Studies on the dynamics of phosphorylated p-tert-butylcalix[6]arenes by using 2D NMR spectroscopy

The overall dynamics of partially phosphorylated and thiophosphorylated p-tert-butylcalix[6]arenes has been studied by NMR spectroscopy. When p-tert-butylcalix[6]arene is monosubstituted with a phosphate or thiophosphate group, the calix[6]arene skeleton is remarkably rigidified. The 1,3- and 1,4-(thio)phosphorylated p-tert-butylcalix[6]arenes are more flexible. 2D NMR spectroscopy showed that these calix[6]arenes adopt syn conformations. A combination of 1H and 31P NMR spectroscopy revealed that the calix[6]arene (thio)phosphates are involved in at least three dynamic processes, viz., macrocyclic ring interconversion, hydrogen bond array reversal, and pinched conformer interconversion. The activation barrier (ΔGm) for macrocyclic ring interconversion depends on the type and number of substituents and ranges from 67 to 86 kJ·mol-1. The activation barrier (ΔGh) for hydrogen bond array reversal depends on the number of hydroxyl groups and ranges from 31 to 45 kJ·mol-1. For the pinched conformer interconversion an activation barrier (ΔGp) ranging from 44 to 55 kJ·mol-1 was found. Pinched conformations have been frequently observed in the solid state, for example, for mono- and 1,4-bis(thiophosphorylated) p-tert-butylcalix[6]arene. These studies, however, show for the first time that these conformations also exist in solution and that differently pinched conformers may rapidly interconvert.