CO 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 CO into valuable materials is an important goal. In recent years, evidence has emerged of low-valent iron-porphyrin complexes able to bind CO and reduce it to carbon monoxide and water. To find out how the porphyrin scaffold and second coordination sphere influence the CO 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 and Fe redox potentials of the complexes. Subsequently, density functional theory calculations were performed on the catalytic cycle of CO 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 CO 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.