Binding properties of neutral diamide ligands for alkaline-earth cations

The complexation of a series of aromatic and alicyclic N,N,N',N'-tetra-n-propyl amides of 1,2-ethylenedioxydiacetic acids with group IIA metal-ion bromides in anhydrous methanol was investigated by ultraviolet absorption spectroscopy. These synthetic ligands were previously found to show selectivity toward divalent over monovalent cations with respect to extraction of ions into bulk organic phase (Borowitz, I.J., Lin, W-O., Wun, T-C., Bittman, R., Weiss, L., Diakiw, V., and Borowitz, G.B. (1977), Tetrahedron, in press). At low concentrations, ligands bearing benzene and naphthalene rings form 1:1 ligand to divalent cation complexes with each of the alkaline-earth metals, but ligands in the cyclohexyl series are stoichiometrically bound to cations in more than one type of complex. Binding isotherms obtained by Scatchard analysis and by the method of continuous variation revealed ligand to divalent ion mole ratios of 2:1, 3:2, and 4:3 for binding of N,N,N',N'-tetra-n-propyl-cis-1,2-cyclohexanedioxydiacetamide with Ca2+, Sr2+, and Ba2+, respectively. In contrast, Scatchard analysis of ultraviolet spectral changes showed that a 1:1 complex is formed between this ligand and Na+ with an apparent association constant of 56 +/- 2M-1; the constant for binding with K+ was smaller (11 M-1). The order of apparent association equilibrium constants for complexation of group IIA cations with this series of neutral ligands was Ca2+ greater than Sr2+ greater than Ba2+ greater than Mg2+; for example, for N,N,N',N'-tetra-n-propyl-1,2-phenylenedioxydiacetamide the apparent binding constants at 25 degrees C were 7.33 +/- 0.25 X 10(4) M-1 for Ca2+, 1.23 +/- 0.03 X 10(4) for Sr2+, 4.42 +/- 0.09 X 10(3) for Ba2+, and 4.04 +/- 0.24 X 10(2) for Mg2+. The divalent cation binding properties of these synthetic diamide ligands are discussed in relation to those of other synthetic ligands and of two naturally occurring ligands.