Scandium carbide/cyanide alloyed cluster inside fullerene cage: synthesis and structural studies of Sc3(μ3-C2)(μ3-CN)@Ih-C80

A new Sc3(C2)(CN)@Ih-C80 metallofullerene encaging a scandium carbide/cyanide alloyed cluster was prepared and investigated. Sc3(C2)(CN)@Ih-C80 was synthesized by the arc-discharging method and isolated by HPLC. Its experimental (13)C NMR spectrum with two signals clearly confirms an icosahedral C80 cage, and theoretically calculated (13)C NMR peaks agree well with the experimental results. Further, theoretical calculations disclosed that the endohedral μ3-C2 and μ3-CN moieties are situated on each side of the triangular shaped Sc3 unit to form a scandium carbide/cyanide alloyed cluster. Kohn-Sham molecular orbitals reveals the electronic structure of (Sc(3+))3(C2)(2-)(CN)(-)@C80(6-), in which two anions, μ3-C2(2-) and μ3-(CN)(-), construct and stabilize this special molecule together. The FTIR and Raman spectra of Sc3(C2)(CN)@Ih-C80 were analyzed by comparison of the experimental and calculated results to further confirm its structure and to uncover cluster-based vibrational modes.

Gadolinium-containing endohedral fullerenes: structures and function as magnetic resonance imaging (MRI) agents

Gadolinium-containing endohedral fullerenes represent a new class of effective relaxation agents for magnetic resonance imaging (MRI). The range of different structures possible for this class of molecules and their properties as MRI agents are reviewed here.

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).

Synthesis and study of hybrid hydrogen-bonded bent-core liquid crystal complexes containing C60- and Si-based proton donors

Mesophasic and electro-optical properties can be manipulated via the ratio of Si- and C₆₀-based moieties in H-bonded bent-core LC complexes.

Prato and Bingel–Hirsch cycloaddition to heptagon-containing LaSc2N@Cs(hept)-C80: importance of pentalene units

In heptagon-containing LaSc₂N@C₈₀both thermodynamically controlled Prato addition and kinetically-controlled Bingel–Hirsch reaction are predicted to functionalize pentalene fragments.

Regioselective cage opening of La2 @D2 (10611)-C72 with 5,6-diphenyl-3-(2-pyridyl)-1,2,4-triazine

The thermal reaction of the endohedral metallofullerene La2 @D2 (10611)-C72 , which contains two pentalene units at opposite ends of the cage, with 5,6-diphenyl-3-(2-pyridyl)-1,2,4-triazine proceeded selectively to afford only two bisfulleroid isomers. The molecular structure of one isomer was determined using single-crystal X-ray crystallography. The results suggest that the [4+2] cycloaddition was initiated in a highly regioselective manner at the C-C bond connecting two pentagon rings of C72 . Subsequent intramolecular electrocyclization followed by cycloreversion resulted in the formation of an open-cage derivative having three seven-membered ring orifices on the cage and a significantly elongated cage geometry. The reduction potentials of the open-cage derivatives were similar to those of La2 @D2 -C72 whereas the oxidation potentials were shifted more negative than those of La2 @D2 -C72 . These results point out that further oxidation could occur easily in the derivatives.

Unprecedented chemical reactivity of a paramagnetic endohedral metallofullerene La@C(s)-C82 that leads hydrogen addition in the 1,3-dipolar cycloaddition reaction

