Facile and Reversible Formation of Iron(III)-Oxo-Cerium(IV) Adducts from Nonheme Oxoiron(IV) Complexes and Cerium(III)

Ceric ammonium nitrate (CAN) or Ce-IV(NH4)(2)- (NO3)(6) is often used in artificial water oxidation and generally considered to be an outer-sphere oxidant. Herein we report the spectroscopic and crystallographic characterization of [(N4Py) Fe-III-O-Ce-IV(OH2)(NO3)(4)](+) (3), a complex obtained from the reaction of [(N4Py) FeII(NCMe)](2+) with 2 equiv CAN or [(N4Py) Fe-IV=O](2+) (2) with CeIII(NO3)(3) in MeCN. Surprisingly, the formation of 3 is reversible, the position of the equilibrium being dependent on the MeCN/water ratio of the solvent. These results suggest that the Fe-IV and Ce-IV centers have comparable reduction potentials. Moreover, the equilibrium entails a change in iron spin state, from S = 1 Fe-IV in 2 to S = 5/2 in 3, which is found to be facile despite the formal spin-forbidden nature of this process. This observation suggests that Fe-IV=O complexes may avail of reaction pathways involving multiple spin states having little or no barrier.