Escaping the Iron Law of Electrochemical CO₂ Reduction Using Pd₁₂L₂₄ Cages as Artificial 2nd Coordination Spheres

The valorization of CO₂ waste into useful materials is a key endeavor in the field of sustainable chemistry. Molecular catalysts that convert CO₂ to CO through electrochemical reduction are difficult to optimize because of scaling relations known as "the iron law". This law states that electronic tuning to decrease activation barriers inevitably slows the overall catalytic reaction. Here, ditopic pyridine building blocks for M₁₂L₂₄ supramolecular cages are modified with mixed moieties to prepare a supramolecular cage that preorganizes FeCl-tetraphenylporphyrin catalysts, a proton source, and CO₂-adduct stabilizing moieties in close proximity. These cages are investigated by cyclic voltammetry (CV), which reveals that high local concentrations impact the CV catalytic response in a manner that aligns with modulation of important intermediates in the accepted catalytic mechanism. Enclosing multiple porphyrins in the cage facilitates comproportionation reactions, enhancing catalytic rates compared to an isoelectronic benchmark catalyst. The combination of both phenol and guanidinium moieties produces a system with an enhanced catalytic rate as compared to a molecular FeTPP analogue, while performing the CO₂RR at milder overpotentials (120 ΔmV). Relying on the combination of mechanistic understanding and robust self-assembly strategies, this work illustrates the strategic formulation of appropriate components to tailor the functional nanospace around an artificial active site.