Electrochemical Conversion of CO2 to CO by a Competent FeI Intermediate Bearing a Schiff Base Ligand

Repurposing First-Row Transition Metal Carbon Dioxide Reduction Electrocatalysts for Electrochemical Carboxylation of Benzyl Chloride

Carbon dioxide (CO2) is an abundant and useful C1 feedstock for electrocarboxylation, a process that incorporates a carboxyl moiety into an organic molecule. In this work, three first-row transition metal CO2 reduction electrocatalysts, NiPDIiPr (1), NiTPA (2), and Fe(salenCl4) (3), were explored as electrocarboxylation catalysts with benzyl chloride as a substrate. The cyclic voltammograms of all three catalysts showed current enhancements in the presence of benzyl chloride under a CO2 atmosphere. Introduction of DMAP as additives showed further current enhancement. Electrolyses with one-compartment cell generated a moderate yield of phenylacetic acid. Addition of MgBr2 was proven to be crucial to the formation of the carboxylate product. While the yield of carboxylation was moderate, this work showed an example of electrocarboxylation of benzyl chloride without using a metal electrode or sacrificial anode, which could lead to a more sustainable carboxylation methodology.

Catalytic CO2 Reduction with Heptacoordinated Polypyridine Complexes: Switching the Selectivity via Metal Replacement

The discovery of molecular catalysts for the CO2 reduction reaction (CO2 RR) in the presence of water, which are both effective and selective towards the generation of carbon-based products, is a critical task. Herein we report the catalytic activity towards the CO2 RR in acetonitrile/water mixtures by a cobalt complex and its iron analog both featuring the same redox-active ligand and an unusual seven-coordination environment. Bulk electrolysis experiments show that the cobalt complex mainly yields formate (52 % selectivity at an applied potential of -2.0 V vs Fc+ /Fc and 1 % H2 O) or H2 (up to 86 % selectivity at higher applied bias and water content), while the iron complex always delivers CO as the major product (selectivity >74 %). The different catalytic behavior is further confirmed under photochemical conditions with the [Ru(bpy)3 ]2+ sensitizer (bpy=2,2'-bipyridine) and N,N-diisopropylethylamine as electron donor, where the cobalt complex leads to preferential H2 formation (up to 89 % selectivity), while the iron analog quantitatively generates CO (up to 88 % selectivity). This is ascribed to a preference towards a metal-hydride vs. a metal-carboxyl pathway for the cobalt and the iron complex, respectively, and highlights how metal replacement may effectively impact on the reactivity of transition metal complexes towards solar fuel formation.

Photoinduced Catalytic Organic-Hydride Transfer to CO2 Mediated with Ruthenium Complexes as NAD+ /NADH Redox Couple Models

The catalytic organic-hydride transfer to CO₂ was first achieved through the photoinduced two-electron reduction of the [Ru(bpy)₂ (pbn)]²⁺ /[Ru(bpy)₂ (pbnHH)]²⁺ (bpy=2,2'-bipyridine, pbn=2-(pyridin-2-yl)benzo[b]-1,5-naphthyridine, and pbnHH=2-(pyridin-2-yl)-5,10-dihydrobenzo[b]-1,5-naphthyridine) redox couple in the presence of 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH). The active species for the catalytic hydride transfer to carbon dioxide giving formate is [Ru(bpy)(bpy⋅^- )(pbnHH)]⁺ formed by one-electron reduction of [Ru(bpy)₂ (pbnHH)]²⁺ with BI⋅.

Mechanism and selectivity of photocatalyzed CO2 reduction by a function-integrated Ru catalyst

