Investigations on the dynamic properties of 25,26,27,28-tetraalkoxycalix[4]arenes: para-substituent- and solvent-dependent properties of paco conformers and determination of thermodynamic parameters of the pinched cone/pinched cone conversion

Abstract

The conformational and dynamic behaviour of the 25,26,27,28-tetraalkoxycalix[4]arenes 1–7 (R = methyl, propyl) has been investigated by X-ray crystallography, dynamic 1D and 2D NMR spectroscopy, as well as with a series of force field (PIMM) and semiempirical (PM3, MNDO) calculations. The stereo-electronic effects of the upper rim substituents of tetramethoxycalix[4]arenes characteristically influence the preferred arrangement of the phenyl rings in the paco conformation. The in/out isomerisation (0001≈AAAA⇌0001≈AAAB) taking place in 4 is strongly hindered in 1 due to the smaller diameter of the upper rim. The ease of rotation of the methoxy group attached to the inverted ring in paco-4 can be altered by using a solvent (CD₂Cl₂), which is able to form a host–guest complex with the calixarene cavity. For 1–3 cone/paco and paco/paco equilibria have been observed by 2D EXSY spectroscopy. Gibbs activation energies for the cone(C_2v)/cone(C_2v) conversion were determined for 1, 2, 5 and 6. An NMR line shape analysis provided Arrhenius and Eyring activation parameters for the cone(C_2v)/cone(C_2v) conversion in 5 and 6.