Electroreductive Iron Catalysis Enabled by a Redox Mediator: Alkyne Semi-Hydrogenation as a Model System

The development of electroreductive Fe-catalyzed processes for organic synthesis has remained scarce compared to other earth-abundant metals despite inherent advantages such as cost, low-toxicity, and accessible redox. Through stability of the Fe center using polydentate and/or redox-active ligand frameworks, pioneering work in reductive chemical catalysis for C–C π-bond hydrogenation and electrocatalytic CO2 reduction sheds light on strategies to harness reduced Fe species electrochemically for organic transformations. Using a tetraphos ligand (P3P), we demonstrate that electroreductive Fe catalysis can be achieved using cobaltocene (Cp2Co) as a redox mediator with alkyne semi-hydrogenation as a model system; the hydrogen evolution reaction (HER) is mitigated by operating at the potential of the redox mediator (Eapp = −1.45 V vs Fc+/0) to access Fe(II/I) reduction as opposed to Fe(II/0) over-reduction. A combination of cyclic voltammetry and controlled potential electrolysis (CPE) studies support a rate-limiting electron transfer step to generate a crystallographically characterized (P3P)Fe(I) which can engage in a nonstereoselective reductive protonation step with internal aryl alkynes and acid. Stoichiometric studies involving a related (P3P)Fe(II)-H ligand suggest that this tandem electrocatalytic system does not operate through a canonical Fe–H mechanism. Lastly, a small survey of diaryl alkynes with unique functional group tolerance is conducted, giving stilbene products with up to 8 turnovers per Fe.