C60 in a peptidic cage: a case of symmetry mismatch studied by crystallography and solid-state NMR

A molecular container with C60 cargo was crystallized and studied by X-ray diffraction revealing a complicated dis­order of the ligand caused by the incompatibility with its site symmetry. The tumbling of the C60 cargo was studied by solid-state NMR which suggested possible phase transitions, positively verified at high temperature by DSC.
A supramolecular complex, formed by encapsulation of C60 fullerene in a molecular container built from two resorcin[4]arene rims zipped together by peptidic arms hydrogen bonded into a cylindrical β-sheet, was studied by X-ray crystallography, solid-state and solution NMR, EPR spectroscopy and differential scanning calorimetry (DSC). The crystal structure, determined at 100 K, reveals that the complex occupies 422 site symmetry, which is compatible with the molecular symmetry of the container but not of the fullerene molecule, which has only 222 symmetry. The additional crystallographic symmetry leads to a complicated but discrete dis­order, which could be resolved and modelled using advanced features of the existing refinement software. Solid-state NMR measurements at 184–333 K indicate that the thermal motion of C60 in this temperature range is fast but has different activation energies at different temperatures, which was attributed to a phase transition, which was confirmed by DSC. Intriguingly, the activation energy for reorientations of C60 in the solid state is very similar for the free and encaged molecules. Also, the rotational diffusion coefficients seem to be very similar or even slightly higher for the encaged fullerene compared to the free molecule. We also found that chemical shift anisotropy (CSA) is not the main relaxation mechanism for the 13C spins of C60 in the studied complex.