Endohedral Cage Research Articles

Exploring the stability and aromaticity of rare earth doped tin cluster MSn16- (M = Sc, Y, La).

Rare earth elements have high chemical reactivity, and doping them into semiconductor clusters can induce novel physicochemical properties. The study of the physicochemical mechanisms of interactions between rare earth and tin atoms will enhance our understanding of rare earth functional materials from a microscopic perspective. Hence, the structure, electronic characteristics, stability, and aromaticity of endohedral cages MSn16- (M = Sc, Y, La) have been investigated using a combination of the hybrid PBE0 functional, stochastic kicking, and artificial bee colony global search technology. By comparing the simulated results with experimental photoelectron spectra, it is determined that the most stable structure of these clusters is the Frank-Kasper polyhedron. The doping of atoms has a minimal influence on density of states of the pure tin system, except for causing a widening of the energy gap. Various methods such as ab initio molecular dynamics simulations, the spherical jellium model, adaptive natural density partitioning, localized orbital locator, and electron density difference are employed to analyze the stability of these clusters. The aromaticity of the clusters is examined using iso-chemical shielding surfaces and the gauge-including magnetically induced currents. This study demonstrates that the stability and aromaticity of a tin cage can be systematically adjusted through doping.

Structures and stabilities of UPbn (n ≤ 18) clusters: A first-principles global optimization calculation

Endohedrally doped clusters have received much attention because they can act as superatoms and have great potential as the building blocks for cluster-assembled materials. We have carried out a comprehensive study based on the combination of the particle swarming optimization algorithm and the relativistic density functional theory, to obtain geometric structures and stabilities of lead (Pb) clusters doped with one uranium (U) atom. A gradual evolution pattern was observed with the increasing number of Pb atoms, from exohedrally doped structures to quasi-endohedral structures, and finally to endohedrally doped structures. At least 12 and 11 Pb atoms were necessary to encapsulate the U atom completely in the calculation without and with the spin-orbit coupling (SOC) effect respectively. The UPb16 cluster was represented as a highly stable endohedral cage structure with the U atom around its center. In addition, the crucial role of the SOC effect was emphasized. We hope that our findings will provide a fundamental understanding of actinide-doped lead clusters, which may be quite different from their light element analogues.

Robust magnetism of the cluster assembled (Fe@In6)Ba2 crystal

Fe@In6 is the smallest transition metal doped endohedral cage cluster. Its anion, with an octahedral geometry of Oh symmetry, has three singly occupied degenerate orbitals with the ground electronic state 4A1g. Global optimization based on the evolutionary algorithm combined with density functional theories shows that the lowest energy structure of the crystal assembled by Fe@In6 and Ba2 is spin polarized, and the crystal structure is slightly deviated from the fcc (face centered cubic) structure due to the spin-polarization. The magnetic moment is 2.45 μB per Fe@In6Ba2 unit, and the magnetism is very robust. The formation enthalpy of the crystal reaches −1.1 eV per atom, and the spin-polarized state is 2.36 eV lower in energy than the non-polarized state. Calculations also demonstrate that the magnetic moments change linearly with the pressures imposing on the polarization direction. The present study provides a new way for designing cluster assembled magnetic materials by transition metal doped clusters.

Nb@Ge13/14]3−: New Family Members of Ge‐Based Intermetalloid Clusters

During the past two decades, single-atom-centered medium-sized germanium clusters [M@Gen ] (M=transition metals, n>12) have been extensively explored, both from theoretical perspectives and experimental gas-phase syntheses. However, the actual structural arrangements of the Ge13 and Ge14 endohedral cages are still ambiguous and have long remained an unresolved problem for experimental implementation. In this work, we successfully synthesize 13-/14-vertex Ge clusters [Nb@Ge13 ]3- (1) and [Nb@Ge14 ]3- (2), which are structurally characterized and exhibit unprecedented topologies, neither classical deltahedra nor 3-connected polyhedral structures. Theoretical analysis indicates that the major stabilization of the Ge backbones arises due to the substantial interaction of Ge 4p-AOs with the endohedral Nb 4d-AOs through three/four-center two-electron bonds with an enhanced electron density accumulated over the shortest Nb-Ge13 contact in 1. Low occupancies of the direct two-center two-electron (2c-2e) Nb-Ge and Ge-Ge σ bonds point to a considerable degree of electron delocalization over the Ge cages revealing their electron deficiency.

