Synergistic Porosity and Charge Effects in a Supramolecular Porphyrin Cage Promote Efficient Photocatalytic CO2 Reduction

We present a supramolecular approach to catalyzing photochemical CO2 reduction in confined spaces through synergistic second-sphere porosity and charge effects. A porous iron porphyrin organic cage bearing 24 cationic groups, FePB-2(P), was made via post-synthetic modification of a versatile alkyne-functionalized supramolecular synthon. FePB-2(P) promotes the photochemical CO2 reduction reaction (CO2RR) with 97% selectivity for CO product, achieving turnover numbers (TON) exceeding 7000 and initial turnover frequencies (TOFmax) reaching 1400 min-1. The cooperativity between second-sphere porosity and charge effects results in a 41-fold increase in activity relative to the parent FeTPP catalyst, which is far greater than analogs that augment catalysis through porosity (FePB-3(N), 4-fold increase) or charge (Fe-p-TMA, 6-fold increase) alone. This work establishes that multiple, synergistic pendants in the secondary coordination sphere can be leveraged as a design element to augment catalysis at primary active sites within confined spaces.