Optimal functionalization of a molecular electrocatalyst for hydride transfer.

Abstract

Optimization of hydride transfer (HT) catalysts to enhance rates and selectivities of (photo)electroreduction reactions could be a crucial component of a sustainable chemical industry. Here, we analyze how ring functionalization of the adsorbed transient intermediate 2-pyridinide (2-PyH^-*)-predicted to form in situ from pyridine (Py) in acidified water at a cathode surface and to be the key to selective CO₂ photoelectroreduction on p-GaP-may enhance catalytic activity. Earlier studies revealed that 2-PyH^-*'s instability results from a protonation side reaction producing adsorbed dihydropyridine (DHP*), which is relatively HT-inactive. Reducing the electron density on 2-PyH^-* could limit this protonation, with the trade-off that it may become less active for HT from 2-PyH^-*-R to CO₂ We explore here how Py functionalization affects the electron distribution and in turn tunes the catalytic performance of 2-PyH^-*. We indeed find that electron-withdrawing groups could enhance the stability of 2-PyH^-* by reducing its electron density on the ring. Furthermore, we find that the change in the number of electrons on the substituting group of the hydride donor is a good descriptor for both the stability against protonation and the magnitude of the HT barrier. We predict that -CH₂-CH₂F is the best candidate substituent, as it significantly improves the stability of 2-PyH^-* with only a small increase in HT barrier. -CH=CH₂ and -CH₂F also could be promising, although they require further investigation due to a larger HT-barrier increase.