Self-driven carbon atom implantation into fullerene embedding metal-carbon cluster.

Hundreds of members have been synthesized and versatile applications have been promised for endofullerenes (EFs) in the past 30 y. However, the formation mechanism of EFs is still a long-standing puzzle to chemists, especially the mechanism of embedding clusters into charged carbon cages. Here, based on synthesis and structures of two representative vanadium-scandium-carbido/carbide EFs, VSc2C@Ih (7)-C80 and VSc2C2@Ih (7)-C80, a reasonable mechanism-C1 implantation (a carbon atom is implanted into carbon cage)-is proposed to interpret the evolution from VSc2C carbido to VSc2C2 carbide cluster. Supported by theoretical calculations together with crystallographic characterization, the single electron on vanadium (V) in VSc2C@Ih (7)-C80 is proved to facilitate the C1 implantation. While the V=C double bond is identified for VSc2C@Ih (7)-C80, after C1 implantation the distance between V and C atoms in VSc2C2@Ih (7)-C80 falls into the range of single bond lengths as previously shown in typical V-based organometallic complexes. This work exemplifies in situ self-driven implantation of an outer carbon atom into a charged carbon cage, which is different from previous heterogeneous implantation of nonmetal atoms (Group-V or -VIII atoms) driven by high-energy ion bombardment or high-pressure offline, and the proposed C1 implantation mechanism represents a heretofore unknown metal-carbon cluster encapsulation mechanism and can be the fundamental basis for EF family genesis.

Steering Single-Electron Metal-Metal Bonds and Hyperfine Coupling between a Transition Metal-Lanthanide Heteronuclear Bimetal Confined in Carbon Cages.

Metal complexes bearing single-electron metal-metal bonds (SEMBs) exhibit unusual electronic structures evoking strong magnetic coupling, and such bonds can be stabilized in the form of dimetallofullerenes (di-EMFs) in which two metals are confined in a carbon cage. Up to now, only a few di-EMFs containing SEMBs are reported, which are all based on a high-symmetry icosahedral (Ih) C80 cage embedding homonuclear rare-earth bimetals, and a chemical modification of the Ih-C80 cage is required to stabilize the SEMB. Herein, by introducing 3d-block transition metal titanium (Ti) along with 4f-block lanthanum (La) into the carbon cage, we synthesized the first crystallographically characterized SEMB-containing 3d-4f heteronuclear di-EMFs based on pristine fullerene cages. Four novel La-Ti heteronuclear di-EMFs were isolated, namely, LaTi@D3h(5)-C78, LaTi@Ih(7)-C80, LaTi@D5h(6)-C80, and LaTi@C2v(9)-C82, and their molecular structures were unambiguously determined by single-crystal X-ray diffraction. Upon increasing the cage size from C78 to C82, the La-Ti distance decreases from 4.31 to 3.97 Å, affording fine-tuning of the metal-metal bonding and hyperfine coupling, as evidenced by an electron spin resonance (ESR) spectroscopic study. Density functional theory (DFT) calculations confirm the existence of SEMB in all four LaTi@C2n di-EMFs, and the accumulation of electron density between La and Ti atoms shifts gradually from the proximity of the Ti atom inside C78 to the center of the LaTi bimetal inside C82 due to the decrease of the La-Ti distance. The electronic properties of LaTi@C2n heteronuclear dimetallofullerenes differ apparently from their homonuclear La2@C2n counterparts, revealing the peculiarity of heteronuclear dimetallofullerenes with the involvement of 3d-block transition metal Ti.

Few-Layer Fullerene Network for Photocatalytic Pure Water Splitting into H2 and H2 O2.

A few-layer fullerene network possesses several advantageous characteristics, including a large surface area, abundant active sites, high charge mobility, and an appropriate band gap and band edge for solar water splitting. Herein, we report for the first time that the few-layer fullerene network shows interesting photocatalytic performance in pure water splitting into H2 and H2 O2 in the absence of any sacrificial reagents. Under optimal conditions, the H2 and H2 O2 evolution rates can reach 91 and 116 µmol g-1 h-1 , respectively, with good stability. This work demonstrates the novel application of the few-layer fullerene network in the field of energy conversion.

