Complexation of the sodium ion by a pendant-arm tetraaza macrocyclic ligand in aqueous solution

Abstract

The first pendant-arm tetraaza macrocyclic sodium(I) complex to be quantitatitatively 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 NEt₄ClO₄ at 298.2 K, and is more stable than its other alkali-metal ion analogues, as shown by potentiometric titration methods. Variable-temperature ²³Na NMR spectroscopy showed sodium ion exchange on this complex to be characterized by k_d(298.2 K)= 70.3 ± 2.3 s^–1, ΔH_d^‡= 64.0 ± 0.7 kJ mol^–1 and ΔS_d^‡= 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⁺ and Ba²⁺ are 2.47, 5.47, 5.00 and 4.72 (all ± 0.05), respectively.