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.