Manganese-Based Catalysts with Varying Ligand Substituents for the Electrochemical Reduction of CO 2 to CO

Steven E. Tignor, Hsin Ya Kuo, Tia S. Lee, Gregory D. Scholes, Andrew Bruce Bocarsly

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Abstract

A series of manganese complexes were synthesized with a variety of ligands and ligand substituents. These complexes were then studied using ultraviolet-visible spectroscopy, cyclic voltammetry, density functional theory calculations, and bulk electrolysis. The UV-vis, cyclic voltammetry, and calculation data show that the bipyridine π∗ level is modulated by the incorporation of different substituents on the bipyridine and through this interaction moderates the observed catalytic activity of the complex toward CO 2 reduction. The calculations were correlated to the experimental UV-vis data and cyclic voltammetry data to demonstrate the relationship among these data, and a Hammett plot showed a good correlation between the substituent identity and the MLCT wavelength from UV-vis (R 2 = 0.96). When aliphatic substituents were placed on the 4,4′-positions of the bipyridine, the location of the bpy π∗ was not significantly altered. However, when more electron withdrawing substituents were placed on the 4,4′-positions the bpy π∗ level was altered more significantly. This alteration in the bpy π∗ level had a profound effect on the rate of CO production determined from bulk electrolysis. While complexes whose bpy π∗ level were similar or more blue shifted in comparison to the parent manganese complex did not display significantly altered efficiencies or rates for the conversion of CO 2 to CO, those species whose bpy π∗ energies were significantly red shifted in comparison to the parent manganese complex displayed far poorer catalysis. This is postulated to be a combination of two factors. First, the singly reduced complex's ability to lose the axial bromide ligand is diminished when electron-withdrawing groups are placed on the bpy ligand due to an increasing gap between the bpy π∗ and the Mn-Br σ∗. Second, the decreased electron density of the HOMO of the doubly reduced complex with electron-withdrawing groups makes the binding of a molecule of CO 2 less energetically favorable.

Original languageEnglish (US)
Pages (from-to)1292-1299
Number of pages8
JournalOrganometallics
Volume38
Issue number6
DOIs
StatePublished - Mar 25 2019

All Science Journal Classification (ASJC) codes

  • Physical and Theoretical Chemistry
  • Organic Chemistry
  • Inorganic Chemistry

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