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

Abstract

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.

Organic reactions for the electrochemical and photochemical production of chemical fuels from CO2 – The reduction chemistry of carboxylic acids and derivatives as bent CO2 surrogates

“…This carboxylate 44 can then be protonated and further reduced in a catalytic cycle as shown in Scheme 9. A Mn variant of this chemistry has also been reported [73] and specifically, the reduction of a (bpy)(CO) 3 Mn I CO 2 H was observed via IR-spectroelectrochemistry [74].…”

Manganese Tricarbonyl Complexes with Asymmetric 2-Iminopyridine Ligands: Toward Decoupling Steric and Electronic Factors in Electrocatalytic CO2 Reduction

“…In particular, this change should affect the energy of the LUMO, the reduction potential, and catalytic activity. [23][24][25][26][27][28][29][30][31][32][33] Introducing steric bulk 23,25 to prevent unwanted reactions of the catalytic species, including dimerization as either Mn Mn, 9 or C(imino) C(imino) bound species, 21 whilst at the same time reducing the risk of increased overpotential is a challenging task. Molecular designs that allow for steric and electronic effects to be decoupled are required.…”

“…23 Indeed, it has recently been shown that the use of bipyridines incorporating bulky groups in the 6,6 -positions 24,25 (or, another bulkier heterocyclic ligand 26 ) largely inhibits dimerization in the catalytic cycle. The result is the formation of the stable 5-coordinate anion via the two-electron transfer (ECE) at the first cathodic wave.…”

Ultraefficient homogeneous catalyst for the CO 2 -to-CO electrochemical conversion

“…solar fuels | CO 2 reduction | electrochemistry | catalysis | contemporary energy challenges T he reductive conversion of CO 2 to CO is an important issue of contemporary energy and environmental challenges (1)(2)(3)(4)(5)(6)(7)(8)(9)(10). Several low-oxidation-state transition metal complexes have been proposed to serve as homogeneous catalyst for this reaction in nonaqueous solvents such as N,N'-dimethylformamide (DMF) or acetonitrile (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23).…”