Fitting of nickel(II) ion into two 14-membered tetraaza macrocycles. Blue-to-yellow conversion and the oxidation and reduction behavior

The novel satd. tetraaza macrocycle isocyclam (I) was prepd. and the behavior of its Ni(II) complexes, which contain a 5,5,6,6 sequence of chelate rings, was compared with that of corresponding complexes of cyclam (II) (sequence: 5,6,5,6). These complexes exist in soln. as a mixt. of blue, octahedral, high-spin and yellow, planar, low-spin species; the thermodn. parameters for the blue-to-yellow conversion depend upon the cyclic nature of the ligand. In analogy with Ni(cyclam)2+, Ni(isocyclam)2+ undergoes 1-electron oxidn. and redn. processes at a Pt electrode in MeCN. The easier redn. and more difficult oxidn. of both blue and yellow 5,5,6,6 complexes with respect to the 5,6,5,6 analogs are related to the differences in the in-plane Ni-N interaction parameters, as inferred from electronic spectra. General redox behavior of Ni(II) complexes with tetraaza macrocycles was reconsidered and a linear correlation between oxidn. and redn. potentials is presented.