Evaluation of bonding,electron affinity,and optical properties of M@C28 (M = Zr,Hf, Th,and U): Role of d‐ and f‐orbitals in endohedral fullerenes from relativistic DFT calculations

The experimentally characterized endohedral metallic fullerenes involving the small C₂₈ cage, has shown to be able to encapsulate zirconium, hafnium, and uranium atoms, among other elements. Here, we explore the formation and nature of concentric bonds from purely d‐ to f‐block elements, given by Zr, Hf, and uranium, along a borderline metal between such blocks, thorium. We explore the interplay of d‐ and f‐orbitals in the chemistry of the early actinides, where the features of a d‐ or f‐block metal can be mixed. Our results indicate that the bonding of Th@C₂₈ involves contributions from both d‐ and f‐type bonds, as characteristic of this early actinide element. Even uranium in U@C₂₈, also exhibits a contribution from d‐type bonds in addition to its relevant f‐block character. Electron affinity and optical properties were evaluated to gain more insights into the variation of these molecular properties in this small endohedral fullerene, along Zr, Hf, Th, and U. The current results, allows to unravel the role of (n − 1)d and (n − 2)f orbitals in confined elements ranging from d‐ to f‐blocks, which can be useful to gain a deeper understanding of the bonding situation in other endohedral species. © 2016 Wiley Periodicals, Inc.     

Bingel–Hirsch Addition on Endohedral Metallofullerenes: Kinetic Versus Thermodynamic Control

An extensive theoretical study of the Bingel–Hirsch addition of bromomalonate on scandium nitride endohedral fullerenes has been carried out. The prototypical and highly symmetrical Sc₃N@I_h‐C₈₀, with a structure that satisfies the isolated pentagon rule (IPR), and the non‐IPR Sc₃N@D₃(6140)‐C₆₈ fullerene show analogous reaction paths despite the distinct topology of the carbon networks and different rotation freedom of the internal nitride cluster. For the two metallofullerenes, our results predict that the reaction takes place under kinetic control yielding open‐cage fulleroids on [6,6] bonds, which is in good agreement with experimental data. The theoretical studies also show that predicting the reactivity of endohedral metallofullerenes is not straightforward and often an accurate analysis of the potential energy surface is required.     

Effect of cage size on the third‐order optical properties of endohedral metallofullerenes Sc3N@C2n (2n = 68, 70, 78, and 80): A theoretical study

Geometrical structures of the investigated endohedral metallofullerenes Sc₃N@C_2n (2n = 68, 70, 78, and 80) were optimized at the B3LYP/6‐31G* level. The analyses of electronic structures display that the contribution of fullerene cage to the lowest unoccupied molecular orbital decreases as the cage size increases. Based on the optimized structures, the time‐dependent density functional theory combined with the sum‐over‐states method was used to investigate their nonlinear optical properties. Calculated third‐order polarizabilities γ and two‐photon absorption (TPA) cross‐section δ do not present the monotone variation with the size of fullerene cage, with largest γ of 0.48 × 10^?34 esu for Sc₃N@C₇₈ in static state, and largest δ of 12.374 GM for Sc₃N@C₇₀ in the wavelength of 902.5 nm. However, the obtained TPA resonant peaks shift red with the size of fullerene cage. By analyzing the electronic origin of the third‐order optical properties, it is found that the charge transfers from the fullerene cage to the encapsulated Sc₃N cluster make important contributions to the studied properties. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

2‐Aminoethanol Extraction as a Method for Purifying Sc3N@C80 and for Differentiating Classes of Endohedral Fullerenes on the Basis of Reactivity

Extraction with 2‐aminoethanol is an inexpensive method for removing empty cage fullerenes from the soluble extract from electric‐arc‐generated fullerene soot that contains endohedral metallofullerenes of the type Sc₃N@C_2n (n=34, 39, 40). Our method of separation exploits the fact that C₆₀, C₇₀, and other larger, empty cage fullerenes are more susceptible to nucleophilic attack than endohedral fullerenes and that these adducts can be readily extracted into 2‐aminoethanol. This methodology has also been employed to examine the reactivity of the mixture of soluble endohedral fullerenes that result from doping graphite rods used in the Krätschmer–Huffman electric‐arc generator with the oxides of Y, Lu, Dy, Tb, and Gd. For example, with Y₂O₃, we were able to detect by mass spectrometry several new families of endohedral fullerenes, namely Y₃C₁₀₈ to Y₃C₁₂₆, Y₃C₁₀₇ to Y₃C₁₂₅, Y₄C₁₂₈ to Y₄C₁₄₆, that resisted reactivity with 2‐aminoethanol more than the empty cage fullerenes and the mono‐ and dimetallo fullerenes. The discovery of the family Y₃C₁₀₇ to Y₃C₁₂₅ with odd numbers of carbon atoms is remarkable, since fullerene cages must involve even numbers of carbon atoms. The newly discovered families of endohedral fullerenes with the composition M₄C_2n (M=Y, Lu, Dy, Tb, and Gd) are unusually resistant to reaction with 2‐aminoethanol. Additionally, the individual endohedrals, Y₃C₁₁₂ and M₃C₁₀₂ (M=Lu, Dy, Tb and Gd), were remarkably less reactive toward 2‐aminoethanol.     

