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

The macrocyclic [Fe(L1)Cl] (, L1 = ,'-dimethyl-2,11-diaza3,3pyridinophane) complex is an active catalyst for the oxidation of water to oxygen using [NH][Ce(NO)] (CAN), NaIO, or Oxone as the oxidant. The mechanism of -catalysed water oxidation was examined by spectroscopic methods and by O-labelling experiments, revealing that Fe[double bond, length as m-dash]O and/or Fe[double bond, length as m-dash]O species are likely to be involved in the reaction. The redox behaviour of and these high-valent Fe[double bond, length as m-dash]O species of L1 has been examined by both cyclic voltammetry and density functional theory (DFT) calculations. In aqueous solutions, the cyclic voltammograms of at different pH show a pH-dependent reversible couple ( = +0.46 V SCE at pH 1) and an irreversible anodic wave ( = +1.18 V SCE at pH 1) assigned to the Fe/Fe couple and the Fe to Fe oxidation, respectively. DFT calculations showed that the value of the half reaction involving [Fe(L1)(O)(OH)]/[Fe(L1)(O)(OH)] is +1.42 V SCE at pH 1. Using CAN as the oxidant at pH 1, the formation of an Fe[double bond, length as m-dash]O 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 and CAN. Using NaIO or Oxone as the oxidant at pH 1, the rate of oxygen evolution was linearly dependent on the concentration of , and a reactive Fe[double bond, length as m-dash]O species with formula [Fe(L1)(O)] generated by oxidation with NaIO or Oxone was suggested by ESI-MS measurements. DFT calculations revealed that [Fe(L1)(O)] is capable of oxidizing water to oxygen with a reaction barrier of 15.7 kcal mol.