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.
Bibliographical noteFunding Information:
This work was supported by a grant from the U.S. National Institutes of Health (GM-38767 to L.Q.). XAS data were collected at the Stanford Synchrotron Radiation Lightsource, which is supported by the U.S. DOE under Contract No. DE-AC02-76SF00515. Use of Beamline 7-3 is supported by the DOE Office of Biological and Environmental Research and the National Institutes of Health, National Institute of General Medical Sciences (including P41GM103393). J.E.M.N.K. thanks the Alexander von Humboldt Foundation for a Feodor Lynen Research Fellowship. We thank Dr. Victor J. Young, Jr. of the University of Minnesota Crystallographic Facility for his valuable advice in the crystallographic analysis.
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- electron transfer
- spin crossover
- water oxidation