Molecular modeling of hydration properties of hydrophobic ions Li<Superscript>+</Superscript>@C<Subscript>60</Subscript> and K<Superscript>+</Superscript>@C<Subscript>60</Subscript>

The Gibbs free energies of solvation (ΔG _s) and the electronic structures of endohedral metallofullerenes M⁺@C₆₀ (M⁺= Li⁺, K⁺) were calculated within the framework of the density functional theory and the polarizable continuum model. In water environment, the equilibrium position of K⁺ is at the center of the fullerene cavity whereas that of Li⁺ is shifted by 0.14 nm toward the fullerene cage. The Li⁺ cation is stabilized by interactions with both the fullerene and solvent. The equilibrium structures of both endohedral metallofullerenes are characterized by very close ΔG _s values. In particular, the calculated ΔG _s values for K⁺@C₆₀ are in the range from −124 to −149 kJ mol⁻¹ depending on the basis set and on the type of the density functional. Molecular dynamics simulations (TIP3P H₂O, OPLS force field, water sphere of radius 1.9 nm) showed that the radial distribution functions of water density around C₆₀ and M⁺@C₆₀ are very similar, whereas orientations of water dipoles around the endohedral metallofullerenes resemble the hydration pattern of isolated metal ions.     

A Complete Guide on the Influence of Metal Clusters in the Diels–Alder Regioselectivity of Ih‐C80 Endohedral Metallofullerenes

The chemical functionalization of endohedral metallofullerenes (EMFs) has aroused considerable interest due to the possibility of synthesizing new species with potential applications in materials science and medicine. Experimental and theoretical studies on the reactivity of endohedral metallofullerenes are scarce. To improve our understanding of the endohedral metallofullerene reactivity, we have systematically studied with DFT methods the Diels–Alder cycloaddition between s‐cis‐1,3‐butadiene and practically all X@I_h‐C₈₀ EMFs synthesized to date: X=Sc₃N, Lu₃N, Y₃N, La₂, Y₃, Sc₃C₂, Sc₄C₂, Sc₃CH, Sc₃NC, Sc₄O₂ and Sc₄O₃. We have studied both the thermodynamic and kinetic regioselectivity, taking into account the free rotation of the metallic cluster inside the fullerene. This systematic study has been made possible through the use of the frozen cage model (FCM), a computationally cheap approach to accurately predicting the exohedral regioselectivity of cycloaddition reactions in EMFs. Our results show that the EMFs are less reactive than the hollow I_h‐C₈₀ cage. Except for the Y₃ cluster, the additions occur predominantly at the [5,6] bond. In many cases, however, a mixture of the two possible regioisomers is predicted. In general, [6,6] addition is favored in EMFs that have a larger charge transfer from the metal cluster to the cage or a voluminous metal cluster inside. The present guide represents the first complete and exhaustive investigation of the reactivity of I_h‐C₈₀‐based EMFs.     

Synthesis and Isolation of the Titanium–Scandium Endohedral Fullerenes—Sc2TiC@Ih‐C80, Sc2TiC@D5h‐C80 and Sc2TiC2@Ih‐C80: Metal Size Tuning of the TiIV/TiIII 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 Sc₄C₂@C₈₀ (the most abundant EMF from this synthesis), Sc₃C₂@C₈₀, isomers of Sc₂C₂@C₈₂, and the family Sc₂C_{2 n} (2 n=74, 76, 82, 86, 90, etc.), as well as Sc₃CH@C₈₀. The Sc–Ti/CH₄ system produces the mixed‐metal Sc₂TiC@C_{2 n} (2 n=68, 78, 80) and Sc₂TiC₂@C_{2 n} (2 n=80) clusterfullerene families. The molecular structures of the new, transition‐metal‐containing endohedral fullerenes, Sc₂TiC@I_h‐C₈₀, Sc₂TiC@D_5h‐C₈₀, and Sc₂TiC₂@I_h‐C₈₀, were characterized by NMR spectroscopy. The structure of Sc₂TiC@I_h‐C₈₀ was also determined by single‐crystal X‐ray diffraction, which demonstrated the presence of a short Ti=C double bond. Both Sc₂TiC‐ and Sc₂TiC₂‐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.     

