Spins Under Pressure: A mechanistic understanding of homogeneous catalysis by high pressure EPR

Catalysis is an extremely important branch of science, which is vital in our modern society. It is estimated that about 90% of all processed chemical compounds have, at some stage of their production, involved the use of a catalyst. As a result catalysis is recognized as a key strategic priority area by EPSRC. In general, catalytic reactions are more energy efficient and, at least in the case of highly selective reactions, lead to reduced waste and undesirable compounds, which is an important consideration with dwindling global reserves of raw materials. New catalysts are being developed for use in alternative energy sources and new conversion technologies, for manufacturing of new materials, for synthesis of molecules such as pure drugs, and for the production of chemicals with minimal energy input. The importance of these developments cannot be overstated. In the past 10 years alone the Nobel Prize in Chemistry was awarded on three separate occasions for the outstanding achievement of scientists whose work has a strong bias in catalysis. Their combined work has revolutionized the field of fine chemical synthesis and chiral feedstock production using well defined and discrete homogeneous organometallic catalysts.

Despite the phenomenal success of these homogeneous catalysts, further improvements and developments of new asymmetric catalysts, bio-catalysts and indeed heterogeneous catalysts will benefit from a greater understanding of the mechanistic pathways involved in the catalytic cycles. Undoubtedly a greater understanding of the mechanism can lead to enhanced performance, even with well established systems. Therefore this advancement in our mechanistic understanding of how catalysts function and operate will require the application and development of new techniques that can probe the catalytic reaction and reveal the inner workings of the mechanism in unsurpassed detail. One approach to address this is the development of a unique high pressure system enabling advanced Electron Paramagnetic Resonance (EPR) methods to be used for the first time to study catalytic reactions under extreme conditions. In many cases, paramagnetic metal centers or reaction intermediates are involved in catalytic cycles, so that EPR spectroscopy and the related hyperfine techniques, such as ENDOR and ESEEM, are ideal characterization tools to study reactions at high pressures as a means to gain further insights into reaction mechanism. Since pressure is a primary thermodynamic parameter of central importance in reaction kinetics, chemical equilibria, molecular conformations and molecular interactions, it is very important in catalysis, and becomes a crucial and available parameter to study the reaction mechanisms. Since the equilibria, selectivity, population of states, conformations of the catalyst - substrate intermediates, role of solvent interactions, can all be affected, HP-EPR will be able to examine these properties. The structure, redox states, electronic and spin states, dynamics, non-covalent interactions, conformation changes, relaxation behavior, can all be analysed by these advanced EPR techniques, using the high pressure facility as a means of controlling and enhancing mechanistic variables in order to facilitate their investigations. Pressure also influences the outcome of most chemical processes, and therefore the HP-EPR facility developed in this project can also be applied to a range of other problems in chemistry involving free radicals, from organic and inorganic reactions, to electron transfer and activation of small molecules. Specific collaborative projects in heterogeneous catalysis, spin crossover phenomena, and electron spin states in condensed media, will all be explored using this new HP-EPR assembly.

Who will benefit from this research?

The project seeks to investigate the mechanistic pathways involved in homogeneous catalysis involving transition metal complexes. The strength of these molecular-based catalysts is the excellent control over the stereo-electronic environment in which substrate molecules are activated and undergo various transformations, and at which products are subsequently desorbed. In several areas (e.g., asymmetric catalysis and single-site olefin polymerization), the sophistication of the ligand "engineering" for desired high selectivity has reached impressive levels. Nevertheless, nature's finest catalysts, enzymes, show how amazingly efficient catalysts can still be and so they demonstrate how much opportunity remains. This opportunity can be realised through a greater mechanistic understanding of the catalytic pathways involved in the reactions, and this in turn requires the development and exploitation of new approaches to catalyst characterisation. This project will provide such a characterisation capability, benefiting academic and industrial scientists in the UK. It is technically diverse requiring the design, construction and application of a high pressure EPR system in both CW and pulsed mode. This facility will be of direct interest to the EPR community, since a wide range of paramagnetic systems can be studied, whilst also beneficial to the broader homogeneous catalysis community within the UK. The ability to investigate paramagnetic systems and reactions of importance to homogeneous catalysis by advanced EPR techniques would represent a significant step-change in our abilities to study paramagnetic inorganic reaction mechanisms, and therefore this will make a strong and positive impact in the dissemination profile of this work. The impact of this work reaches beyond the UK scientific community being of significance to the international research community within both academic and catalysis fields.

How will they benefit?

Different approaches will be employed to guarantee that the various beneficiaries outlined above are identified and engaged from an early stage of the project. In the short term (1-4 years), the academic community will benefit directly from the development of, and access to, the detailed technical aspects of the work. In the longer term, the fundamental research arising from this project will aid in the development of novel homogeneous catalysts. A genuine aspiration of this project is the engagement of industrial partners to explore and test some of the fundamental ideas from the project, in industrially relevant areas. The case for overall societal benefits is also compelling, as results arising from the project can lead directly to realising the improved benefits of catalysis, highlighted as a key strategic priority area by EPSRC. The growth forecasts for the catalysis markets in the UK also suggest that the project could have a major beneficial economic impact on the UK economy. Outreach activities (also outlined in the Pathways to Impact) will aim to engage the general public and school age children. Dr. Mason as a STEM Ambassador will facilitate the opportunities for public engagement on this project. Although the project as described is technically ambitious, there are general concepts (for example, what is a catalysts, how does it work, what is spectroscopy, etc) that easily translate to a general audience or under-16 age group. Training opportunities to use the HP-facility will be provided through a planned workshop, promotion activities through a scientific symposium, while targeted public lectures and exhibitions will be delivered throughout the lifetime of the project, as detailed in the Pathways to Impact.