Four Isomers of the Bis(ethylenediamine)sarcosinatocobalt(III) Ion: Separation, Identification, and Characterization

All four possible isomers of the bis(ethylenediamine)sarcosinatocobalt(III) ion, [Co(sar) (en)2]2+, were separated chromatographically on the column of an SP-Sephadex cation exchanger, and their absorption, circular dichroism, and 13C NMR spectra were measured. The more stable pair of isomers were identified as Δ-[Co(S-sar) (en)2]2+ and Λ-[Co(R-sar) (en)2]2+, while the less stable pair were Δ-[Co(R-sar) (en)2]2+ and Λ-[Co(S-sar) (en)2]2+. The Δ-[Co(S-sar) (en)2]2+ isomer in an aqueous solution showed mutarotation which can be ascribed to a change in configuration (epimerization) about the sarcosinato nitrogen atom. In equiribrium at 25°C, the mixture consisted of 84.8% of the Δ(S) isomer and 15.2% of the Δ(R) isomer. From this result, the free-energy difference between the isomers was estimated to be 4.26 kJ mol−1. The presence of the less stable isomers, which were not found by Buckingham et al., was also confirmed by the 13C NMR measurements.