Chemical Isolation of Isomerically Purified Sc3n@C68

Metallofullerenes with fused pentalenes are rare in arc soot. By far, the majority of endohedral fullerene cages obey the isolated pentagon rule (IPR). In a recent landmark paper (Santiago et al., JACS, 2021, 143, 6037-6042) by the Echegoyen group, they experimentally demonstrated the use of Sc3N@C68 as a new molecular catalyst for Hydrogen Evolution Reactions. Indeed, this fused pentalene metallofullerene (Sc3N@C68) successfully outperformed six other “conventional” (i.e., IPR) endohedrals. Thus, application development for fused pentalene metallofullerenes may open new doors.In this presentation, we discuss our chemical-based purification approach specifically designed for fused pentalene systems such as Sc3N@C68. We find that after two types of selective chemical reaction, we obtain a highly enriched sample of primarily two species, Sc3N@D3(6140)-C68 and Sc3N@Ih(7)-C80. Due to the large retention time difference between C68 and C80 endohedral cages, only a single injection is necessary for a final purification step.

Rational Design of Endohedral Superhalogens without Using Metal Cations and Electron Counting Rules.

Superhalogens, predicted 40 years ago, have attracted considerable attention due to their potential as building blocks of novel materials with various applications. While a large number of superhalogen clusters have been theoretically predicted and experimentally synthesized, they either require the use of a metal cation or electron counting rules. In particular, very rare endohedral cage clusters in defiance of the above requirements have been found to be superhalogens. In this work, motivated by recent experimental advances in endohedral cage clusters, we present a rational design principle for creating a new class of such superhalogens. Focusing on the chemical formula of A@Si20X20 (A = F, Cl, Br, I, BH4, BF4; X = H, F, Cl, Br, I, BO, CN, SCN, CH3), we use first-principles calculations to study 54 different clusters and show that these clusters possess electron affinities as high as 8.5 eV. Some of these clusters with X = BO and CN can even be stable as dianions, with large second electron affinity ∼2 eV. Similarly, Cl@C60 is found to be a superhalogen. This class of superhalogens is different from the conventional ones with chemical formula MXk+1, where X is a halogen and M is a cation with a formal +k oxidation state. Interestingly, the electron affinities of A@Si20X20 are almost independent of the central A moiety, but are guided by the functional group X. The potential of these endohedral superhalogens as electrolytes in Li-ion batteries is discussed.

Cage-Expansion of Fullerenes.

Despite the first proposal on the cage inflation of fullerenes in 1991, the chemical expansion of fullerenes has been still a formidable challenge. Herein, we provide an efficient methodology to expand [60] and [70]fullerene cages by the inclusion of totally C5N unit, giving nitrogen-containing closed structures as C65N and C75N with double fused heptagons. This method consists of two steps commenced with the construction of an opening by the reaction with triazine as a C3N source, followed by the cage reformation using N-phenylmaleimide as a C2 source. We also synthesized endohedral cages, demonstrating that the encapsulated H2O molecule inside the C75N cage prefers the orientation which maximizes the intramolecular interaction with the carbon wall. Additionally, we revealed the existence of a through-space magnetic dipolar interaction between the encapsulated H2 molecule and the embedded N atom.

(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

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.

Structure evolution of chromium-doped boron clusters: toward the formation of endohedral boron cages.

