developed with more and more new functions and practical applications. For gadolinium metallofullerenes, their water-soluble derivatives have been demonstrated with antitumor activity and unprecedented tumor vascular-targeting therapy. Metallofullerene water-soluble derivatives also can be applied to treat reactive oxygen species (ROS)-induced diseases due to their high antioxidative activity. For magnetic metallofullerenes, the internal electron spin and metal species bring about spin sensitivity, molecular magnets, and spin quantum qubits, which have many promising applications. Metallofullerenes are significant candidates for fabricating useful electronic devices because of their various electronic structures. This Review provides a summary of the metallofullerene studies reported recently, in the fields of tumor inhibition, tumor vascular-targeting therapies, antioxidative activity, spin probes, single-molecule magnets, spin qubits, and electronic devices. This is not an exhaustive summary and there are many other important study results regarding metallofullerenes. All of this research has revealed the irreplaceable role of metallofullerene materials.

Crystallographic and Theoretical Investigations of Er2 @C2 n (2 n=82, 84, 86): Indication of Distance-Dependent Metal-Metal Bonding Nature.

Successful isolation and characterization of a series of Er-based dimetallofullerenes present valuable insights into the realm of metal-metal bonding. These species are crystallographically identified as Er2 @Cs (6)-C82 , Er2 @C3v (8)-C82 , Er2 @C1 (12)-C84 , and Er2 @C2v (9)-C86 , in which the structure of the C1 (12)-C84 cage is unambiguously characterized for the first time by single-crystal X-ray diffraction. Interestingly, natural bond orbital analysis demonstrates that the two Er atoms in Er2 @Cs (6)-C82 , Er2 @C3v (8)-C82 , and Er2 @C2v (9)-C86 form a two-electron-two-center Er-Er bond. However, for Er2 @C1 (12)-C84 , with the longest Er⋅⋅⋅Er distance, a one-electron-two-center Er-Er bond may exist. Thus, the difference in the Er⋅⋅⋅Er separation indicates distinct metal bonding natures, suggesting a distance-dependent bonding behavior for the internal dimetallic cluster. Additionally, electrochemical studies suggest that Er2 @C82-86 are good electron donors instead of electron acceptors. Hence, this finding initiates a connection between metal-metal bonding chemistry and fullerene chemistry.

Modular Covalent Graphene Functionalization with C60 and the Endohedral Fullerene Sc3 N@C80 : A Facile Entry to Synthetic-Carbon-Allotrope Hybrids.

Two novel graphene-fullerene hybrid structures, containing C60 and endohedral Sc3 N@C80 bound to graphene, instead of the formerly used graphene oxide, were efficiently synthesized via a reductive activation/exfoliation approach starting from pristine graphite. The structures of these multifunctional hybrid systems were unambiguously characterized by statistical Raman spectroscopy, TG-MS, TG-GC-MS, and LD-TOF mass spectroscopy, confirming the covalent bonding of the respective C60 /Sc3 N@C80 moieties to the pristine graphene. Furthermore, assisted by temperature-dependent Raman spectroscopy studies the corresponding defunctionalization processes were also investigated. Finally, the formation of a carbon allotrope hybrid material on the basis of C60 /Sc3 N@C80 moieties coupled to graphene could be visualized by HRTEM.

Isolation and Crystallographic Characterization of Lu3 N@C2n (2n=80-88): Cage Selection by Cluster Size.

The small Sc3 N cluster has only been found in such small cages as C2n (2n=68, 78, 80, 82), whereas the large M3 N (M=Y, Gd, Tb, Tm) clusters choose those larger cages C2n (2n=82-88). Herein, concrete experimental evidence is presented to establish the size effect of the internal metallic cluster on selecting the outer cage of endohedral metallofullerenes (EMFs) by using a medium-sized metal, lutetium, which possesses an ionic radius between Sc and Gd. A series of lutetium-containing EMFs have been obtained and their structures are unambiguously determined as Lu3 N@Ih (7)-C80 , Lu3 N@D5h (6)-C80 , Lu3 N@C2v (9)-C82 , Lu3 N@Cs (51365)-C84 , Lu3 N@D3 (17)-C86 , and Lu3 N@D2 (35)-C88 by single-crystal X-ray diffraction crystallography. It was confirmed that the encaged Lu3 N cluster always adopts a planar geometry in Lu3 N@C80-88 isomers to ensure substantial metal-cage/metal-nitrogen interactions. As a result, the Lu3 N cluster selects the C2v (9)-C82 cage, which also encapsulates Sc3 N, instead of the Cs (39663)-C82 cage which is more suitable for M3 N (M=Y, Gd, Tb, Tm). However, different from Sc3 N, Lu3 N can also template the C84-88 cages which are absent for Sc3 N-containing EMFs, confirming clearly the size effect of the internal cluster on selecting the outer cage.

