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

Cu^I(14 TMCO) (1), involving a N₃O-macrocycle, reacts with O₂ at cryogenic temperatures to yield an end-on peroxodicopper(II) species. The N₄-analogue Cu^I(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─O_TMCO 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, [Cu^I(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─O_peroxo 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.