Bonding in Endohedral Metallofullerenes as Studied by Quantum Theory of Atoms in Molecules

Metal–cage and intracluster bonding was studied in detail by quantum theory of atoms in molecules (QTAIM) for the four major classes of endohedral metallofullerenes (EMFs), including monometallofullerenes Ca@C₇₂, La@C₇₂, M@C₈₂ (M=Ca, Sc, Y, La), dimetallofullerenes Sc₂@C₇₆, Y₂@C₈₂, Y₂@C₇₉N, La₂@C₇₈, La₂@C₈₀, metal nitride clusterfullerenes Sc₃N@C_2n (2n=68, 70, 78, 80), Y₃N@C_2n (2n=78, 80, 82, 84, 86, 88), La₃N@C_2n (2n=88, 92, 96), metal carbide clusterfullerenes Sc₂C₂@C₆₈, Sc₂C₂@C₈₂, Sc₂C₂@C₈₄, Ti₂C₂@C₇₈, Y₂C₂@C₈₂, Sc₃C₂@C₈₀, as well as Sc₃CH@C₈₀ and Sc₄O_x@C₈₀ (x=2, 3), that is, 42 EMF molecules and ions in total. Analysis of the delocalization indices and bond critical point (BCP) indicators such as G_bcp/ρ_bcp, H_bcp/ρ_bcp, and |V_bcp|/G_bcp, revealed that all types of bonding in EMFs exhibit a high degree of covalency, and the ionic model is reasonable only for the Ca‐based EMFs. Metal–metal bonds with negative values of the electron‐density Laplacian were found in Y₂@C₈₂, Y₂@C₇₉N, Sc₄O₂@C₈₀, and anionic forms of La₂@C₈₀. A delocalized nature of the metal–cage bonding results in a topological instability of the electron density in EMFs with an unpredictable number of metal–cage bond paths and large elipticity values.     

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.     

Electronic Structure and Redox Properties of Metal Nitride Endohedral Fullerenes M3N@C2n (M=Sc,Y, La,and Gd; 2n=80, 84, 88, 92, 96)

An extensive study of the redox properties of metal nitride endohedral fullerenes (MNEFs) based on DFT computational calculations has been performed. The electronic structure of the singly oxidized and reduced MNEFs has been thoroughly analyzed and the first anodic and cathodic potentials, as well as the electrochemical gaps, have been predicted for a large number of M₃N@C_2n systems (M=Sc, Y, La, and Gd; 2n=80, 84, 88, 92, and 96). In particular, calculations that include thermal and entropic effects correctly predict the different anodic behavior of the two isomers (I_h and D_5h) of Sc₃N@C₈₀, which is the basis for their electrochemical separation. Important differences were found in the electronic structure of reduced M₃N@C₈₀ when M=Sc or when M is a more electropositive metal, such as Y or Gd. Moreover, the changes in the electrochemical gaps within the Gd₃N@C_2n series (2n=80, 84, and 88) have been rationalized and the use of Y‐based computational models to study the Gd‐based systems has been justified. The redox properties of the largest MNEFs characterized so far, La₃N@C_2n (2n=92 and 96), were also correctly predicted. Finally, the quality of these predictions and their usefulness in distinguishing the carbon cages for MNEFs with unknown structures is discussed.     

An Expanded Family of Dysprosium–Scandium Mixed‐Metal Nitride Clusterfullerenes: The Role of the Lanthanide Metal on the Carbon Cage Size Distribution

A large family of dysprosium–scandium (Dy‐Sc) mixed‐metal nitride clusterfullerenes (MMNCFs), Dy_xSc_3?xN@C_2n (x=1, 2, 2n=68, 70, 76–86) have been successfully synthesized and isolated. Among these, the C₇₀ and C₈₂‐based MMNCFs are two new cages that have never been isolated for MMNCFs. Synthesis of Dy_xSc_3?xN@C_2n was accomplished by the “selective organic solid” route using guanidinium thiocyanate as the nitrogen source, and their isolation was fulfilled by recycling HPLC. UV/Vis‐NIR spectroscopic study indicates that almost all Dy_xSc_3?xN@C_2n MMNCFs are kinetically stable fullerenes with optical band gaps beyond 1 eV. This feature is distinctly different to their counterparts Dy₃N@C_2n (78≤2n≤88), whose for optical band‐gaps are below 1 eV for relatively large cages such as C₈₄ and C₈₆. An FTIR spectroscopic study in combination with DFT calculations enables reasonable assignments of the cage isomeric structures of all isolated Dy_xSc_3?xN@C_2n (x=1, 2, 2n=68, 70, 76–86) MMNCFs. The carbon cage size distribution of Dy_xSc_3?xN@C_2n (2n=68, 70, 76–86) is compared to the reported Dy₃N@C_2n (78≤2n≤8) homogeneous NCF and Dy_xSc_3?xN@C_2n (78≤2n≤88) MMNCF families, revealing that the medium‐sized Dy metal plays a crucial role on the expanded cage size distribution of MMNCFs. As a result, Dy_xSc_3?xN@C_2n MMNCFs are the largest MMNCF family reported to date.     

