Analysis of sodium, potassium, calcium, and ammonium cation binding and selectivity in one- and two-armed nitrogenpivot lariat ethers

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Abstract
Complexation of sodium, potassium, calcium, and ammonium cations by one- and two-armed (bibracchial) lariat ethers is surveyed and analyzed. A comprehensive list of binding data determined at 25.0±0.1 °C in anhydrous methanol is presented. More than 100 new, experimentally determined (K s ) and calculated (selectivity) values especially for calcium and ammonium are presented. Both cation binding strength and selectivity are evaluated in terms of ring size, sidearm length, number and type of donor groups and other factors. Hitherto unreported divalent calcium cation binding selectivities for bibracchial lariat ethers in aqueous solution rival the cation selectivities of some cryptands. N,N′-bis(Ethoxycarbonylmethyl)-4,13-diaza-18-crown-6 exhibits a Ca2+/K+ selectivity equal to that of [2.2.1]-cryptand and a Ca2+/Na+ selectivity better than that of any cryptand. Two bibracchial systems having >NCH2CONHCHRCOOCH2CH3 sidearms (R=H, s-butyl), have shown Ca2+ over Na+ selectivity in water of ≥103. Cation binding constants determined by extraction and ISE methods are also compared with transport rates determined in bulk organic membranes and by dynamic 23Na-NMR methods.

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