Open-cage Fullerene Research Articles

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.

Open‐Cage Fullerenes: Synthesis, Structure, and Molecular Encapsulation

AbstractFor Abstract see ChemInform Abstract in Full Text.

Open-Cage Fullerene Derivatives Suitable for the Encapsulation of a Hydrogen Molecule

The encapsulation of molecular hydrogen into an open-cage fullerene having a 16-membered ring orifice has been investigated. It is achieved by the pressurization of H2 at 0.6-13.5 MPa to afford endohedral hydrogen complexes of open-cage fullerenes in up to 83% yield. The efficiency of encapsulation is dominantly dependent on both H2 pressure and temperature. Hydrogen molecules inside the C60 cage are observed in the range of -7.3 to -7.5 ppm in 1H NMR spectra, and the formations of hydrogen complexes are further confirmed by mass spectrometry. The trapped hydrogen is released by heating. The activation energy barriers for this process are determined to be 22-24 kcal/mol. The DSC measurement of the endohedral H2 complex reveals that the escape of H2 from the C60 cage corresponds to an exothermic process, indicating that encapsulated H2 destabilizes the fullerene.

Synthesis of Fullerene Derivatives with Novel Structures. Liquid—Phase versus Solid—State Reactions

AbstractFor Abstract see ChemInform Abstract in Full Text.

Floating Single Hydrogen Molecule in an Open‐Cage Fullerene

Open-mouthed fullerene swallows H2 whole: Direct observation of a H2 molecule that is fully encapsulated by a derivative of an aza–thia open-cage fullerene (see model) is achieved by synchrotron X-ray diffraction analysis of single crystals. A 3D electron-density map obtained by the maximum entropy method shows a single H2 molecule (red) floating in the center of a hollow cavity in the open-cage fullerene molecule (yellow).

Encapsulation of Molecular Hydrogen in Fullerene C 60 by Organic Synthesis

In spite of their importance in fundamental and applied studies, the preparation of endohedral fullerenes has relied on difficult-to-control physical methods. We report a four-step organic reaction that completely closes a 13-membered ring orifice of an open-cage fullerene. This process can be used to synthesize a fullerene C60 encapsulating molecular hydrogen, which can be isolated as a pure product. This molecular surgical method should make possible the preparation of a series of C60 fullerenes, encapsulating either small atoms or molecules, that are not accessible by conventional physical methods.

Open-Cage Fullerenes: Synthesis, Structure, and Molecular Encapsulation

The carbon-carbon bond scission of the fullerene cage offers an open-cage fullerene derivative having an opening on the fullerene surface. This account summarizes our recent studies on the syntheses of open-cage C60 derivatives as well as their properties of molecular encapsulation. The photochemical rearrangement of the cyclohexadiene derivative of C60 gives bis(fulleroid), the precursor for the cage scission. The photooxygenative carbon-carbon bond cleavage of bis(fulleroid) affords an open-cage diketone derivative having a 12-membered ring. The reaction of the diketone derivative with aromatic hydrazine or hydrazone allows the ring expansion of the orifice by regioselective cage scission to yield a product bearing a 16-membered ring. The orifice in the product is large enough to pass a hydrogen molecule, producing an endohedral hydrogen complex. In the case of the reaction with ortho-phenylenediamine, two carbon-carbon bonds are sequentially cleaved to afford a product having a 20-membered ring orifice. This product spontaneously encapsulates one water molecule to form an endohedral water complex.

Helium Entry and Escape through a Chemically Opened Window in a Fullerene

(3)He has been inserted into the cavity of an open-cage fullerene derivative close to room temperature, reaching an incorporation fraction of 0.1%. The rate of escape of (3)He from this fullerene was monitored by (3)He NMR to yield the activation barrier and to compare the size of the orifice to those of other open-cage fullerenes. The equilibrium constant was also measured.

Open-cage fullerene derivatives with 15-membered-ring orifices.

The addition reaction of the N-MEM-ketolactam derivative of [60]fullerene with phenyl, p-Br-phenyl, and p-MeO-phenyl hydrazines proceeds regioselectively, affording three open-cage fullerene derivatives bearing a 15-membered-ring orifice on the fullerene cage. Both experimental data and theoretical calculations were utilized for the structure determination of the new [60]fullerene adducts.

Novel open-cage fullerenes having a 12-membered-ring orifice: removal of the organic addends from the rim of the orifice.

Two novel open-cage fullerene derivatives bearing a 12-membered-ring orifice on the fullerene cage have been isolated. Removal of the N-MEM protective group leads to the first open-cage [60]fullerene derivative without organic addends on the rim of the orifice. [structure: see text]

Solid-state NMR spectroscopy of molecular hydrogen trapped inside an open-cage fullerene.

Solid-state 1H experiments were performed an open-cage fullerene hosting molecular hydrogen. The anisotropy of the molecular hydrogen rotation was studied by double-quantum magic-angle-spinning NMR. The time scale of the molecular hydrogen rotation was estimated by spin-lattice relaxation measurements as a function of temperature.