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.     

Theoretical study on the one‐, two‐, and three‐photon absorption properties of exohedral functionalized derivative of Sc3N@C80

Geometrical structures of three investigated molecules Sc₃N@C₈₀, Sc₃N@C₈₀‐Fc, and C₆₀‐Fc were optimized by density functional theory (DFT) at the B3LYP/6‐31G* level. Then the time‐dependent DFT was employed to investigate the excited states of these molecules. After exohedral functionalization by ferrocene (Fc‐) group as the electron donor or replacing C₆₀ with Sc₃N@C₈₀ as the electron acceptor, the wavelengths of the first one‐photon absorption peak and the strongest two‐photon absorption (2PA) and three‐photon absorption (3PA) peaks shift red. The corresponding cross sections of Sc₃N@C₈₀‐Fc in the 2PA and 3PA processes increase as compared with those of Sc₃N@C₈₀, which originate from the contributions of charge transfers from Fc‐ group to C₈₀ cage and simultaneously the transfers from the C₈₀ cage to the encapsulated Sc₃N cluster. When compared with C₆₀‐Fc, the 2PA and 3PA cross sections of Sc₃N@C₈₀‐Fc decrease, which may result from the more negative charge surface of C₈₀ cage in Sc₃N@C₈₀‐Fc molecule which blocks the charge transfers from Fc‐ moiety to the C₈₀ cage in the excitation processes by compared with C₆₀‐Fc. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

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     

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.     

Size effect of fullerene cages and encaged clusters in M<Subscript>3</Subscript>N@C<Subscript>2<Emphasis Type="Italic">n</Emphasis></Subscript>

Based on the calculated findings that the sizes of encaged clusters determine the structures and the stability of C₈₀-based trimetallic nitride fullerenes (TNFs), more extensive density functional theory calculations were performed on M₃N@C₆₈, M₃N@C₇₈ and M₃N@C₈₀ (M=Sc, Y and La). The calculated results demonstrated that the structures and stability undergo a transition with the increasing of the sizes of the cages and clusters. Sc₃N is planar inside the three considered cages, Y₃N is slightly pyramidal inside C₆₈-6140 and C₇₈-5 and planar inside I_h C₈₀-7, however, La₃N is pyramidal inside all the three cages. Those cages with pyramidal clusters inside deformed considerably, compared with their parent cages. In these cases, the bonding of metallic atoms toward the cages does not play an important role, and the encaged cluster tends to be located inside the cages with the largest M-M and M-C distances so that the strain energy can be released mostly. These calculations revealed the size effect of fullerene cages and encaged clusters, and can explain the position priority of M3N inside fullerene cages and the differences in yield of M₃N@C_2n . Supported by the Southwest University, China (Grant No. SWNUB2005002)     

Synthesis,Isolation, and Spectroscopic Characterization of Holmium‐Based Mixed‐Metal Nitride Clusterfullerenes: HoxSc3−xN@C80 (x=1, 2)

The synthesis, isolation and spectroscopic characterization of holmium‐based mixed metal nitride clusterfullerenes Ho_xSc_3?xN@C₈₀ (x=1, 2) are reported. Two isomers of Ho_xSc_3?xN@C₈₀ (x=1, 2) were synthesized by the reactive gas atmosphere method and isolated by multistep recycling HPLC. The isomeric structures of Ho_xSc_3?xN@C₈₀ (x=1, 2) were characterized by laser‐desorption time‐of‐flight (LD‐TOF) mass spectrometry and UV/Vis/NIR, FTIR and Raman spectroscopy. A comparative study of M_xSc_3?xN@C₈₀ (M=Gd, Dy, Lu, Ho) demonstrates the dependence of their electronic and vibrational properties on the encaged metal. Despite the distinct perturbation induced by 4f¹⁰ electrons, we report the first paramagnetic ¹³C NMR study on Ho_xSc_3?xN@C₈₀ (I; x=1, 2) and confirm I_h‐symmetric cage structure. A ⁴⁵Sc NMR study on HoSc₂N@C₈₀ (I, II) revealed a temperature‐dependent chemical shift in the temperature range of 268–308 K.     

