CO₂ reduction to CO on an iron-porphyrin complex with crown-ether appended cation-binding site

With increasing carbon dioxide concentrations in the atmosphere, the utilization and conversion of CO₂ into valuable materials is an important goal. In recent years, evidence has emerged of low-valent iron-porphyrin complexes able to bind CO₂ and reduce it to carbon monoxide and water. To find out how the porphyrin scaffold and second coordination sphere influence the CO₂ reduction on iron-porphyrin complexes, we study the structure, electronic and redox properties of a novel crown-ether appended porphyrin complex with cation (K⁺) binding site. Cyclic voltammetry studies show that the K⁺ binding site does not change the Fe^0/I and Fe^I/II redox potentials of the complexes. Subsequently, density functional theory calculations were performed on the catalytic cycle of CO₂ reduction on the K⁺-bound crown-ether appended iron-porphyrin complex. The work shows that proton-donors such as acetic acid bind the K⁺ strongly and can assist with efficient and fast proton transfer that leads to the conversion of CO₂ to CO and water. In agreement with experiment, the calculations show little perturbations of the redox potentials upon binding K⁺ to the crown-ether scaffold.