Evidence of Sulfur Non‐Innocence in [Co(II)(dithiacyclam)](2+)‐Mediated Catalytic Oxygen Reduction Reactions

In many metalloenzymes, sulfur‐containing ligands participate in catalytic processes, mainly via the involvement in electron transfer reactions. In a biomimetic approach, we now demonstrate the implication of S‐ligation in cobalt mediated oxygen reduction reactions (ORR). A comparative study between the catalytic ORR capabilities of the four‐nitrogen bound [Co(cyclam)](2+) (1; cyclam=1,5,8,11‐tetraaza‐cyclotetradecane) and the S‐containing analog [Co(S(2)N(2)‐cyclam)](2+) (2; S(2)N(2)‐cyclam=1,8‐dithia‐5,11‐diaza‐cyclotetradecane) reveals improved catalytic performance once the chalcogen is introduced in the Co coordination sphere. Trapping and characterization of the intermediates formed upon dioxygen activation at the Co(II) centers in 1 and 2 point to the involvement of sulfur in the O(2) reduction process as the key for the improved catalytic ORR capabilities of 2.