SYNTHESIS AND ELECTROCHEMISTRY OF MONONUCLEAR AND BINUCLEAR EUROPIUM COMPLEXES WITH THE MACROCYCLIC LIGAND AZACYCLOHEXACOSANE-1,4,7,10,14,17,20,23-OCTAACETIC ACID (OHEC)

Mononuclear and binuclear europium complexes with OHEC have been prepa red, where OHEC is 1,4,7,10,14,17,20,23-octaazacyclohexacosane-1,4,7, 10,14,17,20,23-octaacetic acid. Electrochemical analysis of these comp lexes shows that in aqueous solution, Eu(III) binds both at the carbox ylic group and at the nitrogen donor atoms. The mononuclear complex (E u(III)OHEC) undergoes a single-electron reduction to Eu(II); the binuc lear complex (Eu(III)(2)OHEC) shows a two-electron reduction to Eu(II) at more cathodic potentials. The stability constants of Eu(III) are m uch larger than those of Eu(II) complexes. Electrochemical synthesis o f Eu(III) complexes is demonstrated and has considerable advantages ov er the chemical synthesis. In addition to structural information on th e 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.