Structures of Rare Earth Homodinuclear Macrocyclic Complexes with the Polyamine Polycarboxylic Ligand 1,4,7,10,14,17,20,23-Octaazacyclohexacosane-1,4,7,10,14,17,20,23-octaacetic Acid (H8OHEC) in the Crystal and in Solution

The solid-state structures of the four homodinuclear chelate complexes, [Na2(Y2OHEC)(H2O)2] · 7 H2O · C2H5OH (1), [Na2(Gd2OHEC)(H2O)2] · 12 H2O (2), [Na2(Eu2OHEC)(H2O)2] · 11 H2O (3), and [Cs2(La2OHEC)(H2O)2] · 14 H2O (4) (H8OHEC = 1,4,7,10,14,17,20,23-octaazacyclohexacosane-1,4,7,10,14,17,20,23-octaacetic acid), were determined by X-ray crystal structure analysis. Each lanthanide(III) ion is ninefold coordinated by eight donor atoms of the ligand system and the oxygen atom of one water molecule. The structures are compared with those of DOTA- and TETA-coordinated complexes and the conformation of the OHEC ligand system with that of the parent amine and its dinuclear copper complex. NMR investigations of the diamagnetic complex 1indicate a very similar environment of the lanthanide cations in solution and in the solid state. Low-temperature and variable-temperature 1H-NMR measurements prove the existence of two isomers of 1 (3.5:1 ratio) which undergo conformational processes. The rate constants of these processes were deduced from a complete line shape analysis and were used to determine the activation parameters.