Loading-dependent structures of CO2 in the flexible molecular van der Waals host p-tert-butylcalix[4] arene with 1 : 1 and 2 : 1 guest-host stoichiometries

The adsorption of CO2 into the low density form of p-tert-butylcalix[ 4] arene (tBC) has been studied by C-13 solid state NMR, single crystal X-ray diffraction and volumetric adsorption measurements. The experimental results indicate that tBC and carbon dioxide can form two distinct inclusion compounds. At low loadings the structure of the empty low-density form of the tBC framework (space group P2(1)/n) is preserved with the included CO2 molecules located within the conical cavities of the tBC molecules. The ideal composition of this form is therefore 1 : 1 (CO2 : tBC). With higher applied CO2 pressures the guest loading increases and the structure of the tBC framework transforms to a well studied tetragonal (space group P4/n) form. In this form an additional CO2 molecule is located on an interstitial site resulting in an ideal composition 2 : 1 (CO2 : tBC). In agreement with SCXRD and the gas adsorption measurements, 13 C NMR measurements show the change in structure that takes place as a function of sample loading. Inclusion of CO2 is a rather slow activated process that can be accelerated by increasing the temperature and the transition between crystal forms is inhomogeneous over a bulk sample. After gas release, the empty (or near empty) P4/n structure survives, thus providing another low density phase of tBC. The magnitude and temperature variation of the C-13 chemical shift anisotropy of CO2 in both low and high occupancy complexes with tBC indicates restricted motion of the CO2 molecules. The location and dynamics of CO2 molecules inside the tBC structure are discussed and a motional model for CO2 is proposed. The CO2 molecules in the highly loaded compound are shown to exchange rapidly as a single resonance is observed for the two distinct CO2 molecules.