Spin-flow vibrational spectroscopy of molecules with flexible spin density: electrochemistry, ESR, cluster and spin dynamics, and bonding in TiSc2N@C80

Electronic Structure, Aromaticity, and Magnetism of Minimum-Sized Regular Dodecahedral Endohedral Metallofullerenes Encapsulating Rare Earth Atoms

A series of minimally sized regular dodecahedron-embedded metallofullerene REC20 clusters (RE = Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, and Gd) as basic units of nanoassembled materials with tunable magnetism and UV sensitivity have been explored using density functional theory (DFT). The contribution of the 4f orbital of the rare earth atom at the center of the C20 cage to the frontier molecular orbital of REC20 gives the REC20 cluster additional stability. The AdNDP orbitals of the four REC20 superatoms that conform to the spherical jellium model indicate that through natural population analysis and spin density diagrams, we observe a monotonic increase in the magnetic moment from Ce to Gd. This is attributed to the increased number of unpaired electrons in the 4f orbitals of lanthanide rare earth atoms. The UV-visible spectrum of REC20 clusters shows strong absorption in the mid-UV and near-UV bands. REC20 clusters encapsulating lanthanide rare earth atoms stand out for their tunable magnetism, UV sensitivity, and stability, making them potential new self-assembly materials.

Endohedral metallofullerene molecular nanomagnets

Magnetic lanthanide (Ln) metal complexes exhibiting magnetic bistability can behave as molecular nanomagnets, also known as single-molecule magnets (SMMs), suitable for storing magnetic information at the molecular level, thus attracting extensive interest in the quest for high-density information storage and quantum information technologies. Upon encapsulating Ln ion(s) into fullerene cages, endohedral metallofullerenes (EMFs) have been proven as a promising and versatile platform to realize chemically robust SMMs, in which the magnetic properties are able to be readily tailored by altering the configurations of the encapsulated species and the host cages. In this review, we present critical discussions on the molecular structures and magnetic characterizations of EMF-SMMs, with the focus on their peculiar molecular and electronic structures and on the intriguing molecular magnetism arising from such structural uniqueness. In this context, different families of magnetic EMFs are summarized, including mononuclear EMF-SMMs wherein single-ion anisotropy is decisive, dinuclear clusterfullerenes whose magnetism is governed by intramolecular magnetic interaction, and radical-bridged dimetallic EMFs with high-spin ground states that arise from the strong ferromagnetic coupling. We then discuss how molecular assemblies of SMMs can be constructed, in a way that the original SMM behavior is either retained or altered in a controlled manner, thanks to the chemical robustness of EMFs. Finally, on the basis of understanding the structure-magnetic property correlation, we propose design strategies for high-performance EMF-SMMs by engineering ligand fields, electronic structures, magnetic interactions, and molecular vibrations that can couple to the spin states.

Orientational Effects on the Electronic Structure and Polarization in Sc3N@C80

Of particular interest in metal encapsulating fullerenes such as Sc₃N@C₈₀ is not just how the charge is transferred between atoms of the metal nitride core and the carbon cage but how the orientation of the core impacts the electronic structure of the entire molecule. A channel for the electron backdonation is identified which leads to a charge hole on the fullerene cage, just below the valence level, which is pinned to the orientation of the metal nitride. This electron hole is overcompensated by paired electron charge at deeper levels. Reorienting the metal nitride inside the cage results in a change in the surface charge distribution below the valence level and the lowest energy empty levels coupled to the metal ions. The charge separation between the metal nitride core and the carbon cage drives changes in the polarizability of the molecule depending on the orientation of the core. Furthermore, it is found that the electron affinity depends on the orientation of the metal nitride core. This is the result of the overlap between the scandium ions' d-orbitals with the fullerene cage 6:6:6 p orbitals. The electronic and geometric structures of the Sc₃N@C₈₀ metallofullerene are examined by using the density functional theory, and the findings are corroborated by an analysis of the density of states combined with charge density plots.

Potential molecular semiconductor devices: cyclo-Cn (n = 10 and 14) with higher stabilities and aromaticities than acknowledged cyclo-C18

Theoretical calculations reveal that the zero-dimensional allotropes of carbon atoms, cyclo-C_n (n = 10 and 14), have higher thermodynamic, kinetic, optical, and dynamic stabilities and aromaticity than the acknowledged cyclo-C₁₈.

