Infrared spectroscopy of small-molecule endofullerenes

Hydrogen is one of the few molecules that has been incarcerated in the molecular cage of C₆₀ to form the endohedral supramolecular complex H₂@C₆₀. In this confinement, hydrogen acquires new properties. Its translation motion, within the C₆₀ cavity, becomes quantized, is correlated with its rotation and breaks inversion symmetry that induces infrared (IR) activity of H₂. We apply IR spectroscopy to study the dynamics of hydrogen isotopologues H₂, D₂ and HD incarcerated in C₆₀. The translation and rotation modes appear as side bands to the hydrogen vibration mode in the mid-IR part of the absorption spectrum. Because of the large mass difference of hydrogen and C₆₀ and the high symmetry of C₆₀ the problem is almost identical to a vibrating rotor moving in a three-dimensional spherical potential. We derive potential, rotation, vibration and dipole moment parameters from the analysis of the IR absorption spectra. Our results were used to derive the parameters of a pairwise additive five-dimensional potential energy surface for H₂@C₆₀. The same parameters were used to predict H₂ energies inside C₇₀. We compare the predicted energies and the low-temperature IR absorption spectra of H₂@C₇₀.

Molecular surgical synthesis of H₂@C₆₀: recollections

The first synthesis of endohedral fullerene containing molecular hydrogen, H₂@C₆₀, is briefly summarized. The synthesis was conducted according to what we call the 'molecular surgical method', that is, opening a hole on a C₆₀ surface, enlargement of the hole, insertion of a guest species and enclosure of the hole without loss of the encapsulated guest. The entire process involves three chemical reactions to open the hole and four reactions to gradually reduce the hole size and finally close the hole. The total yield of the product, H₂@C₆₀, based on consumed C₆₀ was 9%. The encapsulated molecule of hydrogen exhibited a 6 ppm upfield-shifted ¹H NMR signal when compared with free hydrogen, indicating the aromaticity at the inner centre of the C₆₀ cage.

X-ray observation of a helium atom and placing a nitrogen atom inside He@C60 and He@C70

Single crystal X-ray analysis has been used as a powerful method to determine the structure of molecules. However, crystallographic data containing helium has not been reported, owing to the difficulty in embedding helium into crystalline materials. Here we report the X-ray diffraction study of He@C60 and the clear observation of a single helium atom inside C60. In addition, the close packing of a helium atom and a nitrogen atom inside fullerenes is realized using two stepwise insertion techniques, that is, molecular surgery to synthesize the fullerenes encapsulating a helium atom, followed by nitrogen radio-frequency plasma methods to generate the fullerenes encapsulating both helium and nitrogen atoms. Electron spin resonance analysis reveals that the encapsulated helium atom has a small but detectable influence on the electronic properties of the highly reactive nitrogen atom coexisting inside the fullerene, suggesting the potential usage of helium for controlling electronic properties of reactive species.

Theoretical investigations of sp-sp2 hybridized zero-dimensional fullerenynes

Widely recognized as the quintessential material, sp(2) hybridized carbon material with low dimensions, such as zero-dimensional fullerene, one-dimensional carbon nanotube and two-dimensional graphene, has already compiled an impressive list of superlatives. Quite recently, one-dimensional sp-sp(2) hybridized carbon tubular arrays with a wall thickness of about 40 nm and two-dimensional carbon films with the average thickness of 970 nm have been synthesized successfully. Thus, we expect that the existence of a sp-sp(2) hybridized zero-dimensional carbon allotrope is possible. A novel and stable zero-dimensional carbon allotrope (fullerenyne) with sp-sp(2) hybridization is introduced by means of density functional theory calculation and molecular dynamics confirmation. Unique porous characteristic C(96) fullernenyne with an O(h) symmetry group exhibits exceptionally high stability. We hope that the present study will lead to a further development of a broad new class of carbon materials.

Rational synthesis, enrichment, and (13)C NMR spectra of endohedral C(60) and C(70) encapsulating a helium atom

Endohedral fullerenes encapsulating a helium atom, i.e., He@C(60) and He@C(70), at occupation levels of 30% were prepared by rational chemical synthesis. The existence of weak interactions between the inner helium and the outer fullerene cages was demonstrated by experimental and computational investigations.

