Oxide Clusterfullerenes Research Articles

Crystallographic Characterization of Lu2O@Cs(6)‐C82 and Er2O@Cs(6)‐C82: The Role of Metal Species on Cluster Configuration†

Comprehensive SummaryIntramolecular interactions are fundamental for the unique structures and outstanding properties of metallofullerenes. However, how the internal metal species interplay with the cluster configuration inside oxide clusterfullerenes remains poorly understood. Herein, we successfully captured two oxide clusterfullerenes with different metals, namely, Lu2O@C82 and Er2O@C82, to elucidate the role of metal species in tuning cluster configuration. The two molecules have been fully characterized by mass spectrometry, Vis‐NIR absorption spectroscopy, cyclic voltammetry, and single‐crystal X‐ray diffraction. Crystallographic data show that their isomeric cages were both determined as Cs(6)‐C82. Further structural analysis reveals that the Lu1‐O‐Lu2 angle value in Lu2O@Cs(6)‐C82 is much larger than the Er1‐O‐Er2 angle value in Er2O@Cs(6)‐C82, indicating that the configuration of the endohedral oxide clusters is flexible and dependent on the metal ion radius. Additionally, electrochemical studies suggest that the variation in entrapped metal species causes differences in redox properties, for which the first oxidation and first reduction potentials of Lu2O@Cs(6)‐C82 are shifted positively and negatively, respectively, compared to the counterparts of Er2O@Cs(6)‐C82, resulting in a larger electrochemical band‐gap.

Isolation and Electrochemical Property of Ho2O@C90 Isomers

Four oxide clusterfullerenes Ho2O@C90 (I–IV) were prepared by a modified arc discharge method and isolated by a multistage high performance liquid chromatography procedure. Based on their vis-NIR spectra, the carbon cage symmetries in Ho2O@C90 (I, III, IV) are confirmed to be C 2(43)-C90, C 2(40)-C90 and C 2(41)-C90, respectively. Electrochemical studies display that the first and second reduction potentials of Ho2O@C90 isomers are close to those of Er2C2@C90 ones, but much lower than those of Sm@C90 ones. Moreover, it is found that the electrochemical band gaps of Ho2O@C90 isomers are also very close to those of Er2C2@C90 isomers. A detailed investigation on the redox potentials of C90 based EMFs reveals that endohedral clusters/atoms have much greater effect than carbon cage symmetries on the electrochemical properties of EMFs.

An unprecedented C80 cage that violates the isolated pentagon rule

An unprecedented C1(31876)-C80 cage, stabilized by Lu2O encapsulation, is a key point in the transformation map of oxide cluster fullerenes, filling the missing link in the interconversion process.

Ho2O@D3(85)-C92: Highly Stretched Cluster Dictated by a Giant Cage and Unexplored Isomerization.

For endohedral metallofullerenes (EMFs), it has been well established that the cage shape and size should match those of the endohedral cluster. As a result, sufficient cluster-cage interaction can be achieved, which is essential for mutual stabilization. Nevertheless, how a small endohedral cluster nests in a giant fullerene has been less explored. Herein, we report a pair of large oxide-cluster fullerene (OCF) isomers, denoted as Ho2O@C92-I and -II. Crystallographic studies reveal that major isomer-I possesses a D3(85)-C92 cage with a highly stretched Ho2O cluster inside, which contributes to achieving regular metal-cage contacts. Density functional theory (DFT) computations also reveal the predominant abundance of the D3(85) isomer relative to the other two possible minor species including C1(67) and C2(64) isomers. Moreover, electrochemical (EC) studies verify that the isomers exhibit almost identical redox behaviors, indicating their similar cage structures. On the basis of the remarkable topological similarity of D3(85) and C1(67) isomers, isomer-II is likely to be Ho2O@C1(67)-C92, though it remains to be confirmed. Our studies thus provide new insights into the cage-cluster interplay and cage isomerization, both contributing to a better understanding of large EMFs.

