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.