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

Wai-Pong To; Toby Wai-Shan Chow; Chun-Wai Tse; Xiangguo Guan; Jie-Sheng Huang; Chi-Ming Che

doi:10.1039/c5sc01680k

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

Chem Sci. 2015 Oct 1;6(10):5891-5903. doi: 10.1039/c5sc01680k. Epub 2015 Jul 22.

Authors

Wai-Pong To¹, Toby Wai-Shan Chow¹, Chun-Wai Tse¹, Xiangguo Guan¹, Jie-Sheng Huang¹, Chi-Ming Che^{1

2}

Affiliations

¹ Department of Chemistry and State Key Laboratory of Synthetic Chemistry , The University of Hong Kong , Pokfulam Road , Hong Kong , China . Email: xgguan@hku.hk ; Email: cmche@hku.hk.
² HKU Shenzhen Institute of Research and Innovation , Shenzhen 518053 , China.

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^IV[double bond, length as m-dash]O and/or Fe^V[double bond, length as m-dash]O species are likely to be involved in the reaction. The redox behaviour of 1 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 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[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 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[double bond, length as m-dash]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.