Crystal Structure Analysis, Stability, Phase Transformation and Selective Nucleation Mechanism of Fluralaner Polymorphs

To better understand and control the crystallization of different polymorphs, a comprehensive crystal structure analysis was conducted by using fluralaner as a model compound, and the thermodynamic stability, phase transformation, and selective nucleation mechanisms were studied. Various analytical techniques such as powder X-ray diffraction, thermal analysis, and FT-IR spectra were used to comprehensively characterize Form I, Form Ⅱ, and Form III of fluralaner, and it was found that there is structural similarity between Form I and Form III, which was further confirmed by single crystal X-ray diffraction. However, it was found that Form Ⅱ had unique molecular conformation and packing pattern. The lattice energy was calculated by Materials Studio 7.0 and the thermodynamic stability of three forms was explored by phase transformation experiments, which suggested that the order of thermodynamic stability was Form Ⅱ > Form I > Form III. The selective nucleation of Form I and Form III with similar structure was studied through FT-IR spectra and molecular dynamics simulations. The results indicated that the addition of n-hexane may hinder the solute–solute interaction in the solution, thus resulting in the nucleation of different polymorphs.