First Synthesis and Characterization of CH₄@C₆₀

Abstract

The endohedral fullerene CH 4 @C 60 , in which each C 60 fullerene cage encapsulates a single methane molecule, has been synthesized for the first time. Methane is the first organic molecule, as well as the largest, to have been encapsulated in C 60 to date. The key orifice contraction step, a photochemical desulfinylation of an open fullerene, was completed, even though it is inhibited by the endohedral molecule. The crystal structure of the nickel(II) octaethylporphyrin/ benzene solvate shows no significant distortion of the carbon cage, relative to the C 60 analogue, and shows the methane hydrogens as a shell of electron density around the central carbon, indicative of the quantum nature of the methane. The 1 H spin‐lattice relaxation times ( T 1 ) for endohedral methane are similar to those observed in the gas phase, indicating that methane is freely rotating inside the C 60 cage. The synthesis of CH 4 @C 60 opens a route to endofullerenes incorporating large guest molecules and atoms.

Quasistructural molecules

“…Allowing for the nonrigidity of the lattice, not considered in the Devonshire cell model, introduces two effects changing the quantum treatment of these systems: (a) the host–guest (molecule–crystal) interaction energy is minimized by the relaxation of the lattice, and (b) the orientation‐dependent lattice relaxations, through coupling of the guest rotations with pseudorotation of the host, induce a significant change, in fact a decrease of the effective rotational constants. Recently, quantum studies of molecules trapped in cages found renewed interest, especially with regard to the joint treatment of translation–rotation states. Answering the question whether these systems are quasistructural or not would require detailed experimental high‐resolution spectroscopic studies and variational rovibrational treatments, both hindered by the size of some of these systems.…”

Encapsulation of Mg2inside a C60cage forms an electride

“…Encapsulation of Li + ion inside the C 60 cage was done experimentally by Hiroshi et al 47 Experimentally it is reported that La atom can be inserted into the cavity of C 72 , C 74 , C 82 cages 48–50 . Encapsulation of H 2, 51,52 H 2 O, 53 HF, 54 CH 4 55 molecules inside fullerene were done using the “molecular surgery” technique. Various theoretical works on the encapsulation of different metals and noble gases inside the C 60 cage have been reported 56–59 .…”

Synthesis of Ar@C60 using molecular surgery

“…Upon irradiation of Ar@4 for 35 h under these conditions, at 50 1C using a low-pressure sodium lamp, the bis(hemiketal) Ar@5 was obtained in 26% yield of isolated product. Our previously reported photochemical desulfinylation of CH 4 @4 led to the corresponding bis(hemiketal) CH 4 @5 in only 13% yield under the same conditions, 19 indicating that contraction of the cage-opening is less inhibited by argon, the smaller endohedral species.…”

“…by closure of the orifice of CH 4 @3 to obtain CH 4 @C 60 . 19 Herein, we report that high encapsulation of argon by 3 can be achieved under similar conditions, and closure allows synthesis of Ar@C 60 with a step-change improvement in argon incorporation, mass recovery and yield when compared to the direct encapsulation method.…”

“…Clean and high-yielding oxidation of Ar@3 with dimethyldioxirane gave the sulfoxide Ar@4, and we now applied conditions for photochemical desulfinylation of Ar@4 that were developed for our synthesis of CH 4 @C 60 . 19 Desulfinylation is the first of the 'closure' steps that suture the orifice of the opencage fullerene and a similar photochemical ring-contraction by removal of the sulfinyl group (SO) of other sulfoxide open-cage endofullerenes has been reported, using visible-light irradiation in toluene or benzene. 14,25 Photochemical desulfinylative ring-contraction of sulfoxide 4 re-forms fullerene 2 which is unstable under the reaction conditions, but using a vigorously stirred two-phase mixed solvent system of toluene, acetonitrile and acetic acid (10% v/v aqueous) allows in situ trapping of 2 as the more stable bis(hemiketal) 5.…”