Size‐Selective Complexation and Extraction of Endohedral Metallofullerenes with Cycloparaphenylene

A new strategy for the non‐chromatographic extraction of metallofullerenes from solutions of arc‐processed raw soot is based on the size‐selective complexation with cycloparaphenylene (CPP). [11]CPP has a high affinity for M_x@C₈₂ (x=1, 2); for example, Gd@C₈₂ can be selectively extracted from a fullerene mixture by the addition of [11]CPP. This approach should open new opportunities in metallofullerene chemistry, including for the bulk extraction of metallofullerenes.     

Structural and Electronic Characteristics of Endohedral Metallofullerenes: Y2C2@C82 and Y2@C84 Isomers

A quantum-chemical study was made of the structure and electronic characteristics of the novel endohedral metallofullerene Y₂C₂@C₈₂ in comparison with the Y₂@C₈₄ isomer. The interactions between the encapsulated Y₂C₂ cluster and the C₈₂ fullerene cage are ionic in nature. The electronic spectrum of Y₂C₂@C₈₂ differs greatly from the "parent" C₈₂ fullerene and has a metal-like form. The results are compared with existing experimental data.     

De Novo Design of an Endohedral Heteronuclear Dimetallofullerene (UGd)@C60 with Exceptional Structural and Electronic Properties

Ever since the first synthesis of La@C₈₂ and U@C₂₈, there has been a growing interest in the study of endohedral metallofullerenes (EMFs) because of their great potential in various applications. Here we design a novel heteronuclear EMF (U‐Gd)@C₆₀, by using density functional theory (DFT), which shows an encapsulation energy of about ?5.53 eV, comparable to that of U₂@C_60, La₂@C₈₀, and Lu₂@C₇₆. (U‐Gd)@C₆₀ is found to have a surprising twofold, single‐electron U?Gd bond that results from the strong nanoconfinement of the fullerene, dominated by uranium′s 5f and 6d and gadolinium′s 5d atomic orbitals. The ground state shows an 11‐et high spin state, and the net spins distributed on the U‐pole carbons are relatively scattered, while they are highly concentrated on the Gd‐pole carbons. The exceptional electronic characteristics of this novel EMF, containing both uranium and gadolinium atoms encapsulated, might prove useful for future applications in nuclear energy and biomedicine.     

Supramolecular Formation of Li+@PCBM Fullerene with Sulfonated Porphyrins and Long‐Lived Charge Separation

Lithium‐ion‐encapsulated [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester fullerene (Li⁺@PCBM) was utilized to construct supramolecules with sulfonated meso‐tetraphenylporphyrins (MTPPS^4?; M=Zn, H₂) in polar benzonitrile. The association constants were determined to be 1.8×10⁵ M ^?1 for ZnTPPS^4?/Li⁺@PCBM and 6.2×10⁴ M ^?1 for H₂TPPS^4?/Li⁺@PCBM. From the electrochemical analyses, the energies of the charge‐separated (CS) states were estimated to be 0.69 eV for ZnTPPS^4?/Li⁺@PCBM and 1.00 eV for H₂TPPS^4?/Li⁺@PCBM. Upon photoexcitation of the porphyrin moieties of MTPPS^4?/Li⁺@PCBM, photoinduced electron transfer occurred to produce the CS states. The lifetimes of the CS states were 560 μs for ZnTPPS^4?/Li⁺@PCBM and 450 μs for H₂TPPS^4?/Li⁺@PCBM. The spin states of the CS states were determined to be triplet by electron paramagnetic resonance spectroscopy measurements at 4 K. The reorganization energies (λ) and electronic coupling term (V) for back electron transfer (BET) were determined from the temperature dependence of k_BET to be λ=0.36 eV and V=8.5×10^?3 cm^?1 for ZnTPPS^4?/Li⁺@PCBM and λ=0.62 eV and V=7.9×10^?3 cm^?1 for H₂TPPS^4?/Li⁺@PCBM based on the Marcus theory of nonadiabatic electron transfer. Such small V values are the result of a small orbital interaction between the MTPPS^4? and Li⁺@PCBM moieties. These small V values and spin‐forbidden charge recombination afford a long‐lived CS state.     

