Kinetics of the Trans Effect in Ruthenium Complexes Provide Insight into the Factors That Control Activity and Stability in CO₂ Electroreduction

Abstract

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.

Impact of the Dissolved Anion on the Electrocatalytic Reduction of CO2 to CO with Ruthenium CNC Pincer Complexes

“…The location of the missing electrons in this reaction is not apparent whereas HCO 2 − , CH 4 , and MeOH were not observed. Additionally, catalyst deactivation via CO binding was probed after electrolysis via infrared spectroscopy studies (Figure S18) . No evidence of a Ru bound CO is apparent from these studies based on the lack of a signal at ∼2000 cm −1 .…”

Transition Metal Complexes as Catalysts for the Electroconversion of CO2: An Organometallic Perspective

“…From this point on, the generation of CO follows the ET M pathway (Scheme 23). Further insight on the details of the mechanism can be found in the respective follow‐up report of the group [193] . They provide an extensive kinetic study with a focus on the trans effect of carbonyl ligands on the electronic structure of the metal center.…”

Probing the Effects of Electron Deficient Aryl Substituents and a π‐System Extended NHC Ring on the Photocatalytic CO2 Reduction Reaction with Re‐pyNHC‐Aryl Complexes**

“…[5d] As π-electron withdrawing groups via resonance, nitro (3) and cyano (4) were targeted to compare resonance electron withdrawing group effects with the inductively withdrawing group effects of 2. The bis-CF 3 substituted catalyst (5) was selected to evaluate the role of increasing the number of inductive electron withdrawing groups on catalyst properties. Additionally, an increase in conjugation at the imidazole portion of the ligand was probed with benzimidazole 6.…”

“…[4] Additionally, the strong electron donating properties of these ligands to the metal center may assist in enhancing the rate of the CO 2 reduction reaction (CO 2 RR). [5] As an example, the well-known Re(bpy)(CO) 3 Br (1, where bpy is 2,2'-bipyridine) benchmark PCO 2 RR catalyst is surpassed in rate by the introduction of an NHC group to give Re(pyNHC-PhCF 3 )(CO) 3 Br (2, where py is pyridine) (Figure 1). [5d,6] A significant increase in durability of the Re(pyNHC-aryl)(CO) 3 Br catalyst is also observed relative to the benchmark when an electron deficient aryl group was evaluated.…”