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.     

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.     

Synthesis and Structure of LaSc2N@Cs(hept)‐C80 with One Heptagon and Thirteen Pentagons

The synthesis and single‐crystal X‐ray structural characterization of the first endohedral metallofullerene to contain a heptagon in the carbon cage are reported. The carbon framework surrounding the planar LaSc₂N unit in LaSc₂N@C_s(hept)‐C₈₀ consists of one heptagon, 13 pentagons, and 28 hexagons. This cage is related to the most abundant I_h‐C₈₀ isomer by one Stone–Wales‐like, heptagon/pentagon to hexagon/hexagon realignment. DFT computations predict that LaSc₂N@C_s(hept)‐C₈₀ is more stable than LaSc₂N@D_5h‐C₈₀, and suggests that the low yield of the heptagon‐containing endohedral fullerene may be caused by kinetic factors.     

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.     

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.     

Expanding the Number of Stable Isomeric Structures of the C80 Cage: A New Fullerene Dy3N@C80

The production, isolation, and spectroscopic characterization of a new Dy₃N@C₈₀ cluster fullerene that exhibits three isomers ( 1 – 3 ) is reported for the first time. In addition, the third isomer ( 3 ) forms a completely new C₈₀ cage structure that has not been reported in any endohedral fullerenes so far. The isomeric structures of the Dy₃N@C₈₀ cluster fullerene were analyzed by studying HPLC retention behavior, laser desorption time‐of‐flight (LD‐TOF) mass spectrometry, and UV‐Vis‐NIR and FTIR spectroscopy. The three isomers of Dy₃N@C₈₀ were all large band‐gap (1.51, 1.33, and 1.31 eV for 1 – 3 , respectively) materials, and could be classified as very stable fullerenes. According to results of FTIR spectroscopy, the Dy₃N@C₈₀ (I) ( 1 ) was assigned to the fullerene cage C₈₀:7 (I_h), whereas Dy₃N@C₈₀ (II) ( 2 ) had the cage structure of C₈₀:6 (D_5h). The most probable cage structure of Dy₃N@C₈₀ (III) ( 3 ) was proposed to be C₈₀:1 (D_5d). The significant differences between Dy₃N@C₈₀ and other reported M₃N@C₈₀ (M=Sc, Y, Gd, Tb, Ho, Er, Tm) cluster fullerenes are discussed in detail, and the strong influence of the metal on the nitride cluster fullerene formation is concluded.     

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.     

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.     

Bis[N-(trimethylsilyl)iminobenzoyl]phosphanide des Lithiums und Zinks – Synthese sowie NMR-spektroskopische,strukturelle und quantenchemische Untersuchungen

