Eco-aware selective tandem oxidations. Development of photocatalytic systems using both water and dioxygen as unique oxygen atom sources – TROICAT

Previously we developed two photocatalytic systems combining a photosensitizer and a catalytic partner, either a Ru-based entity for H2O activation, or, a Cu-based entity for O2 activation. For this purpose, in this project we envisage to associate both systems with the goal to use simultaneously water and dioxygen molecules as safe and inexpensive oxygen atom sources to perform two selective oxidations in the same reaction medium. In such a system the electrons released from one site (The Ru catalyst) will be used by se second one.
The different tasks involved mainly i) synthesis of the models dyads (First the Ruphot-Cucat then the Ruphot-Rucat , ii) the full characterisation of the dyads and the different intermediates, iii) studies of their spectroscopic properties and iv) evaluation of their photocatalytic activities and comparison with the activities of the bimolecular system. During this period the partner will study the photophysical properties of all systems in order to bring mechanistic essential information. Finally, the synthesis of the triad will be investigated then its photocatalytic properties. A special care will be given to the intervention of singlet oxygen during the photocatalytic process.

The first 8 months were devoted to the search of the pH D student. Then, until now, the dyad Ruphot-Cucat has been synthesized using the approach reported in scheme 4 (cf attached document). Its synthesis required first the synthesis of the ditopic ligand (scheme 5). Most of the molecules were fully characterized by NMR, Electrospray and elemental analysis. Electrochemical studies of the different complexes were also carried out. More important, an electron transfer from the photosensitizer to the Cucat partner yielding to its reduction has been highlighted by EPR spectroscopy. Indeed, the Cu(II) characteristic signal decrease progressively until total disappearance upon irradiation in the presence of a sacrificial electron donor. This observation pointed out the expected total conversion of Cu(II) to Cu(I). Currently, the photophysical studies are under investigation by our partner.
As far as the photocatalytic studies are concerned, we showed that in an acetonitrile-water mixture, sulfide can be quantitatively converted into its corresponding sulfoxide after 16 hours of irradiation without any trace of overoxidation process since no sulfone could be detected. More important, we also showed that oxidation by singlet oxygen is negligible.

As far as the triade is concerned, its synthesis should start at the beginning of 2018. The approach should be the same as for both dyad: synthesis with full characterization of all the intermediates, spectroscopic and photocatalytic studies of the triade associated to the mechanism determination thanks to photophysical studies done by our partner.

Currently, an article concerning the Ruphot-Cucat is in progress and will be submitted to Inorganic Chemistry. In the course of 2018, a second paper combining the results of the Ruphot-Rucat dyad will also be submitted to the same journal. Finally, due to the originality and novelty of the triad, it is expected that the results (if they are good enough) could be published in a renowned journal.

During recent years, in the wake of increasingly stringent environmental legislation, attention has also been focused on the development of catalytic oxidations for the manufacture of fine chemicals. The traditional methods of many fine chemical oxidations involve stoichiometric quantities of toxic inorganic reagents such as permanganate and dichromate salts. These reactions generates large amount of inorganic salts-containing effluent along with the target products. Thus, currently, there is a considerable pressure to replace these antiquated technologies with cleaner, catalytic alternatives. A clean synthetic technology that should proceed with a high atom economy must be accomplished with a low Environmental Factor (E-Factor). The ideal system for “green” oxidation is the use of water or molecular oxygen as the oxygen atom source.
Moreover, as a consequence of the inevitable end of fossil energy resources associated with a high increase in energy consumption, attempts to develop sustainable energies have emerged all over the word. As a consequence, tremendous efforts were made to take advantage of the exceptional photophysical properties of ruthenium polypyridyl complexes with the final objective to convert solar energy into chemical energy. In our research program aiming to develop new polypyridyl ruthenium-based catalysts for oxidation, we designed a combination of a photosensitizer and a catalytic fragment within the same complex to achieve catalytic light-driven oxidation of sulfides using water as the unique oxygen atom source. However, as most of the photocatalytic systems reported in the literature for oxidation or reduction reactions, a sacrificial reagent is used to either deliver or accept electrons resulting of the reaction. This “electron waste” is also accompanied by the generation of by-products issued of the sacrificial chemical. In order to prevent this drawback, it is envisaged to associate to the oxidative photocatalyst a second system able to add value to the liberated electrons. In particular, bioinspired by monooxygenases, copper and iron complexes would be potentially good candidates to use the released electrons from the ruthenium-based photocatalyst to perform oxidation using molecular dioxygen as oxygen atom source. In this project, it is thus proposed to design a ruthenium and copper (or iron)-based trinuclear complex constituted by the assembly of two catalysts and a photosensitizer initiating the electron transfer from one catalyst to the other. This photocatalyst will be able to perform two different and selective oxidation reactions using both O2 and H2O as oxygen atom sources.
The originality of the proposed project relies mainly on five points:
i)Water and molecular dioxygen as abundant and non-toxic molecules, will be used as the unique oxygen atom sources,
ii)Two selective oxidation reactions will be performed simultaneously in the same pot,
iii) Three metal centers will be covalently associated for an optimized “communication” between them, the photosensitizer playing the role of an electron relay between the two catalytic centers,
iv) The electrons liberated during the oxidation catalyzed by the first catalytic moiety will be used to activate the second catalytic fragment toward O2,
v)Solar (light) energy will be converted into chemical energy.