When metal clusters meet carbon cages: endohedral clusterfullerenes

Fullerenes have the characteristic of a hollow interior, and this unique feature triggers intuitive inspiration to entrap atoms, ions or clusters inside the carbon cage in the form of endohedral fullerenes. In particular, upon entrapping an otherwise unstable metal cluster into a carbon cage, the so-called endohedral clusterfullerenes fulfil the mutual stabilization of the inner metal cluster and the outer fullerene cage with a specific isomeric structure which is often unstable as an empty fullerene. A variety of metal clusters have been reported to form endohedral clusterfullerenes, including metal nitrides, carbides, oxides, sulfides, cyanides and so on, making endohedral clusterfullerenes the most variable and intriguing branch of endohedral fullerenes. In this review article, we present an exhaustive review on all types of endohedral clusterfullerenes reported to date, including their discoveries, syntheses, separations, molecular structures and properties as well as their potential applications in versatile fields such as biomedicine, energy conversion, and so on. At the end, we present an outlook on the prospect of endohedral clusterfullerenes.

Fused-Pentagon-Configuration-Dependent Electron Transfer of Monotitanium-Encapsulated Fullerenes

We introduce monotitanium-based endohedral metallofullerenes (EMFs) using density functional theory calculations. Isomeric C₆₄ fullerenes are initially employed as hosts, and Ti@C₆₄ species show novel features on the electronic structures. Energetically, the preference of titanium residing on triple-fused-pentagon subunits is proposed in theory. More importantly, different from current knowledge on mono-EMFs, electron transfer between titanium and carbon cages is not unified but is essentially dependent on the pentagon distribution of the binding sites, giving rise to variations of the cationic titanium of Ti@C₆₄. Such selective electron-transfer character is extended to the study of the encapsulation of other neighboring metal atoms (i.e., calcium and scandium). Because of their different capabilities to accept d electrons, fullerene cages with distinct fused-pentagon motifs show selective metal encapsulation characters. In addition, some other fullerenes (C₄₄-C₄₈ and C₈₂) are selected as hosts to study the electron-transfer behavior of titanium in smaller fullerenes and larger systems without pentagon adjacency.

Awaking N-hyperfine couplings in charged yttrium nitride endohedral fullerenes

Charged yttrium nitride endohedral fullerenes show particular N-hyperfine couplings that are sensitive to the outer carbon cage.

Synthesis and Isolation of the Titanium-Scandium Endohedral Fullerenes-Sc2 TiC@Ih -C80 , Sc2 TiC@D5h -C80 and Sc2 TiC2 @Ih -C80 : Metal Size Tuning of the Ti(IV) /Ti(III) Redox Potentials

The formation of endohedral metallofullerenes (EMFs) in an electric arc is reported for the mixed-metal Sc-Ti system utilizing methane as a reactive gas. Comparison of these results with those from the Sc/CH4 and Ti/CH4 systems as well as syntheses without methane revealed a strong mutual influence of all key components on the product distribution. Whereas a methane atmosphere alone suppresses the formation of empty cage fullerenes, the Ti/CH4 system forms mainly empty cage fullerenes. In contrast, the main fullerene products in the Sc/CH4 system are Sc4 C2 @C80 (the most abundant EMF from this synthesis), Sc3 C2 @C80 , isomers of Sc2 C2 @C82 , and the family Sc2 C2 n (2 n=74, 76, 82, 86, 90, etc.), as well as Sc3 CH@C80 . The Sc-Ti/CH4 system produces the mixed-metal Sc2 TiC@C2 n (2 n=68, 78, 80) and Sc2 TiC2 @C2 n (2 n=80) clusterfullerene families. The molecular structures of the new, transition-metal-containing endohedral fullerenes, Sc2 TiC@Ih -C80 , Sc2 TiC@D5h -C80 , and Sc2 TiC2 @Ih -C80 , were characterized by NMR spectroscopy. The structure of Sc2 TiC@Ih -C80 was also determined by single-crystal X-ray diffraction, which demonstrated the presence of a short Ti=C double bond. Both Sc2 TiC- and Sc2 TiC2 -containing clusterfullerenes have Ti-localized LUMOs. Encapsulation of the redox-active Ti ion inside the fullerene cage enables analysis of the cluster-cage strain in the endohedral fullerenes through electrochemical measurements.

