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 .