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 Sc3 N@Ih -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 Sc3 N@Ih -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.

Monometallic Endohedral Azafullerene.

Azafullerenes derived from nitrogen substitution of carbon cage atoms render direct modifications of the cage skeleton, electronic, and physicochemical properties of fullerene. Gas-phase ionized monometallic endohedral azafullerene (MEAF) [La@C81N]+ formed via fragmentation of a La@C82 monoadduct was detected in 1999, but the pristine MEAF has never been synthesized. Here, we report the synthesis, isolation, and characterization of the first pristine MEAF La@C81N, tackling the two-decade challenge. Single-crystal X-ray diffraction study reveals that La@C81N has an 82-atom cage with a pseudo C3v(8) symmetry. According to DFT computations, the nitrogen substitution site within the C82 cage is proposed to locate at a hexagon/hexagon/pentagon junction far away from the encapsulated La atom. La@C81N exists in stable monomer form with a closed-shell electronic state, which is drastically different from the open-shell electronic state of the original La@C82. Our breakthrough in synthesizing a new type of azafullerene offers a new insight into the skeletal modification of fullerenes.

Capturing the Missing Carbon Cage Isomer of C84 via Mutual Stabilization of a Triangular Monometallic Cyanide Cluster.

Monometallic cyanide clusterfullerenes (CYCFs) represent a unique branch of endohedral clusterfullerenes with merely one metal atom encapsulated, offering a model system for elucidating structure-property correlation, while up to now only C82 and C76 cages have been isolated for the pristine CYCFs. C84 is one of the most abundant fullerenes and has 24 isomers obeying the isolated pentagon rule (IPR), among which 14 isomers have been already isolated, whereas the C2v(17)-C84 isomer has lower relative energy than several isolated isomers but never been found for empty and endohedral fullerenes. Herein, four novel C84-based pristine CYCFs with variable encapsulated metals and isomeric cages, including MCN@C2(13)-C84 (M = Y, Dy, Tb) and DyCN@C2v(17)-C84, have been synthesized and isolated, fulfilling the first identification of the missing C2v(17)-C84 isomer, which can be interconverted from the C2(13)-C84 isomer through two steps of Stone-Wales transformation. The molecular structures of these four C84-based CYCFs are determined unambiguously by single-crystal X-ray diffraction. Surprisingly, although the ionic radii of Y3+, Dy3+, and Tb3+ differ slightly by only 0.01 Å, such a subtle difference leads to an obvious change in the metal-cage interactions, as inferred from the distance between the metal atom and the nearest hexagon center of the C2(13)-C84 cage. On the other hand, upon altering the isomeric cage from DyCN@C2(13)-C84 to DyCN@C2v(17)-C84, the Dy-cage distance changes as well, indicating the interplay between the encapsulated DyCN cluster and the outer cage. Therefore, we demonstrate that the metal-cage interactions within CYCFs can be steered via both internal and external routes.

Synthesis and Photophysical Properties of [3]Cyclo-1,8-pyrenes via [4 + 2] Cycloaddition Reaction.

Herein, we report the synthesis, characterization, and photophysical properties of the crown-like structure of [3]cyclo-1,8-pyrenes (compounds 9 and 10). Planar pyrenyl arylene-ethynylene macrocycles are used as the precursors to synthesize these pyrene-based cycloarenes by [4 + 2] cycloaddition reaction with good yields. These molecules are confirmed by nuclear magnetic resonance spectroscopy and high-resolution mass spectrometry. The structure of 9 was unambiguously determined by single-crystal X-ray diffraction. Their photophysical properties are investigated by steady-state absorption, fluorescence, and time-resolved fluorescence spectroscopies, combined with theoretical calculations.

Favorite Orientation of the Carbon Cage and a Unique Two-Dimensional-Layered Packing Model in the Cocrystals of Nd@C82(I,II) Isomers with Decapyrrylcorannulene.