Endohedral metallofullerene single molecule magnets

Nanoscale magnetic materials have been attracting enormous interests during the past few decades due to the rapid development of high-density magnetic storage devices. A breakthrough in nanomagnetism is the discovery of single molecule magnets (SMMs), which exhibit slow relaxation and quantum tunneling of the magnetization resulted from a molecular-based blocking anisotropy, and thus have potential applications in high-density data storage, molecular spintronics and quantum computing. Since the report of Mn12-complex as the first SMM by Sessoli et al. in 1993, a large number of SMMs based on organometallic coordination compounds have been synthesized. Compared to the traditional bulk magnets, the magnetic properties of SMMs are determined by their instrinsic spin structures resulting in slow magnetic relaxation. SMM is characterized by a bi-stable magnetic ground state and a certain spin-reversal barrier ( U eff). Additionally, hysteresis loops can be observed below the blocking temperature ( T B). Therefore, SMMs with excellent magnetic properties always have high T B and U eff. U eff depends strongly on spin ground state ( S ) and magneto-crystalline anisotropy ( D ), and can be expressed with the formula U eff =| D|S 2. For transition metal-based SMMs, their low magneto-crystalline anisotropies ( D ) result in relative low U eff. In recent years, lanthanide based SMMs have been studied intensively and become the most promising high-performance SMMs, owning to their strong single ion anisotropy. Encapsulating metal atoms or metallic clusters into fullerene cages generate a special class of fullerenes, the so-called endohedral metallofullerenes (EMFs), for which the encapsulation of metal atoms endow EMFs more fascinating properties compared to empty fullerenes. For instance, EMFs are paramagnetic when the entrapped metal ions have unpaired electrons. Furthermore, EMFs were able to show excellent SMM properties when the encapsulated clusters induce appropriate ligand-fields. EMFs with lanthanide open up a new avenue in the field of SMMs in 2012 when single molecule magnetism was proved for the metal nitride clusterfullerene (NCF) DySc2N@ I h-C80. Fullerene cages are stable in air and thus they can protect the magnetic ions from ambient conditions, and the negatively charged nonmetallic atom(s) within the cluster and the negatively charged carbon atoms of fullerene cages provide magnetic anisotropy which is necessary for single molecule magnetism. Moreover, the molecular structures of EMFs can be modulated by varying the encapsulated metal ions, metal cluster and cage isomer; hence the research on EMF-SMMs has been developed rapidly since 2012. Up to now, metal nitride clusterfullerenes (NCFs), metal carbide clusterfullerenes (CCFs), metal oxide clusterfullerenes (OCFs), metal sulfide clusterfullerenes (SCFs), metal cyanide clusterfullerenes (CYCFs) and dimetallofullerenes (di-EMFs) have been confirmed as SMMs. In this paper, we present an exhaustive review on all kinds of EMF-SMMs reported to date including their 1D, 2D and 3D assemblies. We mainly discuss the influences of the types and the numbers of the encapsulated metal ions and non-metal atoms on the magnetic properties of EMF-SMMs. Finally, the prospects and challenges of EMF-SMMs are also discussed.

(Invited) Trapping Metallic Oxide Cluster inside Fullerene Cage

The sub-nanometer sized void inside fullerene cage permits the accommodation of a single atom, atomic cluster or even small molecule, resulting in the formations of endohedral fullerenes. Particularly, clusterfullerenes can be formed by encapsulating multiple metallic ions in most cases along with non-metal ions (i.e., N3-, C2 2-, S2-, O2-) inside the fullerene cage. Such an association makes clusterfullerene more functional than empty fullerenes and conventional mono-metallofullerenes. Up to date, a variety of clusterfullerenes have been reported, including metal nitrides, carbides, oxides, sulfides, cyanides and so on. Among them, oxide clusterfullerenes (OCFs) can contain variable oxide clusters (i.e., M4O2, M4O3, M3O and M2O; M=Sc or other metal), yielding one of the most versatile families. Thus, OCFs may provide a more convenient platform for developing new functional molecules and for studying the previously less-explored topics such as formation mechanism of clusterfullerenes.In this Account, we review recent progress in the field of OCFs, including their synthesis, isolation, structural and electrochemical studies as well as the preliminary exploration in their potential functions and applications. Thanks for the concrete crystallographic results of an OCF series, we can track the transition of endohedral cluster and fullerene cage. It is suggested that the configuration and internal dynamics of oxide cluster are highly dependent on not only the cage size but also cage structure. On the other hand, based on the experimental observations, two competitive transformation pathways are established for the majority of OCFs, verifying the bottom-up or top-down formation mechanism. It is also found that the redox behaviors of OCFs are more or less comparable to their isoelectronic species with common cage structure and similar cluster geometry, but varied greatly with the cluster variety (i.e., Sc2O vs Sc4O2-3). The mechanism behind such phenomena has been discussed. In addition, the potential of Dy-based OCFs as single molecular magnets (SMMs) is presented theoretically. Nevertheless, experimental advance remains to be achieved. Figure 1

Ho2O@C84: Crystallographic Evidence Showing Linear Metallic Oxide Cluster Encapsulated in IPR Fullerene Cage of D2d(51591)-C84.

