Exploring binding affinity of 1-n-Alkyl-3-methylimidazolium chloride with iron porphyrin and electron uptake ability of the ionic liquid-FeP complex

Abstract

Interaction of ionic liquids with iron porphyrin (FeP) arises in a number of application of ionic liquids such as dye-sensitized solar cells, batteries, and conversion of CO₂ to value-added products, etc. Furthermore, ionic liquid-FeP interactions are thought to be responsible for ionic liquid biodegradation and catalytic breakdown of ionic liquids. Despite the importance of ionic liquid-FeP interactions, there is a lack of information on what conformations ionic liquids adopt when presented to FeP and how thermodynamics of subsequent electron transfer reaction is affected. To begin to answer these questions, electronic structure calculations are performed to assess how the binding propensity of the homologous series of 1-n-alkyl-3-methylimidazolium [C_nmim]Cl (n = 2, 4, 6, 8, 10) to FeP is affected as the alkyl chain length and the initial conformation of the cation presented to FeP are varied. The conceptual density functional theory framework is then invoked to compute the electrophilicity index of the ionic liquid-FeP complex to glean insight into the ability of the complex to acquire an electron. Calculations suggest two equally likely conformations of ionic liquids with similar Gibbs free energy change; however, the enthalpic and entropic contributions differ based on the conformation adopted by ionic liquids which in turn affects the propensity of the subsequent electron transfer process. The importance of results is discussed in terms of experimentally observed alkyl chain length-dependent biodegradability of ionic liquids.