Carbonate binding to copper(ii) in solution: mixed-ligand complex formation and its application to the isolation and separation of the three isomers of [Cu(bpp)(H2O)][ClO4]2 [bpp = 2,6-bis(pyrrolidin-2-yl)pyridine]

Abstract

The binding of the carbonate anion to [Cu(meso-bpp)(H₂O)]²⁺ and rac-[Cu(bpp)(H₂O)]²⁺ [bpp = 2,6-bis(pyrrolidin-2-yl)pyridine] in aqueous solution has been investigated. Formation constants of the carbonato complexes [Cu(meso-bpp)(CO₃)] and rac-[Cu(bpp)(CO₃)] (1.02 × 10³ M⁻¹ and 1.77 × 10³ M⁻¹, respectively, μ = 0.70 M) have been calculated from spectrophotometric measurements. The formation of these Cu²⁺ complexes can also be used for an improved synthesis and an easy isolation of the three diastereoisomers of bpp. The mixture of [Cu(meso-bpp)(H₂O)]²⁺ and rac-[Cu(bpp)(H₂O)]²⁺ is separated by elution from SP Sephadex C-25, either as hydroxo or carbonato derivatives. rac-[Cu(bpp)(H₂O)]²⁺ is then resolved into the enantiomers [Cu(S,S-bpp)(H₂O)]²⁺ and [Cu(R,R-bpp)(H₂O)]²⁺, again on SP Sephadex C-25, by means of L-(+)-tartrate as chiral eluent. The three stereoisomers, meso-bpp, (S,S)-bpp and (R,R)-bpp are liberated from the corresponding copper(II) complexes by ligand displacement using trans-1,2-diaminocyclohexane-N,N,N′,N′-tetraacetic acid (H₄cdta). The structure of the meso isomer was solved by a single crystal X-ray analysis using the perchlorate salt [meso-bppH₂][ClO₄]₂·2H₂O.