Rational design of the molecular Ru-based Water Oxidation Catalysts

Water splitting with sunlight is one of the most promising strategies to obtain renewable fuels in the short to midterm. To achieve this goal one of the crucial elements that need to be fully understood and mastered is the catalytic oxidation of water to molecular oxygen. Two main approaches have been followed so far for the development of water oxidation catalysts (WOCs): the oxide and the molecular way. The latter has generated a wealth of mechanistic information mainly based on Ru complexes and has led to catalysts with turnover frequencies two orders of magnitude higher than those of Oxygen Evolving Center in the Photosystem II.

In this thesis, we developed the synthetic and catalytic chemistry related to Ru complexes bonded to new members of the family of FAME (Flexible Adaptive Multidentate Equatorial) ligands.

The start of the thesis was dedicated to the motivations and objectives of the current studies. In the first part, we showed the development of a new Ru-based WOC, so-called Ru(tPa) complex, where tPa is [2,2′:6′,2′′-terpyridine-6,6′′-diphosphonic acid], which showed significant activity toward water oxidation as well as how second coordination sphere effects are responsible not only to generate the active catalyst but also to reduce energies of activation at the rate-determining step of the catalysis. Later on, we showed alternative ligand designs in order to tune the electrochemical properties of the WOC to decrease the overpotential for catalysis.

Afterward, we performed a detailed analysis of the stability of previously designed molecular WOCs under turnover conditions and consequently, we developed a powerful electroanode based on Ru coordination polymer, which showed high stability and catalytic activity for water oxidation with the achieving current densities above 0.1 A/cm2.

In summary, the present thesis highlights the importance of the rational design of water oxidation catalysts with the balanced ligand environment, the right second coordination sphere setting, as well as the need to evaluate the fate of the catalyst during turnover.

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