​Why Does Nature Choose Ca²⁺ Ion for Water Oxidation? A Hint from Reactivity Studies of Iron(III)-Peroxo Complexes Binding Redox-Inactive Metal Ions

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Department of Chemistry and Nano Science 

by Prof. Wonwoo Nam (wwnam@ewha.ac.kr)

  Photoinduced dioxygen activation by a nonheme iron complex, [(TMC)Fe^II]²⁺ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), in the presence of redox-inactive metal ions (Mⁿ⁺) and an electron donor [1-benzyl-1,4-dinitronicotinamide dimer, (BNA)₂] resulted in the generation of two different intermediates, (TMC)Fe^III-(μ,η²:η²-O₂)-Mⁿ⁺ (1-Mⁿ⁺) and [(TMC)Fe^IV(O)]²⁺ (2), depending on the Lewis acidity of the redox-inactive metal ions. 1-Mⁿ⁺ was formed when Mⁿ⁺ was Ca²⁺ or Sr²⁺, whereas 2 was the product when Mⁿ⁺ was Zn²⁺, Lu³⁺, Y³⁺ or Sc³⁺. Authentic 1-Mⁿ⁺ complexes were synthesized independently and characterized with various spectroscopic methods, and their electrochemical properties and reactions with electron donor and acceptor were investigated. With respect to the electrochemical properties of 1-Mⁿ⁺, we have observed a trend that as the Lewis acidity of Mⁿ⁺ increases, both one-electron oxidation and reduction potentials of 1-Mⁿ⁺ become more positive. When 1-Mⁿ⁺ was reacted with reductant [e.g., (BNA)₂], 1-Mⁿ⁺ was converted to 2 in the case of Mⁿ⁺ = Zn²⁺, Lu³⁺, Y³⁺ and Sc³⁺, but such a conversion was not observed in the case of Mⁿ⁺ = Ca²⁺ and Sr²⁺. In the reaction with an oxidant such as cerium(IV) ammonium nitrate (CAN), 1-Mⁿ⁺ reverted back to [FeII(TMC)]²+ by releasing O₂ in the case of Mⁿ⁺ = Ca²⁺ and Sr²⁺, but such a reaction did not occur in the case of Mⁿ⁺ = Zn²⁺, Lu³⁺, Y³⁺ and Sc³⁺. The release of O₂ in the reaction of 1-Ca²⁺ with CAN was confirmed using ¹⁸O-isotope labeled [(TMC)Fe^III(¹⁸O₂)]⁺-Ca²⁺ (or Sr²⁺) complexes. These results demonstrate that the Lewis acidity of the redox-inactive metal ions is an important factor that modulates not only the electrochemical properties of 1-Mⁿ⁺ but also the reactivities of 1-Mⁿ⁺ with electron donor and acceptor. We have discussed these results in the light of the functional role(s) of Ca²⁺ ion in the oxidation of water to dioxygen by the oxygen-evolving complex (OEC) in Photosystem II (PSII).

*Related article:  

​Redox-inactive metal ions modulate the reactivity and oxygen release of mononuclear non-haem iron(III)-peroxo complexes. Nature Chemistry. 6(10): 934–940 (2014)