FLP Zintl Clusters for the Electrochemical Catalytic Reduction of Small Molecules

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We will harness the untapped reactivity of Zintl ions, polayanionic clusters of earth abundant main-group elements, to affect small molecule activations in stoichiometric and catalytic fashions. Coupling reactions to yield new bonds, especially C-C bonds, occupy a central position in synthetic chemistry. To this end, a rich history of Pd cross-coupling reactions exploited by both academic and industrial chemists has developed. However, the high operational costs and toxicity of metal-based catalysts has spurred an interest in main-group systems that can represent this reactivity. Perhaps the most diverse and successful examples of main-group catalysts are frustrated Lewis pairs (FLPs). Traditional FLP systems rely on a Lewis acidic and a Lewis basic site to activate small molecules, and most homoatomic and heteroatomic bond formation reactions are accessed by bond polarisation mechanisms. However, the scope of these bond formation reactions remains limited and C-C bond formation with CO2 remains unidentified in the field. In this project, a new family of FLPs consisting of clusters capable of multi-site activation will be coupled with electrochemical methods to promote bond formation chemistries.We intend to establish Zintl clusters as the basic component in FLP chemistry. Preliminary investigations are targeted with [P7]3-, not only because of its synthetic accessibility, presence of an NMR handle, and greater stability relative to group 14 clusters, but also because of the wide-spread success of phosphines as the basic component in established FLP systems. Once functionalised with an acidic component, these polyanionic clusters are excellent targets for small molecule activation via a FLP pathway, as they feature both the necessary electron-rich and electron-poor components. With this proximity and the propensity to undergo redox reactions in mind, these systems will be assessed in the electrochemical reduction of small molecules, our most ambition target would be the selective reduction of CO2 to ethane. Recycling the C1 building-block, environmental toxin, and industrial by-product into value-added products reminiscent of the fuels from which CO2 is generated upon combustion.This research project will be collaboratively undertaken by the Mehta and Dryfe groups, and has been divided into two work packages.Work Package 1 (primarily based in the Mehta group):1. Functionalise the group 15 Zintl clusters, namely [P7]3-, with a Lewis acidic component2. Perform insertion chemistry between acidic and basic centres on these clusters akin to FLP chemistry. Small molecules of particular interest include CO2, olefins, carbonyls, carbodiimides, and isocyanates.PhD project call 20203. Reductively couple activated substrates to form new bondsWork Package 2 (primarily based in the Dryfe group):4. Immobilise FLP Zintl material on suitable electrode surfaces, e.g. glassy carbon or edge-plane pyrolytic graphite5. Electrochemically close catalytic cycles by reducing the expected oxidatively coupled cluster product6. Use kinetic analysis methods to determine, and optimise, rates of catalytic processes via fitting of electrochemical data.Zintl clusters capture the imagination of academics because they are molecular models for larger heterogenous systems. Technology we develop with [P7]3- can be expanded to larger polyphosphides, such as [P11]3-, [P16]2-, [P21]3-, and eventually inform reactivity possible with functionalised red phosphorus. The strategy of electrochemically / photochemically converting CO2 to reduced value-added products is referred to as artificial photosynthesis.