Steric effects on the solution chemistry of nickel(II) complexes with N-monomethylated 14-membered tetraaza macrocycles. The blue-to-yellow conversion and the oxidation and reduction behavior

Monomethylated derivatives of isoclam (I; R=R'=H) and cyclam were prepd. The presence of a Me group has substantial effects on the soln. behavior of the corresponding Ni(II) complexes. In particular, it makes the blue (high-spin) to yellow (low-spin) conversion less endothermic and more exoentropic, compared to that of the unsubstituted ligand complexes, as demonstrated through temp.-dependence studies in aq. soln. Moreover, N-methylation of the ligand makes the attainment of the Ni(I) state easier and the access to the Ni(III) more difficult, as shown by a cyclic voltammetry investigation in MeCN. Finally, the M(II) to M(III) oxidn. process was investigated in 10M HClO4 for both Ni and Cu complexes. The different soln. behavior of the complexes with the N-methyl-substituted macrocycles compared to the parent ligand analogs was interpreted in terms of (1) repulsive interactions with apically coordinated solvent mols. and (2) weakening of the metal-nitrogen interactions. Mechanism 1 seems to be operative in the case of the Ni(II) blue-to-yellow conversion; mechanism 2 is effective in the case of oxidn. to Ni(III) and Cu(III) and redn. to Ni(I) processes.