Empty-cage Fullerenes Research Articles

Chemical Isolation of Nanotubular Fullerenes (FullerTubes)

Much literature exists for empty-cage fullerenes with a spheroidal shape. With increasing numbers of carbon atoms, two cage shapes become possible: “tubes” and “balls.” However, little is reported for these nanotubular fullerenes (FullerTubes). This is due to their extremely low yield in typical arc-generated soot extract. Their low abundance renders their isolation quite difficult. HPLC alone is rather challenging using this classical separation method. With our approach, we have discovered a chemical-based approach to selectively react and remove spheroidal fullerenes. In doing so, we obtain samples enriched in FullerTubes by several orders of magnitude. In this presentation, we will discuss the synthesis and separation strategies needed to isolate Fullertubes in macroscopic quantities.

Selective Chemical Reaction of Empty Cage Fullertubes with Variable-Chain Aminoalcohols

Little is published on larger size empty cage fullerenes due to their low yield in soot extracts and inefficient isolation techniques. Even less is known about the newer Fullertubes – molecular carbon structures comprised of fullerene endcaps with a nanotube belt. We have developed a novel method to purify these rarely studied fullertubes and have now opened the door for research into these unique nanoparticles. In this poster presentation, I will discuss the chemical selectivity of various combinations of straight-chain aminoalcohols and solvents to enrich the abundance of Fullertubes in preparation for facile HPLC isolation.

(Invited) What Determines the Composition of Fullerene Porphyrin Cocrystals?

Cocrystallization of endohedral fullerenes and empty cage fullerenes with Ni(octaethylporphyrin) has become a widely used method to obtain X-ray structural information on these molecules. However, the stoichiometry of composition of these materials is difficult to predict. here we will report some new structures and studies of factors that effect the composition of these cocrystals.

Toward the Chemical Isolation of Larger-Size Empty-Cage Fullerenes

A plethora of literature exists for C60 and C70. Isolated in gram quantities since the early 1990s, these empty-cage fullerenes are well-studied due to their (1) high abundance in carbon soot extracts and (2) ease of separation. Depending on the method of synthesis (e.g., electric-arc), C60 and C70 typically comprise 95-98% of the soluble fullerene extract. The remaining 2-5% represents the higher fullerenes, e.g., C76 - C140 and beyond. How does one isolate these higher cage fullerenes given their extremely low abundance? Great question - which will be addressed. Our goal is developing a new separation method to chemically isolate these higher fullerenes. An inherent advantage to chemical methods is the removal of structural isomers, which present co-elution challenges with HPLC alone. In this presentation, we discuss the attractiveness of being able to remove more reactive fullerenes to greatly simplify samples. A successful purification strategy for isolating higher fullerenes would make macroscopic quantities available for the first time.

Reactor R&D: Effect of Plasma Additives on Endohedral Metallofullerene and Empty-Cage Fullerenes

Metallofullerenes encapsulating metallic nitride clusters represent exciting molecules for fundamental study and application development. Mixed-metal nitride clusters are also of interest due to their dissimilar metals. The first step in providing these endohedrals to the scientific community is their synthesis, such as in an electric-arc reactor. Our hypothesis is that we can tune the fullerene product distribution in soot by varying the chemicals that we add to cored graphite rods. In this presentation, our results will demonstrate that plasma additives do permit us to selectively make different types of fullerenes.

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 Sc4C2@C80 (the most abundant EMF from this synthesis), Sc3C2@C80, isomers of Sc2C2@C82, and the family Sc2C2 n (2 n=74, 76, 82, 86, 90, etc.), as well as Sc3CH@C80. The Sc–Ti/CH4 system produces the mixed‐metal Sc2TiC@C2 n (2 n=68, 78, 80) and Sc2TiC2@C2 n (2 n=80) clusterfullerene families. The molecular structures of the new, transition‐metal‐containing endohedral fullerenes, Sc2TiC@Ih‐C80, Sc2TiC@D 5h‐C80, and Sc2TiC2@Ih‐C80, were characterized by NMR spectroscopy. The structure of Sc2TiC@Ih‐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.

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@C2n (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 M4C2n (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.

Isolation and Crystallographic Characterization of Gd3N@D2(35)-C88 through Non-Chromatographic Methods.

While several nonchromatographic methods are available for the isolation and purification of endohedral fullerenes of the type M3N@Ih-C80, little work has been done that would allow other members of the M3N@C2n family to be isolated with minimal chromatography. Here, we report that Gd3N@D2(35)-C88 can be isolated from the multitude of endohedral and empty cage fullerenes present in carbon soot obtained by electric-arc synthesis using Gd2O3-doped graphite rods. The procedure developed utilizes successive precipitation with the Lewis acids CaCl2 and ZnCl2 followed by treatment with amino-functionalized silica gel. The structure of the product was identified by single-crystal X-ray diffraction.

