Dioxygen Reduction at a Cu(I) Complex Supported by a Macrocyclic N ₃ O Ligand System

ABSTRACT

Cu ^I (14 TMCO) ( 1 ), involving a N ₃ O‐macrocycle, reacts with O ₂ at cryogenic temperatures to yield an end‐on peroxodicopper(II) species. The N ₄ ‐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 O ₂ binding. In contrast, an elongated Cu─O _TMCO bond trans to the O ₂ ‐binding site in 1 allows O ₂ ‐coordination. The catalytic reduction of O ₂ to H ₂ O undergoes a full 4e ⁻ /4H ⁺ process in 1 . Notably, [Cu ^I (12‐TMC)] ⁺ ( 3 ), involving a smaller N ₄ ‐macrocycle where the Cu(I) ion is forced to move out of the plane formed by the four nitrogen atoms, also binds O ₂ . However, in contrast to 1 , 3 mediates the catalytic 2e ⁻ /2H ⁺ reduction of O ₂ to H ₂ O ₂ via two ligand‐assisted protonation steps. In this case, the reduction of O ₂ 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 H ₂ O ₂ upon protonation. The present study provides deep mechanistic insights into the transition metal‐mediated O ₂ reduction process and reveals how the first coordination sphere can be tuned to control the 2e ⁻ /2H ⁺ versus 4e ⁻ /4H ⁺ reductions of O ₂ .