The Trans Effect in Electrocatalytic CO₂ Reduction: Mechanistic Studies of Asymmetric Ruthenium Pyridyl-Carbene Catalysts

Abstract

A comprehensive mechanistic study of electrocatalytic CO2 reduction by ruthenium 2,2′:6′,2″-terpyridine (tpy) pyridyl-carbene catalysts reveals the importance of stereochemical control to locate the strongly donating N-heterocyclic carbene ligand trans to the site of CO2 activation. Computational studies were undertaken to predict the most stable isomer for a range of reasonable intermediates in CO2 reduction, suggesting that the ligand trans to the reaction site plays a key role in dictating the energetic profile of the catalytic reaction. A new isomer of [Ru­(tpy)­(Mebim-py)­(NCCH3)]2+ (Mebim-py is 1-methylbenzimidazol-2-ylidene-3-(2′-pyridine)) and both isomers of the catalytic intermediate [Ru­(tpy)­(Mebim-py)­(CO)]2+ were synthesized and characterized. Experimental studies demonstrate that both isomeric precatalysts facilitate electroreduction of CO2 to CO in 95/5 MeCN/H2O with high activity and high selectivity. Cyclic voltammetry, infrared spectroelectrochemistry, and NMR spectroscopy studies provide a detailed mechanistic picture demonstrating an essential isomerization step in which the N-trans catalyst converts in situ to the C-trans variant. Insight into molecular electrocatalyst design principles emerge from this study. First, the use of an asymmetric ligand that places a strongly electron-donating ligand trans to the site of CO2 binding and activation is critical to high activity. Second, stereochemical control to maintain the desired isomer structure during catalysis is critical to performance. Finally, pairing the strongly donating pyridyl-carbene ligand with the redox-active tpy ligand proves to be useful in boosting activity without sacrificing overpotential. These design principles are considered in the context of surface-immobilized electrocatalysis.

Transition metal-based catalysts for the electrochemical CO2reduction: from atoms and molecules to nanostructured materials

“…An analogous trans effect, based on the influence of a trans donor ligand on the CO 2 activation bond, was recently observed for a homogeneous Ru catalyst bearing a 2,2 0 :6 0 ,2 00 -terpyridine and an asymmetric pyridyl-N-heterocyclic carbene (NHC) ligands. 348 A stereochemical control was found to be essential to efficiently catalyze CO 2 RR to CO, since the presence of the strong s-donating NHC ligand in trans to the site for CO 2 bonding significantly boosted the kinetics of the CO dissociation step. Owing to the extreme versatility of this effect, trans coordination of a tertiary amine led to a change in the selectivity of the Fe porphyrins towards formate production, contributing to facilitate the protonation of reduced CO 2 as well as the formate release step (vide supra).…”

A new polypyridyl‐based Ru (II) complex as a highly efficient electrocatalyst for CO2 reduction

“…In recent years, computational study of the catalytic reactions has drawn significant attention from the researchers to confirm the experimental results, and to find a good relationship between the structures and properties of catalysts . Therefore, the theoretical calculations were performed to confirm the proposed electrocatalytic cycle for the CO 2 reduction to CO in the presence of cis ‐[Ru (Me 4 phen) 2 (CH 3 CN) 2 ](PF 6 ) 2 .…”

An NADH-Inspired Redox Mediator Strategy to Promote Second-Sphere Electron and Proton Transfer for Cooperative Electrochemical CO2 Reduction Catalyzed by Iron Porphyrin

“…47 We have also shown that reticular materials can influence CO 2 selectivity and activity by electronic delocalization in the secondary coordination sphere. [48][49] Redox-active ligands based on polypyridines, [50][51][52][53][54][55] imines, 56 and porphyrins [57][58] can also be employed for CO 2 reduction, where ligand-centered reductions offer the advantage of removing electron density away from the metal center to diminish metal hydride formation and off-pathway H 2 evolution. 59 Our laboratory and Miller's laboratory have identified molecular terpyridine-based iron complexes that are highly active CO 2 -to-CO electrocatalysts that operate at extremely low overpotentials due to electronic delocalization.…”