HYDROXYETHYLATION AND ITS EFFECT ON THE RATE OF METAL-ION INGRESS INTO MACROCYCLIC LIGANDS

The hydroxyethylation effect, by which the rate of metal ion uptake by tetraaza macrocyclic ligands is enhanced, was previously known to occ ur when all four nitrogen atoms are hydroxyethylated or when two, on a djacent nitrogens, are so treated. In this work two monohydroxyethylat ed tetraaza macrocycles, (2-hydroxyethyl)-1,4,8,11-tetraazacyclotetrad ecane and 4,8,11-trimethyl-1,4,8,11-tetraazacyclotetradecane have been synthesised as well as one tri-hydroxyethylated macrocycle, thyl)-11- methyl-1,4,8,11-tetraazacyclotetradecane. The stability constants for the monometallic complexes formed from these ligands with Co2+ Ni2+, C u2+, Zn2+, Cd2+, Hg2+ and Pb2+ have been determined in aqueous solutio n at 298.2 K by potentiometric titration. During the course of these t itrations it was noted that the tri-hydroxyethylated macrocycle comple xes establish equilibrium at each titration point within a maximum of 5 min of the addition of each aliquot of base. The mono-hydroxyethylat ed ligands, on the other hand, equilibrate very much more slowly. This indicates that the facilitation of metal ion ingress into tetraaza ma crocycles, afforded by pendant hydroxyethyl groups, requires a minimum of two hydroxyethyl groups located on adjacent nitrogen atoms.