Polarographic study of the rates of the dissociation reactions of the Eu(III)-HAHA complex and of the Eu(III)-, Gd(III)-, and Ho(III)-PAPA complexes.

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Polyaza macrocycles carrying acetate groups, HAHA and PAPA, were found to form solely 1:1 ratio complexes with Eu(III), Gd(III), or Ho(III) ions. The PAPA complexes of the Eu(III), Gd(III), and Ho(III) ions and the HAHA complex of the Eu(III) ion in acetate buffer solutions containing an excess of the Eu(III), Gd(III), or Ho(III) ion produce anodic waves, which are kinetic in nature. They can be ascribed to the free macrocycles formed upon the dissociations of the complexes preceding electron-transfer steps at the electrode surface. From an experimental examination of the nature of these kinetic waves, the reaction mechanisms and rates for the dissociation reactions were determined. The dissociation reactions for the HAHA and PAPA complexes were found to proceed through (A) and (B), respectively. The (kd)o values for the Eu(III)-HAHA and -PAPA complexes were 5.1×10−1 and 3.9×10−2 (sec−1), respectively, and those for the Gd(III)- and Ho(III)-PAPA complexes, 4.04×10−2 and 1.2×10−2 (sec−1). (Remark: Graphics omitted.) The (kd) values for the Eu(III)-HAHA abd -PAPA complexes were 3.70×102 and 4.24×105 (mol−1dm3s−1) and those for the Gd(III)-and Ho(III)-PAPA complexes, 2.45×104 (mol−1dm3s−1).
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