Cation complexation by chemically modified calixarenes. Part 6. Alkali and silver(I) cation complexation by p-tert-butylcalix[5]arene derivatives and X-ray structure determination of a pentaester

Abstract

The preparation of two p-tert-butylcalix[5]arene esters and a p-tert-butylcalix[5]arene carboxylic acid are reported. The cone conformation in solution at 20 °C has been established for all three compounds by ¹H NMR spectroscopy. Crystals of pentaethyl ester 2 were studied by X-ray diffraction and the molecule was found to possess a distorted cone conformation in the solid state. Crystals of 2 are triclinic, space group P with two molecules in a cell of dimensions a= 11.656(2), b= 14.912(3). c= 23.044(5)Å; α= 79.23(2), β= 78.31 (2), γ= 67.52(2)°. The structure was solved by direct methods and refined by least squares to R= 0.084 for 6967 reflections. One phenolic ring (B) is tilted inwards at the upper rim of the cone; the others are tilted outwards, but by varying amounts. The pendant side-chains also adopt very different conformations, resulting in a range of inter-chain O ⋯ O distances around the cavity, from 3.35 Å upwards. The binding properties of the esters towards alkali cations have been assessed by picrate extraction experiments and stability constant determinations. The main conclusions are: (a) both pentamer esters are much more effective in extraction than either their tetramer or hexamer counterparts; (b) the nature of the alkoxy residue in the ester influences the extent of extraction, the tert-butyl ester being more effective than the ethyl ester; (c) extraction favours the larger cations with little discrimination between K⁺, Rb⁺, and Cs⁺; (d) the trends in extraction are mirrored in stability constants with the tert-butyl derivative forming the stronger complexes.