The Driving Force of Photoinduced Charge Separation in Metal‐Cluster‐Encapsulated Triphenylamine‐[80]fullerenes

Understanding photoinduced charge separation in fullerene‐based dye‐sensitized solar cells is crucial for the development of photovoltaic devices. We investigate here how the driving force of the charge separation process in conjugates of M@C₈₀ (M=Sc₃N, Sc₃CH, Sc₃NC, Sc₄O₂, and Sc₄O₃) with triphenylamine (TPA) depends on the nature of the metal cluster. Both singlet and triplet excited‐state electron‐transfer reactions are considered. These results based on TD‐DFT calculations demonstrate that the driving force of charge separation in TPA‐M@C₈₀ can be tuned well by varying the structure of the metal cluster encapsulated inside the fullerene cage.     

Size‐ and Orientation‐Selective Encapsulation of C70 by Cycloparaphenylenes

The size‐ and orientation‐selective formation of the shortest‐possible C₇₀ peapod in solution and in the solid state by using the shortest structural unit of an “armchair” carbon nanotube (CNT), cycloparaphenylene (CPP), has been studied. [10]CPP and [11]CPP exothermically formed 1:1 complexes with C₇₀, thereby giving the resulting peapods. A van′t Hoff plot analysis revealed that the formation of these complexes in 1,2‐dichlorobenzene was mainly driven by entropy, whereas the theoretical calculations suggested that the formation of the complex in the gas phase was predominantly driven by enthalpy. C₇₀ was found to exist in two distinct orientations inside the CPP cavity, namely “lying” and “standing”, depending on the specific size of the CPP. The theoretical calculations and the X‐ray crystallographic analysis revealed that the interactions between [10]CPP and the short axis of C₇₀ in its lying orientation were isotropic and similar to those observed between [10]CPP and C₆₀. However, the interactions between [11]CPP and C₇₀ in its standing orientation were anisotropic, thereby involving the radial deformation of [11]CPP into an ellipsoidal shape. This “induced fit” maximized the van der Waals interactions with the long axis of C₇₀. Theoretical calculations revealed that the deformation occurred readily with low energy loss, thus suggesting that CPPs are highly radially elastic molecules. These results also indicate that the same type of radial deformation should occur in CNT peapods that encapsulate anisotropic fullerenes.     

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.     

Design and Synthesis of a Water‐Stable Anionic Uranium‐Based Metal–Organic Framework (MOF) with Ultra Large Pores

Ionic metal–organic frameworks (MOFs) are a subclass of porous materials that have the ability to incorporate different charged species in confined nanospace by ion‐exchange. To date, however, very few examples combining mesoporosity and water stability have been realized in ionic MOF chemistry. Herein, we report the rational design and synthesis of a water‐stable anionic mesoporous MOF based on uranium and featuring tbo‐type topology. The resulting tbo MOF exhibits exceptionally large open cavities (3.9 nm) exceeding those of all known anionic MOFs. By supercritical CO₂ activation, a record‐high Brunauer‐Emmett‐Teller (BET) surface area (2100 m² g^?1) for actinide‐based MOFs has been obtained. Most importantly, however, this new uranium‐based MOF is water‐stable and able to absorb positively charged ions selectively over negatively charged ones, enabling the efficient separation of organic dyes and biomolecules.     

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.     

Preparation,Structural Determination,and Characterization of Electronic Properties of Bis‐silylated and Bis‐germylated Lu3N@Ih‐C80

Bis‐silylated and bis‐germylated derivatives of Lu₃N@I_h‐C₈₀ ( 3 , 4 , 5 ) were successfully synthesized by the photochemical addition of disiliranes 1 a , 1 b or digermirane 2 , and fully characterized by spectroscopic, electrochemical, and theoretical studies. Interestingly, digermirane 2 reacts more efficiently than disiliranes 1 a and 1 b because of its good electron‐donor properties and lower steric hindrance around the Ge?Ge bond. The 1,4‐adduct structures of 3 , 4 , 5 were unequivocally established by single‐crystal X‐ray crystallographic analyses. The electrochemical and theoretical studies reveal that the energy gaps between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the 1,4‐adducts are remarkably smaller than those of Lu₃N@I_h‐C₈₀, because the electron‐donating groups effectively raise the HOMO levels. It is also observed that germyl groups are slightly more electron‐donating than the silyl groups on the basis of the redox properties and the HOMO–LUMO energies of 4 and 5 . Bis‐silylation and bis‐germylation are effective and versatile methods for tuning the electronic characteristics of endohedral metallofullerenes.     

