Studies on the Dynamics of Phosphorylatedp-tert-Butylcalix[6]arenes by Using 2D NMR Spectroscopy

Abstract

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-tertbutylcalix[6]arenes are more flexible. 2D NMR spectroscopy showed that these calix[6]arenes adopt syn conformations. A combination of 1 H and 31 P 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 (∆G m q ) for macrocyclic ring interconversion depends on the type and number of substituents and ranges from 67 to 86 kJ‚mol -1 . The activation barrier (∆G h q ) 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 (∆G p q ) 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.

A p-tert-butylcalix[6]arene bearing phosphinoyl pendant arms for the complexation and sensitisation of lanthanide ions

“…18 We therefore conclude that conformational interconversion of the macrocyclic ring of B 6 bL 6 occurs via the movement of the dimethylphosphinoylmethoxy substituents of the lower rim through the annulus. The free energy required for the process involving the upper rim is indeed reported to be larger than 88 kJ mol Ϫ1 .…”

Mechanisms and Beyond: Elucidation of Fluxional Dynamics by Exchange NMR Spectroscopy

“…By using adamantane as a guest, 1 H‐ 1 H EXSY measurements gave an entry barrier of Δ G ≠ =16.9±0.4 kcal mol −1 at 22 °C. Calix[6]arene 72 features not only a ring flip often seen in these compounds (see section 4.4), but a hydrogen bond reversal, where the directionality of the hydrogen bonds is in dynamic exchange (between clockwise and anticlockwise). A combination of 31 P‐ 31 P EXSY and rotating‐frame NOE spectroscopy (ROESY) experiments gave a barrier of 10.8 kcal mol −1 for this dynamic process.…”

Lanthanide Complexes with a Calix[8]arene Bearing Phosphinoyl Pendant Arms

“…Indeed, two dynamic processes were detected in large calixarenes with substituted phenol groups: macrocyclic ring interconversion and pinched conformer interconversion. The former takes place by way of two different mechanisms: tert-butylthrough-the-annulus and oxygen-through-the-annulus pathways, [8,33] The first mechanism, in which the aryl groups swing through the annulus in sequence, always operates in unbridged large calixarenes, as demonstrated for p-tert-butylcalix [6]arenes. [30,34,35] Alternatively, the presence of sufficiently bulky groups at the narrow rim, as it is the case for B 8 bL 8 , inhibits oxygen-through-the-annulus interconversion so that ring interconversion can only proceed by the tert-butyl-through-the-annulus route.…”