Chiral Resolution of Hexaamine Cobalt(III) Cages: Substituent Effects on Chiral Discrimination

Abstract

Chiral resolution of the cobalt cage complexes [Co(diNOsar)]3+ and [Co(diAMsarH2)]5+ have been achieved by selective crystallization with the anion bis-μ-(R), (R)-tartratodiantimonate(III) ([Sb2(R,R-tart)2]2–) and also by column chromatography with Na2[Sb2(R,R-tart)2] as eluent. The X-ray crystal structures of Λ-[Co(diNOsar)][Sb2(R,R-tart)2]Cl . 7 H2O and Δ-[Co(diAMsarH2)][Sb2(R,R-tart)2]2Cl . 14 H2O are reported, which reveal an unexpected reversal of chiral discrimination when the cage substituent is changed from nitro (Λ-enantiomer) to ammonio Δ-enantiomer) and shows that the ammonio-substituted cage is capable of forming a three-point hydrogen-bonding interaction with each complex anion, whereas the nitro analogue can only form two hydrogen bonds with each [Sb2(R,R-tart)2]2– anion. During cation exchange chromatography of the racemic cobalt cage complexes with Na2[Sb2(R,R-tart)2] as eluent, Λ-[Co(diNOsar)]3+ elutes first, which implies a tighter ion pairing interaction than for the Δ-enantiomer. On the other hand, Δ-[Co(diAMsarH2)]5+ elutes first during chromatography under identical conditions, which is also consistent with a preferred outer-sphere complex formed between Δ-[Co(diAMsarH2)]5+ and [Sb2(R,R-tart)2]2– relative to Λ-[Co(diAMsarH2)]5+ and [Sb2(R,R-tart)2]2–.