Pendant Catechol Group Improves the Performance of Iron Porphyrin CO₂ Reduction Catalysts

In this work, we prepared chloroiron 5-(2,3-dihydroxyphenyl)-10,15,20-triphenyl porphyrin (Fe(Cat)TPP) and investigated its properties for CO2 reduction. Iron porphyrins make up a class of compounds that are known to efficiently convert CO2 to carbon monoxide (CO). There exist many different porphyrin derivatives that reduce CO2 with some leading examples, including structures that place hydrogen bond donors and/or proton donors near the iron ion active site. Here, the presence of an internal H-bond in the 2,3-dihydroxyphenyl group in Fe(Cat)TPP increases the observed CO2 reduction rate constants by a factor of 10 with respect to the parent chloroiron-5,10,15,20-tetraphenyl porphyrin and a factor of 3 with respect to a porphyrin with only one hydroxyphenyl (i.e., chloroiron 5-(2-hydroxyphenyl)-10,15,20-triphenyl porphyrin). The presence of the internal H-bond is proposed to facilitate the proton-coupled electron transfer process of carbon–oxygen bond breaking, which has been established as the rate-limiting step in CO2-to-CO conversion. The Fe(Cat)TPP molecule, and those like it, are important to improving designs of molecular electrocatalysts. The ongoing development of platforms that can rapidly and selectively mediate the electrochemical transformation of carbon dioxide (CO2) to valorized products is a great technical challenge.