Understanding the Role of Inter- and Intramolecular Promoters in Electro- and Photochemical CO₂ Reduction Using Mn, Re, and Ru Catalysts

Abstract

Conspectus Recycling of carbon dioxide to fuels and chemicals is a promising strategy for renewable energy storage. Carbon dioxide conversion can be achieved by (i) artificial photosynthesis using photoinduced electrons; (ii) electrolysis using electricity produced by photovoltaics; and (iii) thermal CO2 hydrogenation using renewable H2. The focus of our group’s research is on molecular catalysts, in particular coordination complexes of transition metals (e.g., Mn, Re, and Ru), which offer versatile platforms for mechanistic studies of photo- and electrochemical CO2 reduction. The interactions of catalytic intermediates with Lewis or Brønsted acids, hydrogen-bonding moieties, solvents, cations, etc., that function as promoters or cofactors have become increasingly important for efficient catalysis. These interactions may have dramatic effects on selectivity and rates by stabilizing intermediates or lowering transition state barriers, but they are difficult to elucidate and challenging to predict. We have been carrying out experimental and theoretical studies of CO2 reduction using molecular catalysts toward addressing mechanisms of efficient CO2 reduction systems with emphasis on those containing intramolecular (or pendent) and intermolecular (solution phase) additives. This Account describes the identification of reaction intermediates produced during CO2 reduction in the presence of triethanolamine or ionic liquids, the benefits of hydrogen-bonding interactions among intermediates or cofactors, and the complications of pendent phenolic donors/phenoxide bases under electrochemical conditions. Triethanolamine (TEOA) is a common sacrificial electron donor for photosensitizer excited state reductive quenching and has a long history of use in photocatalytic CO2 reduction. It also functions as a Brønsted base in conjunction with more potent sacrificial electron donors, such as 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo­[d]­imidazole (BIH). Deprotonation of the BIH•+ cation radical promotes irreversible photoinduced electron transfer by preventing charge recombination. Despite its wide use, most research to date has not considered the broader reactions of TEOA, including its direct interaction with CO2 or its influence on catalytic intermediates. We found that in acetonitrile, TEOA captures CO2 in the form of a zwitterionic adduct without any metal catalyst. In the presence of ruthenium carbonyl catalysts bearing α-diimine ligands, it participates in metal hydride formation, accelerates hydride transfer to CO2 to form the bound formate intermediate, and assists in the dissociation of formate anion from the catalyst (J. Am. Chem. Soc.202014224132428). Hydrogen bonding and acid/base promoters are understood to interact with key catalytic intermediates, such as the metallocarboxylate or metallocarboxylic acid during CO2 reduction. The former is a high energy species, and hydrogen-bonding or Lewis acid-stabilization are beneficial. We have found that imidazolium-based ionic liquid cations can stabilize the doubly reduced ...

Photocatalytic Reduction of CO2 to CO in Aqueous Solution under Red-Light Irradiation by a Zn-Porphyrin-Sensitized Mn(I) Catalyst

“…An additional advantage of Mn­(I) catalysts over their Re­(I) counterparts is a reduced required overpotential for electrochemical CO 2 reduction. This is the case for catalysts that preferentially undergo the “protonation-first” catalytic mechanism rather than the more common, “reduction-first” pathway. − It has been demonstrated that addition of proton-donating groups, such as amides or alcohols, to the secondary coordination sphere of the Mn complex can promote the protonation-first pathway. , …”

A Mesoionic Carbene–Pyridine Bidentate Ligand That Improves Stability in Electrocatalytic CO2 Reduction by a Molecular Manganese Catalyst

“…Many mechanistic details have been revealed; for example, the rhenium (Re) catalyst can proceed via a pathway producing CO and CO 3 2– or by a faster pathway occurring at a larger overpotential that generates CO and H 2 O . On the other hand, the manganese (Mn) precatalyst exhibits similar trends but deviates from Re with regard to its propensity to rapidly form the dimeric metal–metal-bonded species upon one-electron reduction. − Bulky mesityl substituents near the Mn center slow the rate of dimerization and instead promote the low-overpotential pathway, which ultimately furnishes a more active catalyst. , The dimer itself is an electroactive species as well, whereby the Mn–Mn bond is cleaved by insertion of CO 2 , catalyzing the liberation of CO. , Other works have contributed to the development of a variety of Group 7 metal complexes based on these initial systems and have demonstrated an array of functions within the primary and secondary coordination spheres. ,− …”

Neutral Formazan Ligands Bound to the fac-(CO)3Re(I) Fragment: Structural, Spectroscopic, and Computational Studies

“…Metal complexes with ligands containing the NH functionality have gained importance in catalysis since the NH arm can serve as an anchor for substrate recognition, thus enhancing catalyst selective and activity . A proton source located at the proximity of the metal center has been widely investigated in the proton-coupled electron transfer reduction of small molecules relevant in energy conversion reactions such as hydrogen evolution , and CO 2 reduction. − It was proposed to modulate the redox properties, aid in the stabilization of intermediates, or impact the kinetics due to the increased local proton concentration. Furthermore, deprotonation of the NH group is known to modify the electronic and geometric structure of such complexes. − …”