Rational design of transition metal complexes for the calalytic reduction of carbon dioxide : a synthetic, spectroscopic, and computational approach
Cramer, Hanna Hinrika; Leitner, Walter (Thesis advisor); Neese, Frank (Thesis advisor)
Aachen : RWTH Aachen University (2022)
Dissertation / PhD Thesis
Dissertation, RWTH Aachen University, 2022
The catalytic reduction of carbon dioxide (CO₂) to value-added C1 products is central for establishing sustainable chemical industries and requires both highly active and selective catalysts. This dissertation focuses on 3d transition metal pincer complexes as catalysts for the reduction of CO₂ to derivatives of formic acid, formaldehyde, and methanol. The work connects the catalytic activity and the preferred product reduction level to both the catalyst architecture and the reaction conditions and aims to contribute to a rational catalyst design.A catalytic system based on a cobalt(II) triazine pincer complex was applied in the hydrosilylation of CO₂. Fine adjustment of the reaction conditions enabled the selective arrival at three product levels with the same catalyst and reducing agent, while high temperature, low pressure, and high concentrations favored the reduction beyond the formate level. Synthesis, characterization, and catalytic application of hydride, formate, and silyl complexes enabled the identification of kinetically competent catalytic intermediates. Quantum chemical studies revealed that the cascade reaction proceeds via a series of hydride transfer, oxidative addition, and reductive elimination steps. Increasing kinetic barriers of the three cycles, competing hydride transfer steps, and trapping of formaldehyde as acetal are the key factors for selectivity control and rationalize the influence of the reaction conditions. Molecular volcano plots constructed from a library of iron, cobalt, and nickel pincer complexes provided insight into structure-reactivity relationships. The catalyst activity and selectivity could be described by the hydride affinity as a catalyst property depending on the choice of the metal and the ligand. The relative energy spans of the three steps determine if the reaction rates within the hydrosilylation cascade reaction decrease or increase, leading either to a more controlled arrival at each product level or to a rapid complete reduction to the methanol level. The implications of the volcano plots were confirmed by experimental observations for selected catalyst candidates. In conclusion, this thesis provides insight into molecular control in the reduction of CO₂ to products on different reduction levels both by catalyst design and by the reaction environment and hopes to contribute to the development of future catalytic systems.