Synthesis and electrochemistry of mono- and binuclear europium complexes with the macrocyclic ligand 1,4,7,10,14,17,20,23-octaazacyclohexacosane-1,4,7,10,14,17,20,23-octaacetic acid (OHEC)

Mononuclear and binuclear europium complexes with OHEC have been prepared, where OHEC is 1,4,7,10,14,17,20,23-octaazacyclohexacosane-1,4,1,10,14,17,20,23-octaacetic acid. Electrochemical analysis of these complexes shows that in aqueous solution, Eu(III) binds both at the carboxylic group and at the nitrogen donor atoms. The mononuclear complex (Eu(III)OHEC) undergoes a single-electron reduction to Eu(II); the binuclear complex (Eu(III)₂OHEC) shows a two-electron reduction to Eu(II) at more cathodic potentials. The stability constants of Eu(III) are much larger than those of Eu(II) complexes. Electrochemical synthesis of Eu(III) complexes is demonstrated and has considerable advantages over the chemical synthesis. In addition to structural information on the complex in solution, electrochemistry provides analysis of residual uncomplexed ions and of the preconditions for possible synthesis. This is particularly important in cases such as this one, for which NMR is inaccessible.