Poster Presentation 21st International Conference on Biological Inorganic Chemistry 2025

Spectroscopic and Mechanistic Insights into Mn(III)-O-Ce(IV) Formation (#442)

Anagha Puthiyadath 1 , Timothy A Jackson 1
  1. University of Kansas, Lawrence, KANSAS, United States

Ceric ammonium nitrate (CAN) is a powerful oxidant with a high CeIV/CeIII reduction potential of +1.37 V vs. SCE in aqueous 1 M HNO₃. CAN is widely used as a one-electron oxidant in organic synthesis and a sacrificial oxidant in transition metal catalyzed water oxidation (1-3). The strong Lewis acidity of both CeIV and its reduced form, CeIII can facilitate O–O bond formation by directing a water molecule near a transition-metal center. This resembles the role of CaII in the Mn4CaO5 cluster in the Oxygen Evolving Complex of Photosystem-II (PS-II). Given the increasing application of CAN in water oxidation and importance of Mn in PS-II, there is considerable interest in elucidating the reactive intermediates that form when transition metals interact with CAN (4-6). Previous studies show that CAN oxidation of Fe and Mn complexes produce diverse products, often but not always forming metal-oxygen-cerium adducts. This poses a challenge for correlating product identity with specific ligand characteristics or reaction conditions in CAN-mediated metal oxidation. This study focuses on the reaction of CAN with the mononuclear MnIII-hydroxo complex [MnIII(OH)(PaPy2Q)]+. Unlike expected oxidation, treatment of this MnIII-hydroxo complex with excess CAN does not oxidize the MnIII center; instead, CeIV acts purely as a Lewis acid, converting the MnIII-OH complex into a MnIII-O-CeIV species. This complex was characterized by UV/Vis, NMR, XANES and EXAFS methods. Oxygen and hydrogen atom transfer reactivities of the MnIII-O-CeIV complex has been explored and compared to that of the other reported Mn-O-Ce species. This is the first example of a Mn-O-Ce species that is generated using a stoichiometric amount of CAN. This work can address the important question about the factors that influence product formation.

  1. 1. Piro, N. A.; Robinson, J. R.; Walsh, P. J.; Schelter, E. J., The electrochemical behavior of cerium(III/IV) complexes: Thermodynamics, kinetics and applications in synthesis. Coord. Chem. Rev. 2014, 260, 21-36.
  2. 2. Nair, V.; Deepthi, A., Cerium(IV) Ammonium Nitrate-A Versatile Single-Electron Oxidant. Chem. Rev. 2007, 107 (5), 1862-1891.
  3. 3. Fillol, J. L.; Codolà, Z.; Garcia-Bosch, I.; Gómez, L.; Pla, J. J.; Costas, M., Efficient water oxidation catalysts based on readily available iron coordination complexes. Nat. Chem. 2011, 3 (10), 807-813.
  4. 4. Ellis, W. C.; McDaniel, N. D.; Bernhard, S.; Collins, T. J., Fast Water Oxidation Using Iron. J. Am. Chem. Soc. 2010, 132 (32), 10990-10991.
  5. 5. Grotjahn, D. B.; Brown, D. B.; Martin, J. K.; Marelius, D. C.; Abadjian, M.-C.; Tran, H. N.; Kalyuzhny, G.; Vecchio, K. S.; Specht, Z. G.; Cortes-Llamas, S. A.; Miranda-Soto, V.; van Niekerk, C.; Moore, C. E.; Rheingold, A. L., Evolution of Iridium-Based Molecular Catalysts during Water Oxidation with Ceric Ammonium Nitrate. J. Am. Chem. Soc.Society 2011, 133 (47), 19024-19027
  6. 6. Sridharan, V.; Menéndez, J. C., Cerium(IV) Ammonium Nitrate as a Catalyst in Organic Synthesis. Chem.l Rev. 2010, 110 (6), 3805-3849