Synthesis and Electrochemical Studies of Bingel–Hirsch Derivatives of M3N@Ih‐C80 (M=Sc,Lu)

Bingel–Hirsch derivatives of the trimetallic nitride template endohedral metallofullerenes (TNT‐EMFs) Sc₃N@I_h‐C₈₀ and Lu₃N@I_h‐C₈₀ were prepared by reacting these compounds with 2‐bromodiethyl malonate, 2‐bromo‐1,3‐dipyrrolidin‐1‐ylpropane‐1,3‐dionate bromide, and 9‐bromo fluorene. The mono‐adducts were isolated and their ¹H NMR spectra showed that the addition occurred with high regioselectivity at the [6,6] bonds of the I_h‐C₈₀ fullerene cage. Electrochemical analysis showed that the reductive electrochemistry behavior of these derivatives is irreversible at a scan rate of 100 mV s^?1, which is comparable to the behavior of the pristine fullerene species. The first reduction potential of each derivative is either cathodically or anodically shifted by a different value, depending on the attached addend. Bis‐adducts containing EtOOC‐C‐COOEt and HC‐COOEt addends were isolated by HPLC and in the case of Sc₃N@I_h‐C₈₀ the first reduction potential exhibits a larger shift towards negative potentials when compared to the mono‐adduct. This observation is important for designing acceptor materials for the construction of bulk heterojunction (BHJ) organic solar cells, since the polyfunctionalization not only increases the solubility of the fullerene species but also offers a promising approach for bringing the LUMO energy levels closer for the donor and the acceptor materials.     

Electronic structure and stability of C86 fullerene Isolated‐Pentagon‐Rule isomers

All 19 Isolated‐Pentagon‐Rule isomers of fullerene C₈₆ were investigated by Density Functional Theory (DFT) methods with B3LYP functional at 6‐31G, 6‐31G*, and 6‐31+G* levels. Preliminary distribution of single, double, and delocalized pi‐bonds in molecules of these isomers of fullerene C₈₆ is fulfilled. Obtained results are perfectly supported by DFT quantum–chemical calculations of electronic and geometrical structures of these isomers. The main reason of instability of isomers 1, 3–15, 18, and 19 are phenalenyl‐radical substructures. Thus, there is a possibility to obtain them only as endohedral metallofullerenes or exohedral derivatives. Isomer 2 (C₂) is unstable due to higher local molecular strain. It is shown that empty C₈₆ may be produced and extracted only as isomers 16 (C_s) and 17 (C₂). © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Structural connectivity and formation mechanism of monometallic cluster fullerenes YCN@Cn (n = 68–84)

Excited by the recently experimental reports of monometallic cluster fullerenes, we examined the electronic and geometrical properties of monometallic cluster fullerenes YCN@C_n with size from C₆₈ to C₈₄ by density functional theory and statistical thermodynamic calculations. The calculations demonstrate that the thermodynamically favored isomers of YCN@C_n are in good agreement with available experimental results. Morphology analysis shows that the lowest‐energy YCN@C_n species are structurally connected by C₂ insertion/extrusion and Stone–Wales rotation, which can be promoted under high temperature; enthalpy–entropy interplay can change the relative abundances of low‐energy isomers significantly at high temperature. All the results suggest that there is a structural evolution among these metallic cluster fullerenes in discharge condition, and thus, can rationalize their structural diversity in the soot and partly disclose their formation mechanism. The geometrical structures, electronic properties of these endohedral fullerene were discussed in detail.     

