Significant effect of 2nd sphere interactions on the reductive activation of O2 by non‐heme iron(II) complexes – Application to the electroassisted oxidation of thioanisole

2^nd sphere hydrogen bonds within non-heme iron complexes significantly improves the potential of the reductive O₂ activation process, by generating stabilizing interactions within the Fe/O₂ adduct. O₂ can be activated at a much more favorable potential, which we took advantage of to achieve the electroassisted oxidation of thioanisole by O_2.

Abstract

New mononuclear non-heme Fe^II complexes derived from a well-known pentadentate amine/pyridine ligand were synthesized. The new ligand bears an additional OH group on one pyridyl aimed at developing second sphere hydrogen bonds. Two binding modes of the ligand were observed that can be selectively obtained through the choice of the counterion: the OH group develops an intramolecular H-bond with the 6^th exogenous ligand in the classical mode, or an intermolecular one with the unbound triflate counterion in the alternative mode. The complexes were studied by cyclic voltammetry in the reductive activation of O₂. The addition of an H-bond improves the O₂ activation potential with a gain of 90 mV in the classical case and 220 mV in the alternative one. The difference is ascribed to the development of stabilizing H-bonds within the Fe^III-peroxo intermediate with the proximal O atom of peroxo (classical mode) or the distal one (alternative mode). Ultimately, the complex in the alternative mode activates O₂ at −0.5 V vs SCE (500 mV gain compared to free O₂). We took advantage of this significantly reduced energetic cost to carry out and achieve the electroassisted oxidation of thioanisole by O₂ at −0.5 V vs SCE.

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