Abstract
Ceric ammonium nitrate (CAN) or Ce(IV) (NH(4) )(2) (NO(3) )(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) (OH(2) )(NO(3) )(4) ](+) (3), a complex obtained from the reaction of [(N4Py)Fe(II) (NCMe)](2+) with 2 equiv CAN or [(N4Py)Fe(IV) =O](2+) (2) with Ce(III) (NO(3) )(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.