Dihydropteridine/Pteridine As a 2H+/2e- Redox Mediator for the Catalytic Reduction of CO2 to Methanol Via Hydride-Proton Transfer

The development of renewable energy technologies has been the focus of tremendous recent effort. One particularly desirable approach involves using solar energy to power the production of ?solar fuels? by either splitting H₂O or reducing CO₂. These technologies would form the basis of either a hydrogen or, for example, methanol economy. In either case, the discovery of capable redox catalysts lies at the heart of both of these visions and remains as a grand challenge.

Dyer and coworkers have recently reported the use of a pteridine electrocatalyst, 6,7-dimethyl-4-hydroxy-2-mercaptopteridine (PTE), to catalyze the reduction of CO₂ to CH₃OH with a Faradaic efficiency of 10-23% and at low overpotentials; intermediate 2e^-(formic acid) and 4e^- (formaldehyde) products are also observed. In this contribution, we use computational chemistry to discover that species 3 is the most stable tautomer of PTE and that it undergoes a concerted 2H⁺/2e^- transfer to dominantly form 3a (Scheme 1). We predict that PTE's ability to catalyze the reduction of CO₂ originates from a dearomatization-aromatization process of the 3/3a redox couple, in which 3a acts as a regenerable organo-hydride that reduces CO₂ to CH₃OH via three successive hydride and proton transfer (HTPT) steps (Scheme 1).