2-Aminoethanol Extraction as a Method for Purifying Sc3N@C80 and for Differentiating Classes of Endohedral Fullerenes on the Basis of Reactivity

Fullertubes: A 30-Year Story of Prediction, Experimental Validation, and Applications for a Long-Missing Family of Soluble Carbon Molecules

ConspectusDuring the last 30 years, theoretical scientists imagined segmental families of monolayer carbon tubules with fullerene-based end-caps. These fullertube molecules would possess structural features of both fullerenes (hemispherical end-caps) and tubular belts of single-walled carbon nanotubes (SWCNTs). Yet, their experimental verification remained elusive for decades. It was not until 2020-2023 that segmental families of fullertubes were finally confirmed in the lab. The shocking irony is that these fullertubes were unwittingly coproduced alongside fullerenes (e.g., C60, C70, C84) in both flame and electric arc soot since the 1990s. Yet, nobody knew these "hidden" families of fullertubes were experimentally present in their extracted soot due to their low abundance and the absence of isolation methodology.This eruption of fullertube discoveries in 2020-2023 was brought to fruition by structural data, both DFT and experimental. This "Treasure Trove" of new molecules during this four-year window occurred with only microgram quantities. Typically, milligram levels of purified samples are required for X-ray crystallography and 13C NMR structural analysis. The breakthrough for experimentally verifying the missing fullertubes was an aminopropanol reagent to selectively react with and remove spheroidal carbon (e.g., C60, C70, C84) as hydrophilic derivatives. In contrast, there was suppressed reaction with fullertubes, which remained in organic solvent. It is well established that high symmetry (3-, 5-, and 6-fold) hemispheres for C60-Ih and other fullerenes and metallofullerenes are prerequisite end-caps for fullertubes. For the case of [5,5] C130 fullertubes, this requirement results in only eight 3-, 5-, and 6-fold symmetry structural isomers possible from a total of 39,393 possible isolated pentagon rule (IPR) isomers. From this C130 list of 8 candidate isolated pentagon rule (IPR) high symmetry isomers, surprisingly only one structure matched the DFT polarizability versus chromatographic retention parameter (a new gold standard for isomer identification). The simultaneous emergence of DFT computations of other properties (e.g., total energy, HOMO-LUMO gap, UV-vis) for large carbon molecules provided support for structural determination. Experimental approaches (e.g., mass spectrometry, UV-vis, XPS, Raman, and STEM) provided additional layers of structural elucidation at the microgram level. For the first time, we developed a chemical isolation protocol that would allow the preparation and isolation of soluble pristine fullertubes in the range of C90-C200. To date, applications of SWCNTs for use in nanoscale computer applications requires purities greater than 99.999%. Although this stringent mandate has not yet been demonstrated using SWCNT samples, this high level of purity appears achievable for metallic [5,5] D5d-C120 and semiconductor [10,0] D5h-C120 [10,766] fullertubes. Moreover, commercial production of pristine fullertubes should easily be feasible by the flame method due to its continuous operation and inexpensive feedstock. For application development, theoretical and electrochemical experimental data show that fullertubes exhibit high catalytic activity in oxygen reduction reactions. In the medical sector, pristine fullertube dispersions exhibit antimicrobial effects on Mycobacterium smegmatis and M. abscessus.

Structural Studies of Giant Empty and Endohedral Fullerenes

Structure elucidations of giant fullerenes composed of 100 or more carbon atoms are severely hampered by their extremely low yield, poor solubility and huge numbers of possible cage isomers. High-temperature exohedral chlorination followed by X-ray single crystal diffraction studies of the chloro derivatives offers a practical solution for structure elucidations of giant fullerenes. Various isomers of giant fullerenes have been determined by this method, specially, non-classical giant fullerenes containing heptagons generated by the skeletal transformations of carbon cages. Alternatively, giant fullerenes can be also stabilized by encapsulating metal atoms or clusters through intramolecular electron transfer from the encapsulated species to the outer fullerene cage. In this review, we present a comprehensive overview on synthesis, separation and structural elucidation of giant fullerenes. The isomer structures, chlorination patterns of a series of giant fullerenes C_2n (2n = 100-108) and heptagon-containing non-classical fullerenes derived from giant fullerenes are summarized. On the other hand, giant endohedral fullerenes bearing different endohedral species are also discussed. At the end, we propose an outlook on the future development of giant fullerenes.

