Structural and electronic studies of metal carbide clusterfullerene Sc2C2@Cs-C72

Theoretical Insights into the Metal-Nonmetal Interaction Inside M2O@C 2v (31922)-C80 (M = Sc or Gd)

The metal-nonmetal interaction is complicated but significant in organometallic chemistry and metallic catalysis and is susceptible to the coordination surroundings. Endohedral metallofullerene is considered to be an excellent model for studying metal-nonmetal interactions with the shielding effect of fullerenes. Herein, with the detection of ScGdO@C₈₀ in a previous mass spectrum, we studied the effects of metal atoms (Sc and Gd) on the metal-nonmetal interactions of the thermodynamically stable molecules M₂O@C _2v (31922)-C₈₀ (M = Sc and Gd), where metal atoms M can be the same or different, using density functional theory calculations. The inner metal atom and the fullerene cage show mainly ionic interactions with some covalent character. The Sc atom with higher electronegativity plays a greater important role in the metal-nonmetal interactions than the Gd atom. This study would be useful for the further study of the metal-nonmetal interaction.

Stabilities, Geometries, Electronic Structures, and Conversion Rules of Carbide Cluster Metallofullerenes

Since the discovery of the first carbide cluster metallofullerene (CCMF) Sc₂ C₂ @C₈₄ in 2001, CCMFs have attracted great concerns with variable structures and fascinating characteristics. To date, there are hundreds of studies on CCMFs. Crystallography studies on CCMFs are carried out by single-crystal X-ray diffraction. Theoretical calculations can also be used to study CCMFs in detail without being limited by low experimental yields. This review analyzes the stability of CCMFs reported previously, and indicates that the C₂ unit contributes a lot to their stability. At the same time, the relationship between the structures of inner carbide cluster and cage size is systematically discussed, and the four-electron transfer always occurs. Furthermore, the possible transformation rule between di-EMFs and CCMFs is indicated. Finally, an outlook regarding the future developments and applications of CCMFs is presented.

Ho2C2 Cluster with Flexible Configurations inside a Large C2(61)-C92 Cage

Carbide clusterfullerenes (CCFs) have been of great concern due to their potential applications in materials science, in which the internal carbide cluster plays vital roles in the stability and properties of CCF. However, there still remains a debate about what configuration is ideal for the internal carbide cluster. In this work, we isolated two isomers (I and II) of Ho₂C₉₄ and studied them by means of mass spectrometry, UV-vis-NIR spectroscopy, and cyclic/differential pulse voltammetry. A combined study of single-crystal X-ray diffraction (SC-XRD) and density functional theory (DFT) computation ascertains isomer-I as Ho₂C₂@C₂(61)-C₉₂, in which the Ho₂C₂ cluster displays variable configurations from planar zigzag to folded butterfly with very small distortion energy (∼10 kJ/mol). This study hence confirms that the internal carbide cluster is intrinsically flexible over a broad geometrical range in a relatively large fullerene cage, where the nanoscale compression effect is almost negligible.

Crystallographic evidence and spin activation for the Russian-doll-type metallofullerene Sc4C2@C80

We report unambiguous crystallographic evidence for the Russian-doll-type metallofullerene Sc4C2@Ih-C80. 45Sc NMR further demonstrates the tetrahedron arrangement of the Sc4C2 cluster. Moreover, the electrochemical test reveals the stable oxidation state of Sc4C2@C80. Hence, the Sc4C2@C80 cation radical was studied by electron spin resonance spectroscopy. These results provide better understanding for the previously less-explored Sc4C2@C80.

