Tetra-aza macrocycles containing a pyridine ring. The oxidation and reduction behavior of their nickel(II) complexes

The Ni complexes [NiL](ClO4)2, where L = I (R = R1 = H, Me; R = Me, R1 = H) and II, were prepd. by reaction of NiCl2 with diacetyl- or diformylpyridine and the appropriate triamine under mild conditions (low concns. and low temp.) in aq. EtOH to study the effects of modification of the ligand framework on their redox properties. Insertion of 2 Me groups in the C-backbone in I (R = Me, R1 = H) makes oxidn. to Ni(III) easier, partly as a result of their inductive effect which makes electron abstraction from Ni(II) less difficult because of the increase of electron d. on the adjacent N atoms. However, the advantage of a Me group on the central amine N atom is more than compensated for by the steric effects introduced and this makes oxidn. to Ni(III) of Ni in the complex with I (R = R1 = H) is much easier than that of the other 3 complexes; redn. is particularly sensitive to methylation of the amine N atom and to enlargement of the ring in the Ni-II complex.