Skip to main content

Modular Ligands for Water Splitting

Final Report Summary - WATERSPLIT (Modular Ligands for Water Splitting)

Replacing petroleum by a sustainable fuel energy system and reducing CO2 emissions are among of the most critical issues of today’s society. While the energy consumption per year is predicted to increase from 529 EJ in 2010 to 780 EJ by 2035, the natural supplies of petroleum are diminishing. At the same time CO2 emissions are widely considered as the main factor contributing to global warming. In this context, the use of hydrogen as an optimal energy carrier holds high potential. Its water-solar production is envisioned as one of the most promising approaches with regard to sustainability and energy security, avoiding undesirable emissions. However, fundamental technological challenges still remain to be solved: water-splitting into H2 and O2 is a multi-electron process coupled with a multiple-proton transference in an uphill energy transformation. In this regard, the multi-electron transference has been underlined as a general chemical problem.
Despite these enormous chemical challenges, water oxidation is absolutely required because the reaction produces O2, and water is the only waste-free electron-source substrate that could sustain the multibillion ton scale of the process required in order to supply Earth energy demands. Although, excellent activities has been found when using second and third row transition metals, the development of water oxidation catalysts (WOCs) based on first row transition metals, may be regarded as abundant, environmentally benign, and inexpensive catalyst alternatives. In this regard, the project WaterSplit aimed to study some of these challenging issues. In particular the project aimed for developing well-defined homogeneous coordination complexes based on earth abundant elements as active catalysts for both water oxidation to O2 and water reduction catalyst to H2. Molecularity nature of large majority of water oxidation catalyst and in less measure the water reduction catalysts has been questioned. Therefore, the development and study of molecular species is essential in order to get insights in the operating mechanisms that allows the product formation. Highly chelating and robust ligands may prevent hydrolysis or catalyst decomposition, although this may reduce in part their catalytic activity.
In these regard, we have discovered a new family of well-defined, homogeneous and highly effective water oxidation catalyst based on iron. Our studies showed that catalytic activity is highly sensitive to the geometrical and electronic structure of the metal center. We also detected and spectroscopic characterized a new class of reaction intermediate, and iron-cerium adduct that can be used as a chemical model of the PSII. The designed systems also provided us an excellent starting point to explore additional catalytic transformation such as the hydroxylation of inert C-H bonds in water. We also developed a new family of molecular water reduction (photo- and electrocatalysts based on cobalt. The fundamental knowledge provided by these studies contribute to the titanic endeavor of development of highly efficient water splitting catalysts for the production solar fuels.

Task 1: New Water Reduction Catalysis (WRC). Modular ligands to study the reduction of protons to H2.
We have identified a new family of highly efficient molecular water reduction catalysts that operate in both photochemical and electrochemical conditions. This new family of cobalt complexes is synthesized with highly chelating tetra- and pentadentate aminopyridine ligands. In addition, development of systems based on this ligands offer some direct advantages: i) They are water soluble, ii) They are based on abundant metals in the earth crust, iii) Their electronic and steric properties are easily tuned due to the high modularity of the ligands employed. These properties confer control of the overpotential of catalytic production of H2. And finally iv) the high basicity of the ligands developed favours an heterolytic water reduction mechanism excluding H2 production via bimetallic pathways. As consequence, they may be ideal systems to be implemented into artificial photosynthetic schemes. The high modular character of these ligands opens the access to a large variety of substructures and offers a clear advantage when studying the influence of electronic and steric parameters in the defined transformation.
Our results indicate that this family of complexes is highly active in production of H2, affording up to 800 catalytic cycles of H2 production per molecule of catalyst (TON) under light-driven conditions and more than 5300 TON/h. Electrochemical mechanistic investigations reveal that the reaction is first order in catalyst and second order in acid. Photochemical studies on the effect of the electronic nature of the ligand and ligand coordination index suggests that the reaction is single site and that the active species that leads to the H2 production is LCoII-H. Moreover, computational studies corroborate our experimental evidences on the catalytic cycle. More studies are on-going to fully elucidate the mechanism of H2 production and to explore other target transformations, such as the reduction of organic substrates.

Task 2: New Water Oxidation Catalysis (WOC).
We have obtained a new family of highly efficient molecular water oxidation catalysts. These catalysts are based on neutral tetradentate aminopyridine ligands and iron as active site, which can be seen particularly attractive for designing oxidation catalysts because are based on abundant, environmentally benign and inexpensive elements.
These systems are among the highest turnover number per metal centre described so far for any homogeneous water oxidation reaction based on a 1st row transition metal. Turnover numbers > 2,500 have been obtained using chemical model oxidants. These catalysts contain readily available and modular tetradentate nitrogen ligands which lead to a broad accessibility of catalysts, allowing to extract preliminary data about the scope, efficiency of the reaction and specific mechanistic information on the role of the iron complexes in catalytic water oxidation reactions. In this line, it was found that iron complexes with tetradentate nitrogen based ligands that leave two available cis-positions were effective water oxidation catalysts (for instance [Fe(OTf)2(Pytacn)] and [Fe(OTf)2(mcp)]) when employing CAN as an oxidant). On the contrary, iron complexes with tetradentate nitrogen ligands that leave two available trans-positions, [Fe(OTf)2(tmc)] or with neutral pentadentate nitrogen ligands [Fe(NCCH3)(MePy2CH-tacn)](OTf)2, do not form significant amounts of O2.
A systematic electronic effects observed on the catalytic activity of Fe(OTf)2(E,HPytacn)] (OTf = CF3SO3, E = –H, –Cl, –CO2Et and –NO2) complexes strongly supports that the water oxidation activity originates from molecular complexes operating in a homogeneous phase. Corresponding rates of O2 evolution correlate with the electron-donating nature of the ligand, which in turn is modulated by the substituent located at para position of the pyridine ring. Mechanistic investigations by means of experimental and computational methods point at the O-O bond formation event as the rate-determining step, via nucleophilic attack of a H2O molecule towards a ferryl (FeV=O) species. Electronic principles governing the water oxidation activity of these complexes have been discovered and applied to obtain catalysts with improved performance. Moreover, thanks to this detailed studies we have discovered a new intermediate in the water oxidation reaction. These last detectable intermediate before the rate determining step, the O-O bond formation, is an iron-cerium adduct. Now starting from this new complex we can investigate the role of a Lewis acid in the O-O bond formation step as occurs in the case in the PSII. It is thus envisioned that this study may provide directions for the rational quest and design of more efficient 1st -row water oxidation catalysts.