Entrapping a Group-VB Transition Metal, Vanadium, within an Endohedral Metallofullerene: V(x)Sc(3-x)N@I(h)-C80 (x = 1, 2)

So far the entrapped metals for the isolated endohedral metallofullerenes (EMFs) are primarily limited to rare earth metals, whereas except group-IVB metals, whether it is possible to entrap other d-block transition metals remains unclear. Herein we report the successful entrapment of the group-VB transition metal vanadium(V) into fullerene cage, affording the heretofore unknown V-containing EMFs. Two novel V-containing EMFs--V(x)Sc(3-x)N@C80 (x = 1, 2)--were isolated, and their molecular structures were unambiguously determined by X-ray crystallography to be I(h)(7)-C80 cage entrapping the planar VSc2N/V2ScN clusters. V(x)Sc(3-x)N@I(h)(7)-C80 (x = 1, 2) were further characterized by UV-vis-NIR and ESR spectroscopies and electrochemistry, revealing that the electronic and magnetic properties of V(x)Sc(3-x)N@I(h)(7)-C80 (x = 1, 2) are tunable upon varying the number of entrapped V atoms (i.e., x value). The molecular structures and electronic properties of V(x)Sc(3-x)N@I(h)(7)-C80 (x = 1, 2) were further compared with those of the reported analogous EMFs based on lanthanide metals and the adjacent group-IVB transition metal Ti, revealing the peculiarity of the group-VB transition metal V-based EMFs.

Methane as a Selectivity Booster in the Arc-Discharge Synthesis of Endohedral Fullerenes: Selective Synthesis of the Single-Molecule Magnet Dy2TiC@C80 and Its Congener Dy2TiC2@C80

The use of methane as a reactive gas dramatically increases the selectivity of the arc‐discharge synthesis of M‐Ti‐carbide clusterfullerenes (M=Y, Nd, Gd, Dy, Er, Lu). Optimization of the process parameters allows the synthesis of Dy₂TiC@C₈₀‐I and its facile isolation in a single chromatographic step. A new type of cluster with an endohedral acetylide unit, M₂TiC₂@C₈₀, is discovered along with the second isomer of M₂TiC@C₈₀. Dy₂TiC@C₈₀‐(I,II) and Dy₂TiC₂@C₈₀‐I are shown to be single‐molecule magnets (SMM), but the presence of the second carbon atom in the cluster Dy₂TiC₂@C₈₀ leads to substantially poorer SMM properties.

Electron-spin excitation by implanting hydrogen into metallofullerene: the synthesis and spectroscopic characterization of Sc4C2H@I(h)-C80

A hydrogen-containing endohedral metallofullerene, Sc4C2H@Ih-C80, was synthesized and its paramagnetism was measured by the low-temperature EPR spectrometry with a broad EPR signal. The electron-spin excitation by implanting the H-atom would be a new method for preparation of stable paramagnetic metallofullerenes.

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.

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 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.

Current status and future developments of endohedral metallofullerenes

Endohedral metallofullerenes (EMFs), a new class of hybrid molecules formed by encapsulation of metallic species inside fullerene cages, exhibit unique properties that differ distinctly from those of empty fullerenes because of the presence of metals and their hybridization effects via electron transfer. This critical review provides a balanced but not an exhaustive summary regarding almost all aspects of EMFs, including the history, the classification, current progress in the synthesis, extraction, isolation, and characterization of EMFs, as well as their physiochemical properties and applications in fields such as electronics, photovoltaics, biomedicine, and materials science. Emphasis is assigned to experimentally obtained results, especially the X-ray crystallographic characterizations of EMFs and their derivatives, rather than theoretical calculations, although the latter has indeed enhanced our knowledge of metal-cage interactions. Finally, perspectives related to future developments and challenges in the research of EMFs are proposed. (381 references).

Redox-active scandium oxide cluster inside a fullerene cage: spectroscopic, voltammetric, electron spin resonance spectroelectrochemical, and extended density functional theory study of Sc4O2@C80 and its ion radicals