The electron-deficient nature of boron endows isolated boron clusters with a variety of interesting structural and bonding properties that can be further enriched through metal doping. In the current work, we report the structural and electronic properties of a series of chromium-doped boron clusters. The global minimum structures for CrBn clusters with an even number of n ranging from 8 to 22 are proposed through extensive first-principles swarm-intelligence structure searches. Half-sandwich structures are found to be preferred for CrB8, CrB10, CrB12 and CrB14 clusters and to transform to a drum-like structure at CrB16 cluster. Endohedral cage structures with the Cr atom located at the center are energetically most favorable for CrB20 and CrB22 clusters. Notably, the endohedral CrB20 cage has a high symmetry of D2d and a large HOMO–LUMO gap of 4.38 eV, whose stability is attributed to geometric fit and formation of an 18-electron closed-shell configuration. The current results advance our understanding of the structure and bonding of metal-doped boron clusters.

Structural Evolution and Superatoms in Molybdenum Atom Stabilized Boron Clusters: MoBn (n = 10–24)

Doping transition metal atom is known as an effective approach to stabilize an atomic cluster and modify its structure and electronic properties. We herein report the effect of molybdenum doping on the structural evolution of medium-sized boron clusters. The lowest-energy structures of MoBn (n = 10, 12, 14, 16, 18, 20, 22, 24) clusters are globally searched using genetic algorithm combined with density functional theory calculations. We found that Mo doping has significantly affected the grow behaviors of Bn clusters, leading to a structural evolution from bowl-like to tubular and finally endohedral cage. The size-dependent binding energy, HOMO–LUMO gap, vertical ionization potential and vertical electron affinity show that MoB12, MoB22 and MoB24 clusters have relatively higher stability and enhanced chemical inertness. More interestingly, the endohedral MoB22 cage is identified as an elegant superatom, which satisfies 18-electron closed shell configuration well.

Stabilization of golden cages by encapsulation of a single transition metal atom.

Golden cage-doped nanoclusters have attracted great attention in the past decade due to their remarkable electronic, optical and catalytic properties. However, the structures of large golden cage doped with Mo and Tc are still not well known because of the challenges in global structural searches. Here, we report anionic and neutral golden cage doped with a transition metal atom MAu16 (M = Mo and Tc) using Saunders ‘Kick' stochastic automation search method associated with density-functional theory (DFT) calculation (SK-DFT). The geometric structures and electronic properties of the doped clusters, MAu16q (M = Mo and Tc; q = 0 and −1), are investigated by means of DFT theoretical calculations. Our calculations confirm that the 4d transition metals Mo and Tc can be stably encapsulated in the Au16− cage, forming three different configurations, i.e. endohedral cages, planar structures and exohedral derivatives. The ground-state structures of endohedral cages C2v Mo@Au16−-(a) and C1 Tc@Au16−-(b) exhibit a marked stability, as judged by their high binding energy per atom (greater than 2.46 eV), doping energy (0.29 eV) as well as a large HOMO–LUMO gap (greater than 0.40 eV). The predicted photoelectron spectra should aid in future experimental characterization of MAu16− (M = Mo and Tc).

Effects of manganese doping on the structure evolution of small-sized boron clusters

Atomic doping of clusters is known as an effective approach to stabilize or modify the structures and properties of resulting doped clusters. We herein report the effect of manganese (Mn) doping on the structure evolution of small-sized boron (B) clusters. The global minimum structures of both neutral and charged Mn doped B cluster (n = 10–20 and Q = 0, ±1) have been proposed through extensive first-principles swarm-intelligence based structure searches. It is found that Mn doping has significantly modified the grow behaviors of B clusters, leading to two novel structural transitions from planar to tubular and then to cage-like B structures in both neutral and charged species. Half-sandwich-type structures are most favorable for small (n ⩽ 13) clusters and gradually transform to Mn-centered double-ring tubular structures at clusters with superior thermodynamic stabilities compared with their neighbors. Most strikingly, endohedral cages become the ground-state structures for larger (n ⩾ 19) clusters, among which adopts a highly symmetric structure with superior thermodynamic stability and a large HOMO-LUMO gap of 4.53 eV. The unique stability of the endohedral cage is attributed to the geometric fit and formation of 18-electron closed-shell configuration. The results significantly advance our understanding about the structure and bonding of B-based clusters and strongly suggest transition-metal doping as a viable route to synthesize intriguing B-based nanomaterials.

