Temperature-Dependent EPR Studies on Isolated Scandium Metallofullerenes: Sc@C82(I, II) and Sc@C84

Superatom Molecular Orbitals of Endohedral C82

Understanding superatom molecular orbital (SAMO) states in fullerene derivatives has been in the limelight ever since the first discovery of SAMOs owing to the fundamental interest in this topic as well as to the possible applications in molecular switches and other organic electronics. Nevertheless, very few reports have been published on SAMO states of larger fullerenes so far. Using density functional theory, we attempt to partially remedy this situation by presenting a study on SAMO states in C82 and its Ca and Sc endohedrally doped derivatives, comparing results with previous relevant findings for C60. We find that C82 possesses higher SAMO energies compared to C60, as associated with the symmetry of the molecule, and that endohedral doping leads to energetically favorable side positions of Ca and Sc inside the C82 cage. Among the two, Sc@C82 has more stable SAMO states compared to Ca@C82 as reflected by the shift in the density of states, while the charge states are found to be similar. In the case of the monolayer form, the pz- and 2s-SAMO orbitals overlap with the nearest neighbors, causing parabolic band dispersion with the formation of near free electron states and that the SAMO state energies move closer to the Fermi energy compared to the related molecules. These findings provide promising information about the distribution of SAMO states in C82 fullerene, which can be further relevant in studies of SAMO states of higher fullerenes and for coming applications of these systems.

Embedded nano spin sensor for in situ probing of gas adsorption inside porous organic frameworks

Spin-based sensors have attracted considerable attention owing to their high sensitivities. Herein, we developed a metallofullerene-based nano spin sensor to probe gas adsorption within porous organic frameworks. For this, spin-active metallofullerene, Sc3C2@C80, was selected and embedded into a nanopore of a pyrene-based covalent organic framework (Py-COF). Electron paramagnetic resonance (EPR) spectroscopy recorded the EPR signals of Sc3C2@C80 within Py-COF after adsorbing N2, CO, CH4, CO2, C3H6, and C3H8. Results indicated that the regularly changing EPR signals of embedded Sc3C2@C80 were associated with the gas adsorption performance of Py-COF. In contrast to traditional adsorption isotherm measurements, this implantable nano spin sensor could probe gas adsorption and desorption with in situ, real-time monitoring. The proposed nano spin sensor was also employed to probe the gas adsorption performance of a metal-organic framework (MOF-177), demonstrating its versatility. The nano spin sensor is thus applicable for quantum sensing and precision measurements.

Optically Controlled Molecular Metallofullerene Magnetism via an Azobenzene-Functionalized Metal-Organic Framework

Molecular magnets with optically controlled property have significant applications in data storage and quantum information processing. Herein, we report the optically controlled molecular magnetism of endohedral metallofullerenes, Sc₃C₂@C₈₀ and DySc₂N@C₈₀, by incarcerating them into the pores of a photoswitchable azobenzene-functionalized metal-organic framework (MOF) (^AzoMOF). After ultraviolet (365 nm) irradiation, the isomerization of azobenzene groups in the ^AzoMOF was found to be able to modulate the spin relaxation of Sc₃C₂@C₈₀ and also improve the single-molecule magnet behavior of DySc₂N@C₈₀. The photoisomerization of azobenzene side groups changes the host-guest interaction between metallofullerene and ^AzoMOF pores and endows them with the potential to modulate the magnetic properties with light. These findings offer an effective method to create smart molecular magnetic materials and also promote their applications in information recording, spintronics, and sensors.

An implanted paramagnetic metallofullerene probe within a metal-organic framework

Paramagnetic endohedral metallofullerene can be used as a molecular probe because of its sensitive electron spin characters, one of which is to sense its surroundings. Metal-organic framework (MOF) materials have significant applications in selective adsorption owing to their porous structures. Herein, we report a Sc₃C₂@C₈₀ spin probe implanted in MOF-177 to detect the unusual host-guest interaction between the guest molecules of metallofullerene and the host pores of the MOF. Paramagnetic Sc₃C₂@C₈₀ molecules were incorporated into the pores of MOF-177 via absorption method, and there was strong π-π interaction between oleophilic metallofullerene and aromatic framework. The electron paramagnetic resonance (EPR) signals of Sc₃C₂@C₈₀ in MOF-177 exhibit anisotropic properties caused by the restricted motion of implanted Sc₃C₂@C₈₀. This unusual host-guest interaction between Sc₃C₂@C₈₀ and MOF-177 is gradually strengthened with decreasing temperature as revealed by the EPR signals. In addition, the gas desorption from the MOF-177 pores under subatmospheric pressure can weaken the host-guest interaction and lead to slightly enhanced Sc₃C₂@C₈₀ EPR signals. Furthermore, the changes in the host-guest interaction between Sc₃C₂@C₈₀ and MOF-177 at different temperatures and pressures exhibit reversibility, as shown by cycling EPR measurements. These results will inspire material design and applications of fullerene and MOF complexes.

Spatial aromatic fences of metal-organic frameworks for manipulating the electron spin of a fulleropyrrolidine nitroxide radical

The electron spin properties of a fulleropyrrolidine nitroxide radical incarcerated in the pores of MOF-177 and MIL-53 respectively were investigated for the first time. It was found that the spatial confinement effect and intramolecular interactions in these two solid-state spin systems lead to dramatically distinctive spin dynamics.