Synthesis of Fullerene Derivatives with Novel Structures-Liquid-Phase versus Solid-State Reactions-

Reactions of C60 with various carbon nucleophiles, particularly those with substituted acetylide and fluorenide ions in the liquid phase, give various C60 derivatives. While the reaction with cyanide salts only affords the cyanated C60 in liquid phase, the solid-state mechanochemical reaction selectively gives the dumbbell-shaped dimer. As typically shown by this example, the solid-state reaction was found to be quite advantageous for the formation of various types of fullerene dimers. The reaction of C60 with phthalazine also results in formation of entirely different products depending on the reaction phase. While the solid-state reaction gives a fullerene dimer incorporated in a bicyclic framework, the liquid-phase thermal reaction affords a derivative of an open-cage fullerene. Utilizing this reaction, an attempt has been made for organic synthesis of endohedral fullerene encapsulating molecular hydrogen through a series of procedures, i.e., opening a hole on the fullerene cage, insertion of molecular hydrogen, and reduction of the size of the opening.

HOLEY FULLERENE OPENS WIDE FOR H

JAPANESE CHEMISTS FOR THE first time have prepared an open-cage fullerene derivative with an orifice large enough to allow a hydrogen molecule to be inserted into the cage in 100% yield [ J. Am. Ch...

100% encapsulation of a hydrogen molecule into an open-cage fullerene derivative and gas-phase generation of H2@C60.

By applying high-pressure H2 to a new fullerene derivative, C63NO2SPh2Py (1), having a 13-membered-ring orifice, 100% incorporation of a H2 molecule into the fullerene cage has been achieved for the first time. This result substantiates the theoretical calculations indicating that the energy barrier required for H2 insertion through an orifice in 1 is considerably lower than that for the previously reported derivative with the largest orifice among open-cage fullerenes synthesized thus far. Upon matrix-assisted laser desorption/ionization mass spectroscopy, the removal of organic addends from the fullerene derivative 1 encapsulating H2 and restoration of the pristine C60 cage, which retains approximately one-third of incorporated H2, have been observed.

Synthesis, structure, and properties of novel open-cage fullerenes having heteroatom(s) on the rim of the orifice.

A thermal liquid-phase reaction of fullerene C(60) with 3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine afforded aza-open-cage fullerene derivative 5 having an eight-membered-ring orifice on the fullerene cage. Compound 5 was found to undergo oxidative ring-enlargement reactions with singlet oxygen under photo-irradiation to give azadioxo-open-cage fullerene derivatives 9 and 10, which have a 12-membered-ring orifice, in addition to a small amount of azadioxa-open-cage fullerene derivative 11, which has a 10-membered-ring orifice. A thermal reaction of 9 with elemental sulfur in the presence of tetrakis(dimethylamino)ethylene resulted in further ring-enlargement to give azadioxothia-open-cage fullerene derivative 15, which has a 13-membered-ring orifice. The structures of 5 and 15 were determined by X-ray crystallography, while those of 9, 10, and 11 were confirmed by the agreement of observed (13)C NMR spectra with those obtained by DFT-GIAO calculations. These reactions were rationalized based on the results of molecular orbital calculations. Following electrochemical measurements, compounds 9 and 10, which have two carbonyl groups on the rim of the orifice, were found to be more readily reduced than C(60) itself (the first reduction potential was found to be 0.2 V lower than that of C(60)), while the first reduction potentials of other open-cage fullerene derivatives, 5, 11, and 15, were nearly the same as that of C(60).

The Reaction of Fullerene C60 with 4,6-Dimethyl-1,2,3-triazine: Formation of an Open-Cage Fullerene Derivative.

The Reaction of Fullerene C₆₀ with 4,6-Dimethyl-1,2,3-triazine: Formation of an Open-Cage Fullerene Derivative.

The reaction of fullerene C(60) with 4,6-dimethyl-1,2,3-triazine: formation of an open-cage fullerene derivative.

A thermal reaction of fullerene C(60) with 4,6-dimethyl-1,2,3-triazine (4) in o-dichlorobenzene gave azacyclohexadiene-fused fullerene derivative 5, by the reaction with intermediate azete 11, and then, after flash chromatography over SiO(2), open-cage fullerene derivative 6 having an eight-membered ring orifice on the C(60) cage. Compound 6 is assumed to be formed via addition of diradical intermediate 13 to C(60). Compound 6 underwent a further photochemical reaction with singlet oxygen with the cleavage of one of the double bonds at the rim of the orifice to afford triketone derivative 8 having a 12-membered ring orifice.

Photochemical Reaction of the Open-Cage Fullerene Derivative with Singlet Oxygen

Abstract Photochemical reaction of open-cage fullerene 1 with singlet oxygen gave oxidation product 3, which was shown to have an enol and a carbonyl groups on the 12-membered-ring orifice on the fullerene cage by spectroscopic method and density functional calculations.

The reaction of fullerene C(60) with phthalazine: the mechanochemical solid-state reaction yielding a new C(60) dimer versus the liquid-phase reaction affording an open-cage fullerene.

The reaction of fullerene C(60) with phthalazine was studied both in solution and in the solid state using the high-speed vibration-milling technique. The reaction in solution gave open-cage fullerene derivative 1 in 44% yield by a one-pot reaction. In contrast, the solid-state reaction afforded dimeric derivative 2 as the sole product. Dimeric derivative 2 was found to undergo intramolecular [2 + 2] cycloaddtion between the two C(60) cages located in close proximity to give a new C(60) dimer 6 in quantitative yield. The structures of these new derivatives of C(60) were determined by spectroscopic methods, and the electrochemical behavior of 2 and 6 was also studied.