Revisit of Sin (n = 21–29) Clusters by Ab Initio Global Search

Using our improved genetic algorithm combined with density functional theory calculations, we perform unbiased global search for the most stable structures of Sin clusters with n = 21–29. A new principal direction crossover operation is introduced to search the prolate structures on the potential energy surface. We confirmed previous results for most sized Sin clusters and find more stable structure at n = 23 and 26. In particular, we find a new endohedral cage structure at Si23, which is 0.4 eV lower in energy than previously reported prolate structure and exhibit a distinct peak on the second order difference of total cluster energy. We discuss the competition between prolate and spherical motifs in the medium-sized Sin clusters in light of binding energy and HOMO–LUMO gap. The transition of structural motif from prolate to near spherical in Sin clusters occurs at n = 26. The outer cages are composed of pentagon, hexagon, quadrangle and heptagon, with one to three interior atoms filled inside the cage. These results establish a more complete picture for the structural evolution of the medium-sized silicon clusters. Average bond lengths and Mayer bond order are also discussed.

Design of superhalogens using a core-shell structure model.

Superhalogens, which have larger electron affinity than any halogen, play an important role in physical chemistry and materials design because of their applications in hydrogen storage and lithium-ion batteries. Inspired by the unique geometries and electronic properties of II-VI/III-V cage clusters, particularly the experimentally synthesized B12N12, we propose a core-shell structure model to design new superhalogens. The idea is assessed by conducting ab initio calculations on endohedral cage clusters X@B12N12 (X = F, Cl, Br) and other similar systems. With an exceptionally large electron affinity of 5.36 eV, the stable F@B12N12 cluster behaves as a novel superhalogen that can serve as a building block for Li salts and hyperhalogens. The findings highlight a new route for the discovery of superhalogens and provide useful building blocks for the bottom-up design of materials.

Isolation and Crystallographic Characterization of Gd3N@D2(35)-C88 through Non-Chromatographic Methods.

While several nonchromatographic methods are available for the isolation and purification of endohedral fullerenes of the type M3N@Ih-C80, little work has been done that would allow other members of the M3N@C2n family to be isolated with minimal chromatography. Here, we report that Gd3N@D2(35)-C88 can be isolated from the multitude of endohedral and empty cage fullerenes present in carbon soot obtained by electric-arc synthesis using Gd2O3-doped graphite rods. The procedure developed utilizes successive precipitation with the Lewis acids CaCl2 and ZnCl2 followed by treatment with amino-functionalized silica gel. The structure of the product was identified by single-crystal X-ray diffraction.

Structural, electronic, and magnetic properties of AgnCo (n = 1–9) clusters: A first-principles study

Structural, electronic and magnetic properties of neutral AgnCo (n=1-9) clusters are studied using first principles calculations based on density functional theory. For the ground state structures of the AgnCo clusters, the Co impurity occupies the highest coordinated position. The lowest energy structures for AgnCo (n⩽4) clusters are planar, while from n=5 onwards, AgnCo clusters showed an icosahedral growth except for Ag9Co, which adopts an endohedral cage structure. The stability based on the binding energy showed that AgnCo clusters are energetically favored for the high spin configuration, however, from n=7 onwards, trends to adopt the low spin or non-magnetic configurations are observed. Interestingly, in Ag9Co the silver host quenches the magnetic moment of the encapsulated Co atom. The magnetic orderings between the impurity and the Agn host in AgnCo clusters are discussed.

