Dihydropteridine/Pteridine as a 2H⁺/2e^- Redox Mediator for the Reduction of CO₂ to Methanol: A Computational Study

Conflicting experimental results for the electrocatalytic reduction of CO₂ to CH₃OH on a glassy carbon electrode by the 6,7-dimethyl-4-hydroxy-2-mercaptopteridine have been recently reported [ J. Am. Chem. Soc. 2014 , 136 , 14007 - 14010 , J. Am. Chem. Soc. 2016 , 138 , 1017 - 1021 ]. In this connection, we have used computational chemistry to examine the issue of this molecule's ability to act as a hydride donor to reduce CO₂. We first determined that the most thermodynamically stable tautomer of this aqueous compound is its oxothione form, termed here PTE. It is argued that this species electrochemically undergoes concerted 2H⁺/2e^- transfers to first form the kinetic product 5,8-dihydropteridine, followed by acid-catalyzed tautomerization to the thermodynamically more stable 7,8-dihydropteridine PTEH₂. While the overall conversion of CO₂ to CH₃OH by three successive hydride and proton transfers from this most stable tautomer is computed to be exergonic by 5.1 kcal/mol, we predict high activation free energies (ΔG^‡_HT) of 29.0 and 29.7 kcal/mol for the homogeneous reductions of CO₂ and its intermediary formic acid product by PTE/PTEH₂, respectively. These high barriers imply that PTE/PTEH₂ is unable, by this mechanism, to homogeneously reduce CO₂ on a time scale of hours at room temperature.