First-row transition metal complexes for electrochemical carbon dioxide activation

Kinzel, Niklas Werner; Leitner, Walter (Thesis advisor); Palkovits, Regina (Thesis advisor)

Aachen : RWTH Aachen University (2022)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2022

Abstract

In the endeavor to substitute fossil resources for the production of energy and chemicals with renewable carbon feedstocks and energy sources, the electrochemical reduction of carbon dioxide is considered a "dream reaction". It holds the potential to use CO2 from industrial waste streams or the atmosphere and recycle the C1 building block into the chemical value chain by adding electrons. Molecular coordination complexes, particularly those based on 3d transition metals, can be introduced as catalysts to enable the reaction and expand the scope of accessible products. Thus, the alleviation of the global climatic and socio-economic effects of the greenhouse gas CO2 is combined with producing industrially relevant carbon-containing compounds. In this context, the present study is dedicated to investigating the role of the 3d metal center and the coordinated auxiliary ligands on the structural and electrochemical properties of the corresponding complexes. The obtained results shall be used to conclude the effect of these components on the traversed catalytic mechanism in the electrochemical activation of CO2. A series of mid to late 3d transition metal complexes (from manganese to zinc) was synthesized from the redox-innocent pincer ligand N2,N6-bis(diphenylphosphaneyl)-N2,N6-diphenylpyridine-2,6-diamine. Metal precursors were chosen in their +II or +I oxidation states and coordinated by chloride or acetonitrile (MeCN) ligands. The application of various analytical techniques allowed the correlation of the observed coordination geometries to the electronic configuration of the metal via crystal field theory. A qualitative increase in electronic density at the metal center could be observed throughout the 3d row. Cyclic voltammetry (CV) analyses revealed metal-centered redox processes for iron, cobalt, and nickel down to the zero-valent state. While cobalt and nickel undergo several ligand exchange reactions during this pathway, iron is surmised to dimerize or disproportionate.NMR spectroscopic and CV experiments tracked the ligand exchange between chloride and acetonitrile at the metal center. It was found that a single acetonitrile ligand coordinates cobalt and nickel at the zero-valent state in MeCN, independent from the auxiliary ligand in the starting complex. The π back bonding of MeCN supposedly removes electron density from the metal center, decreasing the potential required to reduce the complex but also its reduction strength. The stability of the complexes was found to be higher when chloride rather than labile acetonitrile ligands are coordinated and increased from iron to nickel, explainable by shorter and, hence, stronger bonds for the later transition metals. Electrochemical analyses under CO2 atmosphere showed the substrate coordination at the oxidation state zero for each of the complexes, yet substantial electron transfer to CO2 is only proposed for iron and cobalt. CV under the addition of the Lewis acid magnesium triflate identified cobalt as promising for the reductive disproportionation of CO2. A CV-based proton source screening revealed methanol as a suitable Brönsted acid for CO2 reduction with the same metal. Controlled potential electrolysis experiments, however, showed hydrogen as the preferred product with only minor amounts of CO. Likely, the probed complexes disintegrate at the electrode surface under formation of heterogeneous species. The para position in the pyridine core of the ligand backbone was identified as a possible weakness of the complex, the protection and further improvement of which will constitute a perspective for this work. Overall, the present study elucidates how the metal center determines the activity and stability of otherwise redox-innocent systems in electroreduction reactions such as carbon dioxide conversion. The in-depth knowledge of the electrochemical behavior of the cobalt and nickel complexes under inert conditions, in contact with CO2, and combined with further co-catalysts provides the means to adjust the properties of the complexes for CO2 activation. These insights will be used to improve the so-far unsatisfactory long-term stability of the complexes under electrocatalytic conditions.