The clusterfullerene Sc(4)O(2)@C(80) with a mixed redox state of scandium was found to be an exciting molecule for endohedral electrochemistry as demonstrated by means of an in situ electron spin resonance (ESR) spectroelectrochemical study of the spin density distribution in its electrochemically generated cation and anion radicals. The compound exhibits two reversible reduction and oxidation steps with a relatively small electrochemical gap of 1.10 V. The ESR spectra of the ion radicals have a rich hyperfine structure caused by two pairs of equivalent Sc atoms. The Sc-based hyperfine structure with large hyperfine coupling constants shows that both oxidation and reduction of Sc(4)O(2)@C(80) are in cavea redox processes, which is the subject of endohedral electrochemistry. The assignment of the experimentally determined a((45)Sc) values to the two types of Sc atoms in the Sc(4)O(2) cluster was accomplished by extended density functional theory and molecular dynamics simulations. Sc atoms adopting a divalent state in the neutral Sc(4)O(2)@C(80) exhibited an especially large coupling constant of 150.4 G in the cation radical, which is the record high a((45)Sc) value for Sc-based endohedral metallofullerenes. Such a high value is explained by the nature of the highest occupied molecular orbital (HOMO) localized on the six-atom Sc(4)O(2) cluster. This HOMO is a Sc-Sc bonding MO and hence has large contributions from the 4s atomic orbitals of Sc(II). We claim that ESR spectroelectrochemistry is an invaluable experimental tool in the studies of metal-metal bonding in endohedral metallofullerenes and in endohedral electrochemistry.

Bonding between strongly repulsive metal atoms: an oxymoron made real in a confined space of endohedral metallofullerenes

Endohedral metallofullerenes (EMFs) are able to encapsulate up to four metal atoms. In EMFs, metal atoms are positively charged because of the electron transfer from the endohedral metal atoms to the carbon cage. It results in the strong Coulomb repulsion between the positively charged ions trapped in the confined inner space of the fullerene. At the same time, in many EMFs, such as Lu(2)@C(76), Y(2)@C(79)N, M(2)@C(82) (M = Sc, Y, Lu, etc.), Y(3)@C(80), or Sc(4)O(2)@C(80), metals do not adopt their highest oxidation states, thus yielding a possibility of the covalent metal-metal bonding. In some other EMFs (e.g., La(2)@C(80)), metal-metal bonding evolves as the result of the electrochemical or chemical reduction, which leads to the population of the metal-based LUMO with pronounced metal-metal bonding character. This article highlights different aspects of the metal-metal bonding in EMFs. It is concluded that the valence state of the metal atoms in dimetallofullerenes is not dependent on their third ionization potential, but is determined by their ns(2)(n- 1)d(1)→ns(1)(n- 1)d(2) excitation energies. Peculiarities of the metal-metal bonding in EMFs are described in terms of molecular orbital analysis as well as topological approaches such as Quantum Theory of Atoms in Molecules and Electron Localization Function. Interplay of Coulomb repulsion and covalent bonding is analyzed in the framework of the Interacting Quantum Atom approach.

Titanium/yttrium mixed metal nitride clusterfullerene TiY2N@C80: synthesis, isolation, and effect of the group-III metal

Titanium/yttrium mixed metal nitride clusterfullerene (MMNCF) TiY(2)N@C(80) has been successfully synthesized, representing the first Ti-containing non-scandium MMNCF. TiY(2)N@C(80) has been isolated by multistep HPLC and characterized by various spectroscopies in combination with DFT computations. The electronic absorption property of TiY(2)N@C(80) was characterized by UV-vis-NIR spectroscopy, indicating the resemblance to that of TiSc(2)N@C(80) with broad shoulder absorptions. The optical band gap of TiY(2)N@C(80) (1.39 eV) is very close to that of TiSc(2)N@C(80) (1.43 eV) but much smaller than that of Y(3)N@C(80)(I(h), 1.58 eV). Such a resemblance of the overall absorption feature of TiY(2)N@C(80) to TiSc(2)N@C(80) suggests that TiY(2)N@C(80) has a similar electronic configuration to that of TiSc(2)N@C(80), that is, (TiY(2)N)(6+)@C(80)(6-). FTIR spectroscopic study and DFT calculations accomplish the assignment of the C(80):I(h) isomer to the cage structure of TiY(2)N@C(80), with the C(1) conformer being the lowest energy structure, which is different from the C(s) conformer assigned to TiSc(2)N@C(80). The electrochemical properties of TiY(2)N@C(80) were investigated by cyclic voltammetry, revealing the reversible first oxidation and first reduction step with E(1/2) at 0.00 and -1.13 V, respectively, both of which are more negative than those of TiSc(2)N@C(80), while the electrochemical energy gap of TiY(2)N@C(80) (1.11 V) is almost the same as that of TiSc(2)N@C(80) (1.10 V). Contrary to the reversible first reduction step, the second and third reduction steps of TiY(2)N@C(80) are irreversible, and this redox behavior is dramatically different from that of TiSc(2)N@C(80), which shows three reversible reduction steps, indicating the strong influence of the encaged group-III metal (Y or Sc) on the electronic properties of TiM(2)N@C(80) (M = Y, Sc).

