Reduction Potentials of Tetraaminecopper(II/I) Couples

The difference in steric strain between the oxidized and the reduced forms of tetraaminecopper complexes is correlated with the corresponding reduction potentials. The experimentally determined data considered range from ?0.54 to ?0.04 V (vs. NHE) in aqueous solution and from ?0.35 to ?0.08 V (vs. NHE) in MeCN. The observed and/or computed geometries of the tetraaminecopper(II) complexes are distorted octahedral or square-pyramidal (4 + 2 or 4+1) with (distorted) square-planar CuN₄ chromophores (Cu^II? N = 1.99–2.06 Å; Cu? O ≈ 2.5 Å; Cu? O ≈ 2.3 Å), those of the tetraaminecopper(I) complexes are (distorted) tetrahedral (four-coordinate; Cu^I? N = 2.12–2.26 Å; tetrahedral twist angle ?? = 30–90°). The reduction potentials of Cu^II/I couples with primary-amine ligands and those with macrocyclic secondary-amine ligands were correlated separately with the corresponding strain energies, leading to slopes of 70 and 61 kJ mol^?1 V^?1, with correlation coefficients of 0.89 and 0.91, respectively. The approximations of the model (entropy, solvation, electronic factors) and the limits of applicability are discussed in detail and in relation to other approaches to compute reduction potentials of transition-metal compounds.