Singly Bonded Monoadduct rather than Methanofullerene: Manipulating the Addition Pattern of Trimetallic Nitride Clusterfullerene through One Endohedral Metal Atom Substitution

Bingel–Hirsch reactions of trimetallic nitride clusterfullerenes (NCFs) generally yield methanofullerene (cyclopropane) adducts instead of singly bonded derivatives, which have been reported for monometallofullerenes. Herein, we report the synthesis and characterization of the Bingel–Hirsch derivative of a mixed metal nitride clusterfullerene (MMNCF) TiY₂N@I_h‐C₈₀. Surprisingly, in contrast to the reported Bingel–Hirsch cyclopropane adducts of the analogous NCF Y₃N@I_h‐C₈₀, the Bingel–Hirsch derivative of TiY₂N@I_h‐C₈₀ is the first singly bonded monoadduct (labeled as TiY₂N@C₈₀‐Mono) to be reported, which was determined unambiguously by single‐crystal X‐ray crystallography. Besides, the reactivity of TiY₂N@I_h‐C₈₀ was found to be significantly improved relative to that of Y₃N@I_h‐C₈₀. Upon substituting one endohedral yttrium (Y) atom of Y₃N@I_h‐C₈₀ with titanium (Ti), the Bingel–Hirsch derivative changes from the cyclopropane to the singly bonded monoadduct, revealing that not only the reactivity but also the addition pattern of NCFs can be manipulated simultaneously through one endohedral metal atom substitution.     

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.     

Molecular Structure and Magnetic and Optical Properties of Endometallonitridofullerene Sc3N@Ih-C80 in Neutral,Radical Anion,and Dimeric Anionic Forms

A series of compounds with Sc₃N@I_h-C₈₀ in the neutral, monomeric, and dimeric anion states have been prepared in the crystalline form and their molecular structures and optical and magnetic properties have been studied. The neutral Sc₃N@I_h-C₈₀ ⋅ 3 C₆H₄Cl₂ ( 1 ) and (Sc₃N@I_h-C₈₀)₃(TPC)₂ ⋅ 5 C₆H₄Cl₂ ( 2 , TPC=triptycene) compounds both crystallized in a high-symmetry trigonal structure. The reduction of Sc₃N@I_h-C₈₀ to the radical anion resulted in dimerization to form diamagnetic singly bonded (Sc₃N@I_h-C₈₀⁻)₂ dimers. In contrast to {[2.2.2]cryptand(Na⁺)}₂(Sc₃N@I_h-C₈₀⁻)₂ ⋅ 2.5 C₆H₄Cl₂ ( 3 ) with strongly disordered components, we synthesized new dimeric phases {[2.2.2]cryptand- (K⁺)}₂(Sc₃N@I_h-C₈₀⁻)₂ ⋅ 2 C₆H₄Cl₂ ( 4 ) and {[2.2.2]cryptand- (Cs⁺)}₂(Sc₃N@I_h-C₈₀⁻)₂ ⋅ 2 C₆H₄Cl₂ ( 5 ) in which only one major dimer orientation was found. The thermal stability of the (Sc₃N@I_h-C₈₀⁻)₂ dimers was studied by EPR analysis of 3 to show their dissociation in the 400–460 K range producing monomeric Sc₃N@I_h-C₈₀^.− radical anions. This species shows an EPR signal with a hyperfine splitting of 5.8 mT. The energy of the intercage C−C bond was estimated to be 234±7 kJ mol⁻¹, the highest value among negatively charged fullerene dimers. The EPR spectra of crystalline (Bu₃MeP⁺)₃(Sc₃N@I_h-C₈₀^.−)₃ ⋅ C₆H₄Cl₂ ( 6 ) are presented for the first time. The salt shows an asymmetric EPR signal, which could be fitted by three lines. Two lines were attributed to Sc₃N@I_h-C₈₀^.−. Hyperfine splitting is manifested above 180 K due to the hyperfine interaction of the electron spin with the three scandium atoms (a total of 22 lines with an average splitting of 5.32 mT are observed at 220 K). Furthermore, each of the 22 lines is additionally split into six lines with an average separation of 0.82 mT. The large splitting indicates intrinsic charge and spin density transfer from the fullerene cage to the Sc₃N cluster. Both the monomeric and dimeric Sc₃N@I_h-C₈₀⁻ anions show an intrinsic shift of the IR bands attributed to the Sc₃N cluster and new bands corresponding to these species appear in the NIR range of their UV/Vis/NIR spectra, which allows these anions to be distinguished from neutral species.     

