Elucidating Electronic and Structural Changes in Metal Porphyrins by 1-n-alkyl-3-methylimidazolium Cations

Metal porphyrins, which are one of the cornerstones of biological metabolism and remediation processes, have been widely studied for their versatile role and chemical reactivity in various environments. These tetrapyrrolic macrocycles that contain transition metals at their core also act as the reactive center of the ubiquitous monooxygenase, cytochrome P-450. As such, both their native and functionalized forms have been successfully utilized in the degradation of recalcitrant contaminants in a variety of aqueous and soil environments. Ionic liquids, although quite promising and possessing benign physicochemical properties, are poorly understood in terms of their biodegradability from a theoretical standpoint. As substrates play a key role in the activation of catalysts through concerted electronic and geometrical effects, it is imperative to gain understanding of their molecular-level interactions with the biocatalytic active center. In this article, a quantum mechanical treatment of ionic liquids 1-n-alkyl-3- methylimidazolium (n = 2, 4, 6, 8, and 10) ([Cnmim]`) is carried out in the presence of a variety of metal porphyrins to understand their binding, which is the first step in the well-known catalytic cycle of cytochromes. Our treatment concerns the interaction strength between the cations and porphyrin molecules and we quantify it in terms of binding energy calculations. The conformations having the alkyl chain of these IL cations facing the porphyrin molecules are destabilized to the greatest extent as the chain length is enhanced along the homologous series. Structures of cation-porphyrin complexes are further analyzed by employing vibrational analysis on the active site molecules to deduce key structural features of the complexes. The reductive abilities of the metal porphyrins considered are also inferred by invoking conceptual DFT in our work through electrophilicity and Fukui reactivity indices.