An Iron Bis(carbene) Catalyst for Low Overpotential CO₂Electroreduction to CO: Mechanistic Insights from Kinetic Zone Diagrams, Spectroscopy, and Theory

Abstract

A common challenge in molecular electrocatalysis is the relationship between maximum activity and the overpotential required to reach that rate, with faster catalysts incurring higher overpotentials. This work follows a strategy based on independent tuning of ligands in the primary coordination sphere to discover a previously unreported iron catalyst for CO2 reduction with higher activity than similar complexes while maintaining the same overpotential. Iron complexes bearing the bis-N-heterocyclic carbene ligand (methylenebis(Nmethylimidazol-2-ylidene, bis-mim) and a redox active 2,2′:6′,2″-terpyridine (tpy) ligand were synthesized and found to catalyze the selective reduction of CO2 to CO at low overpotential with water as the proton source. Mechanistic studies based on kinetic zone diagrams, spectroscopy, and computation enable comparisons with a previously studied pyridyl-carbene analogue. Changing the bidentate ligand donor ability accelerates catalysis at the same overpotential, and changes the nature of the turnover-limiting step of the reaction.

Accessing and Photo-Accelerating Low-Overpotential Pathways for CO2 Reduction: A Bis-Carbene Ruthenium Terpyridine Catalyst

“…TOF max = 0.010 s –1 was calculated from preparative scale electrolysis using eq S8 and the experimentally determined diffusion coefficient ( D = 8.1 × 10 –6 cm/s, Figures S28 and S29). ,, Interestingly, the TOF max for C 2 -Ru-CO + is very similar to the rate constant for CO dissociation ( k CO ) for the monocationic C pic -Ru-CO + ( k CO = 0.040 s –1 ) . Thus, CO dissociation may be rate limiting for the monocationic C 2 -Ru-CO + with TOF max related to k CO as slow CO dissociation is suggested by the reversibility of C 2 -Ru-CO 2+/+ .…”

“…The bis-NHC precursor 1,1′-dimethyl-3,3′-methylenediimidazolium bis­(hexafluorophosphate) was metallated under basic conditions by refluxing with [Ru­(η 6 -C 6 H 6 )­Cl 2 ] 2 in acetonitrile (Scheme ). − NMR spectra of the crude product indicated successful metalation. The intermediate is tentatively assigned as trans -Ru­(bis-mim)­(Cl) 2 (CH 3 CN) 2 ; the highly air- and moisture-sensitive complex was carried on without purification in a reaction with tpy in 1,2-dichloroethane at 85 °C for 12 h, followed by salt metathesis with KPF 6 .…”

“…When glassy carbon was used as the working electrode, a TOF max could be calculated for the experiment using eq S8 and diffusion coefficient for C 2 -Ru-MeCN 2+ (D = 8.1 × 10 −6 cm/s) (Figures S28 and S29). 12,19 Under 455 nm illumination, CO was formed selectively with a FE of 32.3 ± 0.4% with a TOF max = 0.11 s −1 . Analysis of the post electrolysis solution, oxidized by exposure to atmospheric oxygen, by 1 H NMR spectroscopy indicates C 2 -Ru-CO 2+ with little to no degradation observed�but does not account for all initial ruthenium (Figure S42).…”

Role‐Specialized Division of Labor in CO2Reduction with Doubly‐Functionalized Iron Porphyrin Atropisomers

“…Electrocatalytic CO 2 reduction reaction (CO 2 RR) has merited increasing attention by converting CO 2 to fuels and value‐added chemicals [14–27] . Recent efforts have led to the identification of various molecular complexes as CO 2 RR electrocatalysts [28–39] . By studying these molecular catalysts, knowledge of the structure–function relationships has been learned [40–42] .…”

Lewis Acids and Electron-Withdrawing Ligands Accelerate CO Coordination to Dinuclear CuI Compounds

“…An electrochemical zone diagram for EC mechanisms (mechanisms where electrochemical processes are followed by chemical processes) was employed to understand the impact of ligand electronics, scan rate, and added Lewis acids on the coordination of CO to 1–3 . Zone diagrams are qualitative frameworks that map the dependence of voltammogram wave shape to kinetic and thermodynamic parameters. − They have garnered much attention in exploring catalytic systems but are less prevalent in studies of EC mechanisms. − In zone diagrams for EC mechanisms, the thermodynamic parameter is K eq,overall ( y -axis) and the kinetic parameter is λ ( x -axis), which is proportional to k obs /ν, where ν is the voltammetric scan rate (Figure ).…”