Catalytic Reduction of CO 2 by Renewable Organohydrides Chern-Hooi Lim, † Aaron M. Holder, †,‡,§ James T. Hynes, ‡,∥ and Charles B. Musgrave* ,†,‡ † Department of Chemical and Biological Engineering and ‡ Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, United States § National Renewable Energy Laboratory, Golden, Colorado 80401, United States ∥ Chemistry Department, Ecole Normale Supe ́ rieure-PSL Research University, Sorbonne Universite ́ s-UPMC University Paris 06, CNRS UMR 8640 Pasteur, 24 rue Lhomond, 75005 Paris, France ABSTRACT: Dihydropyridines are renewable organohydride reducing agents for the catalytic reduction of CO 2 to MeOH. Here we discuss various aspects of this important reduction. A centerpiece, which illustrates various general principles, is our theoretical catalytic mechanism for CO 2 reduction by successive hydride transfers (HTs) and proton transfers (PTs) from the dihydropyridine PyH 2 obtained by 1H + /1e − /1H + /1e − reductions of pyridine. The Py/PyH 2 redox couple is analogous to NADP + /NADPH in that both are driven to effect HTs by rearomatization. High-energy radical intermediates and their associated high barriers/overpotentials are avoided because HT involves a 2e − reduction. A HT−PT sequence dictates that the reduced intermediates be protonated prior to further reduction for ultimate MeOH formation; these protonations are aided by biased cathodes that significantly lower the local pH. In contrast, cathodes that efficiently reduce H + such as Pt and Pd produce H 2 and create a high interfacial pH, both obstructing dihydropyridine production and formate protonation and thus ultimately CO 2 reduction by HTPTs. The role of water molecule proton relays is discussed. Finally, we suggest future CO 2 reduction strategies by organic (photo)catalysts. T he efficient chemical reduction of CO 2 to fuels has been of interest to scientists for decades, with growing concerns about the impact of CO 2 on climate and future global energy demands motivating increasing efforts to meet this challenge. 1−4 One principal strategy here is to mimic nature’s carbon economy, which photochemically reduces vast quantities of CO 2 to store solar energy and sequester carbon in natural products that serve as materials and fuels. 5,6 However, the molecular structures of the light-harvesting and chemical reduction systems of photo- synthesis are intricate, and their detailed mechanisms are not fully understood; imitating their abilities has posed a difficult challenge. Even attaining the specific goal of developing catalysts that efficiently transform CO 2 into valuable products proves to be enormously difficult. 7−13 One conversion of specific interest, the reduction of CO 2 to methanol (MeOH), is the focus of this Perspective. This conversion has been promoted by Olah as the basis of a MeOH economy. 14,15 Arguments here involve MeOH’s utility as a practical C1 source for chemical synthesis and its attractive properties as a fuel, not demanding the massive changes to the transportation fuels infrastructure required for a hydrogen economy. The partial reduction of CO 2 to methanol is generally preferred over its complete reduction to methane; the former is a more valuable product and is easier to handle and transport as a liquid fuel, which is more compatible with existing transportation fuel technology. The conversion of CO 2 to MeOH is a six-electron reduction described by the overall reaction in eq 1. When this reduction is carried out as a series of six one-electron transfers (ETs) and six proton transfers (PTs), every odd reduction necessarily produces a high-energy radical (open-shell) intermediate. Consequently, the three odd ETs generally result in slow kinetics and low selectivities unless these radicals are stabilized, for example, by conjugation to an aromatic π-system or by orbital mixing with delocalized states of a metal surface. 16 The issue of the difficulty of creating high-energy intermediates by the odd electron reductions is exemplified by the one- electron reduction of CO 2 to CO 2 −• , which involves a very unfavorable reduction potential E 0 of −2.14 V versus SCE. 17 One approach to circumvent this obstacle is to avoid radical intermediates in favor of closed-shell, stable intermediates by performing reductions two electrons at a time as hydride (H − ) transfers (HTs), effectively 2e − /H + transfers. 18 Thus, we can combine the six ETs with three PTs of eq 1 to produce three HTs; this converts the general one-electron reduction route to a two-electron route through HTs, and we can rewrite eq 1 as We begin this contribution by examining CO 2 reduction “catalyzed” 19 by ammonia borane (AB; NH 3 BH 3 ), a compound Received: August 20, 2015 Accepted: November 24, 2015 Perspective pubs.acs.org/JPCL © XXXX American Chemical Society 5078 DOI: 10.1021/acs.jpclett.5b01827 J. Phys. Chem. Lett. 2015, 6, 5078−5092