Mechanism of Water Oxidation Catalyzed by a Mononuclear Iron Complex with a Square Polypyridine Ligand: A DFT Study.

The mononuclear [Cl-Fe(dpa)-Cl] (1) complex containing a square planar tetradentate polypyridine ligand has been reported to catalyze water oxidation in pH = 1 aqueous medium with ceric ammonium nitrate (CAN) as a chemical oxidant. The reaction mechanism of the oxygen evolution driven by this catalyst was investigated by means of density functional calculations. The results showed that one chloride ligand of 1 has to exchange with a water molecule to generate 1, [Cl-Fe(dpa)-OH], as the starting species of the catalytic cycle. The initial one-electron oxidation of 1 is coupled with the release of two protons, generating [Cl-Fe(dpa)═O] (2). Another one-electron transfer from 2 leads to the formation of an Fe═O complex [Cl-Fe(dpa)═O] (3), which triggers the critical O-O bond formation. The electronic structure of 3 was found to be very similar to that of the high-valent heme-iron center of P450 enzymes, termed Compound I, in which a π-cation radical ligand is believed to support a formal iron(IV)-oxo core. More importantly, 3 and Compound I share the same tendency toward electrophilic reactions. Two competing pathways were suggested for the O-O bond formation based on the present calculations. One is the nitrate nucleophilic attack on the iron(V)-oxo moiety with a total barrier of 12.3 kcal mol. In this case, nitrate functions as a co-catalyst for the dioxygen formation. The other is the water nucleophilic attack on iron(V)-oxo with a greater barrier of 16.5 kcal mol. In addition, ligand degradation via methyl hydrogen abstraction was found to have a barrier similar to that of the O-O bond formation, while the aromatic carbon hydroxylation has a higher barrier.