Understanding the chemoselectivity switch in CO2 reduction catalyzed by Co and Fe complexes bearing a pentadentate N5 ligand

The Co and Fe complexes bearing a pentadentate N5 ligand could efficiently catalyze the photochemical CO2 reduction with excellent chemoselectivity favoring CO (Co catalysis) and formate (Fe catalysis). Density functional calculations were carried out to gain mechanistic insights and uncover the origin of the selectivity of these reactions. The pentadentate N5 ligand was delineated to be redox noninnocent that can accept electrons and protons, and the oxidation state of Co and Fe remained at +2 during the reduction. The theoretically established catalytic mechanisms suggest that, for the Co (1) catalysis, a nucleophilic attack on CO2 takes place by the double-reduced catalyst (formally Co0) to generate a CoII-carboxylate complex. Then, a proton is transferred from Et3NH+ to CoII-carboxylate to generate a CoII-COOH intermediate, which was further protonated on its hydroxyl group to cleave the C-O bond and release the CO molecule. For the Fe (2) catalysis, the nucleophilic attack on CO2 by a protonated and double-reduced species (formally Fe0-N-H) generates HCOO-. In this step, two electrons and one pro-ton are transferred to the CO2 moiety from the N5 ligand. Such an orthogonal hydride (electron/proton) transfer mechanism is similar to the formate dehydrogenase metalloenzyme catalysis. The CO formation pathway is favored over the formic acid formation pathway in the Co-catalysis. However, the chemose-lectivity reverses when changing the metal of the catalyst from Co to Fe, which promotes the formation of formic acid over CO by the orthogonal hydride transfer mechanism.(c) 2022 Elsevier Inc. All rights reserved.

Automated summary

Co and Fe complexes with a pentadentate N5 ligand efficiently catalyze the photochemical CO2 reduction, favoring CO (Co catalysis) and formate (Fe catalysis). Mechanistic insights and the origin of selectivity were obtained through density functional calculations. The N5 ligand was found to be redox noninnocent, accepting electrons and protons, while the oxidation state of Co and Fe remained at +2 during reduction.