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

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 CO₂ reduction at moderate overpotential. After synthesizing isomeric pairs of ruthenium complexes featuring 2'-picolinyl-methyl-benzimidazol-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 CO₂ 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 CO₂ electroreduction to CO. In acidic solutions under a N₂ atmosphere, on the other hand, H₂ 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.