title: Judicious Ligand Design in Ruthenium Polypyridyl CO2Reduction Catalysts to Enhance Reactivity by Steric and Electronic Effects.
abstract: A series of RuIIpolypyridyl complexes of the structural design [RuII(R-tpy)(NN)(CH3CN)]2+(R-tpy=2,2':6',2''-terpyridine (R=H) or 4,4',4''-tri-tert-butyl-2,2':6',2''-terpyridine (R=tBu); NN=2,2'-bipyridine with methyl substituents in various positions) have been synthesized and analyzed for their ability to function as electrocatalysts for the reduction of CO2to CO. Detailed electrochemical analyses establish how substitutions at different ring positions of the bipyridine and terpyridine ligands can have profound electronic and, even more importantly, steric effects that determine the complexes' reactivities. Whereas electron-donating groups para to the heteroatoms exhibit the expected electronic effect, with an increase in turnover frequencies at increased overpotential, the introduction of a methyl group at the ortho position of NN imposes drastic steric effects. Two complexes, [RuII(tpy)(6-mbpy)(CH3CN)]2+(trans-[3]2+; 6-mbpy=6-methyl-2,2'-bipyridine) and [RuII(tBu-tpy)(6-mbpy)(CH3CN)]2+(trans-[4]2+), in which the methyl group of the 6-mbpy ligand is trans to the CH3CN ligand, show electrocatalytic CO2reduction at a previously unreactive oxidation state of the complex. This low overpotential pathway follows an ECE mechanism (electron transfer-chemical reaction-electron transfer), and is a direct result of steric interactions that facilitate CH3CN ligand dissociation, CO2coordination, and ultimately catalytic turnover at the first reduction potential of the complexes. All experimental observations are rigorously corroborated by DFT calculations.
authors.affiliation: Department of Chemistry, Ångström Laboratories, Uppsala University, Box 523, 751 20, Uppsala, Sweden.
authors.forename: Ben A
authors.initials: BA
authors.lastname: Johnson
authors.affiliation: Department of Chemistry, Ångström Laboratories, Uppsala University, Box 523, 751 20, Uppsala, Sweden.
authors.forename: Hemlata
authors.initials: H
authors.lastname: Agarwala
authors.affiliation: Department of Chemistry, Ångström Laboratories, Uppsala University, Box 523, 751 20, Uppsala, Sweden.
authors.forename: Travis A
authors.initials: TA
authors.lastname: White
authors.affiliation: Department of Chemistry, Ångström Laboratories, Uppsala University, Box 523, 751 20, Uppsala, Sweden.
authors.forename: Edgar
authors.initials: E
authors.lastname: Mijangos
authors.affiliation: Department of Chemistry, Indian Institute of Technology Hyderabad, Kandi, Sangareddy, 502285, Telangana, India. smaji@iith.ac.in.
authors.forename: Somnath
authors.initials: S
authors.lastname: Maji
authors.affiliation: Department of Chemistry, Ångström Laboratories, Uppsala University, Box 523, 751 20, Uppsala, Sweden. sascha.ott@kemi.uu.se.
authors.forename: Sascha
authors.initials: S
authors.lastname: Ott
keywords: carbon dioxide; electrocatalysis; ligand design; ruthenium; steric hindrance