To date, the experimental studies on Nd-based metallofullerenes are only limited to spectroscopic characterizations. In this work, the molecular structures of Nd@C82(I,II) isomers, including the isomeric symmetry of the C cage and the position of endohedral Nd atom, as well as their unique two-dimensional (2D)-layered crystallographic packing structures were initially and unambiguously elucidated, based on the X-ray structural analyses of the cocrystals of Nd@C82(I) or Nd@C82(II) with cocrystallizing agent decapyrrylcorannulene (DPC). In the V-shaped unit cell, the endohedral Nd atom prefers a site as far away from the DPC molecules as possible because of the unevenly distributed charge on the C cage mainly related to the charge transfers from the endohedral Nd atom, cocrystallizing agent DPC, and solvent toluene molecules to the C82 cage. Apart from charge transfers, multiple C-H···π intermolecular interactions are also confirmed to play important roles both for the orientation of the C cage correlated with the preferential sites of the endohedral Nd atom and for the 2D-layered packing structures within the cocrystals. Density functional theory computations offered theoretical support for the molecular structures of Nd@C82(I,II) isomers, the valence of the endohedral Nd atom (between II+ and III+), and the global ground state, i.e., the Nd@C2v(9)-C82 isomer in the quintet state.

Anomalous Cis-Conformation Regioselectivity of Heterocycle-Fused Sc3 N@D3h -C78 Derivatives.

Endohedral clusterfullerenes exhibit unique chemical properties due to intramolecular electron transfer of the encaged metal cluster to the outer fullerene cages. We report the synthesis of two Sc3 N@D3h -C78 monoadducts 2 a and 2 b through the 1,3-dipolar reaction of Sc3 N@D3h -C78 with carbonyl ylide bearing anomalous cis-conformation regioselectivity. The molecular structures of these monoadducts are unambiguously confirmed by single-crystal X-ray crystallography, revealing that both 2 a and 2 b have cis-conformations with the furan moiety grafted via [6,6]-closed addition patterns. Under the same conditions, the control reaction of C60 with carbonyl ylide affords two monoadducts 3 a and 3 b, which exhibit cis- and trans-conformations, respectively, with [6,6]-closed addition patterns. According to theoretical calculations, the exclusive formation of the cis-only Sc3 N@D3h -C78 monoadducts is a consequence of conjunct effects of thermodynamic stability of adducts, the reactivity of the addition site, and the cis-dipole intermediate from trans 1.

Electrochemical regioselective alkylations of a [60]fulleroindoline with bulky alkyl bromides.

Electrochemical alkylations of a [60]fulleroindoline with different bulky alkyl bromides exhibit different reaction behaviors. The hydroalkylation and dialkylation of the electrochemically generated dianionic [60]fulleroindoline with bulky 2,4,6-tris(bromomethyl)mesitylene give rise to 1,2,3,16-adducts. In comparison, the hydroalkylation of the dianionic [60]fulleroindoline with bulkier diphenylbromomethane still affords a 1,2,3,16-adduct, while the corresponding dialkylation provides a sterically favoured 1,4,9,12-adduct, which is scarcely investigated, as the major product along with the isomeric 1,2,3,16-adduct as the minor product. The structures of these products have been determined by spectroscopic data and single-crystal X-ray diffraction analysis. A plausible reaction mechanism has been proposed to explain the formation of the observed products.

Strain Release of Fused Pentagons in Fullerene Cages by Chemical Functionalization.

According to the isolated pentagon rule (IPR), for stable fullerenes, the 12 pentagons should be isolated from one another by hexagons, otherwise the fused pentagons will result in an increase in the local steric strain of the fullerene cage. However, the successful isolation of more than 100 endohedral and exohedral fullerenes containing fused pentagons over the past 20 years has shown that strain release of fused pentagons in fullerene cages is feasible. Herein, we present a general overview on fused-pentagon-containing (i.e. non-IPR) fullerenes through an exhaustive review of all the types of fused-pentagon-containing fullerenes reported to date. We clarify how the strain of fused pentagons can be released in different manners, and provide an in-depth understanding of the role of fused pentagons in the stability, electronic properties, and chemical reactivity of fullerene cages.

