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–.