Renewable Hydride Donors for the Catalytic Reduction of CO 2 :A Thermodynamic and Kinetic Study Abdulaziz Alherz, † Chern-Hooi Lim, †,‡ Yu-Ching Kuo, † Philip Lehman, † Jennifer Cha, † James T. Hynes, ‡,∥ and Charles B. Musgrave* ,†,‡,§,⊥ † Department of Chemical and Biological Engineering, ‡ Department of Chemistry and Biochemistry, and § Materials Science and Engineering Program, University of Colorado, Boulder, Colorado 80309, United States ∥ PASTEUR, Dé partement de Chimie, E ́ cole Normale Supé rieure, PSL University, Sorbonne Université , CNRS, 75005 Paris, France ⊥ Materials and Chemical Science and Technology Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States * S Supporting Information ABSTRACT: Increasing atmospheric CO 2 concentration and dwin- dling fossil fuel supply necessitate the search for efficient methods for CO 2 conversion to fuels. Assorted studies have shown pyridine and its derivatives capable of (photo)electrochemically reducing CO 2 to methanol, and some mechanistic interpretations have been proposed. Here, we analyze the thermodynamic and kinetic aspects of the efficacy of pyridines as hydride-donating catalytic reagents that transfer hydrides via their dihydropyridinic form. We investigate both the effects of functionalizing pyridinic derivatives with electron- donating and electron-withdrawing groups on hydride-transfer catalyst strength, assessed via their hydricity (thermodynamic ability) and nucleophilicity (kinetic ability), and catalyst recyclability, assessed via their reduction potential. We find that pyridines substituted with electron-donating groups have stronger hydride-donating ability (having lower hydricity and larger nucleophilicity values), but are less efficiently recycled (having more negative reduction potentials). In contrast, pyridines substituted with electron-withdrawing groups are more efficiently recycled, but are weaker hydride donors. Functional group modification favorably tunes hydride strength or efficiency, but not both. We attribute this problematic coupling between the strength and recyclability of pyridinic hydrides to their aromatic nature and suggest several avenues for overcoming this difficulty. 1. INTRODUCTION Concerns about the impact of atmospheric carbon dioxide (CO 2 ) on the climate and ever rising global energy demands have spurred growing efforts toward efficient conversion of CO 2 into useful products, such as fuels (e.g., methanol). 1−4 The solution to this problem via imitating natural CO 2 reduction has yet proven successful. Even the less ambitious goal of developing catalysts that efficiently transform CO 2 into valuable products is extremely challenging. 3,5−9 The conversion of CO 2 into valuable products by reducing CO 2 via a series of one-electron transfers (ETs) and proton transfers (PTs) produces open-shell (radical) high-energy intermediates at every odd electron reduction. This leads to slow kinetics and low selectivities except in cases where these radicals are stabilized. 10 The large energy cost of producing these radical intermediates is demonstrated by the significantly negative reduction potential of −2.14 V vs saturated calomel electrode (SCE) for the one-electron reduction of CO 2 to CO 2 −• . 11,12 Nature circumvents this difficulty by avoiding radical intermediates altogether in favor of closed-shell, stable intermediates by performing reductions of two electron at a time as hydride (H − ) transfers (HTs), which are effectively 2e − /H + reductions. 13 Consequently, the six ETs and six PTs that reduce CO 2 to methanol could in principle be accomplished as three HTs and three PTs, as represented by eqs 1 and 2. 14 + + → + − + CO 6e 6H CH OH H O 2 3 2 (1) + + → + − + CO 3H 3H CH OH H O 2 3 2 (2) Dihydropyridines (DHPs) and their derivatives mimic nature’s approach for reducing CO 2 through the NADP + /NADPH redox couple in the Calvin cycle of photosynthetic organisms. 15−17 Our group recently reported the first detailed theoretical mechanism of converting CO 2 to methanol in aqueous solution catalyzed by 1,2-dihydropyridine (1,2-PyH 2 ) as a renewable organo-hydride. 18,19 Each 1,2-PyH 2 transfers Received: September 1, 2018 Published: October 5, 2018 Article pubs.acs.org/JPCB Cite This: J. Phys. Chem. B 2018, 122, 10179-10189 © 2018 American Chemical Society 10179 DOI: 10.1021/acs.jpcb.8b08536 J. Phys. Chem. B 2018, 122, 10179−10189 Downloaded via UNIV OF COLORADO BOULDER on November 9, 2018 at 23:55:22 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.