Highly Efficient and Reversible Covalent Patterning of Graphene: 2D-Management of Chemical Information.

Patterned graphene-functionalization with a tunable degree of functionalization can tailor the properties of graphene. Here, we present a new reductive functionalization approach combined with lithography rendering patterned graphene-functionalization easily accessible. Two types of covalent patterning of graphene were prepared and their structures were unambiguously characterized by statistical Raman spectroscopy together with scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM-EDS). The reversible defunctionalization processes, as revealed by temperature-dependent Raman spectroscopy, enable the possibility to accurately modulate the degree of functionalization by annealing. This allows for the management of chemical information through complete write/store/erase cycles. Based on our strategy, controllable and efficient patterning graphene-functionalization is no longer a challenge and facilitates the development of graphene-based devices.

Selective Synthesis of Conjugated Chiral Macrocycles: Sidewall Segments of (-)/(+)-(12,4) Carbon Nanotubes with Strong Circularly Polarized Luminescence.

Carbon nanotubes (CNTs) have unusual physical properties that are valuable for nanotechnology and electronics, but the chemical synthesis of chirality- and diameter-specific CNTs and π-conjugated CNT segments is still a great challenge. Reported here are the selective syntheses, isolations, characterizations, and photophysical properties of two novel chiral conjugated macrocycles ([4]cyclo-2,6-anthracene; [4]CAn2,6 ), as (-)/(+)-(12,4) carbon nanotube segments. These conjugated macrocyclic molecules were obtained using a bottom-up assembly approach and subsequent reductive elimination reaction. The hoop-shaped molecules can be directly viewed by a STM technique. In addition, chiral enantiomers with (-)/(+) helicity of the [4]CAn2,6 were successfully isolated by HPLC. The new tubular CNT segments exhibit large absorption and photoluminescence redshifts compared to the monomer unit. The carbon enantiomers are also observed to show strong circularly polarized luminescence (glum ≈0.1). The results reported here expand the scope of materials design for bottom-up synthesis of chiral macrocycles and enrich existing knowledge of their optoelectronic properties.

Covalent Inter-Carbon-Allotrope Architectures Consisting of the Endohedral Fullerene Sc3 N@C80 and Single-Walled Carbon Nanotubes.

Using a reductive sidewall functionalization concept, we prepared for the first time a covalent inter-carbon-allotrope hybrid consisting of single-walled carbon nanotubes (SWCNTs) and the endohedral fullerene Sc3 N@C80 . The new compound type was characterized through a variety of techniques including absorption spectroscopy, Raman spectroscopy, TG-MS, TG-GC-MS, and MALDI-TOF MS. HRTEM investigations were carried out to visualize this highly integrated architecture.

From Cubes to Dice: Solvent-Regulated Morphology Engineering of Endohedral Fullerene Microcrystals with Anomalous Photoluminescence Enhancement.

Despite recent successes in preparing three-dimensional crystals of empty fullerenes, such as C60 and C70 , 3D endohedral fullerene crystals, and especially hollow nanostructures, have been scarcely reported. A universal approach has now been developed to prepare shape-tunable 3D crystals of several metal nitride clusterfullerenes, including cubes and dice (hollow cubes with holes at the center of each face), which can be readily switched by changing the volume ratio of good (mesitylene) and poor (isopropanol) solvents. Synchrotron-based soft X-ray nano-computed tomography was used to unambiguously identify the interior microstructure of the dice-shaped crystals of Tb3 N@C80 , and especially the depth of the hole at each face, confirming that the dice has a solid center and the holes are not interconnected. Owing to the enhanced light absorption, the dice-shaped crystals exhibit enhanced photoluminescence relative to that of the cubes.

Azide Passivation of Black Phosphorus Nanosheets: Covalent Functionalization Affords Ambient Stability Enhancement.

