Selective alkali-metal cation complexation by chemically modified calixarenes. Part 4. Effect of substituent variation on the Na ⁺/K⁺ selectivity in the ester series and x-ray crystal structure of the trifluoroethyl ester

The cation complexing abilities of a series of p-tert-butylcalix[4]arenes bearing ligating ester groups in the cone conformation have been assessed by stability constant measurements in methanol and extraction studies from water into dichloromethane. The cations studied were Na⁺ and K⁺ and variations in the ester function (CO₂R) included R = methyl, ethyl, n-butyl, terf-butyl, benzyl, phenyl, phenacyl, methoxyethyl, trifluoroethyl, methylthioethyl and prop-2-ynyl. The effect of replacing one or two ester functions in the tetraethyl ester by methyl ester, carboxylic acid, ketone and amide functions was also studied. Selectivities for Na⁺ relative to K⁺ in stability constants range from 2 to 2500, the phenacyl derivative having the highest selectivity. X-Ray diffraction analysis was used to probe the conformation of the trifluoroethyl ester 11. Crystals of 11 are monoclinic, space group P2₁/n, in a cell of dimensions a = 13.987(2), b = 16.194(3), c = 27.630(5) Å; β = 98.70(1)°; R = 0.077 for 3172 observed data. The compound possesses a distorted cone conformation.