COMPLEXATION OF THE SODIUM-ION BY A PENDANT-ARM TETRAAZA MACROCYCLIC LIGAND IN AQUEOUS-SOLUTION

The first pendant-arm tetraaza macrocyclic sodium(I) complex to be quantitatively kinetically characterized in aqueous solution, N,N',N'',N'''-tetrakis(2-methoxyethyl)-1,4,7,10-tetraazacyclo-dodecanesodium(I), has an apparent stability constant K = 159 +/- 18 dm3 mol-1 in aqueous 0.100 mol dm-3 NEtClO4 at 298.2 K, and is more stable than its other alkali-metal ion analogues, as shown by potentiometric titration methods. Variable-temperature Na-23 NMR spectroscopy showed sodium ion exchange on this complex to be characterized by k(d) (298.2 K) = 70.3 +/- 2.3 s-1, DELTAH(d)double dagger = 64.0 +/- 0.7 kJ mol-1 and DELTAS(d)double dagger = 5.1 +/- 2.1 J K-1 mol-1, where k(d) is a monomolecular decomplexation rate constant. The protonation of N,N',N'',N--tetrakis(2-methoxyethyl)-1,4,7,10-tetraazacyclododecane is characterized by pK(a) values (+/- 0.05) of 10.92, 8.04 and 2.17, and the log(K/dm3 mol-1) values for its complexes with Mg2+, Ca2+, Sr2+ and Ba2+ are 2.47, 5.47, 5.00 and 4.72 (all +/- 0.05), respectively.