Nickel and cobalt bis(imino)pyridine complexes as catalysts for the hydrogen evolution reaction
- Autores: Bing Jiang
- Directores de la Tesis: Xavier Sala Román (dir. tes.), Antoni Llobet Dalmases (dir. tes.), Nuria Romero (dir. tes.)
- Lectura: En la Universitat Autònoma de Barcelona ( España ) en 2019
- Idioma: español
- Tribunal Calificador de la Tesis: Jordi García-Antón Aviñó (presid.), Eduardo José García Suárez (secret.), Roger Bofill Arasa (voc.), Laia Francàs Forcada (voc.), Marcos Gil Sepulcre (voc.)
- Programa de doctorado: Programa de Doctorado en Química por la Universidad Autónoma de Barcelona
- Materias:
- Enlaces
- Tesis en acceso abierto en: TESEO
- Resumen
Sustainable energy supply is a critical challenge humanity is facing. The dependence of fossil fuels have ignited interest in the development of carbon-free energy sources, among which energy-dense dihydrogen would be one of the most promising when generated from renewable alternatives. In this context, splitting water to produce hydrogen as fuel is of great interest, and hence a large number of catalysts made from earth-abundant transition metals have been developed to accelerate the reaction rate. For the purpose of effectively using metal-atom economy and facilitating mechanistic analysis, molecular complexes as homogenous catalysts got much attention. In this kind of compounds, metal centres are usually required to accommodate multiple redox states and experience metal-hydride intermediates. However, there has been a renaissance in catalysts’ design for the hydrogen evolution reaction (HER) by coordinating the metal centre with non-innocent ligands who can operate as an electron reservoir or/and protonation site. The first Chapter of this PhD Thesis generally introduce the need and techniques for hydrogen production to support the energy demand, reviewing electrocatalysts for the HER based on the non-noble metals nickel and cobalt. The second Chapter describes the objectives of this Thesis, i.e., the synthesis and characterization of redox-active pyridine diimine (PDI)-based ligands and the corresponding nickel and cobalt complexes. Their electrocatalytic evaluation in HER is aimed, as well as the description of the processes from the mechanistic point of view. The third Chapter presents the preparation of a set of pentadentate PDI ligands, synthesis and full characterization of six-coordinated Ni(II) complexes [Ni(Ph2PPrPDI)(Cl)](Cl) (1(Cl)) and [Ni(Ph2PPrPDI(p)Cl)(Cl)](Cl) (2(Cl)) using techniques including mass spectroscopy, UV-vis spectroscopy, X-ray crystallography and elemental (C, H, N) analysis. The electron location upon electrochemical reduction of 1(Cl) was examined by density functional theory (DFT) calculations along with electron paramagnetic resonance (EPR), revealing the metal-based nature for the first one-electron reduction and ligand-based for the second one. The redox and catalytic properties of both complexes were studied. On the one hand, more positive redox potential and therefore lower overpotential were observed for 2(Cl), indicating that the presence of the electron-withdrawing Cl substituent in the PDI ligand scaffold improves the electrocatalytic activity. On the other hand, 1(Cl) is more active for the HER in terms of turnover frequency and rate constant. Short-term stability studies for 1(Cl) and 2(Cl) in the course of HER by CV indicates homogenous catalysis without decomposition, whereas long-term stability analysed by controlled potential electrolysis shows the generation of a new transitory species which can trigger the hydrogen oxidation reaction and reduce the overpotential for the HER. The fourth Chapter focus on the synthesis and characterization of five-coordinated Co complexes bearing the ligands applied in the third Chapter ([CoI(Ph2PPrPDI](BF4), 3(BF4) and [CoI(Ph2PPrPDI(p)Cl](BF4), 4(BF4)). Electrochemistry data shows that these complexes are active for the HER in organic media, undergoing protonation to form Co(III) hydride species which were identified by UV-vis and NMR spectroscopy. On the basis of NMR spectroscopy, the initial Co(I) compound was recovered after a catalytic reaction cycle, demonstrating a homolytic mechanism for HER by this system. Finally, the fifth Chapter lists the most relevant conclusions extracted from the work carried out.
