Nickel(III) and copper(III) complexes with 13- and 14-membered tetra-aza macrocycles. Ring-size and medium effects on the MIII/MII redox couple potentials

The one-electron oxidn. of the NiII and CuII complexes with 13- and 14-membered macrocycles I (n = 2, 3) that produce authentic NiIII and CuIII species was electrochem. investigated and the E1/2 values assocd. with the reversible MIII/MII redox couple were detd. by a differential pulse voltammetry technique. The 13-membered ring favors the attainment of the CuIII state, whereas the 14-membered ring favors the formation of the NiIII complex. This opposite behavior is satisfactorily interpreted in terms of size of the metal ions and aperture of the macrocyclic cave. Moreover the effect of the concn. of the inert electrolyte (NaClO4, 0.1-7.0M) on the E1/2(MIII/MII) values was investigated: the increase of the NaClO4 concn. favors the formation of the CuIII complexes and does not favor the formation of the NiIII species. This behavior is ascribed to the destabilization of NiIII and CuII complexes induced by perchlorate ion through the formation of H bonds with water mols. axially coordinated to the metal ions.