Fullerene C84 is the third-most-abundant species after C60 and C70. In the past decade, a variety of C84-based clusterfullerenes have been well-studied experimentally, which otherwise do not include oxide clusterfullerenes (OCFs). In this work, we report a comprehensive inspection of Ho2O@C84, including its mass, spectroscopic, crystallographic, electrochemical (EC), and density functional theory (DFT) studies. Importantly, crystallographic data reveal an IPR cage of D2d(51591)-C84 with a linear endohedral Ho-O-Ho cluster, indicating that the compression effect of the C84 cage is less pronounced compared to that of a smaller cage. The experimentally observed structure is confirmed by DFT computations, which also verify its superior stability. Further studies suggest that Ho2O@C84 has reduced EC and HOMO-LUMO gaps compared to those of empty species, again demonstrating the effect of cluster encapsulation.

Trapping Metallic Oxide Clusters inside Fullerene Cages.

The sub-nanometer sized void inside a fullerene cage permits the accommodation of a single atom, atomic cluster, or even small molecule, resulting in the formation of endohedral fullerenes. Particularly, clusterfullerenes can be formed by encapsulating multiple metallic ions in most cases along with nonmetal ions (i.e., N3-, C22-, S2-, O2-) inside the fullerene cage. Such an association makes clusterfullerene more functional than empty fullerenes and conventional mono-metallofullerenes. To date, a variety of clusterfullerenes have been reported, including metal nitrides, carbides, oxides, sulfides, cyanides, and so on. Among them, oxide clusterfullerenes (OCFs) can contain variable oxide clusters (i.e., M4O2, M4O3, M3O, and M2O; M = Sc or other metal), yielding one of the most versatile families. Thus, OCFs may provide a more convenient platform for developing new functional molecules and for studying previously less-explored topics such as formation mechanisms of clusterfullerenes. In this Account, we review recent progress in the field of OCFs, including their synthesis, isolation, and structural and electrochemical studies as well as the preliminary exploration into their potential functions and applications. Thanks to the concrete crystallographic results of an OCF series, we can track the transition of endohedral cluster and fullerene cage. It is suggested that the configuration and internal dynamics of the oxide cluster are highly dependent on not only the cage size but also cage structure. On the other hand, based on the experimental observations, two competitive transformation pathways are established for the majority of OCFs, verifying the bottom-up or top-down formation mechanism. It is also found that the redox behaviors of OCFs are more or less comparable to their isoelectronic species with common cage structure and similar cluster geometry but varied greatly with the cluster variety (i.e., Sc2O vs Sc4O2-3). The mechanism behind such phenomena has been discussed. In addition, the potential of Dy-based OCFs as single molecular magnets (SMMs) is presented theoretically. Nevertheless, experimental advance remains to be achieved.

Mixed Dimetallic Cluster Fullerenes: ScGdO@ C3v(8)-C82 and ScGdC2@ C2v(9)-C82.

Mixed-metal cluster fullerenes have been extensively studied in recent years for their rich structural variability of the encaged clusters and have shown great potential in applied studies such as biomedicine and molecular electronic devices. However, the studies in this field have mostly concentrated on the nitride cluster fullerene, and very few other types of mixed-metal cluster fullerenes have been reported so far. Herein, we report the synthesis and isolation of the first mixed-metal oxide cluster fullerene, ScGdO@C82, and a novel mixed dimetallic carbide cluster fullerene, ScGdC2@C82. Spectroscopic and electrochemical studies combined with density functional theory (DFT) calculations assigned the molecular structure of the two cluster fullerenes (CFs) to ScGdO@ C3v(8)-C82 and ScGdC2@ C2v(9)-C82, respectively. DFT calculations also suggested that these two mixed-metal clusters are likely to be found in any of the three isolated pentagon rule Cs(6)-C82, C3v(8)-C82 and/or C2v(9)-C82 cages. The electrochemical studies show that the electrochemical gap of ScGdO@ C3v(8)-C82 and ScGdC2@ C2v(9)-C82 are 1.49 and 1.08 V. Moreover, comparative studies of ScGdO@ C3v(8)-C82 and Sc2O@ C3v(8)-C82, ScGdC2@ C2v(9)-C82 and Sc2C2@ C2v(9)-C82 showed that, despite their close structural resemblance, the replacement of one Sc ion by a Gd ion resulted in notable changes in their electrochemical behaviors as well as their 45Sc NMR spectra.

