The Pattern of Hydroxyphenyl-Substitution Influences CO<sub>2</sub> Reduction More Strongly than the Number of Hydroxyphenyl Groups in Iron-Porphyrin Electrocatalysts
Ana Sonea, Kaitlin L. Branch, Jeffrey J. Warren
Abstract
The development of catalysts that can convert carbon dioxide (CO 2 ) to useful reduced products is a pressing and ongoing challenge. Refinement of the designs of molecular electrocatalysts is of great interest, especially for meso -tetraarylmetalloporphyrins. Iron porphyrins with hydroxyphenyl groups situated near the active site are good electrocatalysts, and herein, we systematically explore how the position and number of meso- 2,6-dihydroxyphenyl groups on iron porphyrins influences CO 2 -to-CO conversion. A series of five iron porphyrins with 2,6-dihydroxyphenyl groups systematically placed at the 5, 10, 15, and 20 porphyrin positions were prepared. The isomer with 5,15-bis(2,6-dihydroxyphenyl) substitution was a superior catalyst for CO 2 reduction electrocatalysis in N,N -dimethylformamide solvent. To our surprise, the previously reported tetrakis(2,6-dihydroxyphenyl)porphyrin iron complex was not the best performing catalyst. We use density functional calculations to explore the factors that distinguish each of the catalysts and show how calculated Fe–C vibrational frequencies are related to observed electrochemical properties and catalyst kinetics. A corresponding analysis of optical spectra and relative reduction potentials illustrates the relationship between the placement of dihydroxyphenyl groups and catalyst performance. We conclude that substituents at the 5 and 15 positions are best at improving catalyst performance, leaving other parts of porphyrin macrocycles open for other modifications.