Structural and Substituent Group Effects on Multielectron Standard Reduction Potentials of Aromatic N-Heterocycles

Abstract

Aromatic N-heterocycles have been used in electrochemical CO2 reduction, but their precise role is not yet fully understood. We used first-principles quantum chemistry to determine how the molecular sizes and substituent groups of these molecules affect their standard redox potentials involving various proton and electron transfers. We then use that data to generate molecular Pourbaix diagrams to find the electrochemical conditions at which the aromatic N-heterocycle molecules could participate in multiproton and electron shuttling in accordance with the Sabatier principle. While one-electron standard redox potentials for aromatic N-heterocycles can vary significantly with molecule size and the presence of substituent groups, the two-electron and two-proton standard redox potentials depend much less on structural modifications and substituent groups. This indicates that a wide variety of aromatic N-heterocycles can participate in proton, electron, and/or hydride shuttling under suitable electrochemical conditions.

Electrochemical Reduction of Carbon Dioxide to Methanol in the Presence of Benzannulated Dihydropyridine Additives

“…Keith has similarly proposed on the basis of quantum chemistry thermodynamic calculations that ANHmediated multi-e -/multi-H + CO2 may be governed by the Sabatier Principle, where Brønsted acidity and hydride donor ability must be balanced to manage both H + and Htransfer to CO2. 33 While phenanthridine (3) and acridine (4) mediate reduction of CO2 with similar %FEs, 4 additionally undergoes a more prominent deposition process (see Figure S6). Access to ortho-(available in 3, not available in 4) or para-positions (available in 4, not available in 3) likely governs the interaction of an ANH at the electrode surface.…”

Electrochemical hydrogenation of α-ketoesters and benzoxazinones using carbon electrodes and a sustainable Brønsted acid

“…23 These observations corroborate computational studies that suggest coupled 2e -/2H + reductions of N-heterocycles are thermodynamically accessible in the range of the reported applied potentials. 24,25 Similarly, the selective hydrogenation of benzannulated pyridines such as phenanthridine to form 1,2dihydrophenanthridine has also been observed when the equivalents of acid present are appropriately controlled. 26 To probe the generality of this process, we sought to investigate the electrochemical hydrogenation of two substrate classes, aketoesters and benzoxazinones (Figure 2), that have been previously explored in the context of biomimetic dihydrophenanthridine-mediated transfer hydrogenation.…”

Advances and challenges in modeling solvated reaction mechanisms for renewable fuels and chemicals

“…Our group has used Pourbaix diagram analyses to study a variety of homogeneous and heterogeneous catalysis systems for CO 2 reduction. Interestingly, we have predicted that reaction conditions for several CO 2 electroreduction processes ranging from homogeneous pyridinium and homogeneous ruthenium‐complexes as well as heterogeneous N‐doped nanocarbons and partially reduced SnO 2 oxides all coincidentally share a similar characteristic—All have Pourbaix boundary lines showing the formation of a new intermediate state near the conditions where CO 2 electrocatalysis has been reported. Experimentally validating these computational predictions has been difficult, in part due to difficulties reproducing experimental data that has been reported in the literature .…”

“…Ahlquist reported agreement with the experimental values only when both implicit solvation and two water molecules are present in the system. Groenenboom et al modeled thermodynamic descriptors for a large set of aromatic N‐heterocycle molecular catalysts for electrochemical CO 2 reduction . Across 27 different molecular catalysts, using one explicit water molecule located at the relevant hydrogen bonding site for each molecule improved direct p K a calculations to reasonably low errors of about 1 p K a unit.…”