How to Stabilize a Heptagon-Containing C80 Cage by Endohedral Derivation

Nonclassical fullerene is a new member of the fullerene family. In the present work, a systematic investigation on La_xSc_3-xN@C₈₀ (x = 0-3) covering both classical and nonclassical C₈₀ cages was performed utilizing density functional theory combined with statistical mechanics. At absolute zero, LaSc₂N@Hept(6)-C_s(2)-C₈₀ with a heptagon-containing nonclassical carbon is the second most stable isomer, whereas at the temperature range of endohedral metallofullerene (EMF) formation, due to the large vibrational frequencies, LaSc₂N@Hept(6)-C_s(2)-C₈₀ is the third most abundant isomer, and its mole fraction is very low, accounting for the low experimental yield of LaSc₂N@Hept(6)-C_s(2)-C₈₀; La₂ScN@Hept(6)-C_s(2)-C₈₀, and La₃N@Hept(6)-C_s(2)-C₈₀ are the overwhelming isomers of the corresponding series, but compared with the cases of Sc₃N@C₈₀ and LaSc₂N@C₈₀, La₂ScN and La₃N clusters suffer much larger constraints from the C₈₀ cages, perhaps preventing the synthesis of La₂ScN@C₈₀ and La₃N@C₈₀ species. Because of the large mole fractions and large electron donation and back-donation of La₂ScN@Hept(6)-C_s(2)-C₈₀ and La₃N@Hept(6)-C_s(2)-C₈₀, it can be inferred that La₂ScN and La₃N clusters may be used to stabilize some other larger nonclassical fullerene cages. This work will provide useful insights into the origins of stabilization of nonclassical fullerene cages by endohedral derivation and guidelines for synthesis EMF with nonclassical cages.

Shape-adaptive single-molecule magnetism and hysteresis up to 14 K in oxide clusterfullerenes Dy2O@C72 and Dy2O@C74 with fused pentagon pairs and flexible Dy-(μ2-O)-Dy angle

Dysprosium oxide clusterfullerenes Dy2O@Cs(10528)-C72 and Dy2O@C2(13333)-C74 are synthesized and characterized by single-crystal X-ray diffraction. Carbon cages of both molecules feature two adjacent pentagon pairs. These pentalene units determine positions of endohedral Dy ions hence the shape of the Dy2O cluster, which is bent in Dy2O@C72 but linear in Dy2O@C74. Both compounds show slow relaxation of magnetization and magnetic hysteresis. Nearly complete cancelation of ferromagnetic dipolar and antiferromagnetic exchange Dy…Dy interactions leads to unusual magnetic properties. Dy2O@C74 exhibits zero-field quantum tunneling of magnetization and magnetic hysteresis up to 14 K, the highest temperature among Dy-clusterfullerenes.

Strain Release of Fused Pentagons in Fullerene Cages by Chemical Functionalization

According to the isolated pentagon rule (IPR), for stable fullerenes, the 12 pentagons should be isolated from one another by hexagons, otherwise the fused pentagons will result in an increase in the local steric strain of the fullerene cage. However, the successful isolation of more than 100 endohedral and exohedral fullerenes containing fused pentagons over the past 20 years has shown that strain release of fused pentagons in fullerene cages is feasible. Herein, we present a general overview on fused-pentagon-containing (i.e. non-IPR) fullerenes through an exhaustive review of all the types of fused-pentagon-containing fullerenes reported to date. We clarify how the strain of fused pentagons can be released in different manners, and provide an in-depth understanding of the role of fused pentagons in the stability, electronic properties, and chemical reactivity of fullerene cages.

Crystallographic characterization of Er2C2@C2(43)-C90, Er2C2@C2(40)-C90, Er2C2@C2(44)-C90, and Er2C2@C1(21)-C90: the role of cage-shape on cluster configuration