Synthesizing unprecedented diamagnetic adducts of an endohedral metallofullerene was achieved by using 1,3-dipolar cycloaddition reaction of paramagnetic La@C(s)-C82 with a simultaneous hydrogen addition. The selective formation of two main products, La@C(s)-C82HCMe2NMeCHPh (2a and 2b), was first detected by HPLC analysis and MALDI-TOF mass spectrometry. 2a and 2b-O, which was readily formed by the oxidation of 2b, were isolated by multistep HPLC separation and were fully characterized by spectroscopic methods, including 1D and 2D-NMR, UV-vis-NIR measurements and electrochemistry. The hydrogen atom was found to be connected to the fullerene cage directly in the case of 2a, and the redox behavior indicated that the C-H bond can still be readily oxidized. The reaction mechanism and the molecular structures of 2a and 2b were reasonably proposed by the interplay between experimental observations and DFT calculations. The feasible order of the reaction process would involve a 1,3-dipolar cycloaddition followed by the hydrogen addition through a radical pathway. It is concluded that the characteristic electronic properties and molecular structure of La@C(s)-C82 resulted in a site-selective reaction, which afforded a unique chemical derivative of an endohedral metallofullerene in high yields. Derivative 2a constitutes the first endohedral metallofullerene where the direct linking of a hydrogen atom has been structurally proven.

Small endohedral metallofullerenes: exploration of the structure and growth mechanism in the Ti@C2n (2n = 26-50) family

Analysis of the structure and the bottom-up growth mechanism in the family of small endohedral metallofullerenes Ti@C_2n (2n = 26–50).

The formation of the smallest fullerene, C₂₈, was recently reported using gas phase experiments combined with high-resolution FT-ICR mass spectrometry. An internally located group IV metal stabilizes the highly strained non-IPR C₂₈ cage by charge transfer (IPR = isolated pentagon rule). Ti@C₄₄ also appeared as a prominent peak in the mass spectra, and U@C₂₈ was demonstrated to form by a bottom-up growth mechanism. We report here a computational analysis using standard DFT calculations and Car–Parrinello MD simulations for the family of the titled compounds, aiming to identify the optimal cage for each endohedral fullerene and to unravel key aspects of the intriguing growth mechanisms of fullerenes. We show that all the optimal isomers from C₂₆ to C₅₀ are linked by a simple C₂ insertion, with the exception of a few carbon cages that require an additional C₂ rearrangement. The ingestion of a C₂ unit is always an exergonic/exothermic process that can occur through a rather simple mechanism, with the most energetically demanding step corresponding to the closure of the carbon cage. The large formation abundance observed in mass spectra for Ti@C₂₈ and Ti@C₄₄ can be explained by the special electronic properties of these cages and their higher relative stabilities with respect to C₂ reactivity. We further verify that extrusion of C atoms from an already closed fullerene is much more energetically demanding than forming the fullerene by a bottom-up mechanism. Independent of the formation mechanism, the present investigations strongly support that, among all the possible isomers, the most stable, smaller non-IPR carbon cages are formed, a conclusion that is also valid for medium and large cages.

Anion radicals of isomeric [5,6] and [6,6] benzoadducts of Sc3N@C80: remarkable differences in endohedral cluster spin density and dynamics

The anion radicals of isomeric [5,6] and [6,6] Sc3N@C80 benzoadducts were studied by electron spin resonance spectroscopy, density functional theory computations, and molecular dynamics. In both compounds the rotation of the Sc3N cluster is frozen and the spin density distribution of the cluster is highly anisotropic, with hyperfine coupling constants of 9.1 and 2 × 33.3 G for the [5,6] adduct and ∼0.6 and 2 × 47.9 G for the [6,6] adduct. Remarkably, the subtle variation of the position of the exohedral group on the surface of the cage results in very pronounced changes in the spin density distribution and the dynamics of the encapsulated Sc3N cluster.

