Work

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

Year: 2017

Type: article

Abstract: Conflicting experimental results for the electrocatalytic reduction of CO 2 to CH 3 OH 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 CO 2 . 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 PTEH 2 . While the overall conversion of CO 2 to CH 3 OH 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 (Δ G ‡ HT ) of 29.0 and 29.7 kcal/mol for the homogeneous reductions of CO 2 and its intermediary formic acid product by PTE/PTEH 2, respectively. These high barriers imply that PTE/PTEH 2 is unable, by this mechanism, to homogeneously reduce CO 2 on a time scale of hours at room temperature.

Source: The Journal of Physical Chemistry B

DOI: 10.1021/acs.jpcb.7b01224

Authors: Chern-Hooi Lim, Aaron M. Holder, James T. Hynes, Charles B. Musgrave