Chemical Reactions of Cationic Metallofullerenes: An Alternative Route for Exohedral Functionalization

The chemistry of cationic forms of clusterfullerenes remain less explored than that of the corresponding neutral or anionic species. In the present work, M₃N@I_h-C₈₀ (M=Sc or Lu) cations were generated by both electrochemical and chemical oxidation methods. The as-obtained cations successfully underwent the typical Bingel–Hirsch reaction that fails with neutral Sc₃N@I_h-C₈₀. Two isomeric Sc₃N@I_h-C₈₀ cation derivatives, [5,6]-open and [6,6]-open adducts, were synthesized, and the former has never been prepared by means of a Bingel–Hirsch reaction with neutral clusterfullerenes. In the case of the Lu₃N@I_h-C₈₀ cation, however, only a [6,6]-open adduct was obtained. Density functional theory (DFT) calculations indicated that the oxidized M₃N@I_h-C₈₀ was much more reactive than the neutral compound upon addition of the diethyl bromomalonate anion. The Bingel–Hirsch reaction of M₃N@I_h-C₈₀ cations occurred by means of an unusual outer-sphere single-electron transfer (SET) process from the diethyl bromomalonate anion to the stable intermediate [M₃N@C₈₀(C₂H₅COO)₂CBr]^.. Remarkably, the diethyl bromomalonate anion was found to act as both a nucleophile and an electron donor.     

Addition reactions of fluoroalkanesulfonyl azides to [60] fullerene under thermal or microwave irradiation condition

The reactions of [60] fullerene with excess fluoroalkanesulfonyl azides R_fSO₂N₃ in o-dichlorobenzene under thermal or microwave irradiation condition afforded monoadduct opened [5,6]-bridged azafulleroids. While, similarly treatment of 2,2,2-trifluoroethyl azides CF₃CH₂N₃ with C₆₀ gave two monoadducts, i.e., opened [5,6]-bridged azafulleroids, closed [6,6]-bridged Aziridino-fullerene, and some multi-addition product. A general mechanism for these addition reactions was proposed.     

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.     

Open versus closed 1,3-dipolar additions of C60: a theoretical investigation on their mechanism and regioselectivity

[reactions: see text] 1,3-Dipolar additions of C60 with dipoles, diazomethane, nitrile oxide, and nitrone have been modeled at the B3LYP/6-31G(d,p)//AM1 level, and their mechanism, regiochemistry, and nature of addition are investigated. All of these reactions lead to the formation of fullerene fused heterocycles; theoretically, these reactions can take up four types of additions, viz., closed [6,6], open [5,6], closed [5,6], and open [6,6] additions, and all of them have been examined. Energetics and thermodynamic analysis of these reactions show that closed [5,6] and open [6,6] additions are not probable and that closed [6,6] additions are the most favored ones and follow a concerted mechanism. Experimental evidence that C60-diazomethane reactions yielded closed [6,6] fullerenopyrazoline provides good support to the theoretical predictions. The observed order of reactivity has been explained based on the double bond character, forcing double bonds in the pentagons of C60, and strain. During the addition, dipoles distort more than C60 and concerted closed [6,6] TSs are found to be more reactant-like or early TS. Inclusion of toluene as solvent through the PCM model increases the reaction rate and exothermicity. NICS values computed at the centers of the reacting benzenoid ring of fullerene clearly reveal, in both open and closed additions, the loss in them of aromaticity during the reaction.     

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.     

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.     

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.     

Revisit to the reaction of [60]fullerene with nitrile ylides generated from imidoyl chlorides and triethylamine

The reaction of [60]fullerene (C₆₀) with nitrile ylides generated from N-benzyl-4-nitrobenzimidoyl chloride/N-(4-chlorobenzyl)-4-nitrobenzimidoyl chloride and triethylamine gave only isomeric monoadducts of C₆₀ with [6,6]-closed structure. No [5,6]-open adduct of C₆₀ could be identified from these reactions. The previously reported [5,6]-open product of C₆₀ should be reassigned as a [6,6]-closed product.     

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.     

Catalytic [2+1] cycloaddition of diazo compounds to [60]fullerene

A catalytic method for the efficient synthesis of 5,6-open and 6,6-closed (2π+1π) fullerene adducts by [2+1] cycloaddition of ethyl 2-alkyl(hetaryl)-2-diazoacetates to [60]fullerene in the presence of a three-component catalyst Pd(acac)₂—PPh₃—Et₃Al have been developed.     

