Stereospecificity in vanadium Schiff base complexes: Formation, crystallization and epimerization processes.

The structures of two stereoisomers of the chiral anion [VO2(N-salicylidene-isoleucinato)](-) possessing three centers of chirality, the vanadium atom (configuration A/C) and the isoleucine moiety (configuration R/S on alpha and beta carbons), are presented. The absolute configuration of all available stereosiomers, CSS, ARR, CSR and ARS, was determined by electronic circular dichroism (ECD), which allows distinguishing between diastereomers, and by vibrational circular dichroism (VCD) capable of differentiating between all four stereoisomers. The comparison of experimental VCD and infrared (IR) spectra with simulated spectra for band assignment revealed the IR spectra of the diastereomers differing significantly in the CH stretching region of the aromatic part in the molecule. Crystallization from binary systems composed of equal ratio of two stereoisomers of isoleucine, unveiled the lower solubility of CSS and ARR stereoisomers, while a longer crystallization time of the CSR and ARS stereoisomers allowed proceeding the vanadium-catalyzed epimerization, leading to the subsequent presence of the CSS and ARR stereoisomers in the product obtained.