Two-dimensional (2D) black phosphorus (BP) has a unique band structure, but it suffers from low ambient stability owing to its high reactivity to oxygen. Covalent functionalization has been demonstrated to passivate the reactive BP effectively, however the reported covalent functionalization methods are quite limited to aryl diazonium and nucleophilic additions affording P-C and P-O-C single bonds, for which the retaining of one unpaired electron in the Group 15 phosphorus atom hampers the passivation effect. Now, covalent azide functionalization of BP nanosheets (BPNSs) is reported, leading to significant enhancement of the ambient stability of BP as confirmed by UV/Vis spectroscopic studies. The most stable configuration of the azide functionalized BPNSs (f-BPNSs) is predicted by theoretical calculations, featuring the grafting of benzoic acid moiety onto BPNSs via the unprecedented P=N double bonds formed through in situ nitrene as a reactive intermediate.

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 Ih -C80 cage. Non-Group-3 transition-metal-containing endohedral fullerenes based on a C80 cage with D5h symmetry, Vx Sc3-x N@D5h -C80 (x=1, 2), have now been synthesized, which exhibit two variable cluster compositions. The molecular structure of VSc2 N@D5h -C80 was unambiguously determined by X-ray crystallography. According to a comparative study with the reported Ti- and V-containing clusterfullerenes based on a Ih -C80 cage and the analogous D5h -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 D5h -C80 -based clusterfullerenes is unraveled.

Stabilization of Giant Fullerenes C2(41)-C90, D3(85)-C92, C1(132)-C94, C2(157)-C96, and C1(175)-C98 by Encapsulation of a Large La2C2 Cluster: The Importance of Cluster-Cage Matching.

Successful isolation and unambiguous crystallographic assignment of a series of higher carbide cluster metallofullerenes present new insights into the molecular structures and cluster-cage interactions of endohedral metallofullerenes. These new species are identified as La2C2@C2(41)-C90, La2C2@D3(85)-C92, La2C2@C1(132)-C94, La2C2@C2(157)-C96, and La2C2@C1(175)-C98. This is the first report for these new cage structures except for D3(85)-C92. Our experimental and theoretical results demonstrate that La2C92-106 are more inclined to exist stably in the carbide form La2C2@C90-104 rather than as the dimetallofullerenes La2@C92-106, which are rationalized by considering a synergistic effect of inserting a C2 unit into the cage, which ensures strong metal-cage interactions by partially neutralizing the charges from the metal ions and by fulfilling the coordination requirement of the La3+ ions as much as possible.

Evidence of Oxygen Activation in the Reaction between an N-Heterocyclic Carbene and M3N@Ih(7)-C80: An Unexpected Method of Steric Hindrance Release.

We herein demonstrate for the first time the unexpected oxygen-involving reaction between M3N@Ih(7)-C80 (M = Sc, Lu) and 1,3-bis(diisopropylphenyl)imidazol-2-ylene (1). By introducing a tiny amount of oxygen into the reaction, unprecedented products (2a for Sc3N@C80 and 3a for Lu3N@C80) with the normal carbene center C2 singly bonded to a triple hexagonal junction (THJ) cage carbon together with an oxygen atom bridging the same THJ carbon atom and a neighboring carbon atom forming an epoxy structure are obtained. In situ mechanism study, in combination with theoretical calculations, reveals that the bond-breaking peroxidation facilitates the formation of the unexpected products 2a and 3a, providing new insight into fullerene chemistry.

Lewis Acid-Base Adducts of Sc2C2@C3v(8)-C82/N-Heterocyclic Carbene: Toward Isomerically Pure Metallofullerene Derivatives.

The addition of a bulky N-heterocyclic carbene (NHC) to Sc2C2@C3v(8)-C82 affords two monoadducts (2a and 2b) quantitatively and regioselectively, representing the first examples of Lewis acid-base pairs of metal carbide cluster fullerenes. 2b is likely a kinetically favorable labile product that cannot be isolated from the solution. The crystallographic results of 2a unambiguously demonstrate that one polarized C-C single bond is formed between the normal carbene site C2N of the NHC and a specific [5,6,6]-carbon atom out of 17 types of nonequivalent cage carbon atoms of Sc2C2@C3v(8)-C82. Theoretical calculations demonstrate that the high regioselectivity, the unexpected addition pattern, and the quantitative formation of monoadducts are synergistic results from the cage geometry and electron distribution on the cage.