World's energetic demands are currently supplied in majority by fossil fuels. Climate change is associated with combustion of these fuels due to CO2 and methane emissions. The cleanest alternative energy source is sunlight that can be used in two ways, either directly producing electric current or promoting a chemical transformation. In this last case, a very promising, clean and sustainable alternative is water splitting into dihydrogen (H2) and dioxygen (O2). These two gases could be further recombined with only one by-product, water. Several challenges persist and one of them is the production of catalysts for the most difficult reaction in the process, the oxidation of water to produce O2. The most studied homogeneous catalysts for this reaction are ruthenium complexes but many aspects of the field still need to be investigated. The understanding of reaction mechanisms would help in the planning of more efficient and robust catalysts. Our proposal aims at answering two fundamental questions: 1) is it possible to achieve a two protons-two electrons RuII/RuIV oxidation with a lower potential than previously observed? If this is achieved, we would expect a much lower O2-evolving potential and this class of ruthenium complexes could be very competitive with the best catalysts known; 2) is it possible to promote ligand-based water oxidation (not on the metal center)? This is a completely new approach to the problem and, if successful, this mechanism could be extended to first row transition metals. To achieve our goal, we will investigate the pH-dependent electrochemistry of five new complexes bearing at least one imidazole unit. Moreover, the mechanism of water oxidation will also be determined by kinetic studies using CeIV as the sacrificial oxidant with O2 monitoring. Besides the spectroscopic methods, theoretical calculations will be used to support experimental data. For that, transition state activation energies will be calculated using DFT with standard methods. With this approach, different pathways will be compared to the experiment. Moreover, the structures of key intermediates will also be studied. Using DFT, theoretical estimates of pKa values and redox potentials will be obtained in order to get insight into the electronic structures of the catalyst in different oxidation/protonation states.
News published in Agência FAPESP Newsletter about the scholarship: