Comparative kinetic studies of a series of new ruthenium complexes provide a platform for understanding how strong trans effect ligands and redox-active ligands work together to enable rapid electrochemical CO 2 reduction at moderate overpotential. After synthesizing isomeric pairs of ruthenium complexes featuring 2′-picolinyl-methylbenzimidazol-2-ylidene (Mebim-pic) as a strong trans effect ligand and 2,2′:6′,2″-terpyridine (tpy) as a redox-active ligand, chemical and electrochemical kinetic studies examined how complex geometry and charge affect the individual steps and overall catalysis of CO 2 reduction. The relative trans effect of picoline vs the N-heterocyclic carbene (NHC) was quantified through a kinetic analysis of reductively triggered chloride dissociation, revealing that chloride loss is 1000 times faster in the isomer with the NHC trans to chloride. The kinetics of CO dissociation from a site trans to the NHC were examined in a systematic study of isostructural carbonyl complexes across four different overall charges. The rate constants for CO loss span 12 orders of magnitude and are fastest upon two-electron reduction, leading to a hypothesis that redox-active ligands play a key role in promoting reductive CO dissociation during catalysis. Analogous studies of complexes featuring the picoline ligand trans to the carbonyl reveal the importance of the trans effect of the CO ligand itself, with picoline ligand dissociation observed upon reduction. The complexes with NHC trans to the active site proved to be active electrocatalysts capable of selective CO 2 electroreduction to CO. In acidic solutions under a N 2 atmosphere, on the other hand, H 2 evolution proceeds via an intermediate that positions a hydride ligand trans to picoline. The mechanistic insight and quantitative kinetic parameters that arise from these studies help establish general principles for molecular electrocatalyst design.

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.

A ruthenium catalyst bearing a bidentate bis­(carbene) ligand is prepared and studied as a catalyst for CO2 electroreduction. The catalyst [Ru­(tpy)­(bis-mim)­(MeCN)]­[PF6]2 (tpy) is 2,2′,:6′,2″-terpyridine; bis-mim is (methylenebis­(N-methylimidazol-2-ylidene)) mediates reduction of CO2 into CO with a turnover frequency of 630 s–1 and Faradaic efficiency (FE) of 30% at an overpotential of 730 mV. The strongly donating bis­(carbene) ligand also enables access to a pathway operating at a lower overpotential of ca. 310 mV. While low-overpotential catalysis is slow in the dark (TOF = 0.01 s–1), visible light illumination increases the rate 10-fold (TOF = 0.11 s–1). A full mechanistic picture is developed using kinetic analysis from cyclic voltammetry, spectroelectrochemistry, and computational methods, with the bis-mim ligand facilitating rapid CO2 activation at low overpotentials. Comparisons with other ruthenium catalysts yield insight into the ability to tune the rate of chemical steps (e.g., ligand dissociation and CO2 nucleophilic attack) and the overpotential by tailoring the primary coordination sphere while retaining the “redox-active” tpy ligand.