Electro and photo-assisted carbon dioxide reduction on molecular catalysts

The general context of this thesis is on CO₂ valorisation, and recounts fundamental research aimed at finding new molecular catalysts in order to achieve CO₂ conversion. The results obtained provide additional knowledge in view of developing an efficient and selective catalytic CO₂ reduction process. The first chapter begins with the general picture of CO₂ utilisation and contains a bibliographical overview on the use of molecular catalysts for CO₂ photo- and electroreduction. This short review demonstrates the wide variety of transition metal complexes used as catalysts, in particularly those based on rhenium. An analysis on the current state of CO₂ reduction research using molecular catalyst complexes of the manganese carbonyl type is then reported. In Chapter II, the discussion begins with a brief overview of the research developed during this thesis relating to new complexes of the general formula: [Mn(L)(CO)₃(X)] (X = Br, n = 0 ou CH₃CN, n = 1; L = terpyridine derivatives). The interest in these complexes is twofold, as their physico-chemical properties give them potential catalytic CO₂ reduction applications, as well as applications in the field relating to controlled CO release molecules. Developing new complexes based on ligands derived from terpy is one of the main routes that were chosen for catalyst performance optimisation and improvement. In this context, Mn complexes were synthesised, their photo- and electrochemical properties were studied in detail, before testing their activity with respect to catalytic CO₂ reduction. The most remarkable result comes from the unique ability of these species to release one of their carbonyl ligands in a controlled fashion, which led to the discovery of novel Mn dicarbonyl complexes which are selective catalysts for CO₂ reduction, and also to new molecules which are applicable in the release of small quantities of CO for therapeutic purposes. Chapter III covers the study of the complex [Mn(phen-dione)(CO)₃(X)]ⁿ⁺ (X = Br, n = 0 ou CH₃CN, n = 1). This complex was synthesised with the aim in mind of obtaining complexes soluble in aqueous media. While this objective was ambitious, it was soon found that these species showed no catalytic activity for CO₂ reduction in the target aqueous medium. Nevertheless, electrocatalytic activity was demonstrated in hydro-organic media and photo-catalytic applications in organic media proved promising. This work allowed us to gain a better understanding of the physicochemical properties and catalytic mechanisms of a Mn complex containing a redox-active ligand.The results presented in Chapter IV are based on the study of another new Mn carbonyl complex family. The study concentrated mainly on the complex [Mn(pyperNH)(CO)₃(CH₃CN)]⁺. The pyperNH ligand, with its aromatic system and NH functional group, plays a major role and is redox-active. After a section dedicated to its synthesis and characterisation, this chapter devotes a large part to the role that different experimental parameters can play on the efficiency of the catalytic CO₂ reduction reaction. The Mn complexes described in this chapter have remarkable redox properties. A preliminary study of the catalytic activity of these complexes for CO₂ reduction has allowed us to have an initial idea about the potential role of the N-H functional group of the ligand on the catalytic mechanism and reaction selectivity. This research was extended beyond the pyperNH ligand into a complex class of similar ligand structures, and the preliminary results obtained are original and promising, and open the way towards new perspectives. (author)