Evidence of Sulfur Non‐Innocence in [CoII(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+ (cyclam = 1,5,8,11-tetraaza-cyclotetradecane) (1) and the S-containing analog [Co(S2N2-cyclam)]2+ (S2N2-cyclam = 1,8-dithia-5,11-diaza-cyclotetradecane, (2) 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 O2 reduction process as the key for the improved catalytic ORR capabilities of 2.