Iron-oxide-supported nanocarbon in lithium-ion batteries, medical, catalytic, and environmental applications

Owing to the three different orbital hybridizations carbon can adopt, the existence of various carbon nanoallotropes differing also in dimensionality has been already affirmed with other structures predicted and expected to emerge in the future. Despite numerous unique features and applications of 2D graphene, 1D carbon nanotubes, or 0D fullerenes, nanodiamonds, and carbon quantum dots, which have been already heavily explored, any of the existing carbon allotropes do not offer competitive magnetic properties. For challenging applications, carbon nanoallotropes are functionalized with magnetic species, especially of iron oxide nature, due to their interesting magnetic properties (superparamagnetism and strong magnetic response under external magnetic fields), easy availability, biocompatibility, and low cost. In addition, combination of iron oxides (magnetite, maghemite, hematite) and carbon nanostructures brings enhanced electrochemical performance and (photo)catalytic capability due to synergetic and cooperative effects. This work aims at reviewing these advanced applications of iron-oxide-supported nanocarbon composites where iron oxides play a diverse role. Various architectures of carbon/iron oxide nanocomposites, their synthetic procedures, physicochemical properties, and applications are discussed in details. A special attention is devoted to hybrids of carbon nanotubes and rare forms (mesoporous carbon, nanofoam) with magnetic iron oxide carriers for advanced environmental technologies. The review also covers the huge application potential of graphene/iron oxide nanocomposites in the field of energy storage, biomedicine, and remediation of environment. Among various discussed medical applications, magnetic composites of zero-dimensional fullerenes and carbon dots are emphasized as promising candidates for complex theranostics and dual magneto-fluorescence imaging.

Regioselective benzyl radical addition to an open-shell cluster metallofullerene. Crystallographic studies of cocrystallized Sc3C2@Ih-C80 and its singly bonded derivative

The endohedral fullerene once erroneously identified as Sc3@C82 was recently shown to be Sc3C2@Ih-C80, the first example of an open-shell cluster metallofullerene. We herein report that benzyl bromide (1) reacts with Sc3C2@ Ih-C80 via a regioselective radical addition that affords only one isomer of the adduct Sc3C2@Ih-C80(CH2C6H5) (2) in high yield. An X-ray crystallographic study of 2 demonstrated that the benzyl moiety is singly bonded to the fullerene cage, which eliminates the paramagnetism of the endohedral in agreement with the ESR results. Interestingly, X-ray results further reveal that the 3-fold disordered Sc3C2 cluster adopts two different configurations inside the cage. These configurations represent the so-called "planar" form and the computationally predicted, but not crystallographically characterized, "trifoliate" form. It is noteworthy that this is the first crystallographic observation of the "trifoliate" form for the Sc3C2 cluster. In contrast, crystallographic investigation of a Sc3C2@Ih-C80/Ni(OEP) cocrystal, in which the endohedral persists in an open-shell structure with paramagnetism, indicates that only the former form occurs in pristine Sc3C2@ Ih-C80. These results demonstrate that the cluster configuration in EMFs is highly sensitive to the electronic structure, which is tunable by exohedral modification. In addition, the electrochemical behavior of Sc3C2@Ih-C80 has been markedly changed by the radical addition, but the absorption spectra of the pristine and the derivative are both featureless. These results suggest that the unpaired electron of Sc3C2@Ih-C80 is buried in the Sc3C2 cluster and does not affect the electronic configuration of the cage.

Endohedral fullerene with μ3-carbido ligand and titanium-carbon double bond stabilized inside a carbon cage

In all metallofullerenes known before this work, metal atoms form single highly polar bonds with non-metal atoms in endohedral cluster. This is rather surprising for titanium taking into account the diversity of organotitanium compounds. Here we show that the arc-discharge synthesis of mixed titanium-lutetium metallofullerenes in the presence of ammonia, melamine or methane unexpectedly results in the formation of TiLu2C@I(h)-C80 with an icosahedral Ih(7) carbon cage. Single-crystal X-ray diffraction and spectroscopic studies of the compound reveal an unprecedented endohedral cluster with a μ3-carbido ligand and Ti-C double bond. The Ti(IV) in TiLu2C@I(h)-C80 can be reversibly reduced to the Ti(III) state. The Ti = C bonding and Ti-localized lowest unoccupied molecular orbital in TiLu2C@Ih-C80 bear a certain resemblance to titanium alkylidenes. TiLu2C@I(h)-C80 is the first metallofullerene with a multiple bond between a metal and the central, non-metal atom of the endohedral cluster.

