The oxidation of water is a challenging transformation that nature carries out with a highly oxidized metal-oxo cluster under ambient conditions. Efforts to mimic nature and carry out catalytic water oxidation often use Ce^IV as a sacrificial oxidant.

Lawrence Que, Jr. and colleagues, University of Minnesota, Minneapolis, MN, USA, have investigated the reversible formation of iron-oxo-cerium adducts. The team observed a reaction between synthetic Fe^IV=O complexes and Ce^III to form Fe^III–O–Ce^IV adducts (pictured). This shows that Fe^IV=O can oxidize Ce^III to Ce^IV. The researchers also determined that the ion spin state changes: the S=1 iron(IV) center of the precursor is converted to a S=5/2 iron(III) center in the product.

Surprisingly, the formation of the Fe^III–O–Ce^IV adduct from its Fe^IV=O precursor is reversible. The position of the equilibrium between the two could be shifted by changing the solvent/water ratio, which modulates the Ce^IV/III redox potential. These results suggest that the Fe^IV and Ce^IV centers have comparable reduction potentials, and provide an indirect way to assess the Fe^IV/III reduction potential.

Moreover, the interconversion between the two iron spin states is quite easy, despite being a spin-forbidden process. This transformation resembles the reaction of S=1 Fe^IV=O with C–H bonds to form S=5/2 Fe^III–OH and suggests a parallel between the H atom and Ce^III in such reactions.

• Facile and Reversible Formation of Iron(III)-Oxo-Cerium(IV) Adducts from Nonheme Oxoiron(IV) Complexes and Cerium(III),
  Apparao Draksharapu, Waqas Rasheed, Johannes E. M. N. Klein, Lawrence Que,
  Angew. Chem. Int. Ed. 2017.
  DOI: 10.1002/anie.201704322