Formation Of Metallofullerenes Research Articles

Metallofullerenes as potential candidates for the explanation of astrophysical phenomena.

Detection of complex organic species in space has been one of the biggest challenges of the astrophysical community since the beginning of space exploration, with C60-fullerene representing one of the largest molecules so far detected. The presence of small metal-containing organic molecules, like MgNC or CaCN, in space, promoted the idea that C60 may also interact with metals and form metallofullerenes based on the fact that in certain circumstellar and interstellar environments, the ingredients for the formation of metallofullerenes, i.e., metal and fullerenes, are abundant. In this perspective, we summarized the current effort to explore the presence of metallofullerenes in space, which started soon after the discovery of fullerenes about 40 years ago. Several implications of astrophysical phenomena were briefly discussed and shown to be addressable as the possible consequence of metallofullerenes' presence. We highlighted the spectral fingerprints that might be followed to achieve the future detection of cosmic metallofullerenes from a combined effort of laboratory and quantum chemical calculations. These results are expected to gain great importance with the James Webb Space Telescope (JWST), whose capability of unprecedented high sensitivity and high spectral resolution in the far- to mid-infrared range could aid the unequivocal detection of metallofullerenes in space.

(Invited) Intramolecular Reactions for Gas-Phase Formation of Carbon-Entrapped Clusterfullerenes

Fullerenes that encapsulate clusters of atoms represent a fundamental interest in chemistry, materials, and carbon science due to their unique properties and nanoscale structures. Numerous cages that feature a combination of carbon, metal, and heteroatom-based clusters have been discovered since, for example, Sc3NC@C80 was first reported. Nanocarbon reactions that underlie formation of such compounds, however, are not well understood. Here, we experimentally investigate intramolecular reactions of metallofullerenes in the gas phase by means of laser-based techniques, analyzed by high magnetic field Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. A variety of structurally defined clusterfullerene cages (e.g., Sc3N@C68, Sc3N@C78, etc.) are probed under energetic conditions to decipher reactions that may lead to encapsulation of C atoms, as well as clusterfullerenes that include metal-loss products. An aim of the present work is to identify mechanistic processes and principles that guide formation of metallofullerenes that entrap carbon, metal, and heteroatom-based clusters.

Key heptagon-containing metallofullerene intermediates in the LaSc2N@D5h(6)-C80 formation pathway: A theoretical study

Endohedral metallofullerene formation mechanisms have been proposed to involve insertion/extrusion of C2 fragment and Stone-Wales transformation (SWT). The isolation and crystallographic characterization of two isomeric LaSc2N@Ih(7)-C80 and LaSc2N@Cs(hept)-C80, which are closely related via a single step SWT, are reported to provide a clue for the trimetallic nitride template (TNT) formation pathway. Herein, we investigate three heptagon-containing isomers of LaSc2N@C80 by quantum chemistry calculations based on structural interconnections, revealing these heptagon-containing TNT links for the formation of LaSc2N@C80:D5h(6)-C80. Meanwhile, electrochemical redox properties, IR and NMR spectra of LaSc2N@C80:D5h(6)-C80 and heptagon-containing TNT isomers are simulated to help experimental identification and characterization.

Quantum Chemical Insight into La2C96: Metal Carbide Fullerene La2C2@C94 versus Dimetallofullerene La2@C96.

A family of dilanthanum-containing endohedral metallofullerene La2C2n (n = 46-51) was synthesized recently. In the present work, a systematical investigation on La2C96 series including the carbide clusterfullerene form La2C2@C94 and the conventional dimetallofullerene form La2@C96 was implemented by density functional theory, combined with statistical mechanics. Three isomers, i.e., La2@D2(191838)-C96, La2C2@Cs(153479)-C94, and La2C2@C1(153491)-C94 were disclosed to be thermodynamically stable at the temperature region of endohedral metallofullerene formation. La2@D2(191838)-C96 is the prevailing isomer at low temperature, while La2C2@Cs(153479)-C94 and La2C2@C1(153491)-C94 are the most and second-most abundant isomers at high temperature. Interestingly, the highest occupied molecular orbital (HOMO) of La2C2@C1(153491)-C94 is distributed on one pole of the cage, and the lowest unoccupied molecular orbital (LUMO) of this isomer is mainly located on the equator of the cage, which can facilitate synthesis of regioselective derivatives. This work will provide useful information for further experimental identification and application of La2C96.

An atlas of endohedral Sc2S cluster fullerenes.