Endohedral metallofullerenes based on spherical I(h)-C(80) cage: molecular structures and paramagnetic properties

Fullerenes are carbon cages assembled from fused hexagons andpentagons that have closed networks and conjugated π systems. The curve of the fullerene structure requires that the constituent carbon atoms take on a pyramidal shape and produces extra strain energy. However, the highly symmetrical geometry of the fullerene decreases the surface tension in these structures, so highly symmetrical fullerenes are usually very stable. For example, C60 with icosahedral symmetry (Ih) is the most stable fullerene molecule. However, another highly symmetrical fullerene, Ih-C80, is extremely unstable. The reason for this difference is the open-shell electronic structure of Ih-C80, which has a 4-fold degenerate HOMO occupied by only two electrons. Predictably, once the degenerate HOMO of Ih-C80 accepts six more electrons, it forms a closed-shell electronic structure similar to Ih-C60 and with comparable stability. Because the hollow structure of fullerenes can encapsulate metal atoms and those internal metals can transfer electrons to the fullerene cage, the encapsulation of metal clusters may provide an ideal technique for the stabilization of the Ih-C80 fullerenes. In this Account, we focus on the molecular structures and paramagnetic properties of spherical Ih-C80 endohedral fullerenes encaging a variety of metal moieties, such as metal atoms (Mn), metal nitride (M3N), metal carbide (MnC2), metal carbonitride (M3CN), and metal oxides (M4Om). We introduce several types of endohedral metallofullerenes such as Sc4C2@Ih-C80, which exhibits a Russian-doll-like structure, and Sc3CN@Ih-C80, which encapsulates a planar metal carbonitride cluster. In addition, we emphasize the paramagnetic properties of Ih-C80-based metallofullerenes, such as Sc3C2@Ih-C80, Y2@C79N, and M3N@Ih-C80, to show how those spin-active species can present a controllable paramagnetism. This Account highlights an inspiring molecular world within the spherical Ih-C80 cages of various metallofullerenes.

Endohedral metal or a fullerene cage based oxidation? Redox duality of nitride clusterfullerenes Ce(x)M(3-x)N@C(78-88) (x = 1, 2; M = Sc and Y) dictated by the encaged metals and the carbon cage size

Redox behavior of endohedral metallofullerenes, in particular their oxidation process, can be classified as a fullerene-based or endohedral species-based process according to the mechanism of the electron transfer. Here we report on the phenomenon of the strain-driven electrochemical behavior achieved by encapsulating the cerium-containing clusters into a series of carbon cages ranging from C78 to C88. The Ce-based mixed metal nitride clusterfullerenes CexM3-xN@C2n (x = 1, 2; M = Sc or Y; 2n = 78-88) were synthesized and characterized. The magnitude of the inherent strain caused by the limited inner space of the carbon cage for the relatively large nitride clusters can be varied by choosing different scaffold metals (Sc, Lu, or Y) to tailor the size of the encaged CexM3-xN cluster and by matching the nitride cluster with different fullerene cages in the size range from C78 to C88. The redox properties of CexM3-xN@C2n were investigated by cyclic and square wave voltammetry. The mechanism of the electrochemical oxidation of Ce-based mixed metal nitride clusterfullerenes, in particular whether the fullerene-based oxidation or the Ce(III) → Ce(IV) process is observed, is found to be dependent on the scaffold metal and the size of the fullerene cage. The endohedral oxidation of Ce(III) to Ce(IV) was observed for a number of compounds as revealed by the negative shift of their oxidation potentials with respect to the values measured for the non-Ce analogues. Experimental studies are supported by DFT calculations. We conclude that the prerequisites for the Ce-based endohedral oxidation process are suitable inherent cluster-cage strain and sufficiently high oxidation potential of the fullerene cage.