Fullertubes: Cylindrical Carbon with Half-Fullerene End-Caps and Tubular Graphene Belts, Their Chemical Enrichment, Crystallography of Pristine C90-D5h(1) and C100-D5d(1) Fullertubes, and Isolation of C108, C120, C132, and C156 Cages of Unknown Structures

We report a chemical separation method to isolate fullertubes: a new and soluble allotrope of carbon whose structure merges nanotube, graphene, and fullerene subunits. Fullertubes possess single-walled carbon nanotube belts resembling a rolled graphene midsection, but with half-fullerene end-caps. Unlike nanotubes, fullertubes are reproducible in structure, possess a defined molecular weight, and are soluble in pristine form. The high reactivity of amines with spheroidal fullerene cages enables their removal and allows a facile isolation of C₉₆-D_3d(3), C₉₀-D_5h(1), and C₁₀₀-D_5d(1) fullertubes. A nonchromatographic step (Stage 1) uses a selective reaction of carbon cages with aminopropanol to permit a highly enriched sample of fullertubes. Spheroidal fullerenes are reacted and removed by attaching water-soluble groups onto their cage surfaces. With this enriched (100-1000 times) fullertube mixture, Stage 2 becomes a simple HPLC collection with a single column. This two-stage separation approach permits fullertubes in scalable quantities. Characterization of purified C₁₀₀-D_5d(1) fullertubes is done with samples isolated in pristine and unfunctionalized form. Surprisingly, C₆₀ and C₁₀₀-D_5d(1) are both purplish in solution. For X-ray crystallographic analysis, we used decapyrrylcorannulene (DPC). Isomerically purified C₉₀ and C₁₀₀ fullertubes were mixed with DPC to obtain black cocrystals of 2DPC{C₉₀-D_5h(1)}·4(toluene) and 2DPC{C₁₀₀-D_5d(1)}·4(toluene), respectively. A serendipitous outcome of this chemical separation approach is the enrichment and purification of several unreported larger carbon species, e.g., C₁₂₀, C₁₃₂, and C₁₅₆. Isolation of these higher cage species represents a significant advance in the unknown experimental arena of C₁₀₀-C₂₀₀ structures. Our findings represent seminal experimental evidence for the existence of two mathematically predicted families of fullertubes: one family with an axial hexagon with the other series based on an axial pentagon ring. Fullertubes have been predicted theoretically, and herein is their experimental evidence, isolation, and initial characterization.

Isolation and Crystallographic Characterization of Two, Nonisolated Pentagon Endohedral Fullerenes: Ho3 N@C2 (22010)-C78 and Tb3 N@C2 (22010)-C78

Purified samples of Ho₃ N@C₂ (22010)-C₇₈ and Tb₃ N@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 Ho₂ O₃ or Tb₄ O₇ . 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 Ho₃ N or Tb₃ N unit is planar. Ho₃ N@C₂ (22010)-C₇₈ and Tb₃ N@C₂ (22010)-C₇₈ use the same cage previously found for Gd₃ N@C₂ (22010)-C₇₈ rather than the IPR-obeying cage found in Sc₃ N@D_3h -C₇₈ .

Purification of Uranium-based Endohedral Metallofullerenes (EMFs) by Selective Supramolecular Encapsulation and Release

Supramolecular nanocapsule 1⋅(BArF)₈ is able to sequentially and selectively entrap recently discovered U₂ @C₈₀ and unprecedented Sc₂ CU@C₈₀ , simply by soaking crystals of 1⋅(BArF)₈ in a toluene solution of arc-produced soot. These species, selectively and stepwise absorbed by 1⋅(BArF)₈ , are easily released, obtaining highly pure fractions of U₂ @C₈₀ and Sc₂ CU@C₈₀ in one step. Sc₂ CU@C₈₀ represents the first example of a mixed metal actinide-based endohedral metallofullerene (EMF). Remarkably, the host-guest studies revealed that 1⋅(BArF)₈ is able to discriminate EMFs with the same carbon cage but with different encapsulated cluster and computational studies provide support for these observations.

Transformation of doped graphite into cluster-encapsulated fullerene cages

An ultimate goal in carbon nanoscience is to decipher formation mechanisms of highly ordered systems. Here, we disclose chemical processes that result in formation of high-symmetry clusterfullerenes, which attract interest for use in applications that span biomedicine to molecular electronics. The conversion of doped graphite into a C₈₀ cage is shown to occur through bottom-up self-assembly reactions. Unlike conventional forms of fullerene, the iconic Buckminsterfullerene cage, I_h-C₆₀, is entirely avoided in the bottom-up formation mechanism to afford synthesis of group 3-based metallic nitride clusterfullerenes. The effects of structural motifs and cluster–cage interactions on formation of compounds in the solvent-extractable C₇₀–C₁₀₀ region are determined by in situ studies of defined clusterfullerenes under typical synthetic conditions. This work establishes the molecular origin and mechanism that underlie formation of unique carbon cage materials, which may be used as a benchmark to guide future nanocarbon explorations.

An understanding of how caged carbon materials self-assemble from doped graphite is a long-standing challenge. Here, the authors show that distinct bottom-up processes lead to the synthesis of high-symmetry clusterfullerenes.

Non-Chromatographic Purification of Endohedral Metallofullerenes

The purification of endohedral metallofullerenes by high performance liquid chromatography is very time-consuming and expensive. A number of rapid and inexpensive non-chromatographic methods have thus been developed for large-scale purification of metallofullerenes. In this review, we summarize recent advances in non-chromatographic purification methods of metallofullerenes. Lewis acid-based complexation is one of the most efficient and powerful methods for separation of metallofullerenes from empty fullerenes. The first oxidation potential of metallofullerenes is a critical factor that affects the separation efficiency of the Lewis acid-based method. Supramolecular methods are effective for separation of fullerenes and metallofullerenes that are different in size and shape. Chemical/electrochemical reduction and exohedral functionalization are also utilized to separate and purify metallofullerenes on a large scale.

