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

Ceric ammonium nitrate (CAN) or CeIV (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)FeIII -O-CeIV (OH2 )(NO3 )4 ]+ (3), a complex obtained from the reaction of [(N4Py)FeII (NCMe)]2+ with 2 equiv CAN or [(N4Py)FeIV =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 FeIV and CeIV centers have comparable reduction potentials. Moreover, the equilibrium entails a change in iron spin state, from S=1 FeIV 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 FeIV =O complexes may avail of reaction pathways involving multiple spin states having little or no barrier.