For endohedral metallofullerenes (EMFs), that is, fullerenes encapsulating metallic species, cage size is known to be an important factor for cluster configuration adoption; however, the impact of the cage shape on the cluster geometry fitting remains poorly understood. Herein, for the first time, four dierbium-carbide EMFs with C₉₀ cages, namely, Er₂C₂@C₂(43)-C₉₀, Er₂C₂@C₂(40)-C₉₀, Er₂C₂@C₂(44)-C₉₀, and Er₂C₂@C₁(21)-C₉₀, were successfully synthesized and fully characterized using a combination of mass spectrometry, single-crystal X-ray diffractometry, vis-NIR, Raman and photoluminescence spectroscopies, and cyclic voltammetry. In particular, the fullerene cages of C₂(43)-C₉₀ and C₂(44)-C₉₀ are crystallographically identified for the first time. Interestingly, the ErEr distance of the major sites in Er₂C₂@C₂(43)-C₉₀, Er₂C₂@C₂(40)-C₉₀, Er₂C₂@C₂(44)-C₉₀, and Er₂C₂@C₁(21)-C₉₀ is 3.927, 4.058, 4.172, and 4.651 Å, respectively, which increases gradually with an increase in the major axis of the cage. Moreover, the bond length of the inner C₂-unit decreases progressively with an increase in the ErEr distance, indicating that the inserted C₂-unit can serve as a molecular spring to support the strong metal-cage interactions within cages with the same size but different shapes. Hence, the role of cage shape on the cluster configuration is unveiled safely for the as-obtained Er₂C₂@C₉₀ isomers.

Encapsulation of Monometal Uranium into Fullerenes C2n (2n = 70-74): Important Ionic U4+C2n4- Characters and Covalent U-Cage Bonding Interactions

By using density functional theory calculations combined with statistical thermodynamic analyses, the stabilization performance of a series of fullerene cages C_2n (2n = 70-74) via encapsulating monometal uranium was systematically and thoroughly investigated. Results indicate that fullerene cages D_5h(8149)-C₇₀ and D_3h(14246)-C₇₄ obeying the isolated pentagon rule and C₂(10612)-C₇₂ featured with one pentalene moiety were the most promising candidates to encage uranium. Subsequent Mulliken spin density distribution and frontier molecular orbital analyses suggest that four formal electron transfer occurs from monometal U to above the carbon cages. There also exists a high degree of covalent character between the atom U and fullerenes C_2n based on Mayer bond order and quantum theory of atoms in molecule (QTAIM) analyses, indicative of the cooperative stabilization by both ionic and covalent bonding interactions. In addition, investigations on the above-mentioned U@C_2n isomers and other favorable candidates (U@C_s(8094)-C₇₀, U@C₁(10610)-C₇₂, U@C₁(13393)-C₇₄, and U@C₁(14049)-C₇₄) reveal that these isomers could be closely linked via simple C₂ addition and Stone-Wales transformation. These results will provide a systematic understanding on U-based endohedral metallofullerenes (EMFs) and also might be helpful for further exploration of EMF growth mechanisms.

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.

Facile Access to Y2C2n (2n = 92-130) and Crystallographic Characterization of Y2C2@C1(1660)-C108: A Giant Nanocapsule with a Linear Carbide Cluster

A series of giant metallofullerenes Y₂C_2n (2n = 92-130) have been successfully obtained through the treatment of the fraction enriched by 1,2-dichlorobenzene with SnCl₄. Subsequent chromatographic separation gives a pure sample with a composition of Y₂C₁₁₀. Crystallographic results reveal that this endohedral takes the carbide form, namely Y₂C₂@C₁(1660)-C₁₀₈, representing as the largest metallofullerene that has been characterized by crystallography to date. Despite the disorder of the metal cluster, the major Y₂C₂ adopts a previously predicted linear configuration, indicating that the compression of the internal cluster by the cage is almost negligible in this giant cage. Electrochemical studies suggest that Y₂C₂@C₁(1660)-C₁₀₈ is a good electron donor instead of an electron acceptor.

Scandium carbides/cyanides in the boron cage: computational prediction of X@B80 (X = Sc2C2, Sc3C2, Sc3CN and Sc3C2CN)

As the first study on metal carbide/cyanide boron clusterfullerenes, the geometries, energies, stabilities and electronic properties of four novel scandium cluster-containing B80 buckyball derivatives, namely Sc2C2@B80, Sc3C2@B80, Sc3CN@B80 and Sc3C2CN@B80, were investigated by means of density functional theory computations. The rather favorable binding energies, which are very close to those of the experimentally abundant carbon fullerene analogues, suggest a considerable possibility to realize these doped boron clusterfullerenes. Their intracluster and cluster-cage bonding natures were thoroughly revealed by various theoretical approaches. In contrast to carbon clusterfullerenes, in which the encaged non-metal atoms mainly play a stabilizing role in the metal clusters, the encapsulated carbon and nitrogen atoms inside the B80 cage covalently bond to the boron framework, resulting in strong cluster-cage interactions. Furthermore, infrared spectra and (11)B nuclear magnetic resonance spectra were simulated and fingerprint peaks were proposed to assist future experimental characterization.