The phosphine-substituted Ru(II) polypyridyl complex, [Ru^II-(tpy)(pqn)(MeCN)]²⁺ (RuP), was disclosed to be an efficient photocatalyst for the reduction of CO₂ to CO with excellent selectivity. In this work, density functional calculations were performed to elucidate the reaction mechanism and understand the origin of selectivity. The calculations showed that RuP was first excited to the singlet excited state, followed by intersystem crossing to produce a triplet species (3RuIII(L˙-)-S), which was then reduced by the sacrificial electron donor BIH to generate a Ru^II(L˙^-) intermediate. The ligand of Ru^II(L˙^-) was further reduced to produce a Ru^II(L^2-) intermediate. The redox non-innocent nature of the tpy and pqn ligands endows the Ru center with an oxidation state of +2 after two one-electron reductions. Ru^II(L^2-) nucleophilically attacks CO₂, in which two electrons are delivered from the ligands to CO₂, affording a Ru^II-COOH species after protonation. This is followed by the protonation of the hydroxyl moiety of Ru^II-COOH, coupled with the C-O bond cleavage, resulting in the formation of Ru^II-CO. Ultimately, CO is dissociated after two one-electron reductions. Protonation of Ru^II(L^2-) to generate a Ru^II-hydride, a critical intermediate for the production of formate and H₂, turns out to be kinetically less favorable, even though it is thermodynamically more favorable. This fact is due to the presence of a Ru²⁺ ion in the reduced catalyst, which disfavors its protonation.

Computational Study of CO2 Reduction Catalyzed by Iron(I) Complex at Different Spin States: Cooperativity of Hydrogen Bonding and Auxiliary Group Effect

To explore alternative approaches to the CO₂ reduction to formate and provide an insight into the spin state effect on the CO₂ reduction, we theoretically designed a kind of low-valence iron(I) model complex, whose doublet, quartet, and sextet states are denoted as ² Fe(I), ⁴ Fe(I), and ⁶ Fe(I), respectively. This complex is featured with an iron(I) center, which bonds to a 1,2-ethanediamine (en) and a 2-hydroxy-biphenyl group. Reaction mechanisms for the CO₂ reduction to formate catalyzed by this iron(I) model complex were explored using density functional theory (DFT) computations. Studies showed that the univalent iron(I) compound can efficiently fix and activate a CO₂ molecule, whereas its oxidized forms with trivalent iron(III) or bivalent iron(II) cannot activate CO₂. For the iron(I) compound, it was found that the lowest spin state ² Fe(I) is the most favorable for the CO₂ reduction as the reactions barriers involving ² Fe(I), ⁴ Fe(I), and ⁶ Fe(I) are 25.6, 37.2, and 35.9 kcal/mol, respectively. Yet, a photosensitizer-free visible-light-mediated high-low spin shift from ⁴ Fe(I) and ⁶ Fe(I) to ² Fe(I) is likely through the reverse intersystem crossing (RIC) because the ⁴ Fe(I) and ⁶ Fe(I) compounds have strong absorption in the visible-light range. Notably, the synergistic interaction between the hydrogen bonding from the auxiliary hydroxyl group in the 2-hydroxy-biphenyl moiety to CO₂ and an intermediate five-membered ring promotes the proton transfer, leading to the formation of the -COOH moiety from CO₂ and the Fe-O bond. With the addition of H₂, one H₂ molecule is split by the Fe-O bond and thus serves as H atom sources for both the CO₂ reduction and the recovery of the auxiliary hydroxyl group. The present theoretical study provides a novel solution for the challenging CO₂ reduction, which calls for further experimental verifications.

Carbon Dioxide Reduction Mediated by Iron Catalysts: Mechanism and Intermediates That Guide Selectivity

The reduction of carbon dioxide represents an ambitious target, with potential impact on several of the United Nations' sustainable development goals including climate action, renewable energy, sustainable cities, and communities. This process shares a common issue with other redox reactions involved in energy-related schemes (i.e., proton reduction to hydrogen and water oxidation to oxygen), that is, the need for a catalyst in order to proceed at sustainable rates. Moreover, the reduction of CO₂ faces an additional selectivity complication, since several products can be formed, including carbon monoxide, formic acid/formate, methanol, and methane. In this Mini-Review, we will discuss iron-based molecular catalysts that catalyze the reduction of CO₂, focusing in particular on the selectivity of the processes, which is rationalized and guided on the basis of the reaction mechanism. Inspired by the active sites of carbon monoxide dehydrogenases, several synthetic systems have been proposed for the reduction of CO₂; these are discussed in terms of key intermediates such as iron hydrides or Fe-CO₂ adducts, where the ligand coordination motif, together with the presence of co-additives such as Brønsted acids, nucleophiles, or CO₂ trapping moieties, can guide the selectivity of the reaction. A mechanistic comparison is traced with heterogeneous iron single-atom catalysts. Perspectives on the use of molecular catalysts in devices for sustainable reduction of CO₂ are finally given.