Efficient cage-opening cascade process for the preparation of water-encapsulated [60]fullerene derivatives

Cage-opened fullerenes with 18-membered-ring orifices have been prepared starting from fullerene-mixed peroxides. The key transformation involves a cascade sequence including a deketalization, a S(N)2' epoxide opening reaction, and a formal 3,3-sigma rearrangement. The 18-membered-ring orifice is big enough for water encapsulation under mild conditions. Single crystal X-ray structures were obtained for both the empty and water-encapsulated fullerene derivatives.

Extraction and derivatization in single drop coupled to MALDI-FTICR-MS for selective determination of small molecule aldehydes in single puff smoke

Extraction and derivatization in single drop (EDSD) coupled to matrix-assisted laser desorption/ionization Fourier transform ion cyclotron resonance mass spectrometry (MALDI-FTICR-MS) was utilized to determine small molecular aldehydes (SMAs) in single puff smoke. A methanol solution of diphenylamine, 2,5-dihydroxybenzoic acid was used for extraction/derivatization of SMAs and a microdrop of the extraction solution containing the ionic derivatization products of SMAs could be directly deposited in the MALDI target to perform the measurement of SMAs in the cigarette smoke on puff level by MALDI-FTICR-MS. As a result, a consecutive operation of sample pre-treating and MALDI sample plate preparation was achieved. SMAs were derivatized by the reaction with diphenylamine and transformed a stable Schiff base bearing a quaternary ammonium group, and the sensitivity of analysis for SMAs was significantly improved. Good sample homogeneity in in-dot was achieved by adding diphenylamine into the solvent for EDSD and a satisfying repeatability (R.S.D.=7.3% for formaldehyde, n=5) of MALDI-FTICR-MS signals was obtained. This solvent- and reagent-minimized approach with an automatic potential provided a simple, rapid, and accurate procedure for the determination of SMAs in smoke.

Surgery of fullerenes

Recent attempts at the synthesis of endohedral fullerenes by organic reactions, so-called "molecular surgery" methods, are surveyed. The creation of an opening on the surface of fullerene cages allowed insertion of He, H(2), H(2)O, or CO within the cages. An effective route to "suture" an opening was established to realize a new endohedral fullerene, H(2)@C(60). Further development of this operation as well as the properties and reactions of H(2)@C(60) are summarized. Also the application of the encapsulated H(2) molecule as an NMR probe for the study of aromaticity of ionic fullerenes is described.

Synthesis and properties of annulenic subunits of graphyne and graphdiyne nanoarchitectures

This report describes the synthetic strategies toward and optoelectronic properties of substructures of the non-natural, planar carbon networks graphyne and graphdiyne, which are based on the dehydrobenzo[12]annulene and dehydrobenzo[18]annulene framework, respectively.

Synthesis of [59]fullerenones through peroxide-mediated stepwise cleavage of fullerene skeleton bonds and X-ray structures of their water-encapsulated open-cage complexes

Fullerene skeleton modification has been investigated through selective cleavage of the fullerene carbon-carbon bonds under mild conditions. Several cage-opened fullerene derivatives including three [59]fullerenones with an 18-membered-ring orifice and one [59]fullerenone with a 19-membered-ring orifice have been prepared starting from the fullerene mixed peroxide 1, C60(OOtBu)6. The prepositioned tert-butyl peroxy groups in 1 serve as excellent oxygen sources for formation of hydroxyl and carbonyl groups. The cage-opening reactions were initiated by photoinduced homolysis of the tBu-O bond, followed by sequential ring expansion steps. A key step of the ring expansion reactions is the oxidation of adjacent fullerene hydroxyl and amino groups by diacetoxyliodobenzene (DIB). Aminolysis of a cage-opened fullerene derivative containing an anhydride moiety resulted in multiple bond cleavage in one step. A domino mechanism was proposed for this reaction. Decarboxylation led to elimination of one carbon atom from the C60 cage and formation of [59]fullerenones. The cage-opened [59]fullerenones were found to encapsulate water under mild conditions. All compounds were characterized by spectroscopic data. Single-crystal structures were also obtained for five skeleton-modified derivatives including two water-encapsulated fulleroids.