Manipulating Reactivity Differences for Isolating New Endohedral Metallofullerenes

Much excitement is due to the production of new types of endohedral clusters inside fullerene cages. Scientists are also investigating the effect of increasing cage size versus the type and geometry of the entrapped cluster. Along with the enthusiasm of synthesizing diverse families of metallofullerenes comes the technical hurdle of isolating purified compounds from increasingly complex mixtures in raw soot extract. Based on reactivity differences between empty-cage fullerenes and endohedrals, one can fractionate and obtain separate samples of fullerenes and endohedrals. Our newest results show even further selectivity such that we can now achieve a separation of endohedrals from other endohedrals. An advantage of our new separation science is the non-HPLC approach. I will discuss the complexation/decomplexation approach with Lewis acids to distinguish between different classes of endohedrals. Further selectivity can be achieved by manipulating a binding step to amines, followed by kinetic differences during their release.

Enhanced Dipole Moments in Trimetallic Nitride Template Endohedral Metallofullerenes with the Pentalene Motif

Although not found to date in empty-cage fullerenes, the fused pentagon motifs (pentalenes) are allowed in endohedral metallofullerenes (EMFs). We have found that members of the trimetallic nitride template (TNT) EMF Y3N@C2n (n = 39-44) family that contain pentalene motifs exhibit significant dipole moments. This finding is predicted to be significant for other EMFs with a metal atom orientated toward the pentalene motif. Chromatographic retention data and computational results for Y3N@C2-C78, Y3N@Cs-C82, and Y3N@Cs-C84 are examples that pentalene groups lead to a significant induced dipole moment (∼1D). A special case is the Y3N@C2-C78 that contains two pentalenes in a relatively small cage. The (13)C NMR spectrum for Y3N@C2-C78 exhibits strongly deshielded signals for the fullerene cage (155-170 ppm) supporting the presence of the pentalene motif. In addition, a lengthening of the covalent M-N bond in the internal M3N cluster is found for all reported TNT EMFs that contain one or two pentalene motifs.

Selective Synthesis, Isolation, and Crystallographic Characterization of LaSc2N@Ih-C80

The successful preparation and isolation of the mixed-metal endohedral fullerene, LaSc(2)N@I(h)-C(80), and its structural characterization by single-crystal X-ray diffraction are reported. Results from chemically adjusting plasma temperature, energy, and reactivity (CAPTEAR) experiments indicate that a 10 wt % addition of Cu(NO(3))(2)·2.5H(2)O to a mixture of La(2)O(3) and Sc(2)O(3) decreases the amount of C(60) and C(70) found in soot extracts by an order of magnitude. By combining a stoichiometric 2-fold excess of La to Sc atoms in the plasma reactor, an extract containing a greater abundance of LaSc(2)N@I(h)-C(80) relative to Sc(3)N@I(h)-C(80) was obtained. Alternatively, the stir and filter approach (SAFA method) can be used to remove the empty cage fullerenes from a carbon soot sample prepared without using Cu(NO(3))(2)·2.5H(2)O. LaSc(2)N@I(h)-C(80) has been characterized by UV/vis absorption spectroscopy and by single-crystal X-ray diffraction. Ordered crystals with nearly identical orientations of the endohedral relative to the porphyrin have been obtained by cocrystallization of LaSc(2)N@I(h)-C(80) with either Ni(II)(OEP) or H(2)(OEP). The LaSc(2)N unit is planar, although earlier computations suggested that it would be pyramidal.

Functionalization of Endohedral Metallofullerenes

Since the serendipitous discovery of C60 and subsequent finding of other fullerenes, scientists have focused on the discovery of the physical and chemical properties of these nanomaterials and their incorporation in biomaterials and nanodevices [1,2]. The development of a particular family of fullerenes which incarcerates a metal, metals, or a metallic cluster in the interior of their carbon cages, the endohedral metallofullerenes, is quickly becoming one of the most interesting research areas within the field of carbon-based compounds because of the scope of electronic properties not accessible with empty-cage fullerenes [3-7]. Even though mass spectrometric evidence of their existence was already available in the first paper reporting the discovery of Buckminsterfullerene in 1985 [8, 9], there was controversy about the location of the metals, since fullerenes with exohedral metals were also observed. It was not until 1991 that Smalley and coworkers unequivocally showed that a lanthanum atom was encapsulated in a C82 fullerene shell as La@C82 [10], and since then, many analogous species have been isolated. However, the exploration of the physical, chemical and electronic properties of endohedral metallofullerenes has become a reality only in the last few years when technological advances have been made in the production of macroscopic quantities of these nanomaterials [11, 12]. In this section, an overall view of the effect of the encapsulated metal species on the exohedral reactivity of the carbon cages will be addressed and the differences between empty cage fullerenes and endohedral metallofullerenes will be emphasized. It is our intent to suggest that endohedral metallofullerenes are the fullerenes of the future. Keywords: Fullerenes, Endohedral fullerenes, Trimetallic nitride endohedral fullerenes

