Encapsulated Metal Atoms Research Articles

Ln@C60 endohedral fullerenes: A DFT analysis for the complete series from lanthanum to lutetium

We report on a computational study of the complete series of lanthanide atoms encapsulated in C60 fullerene molecule (Ln@C60) by using density functional theory with the PBE functional complemented with the Grimme’s dispersion correction (PBE-D). The goal of the present survey is to account for the influence of step-by-step adding 4f-electrons in lanthanides on the geometrical and electronic properties of Ln@C60 endohedral species. In order to compare the results obtained, we tested two double-numerical basis sets: DN and DND, which are equivalent to the 6-31G and 6-31G(d) Pople-type basis sets. The electronic parameters analyzed comprised the HOMO, LUMO and HOMO-LUMO gap energies, as well as the charge and spin of the encapsulated metal atom. The La and Lu atoms exhibit no tangible spin contribution, whereas in the remaining Ln@C60 species unpaired electrons are found on both lanthanide atom and C60 cage. An intermediate case is the Yb@C60 complex, where only a minor presence of unpaired electrons on the central metal was found.

Иттрий в фуллеренах

The study of the literature data showed that yttrium is encapsulated in fullerene molecules in the form of metal atoms, clusters of nitrides, carbides, sulfides and other compounds. Endohedral metallofullerenes are capable of encapsulating up to four metal atoms. In the molecules of these compounds, the metal atoms are positively charged due to the transfer of an electron from the endohedral metal atom to the fullerene carbon cage. First of all, the main experimental and theoretical achievements described in early (up to 2000) works are considered. Achievements in the production, separation (isolation) and various spectroscopic characteristics of endohedral metallofullerenes have been thoroughly studied in an attempt to elucidate their structural, electronic, and solid-state properties. It is shown that endohedral metallofullerenes in electrical conductivity can be metals, semiconductors with small gaps, or insulators, depending on the size of the fullerene, the type and number of encapsulated metal atoms. Other electronic and magnetic properties of metallofullerenes are also interesting. Also, some promising applications of metallofullerenes are considered. In addition, when analyzing the literature on the synthesis and properties of metalloendofullerenes, a very large number of publications related to the exohedral functionalization of metallofullerenes attracts attention. First of all, it should be noted that the main problem hindering the development of science, technology and the use of fullerenes, endohedral metallofullerenes and nanotubes was the difficulty of obtaining high-purity samples. When metals are introduced into the electric arc process, the situation becomes more complicated due to the presence of many isomers of both fullerenes and endohedral metallofullerenes. Exohedral functionalization helps to solve the problem of separation of synthesis products, on the one hand, and leads to the production of substances with new useful properties and potential applications in materials science and medicine. It is noted that currently the most productive and widespread method for the production of endohedral fullerenes is the electric arc process. The quantitative and qualitative output of metallofullerenes is significantly influenced by the conditions of the process in the reactor.

Antiaromaticity-aromaticity transition of cyclo[16]carbon upon metal encapsulation.

In contrast to aromatic compounds with particular stability, antiaromatic compounds are usually less stable due to their high reactivity and unfavorable formation energies. Cyclo[16]carbon (C16) is a carbon ring molecule with a dual antiaromatic character. In this study, we demonstrate that C16 can be transformed into highly aromatic molecules upon metal encapsulation. The geometrical characteristics, electronic properties and thermodynamic stability of MC16 compounds (M = Ca, Sc, Ti, V, Ce, U) are fully investigated from a theoretical perspective. Based on natural population analysis, atom-in-molecules theory and localized molecular orbital analysis, the nature of the metal-carbon interaction in the MC16 compounds is investigated. It has been proved that the bonding between Ca and C16 corresponds to a typical ionic interaction, while other metal atoms form polar covalent bonds with C16. By analyzing the frontier molecular orbitals and magnetic response of MC16, we have found that all the encapsulated metal atoms donate two electrons to the in-plane π orbitals via either electron transfer or orbital hybridization, which makes the in-plane π orbitals completely satisfy the 4n + 2 (n = 4) Hückel aromaticity rule. The U atom formally transfers four electrons to the carbon ring in total, two to the in-plane π orbitals and two to the out-of-plane π orbitals, which results in the remarkable dual aromaticity feature of UC16. The transformation of aromaticity can be utilized to develop new strategies for the synthesis of novel carbon ring molecules.