Structural interconnections and the role of heptagonal rings in endohedral trimetallic nitride template fullerenes

Recent experiments indicate that fullerene isomers outside the classical definition can also encapsulate metallic atoms or clusters to form endohedral metallofullerenes. Our systematic study using DFT calculations, suggests that many heptagon‐including nonclassical trimetallic nitride template fullerenes are similar in stability to their classical counterparts, and that conversion between low‐energy nonclassical and classical parent cages via Endo–Kroto insertion/extrusion of C₂ units and Stone–Wales isomerization may facilitate the formation of endohedral trimetallic nitride fullerenes. Close structural connections are found between favored isomers of trimetallic nitride template fullerenes from C₇₈ to C₈₂. It appears that the lower symmetry and local deformations associated with introduction of a heptagonal ring favor encapsulation of intrinsically less symmetrical mixed metal nitride clusters. © 2016 Wiley Periodicals, Inc.     

Preferential Formation of Mono‐Metallofullerenes Governed by the Encapsulation Energy of the Metal Elements: A Case Study on Eu@C2n (2n=74–84) Revealing a General Rule

Encapsulating one to three metal atoms or a metallic cluster inside fullerene cages affords endohedral metallofullerenes (EMFs) classified as mono‐, di‐, tri‐, and cluster‐EMFs, respectively. Although the coexistence of various EMF species in soot is common for rare‐earth metals, we herein report that europium tends to prefer the formation of mono‐EMFs. Mass spectroscopy reveals that mono‐EMFs (Eu@C_2n) prevail in the Eu‐containing soot. Theoretical calculations demonstrate that the encapsulation energy of the endohedral metal accounts for the selective formation of mono‐EMFs and rationalize similar observations for EMFs containing other metals like Ca, Sr, Ba, or Yb. Consistently, all isolated Eu‐EMFs are mono‐EMFs, including Eu@D_3h(1)‐C₇₄, Eu@C_2v(19138)‐C₇₆, Eu@C_2v(3)‐C₇₈, Eu@C_2v(3)‐C₈₀, and Eu@D_3d(19)‐C₈₄, which are identified by crystallography. Remarkably, Eu@C_2v(19138)‐C₇₆ represents the first Eu‐containing EMF with a cage that violates the isolated‐pentagon‐rule, and Eu@C_2v(3)‐C₇₈ is the first C₇₈‐based EMF stabilized by merely one metal atom.     

Geometrical and Electronic Rules in Fullerene‐Based Compounds

The discovery of buckminsterfullerene C₆₀ opened up a new scientific area and stimulated the development of nanoscience and nanotechnology directly. Fullerene science has since emerged to include fullerenes, endohedral fullerenes (mainly metallofullerenes), exofullerenes, and carbon nanotubes as well. Herein, we look back at the development of fullerene science from the perspective of epistemology by highlighting the proposed main rules or criteria for understanding and predicting the structures and stability of fullerene‐based compounds. We also point out that a rule or criterion may contribute significantly to the corresponding discipline and suggest that two unsolved issues in fullerene science are the addition patterns of fullerene derivatives and the structures and stability of nonclassical fullerenes.     

Controllable Growth of Fullerene Nanostructures

One‐dimensional fullerene nanostructures with well‐defined morphology have been prepared by a controllable method. Fullerene molecules, such as C₆₀ derivatives and endohedral metallofullerenes, are introduced into the pores of anodic aluminum oxide (AAO) templates under a direct current (DC) electric field. Then several nanostructures such as porous‐wall and solid‐wall fullerene nanowires and nanotubes were fabricated in the pores. The morphology of the fullerene nanostructures is well controllable, and the fullerene nanotubes can be further fabricated through filling nickel atoms inside to form fullerene‐metal composite structures. The results provide, in principle, a step toward broader applications of fullerene‐related materials in nanoscience and nanotechnology.     

La2@Cs(17 490)‐C76: A New Non‐IPR Dimetallic Metallofullerene Featuring Unexpectedly Weak Metal–Pentalene Interactions

Although all the pure‐carbon fullerene isomers above C₆₀ reported to date comply with the isolated pentagon rule (IPR), non‐IPR structures, which are expected to have different properties from those of IPR species, are obtainable either by exohedral modification or by endohedral atom doping. This report describes the isolation and characterization of a new endohedral metallofullerene (EMF), La₂@C₇₆, which has a non‐IPR fullerene cage. The X‐ray crystallographic result for the La₂@C₇₆/[Ni^II(OEP)] (OEP=octaethylporphyrin) cocrystal unambiguously elucidated the C_s(17 490)‐C₇₆ cage structure, which contains two adjacent pentagon pairs. Surprisingly, multiple metal sites were distinguished from the X‐ray data, which implies dynamic behavior for the two La³⁺ cations inside the cage. This dynamic behavior was also corroborated by variable‐temperature ¹³⁹ La NMR spectroscopy. This phenomenon conflicts with the widely accepted idea that the metal cations in non‐IPR EMFs invariably coordinate strongly with the negatively charged fused‐pentagon carbons, thereby providing new insights into modern coordination chemistry. Furthermore, our electrochemical and computational studies reveal that La₂@C_s(17 490)‐C₇₆ has a larger HOMO–LUMO gap than other dilanthanum‐EMFs with IPR cage structures, such as La₂@D_3h(5)‐C₇₈ and La₂@I_h(7)‐C₈₀, which implies that IPR is no longer a strict rule for EMFs.     

