CATION COMPLEXATION BY CHEMICALLY-MODIFIED CALIXARENES .6. ALKALI AND SILVER(I) CATION COMPLEXATION BY P-TERT-BUTYLCALIX[5]ARENE DERIVATIVES AND X-RAY STRUCTURE DETERMINATION OF A PENTAESTER

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-degrees-C has been established for all three compounds by H-1 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 P1BAR, with two molecules in a cell of dimensions a = 11.656(2), b = 14.912(3), c = 23.044(5) angstrom; alpha = 79.23(2), beta = 78.31 (2), gamma = 67.52(2)-degrees. 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 angstrom 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.