Isolation of Four Isomers of C96 and Crystallographic Characterization of Nanotubular D3d(3)‐C96 and the Somewhat Flat‐Sided Sphere C2(181)‐C96

Tube versus sphere: Two differently shaped fullerenes, nanotubular D3d(3)-C96 and the somewhat flat-sided C2(181)-C96 are the largest, pristine, empty-cage fullerenes to be identified structurally by X-ray crystallography to date.

Fullerenes in Medicine; Will it ever Occur?

Fullerenes represent a group of compounds having unique properties that make them attractivecandidates for use as a platform for developing new medical applications. The carbon cage(usually C60 and C70) of empty cage fullerenes (Figure 1-left/middle) are being developed astherapeutics for disease processes such as multiple sclerosis, neurodegeneration, HIV infection,cancer, radiation exposure, ischemia, allergic disease, infectious disease, and generalinflammation. Metallo-fullerenes (Figure 1-right; that have metals enclosed within the carboncage) are being developed into new biomarker homing, diagnostic contrast agents for MRI.Lastly, a new class of theranostics are being developed that combine cell targetingcapabilities/imaging with a therapeutic payload. Their inherent properties combined with theirability to be derivatized with side chains results in almost limitless new chemical structuresmaking them ideal platform molecules for new solutions to basic biological problems. However,one of the biggest obstacles that have kept this class of compounds from potentially improvinghuman health and reducing health care costs is the concern about toxicity. This has been due, inpart, to the lack of standard structural relationships that affect biological outcomes of FullereneDerivatives (FD).

Selective complexation and reactivity of metallic nitride and oxometallic fullerenes with Lewis acids and use as an effective purification method.

Metallic nitride fullerenes (MNFs) and oxometallic fullerenes (OMFs) react quickly with an array of Lewis acids. Empty-cage fullerenes are largely unreactive under conditions used in this study. The reactivity order is Sc(4)O(2)@I(h)-C(80) > Sc(3)N@C(78) > Sc(3)N@C(68) > Sc(3)N@D(5h)-C(80) > Sc(3)N@I(h)-C(80). Manipulations of Lewis acids, molar ratios, and kinetic differences within the family of OMF and MNF metallofullerenes are demonstrated in a selective precipitation scheme, which can be used either alone for purifying Sc(3)N@I(h)-C(80) or combined with a final high-performance liquid chromatography pass for Sc(4)O(2)@I(h)-C(80), Sc(3)N@D(5h)-C(80), Sc(3)N@C(68), or Sc(3)N@C(78). The purification process is scalable. Analysis of the experimental rate constants versus electrochemical band gap explains the order of reactivity among the OMFs and MNFs.

M2@C79N (M = Y, Tb): isolation and characterization of stable endohedral metallofullerenes exhibiting M-M bonding interactions inside aza[80]fullerene cages.

Y2@C79N and Tb2@C79N have been prepared by conducting the Kratschmer-Huffman electric-arc process under 20 Torr of N2 and 280 Torr of He with metal oxide-doped graphite rods. These new heterofullerenes were separated from the resulting mixture of empty cage fullerenes and endohedral fullerenes by chemical separation and a two-stage chromatographic process. Crystallographic data for Tb2@C79N x Ni(OEP) x 2 C6H6 demonstrate the presence of an 80-atom cage with idealized I(h) symmetry and two, widely separated Tb atoms inside with a Tb-Tb separation of 3.9020(10) A for the major terbium sites. The EPR spectrum of the odd-electron Y2@C79N indicates that the spin density largely resides on the two equivalent yttrium ions. Computational studies on Y2@C79N suggest that the nitrogen atom resides at a 665 ring junction in the equator on the fullerene cage and that the unpaired electron is localized in a bonding orbital between the two yttrium ions of this stable radical. Thus, the Tb-Tb bond length of the single-electron bond is an exceedingly long metal-metal bond.

