Efficient microwave-assisted synthesis of 1-hexylpyridin-1-ium bromide: Anion exchange to acetate and fundamentals tools to probe its interaction with RuCl3

Abstract

The study presents a robust and efficient method for synthesizing high-purity 1-hexylpyridin-1-ium acetate (1-HP-Ac), a room-temperature ionic liquid (RTIL), using microwave synthesis. The study also explores its interaction with ruthenium trichloride (RuCl3). Initially, we employed classical solvothermal methods to obtain 1-hexypyridin-1-ium bromide (1-HP-Br), a precursor for 1-HP-Ac. Issues related to impurity and time constraints necessitated a transition to microwave-assisted synthesis, which emerged as an eco-friendly, energy-efficient approach. Microwave synthesis proved superior by delivering shorter reaction times (30 min), higher yields, and eliminating the need for solvents or inert atmospheres. Various characterization techniques (1H NMR, 13C NMR, FTIR, TGA, and HRMS) and matching quantum mechanical NMR predictions confirmed the successful synthesis of the hexylpyridinium cation and anion exchange transforming 1-HP-Br into its acetate form. The interaction between the synthesized RTIL and RuCl3 was monitored through NMR-based titrations and molecular dynamics (MD) modeling. The study explored the effects of increasing amounts of either RTIL or RuCl3 with the critical inflection point at a 1:2 ratio of 1-HP-Ac to RuCl3 for effective interactions and potential changes in average molecular conformation. Specifically, examining Karplus J-coupling constants provided information about dihedral angles within 1-HP-Ac. The data suggested that RuCl3 has a discernible influence on these angles, particularly for aliphatic protons. With thermal stability up to 200 °C, 1-HP-Ac is a fitting candidate for high-temperature reactions. MD simulation quantifies the solvation environment created by the IL as well as elucidates interactions of 1-HP-Ac with the RuCl3 crystal structure. The insights gained from our research into 1-HP-Ac's interaction with RuCl3, combined with the efficiency of microwave-assisted synthesis, open new avenues for further studies in catalysis, material science, and related fields..