Bio-inspired catalysts for the activation of aliphatic C-H bonds: toward the development of novel green processes

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Nature has long been a source of inspiration for both scientists and artists. Some of the most ingeniously designed products and gadgets familiar to millions of people worldwide owe their origin to seemingly simple forms and patterns found in plants and wildlife. Although we lately learned plenty of information about how nature proceeds to perform various efficient chemical transformations, biological systems are complicated and still far from being fully understood. An example of such complex systems is the mild way nature converts atmospheric nitrogen to ammonia. For 100 years, ammonia has been exclusively produced from nitrogen and hydrogen gas via the Haber-Bosch process, a technique that requires very harsh conditions (350-550 °C, 150-350 atm). In contrast, some enzymes found in nature known as nitrogenases are able to easily achieve this transformation at ambient temperature and pressure. As it is the case for the breakage of the strong nitrogen triple bond, the controlled activation and functionalization of strong C-H bonds in hydrocarbons also remains a challenge. The vast majority of precious raw materials like liquid and gaseous hydrocarbons are simply burnt as fuels for heating and transportation, and only a modest portion of these C-H bond containing molecules (alkanes and aromatics) are converted into chemical intermediates on a large-scale for use in the chemical industry. The transition metal-catalyzed dehydrogenation of alkanes, which produces olefins, the major feedstock of chemical industry, is a green approach to bond construction strategies. This reaction allows the substitution of highly reactive mutagenic halogenated starting reagents that are currently used by industry by simple C and H containing compounds, and prevents waste production. The dehydrogenation of alkanes also generates hydrogen gas, which is a coveted non polluting energy source. Importantly, due to the reversibility of the reaction, catalysts able to dehydrogenate alkanes can potentially find applications in the field of hydrogen storage. There is a need for greener catalysts to be developed. For example, one of the technologies currently used by industry and which accounts for over 90% of ethylene production in North America requires temperatures in excess of 800 °C and produces 1-3 tons of CO2/ton of ethylene. Transition metal complexes as homogeneous catalysts for the dehydrogenation of alkanes allow milder reaction conditions as well as selectivity, but are mostly based on precious metals such as Ir and Rh. Using nature as a source of inspiration, our group will study complexes based on inexpensive metals, which have traditionally been overlooked to achieve the dehydrogenation of alkyl chains, and that are responsible for the activity of many enzymes, such as iron (Fe) or tungsten (W). Research on alternatives to the use of noble metals is a novel approach in the field of C-H activation catalyst development. Avoiding precious metals will lead to cheaper catalysts with minimum environmental and toxicological impact, and which can accomodate a huge scale of usage (high abundance). In this program, we propose to investigate the reactivity of W and/or Fe mono- and multi-metallic species bearing rigid and highly tuneable ligands, leading to carefully engineered alkane dehydrogenation catalysts that can be efficient under mild conditions. The objective of this research is not only to decrease the costs associated with the dehydrogenation of alkane process, but also to contribute to the development of greener routes to bond formation as well as to get a better understanding of the transformations occurring at the active site of enzymes.