Trivalent nickel bis(triaza macrocyclic) complexes. Ligand ring size and medium effects on the nickel(III)/nickel(II) redox couple potential

The formation of NiL23+ (L = 1,4,7-triazacyclononane (I), 1,4,8-triazacyclodecane (II), 1,4,8-triazacycloundecane (III)) was studied in CH3CN, DMSO and H2O. ESR spectra of frozen CH3CN solns. of NiL23+ are typical of a low-spin d7 cation and display a rhombic (macrocycles I and II) pattern or axial (III) patterns. Examn. of E1/2(Ni(II)/Ni(III)) values, detd. by voltammetric techniques, shows that the easiest attainment of the trivalent state occurs with the smallest size macrocycle I. In particular, [NiL2]3+ (L = I) formed at an even less pos. potential than [Ni(cyclam)]3+, considered until now as the most stable Ni(III) species among polyaza macrocyclic complexes. This definitively demonstrates the superiority of 6-coordination in stabilizing the Ni(III) oxidn. state. The role of the solvent on the formation of trivalent complexes, at the level of both the coordination and the solvation sphere, was studied and discussed. In aq. soln., the increase of the concn. of the supporting electrolyte (NaClO4) may disfavor or favor the access to the trivalent state, according to whether water mols. are present or not in the redox equil., thus introducing a novel element of selectivity