Structural identification is a difficult task in the study of metallofullerenes, but understanding of the mechanism of formation of these structures is a pre-requisite for new high-yield synthetic methods. Here, systematic density functional theory calculations demonstrate that metal sulfide fullerenes Sc2S@Cn have similar cage geometries from C70 to C84 and form a close-knit family of structures related by Endo-Kroto insertion/extrusion of C2 units and Stone-Wales isomerization transformations. The stabilities predicted for favoured isomers by DFT calculations are in good agreement with available experimental observations, have implications for the formation of metallofullerenes, and will aid structural identification from within the combinatorially vast pool of conceivable isomers.

Self-assembly of endohedral metallofullerenes: a decisive role of cooling gas and metal-carbon bonding.

The endohedral metallofullerene (EMF) self-assembly process in Sc/carbon vapor in the presence and absence of an inert cooling gas (helium) is systematically investigated using quantum chemical molecular dynamics simulations. It is revealed that the presence of He atoms accelerates the formation of pentagons and hexagons and reduces the size of the self-assembled carbon cages in comparison with analogous He-free simulations. As a result, the Sc/C/He system simulations produce a larger number of successful trajectories (i.e. leading to Sc-EMFs) with more realistic cage-size distribution than simulations of the Sc/C system. The main Sc encapsulation mechanism involves nucleation of several hexagons and pentagons with Sc atoms already at the early stages of carbon vapor condensation. In such proto-cages, both Sc-C σ-bonds and coordination bonds between Sc atoms and the π-system of the carbon network are present. Sc atoms are thus rather labile and can move along the carbon network, but the overall bonding is sufficiently strong to prevent dissociation even at temperatures around 2000 kelvin. Further growth of the fullerene cage results in the encapsulation of one or two Sc atoms within the fullerene. In agreement with experimental studies, an extension of the simulations to Fe and Ti as the metal component showed that Fe-EMFs are not formed at all, whereas Ti is prone to form Ti-EMFs with small cage sizes, including Ti@C28-Td and Ti@C30-C2v(3).

Bottom-up formation of endohedral mono-metallofullerenes is directed by charge transfer.

An understanding of chemical formation mechanisms is essential to achieve effective yields and targeted products. One of the most challenging endeavors is synthesis of molecular nanocarbon. Endohedral metallofullerenes are of particular interest because of their unique properties that offer promise in a variety of applications. Nevertheless, the mechanism of formation from metal-doped graphite has largely eluded experimental study, because harsh synthetic methods are required to obtain them. Here we report bottom-up formation of mono-metallofullerenes under core synthesis conditions. Charge transfer is a principal factor that guides formation, discovered by study of metallofullerene formation with virtually all available elements of the periodic table. These results could enable production strategies that overcome long-standing problems that hinder current and future applications of metallofullerenes.

Studies of Top-Down Mechanistic Processes of Fullerene and Metallofullerene Formation for Cages Greater Than Ninety Carbons

The elucidation of a mechanistic theory for fullerene formation in electric-arc plasmas, laser ablation, and interstellar space has been a subject of considerable interest over the past two decades. For many years, a “bottom-up” theory of fullerene formation from C2 units was dominant. However, in recent years evidence is emerging suggesting a “top-down mechanism” - that is, fullerenes are formed via shrinkage of giant fullerenes generated from graphene. Previously, we reported molecular structural evidence implying a top-down mechanism for the asymmetric non-IPR metal carbide metallofullerene, M2C2@C1 (51383)-C84 (M=Y,Gd) as a missing link in the mechanism for the formation of smaller, more symmetric metallofullerene cages. In this paper, we extend the study mechanism to include larger fullerenes, namely the C3v (134)C94 and D3 (85)C92 cages.

Metallofullerene and fullerene formation from condensing carbon gas under conditions of stellar outflows and implication to stardust

Carbonaceous presolar grains of supernovae origin have long been isolated and are determined to be the carrier of anomalous (22)Ne in ancient meteorites. That exotic (22)Ne is, in fact, the decay isotope of relatively short-lived (22)Na formed by explosive nucleosynthesis, and therefore, a selective and rapid Na physical trapping mechanism must take place during carbon condensation in supernova ejecta. Elucidation of the processes that trap Na and produce large carbon molecules should yield insight into carbon stardust enrichment and formation. Herein, we demonstrate that Na effectively nucleates formation of Na@C60 and other metallofullerenes during carbon condensation under highly energetic conditions in oxygen- and hydrogen-rich environments. Thus, fundamental carbon chemistry that leads to trapping of Na is revealed, and should be directly applicable to gas-phase chemistry involving stellar environments, such as supernova ejecta. The results indicate that, in addition to empty fullerenes, metallofullerenes should be constituents of stellar/circumstellar and interstellar space. In addition, gas-phase reactions of fullerenes with polycyclic aromatic hydrocarbons are investigated to probe "build-up" and formation of carbon stardust, and provide insight into fullerene astrochemistry.

