Mechanism of water oxidation by non-heme iron catalysts when driven with sodium periodate

Iron tris(2-methylpyridyl)amine (TPA) and iron 1-(bis(2-methylpyridyl)amino)-2-methyl-2-propanoate (BPyA) salts are characterized as water oxidation catalysts (WOCs) using sodium periodate. Under the conditions used, these complexes serve as homogeneous WOCs as demonstrated via kinetic analysis and dynamic light scattering (DLS). The Fe(BPyA) salt serves as both a mononuclear and dinuclear catalyst, with the mononuclear form showing higher catalytic activity. Based on the H/D kinetic isotope effect and pH dependence, the rate determining step (RDS) in water oxidation (WO) by Fe(BPyA) is nucleophilic attack by water during O-O bond formation. In contrast, Fe(TPA) shows complex kinetic behavior due to the formation of multiple oxidation states of the complex in solution, each of which exhibits catalytic activity for WO. The RDS in WO by Fe(TPA) follows an equilibrium established between monomeric and dimeric forms of the catalyst. Under acidic conditions formation of the monomer is favored, which leads to an increase in the WO rate.