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 O-2 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.