Manganese Catalysts with Bulky Bipyridine Ligands for the Electrocatalytic Reduction of Carbon Dioxide: Eliminating Dimerization and Altering Catalysis

With
the goal of improving previously reported Mn bipyridine electrocatalysts
in terms of increased activity and reduced overpotential, a bulky
bipyridine ligand, 6,6′-dimesityl-2,2′-bipyridine (mesbpy),
was utilized to eliminate dimerization in the catalytic cycle. Synthesis,
electrocatalytic properties, X-ray diffraction (XRD) studies, and
infrared spectroelectrochemistry (IR-SEC) of Mn­(mesbpy)­(CO)3Br and [Mn­(mesbpy)­(CO)3(MeCN)]­(OTf) are reported.
Unlike previously reported Mn bipyridine catalysts, these Mn complexes
exhibit a single, two-electron reduction wave under nitrogen, with
no evidence of dimerization. The anionic complex, [Mn­(mesbpy)­(CO)3]−, is formed at 300 mV more positive potential
than the corresponding state is formed in typical Mn bipyridine catalysts.
IR-SEC experiments and chemical reductions with KC8 provide
insights into the species leading up to the anionic state, specifically
that both the singly reduced and doubly reduced Mn complexes form
at the same potential. When formed, the anionic complex binds CO2 with H+, but catalytic activity does not occur
until a ∼400 mV more negative potential is present. The Mn
complexes show high activity and Faradaic efficiency for CO2 reduction to CO with the addition of weak Brønsted acids. IR-SEC
experiments under CO2/H+ indicate that reduction
of a Mn­(I)–CO2H catalytic intermediate may be the
cause of this unusual “over-reduction” required to initiate
catalysis.