Non-IPR Chlorofullerenes

The rule per excellence governing the stability of fullerene cages is the well-known Isolated Pentagon Rule, or IPR. Fullerenes violating the pentagon rule have been obtained in situ through (i) encapsulation of metallic clusters inside the cavity or by (ii) external addition of halogenated species. The endohedral non-IPR cages (type i) appeared in the 2000, with the simultaneous discovery of Sc2@C66 and Sc3N@C68;[ 1 ] the halogenated IPR-violating fullerenes (type ii) appeared in 2004 with the characterization of the first empty non-IPR cage C50Cl10.[ 2 ] From then on, the list of non-IPR cages synthesized with any of the two functionalizations has increased significantly. It is important to note that there exists no IPR-violating isomer obtained both as metallo-endohedral and chlorinated fullerene, suggesting that isomers encapsulating metals are not suitable to be obtained with halogen derivatization. The factors that govern the formation of metalloendohedral fullerenes are well-known.[ 3 ] Now, we intend to discover which are the elements that favor the formation of chlorinated species. With this goal, we have analyzed several isomers for C66 and other fullerenes that have been observed as chloro- and endofullerenes in order to unravel the features of the non-IPR isomers that make them suitable to be trapped as halogenated fullerenes. (1) Stevenson, S.; Fowler, P. W.; Heine, T.; Duchamp, J. C.; Rice, G.; Glass, T.; Harich, K.; Hajdu, E.; Bible, R.; Dorn, H. C. Nature 2000, 408, 427. (2) Xie, S.-Y.; Gao, F.; Lu, X.; Huang, R.-B.; Wang, C.-R.; Zhang, X.; Liu, M.-L.; Deng, S.-L.; Zheng, L.-S. Science 2004, 304, 699. (3) a) Rodriguez-Fortea, A.; Alegret, N.; Balch, A. L.; Poblet, J. M. Nature Chem. 2010, 2, 955; b) Campanera, J. M.; Bo, C.; Poblet, J. M. Angew. Chem., Int. Ed. 2005, 44, 7230. Figure 1

Structure and bonding in a bimetallic endohedral cage, [Co2@Ge16]z−

We report here mass spectrometric evidence for the new bimetallic cluster, [Co2@Ge16]z− which is observed as a mono-anion in the mass spectrum but as a tetra-anion in the solid state. Density functional theory suggests that at both oxidation levels it adopts a D2h-symmetric structure based on two pentagonal bipyramids. The 3-connected vertices of the cage are characteristic of an electron-precise (5n = 80) count, but assigning 80 of the 86 valence electrons to the cage would imply an unrealistic degree of charge separation. We resolve this dilemma by showing that many of the electron pairs fulfill a dual role, contributing to the stable 80-electron count at the cage and to the 18-electron count at each metal. Thus it is not possible in this case to partition the charge into mutually exclusive subsets on the metal and on the cluster.

Room-Temperature Ordered Spin Structures in Cluster-Assembled Single V@Si12 Sheets

Since most of the existing pristine two-dimensional (2D) materials are either intrinsically nonmagnetic or magnetic with small magnetic moment per unit cell and weak strength of magnetic coupling, introducing transition metal atoms in various nanosheets has been widely used for achieving a desired 2D magnetic material. However, the problem of surface clustering for the doped transition metal atoms is still challenging. Here we demonstrate via first-principles calculations that the recently experimentally characterized endohedral silicon cage V@Si12 clusters can construct two types of single cluster sheets exhibiting hexagonal porous or honeycomb-like framework with regularly and separately distributed V atoms. For the ground state of these two sheets, the preferred magnetic coupling is found to be ferromagnetic due to a free-electron-mediated mechanism. By using external strain, the magnetic moments and strength of magnetic coupling for these two sheets can be deliberately tuned, which would be propitious to their advanced applications. More attractively, our first-principles molecular dynamics simulations show that both the structure and strength of ferromagnetic coupling for the pristine porous sheet are stable enough to survive at room temperature. The insights obtained in this work highlight a new avenue to achieve 2D silicon-based spintronics nanomaterials.