Chromatographic Separation of Isomers of Meridional Tris(amino-acidato) Cobalt(III) Homochelates and Their Absolute Configuration

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Abstract
Chromatographic separation of enantiomers or diastereomers was undertaken for four kinds of meridional [Co(AA)3] complexes through the column packed with antimony d-tartrate form of the anion exchange resin, where AA stands for D-alanine, L-alanine, glycine and β-alanine. From the CD spectra of the separated isomers, it was revealed that the δ-isomer was eluted first for the three α-aminoacidato chelates. As to the case of mer-[Co(β-ala)3] a single crystal X-ray analysis was made for the second eluted isomer and determined its absolute configuration to be Δ. Thus, it was concluded that in all four cases the Δ-isomer has greater affinity toward antimony d-tartrate anion than the δ-isomer. This makes a contrast to the case of the corresponding facial [Co(AA)3] in which the δ-isomer has greater affinity toward antimony d-tartrate than the Δ-isomer. Based on the values of the separation factors for the four enantiomeric pairs, the association mode effective for chiral discrimination was discussed.