Synthesis, Isolation and Characterization of Two Isomers of Dy2O@C82

Recent theoretical studies predicted that the magnetic properties of Dy2O@C82 may be superior than any other clusterfullerenes decovered so far, but the synthesis and characterization have never been reported. Here, we report that the clusterfullerenes Dy2O@C82 have been synthesized by arc-discharging method with graphite rods packed with graphite powder and metal oxide(Dy2O3) under a He/CO2(8:1) atmosphere. Two isomers of Dy2O@C82 endohedral fullerene have been isolated and purified by high performance liquid chromatography by multistage separation on different columns. The purified Dy2O@C82 were characterized by mass spectrometry, UV-Vis-NIR absorption spectroscopy. One isomer shows large resemblance to those of previously reported metallic fullerenes, such as Er2@C s-C82, Sc2O@C s-C82, and another one is similar to Y2C2@C 3v-C82. This work will lead to the detailed study of the unique molecule magnetic properties of dysprosium-based oxide cluster fullerenes and their potential of application in high density information storage, quantum calculation and spintronics.

Sc3O@Ih(7)-C80: A Trimetallic Oxide Clusterfullerene Abundant in the Raw Soot

The trimetallic oxide clusterfullerene (OCF) Sc3O@C80 has been obtained with rather high abundance in the raw soot. Most of the formed product, however, remained nonextracted in the soot so that only a small amount of it was isolated and purified. The tiny quantity of pure product acquired made only possible characterization by UV–vis-NIR spectroscopy. DFT computations predict Sc3O@Ih(7)-C80 to be the isolated isomer and provide further information about the electronic structure and other (magnetic and electrochemical) properties of this singular OCF. Significant spin density on the endohedral Sc ions and in cavea redox processes are two main features of Sc3O@Ih(7)-C80, which is isoelectronic to the anion of the prototypical nitride Sc3N@Ih(7)-C80. Polymerization is predicted to be a favored process that could explain the very low yields obtained once the product is purified.

(Invited) Butterfly inside Carbon Cage: Preparation, Structural Characterization and Chemical Properties of Novel Dimetallic Oxide Cluster Fullerenes

Fullerenes are spherical molecules composed of pentagons and hexagons. Endohedral fullerenes, which possess inner space capable of encapsulation of variable clusters and molecules, have generated considerable recent research interest. By introducing CO2 as the reactive gas source, the synthesis of extensive families of oxide clusterfullerenes was demonstrated in this study. Sc2O@C70, Sc2O@C2n (n=38-42) and mixed dimetaillic fullerenes ScGdO@C82, have been isolated and characterized by mass spectrometry, UV−Vis−NIR absorption spectroscopy, cyclic voltammetry, 45Sc NMR, DFT calculations and single crystal X-ray diffraction. Crystallographic studies unambiguously assigned the cage structures of these metallic oxide cluster fullerenes to Sc2O@C2(7892)-C70，Sc2O@T d(19151)-C76, Sc2O@D 3h(5)C78, Sc2O@C 2v(3)C78, Sc2O@C 2v(5)-C80 and Sc2O@C 3v(8)-C82. Among them, Sc2O@C2(7892)-C70 presents the first detailed structural analysis of is a Non-IPR C70 endohedral fullerene and Sc2O@T d(19151)-C76 is the first tetrahedral fullerene cage characterized by single crystal X-ray diffraction. The comparative crystallographic studies of these novel structures also demonstrate that Sc2O cluster is flexible inside the carbon cage and the Sc-O-Sc angle varies with the different cage size or symmetry. The discovery and characterization of these novel structures provide examples of stabilization of new endohedral fullerene structures resulting from the combined effect of electron transfer and the geometrical fit between Sc2O cluster and the cage.

Sc2O@C(3v)(8)-C82: A Missing Isomer of Sc2O@C82.