Organometallic complexes of graphene: toward atomic spintronics using a graphene web

Graphene|metal|ligand systems open a new realm in surface magnetochemistry. We show that by trapping metal atoms in the two-dimensional potential lattice of a graphene-ligand interface it is possible to build a chemical analogue of an optical lattice, a key setup in quantum information and strongly correlated systems. Employing sophisticated first-principles calculations, we studied electronic and dynamic properties of graphene|metal|ligand assemblies and showed that there is a general principle--spin-charge separation in π-d systems--that underlies the possibility of synthesizing and controlling such systems. We find that ligands can work as a local gate to control the properties of trapped metal atoms and can impose bosonic or fermionic character on such atomic nets, depending on the ligand's nature. Remarkably, the magnetization energy in such systems reaches record-high values of ca. 400 meV, which makes the respective magnetic phenomena utilizable at room temperature. Accompanied by spin polarization of the graphene π-conjugated system it leads to spin-valve materials and brings the realization of quantum computing one step closer.

A molecular switch based on current-driven rotation of an encapsulated cluster within a fullerene cage

By scanning tunneling microscopy imaging and electronic structure theory, we investigate a single-molecule switch based on tunneling electron-driven rotation of a triangular Sc3N cluster within an icosahedral C80 fullerene cage among three pairs of enantiomorphic configurations. Bias-dependent action spectra and modeling implicate the antisymmetric stretch vibration of Sc3N cluster as the gateway for energy transfer from the tunneling electrons into the cluster rotation. Hierarchical switching of conductivity among multiple stationary states while maintaining a constant molecular shape, offers an advantage for the integration of endohedral fullerene-based single-molecule switches into multiple logic state molecular devices.

A Multibox Splitting Scheme: Robust Approximation For ab Initio Molecular Dynamics

In this work, the multibox (M-box) simulation scheme is introduced, which can be considered as a generalization of the QM/MM scheme for multifragment (molecular) systems. This scheme exploits the natural locality of multifragment molecular-based systems by mapping the system into force-coupled block subspaces. Where defined in this way, the entire system can be fully modeled under a quantum mechanical force field. This allows the description of each subspace explicitly by means of a robust electronic structure theory without the requirement for large computational resources. An adequate block-to-block coupling by means of shared subsystem fragments is applied to preserve the long-distance structural correlation in the system during a molecular dynamic (MD) simulation. Since electronic structure descriptions play a central role in the formulation of several parametric models for charge or energy transport, we expect that this space-time correlated scheme can become a reliable computational tool for charge/energy transport/transfer applications. The efficiency of the method is demonstrated by performing statistical and time-resolved analysis using both the multifragment box and full ab initio approaches. We illustrate the method using as examples the melting process of a one-dimensional benzene chain (weak interaction situation) and NVE dynamics for the CnHn polymeric chain (strong interaction situation). We also have extended the threshold of applicability of our model, demonstrating how it can cope with MD simulation with more complex systems and processes.

Fullerenes encaging metal clusters--clusterfullerenes

Clusterfullerenes represent a novel branch of endohedral fullerenes, which are characterized by a robust fullerene cage with metal clusters encaged in its hollow. Since the discovery of nitride clusterfullerenes (NCFs) in 1999, the family of clusterfullerenes has been significantly expanded within the past decade, with new members including carbide clusterfullerenes (CCFs), hydrocarbide clusterfullerenes (HCCFs), oxide clusterfullerenes (OCFs), sulfide clusterfullerenes (SCFs), and carbonitride clusterfullerenes (CNCFs). We first present the classification of clusterfullerenes and list all the clusterfullerenes reported to date. For each type of clusterfullerenes, we review in detail their synthesis, separation, intriguing molecular structures and properties. For NCFs, as the first and most important clusterfullerenes, we point out the significance of their discovery and focus on their new synthesis and separation methods as well as the new advances. Finally the potential applications of clusterfullerenes are addressed. We conclude that clusterfullerenes appear to be the fastest growing family of endohedral fullerenes up to now, and emphasize the importance of exploring new structures and chemical functionalizations of clusterfullerenes.