Dimerization of Metallofullerenes to Obtain Materials with Enhanced Nonlinear Optical Properties.

In this work, we theoretically designed a new class of fullerene derivatives, metallofullerene heterodimers, through cross dimerization of metallofullerene monomers Li@C60 , Y@C82 and Y2 @C80 . The molecular structures, electronic structures, linear and nonlinear optical properties of the heterodimers were studied by using density functional theory calculations. We found that two fullerene cages are connected through a relatively weak carbon-carbon single bond. Inter-cage electron transfer takes place in the heterodimers because of the different electron accepting abilities of the metallofullerene moieties. The first hyperpolarizability of the metallofullerenes increases remarkably as a result of dimerization. Time-dependent density functional theory calculations reveal that the enhanced first hyperpolarizability of the dimer is associated with charge-transfer transitions. This study demonstrates that covalent dimerization is an efficient means to improve the nonlinear optical response of metallofullerenes.

Formation of Nickel Clusters Wrapped in Carbon Cages: Toward New Endohedral Metallofullerene Synthesis.

Despite the high potential of endohedral metallofullerenes (EMFs) for application in biology, medicine and molecular electronics, and recent efforts in EMF synthesis, the variety of EMFs accessible by conventional synthetic methods remains limited and does not include, for example, EMFs of late transition metals. We propose a method in which EMF formation is initiated by electron irradiation in aberration-corrected high-resolution transmission electron spectroscopy (AC-HRTEM) of a metal cluster surrounded by amorphous carbon inside a carbon nanotube serving as a nanoreactor and apply this method for synthesis of nickel EMFs. The use of AC-HRTEM makes it possible not only to synthesize new, previously unattainable nanoobjects but also to study in situ the mechanism of structural transformations. Molecular dynamics simulations using the state-of-the-art approach for modeling the effect of electron irradiation are performed to rationalize the experimental observations and to link the observed processes with conditions of bulk EMF synthesis.

Purification of Uranium-based Endohedral Metallofullerenes (EMFs) by Selective Supramolecular Encapsulation and Release.

Supramolecular nanocapsule 1⋅(BArF)8 is able to sequentially and selectively entrap recently discovered U2 @C80 and unprecedented Sc2 CU@C80 , simply by soaking crystals of 1⋅(BArF)8 in a toluene solution of arc-produced soot. These species, selectively and stepwise absorbed by 1⋅(BArF)8 , are easily released, obtaining highly pure fractions of U2 @C80 and Sc2 CU@C80 in one step. Sc2 CU@C80 represents the first example of a mixed metal actinide-based endohedral metallofullerene (EMF). Remarkably, the host-guest studies revealed that 1⋅(BArF)8 is able to discriminate EMFs with the same carbon cage but with different encapsulated cluster and computational studies provide support for these observations.

Biomedical Applications of Metal-Encapsulated Fullerene Nanoparticles.

The carbonaceous nanomaterials known as metallofullerenes have attracted considerable attention due to their attractive properties. The robust nature of the "Trojan Horse" fullerene cage provides an important structural component, which isolates the metal cluster from the bioenvironment. The large carbon surface area is ideally suited for multiple exo-functionalization approaches to modify the hydrophobic cage for a more hydrophilic bioenvironment. Additionally, peptides and other agents are readily covalently attached to this nanoprobe for targeting applications. The recent progress in developing metallofullerenes for next-generation biomedical applications is described. Of special interest are magnetic resonance imaging (MRI) contrast agents. Several recent studies reported cumulative gadolinium deposition in the brain and bones of individuals using commercial clinical MRI contrast agents. Gadolinium-based metallofullerenes provide 2-3 orders of magnitude improvement in MRI relaxivity and potentially lower clinical levels of toxic Gd3+ ions deposited. Other potential biomedical applications are also reviewed herein.

