Tuning thermodynamics and electronic properties of 1-hexyl-3-methylimidazolium based organic ionic liquids with iron porphyrin

Ionic liquid-iron porphyrin (IL-FeP) complexes are systems with diverse applications, particularly in oxygen reduction reactions and the biodegradation of ionic liquids by cytochrome P450 enzymes, where FeP serves as the active site. Despite the importance of such systems, there is currently a lack of information on the conformational preference of ionic liquids binding to FeP and the influence that such binding conformations exert on the electronic properties of FeP. In addition, the role of binding interactions in facilitating biodegradation of ionic liquids is not yet fully understood. In this article, we address the knowledge gap by employing density functional theory calculations to identify the most stable structures of IL-FeP complexes. We considered four ionic liquids containing the cation 1-hexyl-3-methylimidazolium paired with acetate, methanesulfonate, ethylsulfate, and butylsulfate anions. Thermodynamic analysis of binding was supplemented with energy decomposition analysis, non-covalent interaction analysis, and natural bond orbital analysis to glean insight into the energetic factors driving binding stability. Our results reveal conformations in which one of the oxygen atoms in the anions is positioned directly above Fe, and the imidazolium ring aligns approximately parallel to the FeP plane, which are characterized by strong binding energies, irrespective of the organic anion involved. Electrostatic interactions between the IL ions and FeP play a dominant role in determining the binding conformations. The propensity to acquire an electron by an IL-FeP complex is significantly reduced when oxygen in the anion is in close proximity to Fe. These findings enhance our understanding of the fundamental interactions governing IL-FeP complexes, providing a foundation for tailoring ionic liquids for specific applications in catalysis and biodegradation.