TY - JOUR
T1 - Mechanistic contrasts between manganese and rhenium bipyridine electrocatalysts for the reduction of carbon dioxide
AU - Riplinger, Christoph
AU - Sampson, Matthew D.
AU - Ritzmann, Andrew M.
AU - Kubiak, Clifford P.
AU - Carter, Emily A.
N1 - Publisher Copyright:
© 2014 American Chemical Society.
PY - 2014/11/19
Y1 - 2014/11/19
N2 - [Re(bpy)(CO)3]- is a well-established homogeneous electrocatalyst for the reduction of CO2 to CO. Recently, substitution of the more abundant transition metal Mn for Re yielded a similarly active electrocatalyst, [Mn(bpy)(CO)3]-. Compared to the Re catalyst, this Mn catalyst operates at a lower applied reduction potential but requires the presence of a weak acid in the solution for catalytic activity. In this study, we employ quantum chemistry combined with continuum solvation and microkinetics to examine the mechanism of CO2 reduction by each catalyst. We use cyclic voltammetry experiments to determine the turnover frequencies of the Mn catalyst with phenol as the added weak acid. The computed turnover frequencies for both catalysts agree to within one order of magnitude of the experimental ones. The different operating potentials for these catalysts indicate that different reduction pathways may be favored during catalysis. We model two different pathways for both catalysts and find that, at their respective operating potentials, the Mn catalyst indeed is predicted to take a different reaction route than the Re catalyst. The Mn catalyst can access both catalytic pathways, depending on the applied potential, while the Re catalyst does not show this flexibility. Our microkinetics analysis predicts which intermediates should be observable during catalysis. These intermediates for the two catalyzed reactions have qualitatively different electronic configurations, depending on the applied potential. The observable intermediate at higher applied potentials possesses an unpaired electron and therefore should be EPR-active; however, the observable intermediate at lower applied potentials, accessible only for the Mn catalyst, is diamagnetic and therefore should be EPR-silent. The differences between both catalysts are rationalized on the basis of their electronic structure and different ligand binding affinities.
AB - [Re(bpy)(CO)3]- is a well-established homogeneous electrocatalyst for the reduction of CO2 to CO. Recently, substitution of the more abundant transition metal Mn for Re yielded a similarly active electrocatalyst, [Mn(bpy)(CO)3]-. Compared to the Re catalyst, this Mn catalyst operates at a lower applied reduction potential but requires the presence of a weak acid in the solution for catalytic activity. In this study, we employ quantum chemistry combined with continuum solvation and microkinetics to examine the mechanism of CO2 reduction by each catalyst. We use cyclic voltammetry experiments to determine the turnover frequencies of the Mn catalyst with phenol as the added weak acid. The computed turnover frequencies for both catalysts agree to within one order of magnitude of the experimental ones. The different operating potentials for these catalysts indicate that different reduction pathways may be favored during catalysis. We model two different pathways for both catalysts and find that, at their respective operating potentials, the Mn catalyst indeed is predicted to take a different reaction route than the Re catalyst. The Mn catalyst can access both catalytic pathways, depending on the applied potential, while the Re catalyst does not show this flexibility. Our microkinetics analysis predicts which intermediates should be observable during catalysis. These intermediates for the two catalyzed reactions have qualitatively different electronic configurations, depending on the applied potential. The observable intermediate at higher applied potentials possesses an unpaired electron and therefore should be EPR-active; however, the observable intermediate at lower applied potentials, accessible only for the Mn catalyst, is diamagnetic and therefore should be EPR-silent. The differences between both catalysts are rationalized on the basis of their electronic structure and different ligand binding affinities.
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U2 - 10.1021/ja508192y
DO - 10.1021/ja508192y
M3 - Article
C2 - 25327956
AN - SCOPUS:84913555609
SN - 0002-7863
VL - 136
SP - 16285
EP - 16298
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
IS - 46
ER -