Tuning Second Coordination Sphere Interactions in Polypyridyl–Iron Complexes to Achieve Selective Electrocatalytic Reduction of Carbon Dioxide to Carbon Monoxide

The development of noble metal-free catalysts capable of electrochemically converting carbon dioxide (CO₂) selectively into value added compounds remains one of the central challenges in sustainable energy science. Here, we present a systematic study of Fe(II) complexes of the functionalized ligands bpy^RPY2Me (bpyPY2Me = 6-(1,1-di(pyridin-2-yl)ethyl)-2,2′-bipyridine) in pursuit of water-stable molecular Fe complexes that are selective for the catalytic formation of CO from CO₂. Taking advantage of the inherently high degree of tunability of this ligand manifold, we followed a bio-inspired approach by installing protic functional groups of varying acidities (–H, –OH, –OMe, –NHEt, and –NEt2) into the ligand framework to systematically modify the second coordination sphere of the Fe center. This family of [(bpy^RPY2Me)Fe(II)] complexes was characterized using single-crystal X-ray analysis, 1H NMR spectroscopy, and mass spectrometry. Comparative catalytic evaluation of this set of compounds via voltammetry and electrolysis experiments identified [(bpy^NHEtPY2Me)Fe]²⁺ in particular as an efficient, iron-based, non-heme CO₂ electro-reduction catalyst that displays significant selectivity for the conversion of CO₂ to CO in acetonitrile solution with 11 M H₂O. We propose that the NH group acts as a local proton source for cleaving the C–O bond in CO₂ to form CO. Interestingly, the complex with the most acidic functional group in the second coordination sphere, [(bpy^OHPY2Me)Fe]²⁺, favors formation of H₂ over CO. Our results correlate the selectivity of water versus carbon dioxide reduction to the acidity of the second coordination sphere functional group and emphasize the continued untapped potential that synthetic molecular chemistry offers in the pursuit of next-generation CO₂ reduction electrocatalysts.