First synthesis and characterisation of CH4@C60.

The endohedral fullerene CH4@C60, in which each C60 fullerene cage encapsulates a single methane molecule, has been synthesised for the first time. The key orifice contraction step, a photochemical desulfinylation of an open fullerene, was successfully completed, even though it is significantly inhibited by the presence of the endohedral molecule. The 13C NMR resonance for the cage nuclei in CH4@C60 is deshielded by Δδ = +0.52 ppm relative to C60. The crystal structure of the nickel(II) octaethylporphyrin / benzene solvate shows no significant distortion of the carbon cage, relative to the C60 analogue, and shows the methane hydrogens as a shell of electron density around the central carbon, indicative of the quantum nature of the methane, existing in a set of quantised rotational-translational states at 100 K. The 1H and 13C spin-lattice relaxation times (T1) for endohedral methane have been measured. The 1H T1 values are similar to those observed in the gas phase, also indicating that methane is freely rotating inside the C60 cage. The 1H relaxation rate constant T1-1 increases with increasing temperature and suggests a significant spin-rotation contribution to the relaxation. The successful synthesis of CH4@C60 opens a route to endofullerenes incorporating larger guest molecules than those encapsulated previously.