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.

Chemistry of endohedral metallofullerenes: the role of metals

Recent breakthroughs achieved in the chemical functionalization of endohedral metallofullerenes (EMFs), especially single crystallographic X-ray characterizations of their derivatives, have presented fundamentally new insights into the structures and properties of these metal-carbon hybrid molecules, and have also brought immense potential applications. In particular, the interplay between the encapsulated metallic species and the fullerene cage has been well investigated. On one hand, the position and motion of the encapsulated metals can be effectively controlled by exohedral modification. On the other hand, the cage structures, the chemical behaviours of cage carbons and thus the chemical reactivity of the whole molecule are also apparently influenced by the electronic configuration and geometrical conformation of the internal metals via strong metal-cage interactions. In this article, we contribute a systematic review of the important chemical transformations of EMFs reported to date, including disilylation, 1,3-dipolar cycloaddition with ylides, cyclopropanation with carbenes and carbanions, cycloaddition with dienes and benzyne, radical reactions, and other miscellaneous reactions, in addition to noncovalent interactions such as supramolecular complexation. The roles that internal metals play in controlling the reactivity of cage carbons are particularly emphasized. Finally, some applicable materials based on EMFs and their derivatives are summarized and practical perspectives are proposed.

Metal sulfide in a C82 fullerene cage: a new form of endohedral clusterfullerenes

The row of endohedral fullerenes is extended by a new type of sulfur-containing clusterfullerenes: the metal sulfide (M(2)S) has been stabilized within a fullerene cage for the first time. The new sulfur-containing clusterfullerenes M(2)S@C(82)-C(3v)(8) have been isolated for a variety of metals (M = Sc, Y, Dy, and Lu). The UV-vis-NIR, electrochemical, and FTIR spectroscopic characterization and extended DFT calculations point to a close similarity of the M(2)S@C(82) cage isomeric and electronic structure to that of the carbide clusterfullerenes M(2)C(2)@C(2n). The bonding in M(2)S@C(82) is studied in detail by molecular orbital analysis as well as with the use of quantum theory of atom-in-molecules (QTAIM) and electron localization function (ELF) approaches. The metal sulfide cluster formally transfers four electrons to the carbon cage, and metal-sulfur and metal-carbon cage bonds with a high degree of covalency are formed. Molecular dynamics simulations show that Sc(2)S cluster exhibits an almost free rotation around the C(3) axis of the carbon cage, resulting thus in a single line (45)Sc NMR spectrum.

Controlling intermolecular spin interactions of La@C(82) in empty fullerene matrices

The ESR properties and crystal structures of solid-state La@C(82) in empty fullerene matrices were investigated by changing the concentration of La@C(82) and the species of an empty fullerene matrix: C(60), C(70), C(78)(C(2v)(3)), C(82)(C(2)) and C(84)(D(2d)(4)). The rotational correlation time of La@C(82) molecules tended to be shorter when La@C(82) is dispersed in larger fullerene matrices because large C(2n) molecules provide more space for La@C(82) molecules for rotating. La@C(82) dispersed in a hcp-C(82) matrix showed the narrowest hyperfine structure (hfs) due to the ordered nature of La@C(82) molecules in the C(82) crystal. On the other hand, in a C(60) matrix, La@C(82) molecules formed clusters because of the large different solubility, which leads to the ESR spectra being broad sloping features due to strong dipole-dipole and exchange interactions.

Dynamics of paramagnetic metallofullerenes in carbon nanotube peapods

We filled SWNTs with the paramagnetic fullerene Sc@C82 to form peapods. The interfullerene 1D packing distance measured using TEM is d = 1.1 +/- 0.02 nm. The Sc@C82 in SWNT peapods continuously rotated during the 2 s TEM exposure time, and we did not see the Sc atoms. However, Sc@C82 metallofullerenes in MWNT peapods have periods of fixed orientation, indicated by the brief observation of Sc atoms. La@C82 peapods were also prepared and their rotational behavior examined. The interfullerene 1D packing of both La@C82 and Sc@C82 peapods is identical and thus independent of the charge transfer state for these paramagnetic fullerenes. The La@C82 metallofullerenes in the peapods have fixed orientations for extended periods of time, up to 50 s in some cases. The La@C82 spontaneously rotates rapidly between fixed orientations.

Magnetic Properties and Crystal Structure of Solvent-Free Sc@C82 Metallofullerene Microcrystals

Magnetic properties of solvent-free crystals of the endohedral Sc@C82 are investigated by SQUID and X-ray powder diffraction. We find that the crystal is a paramagnet and the magnetic susceptibility decreases from 150 K with evidence of antiferromagnetic-like interactions by slow cooling. X-ray crystal analysis reveals the presence of a phase transition at 150 K, which is attributed to an orientational ordering transition of the fullerene molecules. At low temperatures we find a magnetic metastable state that can be controlled by the cooling rate. The metastable state can be formed by rapid cooling. The direction of Sc@C82 molecular axis in the crystals is disordered in the metastable state, and the susceptibility is higher than that in the slow cooling case at low temperature.