The impact of second coordination sphere functional group extension on product selectivity for manganese bipyridyl CO2 reduction electrocatalysts

Abstract

Utilizing the well established manganese bipyridyl class of homogeneous electrocatalyst, four new ligands are studied to probe the influence of distal, outer coordination sphere, H-bonding and steric effects on product selectivity for proton-coupled electrocatalytic CO₂ reduction. The presence of a simple acetate functional group in the second coordination sphere provides a high selectivy for CO₂-to-CO conversion irrespective of proton source (H₂O vs. PhOH) or applied potential. The o-(methoxybenzoate)phenyl second/outer coordination sphere at the bipyridyl 6,6′-positions imparts poor product selectivity. In contrast, upon conjugation of the acetate functional group with the N-Boc-alanine moiety, a CO : HCO₂⁻ product selectivity of ∼1 : 1 is observed at the high overpotential catalytic wave (for both H₂O and PhOH acids). Computed enthalpy and free energy of activation parameters suggest that selective CO₂ insertion at the manganese hydride transition state is favored, over protonation, consistent with negligible hydrogen production during controlled potential electrolysis studies.