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.