Structural Studies of Giant Empty and Endohedral Fullerenes.

Structure elucidations of giant fullerenes composed of 100 or more carbon atoms are severely hampered by their extremely low yield, poor solubility and huge numbers of possible cage isomers. High-temperature exohedral chlorination followed by X-ray single crystal diffraction studies of the chloro derivatives offers a practical solution for structure elucidations of giant fullerenes. Various isomers of giant fullerenes have been determined by this method, specially, non-classical giant fullerenes containing heptagons generated by the skeletal transformations of carbon cages. Alternatively, giant fullerenes can be also stabilized by encapsulating metal atoms or clusters through intramolecular electron transfer from the encapsulated species to the outer fullerene cage. In this review, we present a comprehensive overview on synthesis, separation and structural elucidation of giant fullerenes. The isomer structures, chlorination patterns of a series of giant fullerenes C2n (2n = 100-108) and heptagon-containing non-classical fullerenes derived from giant fullerenes are summarized. On the other hand, giant endohedral fullerenes bearing different endohedral species are also discussed. At the end, we propose an outlook on the future development of giant fullerenes.

Transition metal atoms encapsulated within microporous Silicalite-1 zeolite: A systematic computational study

Single-atom catalysts (SACs) have emerged as a novel class of catalysts that show unique catalytic properties thanks to their easy tuneability. In this context, zeolites are widely used as support materials for SACs, since the steric restrictions from their channels can effectively suppress the aggregation of the metal particles and provide encapsulated metal atoms with exceptional stability. Although significant efforts have been made to study zeolite-supported SACs, only a few transition metal atoms have been tested, and the determination of precise locations of single metal atoms within the zeolite matrix by experiments remains a nontrivial task. Herein we provide a thorough dispersion corrected density functional theory study of the energetic, structural, magnetic and electronic properties for the full sets of 3d, 4d and 5d transition metal atoms encapsulated on MFI-type Silicalite-1 zeolite, namely TM@S-1. Most TM atoms prefer to adsorb in the center of the S-1 channels, weakly interacting with the zeolite, with the exception of Group 3 − Sc, Y, La − , Group 10 − Ni, Pd, Pt − , Ru and Rh atoms, making them good candidates as stable SACs with superior catalytic performance. The optimized geometry files for the full set of TM@S-1 have been uploaded to a public online repository, paving the road for further catalytic studies of these systems. The behavioral trends gained from this study provide a solid theoretical background that can be used as the basis for interpreting, understanding, and discussing already observed experimental features, and future findings as well. • Only 4 specific sites in S-1 are preferred by the entire set of TM adatoms. • TM@S-1 of groups 3, 10, 11 & 12 more resistant against sintering than other groups. • Weakest Eads for group 11 and 12 TMs with a full occupancy of d orbitals. • Similar behavior for TMs of groups 6 and 7 with half occupancy of orbitals. • All TMs n-dope S-1, except Pt, which p-dopes it.

