Elektrochemische Untersuchungen einkerniger Mangan- und Eisen-Komplexe für die Wasseroxidationskatalyse
- Electrochemical investigations of mononuclear manganese and iron complexes for water oxidation catalysis
Rohner, Stefan Sebastian; Leitner, Walter (Thesis advisor); Eichel, Rüdiger-Albert (Thesis advisor)
Dissertation / PhD Thesis
Dissertation, RWTH Aachen University, 2019
In the present work, various mononuclear manganese and iron complexes were investigated in the context of water oxidation catalysis. The aim of the work was to understand how a change of the electronic properties of the ligand influences the fundamental electrochemical water oxidation behavior. In addition, the formation of the respective catalytically active species and the mechanism of water oxidation were studied in detail. At the beginning, mononuclear manganese-bipyridine complexes were used. However, they did not show catalytic water oxidation neither when using chemical oxidants nor electrochemically. In the second part of the work the dpaq ligand system was used, which has stronger σ-donor properties due to an anionic carboxamide group. Starting from the known [FeIII(dpaqH)(OH2)](ClO4)2-WOC, the influence of different substituents in the 5-position of the ligand on water oxidation was investigated. It was shown that electron-withdrawing substituents increase the catalytic current, but only with a simultaneous increase of the overpotential, while electron-donating substituents reduce the catalytic activity. An increased catalytic current with a concomitant decreased overpotential was finally achieved by the introduction of a pyrene group into the ligand framework which led to an increased π-conjugation. In addition to the Fe complexes, the analogous [MnII(dpaqR)](ClO4) complexes were investigated and also showed a catalytic current. The influence of the different substituents in the ligand framework on the overpotential followed the same trend. In contrast to the Fe complexes, however, no reversible reduction waves were visible, so that a fundamental structural change of the starting complexes must be assumed under the measurement conditions. In the third part of the work, the tetradentate dpqma ligand was used to synthesize a new mononuclear [MnII(dpqma)Br2] complex, which showed a catalytic current in CV measurements in a borate buffer. However, when using Cerium (IV) ammonium nitrate (CAN) as the oxidant, no evolution of oxygen could be detected. Using EPR spectroscopy, it was shown that the dpqma ligand presumably dissociates after the addition of CAN, followed by the formation of a μ-oxo bridged MnIIIMnIV dimer. After a longer reaction time, the EPR measurements indicated the formation of MnOx compounds. In addition, the formation of MnO4- ions was detected by UV/Vis spectroscopy. This showed that the [MnII(dpqma)Br2] complex is not a suitable water oxidation catalyst when using CAN as the oxidant due to the poor stability under the acidic oxidative reaction conditions.