Ligand-Controlled Product Selectivity in Electrochemical Carbon Dioxide Reduction Using Manganese Bipyridine Catalysts

Electrocatalysis
is a promising tool for utilizing carbon dioxide
as a feedstock in the chemical industry. However, controlling the
selectivity for different CO2 reduction products remains
a major challenge. We report a series of manganese carbonyl complexes
with elaborated bipyridine or phenanthroline ligands that can reduce
CO2 to either formic acid, if the ligand structure contains
strategically positioned tertiary amines, or CO, if the amine groups
are absent in the ligand or are placed far from the metal center.
The amine-modified complexes are benchmarked to be among the most
active catalysts for reducing CO2 to formic acid, with
a maximum turnover frequency of up to 5500 s–1 at
an overpotential of 630 mV. The conversion even works at overpotentials
as low as 300 mV, although through an alternative mechanism. Mechanistically,
the formation of a Mn–hydride species aided by in situ protonated
amine groups was determined to be a key intermediate by cyclic voltammetry, 1H NMR, DFT calculations, and infrared spectroelectrochemistry.