Efficient Synthesis of Open-Cage Fullerene Derivatives Having 16-Membered-Ring Orifices

The synthesis of 10 new open-cage fullerene derivatives with a 16-membered-ring orifice is being reported. These compounds, derived from the regioselective addition reaction of an aromatic hydrazine or hydrazone to isomeric diketone derivatives of C(60), were isolated in moderate to excellent yields.

Open-Cage Fullerene Derivatives Having 11-, 12-, and 13-Membered-Ring Orifices: Chemical Transformations of the Organic Addends on the Rim of the Orifice

A novel open-cage [60]fullerene derivative, having two sulfur atoms on the rim of its 13-membered-ring orifice, has been isolated and characterized. Extensive studies on the N-MEM group reactivity of this as well as other previously reported open-cage [60]fullerene derivatives led to several new open-cage [60]fullerene adducts.

An Orifice-Size Index for Open-Cage Fullerenes

In the field of open-cage fullerenes, there was a lack of a universal standard that could correlate and quantify the orifice size of open-cage fullerenes. One cannot compare the relative orifice size by simple comparison of the number of atoms that composes the orifice. We present a general term for easy estimation of relative orifice size by defining an index for open-cage fullerenes. We estimated the corresponding effective areas A(area) for orifices of open-cage fullerenes by matching calculated activation energies Ea(calcd) for hydrogen release from open-cage fullerenes (B3LYP/6-31G**//B3LYP/3-21G) to the computed energies required for a hydrogen molecule passing through a cyclo[n]carbon ring. Then we define an index K(orifice) based on experimental hydrogen release rate, where K(orifice) = ln k/k degrees (k is rate constant of hydrogen-release rate of any open-cage fullerenes taken for comparison at 160 degrees C; k degrees is the hydrogen release rate from H2@4a taken as the standard compound). We synthesized several open-cage fullerenes and studied kinetics of a set of H2-encapsulated open-cage fullerenes to evaluate their K values. A correlation of the index K(orifice) with the effective areas A(area) showed a good linear fit (r2 = 0.972) that demonstrated a good interplay between experiment and theory. This allows one to estimate K(orifice) index and/or relative rate k of hydrogen release through computing activation energy Ea(calcd) for a designed open-cage fullerene.

Approaches to Open Fullerenes: Synthesis and Kinetic Stability of Diels−Alder Adducts of Substituted Isobenzofurans and C60

We have examined the reactions of 1,3-disubstituted isobenzofurans with the fullerene C60 in the context of an approach to open a large orifice on the fullerene framework. A variety of substituted isobenzofurans (6a-h), generated from the reaction of 1,4-substituted 1,4-epoxynaphthalenes 3a-h with 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine (4a) or 1,2,4,5-tetrazine (4b), were added to C60 to afford the Diels-Alder adducts 7a-h. The thermal stability of these adducts toward retro-Diels-Alder fragmentation differs greatly in solution from that in the solid state. In solution, the relatively facile retro-Diels-Alder fragmentation of monoadducts 7a and 7c, to give C60 and the free isobenzofurans 6a and 6c, have rate constants (and activation barriers) of k=9.29x10(-5) s-1 at 70 degrees C (Ea=32.6 kcal mol-1) and k=1.36x10(-4) s-1 at 40 degrees C (Ea=33.7 kcal mol-1), respectively, indicating that the addition of isobenzofurans to C60 is readily reversible at those temperatures. In the solid state, thermogravimetric analysis of adduct 7a indicates that its decomposition occurs only within the temperature range of 220-300 degrees C. As a result, these compounds can be stored at room temperature in the solid state for several weeks without significant decomposition but have to be handled within several hours in solution.

Fine tuning of the orifice size of an open-cage fullerene by placing selenium in the rim: insertion/release of molecular hydrogen

A newly synthesized open-cage fullerene containing selenium in the rim of the 13-membered-ring orifice allows milder conditions for hydrogen insertion, and the rate for hydrogen release is ca. three times faster than its sulfur analogue.

