Towards a deeper understanding of the coordination chemistry in pyridinium-based ionic liquids-iron systems: Insights from quantum chemical and semi-empirical investigations

Abstract

In recent times, the utilization of ionic liquids (ILs) in mitigating corrosion has been a subject of interest, but the detailed examination of their coordination with iron remains an understudied area. This research utilizes SCC-DFTB simulations to probe the interaction profiles and coordination chemistry of six ILs-specifically, 4-(Dimethylamino)-1-(2-hydroxyethyl)pyridinium bromide (P2CH), 4-(Dimethylamino)-1-(3-hydroxypropyl)pyridinium bromide (P3CH), 4-(Dimethylamino)-1-(2-methoxyethyl)pyridinium bromide (P2CM), 4-(dimethylamino)-1-ethylpyridinium bromide (P2C), 1-butyl-4-(dimethylamino) pyridinium bromide (P4C), and 4-(dimethylamino)-1-pentylpyridinium bromide (P5C)-and their interaction with the Fe(1 1 0) iron surface. Quantum chemical calculations were conducted to understand their inherent reactivity independent of external factors. Our findings revealed comparable reactivity patterns across all ILs, with P2CM distinguishing itself through exceptional electron donating and accepting abilities, and a minimal energy gap. The study of IL-Iron coordination was carried out in three distinct interaction configurations. Molecular orientation and N-alkylation of the pyridinium component were found to be significant in controlling interaction intensities. Particularly, the ILs demonstrated maximum affinity in parallel alignment with the iron surface, and P3CH exhibited an extraordinary interaction through three Fe-C bonds. The pattern of increasing adsorption strength was identified as P2CH ˂ P2CM ˂ P2C ˂ P4C ˂ P5C ˂ P3CH, with P2CH forming bonds consistently across configurations. Interestingly, even when ILs did not establish bonds with Fe atoms, negative interaction energies were observed, implying additional coordination mechanisms beyond mere covalent bonds. This study emphasizes the crucial role of molecular structures in determining IL-metal surface interactions and supports the design of novel ILs with enhanced corrosion resistance abilities.