Purification of Trimetallic Nitride Templated Endohedral Metallofullerenes by a Chemical Reaction of Congeners with Eutectic 9-Methylanthracene

The availability of pure samples is a major hurdle in the study of trimetallic nitride templated endohedral metallofullerenes (TNT EMFs). Current HPLC methods of purification are costly, nonrecyclable, and take considerable time. Reported herein is a solvent-free reaction of crude soot extract (empty-cage and Sc or Lu TNT EMFs) in molten 9-methylanthracene that affords almost complete conversion of empty-cage fullerenes to 9-methylanthracene adducts, while leaving TNT EMFs unreacted. After the recovered extract is washed with diethyl ether, the washed extract can then be applied to a silica gel column and subsequently flushed with toluene. To illustrate for Sc3N@C80, the sample was purified to ∼60% in a time period of 99%) in an expeditious and less costly manner.

Chemically Adjusting Plasma Temperature, Energy, and Reactivity (CAPTEAR) Method Using NOx and Combustion for Selective Synthesis of Sc3N@C80 Metallic Nitride Fullerenes

Goals are (1) to selectively synthesize metallic nitride fullerenes (MNFs) in lieu of empty-cage fullerenes (e.g., C60, C70) without compromising MNF yield and (2) to test our hypothesis that MNFs possess a different set of optimal formation parameters than empty-cage fullerenes. In this work, we introduce a novel approach for the selective synthesis of metallic nitride fullerenes. This new method is "Chemically Adjusting Plasma Temperature, Energy, and Reactivity" (CAPTEAR). The CAPTEAR approach with copper nitrate hydrate uses NOx vapor from NOx generating solid reagents, air, and combustion to "tune" the temperature, energy, and reactivity of the plasma environment. The extent of temperature, energy, and reactive environment is stoichiometrically varied until optimal conditions for selective MNF synthesis are achieved. Analysis of soot extracts indicate that percentages of C60 and Sc3N@C80 are inversely related, whereas the percentages of C70 and higher empty-cage C2n fullerenes are largely unaffected. Hence, there may be a "competitive link" in the formation and mechanism of C60 and Sc3N@C80. Using this CAPTEAR method, purified MNFs (96% Sc3N@C80, 12 mg) have been obtained in soot extracts without a significant penalty in milligram yield when compared to control soot extracts (4% Sc3N@C80, 13 mg of Sc3N@C80). The CAPTEAR process with Cu(NO3)2.2.5H2O uses an exothermic nitrate moiety to suppress empty-cage fullerene formation, whereas Cu functions as a catalyst additive to offset the reactive plasma environment and boost the Sc3N@C80 MNF production.

An Alternative Approach for Separating Metallic Nitride Fullerenes (MNFs) from Nanomaterial Mixtures

Metallic nitride fullerenes (MNFs) are of interest to fundamental studies and applications. However, MNF sample availability for these studies has been limited. The lack of samples is related to the currently used chromatographic (HPLC) separation protocol, a tedious approach that is solvent intensive, expensive, and has a limited throughput. To address these drawbacks, we developed an alternative method (non-chromatographic) for removing contaminant C60 and C70 empty-cage fullerenes and other classical metallofullerenes (i.e. non-nitrogen containing endohedrals) from soot extracts. Specifically, we present a purification approach based on differences of cage reactivity of MNFs versus other fullerenes. Results indicate that reactive silica can preferentially bind to contaminant fullerenes and metallofullerenes using our Stir and Filter Approach (SAFA). With this process, fullerene contaminants are immobilized onto reactive silica, while unreacted MNFs remain in the filtrate. In this manner, purified samples of Sc3N@C80 are readily obtained.

Nonchromatographic “Stir and Filter Approach” (SAFA) for Isolating Sc3N@C80 Metallofullerenes

Separation difficulties have led to a paucity of purified metallic nitride fullerenes (MNFs). Fundamental research and application development has been hampered with limited sample availability. Separation techniques designed to remove contaminant empty-cage fullerenes (e.g., C(60), C(70)...C(2)(n)) and classical metallofullerenes (e.g., non-MNFs) traditionally require expensive and tedious chromatographic methods. Our motivation is an alternative purification approach to minimize dependence on HPLC. Herein we report the use of cyclopentadienyl (CPD) and amino functionalized silica to selectively bind contaminant fullerenes. This "Stir and Filter Approach" (SAFA) provides purified MNF samples at ambient and reflux conditions. Under reflux conditions, purified MNFs (80% recovery, 41 h) are obtained using CPD silica. However, at room temperature, there is an equilibrium established between fullerenes and CPD silica, and no purified MNF samples are obtained using SAFA. In contrast, purified MNF samples (99+%) are readily obtained at room temperature using amino, diamino, and triamino silica at recoveries of 93% (11 h), 76% (9 h), and 50% (6 h), respectively.