Molecular dynamics study of the initial stages of catalyzed single-wall carbon nanotubes growth: force field development

Effective force fields for Ni-C interactions developed by Yamaguchi and Maruyama for the formation of metallofullerenes are modified to simulate the catalyzed growth of single-wall carbon nanotubes on Ni(n) clusters with n >20, and the reactive empirical bond order Brenner potential for C-C interactions is also revised to include the effect of the metal atoms on such interactions.

Contrasting Behaviors of Metal Fullerides C60Rhx and C60Mx (M = La, Y) in the Metallofullerene Formation by Laser Ablation

Contrasting behaviors of Rh with Y(La) in the formation of metallofullerenes by excimer laser ablation of metal fullerides C60Rhx and C60Mx (M = La, Y) were observed. For Rh, two sequences of rhodium metallofullerenes, C2nRh and C2n+1Rh with even and odd numbers of carbon atoms, are formed in both the negative and positive ion channels, but for La and Y, only the metallofullerene sequence with even numbers of carbon atoms is formed and only in the positive ion channel. Evidence of the encapsulation of La and Y atoms in fullerene cages forming endohedral fullerenes has been observed, whereas the Rh atom is most probably incorporated into the network of the fullerene cage or pseudo-fullerene forming substitutional metallofullerenes. Density functional theory calculation was performed to determine the structures of the model metallofullerenes C53Rh and C54M (M = Rh and Y). The calculated results are in general consistent with the experimental observations.

Formation of metallofullerenes by laser ablation of externally doped fullerenes C60Mx (M=Sm, Pt and Ni)

Photofragmentation of metal fullerides C60Mx (M=Sm, Pt and Ni) has been studied by excimer laser ablation–TOF mass spectrometry. Metallofullerenes of the type CnM (n<60) have been observed in both the positive and negative ionic modes, with C59M being the most prominent species. It is supposed that the metal atom is incorporated into the network of the fullerene cage to replace one carbon atom of the cage, forming substitutional metallofullerene. The occurrence of the C59M, C58M, C57M clusters in the mass spectra is confirmed by the coincidence of the intensity distribution of the mass peaks with the isotopic abundance pattern calculated from the natural abundance of isotopes of C and M. Odd-numbered high-carbon clusters are observed in our laser ablation study of all the metal fullerides in the negative ion channel. The evolution of the mass spectra of these samples with laser irradiation shots indicates that the transformation process from an externally doped fullerene to the substitutionally doped fullerene involves the loss of metal carbide, MC. The structures of metallofullerenes C2n+1M and C2nM with even and odd total numbers of atoms respectively are discussed. Formation mechanisms with the participation of odd-numbered all-carbon fullerene clusters as intermediates are supposed.

Formation of substitutional metallofullerenes by laser ablation of externally doped fullerenes C60Mx (M=Sm, Pt, Ni and Rh)

Photofragmentation study of metal fullerides C60Mx(M = Sm, Pt, Ni and Rh) by excimer laser ablation-TOF mass spectrometry shows that metallofullerenes C2nM and C2n+1M formed in both the positive and negative ionic modes. The isotopic distributions of the metallofullerenes C2nM and C2n+1M are consistent with the calculated spectra based on the natural abundance of isotopes of C and M, confirming the formation of metallofullerenes. The metal atom is supposed to be incorporated into the network of the fullerene cage to replace one carbon atom of the cage forming substitutional metallofullerene. Odd-numbered high carbon clusters are observed in our laser ablation study of all the metal fullerides in the negative ion channel. Evolution of mass spectrum with irradiation laser shots shows that the formation of the substitutionally doped fullerenes is closely related to the production of metal carbide (MC). The structures, as well as formation mechanism, of metallofullerenes C2n+1M and C2nM with even and odd numbers of total atoms respectively are discussed based on the structural optimization of the odd-numbered high carbon clusters.