Endohedral metallofullerene single molecule magnets

Nanoscale magnetic materials have been attracting enormous interests during the past few decades due to the rapid development of high-density magnetic storage devices. A breakthrough in nanomagnetism is the discovery of single molecule magnets (SMMs), which exhibit slow relaxation and quantum tunneling of the magnetization resulted from a molecular-based blocking anisotropy, and thus have potential applications in high-density data storage, molecular spintronics and quantum computing. Since the report of Mn12-complex as the first SMM by Sessoli et al. in 1993, a large number of SMMs based on organometallic coordination compounds have been synthesized. Compared to the traditional bulk magnets, the magnetic properties of SMMs are determined by their instrinsic spin structures resulting in slow magnetic relaxation. SMM is characterized by a bi-stable magnetic ground state and a certain spin-reversal barrier ( U eff). Additionally, hysteresis loops can be observed below the blocking temperature ( T B). Therefore, SMMs with excellent magnetic properties always have high T B and U eff. U eff depends strongly on spin ground state ( S ) and magneto-crystalline anisotropy ( D ), and can be expressed with the formula U eff =| D|S 2. For transition metal-based SMMs, their low magneto-crystalline anisotropies ( D ) result in relative low U eff. In recent years, lanthanide based SMMs have been studied intensively and become the most promising high-performance SMMs, owning to their strong single ion anisotropy. Encapsulating metal atoms or metallic clusters into fullerene cages generate a special class of fullerenes, the so-called endohedral metallofullerenes (EMFs), for which the encapsulation of metal atoms endow EMFs more fascinating properties compared to empty fullerenes. For instance, EMFs are paramagnetic when the entrapped metal ions have unpaired electrons. Furthermore, EMFs were able to show excellent SMM properties when the encapsulated clusters induce appropriate ligand-fields. EMFs with lanthanide open up a new avenue in the field of SMMs in 2012 when single molecule magnetism was proved for the metal nitride clusterfullerene (NCF) DySc2N@ I h-C80. Fullerene cages are stable in air and thus they can protect the magnetic ions from ambient conditions, and the negatively charged nonmetallic atom(s) within the cluster and the negatively charged carbon atoms of fullerene cages provide magnetic anisotropy which is necessary for single molecule magnetism. Moreover, the molecular structures of EMFs can be modulated by varying the encapsulated metal ions, metal cluster and cage isomer; hence the research on EMF-SMMs has been developed rapidly since 2012. Up to now, metal nitride clusterfullerenes (NCFs), metal carbide clusterfullerenes (CCFs), metal oxide clusterfullerenes (OCFs), metal sulfide clusterfullerenes (SCFs), metal cyanide clusterfullerenes (CYCFs) and dimetallofullerenes (di-EMFs) have been confirmed as SMMs. In this paper, we present an exhaustive review on all kinds of EMF-SMMs reported to date including their 1D, 2D and 3D assemblies. We mainly discuss the influences of the types and the numbers of the encapsulated metal ions and non-metal atoms on the magnetic properties of EMF-SMMs. Finally, the prospects and challenges of EMF-SMMs are also discussed.

The spin-dependent transport properties of endohedral transition-metal-fullerene X@C66H4 (X=Fe, Co, Mn, Ni)

Inspired by recent experiments on the successful synthesis of hydrofullerene C66H4 in Tian et al. (2019) [12] with two negatively curved heptagons. Based on the density functional theory and nonequilibrium Green's function method, we report the spin-dependent transport through transition-metal-atom-encapsulated C66H4 hydrofullerene, i.e., X@C66H4(X=Fe, Co, Mn, Ni), contacted by single gold atoms via semi-infinite non-magnetic Au electrodes. It is found that, Mn- and Fe-doped systems show highly spin-polarized transmission as well as considerable magnetic moments whereas Ni-doped systems show completely spin-unpolarized transmission and nonmagnetic. Interestingly, Co-doped systems show two spin states, i.e., spin-polarized and spin-unpolarized ones. Further analysis shows that, for Mn-, Fe- and Ni-doped systems, the spin-polarized/unpolarized state is caused by the finite/(nearly-)zero magnetism of the encapsulated metal atom. While the magnetism in Co-doped systems is quenched for the top hexagonal doping case, but not for the side heptagonal doping one, which induces the spin-unpolarized/spin-polarized state. And the screening effect of electrodes on the magnetism of Co is the underlying physical mechanism. Our findings would be beneficial to the design of spintronics devices.

Supramolecular Complexes of C80-Based Metallofullerenes with [12]Cycloparaphenylene Nanoring and Altered Property in a Confined Space

Metallofullerenes have a cage-shaped molecular structure and special properties derived from encapsulated metal atoms. Owing to the ball-like fullerene cage, it is still a challenge to manipulate the metallofullerene molecule and to control its properties. Herein, we use the molecular carbon nanoring of [12]cycloparaphenylene to hoop the C80-based metallofullerene Y3N@C80 and azafullerene Y2@C79N, resulting in a stable supramolecular complex, induced molecule orientation, changed assembly behavior, and tunable spin state. These supramolecular complexes and their host–guest interactions were systematically characterized by DFT-calculations, spectroscopy, NMR, and electrochemical analysis. Scanning tunneling microscopy was employed to reveal that the metallofullerene guests dominate the assembly process on the Au(111) surface and the [12]CPP nanoring can change the weak van der Waals forces and influence self-assembly. Moreover, the electron spin in paramagnetic Y2@C79N was employed to percept the host–gues...

