Catalysts with Metal-Ligand Cooperation for Sustainable Energy – CatEngy

Currently, bifunctional complexes featuring acidic (metal) and basic (ligand) sites are associated to highly efficient hydrogenation and dehydrogenation catalytic processes.
Recently, Partner 1 set up a straightforward and innovant experimental procedure for the formation of unprecedented bifunctional tethered oxocyclohexadienyl ruthenium complexes. Based on their preliminary results, Partner 1 will design a large variety of bifunctional tethered oxocyclohexadienyl ruthenium complexes. It is important to note that the reported synthetical strategy developed by Partner 1 allows many possibilities for the tuning of the chelating ligands of the respective complexes, in order to improve and optimize the catalytic performances of the designed complexes.
There is a great interest in replacing precious metals by abundant and environmentally friendly elements as iron. Even if their efficiency are still low, hydrogenation of bicarbonates and CO2 to formates catalyzed by iron bifunctional cyclopentadienone complexes were performed. By following the strategy of rigidifying the entire structure we developed for Ru-based complexes, we will prepare configurationally stable tethered cyclopentadienone iron complexes. Inspired by reported procedure, the latter catalysts will be design starting from non-expensive commercially available chemicals.
Using appropriate acetylenic substrates, homo- and cross-coupling of alkynes via Fe(CO)5-promoted cyclocarbonylation will provide a direct method for selective and high yield construction of tethered cyclopentadienone iron catalysts. It has been demonstrated that cyclopentadienones act as ligands in the presence of Fe2(CO)9 to end up with the corresponding cyclopentadienone iron carbonyl complexes. In a first approach we wil select the synthetical pathway which use commercially available cyclopropenones and a wide range of alkynes in a catalyzed cycloadditions, providing selectively cyclopentadienones in high yield on a multigram scale.

On his arrival at the laboratory (03/04/2019), Emmanuel Puig, post-doctoral fellow recruited by partner 1, began the synthesis of ruthenium oxocyclohexadienyl complexes, drawing inspiration from previous work carried out in our group by Manel Kéchaou-Perrot ( thesis defended on September 26, 2013). The synthesis conditions were optimized in order to obtain substantial amounts of ruthenium complexes for the catalytic tests. The chemical reactivity towards hydrogen H2, acids and ROH alcohols of the complexes was achieved. The structure by X-ray diffraction of the first oxocyclohexadienyl chelate dimer, starting product of many catalytic precursors, was obtained. The products were perfectly identified by the various analysis techniques. Since June 2019, batches of ruthenium complexes have been sent regularly to our Rennes partners (P2) for the various catalytic tests to be carried out.
The catalysts have been evaluated in ketone hydrogenation test reactions. They have shown interesting catalytic activity, especially when used in hydrogen transfer using formic acid as the source of hydrogen. The outstanding result to be highlighted is that these catalysts work in the absence of any other additive, in particular in the absence of base, which was one of the working hypotheses and a first objective of this project has therefore been achieved. The catalysts were also evaluated in the Guerbet reactions. Here again, they have demonstrated that they are able to catalyze this reaction, however with activity and selectivity below the current state of the art. Very recently, studies have started in the hydrogenation of CO2. Here again, the catalysts make it possible to carry out this reaction with still modest results but encouraging for the rest of the project in view of the changes envisaged concerning the catalysts.

Frequent exchanges between the two partners have made it possible to identify avenues for improving the catalysts tested so far and in view of the work carried out, the synthesis of a second generation of ruthenium catalysts will be initiated very soon.
Given the very innovative nature of our complexes and the modularity that it is possible to incorporate into the various syntheses that we have developed, we will try to achieve our synthesis objectives for the ruthenium complexes before the end Emmanuel Puig's contract (02/27/2021).
Despite the events that we have gone through since the start of the CatEngy project (the coordinator's stroke, confinement period linked to COVID-19) and which resulted in a total cumulative delay of about 8 months out of the past 24 months, we have tried to adapt to the different situations and we can consider that the project is going well. The preliminary results obtained in the synthesis and reactivity of our tethered oxocyclohexadienyl ruthenium complexes as well as the first results in catalysis will very soon be the subject of the writing of publications in collaboration with our theoretical partners from Pau, Dr Jean-Marc Sotiropoulos and Dr Karinne Miqueu .
Over the next period, exchanges between the two partners are expected with, among the main objectives, the collection of results in catalysis, their comparison and their rationalization according to the structure of the tested complexes.

