Formation of nickel(III) complexes with n-dentate amine macrocycles (n = 4, 5, 6). ESR and electrochemical studies

Low-spin Ni(III) complexes with the complete series of 12- to 16-membered cyclic tetraamines, with the novel series of 15- to 17-membered cyclic pentaamines, and with the 18-membered cyclic hexamine were prepd. in MeCN by anodic and/or chem. oxidn. of the corresponding Ni(II) complexes. Spin state and stereochem. of the Ni(III) complexes were deduced from the ESR spectra of the frozen solns. (77 K). The half-wave potential values, E1/2, assocd. to the 2+/3+ quasi-reversible redox change in the considered complexes were obtained through voltammetry expts. at a Pt electrode. E1/2 Values show that for tetraamine complexes the attainment of the Ni(III) state is easiest with the 14-membered macrocycle and is dramatically affected by contraction or expansion of the ligand cavity. This ring-size-dependent redox selectivity is drastically reduced in the complexes with pentadentate macrocycles. The synthesis of this novel series of cyclic pentaamines is described in detail.