By introducing CO2 as the oxygen source during the arcing process, a new isomer of Sc2O@C82, Sc2O@C(3v)(8)-C82, previously investigated only by computational studies, was discovered and characterized by mass spectrometry, UV-vis-NIR absorption spectroscopy, cyclic voltammetry, (45)Sc NMR, density functional theory (DFT) calculations, and single-crystal X-ray diffraction. The crystallographic analysis unambiguously elucidated that the cage symmetry was assigned to C(3v)(8) and suggests that Sc2O cluster is disordered inside the cage. The comparative studies of crystallographic data further reveal that the Sc1-O-Sc2 angle is in the range of 131.0-148.9°, much larger than that of the Sc2S@C(3v)(8)-C82, demonstrating a significant flexibility of dimetallic clusters inside the cages. The electrochemical studies show that the electrochemical gap of Sc2O@C(3v)(8)-C82 is 1.71 eV, the largest among those of the oxide cluster fullerenes (OCFs) reported so far, well correlated with its rich abundance in the reaction mixture of OCF synthesis. Moreover, the comparative electrochemical studies suggest that both the dimetallic clusters and the cage structures have major influences on the electronic structures of the cluster fullerenes. Computational studies show that the cluster can rotate and change the Sc-O-Sc angle easily at rather low temperature.

Sc2O@C(2v)(5)-C80: Dimetallic Oxide Cluster Inside a C80 Fullerene Cage.

A new oxide cluster fullerene, Sc2O@C(2v)(5)-C80, has been isolated and characterized by mass spectrometry, UV-vis-NIR absorption spectroscopy, cyclic voltammetry, (45)Sc NMR, DFT calculations, and single crystal X-ray diffraction. The crystallographic analysis unambiguously elucidated that the cage symmetry was assigned to C(2v)(5)-C80 and suggests that the Sc2O cluster is ordered inside the cage. The crystallographic data further reveals that the Sc1-O-Sc2 angle is much larger than that found in Sc2O@T(d)(19151)-C76 but almost comparable to that in Sc2O@Cs(6)-C82, suggesting that the endohedral Sc2O unit is flexible and can display large variation in the Sc-O-Sc angle, which depends on the size and shape of the cage. Computational studies show that there is a formal transfer of four electrons from the Sc2O unit to the C80 cage, i.e., (Sc2O)(4+)@(C80)(4-), and the HOMO and LUMO are mainly localized on the C80 framework. Moreover, thermal and entropic effects are seen to be relevant in the isomer selection. Comparative studies between the recently reported Sc2C2@C(2v)(5)-C80 and Sc2O@C(2v)(5)-C80 reveal that, despite their close structural resemblance, subtle differences exist on the crystal structures, and the clusters exert notable impact on their spectroscopic properties as well as interactions between the clusters and corresponding cages.

Synthesis of an Extensive Family of Sc2o@C2n (n=35-47) and Chemical Insight into the Small-Size Species

An extensive family of oxide cluster fullerenes (OCFs) Sc2O@C2n (n=35-47) has been facilely produced for the first time by introducing CO2 as the oxygen source. Among this family, Sc2O@C70 was identified as the smallest OCF and therefore isolated and characterized by mass spectrometry, 45Sc NMR, UV-Vis-NIR absorption spectroscopy, cyclic voltammetry and DFT calculations. The combined experimental and computational studies reveal a non-IPR isomer Sc2O@C2(7892)-C70 with reversible oxidative behavior and lower bandgap relative to that of Sc2S@C2 (7892)-C70, demonstrating a typical example of unexplored OCF and underlining its cluster-dependant electronic properties. As for other OCF members, the related studies are still on the way. Figure 1

Facile Synthesis of an Extensive Family of Sc2O@C2n(n= 35–47) and Chemical Insight into the Smallest Member of Sc2O@C2(7892)–C70

An extensive family of oxide cluster fullerenes (OCFs) Sc2O@C2n (n = 35–47) has been facilely produced for the first time by introducing CO2 as the oxygen source. Among this family, Sc2O@C70 was identified as the smallest OCF and therefore isolated and characterized by mass spectrometry, 45Sc nuclear magnetic resonance, UV–vis–near-infrared absorption spectroscopy, cyclic voltammetry, and density functional theory calculations. The combined experimental and computational studies reveal a non-isolated pentagon rule isomer Sc2O@C2(7892)–C70 with reversible oxidative behavior and lower bandgap relative to that of Sc2S@C2(7892)–C70, demonstrating a typical example of unexplored OCF and underlining its cluster-dependent electronic properties.

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.