THERMODYNAMIC AND KINETIC-STUDIES OF LANTHANIDE COMPLEXES OF 1,4,7,10,13-PENTAAZACYCLOPENTADECANE-N,N',N'',N''',N''''-PENTAACETIC ACID AND 1,4,7,10,13,16-HEXAAZACYCLOOCTADECANE-N,N',N'',N''',N'''',N'''''-HEXAACETIC ACID

The macrocyclic pentaamino pentacarboxylate ligand PEPA (5) and the hexaamino hexacarboxylate HEHA (6) were synthesized, and their complexation with lanthanides was investigated. pH-Metric titrations were used for the equilibrium study, and the ligand-exchange reactions of M(III)-Arsenazo III with macrocyclic ligands were employed for the kinetic study. The results are compared with those for known lanthanide chelating agents, namely, linear DTPA (2) and macrocyclic DOTA (3) and TETA (4). The stability trends of PEPA (5) and HEHA (6) complexes relative to the lanthanide + 3 ion size are almost parallel to those of 2 and 4. Measurements of the thermodynamic parameters DELTA-H and DELTA-S show that DELTA-S is the major contributor to the complex stability as expected. 6 yields with few exceptions more stable 1:1 complexes than 2 at pH approximately 7. The relative complexation rates of 6 are 10 times slower than those of 2 but 100 times faster than those of 3.