CO binding at Iron porphyrins: Reflecting on Heme model complexes to guide future electrocatalyst design

Homogeneous CO₂ reduction beyond CO or formate is rare, yet the limited existing examples identify the crucial role of CO-bound intermediates. Understanding the structure-property relationships that govern CO binding to Fe porphyrins is therefore essential for enabling the electrocatalytic production of highly reduced products from CO₂ in these systems. Long before the emergence of Fe porphyrins as electrocatalysts, they were used as synthetic models to unravel the factors governing CO/O₂ discrimination in hemoproteins. In this context, there is a rich literature discussing molecular design of primary and secondary coordination spheres and the systematic dissection of factors that influence CO binding and activation at Fe porphyrin sites. The current review draws on these foundations to evaluate how primary coordination sphere modifications-through electronic tuning and axial ligation-govern CO affinity and activation. We further highlight how second coordination sphere design principles may be used to further promote productive CO binding and activation, drawing inspiration from both synthetic and biological systems. By re-examining these works through the lens of modern electrocatalyst development, we hope to support investigators tackling the challenge of homogeneous CO₂ reduction beyond CO.