Capturing the Unconventional Metallofullerene M@C66 by Trifluoromethylation: A Theoretical Study.

The endohedral metallofullerenes M@C66 (M=rare-earth metal) have a unique structure that violates the well-known "isolated pentagon rule" of fullerene science. Although the synthesis of M@C66 has been achieved by using the arc discharge method, the solvent extraction and purification of M@C66 remain challenges because of their radical character and extremely high reactivity. In this paper, the possibility of capturing these missing metallofullerenes by exohedral functionalization of the C66 cage is demonstrated theoretically. Stable trifluoromethylated derivatives of Y@C66 are revealed by density functional theory calculations. Mono- or poly-trifluoromethylation of Y@C66 results in a closed-shell electronic configuration and a large band gap. Thus Y@C66 can be greatly stabilized through trifluoromethylation. The trifluoromethyl group prefers to be attached to the fused pentagon region to relieve local steric strain. The mechanism of isomerization of Y@C66 (CF3 )3 is also investigated and it is found that the attached trifluoromethyl group can migrate from a carbon atom to another via a transition state.

Use of trimetasphere metallofullerene MRI contrast agent for the non-invasive longitudinal tracking of stem cells in the lung.

Magnetic Resonance Imaging (MRI) is a commonly used, non-invasive imaging technique that provides visualization of soft tissues with high spatial resolution. In both a research and clinical setting, the major challenge has been identifying a non-invasive and safe method for longitudinal tracking of delivered cells in vivo. The labeling and tracking of contrast agent labeled cells using MRI has the potential to fulfill this need. Contrast agents are often used to enhance the image contrast between the tissue of interest and surrounding tissues with MRI. The most commonly used MRI contrast agents contain Gd(III) ions. However, Gd(III) ions are highly toxic in their ionic form, as they tend to accumulate in the liver, spleen, kidney and bones and block calcium channels. Endohedral metallofullerenes such as trimetallic nitride endohedral metallofullerenes (Trimetasphere®) are one unique class of fullerene molecules where a Gd3N cluster is encapsulated inside a C80 carbon cage referred to as Gd3N@C80. These endohedral metallofullerenes have several advantages over small chelated Gd(III) complexes such as increased stability of the Gd(III) ion, minimal toxic effects, high solubility in water and high proton relativity. In this study, we describe the evaluation of gadolinium-based Trimetasphere® positive contrast agent for the ​in vitro labeling and in vivo tracking of human amniotic fluid-derived stem cells within lung tissue. In addition, we conducted a 'proof-of-concept' experiment demonstrating that this methodology can be used to track the homing of stem cells to injured lung tissue and provide longitudinal analysis of cell localization over an extended time course.

Eu@C72: Computed Comparable Populations of Two Non-IPR Isomers.

Relative concentrations of six isomeric Eu@C 72 -one based on the IPR C 72 cage (i.e., obeying the isolated-pentagon rule, IPR), two cages with a pentagon-pentagon junction (symmetries C 2 and C 2 v ), a cage with one heptagon, a cage with two heptagons, and a cage with two pentagon-pentagon fusions-are DFT computed using the Gibbs energy in a broad temperature interval. It is shown that the two non-IPR isomers with one pentagon-pentagon junction prevail at any relevant temperature and exhibit comparable populations. The IPR-satisfying structure is disfavored by both energy and entropy.

Stereodivergent-at-Metal Synthesis of [60]Fullerene Hybrids.

Chiral fullerene-metal hybrids with complete control over the four stereogenic centers, including the absolute configuration of the metal atom, have been synthesized for the first time. The stereochemistry of the four chiral centers formed during [60]fullerene functionalization is the result of both the chiral catalysts employed and the diastereoselective addition of the metal complexes used (iridium, rhodium, or ruthenium). DFT calculations underpin the observed configurational stability at the metal center, which does not undergo an epimerization process.