Supramolecular poly(ether ketone)–polyisobutylene pseudo‐block copolymers

The association behavior of telechelic hydrogen‐bonded poly(ether ketone) (PEK) and polyisobutylene (PIB) polymers and the formation of a new class of pseudo‐block copolymers is reported. The attachment of complementing hydrogen bonds (thymine/2,6‐diaminotriazine and cytosine/2,6‐diaminotriazine) onto the respective PIB and PEK polymers leads to a dramatic increase in the miscibility between the normally immiscible PEK and PIB polymers. The structure formation in the liquid state was studied by dynamic NMR spectroscopy as well as in the solid state via solid‐state NMR‐spectroscopy, DSC, and TEM methods. The polymers form a nanophase structure with a periodicity of 70 nm with the microphase separation occurring specifically within the donor–acceptor pair with the higher binding constant (thymine/2,6‐diaminotriazine) and not within the weaker bonded cytosine/2,6‐diaminotriazine pair. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 162–172, 2004     

Selective Encapsulation and Sequential Release of Guests Within a Self‐Sorting Mixture of Three Tetrahedral Cages

A mixture of two triamines, one diamine, 2‐formylpyridine and a Zn^II salt was found to self‐sort, cleanly producing a mixture of three different tetrahedral cages. Each cage bound one of three guests selectively. These guests could be released in a specific sequence following the addition of 4‐methoxyaniline, which reacted with the cages, opening each in turn and releasing its guest. The system here described thus behaved in an organized way in three distinct contexts: cage formation, guest encapsulation, and guest release. Such behavior could be used in the context of a more complex system, where released guests serve as signals to other chemical actors.     

Substrate‐Independent Magnetic Bistability in Monolayers of the Single‐Molecule Magnet Dy2ScN@C80 on Metals and Insulators

Magnetic hysteresis is demonstrated for monolayers of the single‐molecule magnet (SMM) Dy₂ScN@C₈₀ deposited on Au(111), Ag(100), and MgO|Ag(100) surfaces by vacuum sublimation. The topography and electronic structure of Dy₂ScN@C₈₀ adsorbed on Au(111) were studied by STM. X‐ray magnetic CD studies show that the Dy₂ScN@C₈₀ monolayers exhibit similarly broad magnetic hysteresis independent on the substrate used, but the orientation of the Dy₂ScN cluster depends strongly on the surface. DFT calculations show that the extent of the electronic interaction of the fullerene molecules with the surface is increasing dramatically from MgO to Au(111) and Ag(100). However, the charge redistribution at the fullerene‐surface interface is fully absorbed by the carbon cage, leaving the state of the endohedral cluster intact. This Faraday cage effect of the fullerene preserves the magnetic bistability of fullerene‐SMMs on conducting substrates and facilitates their application in molecular spintronics.     

Structure and Stability of Endohedral Complexes X@（HBNH）12

Using quantum chemistry methods B3LYP/6-31＋＋G（d,p） to optimize endohedral complexes X@（HBNH）12 （X=Li^0/＋, Na^0/＋, K^0/＋, Be^0/2＋, Mg^0/2＋, Ca^0/2＋, H and He）, the geometries with the lowest energy were achieved. Inclusion energy, standard equilibrium constant, natural charge, spin density, ionization potentials, and HOMO-LUMO energy gap were also discussed. The calculation predicted that X=Na^0/＋, K^0/＋, Mg^0/2＋, Ca^0/2＋, H and He are nearly located at the center of （HBNH）12 cluster. Li^＋ lies in less than 0.021 nm departure from the center. Li and Be^0/2＋ dramatically deviate from the center. （HBNH）12 prefers to enclose Li^＋, Be^2＋, Mg^2＋, and Ca^2＋ in it than others. Moreover, M@（HBNH）12 （M=Li, Na, K） species are ＂superalkalis＂ in that they possess lower first ionization potentials than the Cs atom （3.9 eV）.     

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.