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

Abstract

The adsorption of CO₂ into the low density form of p-tert-butylcalix[4]arene (tBC) has been studied by ¹³C 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 groupP2₁/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 groupP4/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, ¹³C 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 ¹³C 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.