Water oxidation catalysed by iron complex of N,N′-dimethyl-2,11-diaza[3,3](2,6)pyridinophane. Spectroscopy of iron–oxo intermediates and density functional theory calculations

Abstract

The macrocyclic [Fe^III(L1)Cl₂]⁺ (1, L1 = N,N′-dimethyl-2,11-diaza[3,3](2,6)pyridinophane) complex is an active catalyst for the oxidation of water to oxygen using [NH₄]₂[Ce^IV(NO₃)₆] (CAN), NaIO₄, or Oxone as the oxidant. The mechanism of 1-catalysed water oxidation was examined by spectroscopic methods and by ¹⁸O-labelling experiments, revealing that Fe^IVO and/or Fe^VO species are likely to be involved in the reaction. The redox behaviour of 1 and these high-valent FeO species of L1 has been examined by both cyclic voltammetry and density functional theory (DFT) calculations. In aqueous solutions, the cyclic voltammograms of 1 at different pH show a pH-dependent reversible couple (E_1/2 = +0.46 V vs. SCE at pH 1) and an irreversible anodic wave (E_pa = +1.18 V vs. SCE at pH 1) assigned to the Fe^III/Fe^II couple and the Fe^III to Fe^IV oxidation, respectively. DFT calculations showed that the E value of the half reaction involving [Fe^V(L1)(O)(OH)]²⁺/[Fe^IV(L1)(O)(OH₂)]²⁺ is +1.42 V vs. SCE at pH 1. Using CAN as the oxidant at pH 1, the formation of an Fe^IVO reaction intermediate was suggested by ESI-MS and UV-vis absorption spectroscopic measurements, and the rate of oxygen evolution was linearly dependent on the concentrations of both 1 and CAN. Using NaIO₄ or Oxone as the oxidant at pH 1, the rate of oxygen evolution was linearly dependent on the concentration of 1, and a reactive Fe^VO species with formula [Fe^V(L1)(O)₂]⁺ generated by oxidation with NaIO₄ or Oxone was suggested by ESI-MS measurements. DFT calculations revealed that [Fe^V(L1)(O)₂]⁺ is capable of oxidizing water to oxygen with a reaction barrier of 15.7 kcal mol⁻¹.