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.     

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.     

Addition of S‐Heterocyclic Carbenes to Fullerenes: Formation and Characterization of Dithiomethano‐Bridged Derivatives

The reactions of novel S‐heterocyclic carbenes (SHCs), which were prepared by the cycloaddition of disilenes and digermenes to CS₂, with C₆₀ and Sc₃N@I_h‐C₈₀ afforded the corresponding methano‐bridged fullerenes. The [6,6]‐closed and [6,6]‐open structures were characterized for the SHC adducts of C₆₀ and Sc₃N@I_h‐C₈₀, respectively. These derivatives exhibited relatively low oxidation potentials, indicative of the electron‐donating effects of the SHC addends. The electronic properties of the SHC derivatives were clarified by the density functional theory calculations.     

Preparation,Structural Determination,and Characterization of Electronic Properties of [5,6]‐ and [6,6]‐Carbosilylated Sc3N@Ih‐C80

Photochemical carbosilylation of Sc₃N@I_h ‐C₈₀ with silirane 1 afforded two corresponding [5,6]‐adducts, 2 and 3 , and a [6,6]‐adduct, 4 . The structural and electronic properties of these products were characterized by means of spectroscopic, electrochemical, and theoretical methods. The structure of 2 was disclosed by means of single‐crystal X‐ray crystallographic analysis. Thermal isomerization of 3 to 2 was observed, whereas that of 2 to 3 proceeded less efficiently at 100 °C. Upon heating under the same conditions, adduct 4 underwent facile decomposition to afford Sc₃N@I_h ‐C₈₀, or isomerized into small amounts of 2 and 3 . The relative stabilities of 2 , 3 , and 4 were rationalized through the results of theoretical calculations. In contrast, adducts 2 , 3 , and 4 were stable under the photolytic conditions employed for carbosilylation. The photochemical functionalization of Sc₃N@I_h ‐C₈₀ represents a convenient synthetic method to obtain thermally labile fullerene‐based products.     

Metal Nitride Cluster Fullerene M3N@C80 (M=Y,Sc) Based Dyads: Synthesis,and Electrochemical,Theoretical and Photophysical Studies

The first pyrrolidine and cyclopropane derivatives of the trimetallic nitride templated (TNT) endohedral metallofullerenes I_h‐Sc₃N@C₈₀ and I_h‐Y₃N@C₈₀ connected to an electron‐donor unit (i.e., tetrathiafulvalene, phthalocyanine or ferrocene) were successfully prepared by 1,3‐dipolar cycloaddition reactions of azomethine ylides and Bingel–Hirsch‐type reactions. Electrochemical studies confirmed the formation of the [6,6] regioisomers for the Y₃N@C₈₀‐based dyads and the [5,6] regioisomers in the case of Sc₃N@C₈₀‐based dyads. Similar to other TNT endohedral metallofullerene systems previously synthesized, irreversible reductive behavior was observed for the [6,6]‐Y₃N@C₈₀‐based dyads, whereas the [5,6]‐Sc₃N@C₈₀‐based dyads exhibited reversible reductive electrochemistry. Density functional calculations were also carried out on these dyads confirming the importance of these structures as electron transfer model systems. Furthermore, photophysical investigations on a ferrocenyl–Sc₃N@C₈₀‐fulleropyrrolidine dyad demonstrated the existence of a photoinduced electron‐transfer process that yields a radical ion pair with a lifetime three times longer than that obtained for the analogous C₆₀ dyad.     

