Resumen de NHC-stabilized Rh Nanoparticles as efficient catalysts for the hydrogenation of aromatics and selective H/D exchange in P-based ligands
Transition metal nanoparticles (M-NPs) have gained significant interest in catalysis during the recent years and have been considered at the frontier between homogeneous and heterogeneous catalysis, potentially combining the advantage of both. Because of their small size, and consequently high surface to volume ratio, they have exhibited high activities, even under mild reaction conditions. They are soluble in a wide range of organic solvents that are commonly used as reaction media in catalysis. For their preparation, the use of stabilizers is required to avoid the formation of bulk metal, which is the thermodynamically favoured product during the aggregation of metal atoms. In this context, the utilization of small molecules such as ligands for NPs stabilization is of special interest since parameters such as their steric and electronic properties have been extensively studied in homogeneous catalysis and allows the tuning of the final metallic catalyst performance. In the field of ligand-stablized M-NPs, N-heterocyclic carbenes (NHCs) have demonstrated to be efficient compounds for the formation of stable M-NPs due to their high σ-donor properties. For these reasons, M-NPs have been considered promising catalysts for different processes such as selective hydrogenation and C-H activation reactions, among others.
The final goal of this thesis was the synthesis and characterization of Rh NPs stabilized by N-heterocyclic carbene (NHC) ligands for their application in the hydrogenation of compounds bearing aromatic rings such as aromatic ketones, phenols and pyridines. Besides, the catalytic performance of these Rh NPs towards the selective deuteration of P-containing molecules such as phosphines and phosphine oxides has also been explored to get insights into the coordination mode of these P-ligands at metal surfaces.
In Chapter 1 a general introduction on the synthesis and application of M-NPs in catalytic reactions will be exposed. In this part, some examples on the use of M-NPs in reactions such as oxidation of alkanes and alkenes, Fischer-Tropsch, hydrosilylation of alkenes and akynes, C-C coupling (Heck and Suzuky-Miyaura), hydroformylation of alkenes, transfer hydrogenation of ketones, olefins, nitroarenes and α,β-unsaturated carbonyl compounds hydrogenation and H/D exchange of N- and P-based compounds will be described.
Chapter 2 sets out the general objectives of the thesis.
The synthesis of Rh NPs formed by the utilization of different stabilizing agents will be exposed in the introduction part of Chapter 3, with special emphasis on the methods previously reported for the formation of M-NPs stabilized by NHC ligands. Results regarding the synthesis and characterization of Rh NPs stabilized by 1,3-Bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene (IPr) will be exposed in this chapter. The effect of the amount of ligand used on the Rh NPs formation will be investigated and the availability of sites on the NPs surface where catalytic reactions can take place will be examined by CO adsorption experiments.
The research in Chapter 4 deals with the application of these Rh NPs stabilized by IPr in the selective hydrogenation of aromatic ketones, phenols and N-heteroaromatic compounds as such as pyridines and quinoline. A review of the previously reported results regarding these catalytic reactions using M-NPs will be described, followed by the catalytic results obtained using these NHC-stabilized Rh NPs.
In the case of aromatic ketones, the preferential reduction of the arene ring over the ketone moiety will be investigated. Parameters such as the arene-ketone distance and the presence of different substituents close to the ketone group and in an aromatic position will also be examined.
In the case of phenols, the selective formation of cyclohexanones by partial reduction of the arene ring will be examined, studying the effect of the presence of different substituents in ortho and para positions of the arene ring.
Finally, in the case of N-heteroaromatic substrates such as pyridine derivatives, the catalytic activity of these Rh NPs will be examined. The selective reduction of substituted pyridines bearing phenyl and ketone moieties will also be investigated. The complete reduction of quinoline was also aimed at during this study.
Chapter 5 describes the application of Rh and Ru NPs stabilized by IPr and poly(N-vinyl-2-pyrrolidone) (PVP) in the selective H/D exchange of P-containing substrates. The catalytic application of M-NPs in this reaction provided valuable information on the coordination mode of these substrates at the surface of these NPs. In this context, a detailed review exposing the reported results concerning the application of M-NPs in the selective H/D exchange reaction of N- and P-containing molecules will be presented in the introduction of this chapter. The research in this chapter explores the difference in catalytic activity and selectivity of Rh and Ru NPs formed by stabilization of IPr and PVP in the selective deuteration of various phosphines and diphosphines bearing aryl/alkyl positions and phosphine oxides. Indeed, coordination mode for these substrates at the surface of these NPs will be proposed based on the results obtained for these selective deuteration reactions.
The final remarks and conclusions extracted from the results obtained in this thesis will be described in Chapter 6.
