Francesca Benevelli

Chloroform encapsulated in p-tert-butylcalix[4]arene: Structure and dynamics

A complex of p-tert-butylcalix[4]arene and chloroform has been prepared and characterized in the solid state using, for the first time for any calixarene complex, the Rietveld powder XRD method, as well as variable-temperature 2H and 13C solid-state NMR. The 2H NMR spectra are composed of a central isotropic peak, attributed to a small fraction of CDCl3 in the free motion regime inside the crystal lattice, and a quadrupolar powder pattern. 2H spin–lattice relaxation times lead to activation energies for the molecular motion of 4.9±0.5 and 6.0±0.6 kJ mol−1 for the two spectral components, respectively. The temperature dependence of the static 2H spectra shows a very unusual increase in the effective quadrupolar coupling constant with temperature, caused by the change in the geometry of the complex. We show that the complexation is caused by weak dispersion interactions. The 13C MAS NMR spectrum of the guest molecule changes upon prolonged heating, while thermal analysis confirms the presence of an exothermic transition attributed to an intermediate state before the choroform is released.

X-ray diffraction and 13C solid-state NMR studies of the solvate of tetra(C-undecyl)calix[4]resorcinarene with dimethylacetamide

Single crystal X-ray diffraction (XRD) analysis of the complex of calix[4]resorcinarene (CAV11) with dimethylacetamide (DMA) shows that the CAV11 molecules form centrosymmetric dimers, held by van der Waals interactions of the C-undecyl aliphatic chains. The dimers are associated in layers with hydroxyl groups at the surfaces. The DMA molecules are found between the CAV11 layers, above the cavities formed by the resorcine units and distant from them. They join the layers of CAV11 by molecular forces, in particular by various hydrogen bonds, and are relatively immobile. The 13C CP/MAS spectra are in reasonable agreement with the XRD results.

Complexation behaviour of p-tert-butylcalix[4]arene and p-tert-butylcalix[6]arene towards acetone

Novel 1:1 complexes of acetone with p-tert-butylcalix[4]arene and p-tert-butylcalix[6]arene were prepared and characterized by TGA, DSC, PXRD and 13C and 2H solid-state NMR. The behaviour of both molecules towards acetone is very similar, with the same orientation of the guest, and the same activation energy for the rotation of the C–D bond of acetone about its threefold axis. This is interpreted in terms of the pinched-cone conformation of p-tert-bultylcalix[6]arene in the complex, which can be viewed as two edge-sharing p-tert-bultylcalix[4]arene molecules in the cone conformation.