A theoretical study on ionic liquid endohedral C540 fullerene

The effect of the confinement of ionic liquid (choline benzoate) cluster inside C540 fullerene has been studied through both molecular dynamic and density functional theory simulations.

Highly regioselective 1,3-dipolar cycloaddition of diphenylnitrilimine to Sc3N@Ih-C80 affording a very stable, unprecedented pyrazole-ring fused derivative of endohedral metallofullerenes

The first 1,3-dipolar cycloaddition of diphenylnitrilimine to Sc₃N@C₈₀ affords an unprecedented pyrazole-ring fused derivative of endohedral metallofullerenes in a highly regioselective manner.

The role of aromaticity in determining the molecular structure and reactivity of (endohedral metallo)fullerenes

The molecular structure and chemical reactivity of endohedral metallofullerenes can be greatly predicted and rationalized by their local and global aromaticity.

Hemicarceplex formation allows ready identification of the isomers of the metallofullerene Sc3N@C80using1H and13C NMR spectroscopy

A cyclotriveratrylene-based molecular cage forms hemicarceplexes that significantly increase the solubility of commercially available Sc₃N@C₈₀in CDCl₃.

Orientational variation, solvate composition and molecular flexibility in well-ordered cocrystals of the fullerene C70 with bis(ethylenedithio)tetrathiafulvalene

Cocrystallization of C70 with bis(ethylenedithio)tetrathiafulvalene (ET) produces two different solvates, C70·ET·C6H6 and 2C70·2ET·CS2, which show distinctly different overlap between the fullerene and ET molecules.

Metallofullerenes: a new class of MRI agents and more?

Metallofullerenes have incited research endeavors across many disciplines owing to their wide range of properties obtainable by altering the metal component inside the fullerene cage or by a variety of surface functionalities. With a metal component of gadolinium, gadofullerenes have particularly shown promise in MRI applications owing to their high proton relaxivity and isolation of the metal from the biological environment. This article aims to give a perspective on the development of metallofullerenes as MRI contrast agents and further applications that distinguish them as a new class of imaging agent.

Spectroscopy of light-molecule endofullerenes

Molecular endofullerenes are supramolecular systems consisting of fullerene cages encapsulating small molecules. Although most early examples consist of encapsulated metal clusters, recently developed synthetic routes have provided endofullerenes with non-metallic guest molecules in high purity and macroscopic quantities. The encapsulated light molecule behaves as a confined quantum rotor, displaying rotational quantization as well as translational quantization, and a rich coupling between the translational and rotational degrees of freedom. Furthermore, many encapsulated molecules display spin isomerism. Spectroscopies such as inelastic neutron scattering, nuclear magnetic resonance and infrared spectroscopy may be used to obtain information on the quantized energy level structure and spin isomerism of the guest molecules. It is also possible to study the influence of the guest molecules on the cages, and to explore the communication between the guest molecules and the molecular environment outside the cage.

An improbable monometallic cluster entrapped in a popular fullerene cage: YCN@C(s)(6)-C82

Since the first proposal that fullerenes are capable of hosting atoms, ions, or clusters by the late Smalley in 1985, tremendous examples of endohedral metallofullerenes (EMFs) have been reported. Breaking the dogma that monometallofullerenes (mono-EMFs) always exist in the form of M@C2n while clusterfullerenes always require multiple (two to four) metal cations to stabilize a cluster that is unstable as a single moiety, here we show an unprecedented monometallic endohedral clusterfullerene entrapping an yttrium cyanide cluster inside a popular C82 cage--YCN@C(s)(6)-C82. X-ray crystallography and (13)C NMR characterization unambiguously determine the cage symmetry and the endohedal cyanide structure, unexpectedly revealing that the entrapped YCN cluster is triangular. The unprecedented monometallic clusterfullerene structure unveiled by YCN@C(s)(6)-C82 opens up a new avenue for stabilizing a cluster by a single metal cation within a carbon cage, and will surely stimulate further studies on the stability and formation mechanism of EMFs.