Redox-inactive metal ions modulate the reactivity and oxygen release of mononuclear non-haem iron(III)–peroxo complexes

Abstract

Redox-inactive metal ions that function as Lewis acids play pivotal roles in modulating the reactivity of oxygen-containing metal complexes and metalloenzymes, such as the oxygen-evolving complex in photosystem II and its small-molecule mimics. Here we report the synthesis and characterization of non-haem iron(III)–peroxo complexes that bind redox-inactive metal ions, (TMC)FeIII–(μ,η2:η2-O2)–Mn+ (Mn+ = Sr2+, Ca2+, Zn2+, Lu3+, Y3+ and Sc3+; TMC, 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane). We demonstrate that the Ca2+ and Sr2+ complexes showed similar electrochemical properties and reactivities in one-electron oxidation or reduction reactions. However, the properties and reactivities of complexes formed with stronger Lewis acidities were found to be markedly different. Complexes that contain Ca2+ or Sr2+ ions were oxidized by an electron acceptor to release O2, whereas the release of O2 did not occur for complexes that bind stronger Lewis acids. We discuss these results in the light of the functional role of the Ca2+ ion in the oxidation of water to dioxygen by the oxygen-evolving complex.

Substrate and Lewis Acid Coordination Promote O–O Bond Cleavage of an Unreactive L2CuII2(O22–) Species to Form L2CuIII2(O)2 Cores with Enhanced Oxidative Reactivity

“…47 Nam and co-workers expanded that reactivity to other Lewis acids, where depending on the nature of the Lewis acid identity, the iron(III)-peroxo-LA adduct could be reduced to liberate dioxygen (LA: Ca 2+ , Sr 2+ ) or led to O–O cleavage (LA: Zn 2+ , Lu 3+ , Y 3+ , Sc 3+ ). 48 To the best of our knowledge, the reactivity reported herein is the first example of Lewis acid mediated reversible O–O bond cleavage/formation, which is reminiscent of the chemistry occurring in water oxidation process catalyzed by the Mn 4 Ca 2+ cluster in photosystem II (Figure 6, left).…”

Tuning the Redox Properties of a Nonheme Iron(III)–Peroxo Complex Binding Redox‐Inactive Zinc Ions by Water Molecules

“…[9, 10] The Lewis acidity of the redox-inactive metal ions was shown to be an important factor that determines the redox potentials and reactivities of 1 -M n + with electron donors and acceptors. [9b] For example, as the Lewis acidity of the redox-inactive metal ions increased, the one-electron oxidation and reduction potentials of 1 -M n + became more positive. Further, the 1 -M n + complexes binding Ca 2+ and Sr 2+ ions showed similar redox potentials and reactivities in the one-electron oxidation and reduction reactions.…”

“…Furthermore, the 1 -M n + complexes binding Ca 2+ and Sr 2+ ions were oxidized by an oxidant [e.g., cerium(IV) ammonium nitrate (CAN)] to release O 2 , whereas no release of O 2 occurred for complexes binding stronger Lewis acids ( 1 -M n + ; M n + = Zn 2+ , Lu 3+ , Y 3+ , and Sc 3+ ). [9b] …”

“…[8, 9b] Upon addition of H 2 O (0–2.8 m ) to a solution of 1 -Zn 2+ in CH 3 CN (MeCN) at 273 K, the absorption band at 650 nm for 1 -Zn 2+ was red-shifted to 760 nm with a clean isosbestic point at 765 nm, and the shift of the absorption band was found to depend on the amount of H 2 O added (Figure 2a). The spectral change of 1 -Zn 2+ exhibits a sigmoidal curvature (Figure 2b), suggesting that more than one H 2 O molecule is coordinated to 1 -Zn 2+ .…”

Nuclear Resonance Vibrational Spectroscopic Definition of Peroxy Intermediates in Nonheme Iron Sites

“…57 Fe(CF 3 SO 3 ) 2 ·2CH 3 CN was obtained as a white solid power. [(TMC) 57 Fe II (CH 3 CN)](CF 3 SO 3 ) 2 and 1 were prepared according to methods described in the literature: 31 [(TMC) 57 Fe II (CH 3 CN)](CF 3 SO 3 ) 2 was synthesized by reacting 57 Fe(CF 3 SO 3 ) 2 ·2CH 3 CN with the tetramethylcyclam ligand (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) under an Ar atmosphere in CH 3 CN at 25 °C. Then, 1 was generated by reacting [(TMC) 57 Fe II (CH 3 CN)](CF 3 SO 3 ) 2 (69.3 mg, 0.10 mmol) with 5 equiv.…”