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

The macrocyclic [Fe-III(L1)Cl-2](+) (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 O-18-labelling experiments, revealing that Fe-IV=O and/or Fe-V=O species are likely to be involved in the reaction. The redox behaviour of 1 and these high-valent Fe=O 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-IV=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 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-V=O 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).