Full Exploration of the Diels–Alder Cycloaddition on Metallofullerenes M3N@C80 (M=Sc,Lu, Gd): The D5h versus Ih Isomer and the Influence of the Metal Cluster

In this work a detailed investigation of the exohedral reactivity of the most important and abundant endohedral metallofullerene (EMF) is provided, that is, Sc₃N@I_h‐C₈₀ and its D_5h counterpart Sc₃N@D_5h‐C₈₀, and the (bio)chemically relevant lutetium‐ and gadolinium‐based M₃N@I_h/D_5h‐C₈₀ EMFs (M=Sc, Lu, Gd). In particular, we analyze the thermodynamics and kinetics of the Diels–Alder cycloaddition of s‐cis‐1,3‐butadiene on all the different bonds of the I_h‐C₈₀ and D_5h‐C₈₀ cages and their endohedral derivatives. First, we discuss the thermodynamic and kinetic aspects of the cycloaddition reaction on the hollow fullerenes and the two isomers of Sc₃N@C₈₀. Afterwards, the effect of the nature of the metal nitride is analyzed in detail. In general, our BP86/TZP//BP86/DZP calculations indicate that [5,6] bonds are more reactive than [6,6] bonds for the two isomers. The [5,6] bond D _5h ‐b , which is the most similar to the unique [5,6] bond type in the icosahedral cage, I _h ‐a , is the most reactive bond in M₃N@D_5h‐C₈₀ regardless of M. Sc₃N@C₈₀ and Lu₃N@C₈₀ give similar results; the regioselectivity is, however, significantly reduced for the larger and more electropositive M=Gd, as previously found in similar metallofullerenes. Calculations also show that the D_5h isomer is more reactive from the kinetic point of view than the I_h one in all cases which is in good agreement with experiments.     

Blending Non‐Group‐3 Transition Metal and Rare‐Earth Metal into a C80 Fullerene Cage with D5h Symmetry

Rare‐earth metals have been mostly entrapped into fullerene cages to form endohedral clusterfullerenes, whereas non‐Group‐3 transition metals that can form clusterfullerenes are limited to titanium (Ti) and vanadium (V), and both are exclusively entrapped within an I_h‐C₈₀ cage. Non‐Group‐3 transition‐metal‐containing endohedral fullerenes based on a C₈₀ cage with D_5h symmetry, V_xSc_3?xN@D_5h‐C₈₀ (x=1, 2), have now been synthesized, which exhibit two variable cluster compositions. The molecular structure of VSc₂N@D_5h‐C₈₀ was unambiguously determined by X‐ray crystallography. According to a comparative study with the reported Ti‐ and V‐containing clusterfullerenes based on a I_h‐C₈₀ cage and the analogous D_5h‐C₈₀‐based metal nitride clusterfullerenes containing rare‐earth metals only, the decisive role of the non‐Group‐3 transition metal on the formation of the corresponding D_5h‐C₈₀‐based clusterfullerenes is unraveled.     

Pivotal Role of Nonmetal Atoms in the Stabilities,Geometries, Electronic Structures,and Isoelectronic Chemistry of Sc3X@C80 (X = C,N, and O)

The thermodynamic and dynamic stabilities of Sc₃X@C₈₀ (X = C, N, and O) are explored via density functional theory combined with statistical thermodynamic analysis and ab initio molecular dynamics. It is the first time to comprehensively consider the effect of nonmetal atoms on trimetallic endohedral clusterfullerenes. Relative to Sc₃X@I_h (31924)-C₈₀ (X = N and O) with general six-electron transfer, an intriguing electronic structure of unexplored Sc₃C@D_5h (31923)-C₈₀ with thermodynamic and dynamic stabilities is clearly disclosed. Natural bond orbitals and charge decomposition analysis simultaneously suggest that one unpaired electron appears on the cage for neutral Sc₃C@D_5h (31923)-C₈₀, which could be prospectively stabilized by effective exohedral derivatization and ionization in the future. Moreover, isoelectronic endohedral clusterfullerenes, (Sc₃C@C₈₀)⁻, Sc₃N@C₈₀, and (Sc₃O@C₈₀)⁺, are also uniquely taken into account. The geometries, electronic structures, reactivities, and reactive sites of isoelectronic species are examined, and it turns out that all the three isoelectronic species would rather electrophilic than nucleophilic reactions. © 2019 Wiley Periodicals, Inc.     

