Nitrogen-based ligands for iron, zinc and cobalt catalyzed selective processes

Resumen

Organometallic compounds have become an established synthetic tool for both fine and bulk chemicals. The reactivity and selectivity of the active catalyst are influenced by the choice of the metal center and the design of the surrounded ligands. Traditionally, second and third-row transition metals have been applied in organometallic chemistry, particularly the platinum-group metals (PGMs) have proven to be efficient for a large number of applications. However, the study of the organometallic chemistry of Earth-abundant metals has increased over the past few years as consequence of their low cost, ready availability, comparable low toxicity and sustainability. The final goal of this thesis is the development of novel families of ligands and their coordination to earth-abundant metals (Fe, Zn and Co) exploring their performance in asymmetric transfer hygrogenation and asymmetric hydrogenation of ketones and the synthesis of cyclic carbonates from CO2 and epoxides. Chapter 1 contains a general introduction to the importance of catalysis and the general considerations to be considered when earth-abundant metals are employed in catalysis. Chapter 2 sets out the general objectives of this thesis. The research in Chapter 3 describes the synthesis of chiral PNNP ligands containing a pyrrolidine backbone and their related Fe(II) complexes. The X-Ray structures of some Fe-PNNP complexes were also elucidated. The combination of the Fe3(CO)12 as iron precursor with a chiral PNNP ligand was found to catalyze the asymmetric transfer hydrogenation of a variety of ketones with moderates to high yields and enantioselectivities . The above mentioned system was found to be heterogeneous rather than homogeneous, having modified iron particles acting as active catalyst. The research in Chapter 4 describes the synthesis of a new family of ligands containing a N2O2, N2NH2, N4 and N4(NH) ligand scaffolds with a pyrrolidine backbone. In addition, the corresponding Zn(II) complexes were prepared and fully characterized. The X-Ray structures of a tetranuclear and mononuclear Zn(II) complexes were also elucidated. These Zn(II) complexes were tested in the coupling of CO2 with terminal and internal epoxides and were found to be highly active catalysts for these transformation. It should be highlighted that excellent activity and total selectivity to the corresponding cyclic carbonate were obtained. Importantly, one of the Zn(II)/TBAI systems provided the highest conversion obtained for the coupling of CO2 with the trans-2,3-epoxybutane as substrate using Zn(II) complexes. Additionally, recycling experiments (up to 5 cycles) for one of the catalytic systems were carried out in order to ascertain the robustness of the system. The research in Chapter 5 explores the synthesis of chiral C^N^C type pincer ligands, which were obtained in two steps in moderate to good yields. These ligands were coordinated to cobalt using the Co(OAc)2 as cobalt precursor in good yields (72-78%). The resulting cobalt complexes were characterized by NMR, IR, Magnetic Susceptibility, ICP and EA. With all the collected data in hand, it was proposed the coordination of the Co(OAc)2 moiety to the pyridine of the C^N^C ligand maintaining the imidazole moieties uncoordinated. These cobalt C^N^C complexes were tested in the asymmetric hydrogenation of ketones with moderate to high yields, although low to moderate ee¿s were obtained. Initial attempts to obtain a well defined Co(II)-pyridine biscarbene complex having both the carbene and the pyridine coordinated to the metal center were accomplished, although still further investigation need to be performed.