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.     

Rigid Tether Directed Regioselective Synthesis and Crystallographic Characterization of Labile 1,2,3,4‐Bis(triazolino)[60]fullerene and Its Thermolized Derivatives

Labile bis‐triazoline adducts of C₆₀ are supposed to be the precursors of bis‐azafulleroids, but the formation mechanism is still unclear because of the incomplete isolation of the thermolized products and the lack of X‐ray structures. A rigid‐tethered reagent 1,2‐bis(azidomethyl)benzene ( 1 ) was used to regioselectively synthesize the labile 1,2,3,4‐bis(triazolino)[60]‐fullerene ( 2 ), the structure of which was determined by single‐crystal X‐ray crystallography. Further thermolysis of 2 produces four products (3 a – 3 d ), which were all characterized by X‐ray crystallography. Although 3 a and 3 b have traditional bis‐azafulleroid structures, as proposed previously, 3 c and 3 d show unprecedented structures with either the coexistence of [5,6]‐open and [6,6]‐closed patterns or an oxidized structure with an 11‐membered ring on the cage. A thermolysis mechanism is proposed to clarify long‐term confusion about the transformation process from bis‐triazoline adducts to bis‐azafulleroids of C₆₀.     

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.     

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.     

Azide addition to an endohedral metallofullerene: formation of azafulleroids of Sc3N@I(h)-C80

The reaction of an organic azide with an endohedral metallofullerene has been investigated for the first time. Isomeric [5,6]- and [6,6]-azafulleroids can be obtained from the thermal reaction of Sc(3)N@I(h)-C(80) with 4-isopropoxyphenyl azide, while photoirradiation leads exclusively to the [6,6]-azafulleroid. An unprecedented thermal interconversion between the two isomeric azafulleroids has also been discovered.     

Determination of the Electron Density Distribution in the Bonds of a Fullerene Derivative by High-Resolution X-Ray Structure Analysis

Bent bonds in the strained fullerene system , restricted to the [5,6] bonds, were detected by high-resolution X-ray structure analysis of the 1,2-dihydro[60]fullerene derivative 1 . In addition the maxima of electron densities are higher in the [6,6] bonds than in the [5,6] bonds—an important finding with respect to the question of the extent of electron delocalization in fullerenes.     

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.     

C60与叠氮化合物的单加成反应

本文从实验以及理论研究两方面介绍了C60与叠氧化合物的单加成反应。依照叠氮化合物的不同,C60与叠氮合物的单加成反应可分为烷基叠氮化物与C60的反应,酰基叠氧化物与C60的反应以及苯基叠氮化与C60的反应三类,而反应产物则为C60亚氨基[6,6]闭环衍生物和C60亚氨基[5,6]开环衍生物两类,不同类型的反应具有不同的反应机理,某些C60亚氨基[5,6]开环衍生物可以转化为C60亚氨基[6,6]闭环衍生物。本文还介绍了碳纳米管与叠氮化合物的加成反应。     

Diels–Alder Reaction on Free C68 Fullerene and Endohedral Sc3N@C68 Fullerene Violating the Isolated Pentagon Rule: Importance of Pentagon Adjacency

The reaction mechanism and regioselectivity of the Diels–Alder reactions of C₆₈ and Sc₃N@C₆₈, which violate the isolated pentagon rule, were studied with density functional theory calculations. For C₆₈, the [5,5] bond is the most favored thermodynamically, whereas the cycloaddition on the [5,6] bond has the lowest activation energy. Upon encapsulation of the metallic cluster, the exohedral reactivity of Sc₃N@C₆₈ is reduced remarkably owing to charge transfer from the cluster to the fullerene cage. The [5,5] bond becomes the most reactive site thermodynamically and kinetically. The bonds around the pentagon adjacency show the highest chemical reactivity, which demonstrates the importance of pentagon adjacency. Furthermore, the viability of Diels–Alder cycloadditions of several dienes and Sc₃N@C₆₈ was examined theoretically. o‐Quinodimethane is predicted to react with Sc₃N@C₆₈ easily, which implies the possibility of using Diels–Alder cycloaddition to functionalize Sc₃N@C₆₈.