Computational Chemistry for Homogeneous Redox Catalysis

Laburpena

Redox chemistry is based on the use of oxidants or reductants to promote challenging reactions. The main characteristic of redox reaction is that the processes are based on electron transfers, and therefore, the reactivity, and more specifically the reaction mechanisms, is very different from the classic two electron chemistry (acid-base catalysis, cross-coupling reactions, etc.). In this context, new methodologies such as artificial photosynthesis, reductive and oxidative couplings have appeared recently to promote a more sustainable chemistry and to solve challenging problems, such as the fossil fuel based energetic scheme (in the first case) and the poor atom-economy of classic cross-coupling catalysis (in the second cases). With the raise of computer power, computational chemistry is now a very interesting field that can model a variety of reactions and can predict and improve the reactivity along the chemical space. This, in conjunction with the need to develop mechanistic studies to understand the redox catalyzed reactions, prompted us to perform a wide computational study of water oxidation and oxidative coupling reactions. Due to the size of the systems and the presence of transition metals, Density Functional Theory was selected as the computational methodology. Additionally, reactivity based on electron transfers is still poorly studied from a mechanistic point of view, and developing new theories that support the experimental evidence would be very desirable to facilitate the rational design of new homogeneous redox catalyzed reactions. In this thesis, we aim to study a wide variety of redox-based reactions in two different fields, the water oxidation and the oxidative coupling. Although both topics could seem very different, they share the same basis regarding the mechanistic features, in which transition metal chemistry depends on different electronic states and is the key to explain the reactivity. The first chapter presents an overview of redox reactivity and the previous theoretical background on the field. Then, chapter 2 describes all the theoretical methods used in the reaction mechanistic studies throughout chapters 3 and 4 and it also illustrates the development of two new theoretical methodologies, one to apply quasi-harmonic corrections to the free energy in an easy way and the other to calculate standard redox potentials from the Born-Haber thermodynamic cycle, including entropy contributions to the cohesive energy. Chapter 3 covers the computational study on two different oxidative coupling reactions, the rhodium/copper catalyzed oxidative coupling of benzoic acid and alkyne and the ruthenium/copper catalyzed homocoupling of carbazole under aerobic conditions. The specific effect of the oxidant was unraveled in the first case, demonstrating a cooperative effect between rhodium and copper, leading to a new type of elementary step, the cooperative reductive elimination. In the second part, we collaborated with an experimental group and we also probed the complicated integration of ruthenium, copper and molecular dioxygen in the C-H activation and reductive elimination for the carbazole homocoupling. These results were in very good agreement with the experimental kinetic measurements. These studies would help in the experimental design of new oxidative couplings, giving light to the oxidant effect well-known on the field. In chapter 4, we analyzed computationally the mechanism of water oxidation for different homogeneous copper catalysts. In collaboration with an experimental group, we developed in the first part a new family of mononuclear copper-based water oxidation catalyst. The activation of the catalyst is based on a ligand oxidation, and the oxygen-oxygen bond formation step occurs through an unprecedented mechanism, the single electron transfer water nucleophilic attack (SET-WNA). The mechanistic knowledge led to the rational design of new ligands that decreased the overpotential for the water oxidation reaction until 170 mV. Finally, the last part of this chapter extends the applicability of the new reported mechanism to other important copper catalysts, redefining the mechanistic scenario of the oxygen-oxygen bond formation, especially for first-row transition metal catalysts. In conclusion, this Thesis provides a deep understanding of redox reactions. These processes are based on electron transfers and therefore, this was very challenging from a computational perspective. The results explain successfully several quations of the different mechanisms that can help in the rational design of new reactions.