Chiral recognition in ion pairing of an optically active tris(ethylenediamine)cobalt(III) cation and application to chromatographic resolution of metal complex anions

Abstract

A series of cobalt(III) complex anions have been completely resolved by column chromatography on a DEAE-Sephadex A-25 column using Λ-[Co(en)₃]Cl₃·H₂O (en = ethylenediamine) as an eluting agent. By this method the Δ-complex anions are always eluted faster than the Λ-complex anions. In order to explain this elution order, the diastereomeric ion-pair formation constants for [Co(en)₃]³⁺–cis(N)-[Co(ida)₂]^–(ida = iminodiacetate) and [Co(en)₃]³⁺–[Co(edta)]^–(edta = ethylenediaminetetra-acetate) systems have been determined by the conductance method. The results were consistent with those of the elution order: the ion-pair formation constants of the faster-eluted Δ-complex anions with Λ-[Co(en)₃]³⁺ ion were significantly larger than those of the Λ-complex anions with the same cation for both the above systems. The favourable ion-pair mode of the complex anions with Λ-[Co(en)₃]³⁺ is discussed on the basis of the hydrogen bonds between the co-ordinated oxygen atoms of the complex anions and the N–H protons of the complex cation. By taking advantage of the ability of the Λ-[Co(en)₃]³⁺ cation to discriminate the chirality of complex anions, a chromatographic resolution method using Λ-[Co(en)₃]Cl₃·H₂O as eluting agent is proposed for various metal complex anions.