Eu@C72: Computed Comparable Populations of Two Non-IPR Isomers

Relative concentrations of six isomeric Eu@C 72 -one based on the IPR C 72 cage (i.e., obeying the isolated-pentagon rule, IPR), two cages with a pentagon-pentagon junction (symmetries C 2 and C 2 v ), a cage with one heptagon, a cage with two heptagons, and a cage with two pentagon-pentagon fusions-are DFT computed using the Gibbs energy in a broad temperature interval. It is shown that the two non-IPR isomers with one pentagon-pentagon junction prevail at any relevant temperature and exhibit comparable populations. The IPR-satisfying structure is disfavored by both energy and entropy.

An atlas of endohedral Sc2S cluster fullerenes

Low-energy Sc₂S@C_n isomers are connected by an intricate web of Stone–Wales isomerization and Endo–Kroto C₂ insertions, giving clues to their formation.

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.

Endohedrally stabilized C70 isomer with fused pentagons characterized by crystallography

In this study, we report for the first time, the X-ray structure of an unconventional endohedral C₇₀ fullerene, Sc₂O@C₂(7892)-C₇₀.

Beyond the Butterfly: Sc2C2@C(2v)(9)-C86, an Endohedral Fullerene Containing a Planar, Twisted Sc2C2 Unit with Remarkable Crystalline Order in an Unprecedented Carbon Cage

The synthesis, isolation, and characterization of a new endohedral fullerene, Sc2C88, is reported. Characterization by single crystal X-ray diffraction revealed that it is the carbide Sc2C2@C(2v)(9)-C86 with a planar, twisted Sc2C2 unit inside a previously unseen C(2v)(9)-C86 fullerene cage.

Theoretical Insight into the Ambiguous Endohedral Metallofullerene Er3C74: Covalent Interactions among Three Lanthanide Atoms

All of C74-based endohedral metallofullerenes (EMFs) are found to be monometallofullerenes with the same D3h(14246)-C74 cage so far. An opening question is whether other C74 cages could survive during the production of some novel C74-EMFs. Theoretically, we studied the trimetallic endohedral fullerene Er3C74, the existence of which had been proven without any further characterizations. Two thermodynamically stable Er3C74 isomers were obtained, both of which could be expressed as Er3@C74, meaning that previously synthesized Er3C74 is indeed an endohedral trierbium fullerene. Besides the isomer with well-known D3h(14246)-C74 cage which obeys isolated pentagon rule (IPR), another one possesses the C1(13771)-C74 cage with two adjacent pentagons. Notably, it is the first time an endohedral metallofullerene containing the C1(13771)-C74 cage has been reported. Frontier orbitals analysis, bonding analysis in terms of quantum theory of atoms-in-molecule (QTAIM) and Mayer bond order, together with two-dimensional maps of electron localization function (ELF) and Laplacian of electron density of Er3@D3h(14246)-C74 and Er3@C1(13771)-C74 show obvious covalent interactions not only between metallic atoms and carbon cage but also among three erbium atoms. Finally, simulated IR spectra of Er3@D3h(14246)-C74 and Er3@C1(13771)-C74 were simulated, which should be useful to distinguish those two isomers.

Sc2S@C68: an obtuse di-scandium sulfide cluster trapped in a C2v fullerene cage

Density functional theory calculations combined with statistical thermodynamics treatments revealed that the mass-spectrum detected Sc₂S@C₆₈ should possess the C_2v(6073)-C₆₈ cage.