(Invited) Isolation and Structural Characterization of Lu2C2n

Endohedral metallofullerenes (EMFs) with a composition of M2C2n may exist as di-EMFs, M2@C2n, or as carbide cluster metallofullerenes (CCMFs), M2C2@C2n-2. EMFs containing such metal atoms with a small ionic radius as scandium were found to prefer the CCMF form some of which had been incorrectly assigned as di-EMFs.[1] Lu-containing EMFs remain largely unexplored in spite that lutecium also takes small ionic radius so that . In this work, we report the single-crystal X-ray crystallographic results of a series of Lu-containing EMFs to show that all of them are di-EMFs, namely Lu2@C2n (2n=76-86). Theoretical results confirm the formation of direct Lu-Lu bonding between the two encapsulated metal atoms.[ 2 ] References： [1] X. Lu, T. Akasaka, S. Nagase, Acc. Chem. Res. 2013, 46, 1627-1635. [2] W. Shen, L. Bao, Y. Wu, C. Pan, S. Zhao, H. Fang, Y. Xie, P. Jin, P. Peng, F.-F. Li, X. Lu, J. Am. Chem. Soc. 2017, 139, 9979−9984.

(Invited) Relative Stabilities for Isomeric and Non-Isomeric Endohedrals

There are several stability problems important for fullerenic species, namely relative populations of (i) isomeric endohedrals, (ii) non-isomeric metallofullerenes differing in the number of the encapsulated metal atoms placed in still isomeric carbon cages, and (iii) monometallofullerenes with different metals encapsulated in one selected carbon cage. The related computational approaches are illustrated on endohedrals with encapsulated La and its congeners, evaluated using various density-functional theory (DFT) treatments or the second order Moller-Plesset perturbation theory (MP2). One of the illustrative examples treats encapsulations in the T_d IPR C76 cage using the metal series Al, Sc, Y, La, and Ac. Three formal reaction steps can be considered with the encapsulations: (i) double- (or triple-) ionization of the free metal, (ii) double (or triple-) charging of the empty cage, and (iii) placing the metal di- (or tri-) cation into the di- or (tri-) anionic cage. The (ii) energy change is identical for all members of the series, and the (iii) terms should be similar as they are controlled by electrostatics. Hence, only the (i) energy term comes as a differentiating factor. In other words, the free-metal ionization potentials should actually represent the critical yield-controlling parameter.The computed relative potential-energy changes upon encapsulation δE and the relative observed ionization potentials of the free atoms δIP should (according to the above three-step analysis) be correlated. Interestingly, Ac has the second best encapsulation energy after La in the series.

The chirality of metal-encapsulated silicene-like nanotubes

ABSTRACTThe energy stability and electronic structures of (3,3) armchair metal-encapsulated SiNTs (M@SiNTs) are systematically studied by using DFT-GGA calculations. Results show that most the metal-encapsulated SiNTs with the same space group can be stabilized through doping by metal atoms with silicene-like structures. Compared with the different chirality, (4,0) zigzag M@SiNTs are more stable than (3,3) armchair M@SiNTs. The band gaps of (3,3) and (4,0) SiNTs are 1.06 eV and 0.06 eV, they are both semiconductors. But most (3,3) armchair and (4,0) zigzag M@SiNTs are conductors, except (3,3) Sn@SiNT, (4,0) K@SiNT and (4,0) Ba@SiNT are semiconductors with small value of band gaps. The interaction between encapsulated metal and silicon atoms makes out orbital hybridizations, and decreases the gap between valence band and conduction band. Finally, electronic structural analysis shows that metal atoms gain electrons and Si atoms lose electrons as a whole, some electrons transfer from Si to metal atoms.

