Hydroxyethylation and its effect on the rate of metal ion ingress into macrocyclic ligands

Abstract

The hydroxyethylation effect, by which the rate of metal ion uptake by tetraaza macrocyclic ligands is enhanced, was previously known to occur when all four nitrogen atoms are hydroxyethylated or when two, on adjacent nitrogens, are so treated. In this work two mono-hydroxyethylated tetraaza macrocycles, 1-(2-hydroxyethyl)-1,4,8,11-tetraazacyclotetradecane and 1-(2-hydroxyethyl)-4,8,11-trimethyl-1,4,8,11-tetraazacyclotetradecane have been synthesised as well as one tri-hydroxyethylated macrocycle, 1,4,8-tris(2-hydroxyethyl)-11-methyl-1,4,8,11-tetraazacyclotetradecane. The stability constants for the monometallic complexes formed from these ligands with Co 2+, Ni 2+, Cu 2+, Zn 2+, Cd 2+, Hg 2+ and Pb 2+ have been determined in aqueous solution at 298.2 K by potentiometric titration. During the course of these titrations it was noted that the tri-hydroxyethylated macrocycle complexes establish equilibrium at each titration point within a maximum of 5 min of the addition of each aliquot of base. The mono-hydroxyethylated ligands, on the other hand, equilibrate very much more slowly. This indicates that the facilitation of metal ion ingress into tetraaza macrocycles, afforded by pendant hydroxyethyl groups, requires a minimum of two hydroxyethyl groups located on adjacent nitrogen atoms.