Synthesis and Properties of Endohedral C₆₀ Encapsulating Molecular Hydrogen

Abstract

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.

The dipolar endofullerene HF@C60

“…The downfield shift of 0.05 ppm relative to empty C 60 deviates from the empirical correlation drawn previously between the van der Waals radius of the endohedral guest and a downfield 13 C chemical shift (H 2 O@C 60 , 0.11 ppm; H 2 @C 60 , 0.08 ppm; He@C 60 0.02 ppm). 2,3,30 A series of 19 F NMR spectra of HF@C 60 in the polycrystalline solid state were acquired at 263 K, using magic-angle spinning at a range of rotation frequencies, without 1 H decoupling ( Figure 2 and Supplementary Figures 5 and 6). All spectra display a prominent splitting due to a 1 H-19 F J-coupling of 506 ± 0.5 Hz.…”

“…1 The dihydrogen and water endofullerenes H 2 @C 60 , H 2 O@C 60 , and their isotopologues, have been synthesized by the procedure known as 'molecular surgery', in which synthetic operations are used to open a hole in the fullerene allowing encapsulation of the guest, followed by a suturing technique to reform the pristine fullerene shell. [2][3][4] Recently the approach has been extended to C 70 and C 59 N. [5][6][7] The confined molecules display quantization of their coupled translational and rotational degrees of freedom, and exhibit phenomena such as nuclear spin isomerism and orthopara conversion. [8][9][10][11][12] Recently it was shown that nuclear spin conversion of the encapsulated water molecules in H 2 O@C 60 leads to a change in the dielectric constant of the material.…”

Interaction potential and infrared absorption of endohedral H2 in C60

“…This method [1][2][3] results in sample quantities of the order of 100 mg and has made possible nuclear magnetic resonance (NMR), 4,5 infrared (IR), 6 and inelastic neutron scattering (INS) (Refs. 7 and 8) spectroscopic investigations of the trapped H 2 dynamics and stimulated theoretical investigations.…”

Evaluation of hydrogen dissolved in rubber materials under high-pressure exposure using nuclear magnetic resonance

“…1 There are several reports on the evaluation of gas absorbed in nanostructures using NMR and 129 Xe as a probe molecule for the characterization of zeolite 2,3 or polymers. [4][5][6][7][8] Hydrogen gas was found to have been trapped in absorption materials, such as fullerene 9 and carbon nanotubes. 10 NMR measurement is more suitable for hydrogen gas than for other gases, such as CO 2 , because of its high natural abundance, which is the target for evaluation.…”