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

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 <i>vs.</i> PhOH) or applied potential. The <i>o</i>-(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 <i>N</i>-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.