Resumen de Mechanistic studies of water oxidation catalyzed by homogeneous iron and ruthenium complexes and light-driven organic reductions with a dual cobalt/copper catalytic system
The transition to a green and sustainable energy-based scheme is one of the most important challenges that faces our society. Natural photosynthesis is the process by which sunlight energy is stored into chemical bonds to sustain life, producing only O2 as a by-product. Therefore, an appealing approach is the application of artificial photosynthetic (AP) schemes to produce the so-called solar fuels and fine solar chemicals from CO2 and water using sunlight as driving force. However, both CO2 reduction and water oxidation (WO) are challenging processes and remain bottlenecks for the development of efficient AP. In addition, a viable artificial photosynthetic approach should also rely on inexpensive and long-lasting photocatalytic materials. In this regard, new sustainable, modular, robust and efficient catalytic platforms are needed. Moreover, it is important to notice that to design efficient and robust artificial photosynthetic systems, a fundamental understanding of the factors that control both the catalytic activity and selectivity is necessary.
Therefore, this thesis entails a fundamental understanding of the mechanisms involved in AP schemes and their application to produce fine chemicals. In this regard, we describe the synthesis and characterization of well-defined molecular iron and ruthenium WO catalysts, which, led to a better mechanistic understanding of how to access and stabilize the high oxidation states required at the metal center for O-O bond formation. Ultimately, this work resulted in the isolation and comprehensive characterization the key intermediates, as well as a thorough experimental and theoretical description of the catalytic cycle. Of particular note is the isolation and direct characterization of a high-valent RuIV-peroxo intermediate after the O-O bond formation in ruthenium catalyzed WO.
Furthermore, the development of light-driven selective organic transformations based on a dual copper-cobalt photocatalytic system is also described. To remark is the unprecedented selectivity for the reduction of aromatic ketones in front of aliphatic aldehydes. Here, mechanistic studies point to a substrate-dependent mechanism for the reactivity of the postulated [Co-H] species, which is dependent on the reduction potential of the substrate. Detailed reactivity and mechanistic studies based on spectroscopic techniques (UV-vis, XAS, NMR, etc.) together with DFT calculations point to the formation of a CoI intermediate, which followed by protonation forms the [Co-H] active species. These examples pave the way for novel sustainable light-driven reductive transformations using earth-abundant elements as catalysts.