New Trifluoromethylated Derivatives of Metal Nitride Clusterfullerenes: Sc3N@Ih‐C80(CF3)14 and Sc3N@D5h‐C80(CF3)16

Trifluoromethylated derivatives of Sc₃N@I_h‐C₈₀ and Sc₃N@D_5h‐C₈₀ were synthesized by the reaction with CF₃I at 440 °C. HPLC separation of the mixture of Sc₃N@D_5h‐C₈₀(CF₃)_n derivatives resulted in isolation and X‐ray structure determination of Sc₃N@D_5h‐C₈₀(CF₃)₁₆, which represents a precursor of the known Sc₃N@D_5h‐C₈₀(CF₃)₁₈. Among the CF₃ derivatives of Sc₃N@I_h‐C₈₀, two new isomers of Sc₃N@I_h‐C₈₀(CF₃)₁₄ ( Sc‐14‐VI and Sc‐14‐VII ) were isolated by HPLC, and their molecular structures were determined by X‐ray diffraction, thus enabling a comprehensive comparison of altogether seven isomers. Two types of addition patterns with different orientations of the Sc₃N cluster relative to the I_h‐C₈₀ fullerene cage were established. In particular, Sc‐14‐VII represents a direct precursor of the known Sc₃N@I_h‐C₈₀(CF₃)₁₆‐ II . All molecular structures exhibit an ordered position of a Sc₃N cluster inside the fullerene C₈₀ cage.     

An Efficient Method to Separate Sc3N@C80 Ih and D5h Isomers and Sc3N@C78 by Selective Oxidation with Acetylferrocenium [Fe(COCH3C5H4)Cp]+

Based on the different oxidation potentials of endohedral fullerenes Sc₃N@C₈₀ I_h and D_5h and Sc₃N@C₇₈, an efficient and useful method that avoids HPLC has been developed for their separation. Selective chemical oxidation of the Sc₃N@D_5h‐C₈₀ isomer and Sc₃N@C₇₈ by using an acetylferrocenium salt [Fe(COCH₃C₅H₄)Cp]⁺ followed by column chromatographic separation and reduction with CH₃SNa resulted in the isolation of pure Sc₃N@I_h‐C₈₀, Sc₃N@C₇₈, and a mixture of Sc₃N@D_5h‐C₈₀ and Sc₃N@C₆₈.     

Essential Factors for Control of the Equilibrium in the Reversible Rearrangement of M3N@Ih‐C80 Fulleropyrrolidines: Exohedral Functional Groups versus Endohedral Metal Clusters

The effects of exohedral moieties and endohedral metal clusters on the isomerization of M₃N@I_h‐C₈₀ products from the Prato reaction through [1,5]‐sigmatropic rearrangement were systematically investigated by using three types of fulleropyrrolidine derivatives and four different endohedral metal clusters. As a result, all types of derivatives provided the same ratios of the isomers for a given trimetallic nitride template (TNT) as the thermodynamic products, thus indicating that the size of the endohedral metal clusters inside C₈₀ was the single essential factor in determining the equilibrium between the [6,6]‐isomer (kinetic product) and the [5,6]‐isomer. In all the derivatives, the [6,6]‐ and [5,6]‐Prato adducts with larger metal clusters, such as Y₃N and Gd₃N, were equally stable, which is in good agreement with DFT calculations. The reaction rate of the rearrangement was dependent on both the substituent of exohedral functional groups and the endohedral metal‐cluster size. Further DFT calculations and ¹³C NMR spectroscopic studies were employed to rationalize the equilibrium in the rearrangement between the [6,6]‐ and [5,6]‐fulleropyrrolidines.     