Formation of odd-numbered fullerene-related species and its relation to the formation of metallofullerenes

Odd-numbered carbon clusters in the fullerene regime are formed through laser ablation of various fullerene-containing materials: organic fullerene derivatives, metal fullerides, and guest-host fullerene materials. The formation mechanism of odd-numbered “fullerene” species is briefly discussed. The structural properties are calculated with genetic algorithm and ab initio methods, and the results show that the odd-numbered fullerene species are pseudo-open-caged fullerenes. In the laser ablation of metal fullerides, substitutional or endohedral metallofullerenes are observed. A new way for metallofullerene preparation through the odd-numbered fullerene intermediates is suggested and realized experimentally.

Formation of metallofullerenes from laser ablation of lanthanum- and yttrium-containing C 60 materials

Photofragmentation of composite materials composed of M 2O 3 (M=La,Y) and ozonized C 60 precipitate as well as metal fullerides C 60M x (M=La,Y) has been studied by XeCl excimer laser ablation-TOF mass spectrometry. Evidence of the encapsulation of La and Y atoms in fullerene cages forming endohedral fullerenes has been observed in both kinds of materials. Under similar experimental conditions but with pristine C 60 replacing the ozonized C 60, no endo-metallofullerenes have been observed. Odd-numbered `fullerene' clusters have been detected in the negative ion mass spectra of these samples. Formation of endohedral metallofullerenes is supposed to be related to the odd-numbered `fullerene' intermediates.

Kinetic stability of metallofullerenes as predicted by the bond resonance energy model

Endohedral metallofullerenes consist of a negatively charged fullerene cage and one or more metal cations within it. The minimum bond resonance energy (min BRE) proved to be a useful index for predicting the degree of kinetic stability of metallofullerenes. It was found that all metallofullerenes so far extracted and structurally identified have a negatively charged carbon cage with a min BRE> − 0.100 ∣β∣. All isolable or extractable metallofullerenes are then presumed to be not only thermodynamically but also kinetically stable in this sense. It is noteworthy that some of the carbon cages employed for the formation of metallofullerenes are kinetically very unstable when they are deprived of negative charge. All molecular anions of C84 with − 1 to − 6 formal charges are predicted to be kinetically stable with a min BRE> − 0.100 ∣β∣.

Laser Ablation Mass Spectrometry of Pyrolyzed Koppers Coal-Tar Pitch: A Precursor for Fullerenes and Metallofullerenes

Pyrolyzed Koppers coal-tar pitch samples consisting mainly of polycyclic aromatic hydrocarbons were studied by direct laser vaporization Fourier transform ion cyclotron resonance and time-of-flight mass spectrometry. Fullerene cations were readily formed from these carbonaceous precursors over a wide range of low-laser ablation power densities. The laser-desorption ionization of the pyrolyzed coal-tar pitches shows the presence of a range of polyaromatic hydrocarbons some containing as many as 70 carbon atoms, which should require only small amounts of laser energy to convert them into fullerenes. Metallofullerene formation also occurs with low power laser ablation of pyrolyzed mixtures of metal oxides, metal sulfides, or metal carbonates and the Koppers coal-tar pitch. Ablation with two different laser pulse widths of 230 μs and 8 ns, at a laser wavelength of 1064 nm, shows that only the long-pulse low-power (200−2000 kW cm-2) laser radiation produces metallofullerene ions. The short-pulse high-power (1−...

Kinetic Model of Metallofullerene Formation in Contact Arc Generator

A kinetic model of the metallofullerene growth process in a contact are generator is developed. the metals chosen for the kinetic analysis were lanthan and calcium. An optimum starting composition of composite electrode was found. In addition, it was discovered that small amounts of metal impurities reduced the fullerene yield just slightly.

Formation of metallofullerenes with higher group elements

Abstract The formation of metallofullerenes was examined for 23 elements, and some experimental evidence was obtained for metallofullerenes with group-4, Zr and Hf, and group-5, Nb, elements. Relative yields of the metallofullerenes eluted at the retention time around those for M@C 82 (M=group-2 and -3 elements) were found to decrease exponentially as the group number increased. The yields for Fe and Co, namely group-8 and -9 elements, were estimated to be less than one hundredth of that for M@C 82 (M=lanthanoid)

Production of Carbon Clusters by Means of Arc Discharge and Their Application. 5. A Molecular Dynamics Simulation of the Fullerene Formation Process.

Simulations of the molecular dynamics of the fullerene formation process are described. Starting from 500 randomly distributed gas-phase carbon atoms, the formation of cage-like carbon structure is demonstrated. Combined with annealing simulations of imperfect cage structures with the intension of forming perfect C60 and C70 structures, a fullerene formation model through random-cage structure is proposed. In addition, a simulation of the molecular dynamics of endohedral metallofullerene formation is also demonstrated.