Piperazine-functionalized C60 and diiodine or iodine monochloride as components in forming supramolecular assemblies

The reaction of the piperazine mono-adduct, N(CH₂CH₂)₂NC₆₀, with diiodine produced well ordered, black crystals of (I₂N(CH₂CH₂)₂NI₂)C₆₀·2.884(C₆H₆)·0.116I₂, which contains two nearly linear N-I-I units. Reaction of N(CH₂CH₂)₂NC₆₀ with iodine monochloride produced two materials: the dihalogen adduct, (ClIN(CH₂CH₂)₂NICl)C₆₀·2.3(CS₂)·0.7(CH₂Cl₂), when crystallization occurred rapidly from carbon disulfide/dichloromethane solution or the salt, [(N(CH₂CH₂)₂NH)C₆₀⁺][ICl₂^-]·CS₂, when crystallization happened more slowly from toluene/dichloromethane solution where hydrolysis of the iodine monochloride by adventitious water presumably occurred.

Piperazine Functionalization of C70 for Incorporation into Supramolecular Assemblies

The photochemical reaction of piperazine with C₇₀ produces a mono-adduct (N(CH₂ CH₂ )₂ NC₇₀ ) in high yield (67 %) along with three bis-adducts. These piperazine adducts can combine with various Lewis acids to form crystalline supramolecular aggregates suitable for X-ray diffraction. The structure of the mono-adduct was determined from examination of the adduct I₂ N(CH₂ CH₂ )₂ NI₂ C₇₀ that was formed by reaction of N(CH₂ CH₂ )₂ NC₇₀ with I₂ . Crystals of polymeric {Rh₂ (O₂ CCF₃ )₄ N(CH₂ CH₂ )₂ NC₇₀ }_n ⋅nC₆ H₆ that formed from reaction of the mono-adduct with Rh₂ (O₂ CCF₃ )₄ contain a sinusoidal strand of alternating molecules of N(CH₂ CH₂ )₂ NC₇₀ and Rh₂ (O₂ CCF₃ )₄ connected through Rh-N bonds. Silver nitrate reacts with N(CH₂ CH₂ )₂ NC₇₀ to form black crystals of {(Ag(NO₃ ))₄ (N(CH₂ CH₂ )₂ NC₇₀ )₄ }_n ⋅7nCH₂ Cl₂ that contain parallel, nearly linear chains of alternating (N(CH₂ CH₂ )₂ NC₇₀ molecules and silver ions. Four of these {Ag(NO₃ )N(CH₂ CH₂ )₂ NC₇₀ }_n chains adopt a structure that resembles a columnar micelle with the ionic silver nitrate portion in the center and the nearly non-polar C₇₀ cages encircling that core. Of the three bis-adducts, one was definitively identified through crystallization in the presence of I₂ as ¹² {N(CH₂ CH₂ )₂ N}₂ C₇₀ with addends on opposite poles of the C₇₀ cage and a structure with C_2v symmetry. In ¹² {I₂ N(CH₂ CH₂ )₂ N}₂ C₇₀ , individual ¹² {I₂ N(CH₂ CH₂ )₂ N}₂ C₇₀ units are further connected by secondary I2⋅⋅⋅N2 interactions to form chains that occur in layers within the crystal. Halogen bond formation between a Lewis base such as a tertiary amine and I₂ is suggested as a method to produce ordered crystals with complex supramolecular structures from substances that are otherwise difficult to crystallize.

Synthesis and Isolation of the Titanium-Scandium Endohedral Fullerenes-Sc2 TiC@Ih -C80 , Sc2 TiC@D5h -C80 and Sc2 TiC2 @Ih -C80 : Metal Size Tuning of the Ti(IV) /Ti(III) 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 Sc4 C2 @C80 (the most abundant EMF from this synthesis), Sc3 C2 @C80 , isomers of Sc2 C2 @C82 , and the family Sc2 C2 n (2 n=74, 76, 82, 86, 90, etc.), as well as Sc3 CH@C80 . The Sc-Ti/CH4 system produces the mixed-metal Sc2 TiC@C2 n (2 n=68, 78, 80) and Sc2 TiC2 @C2 n (2 n=80) clusterfullerene families. The molecular structures of the new, transition-metal-containing endohedral fullerenes, Sc2 TiC@Ih -C80 , Sc2 TiC@D5h -C80 , and Sc2 TiC2 @Ih -C80 , were characterized by NMR spectroscopy. The structure of Sc2 TiC@Ih -C80 was also determined by single-crystal X-ray diffraction, which demonstrated the presence of a short Ti=C double bond. Both Sc2 TiC- and Sc2 TiC2 -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.