Turning on the Protonation-First Pathway for Electrocatalytic CO2 Reduction by Manganese Bipyridyl Tricarbonyl Complexes

Ken T. Ngo, Meaghan McKinnon, Bani Mahanti, Remya Narayanan, David Grills, Mehmed Z. Ertem, Jonathan Rochford

Research output: Contribution to journalArticle

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Abstract

Electrocatalytic reduction of CO2 to CO is reported for the complex, {fac-MnI([(MeO)2Ph]2bpy)(CO)3(CH3CN)}(OTf), containing four pendant methoxy groups, where [(MeO)2Ph]2bpy = 6,6′-bis(2,6-dimethoxyphenyl)-2,2′-bipyridine. In addition to a steric influence similar to that previously established [Sampson, M. D. et al. J. Am. Chem. Soc. 2014, 136, 5460-5471] for the 6,6′-dimesityl-2,2′-bipyridine ligand in [fac-MnI(mes2bpy)(CO)3(CH3CN)](OTf), which prevents Mn0-Mn0 dimerization, the [(MeO)2Ph]2bpy ligand introduces an additional electronic influence combined with a weak allosteric hydrogen-bonding interaction that significantly lowers the activation barrier for C-OH bond cleavage from the metallocarboxylic acid intermediate. This provides access to the thus far elusive protonation-first pathway, minimizing the required overpotential for electrocatalytic CO2 to CO conversion by Mn(I) polypyridyl catalysts, while concurrently maintaining a respectable turnover frequency. Comprehensive electrochemical and computational studies here confirm the positive influence of the [(MeO)2Ph]2bpy ligand framework on electrocatalytic CO2 reduction and its dependence upon the concentration and pKa of the external Brønsted acid proton source (water, methanol, trifluoroethanol, and phenol) that is required for this class of manganese catalyst. Linear sweep voltammetry studies show that both phenol and trifluoroethanol as proton sources exhibit the largest protonation-first catalytic currents in combination with {fac-MnI([(MeO)2Ph]2bpy)(CO)3(CH3CN)}(OTf), saving up to 0.55 V in overpotential with respect to the thermodynamically demanding reduction-first pathway, while bulk electrolysis studies confirm a high product selectivity for CO formation. To gain further insight into catalyst activation, time-resolved infrared (TRIR) spectroscopy combined with pulse-radiolysis (PR-TRIR), infrared spectroelectrochemistry, and density functional theory calculations were used to establish the v(CO) stretching frequencies and energetics of key redox intermediates relevant to catalyst activation.

Original languageEnglish
Pages (from-to)2604-2618
Number of pages15
JournalJournal of the American Chemical Society
Volume139
Issue number7
DOIs
Publication statusPublished - Feb 22 2017

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2,2'-Dipyridyl
Protonation
Carbon Monoxide
Manganese
Catalysts
Chemical activation
Ligands
Phenols
Protons
Trifluoroethanol
Spectroelectrochemistry
Infrared radiation
Radiolysis
Dimerization
Acids
Phenol
Voltammetry
Electrolysis
Stretching
Density functional theory

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

Cite this

Turning on the Protonation-First Pathway for Electrocatalytic CO2 Reduction by Manganese Bipyridyl Tricarbonyl Complexes. / Ngo, Ken T.; McKinnon, Meaghan; Mahanti, Bani; Narayanan, Remya; Grills, David; Ertem, Mehmed Z.; Rochford, Jonathan.

In: Journal of the American Chemical Society, Vol. 139, No. 7, 22.02.2017, p. 2604-2618.

Research output: Contribution to journalArticle

Ngo, Ken T. ; McKinnon, Meaghan ; Mahanti, Bani ; Narayanan, Remya ; Grills, David ; Ertem, Mehmed Z. ; Rochford, Jonathan. / Turning on the Protonation-First Pathway for Electrocatalytic CO2 Reduction by Manganese Bipyridyl Tricarbonyl Complexes. In: Journal of the American Chemical Society. 2017 ; Vol. 139, No. 7. pp. 2604-2618.
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