Acyl- and Alkylidenephosphanes. XXXV. Bis[ N -(trimethylsilyl)iminobenzoyl]phosphanides of Lithium and Zinc – Syntheses as well as NMR Spectroscopic, Structural, and Quantumchemical Studies From the reaction of bis(tetrahydrofuran)lithium bis(trimethylsilyl)phosphanide with two equivalents of benzonitrile in 1,2-dimethoxyethane, the yellow dme complex ( 2 a ) of lithium bis[N-(trimethylsilyl)iminobenzoyl]phosphanide ( 2 ) was obtained in 69% yield. However, the intermediate {1-[N-lithium-N-(trimethylsilyl)amido]benzylidene}trimethylsilylphosphane ( 1 ), formed by an analogous 1 : 1 addition in diethyl ether, turned out to be unstable and as a consequence could be characterized by nmr spectroscopic methods only; attempts to isolate the compound failed, but small amounts of the neutral complex 2 b , with the ligands benzonitrile and tetrahydrofuran coordinated to lithium, precipitated. The reaction of compound 2 with zinc(II) chloride in diethyl ether gives the orange-red spiro-complex zinc bis{bis[N-(trimethylsilyl)iminobenzoyl]phosphanide} ( 3 ); this complex is also formed from bis[N-(trimethylsilyl)iminobenzoyl]phosphane ( 4 ), easily amenable by a lithium hydrogen exchange of 2 a with trifluoroacetic acid [18], and zinc bis[bis(trimethylsilyl)amide]. As derived from nmr spectroscopic studies and x-ray structure determinations, compounds 2 a {δ³¹P +63.3 ppm; P2₁/n; Z = 4; R₁ = 0.067}, 2 b {δ³¹P +63.3 ppm; P2₁/c; Z = 4; R₁ = 0.063}, 3 {δ³¹P +58.2 ppm; C2/c; Z = 4; R₁ = 0.037} and 4 {δ³¹P +58.1 ppm [18]} exist as cyclic 3-imino-2λ³σ²-phosphapropenylamides and -propenylamine, respectively, in solution as well as in the solid state. Unlike hydrogen derivative 4 the bis[N-(trimethylsilyl)iminobenzoyl]phosphanide fragments N,N′-coordinating either a lithium or a zinc cation are characterized by almost completely equalized bond lengths; typical mean distances and angles are: PC 180.3 and 178.7; CN 130.5 and 131.8; N–Si 175.3 and 179.3; N–Li 202.3; N–Zn 203.5 pm; CPC 108.8° and 110.5°; PCN 130.9° and 132.9°; CN–Li 113.0°, CN–Zn 117.4°; N–Li–N 104.6°; N–Zn–N 108.8°. Alterations in the shape of the six membered chelate rings, caused by an exchange of the 3-imino-2λ³σ²-phosphapropenylamide or related 2λ³σ²-phospha-1,3-dionate units for the corresponding phosphorus free ligands, are discussed in detail. The results of quantumchemical DFT-B3LYP calculations coincide very well with the experimentally obtained findings.     

Metallofullerene photoswitches driven by photoinduced fullerene-to-metal electron transfer

We report on the discovery and detailed exploration of the unconventional photo-switching mechanism in metallofullerenes, in which the energy of the photon absorbed by the carbon cage π-system is transformed to mechanical motion of the endohedral cluster accompanied by accumulation of spin density on the metal atoms. Comprehensive photophysical and electron paramagnetic resonance (EPR) studies augmented by theoretical modelling are performed to address the phenomenon of the light-induced photo-switching and triplet state spin dynamics in a series of Y_xSc_3−xN@C₈₀ (x = 0–3) nitride clusterfullerenes. Variable temperature and time-resolved photoluminescence studies revealed a strong dependence of their photophysical properties on the number of Sc atoms in the cluster. All molecules in the series exhibit temperature-dependent luminescence assigned to the near-infrared thermally-activated delayed fluorescence (TADF) and phosphorescence. The emission wavelengths and Stokes shift increase systematically with the number of Sc atoms in the endohedral cluster, whereas the triplet state lifetime and S₁–T₁ gap decrease in this row. For Sc₃N@C₈₀, we also applied photoelectron spectroscopy to obtain the triplet state energy as well as the electron affinity. Spin distribution and dynamics in the triplet states are then studied by light-induced pulsed EPR and ENDOR spectroscopies. The spin–lattice relaxation times and triplet state lifetimes are determined from the temporal evolution of the electron spin echo after the laser pulse. Well resolved ENDOR spectra of triplets with a rich structure caused by the hyperfine and quadrupolar interactions with ¹⁴N, ⁴⁵Sc, and ⁸⁹Y nuclear spins are obtained. The systematic increase of the metal contribution to the triplet spin density from Y₃N to Sc₃N found in the ENDOR study points to a substantial fullerene-to-metal charge transfer in the excited state. These experimental results are rationalized with the help of ground-state and time-dependent DFT calculations, which revealed a substantial variation of the endohedral cluster position in the photoexcited states driven by the predisposition of Sc atoms to maximize their spin population.

Photoexcitation mechanism of Y_xSc_3−xN@C₈₀ metallofullerenes is studied by variable-temperature photoluminescence, advanced EPR techniques, and DFT calculations, revealing photoinduced rotation of the endohedral cluster.