Endohedral metallofullerenes (M@C60) as efficient catalysts for highly active hydrogen evolution reaction

The cage structure of C60 fullerenes with encapsulated metal atoms, i.e. endohedral fullerenes (M@C60) possess unique electronic properties with novel applications. By using density functional theory (DFT), we for the first time predict endohedral M@C60 (M=Na, K, Rb, Cs, Sc, Ti, Mn, Fe) fullerenes from 20 possible candidates as promising high performance hydrogen evolution reaction (HER) catalysts. For the pristine C60, the Gibbs free energy is too positive to prevent the adsorption of H-atoms on surface carbon atoms. However, when a metal atom is embedded in the C60 cage, the H-atom binding free energy on M@C60 can be optimized to ideal value for HER (ΔGH=0). The catalytic active site is non-metal C-atom and the HER performance of M@C60 are even better than those of the state-of-the-art Pt and MoS2 catalysts. The excellent catalytic activities are attributed to the charge transfer between the metal atom and C60 cage. Since the endohedral fullerenes can be easily realized in experiment, our findings highlight a new class of low-cost and efficient HER catalyst for experimental validation studies toward hydrogen production.

Functionalization of Endohedral Metallofullerenes with Reactive Silicon and Germanium Compounds.

Exohedral derivatization of endohedral metallofullerenes (EMFs) has been exploited as a useful method for characterizing the structural and chemical properties of EMFs, and for functionalizing them for potential applications. The introduction of heteroatoms, such as electropositive silicon atoms, to fullerene cages is a novel functionalization method that remarkably affects the electronic characteristics of fullerenes. This review comprehensively describes the results of the reactions of monometallofullerene, dimetallofullerene, and trimetallic nitride template EMFs with disilirane, silirane, silylene, and digermirane, which afforded the corresponding silylated and germylated fullerenes. Several examples emphasize that exohedral functionalization regulates the dynamic behaviors of the encapsulated metal atoms and clusters in the fullerene cages. The electronic effects of silyl and germyl groups are represented by comparing the redox properties of silylated and germylated EMFs with those of other EMFs derivatized with carbon-atom-based functional groups.

Stability issues in computational screening of carbon nanostructures: illustrations on La endohedrals

Three important stability problems frequently met in calculations of carbon nanostructures are outlined. The three relative-stability issues concern: isomeric carbon clusters, non-isomeric metallofullerenes differing in the number of the encapsulated metal atom yet placed in still isomeric carbon cages, and monometallofullerenes with different metals encapsulated in one selected carbon cage. The related computational approaches are illustrated with endohedrals based on La and its congeners located in , and cages, both IPR (isolated-pentagon rule) and non-IPR ones, evaluated using various DFT treatments. It is also concluded that among the La congeners, Ac can produce the second best encapsulation energy, just after La.

Magnetic Silicon Fullerenes: Experimental Exploration and Theoretical Insight

The present article summarizes progress in research on silicon clusters with encapsulated metal atoms, and specifically focuses on the recent identification of magnetic silicon fullerenes. Considering that C $$_{20}$$ forms the smallest known fullerene, the Si $$_{20}$$ cluster is of particular interest in this context. While the pure hollow Si $$_{20}$$ cage is unstable due to the lack of $$sp^2$$ hybridization, endohedral doping with a range of metal atoms has been considered to be an effective way to stabilize the cage structure. In order to seek out suitable embedded atoms for stabilizing Si $$_{20}$$ , a broad search has been made across elements with relatively large atomic radius. The rare earth elements have been found to be able to stabilize the Si $$_{20}$$ cage in the neutral state by forming R@Si $$_{20}$$ fullerene cages. Among these atoms, Eu@Si $$_{20}$$ has been reported to yield a stable magnetic silicon fullerene. The central europium atom has a large magnetic moment of nearly 7.0 Bohr magnetons. In addition, based on a stable Eu $$_2$$ Si $$_{30}$$ tube, a magnetic silicon nanotube has been constructed and discussed. These magnetic silicon fullerenes and nanotubes may have potential applications in the fields of spintronics and high-density magnetic storage.

Role of encapsulated metal cation in the reactivity and regioselectivity of the C₆₀ Diels-Alder reaction.