Isolated pentagon rule violating endohedral metallofullerenes explained using the Hückel rule: A statistical mechanical study of the C84 Isomeric Set

Fullerenes and their structure and stability have been a major topic of discussion and research since their discovery nearly 30 years ago. The isolated pentagon rule (IPR) has long served as a guideline for predicting the most stable fullerene cages. More recently, endohedral metallofullerenes have been discovered that violate the IPR. This article presents a systematic, temperature dependent, statistical thermodynamic study of the 24 possible IPR isomers of C₈₄ as well as two of the experimentally known non‐IPR isomers (51365 and 51383), at several different charges (0, ?2, ?4, and ?6). From the results of this study, we conclude that the Hückel rule is a valid simpler explanation for the stability of fused pentagons in endohedral metallofullerenes. © 2014 Wiley Periodicals, Inc.     

Small Cage Uranofullerenes: 27 Years after Their First Observation

The tetravalently stabilized fullerene cage of C₂₈ is historically the most elusive small fullerene cage observed by employing the laser vaporization synthesis methodology. Its first observation reported by Smalley et al. in 1992 suggests that C₂₈ is potentially the smallest and most stable fullerene ever observed. By using the Krätschmer?Huffman arc discharge synthesis method, we have recently succeeded in synthesizing a series of uranium‐endohedral fullerenes which differ from those reported by Smalley and co‐workers. Intrigued by this interesting mismatch, we tuned our experimental conditions to favor the formation and detection of these missing species. Experiments done using solvents of varying polarity allowed the observation of several empty and uranofullerenes. Extractions with pyridine and o‐DCB allowed for observation of small U@C_2n (2n=28, 60, 66, 68, 70) by high resolution Fourier‐Transform Ion Cyclotron Resonance Mass Spectrometry (FT‐ICR MS). This is the first time that U@C₂₈ is observed in soot produced by the Krätschmer‐Huffman arc‐discharge methodology. Carbon cage selection and spin density distribution on the endohedral metallofullerenes (EMFs) U@C₆₀, U@C₇₀, and U@C₇₂ were studied by means of density functional theory (DFT) calculations. A plausible pathway for the formation of U@D_3h‐C₇₄ from U@D_5h‐C₇₀ through two C₂ insertions and one Stone‐Wales rearrangement is proposed.     

Optically Active Macrocyclic cis‐3 Bis‐Adducts of C60: Regio‐ and Stereoselective Synthesis,Exciton Chirality Coupling,and Determination of the Absolute Configuration,and First Observation of Exciton Coupling between Fullerene Chromophores in a Chiral Environment

