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Calix[4]arene units functionalized at the 1,3-positions of the upper rim with amido groups, –NHC(O)X, act as neutral, hydrogen-bonding receptors for hydrogen sulfate, dihydrogen phosphate, acetate, benzoate, nicotinate, oxalate, terephthalate, isophthalate and fumarate with binding strength and selectivity tunable by varying the electron-withdrawing ability of the terminal substituent X (X = CH 2 Cl, CHCl 2 , CCl 3 ).
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Cameron et al. (1997) studied this question.
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