Crystallographic Evidence for Direct Metal–Metal Bonding in a Stable Open‐Shell La2@Ih‐C80 Derivative

Endohedral metallofullerenes (EMFs) have novel structures and properties that are closely associated with the internal metallic species. Benzyl radical additions have been previously shown to form closed‐shell adducts by attaching an odd number of addends to open‐shell EMFs (such as Sc₃C₂@I_h‐C₈₀) whereas an even number of groups are added to closed‐shell EMFs (for example Sc₃N@I_h‐C₈₀). Herein we report that benzyl radical addition to the closed‐shell La₂@I_h‐C₈₀ forms a stable, open‐shell monoadduct instead of the anticipated closed‐shell bisadduct. Single‐crystal X‐ray diffraction results show the formation of a stable radical species. In this species, the La?La distance is comparable to the theoretical value of a La?La covalent bond and is shorter than reported values for other La₂@I_h‐C₈₀ derivatives, providing unambiguous evidence for the formation of direct La?La bond.     

Isolation and Crystallographic Characterization of Two,Nonisolated Pentagon Endohedral Fullerenes: Ho3N@C2(22010)-C78 and Tb3N@C2(22010)-C78

Purified samples of Ho₃N@C₂(22010)-C₇₈ and Tb₃N@C₂(22010)-C₇₈ have been isolated by two distinct processes from the rich array of fullerenes and endohedral fullerenes present in carbon soot from graphite rods doped with Ho₂O₃ or Tb₄O₇. Crystallographic analysis of the endohedral fullerenes as cocrystals with Ni(OEP) (in which OEP is the dianion of octaethylporphyrin) shows that both molecules contain the chiral C₂(22010)-C₇₈ cage. This cage does not obey the isolated pentagon rule (IPR) but has two sites where two pentagons share a common C−C bond. These pentalene units bind two of the metal ions, whereas the third metal resides near a hexagon of the cage. Inside the cages, the Ho₃N or Tb₃N unit is planar. Ho₃N@C₂(22010)-C₇₈ and Tb₃N@C₂(22010)-C₇₈ use the same cage previously found for Gd₃N@C₂(22010)-C₇₈ rather than the IPR-obeying cage found in Sc₃N@D_3h-C₇₈.     

Two Regioisomers of Endohedral Pyrrolidinodimetallofullerenes M2@Ih‐C80(CH2)2NTrt (M=La,Ce; Trt=trityl): Control of Metal Atom Positions by Addition Positions

The two regioisomers of endohedral pyrrolidinodimetallofullerenes M₂@I_h‐C₈₀(CH₂)₂NTrt (M=La, Ce; Trt=trityl) were synthesized, isolated, and characterized. X‐ray crystallographic analyses of [6,6]‐La₂@I_h‐C₈₀(CH₂)₂NTrt and [6,6]‐Ce₂@I_h‐C₈₀(CH₂)₂NTrt revealed that the encapsulated metal atoms are located at the slantwise positions on the mirror plane that parallels the pyrrolidine ring. Paramagnetic NMR analyses of [6,6]‐ and [5,6]‐Ce₂@I_h‐C₈₀(CH₂)₂NTrt were also carried out to clarify the metal positions. As for the [6,6]‐adduct, the metal positions obtained by paramagnetic NMR analysis agree well with the X‐ray structure. In contrast, paramagnetic NMR analysis of the [5,6]‐adduct showed that the two Ce atoms are collinear with the pyrrolidine ring. We also compared the observed paramagnetic effects of the pyrrolidinodimetallofullerenes with those of other cerium‐encapsulating fullerene derivatives such as bis‐silylated Ce₂@I_h‐C₈₀ and a carbene adduct of Ce₂@I_h‐C₈₀. We found that the metal positions can be explained by the electrostatic potential maps of the corresponding [6,6]‐ and [5,6]‐adducts of [I_h‐C₈₀(CH₂)₂NTrt]^6?. These findings clearly show that metal positions inside fullerene cages can be controlled by means of the addition positions of the addends. In addition, the radical anions of the pyrrolidinodimetallofullerenes were prepared by bulk controlled‐potential electrolysis and characterized by X‐band EPR spectral study.     