The outside knows the difference inside: trapping helium by immediate reduction of the orifice size of an open-cage fullerene and the effect of encapsulated helium and hydrogen upon the NMR of a proton directly attached to the outside

A methodology to entrap He inside an open-cage fullerene by immediate reduction of the size of an orifice was developed, and the effects of encapsulated He and H(2) on the chemical shift of a proton directly attached to the outer fullerene sphere were revealed.

Switch of Electronic Reactivity in Fullerene C60: Activation of Three trans-4 Positions via Temporary Saturation of the cis-1 Positions

[Structure: see text] The regioselective functionalization of C60 with a trans-4,trans-4,trans-4 trisaddition pattern is not feasible directly. We have found an indirect approach taking advantage of the modified electronic reactivity of cis-1 bisadducts. The cis-1 addition pattern electronically activates three trans-4 C=C bonds on the opposite hemisphere of C60, allowing further highly regioselective additions at these positions. Thermal removal of the cis-1 blocking unit results in a trans-4,trans-4,trans-4 trisadduct with C3v symmetry.

Approaches to Open Fullerenes: A 1,2,3,4,5,6-Hexaadduct of C60

Complete saturation of a single six-membered ring on fullerene C60 has been achieved. The critical step in this first synthesis of a fully characterized 1,2,3,4,5,6-hexaadduct consisted of a remarkable double 5-exo-trig addition of alkoxyl radicals promoted by lead tetraacetate. Two possible opening pathways ([2 + 2 + 2] retrocycloadditions) for the newly synthesized compound were explored using quantum mechanical calculations. We found that the oxa bridges in the hexaadduct prevent ring opening through the retro[2 + 2 + 2] mechanism due to the high activation barrier and endothermicity of the reaction.

Synthesis and Properties of Endohedral C60 Encapsulating Molecular Hydrogen

We report the details of our study to synthesize a new endohedral fullerene, H2@C60, in more than 100 mg quantities by closure of the 13-membered ring orifice of an open-cage fullerene using four-step organic reactions. The 13-membered ring orifice in a previously synthesized open-cage fullerene incorporating hydrogen in 100% yield was reduced to a 12-membered ring by extrusion of a sulfur atom at the rim of the orifice, and the ring was further reduced into an eight-membered ring by reductive coupling of two carbonyl groups also at the orifice. Final closure of the orifice was completed by a thermal reaction. Purification of H2@C60 was accomplished by recycle HPLC. A gradual downfield shift of the NMR signal for the encapsulated hydrogen observed upon reduction of the orifice size was interpreted based on the gauge-independent atomic orbital (GIAO) and the nucleus-independent chemical shift (NICS) calculations. The spectral as well as electrochemical examination of the properties of H2@C60 has shown that the electronic interaction between the encapsulated hydrogen and outer C60 pi-system is quite small but becomes appreciable when the outer pi-system acquires more than three extra electrons. Four kinds of exohedral derivatives of H2@C60 were synthesized. The tendency in the shift of the NMR signal of the inner hydrogen was found to be quite similar to that observed for the 3He NMR signal of the corresponding derivatives of 3He@C60.

Tether-Directed Selective Synthesis of Fulleropyrrolidine Bisadducts

Selective synthesis of C60 bisadducts has been achieved by using the Prato 1,3-dipolar cycloaddition of tethered bis-azomethine ylides. New bis(benzaldehydes) 1-4 tethered by a rigid linker were prepared and used to direct the second cycloaddition of azomethine ylide to C60. Equatorial, trans-4, trans-3, trans-2, and trans-1 bisadducts have been selectively prepared with this approach. However, the introduction of chiral centers in the pyrrolidine rings in the course of the reaction complicated the chemistry, as a number of stereoisomers theoretically could be formed. The structure determination of the isomeric bisadducts was made based on spectroscopic data and theoretical calculations. To our best knowledge, this represents the first example of a systematic study on tether-directed selective synthesis of C60 fulleropyrrolidine bisadducts.