Dioxygen Reduction at a Cu(I) Complex Supported by a Macrocyclic N3O Ligand System.

CuI(14 TMCO) (1), involving a N3O-macrocycle, reacts with O2 at cryogenic temperatures to yield an end-on peroxodicopper(II) species. The N4-analogue CuI(14-TMC) (2) remains unreactive under similar conditions. 2 contains an average short Cu─N bond length, resulting in a dominating trans-effect, which inhibits O2 binding. In contrast, an elongated Cu─OTMCO bond trans to the O2-binding site in 1 allows O2-coordination. The catalytic reduction of O2 to H2O undergoes a full 4e-/4H+ process in 1. Notably, [CuI(12-TMC)]+ (3), involving a smaller N4-macrocycle where the Cu(I) ion is forced to move out of the plane formed by the four nitrogen atoms, also binds O2. However, in contrast to 1, 3 mediates the catalytic 2e-/2H+ reduction of O2 to H2O2 via two ligand-assisted protonation steps. In this case, the reduction of O2 involves the initial formation of a side-on peroxodicopper(II) species, where the weak Cu─Operoxo bond distance resulting from the strong σ-donation from the trans Cu─N bond presumably leads to the release of H2O2 upon protonation. The present study provides deep mechanistic insights into the transition metal-mediated O2 reduction process and reveals how the first coordination sphere can be tuned to control the 2e-/2H+ versus 4e-/4H+ reductions of O2.