A series of optically active cis‐3 bis‐adducts, such as (R,R,^fC)‐ 16 (Scheme 6), was obtained regio‐ and diastereoselectively by Bingel macrocyclization of C₆₀ with bis‐malonates, which contain optically active tethers derived from 1,2‐diols. The absolute configuration of the inherently chiral addition pattern in cis‐3 bis‐adducts had previously been determined by comparison of calculated and experimental circular dichroism (CD) spectra. Full confirmation of these earlier assignments was now obtained by an independent method based on semiempirical AM1 (`Austin Model 1') and OM2 (`Orthogonalization Method 2') calculations combined with ¹H‐NMR spectroscopy. It was found computationally that bis‐malonates [CHR(OCOCH₂COOEt)]₂, which contain (R,R)‐ or (S,S)‐butane‐2,3‐diol derivatives as optically active tethers, preferentially form out‐out cis‐3 bis‐adducts of C₆₀ as a single diastereoisomer in which the alkyl groups R adopt a gauche conformation, while the two glycolic H‐atoms are in an antiperiplanar (ap) and the ester linkages to the fullerene in a gauche relationship (Figs. 2 and 5). In contrast, in the less favorable diastereoisomer, which should not form, the alkyl groups R adopt an ap and the H‐atoms a gauche conformation, while the ester bridges to the fullerene remain, for geometric reasons, locked in a gauche conformation. According to the OM2 calculations, the geometry of the fully staggered tether in the free bis‐malonates closely resembles the conformation of the tether fragment in the bis‐adducts formed. These computational predictions were confirmed experimentally by the measurement of the coupling constant between the vicinal glycolic H‐atoms in the ¹H‐NMR spectrum. For (R,R,^fC)‐ 16 , ³J(H,H) was determined as 7.9 Hz, in agreement with the ap conformation, and, in combination with the calculations, this allowed assignment of the ^fC‐configuration to the inherently chiral addition pattern. This conformational analysis was further supported by the regio‐ and diastereoselective synthesis of cis‐3 bis‐adducts from bis‐malonates, including tethers derived from cyclic glycol units with a fixed gauche conformation of the alkyl residues R at the glycolic C‐atoms. Thus, a bis‐malonate of (R,R)‐cyclohexane‐1,2‐diol provided exclusively cis‐3 bis‐adduct (R,R,^fC)‐ 20 in 32% yield (Scheme 7). Incorporation of a tether derived from methyl 4,6‐O,O‐benzylidene‐α‐D ‐glucopyranoside into the bis‐malonate and Bingel macrocyclization diastereoselectively produced the cis‐3 stereoisomer (α,D ,^fA)‐ 22 (Scheme 8) as the only macrocyclic bis‐adduct. If the geometry of the alkyl groups R at the glycolic C‐atoms of the tether component deviates from a gauche relationship, as in the case of tethers derived from exo cis‐ and trans‐norbornane‐2,3‐diol or from trans‐cyclopentane‐1,2‐diol, hardly any macrocyclic product is formed (Schemes 5 and 9). The absolute configurations of the various optically active cis‐3 bis‐adducts were also assigned by comparison of their CD spectra, which are dominated by the chiroptical contributions of the inherently chiral fullerene chromophore (Figs. 1, 3, and 4). A strong chiral exciton coupling was observed for optically active macrocyclic cis‐3 bis‐adducts of C₆₀ with two appended 4‐(dimethylamino)benzoate ((S,S,^fC)‐ 26 ; Fig. 6) or meso‐tetraphenylporphyrin ((R,R,^fC)‐ 28 ; Fig. 7) chromophores. Chiral exciton coupling between two fullerene chromophores was observed for the first time in the CD spectrum of the threitol‐bridged bis‐fullerene (R,R)‐ 35 (Fig. 9).     

The Regioselectivity of Bingel–Hirsch Cycloadditions on Isolated Pentagon Rule Endohedral Metallofullerenes

In this work, the Bingel–Hirsch addition of diethylbromomalonate to all non‐equivalent bonds of Sc₃N@D_3h‐C₇₈ was studied using density functional theory calculations. The regioselectivities observed computationally allowed the proposal of a set of rules, the predictive aromaticity criteria (PAC), to identify the most reactive bonds of a given endohedral metallofullerene based on a simple evaluation of the cage structure. The predictions based on the PAC are fully confirmed by both the computational and experimental exploration of the Bingel–Hirsch reaction of Sc₃N@D_5h‐C_80, thus indicating that these rules are rather general and applicable to other isolated pentagon rule endohedral metallofullerenes.     

Isolation and Structure Determination of a Missing Endohedral Fullerene La@C70 through In Situ Trifluoromethylation

D_5h‐symmetric fullerene C₇₀ (D_5h‐C₇₀) is one of the most abundant members of the fullerene family. One longstanding mystery in the field of fullerene chemistry is whether D_5h‐C₇₀ is capable of accommodating a rare‐earth metal atom to form an endohedral metallofullerene M@D_5h‐C₇₀, which would be expected to show novel electronic properties. The molecular structure of La@C₇₀ remains unresolved since its discovery three decades ago because of its extremely high instability under ambient conditions and insolubility in organic solvents. Herein, we report the single‐crystal X‐ray structure of La@C₇₀(CF₃)₃, which was obtained through in situ exohedral functionalization by means of trifluoromethylation. The X‐ray crystallographic study reveals that La@C₇₀(CF₃)₃ is the first example of an endohedral rare‐earth fullerene based on D_5h‐C₇₀. The dramatically enhanced stability of La@C₇₀(CF₃)₃ compared to La@C₇₀ can be ascribed to trifluoromethylation‐induced bandgap enlargement.