The writing of a publication on the reactivity of bifonctional tethered oxocyclohexadienyl ruthenium complexes is in progress. Another publication is in preparation which will deal with the preliminary catalytic results.

Catalysts with Metal-Ligand Cooperation for Sustainable Energy (CatEngy) project addresses some challenges of a sustainable chemistry and production of alternative energy sources for future generations. Hydrogen and alcohols in particular bio-ethanol are two energy vectors considered as serious candidates for future utilizations.
The safe chemical storage of hydrogen, H2, is a topic of intense research. Although the concept of hydrogen storage through CO2 hydrogenation into formic acid is not recent, efficient catalysts are rare. Formic acid, HCOOH, is a safe and easy to store and handle liquid fulfilling some of the criteria for hydrogen storage. Efficient formic acid dehydrogenation and its reverse CO2 hydrogenation are thus the two key transformations to develop for the reversible storage of hydrogen. CatEngy project aims at studying the catalytic hydrogenation of CO2. Main challenges will be to achieve efficient reactions with ruthenium and iron catalysts under mild conditions, and, efficient reactions under base free conditions.
Alcohols, considered as safe chemical reservoirs of hydrogen as well, can be used as more conventional fuels for thermic engines. Standart engines do tolerate ethanol but only when added in low amounts to conventional combustibles. Two issues related to ethanol as engine fuel are its low energy density and its hydrophilicity. One way to address these issues is to upgrade ethanol into butanol. For this reason, Guerbet reaction is knowing a revival as it allows for the upgrade of primary and secondary alcohols into higher alcohols by an atom economic “hydrogen borrowing” reaction. CatEngy project aims at studying the upgrade of ethanol into higher alcohols for fuel engines. Main challenges will be to achieve efficient and selective transformations under mild conditions with ruthenium and iron catalysts, and, efficient transformation of bio-ethanol.
To achieve these goals, CatEngy project will develop innovant, efficient and selective homogeneous tethered catalysts with metal-ligand cooperation (= bifunctional catalysts).
Currently, bifunctional complexes featuring acidic (metal) and basic (ligand) sites are undoubtedly associated to highly efficient hydrogenation and dehydrogenation catalytic processes. Recently, Partner 1 set up a straightforward and innovant experimental procedure for the formation of bifunctional tethered (= chelating) oxocyclohexadienyl ruthenium complexes which are thermally robust, chemically inert to air, and can be handle in water. The synthetic strategy developed by Partner 1 offers many possibilities for the fine tuning of the chelating ligands of the targeted catalysts Cat A. Therefore, it will be possible to exploit the steric and electronic diversity of properties of Cat A to optimize the catalytic performances of the designed complexes.
There is a great interest in replacing precious metals by more abundant and environmentally friendly elements such as iron. Recently, even if their efficiencies are low, hydrogenation of bicarbonates and CO2 to formates catalyzed by iron bifunctional cyclopentadienone complexes have been reported. Following the strategy consisting in rigidifying the structure of the targeted complexes, we will prepare configurationally stable tethered cyclopentadienone iron, Cat B. Inspired by reported procedure, the latter catalysts will be designed following two main synthetic strategies.
From the synthetical part of CatEngy project, the main challenges will be to set up the optimum experimental conditions to prepare in a straightforward way the bifunctional configurationally stable catalysts in high yield with cheap commercially available chemicals.
This collaborative project is relevant to the main challenges addressing energy resources for future generations. The success of the project should bring new progresses in the synthesis of energy sources such as hydrogen and alcohols by overcoming some bottlenecks preventing future implémentations.