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.     

Dearomative Ring-Fused Azafulleroids and Carbazole-Derived Metallofullerenes: Reactivity Dictated by Encapsulation in a Fullerene Cage

Herein, we report divergent additions of 2,2′-diazidobiphenyls to C₆₀ and Sc₃N@I_h-C₈₀. In stark contrast to that of the previously reported bis-azide additions, the unexpected cascade reaction leads to the dearomative formation of azafulleroids 2 fused with a 7-6-5-membered ring system in the case of C₆₀. In contrast, the corresponding reaction with Sc₃N@I_h-C₈₀ switches to the C−H insertion pathway, thereby resulting in multiple isomers, including a carbazole-derived [6,6]-azametallofulleroid 3 and a [5,6]-azametallofulleroid 4 and an unusual 1,2,3,6-tetrahydropyrrolo[3,2-c]carbazole-derived metallofullerene 5 , whose molecular structures have been unambiguously determined by single-crystal X-ray diffraction analyses. Among them, the addition type of 5 is observed for the first time in all reported additions of azides to fullerenes. Furthermore, unexpected isomerizations from 3 to 5 and from 4 to 5 have been discovered, providing the first examples of the isomerization of an azafulleroid to a carbazole-derived fullerene rather than an aziridinofullerene. In particular, the isomerism of the [5,6]-isomer 4 to the [5,6]-isomer 5 is unprecedented in fullerene chemistry, contradicting the present understanding that isomerization generally occurs between [5,6]- and [6,6]-isomers. Control experiments have been carried out to rationalize the reaction mechanism. Furthermore, representative azafulleroids have been applied in organic solar cells, thereby resulting in improved power conversion efficiencies.     

Unexpected formation of 1,2- and 1,4-bismethoxyl Sc3N@Ih-C80 derivatives via regioselective anion addition: an unambiguous structural identification and mechanism study

An attempt to achieve heterocyclic cycloadducts of Sc₃N@I_h-C₈₀via reaction with Ph₂C O, PhC CPh or PhC N in the presence of tetrabutylammonium hydroxide (TBAOH) stored in CH₃OH led to the formation of the unexpected bismethoxyl adducts of Sc₃N@I_h-C₈₀ (1 and 2). Further studies reveal that TBAOH in CH₃OH can boost the CH₃O⁻ addition efficiently, regardless of the presence of other reagents. Single-crystal X-ray diffraction results firmly assign the molecular structures of 1 and 2 as respective 1,4- and 1,2-bismethoxyl adducts, and reveal unusual relationships between the internal Sc₃N cluster and the addition modes, in addition to the unusual packing mode in view of the orientation of the methoxyl groups. Electrochemical results demonstrate smaller electrochemical gaps for 1 and 2, relative to that of Sc₃N@I_h-C₈₀, confirming their better electroactive properties. Finally, a plausible reaction mechanism involving anion addition and a radical reaction was proposed, presenting new insights into the highly selective reactions between the methoxyl anion and metallofullerenes. 1 and 2 represent the first examples of methoxyl derivatives of metallofullerenes. This work not only presents a novel and facile strategy for the controllable synthesis of alkoxylated metallofullerene derivatives, but also provides new non-cycloadducts for the potential applications of EMFs.

An attempt to achieve heterocyclic cycloadducts of Sc₃N@I_h-C₈₀via reaction with Ph₂C O, PhC CPh or PhC N in the presence of tetrabutylammonium hydroxide (TBAOH) stored in CH₃OH led to the formation of the unexpected bismethoxyl adducts of Sc₃N@I_h-C₈₀ (1 and 2).