Electronic Structure and Bonding Contributions to Bond Activation in Low-Valent Iron Complexes

Catalysis has an extraordinary significance for the industrial-scale synthesis of most chemicals. This is particularly relevant for fine chemicals such as pharmaceuticals or agricultural products that often have complicated chemical structures. Multiple chemical bonds that constitute molecules can be constructed more energy-efficiently using transition metal-catalysed reactions. A wide range of reactivities of these metals find application in various industrial processes, including ammonia synthesis, hydrogenation of vegetable oil and alcohol oxidation. Of note are bond activation processes such as C-H functionalisation, which have attracted significant attention recently due to their importance for synthesising natural products, material science, and biomedicine. These reactions can modify inert carbon-hydrogen bonds within the molecule with high selectivity at ambient pressure and temperature. Most C-H-activation reactions relevant
to the synthesis of fine chemicals use platinum group metals as catalysts. Within this group, palladium is most commonly used. Due to low abundance, the cost of the reaction is prohibitively high, and the price of the metal is subject to fluctuations due to geopolitical tension and is expected to rise in the future due to dwindling supply. Moreover, palladium is highly toxic and must be removed from the product. Therefore, the potential scale-up of the transition-metal catalysed reactions for industrial applications is possible only for very few products with high added value.
Base metals such as iron can serve as cheaper and more sustainable alternatives to noble metals catalysts with over 1000 times lower CO2 footprint. Recent research confirms that iron-based catalysts demonstrate excellent performance in a variety of industrially important reactions with an impressive potential for future development. However, this area remains largely underdeveloped, with a lack of deep understanding of how iron catalysts work and how their performance can be improved by tuning the parameters of the reaction and catalyst structure. Iron's reactivity is flexible, with several reaction pathways possible. The classical mechanism of the bond activation involves 2-electron transfer from the metal centre to the bond, which then splits in half. This pattern is typical for noble metals but much less commonly observed for iron. 1-electron processes are more prevalent for iron catalysts when only a single electron is transferred from the metal centre to the substrate. Radicals with unpaired electrons form, which are highly reactive. This pathway is ubiquitous for oxidation reactions, for example, in the decomposition of hydrogen peroxide catalysed by transition metals.
This project aims to expand the limits of the iron-catalysed reactions by establishing the relationship between low-valent iron catalyst structure and its reactivity (1 vs 2 electron pathways) using advanced characterisation methods, including Electron Paramagnetic Resonance (EPR) and Mössbauer spectroscopies. A fundamental understanding of reaction mechanisms will help advance the field of strong bond activation with iron to develop more sustainable industrial processes. This project contributes to several EPSRC strategies, including Manufacturing the Future (developing new routes to known and novel chemical space for the pharmaceutical and agrochemical sectors), Healthcare Technologies (new chemical space) and Physical Sciences (Chemistry and understanding) as well as contributing to "transforming to a sustainable society" (by using Earth-abundant iron catalysts). Also, this project falls with several EPSRC research
areas, such as Catalysis (developing new catalysts and processes), Chemical Reaction Dynamics and Mechanism (fundamental understanding of iron-catalysis and bond activation), Computational Chemistry (understanding structure-activity relationships), and Synthetic Organic Chemistry (new methods and new reactivity).