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Computational Modelling and Design of Sustainable Catalysts for Water Oxidation

Final Report Summary - COMPUWOC (Computational Modelling and Design of Sustainable Catalysts for Water Oxidation)

This project aimed at the computational design of active and robust catalysts for the oxidation of water (Figure, A). This chemical process is a key component of artificial photosynthesis devices, which generate energy from renewable sources including water and sunlight. The execution of the project yielded unexpected results and failures that were tackled by following the risk management plan of the original proposal. Following this plan, the research activities and goals of the project were diversified by including three other reactions of high socioeconomic interest; namely 1) carbon dioxide reduction, enabling the production of valuable chemicals from this greenhouse effect gas, 2) functionalization of cheap chemical feedstocks into added-value products (Figure, B), and 3) cross-coupling reactions, enabling the synthesis of pharmaceuticals (Figure, C). The deliverables of the project included structures, energies, kinetic parameters and spectroscopic properties, which, together, allowed for a mechanistic understanding of the catalysts at the molecular level. The project was developed in close collaboration with experimental groups, both at the host institution (Tilset group, Univ. of Oslo, Norway) and abroad (Hazari and Crabtree groups, Yale Univ., USA; Pérez, Univ. of Huelva, Spain). This enabled the benchmarking of the computational methods used in the calculations and the transfer of the knowledge gained in the supercomputers to the real laboratory. State-of-the-art methods, mostly in the framework of the density functional theory (DFT), were used. Theoretical collaborators included the groups of Prof. Ruud (Univ. of Tromsø, Norway), Lledós (A. Univ. of Barcelona, Spain), Pedersen (Univ. of Oslo, Norway), Reiher (ETH Zürich, Switzerland) and Rovira (Univ. of Barcelona, Spain). The main work, results and impact of the project can be summarized as follows:

1) In catalytic water oxidation, the electronic structure, redox properties and UV-Vis spectra of a series of oxo-bridged dinuclear complexes were characterized (J. Am. Chem. Soc. 2016, p. 15917; 2017, p. 9672; Angew. Chem. Int. Ed. 2017, p. 13047) . Experiments were combined with advanced DFT, TD-DFT and DMRG-CASSCF methods. This work advanced the state-of-the-art of the field because, for the first time, the highly oxidized species required in water oxidation were characterized for metal centers supported by purely organic ligands in the iridium III, IV and V oxidation states. Copper catalysts were also investigated and yielded promising results, including the C-H oxidation of inert organic substrates (Chem. Sci., 2017, 8, 8373). The dynamic solvation structure of these catalysts in water was explored by means of DFT molecular dynamics calculations.

2) In the functionalization of simple and cheap organic molecules (e.g. acetylene), the structure, reactivity and catalytic properties of transition metal complexes based on gold were determined (Angew. Chem. Int. Ed. 2013, p. 1660; Dalton Trans. 2016, p. 5504 and 14719; ACS Catal. 2017, p. 5023). Despite its noble character, this metal has become very popular in catalytic applications. Our strategy focused on the rare gold(III) state, which had been proposed as a transient intermediate in this chemistry. DFT calculations were used to determine the underlying reaction mechanisms. This knowledge was used to design experiments aimed at the characterization and development of these catalytic systems.

3) In catalytic cross-coupling reactions, the kinetic and thermodynamics parameters of the mechanism were determined for the key steps occurring inside and outside the catalytic cycle (J. Am. Chem. Soc., 2014, p. 7300; Organometallics, 2015, p. 381; ACS Catal., 2015, p. 3680 and p. 5596; J. Am. Chem. Soc., 2017, p. 922). The calculations were combined with the experiments to develop a design model of the catalyst enabling the optimization of both catalytic activity and robustness. This research line yielded one of the most efficient catalysts known to date for the Suzuki-Miyaura reaction. The substitution of palladium by nickel, which is a cheaper and less toxic metal, was also explored to identify and prevent the key factors diminishing the catalyst robustness in the latter case.

Other secondary research lines included: Location of hydride positions in polynuclear complexes (Angew. Chem., Int. Ed., 2014, p. 12808; Dalton Trans., 2015, p. 18403; J. Organomet. Chem., 2017, p. 17), prediction of NMR (nuclear magnetic resonance) parameters (Inorg. Chem., 2015, p. 11411; Organometallics, 2016, p. 3154), mechanistic studies on the chorismate mutase enzyme (FEBS Open Bio, 2017, p. 789) and the design of new catalysts for hydroamination (ACS Catal., 2016, p. 5651) and oxygen activation (Chem. Eur. J., 2017, p. 5232).

Beyond the results accomplished during these four years, the execution of the project has also opened other promising research lines that are currently active and will be further pursued in the near future. These include:

1) In the oxo-bridged dinuclear complexes related to water oxidation, the substitution of the anionic chloride ligands by neutral aqua ligands under acidic conditions will be explored. This will open the possibility of oxidizing the complexes further by introducing hydroxo and oxo ligands. The dissociation of the dimers into mononuclear iridium species or their aggregation into polynuclear species will be also investigated. The presence of oxyl character in these species may also open interesting possibilities in oxidative C-H activation.

2) Also in the field of catalytic water oxidation, I recently started a new collaboration with the experimental group of Tilley at UC Berkeley. The project focuses on the activation of cubic cobalt oxides by replacing one cobalt by a different metal (e.g. manganese) or by appending one extra cobalt center in a dangling position. The goal of this strategy is to boost the oxidizing properties of the system by breaking the symmetry of its electronic structure. In this framework, DFT models will be used to determine the charge and spin multiplicity of the ground states and to follow the intramolecular electron flow along the reaction pathways.

3) In the catalytic photoreduction of carbon dioxide, the cyclometallated ruthenium complexes will be incorporated into nanoporous MOF (Metal Organic Framework) materials. This will enable the uptake of the gas by adsorption and its transformation into a series of valuable products, including formic acid, formaldehyde and methanol. Computational models will be used to estimate the influence of the ligand modification required to support the catalyst onto the MOF, the confinement effects inside the nano-scale pores and the viability of the resulting materials. The use of similar systems for the oxidation of natural gas to methanol will be also explored.

In addition to the classical dissemination channels, including the publication of articles in high-impact journals and the invited contributions to international conferences, the results of the project were presented and constantly updated in a new website opened with this purpose:

In summary, this grant has allowed me to develop my original project idea and to enrich it by adding new research lines. The execution of the project has also involved a significant expansion of my international network of collaborators. The positive impact of this Marie Curie action is well reflected on the list of articles published and on my recent promotion to a permanent Researcher position at the host institution.