Endohedral metallofullerene has novel properties because of the interaction between the encapsulated metal atom or cation and fullerene. Experiments have demonstrated that the insertion of Li(+) into C60 can greatly promote the reactivity of the Diels-Alder (DA) cycloaddition of cyclopentadiene (CpH) to C60. However, the reaction is sufficiently fast that its quantitative kinetic data cannot be obtained experimentally. In addition, knowledge regarding the effects of other alkali metal cations and metal cations with more charges on the reactivity and regioselectivity of C60 is almost nonexistent. In the current study, DA cycloadditions of CpH to M(+)@C60 (where M = Li, Na, K, Rb, and Cs) and Ca(2+)@C60 were investigated via density functional theory in the gas phase and in solvent. Via careful discussion and comparison with the results of C60, we concluded the following for the DA reaction of CpH to C60 and, more generally, for DA reactions of other fullerenes: (1) the encapsulated metal cations enhance the reactivity; (2) among alkali metal cations, Na(+) could be the best catalyst; (3) Ca(2+) is more favorable in promoting the reactivity than any alkali metal cation; (4) encapsulated metal cations with more positive charges enhance the reactivity of the 6-5 bond in C60, which is significant when the 6-5 adduct is the target product.

ChemInform Abstract: Emergence of Highly Elaborated π‐Space and Extending Its Functionality Based on Nanocarbons: New Vistas in the Fullerene World

AbstractReview: [151 refs.

Thermoelectricity at the molecular scale: a large Seebeck effect in endohedral metallofullerenes.

Single molecule devices provide a unique system to study the thermoelectric energy conversion at an atomistic level and can provide valuable information for the design of organic thermoelectric materials. Here we present a comprehensive study of the thermoelectric transport properties of molecular junctions based on C(82), Gd@C(82), and Ce@C(82). We combine precise scanning tunneling microscope break-junction measurements of the thermopower and conductance with quantitatively accurate self-energy-corrected first-principles transport calculations. We find that all three fullerene derivatives give rise to a negative thermopower (n-conducting). The absolute value, however, is much larger for the Gd@C(82) and Ce@C(82) junctions. The conductance, on the other hand, remains comparable for all three systems. The power factor determined for the Gd@C(82) based junction is so far the highest obtained for a single-molecule device. Although the encapsulated metal atom does not directly contribute to the transport, we show that the observed enhancement of the thermopower for Gd@C(82) and Ce@C(82) is elucidated by the substantial changes in the electronic- and geometrical structure of the fullerene molecule induced by the encapsulated metal atom.

Emergence of Highly Elaborated π-Space and Extending Its Functionality Based on Nanocarbons: New Vistas in the Fullerene World

Abstract How the electronic properties and chemical reactivity of empty fullerenes can be changed upon endohedral metal-doping? How the dynamic behavior and position of encapsulated metal atoms can be controlled? How its functionality can be extended by chemical treatments? We have addressed all aspects of endohedral metallofullerenes, including the synthesis, isolation, structure, chemical properties, and theoretical considerations and their applications in the related fields to answer these questions.

The role of the encapsulated atom in the vibrational spectrа of La@C60–Lu@C60 lanthanide endofullerenes

Instead of the four experimentally derived infrared (IR) lines observed for C60 fullerene, the IR spectrum of Ln@C60 endofullerenes contains a variety of lines in the 20–1500cm−1 range that correspond to the metal–cage and cage vibrational modes. Here, we present the frequencies and intensities of these lines for lanthanide endofullerenes calculated using the quantum-chemistry PRIRODA software package. Additionally, their structures, symmetries, spins and dipole moments are investigated. The lines found in the near IR (20–160cm−1) range correspond to the translational motion of encapsulated lanthanide atoms (metal–cage vibrational modes). The far-IR range corresponds to the experimentally observed absorption region in the IR spectrum of endofullerenes with higher symmetry. This correspondence indicates that the frequencies are determined by the structure and symmetry of the carbon-cage fragment adjacent to the metal atom, as only the nearest atoms of the carbon cage are involved in these vibrations. It is shown that the magnitude of the splitting of the vibrational frequencies of the encapsulated metal atom is determined by the symmetry of its position in the fullerene. For isomers of the same symmetry, the metal-atom vibrational frequencies depend on the valence of the lanthanide metal.