Synthesis, structural and conformational analysis, and IR spectra of ethyl 4-chloro-7-iodoquinoline-3-carboxylate

Abstract

In this work, we report the synthesis of a novel quinoline derivative, ethyl 4-chloro-7-iodoquinoline-3-carboxylate (CIQC), and its structural, conformational and vibrational characterization. The compound was studied in its neat solid phases (crystalline and low-temperature amorphous phases) and as an isolated species in a cryogenic argon matrix (at similar to 15 K). Infrared spectroscopy and single crystal X-ray diffraction were the chosen experimental techniques. The conformational space and the vibrational spectra of the isolated molecules of the compound were also investigated theoretically at the B3LYP/LANL2DZ-Fcc-pVDZ level of approximation. The CIQC molecule exists in four different conformers, with predicted populations of 4225:17:16% at room temperature (rt). The rt equilibrium conformational mixture was successfully trapped in an argon matrix, at 15K, and the vibrational signatures of the conformers were determined. Upon annealing of the matrix of the compound at higher temperatures (similar to 40 K), conversion of the higher energy forms into the most stable conformer was found to take place, in consonance with the low predicted barriers for conformational isomerization. Sublimation of the host matrix argon atoms (at similar to 43 K) led to production of a low-temperature amorphous state of CIQC, containing the lowest but also the higher energy conformers. At T similar to 233-243 K, the amorphous rearranged to the crystalline state, whose molecular unit corresponds to the most stable CIQC conformer, as shown both by infrared spectroscopy and single crystal X-ray diffraction. At room temperature CIQC crystallizes in the hexagonal P6(3)/m space group, with a=b=17.7928(2) angstrom, and c=6.9830(1) angstrom. The molecules lie on a crystallographic mirror plane and are stacked in layers along the c-axis. The main packing motif consists of a group of three molecules, related by a three-fold rotation, joined together by weak C-H ... O hydrogen bonds