Ligand Modification Transforms a Catalase Mimic into a Water Oxidation Catalyst

The catalytic reactivity of the high‐spin Mn^II pyridinophane complexes (Py₂NR₂)Mn(H₂O)₂]²⁺ (R=H, Me, tBu) toward O₂ formation is reported. With small macrocycle N‐substituents (R=H, Me), the complexes catalytically disproportionate H₂O₂ in aqueous solution; with a bulky substituent (R=tBu), this catalytic reaction is shut down, but the complex becomes active for aqueous electrocatalytic H₂O oxidation. Control experiments are in support of a homogeneous molecular catalyst and preliminary mechanistic studies suggest that the catalyst is mononuclear. This ligand‐controlled switch in catalytic reactivity has implications for the design of new manganese‐based water oxidation catalysts.