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]

The binding of the carbonate anion to [Cu(<i>meso</i>-bpp)(H₂O)]²⁺ and <i>rac</i>-[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(<i>meso</i>-bpp)(CO₃)] and </i>rac</i>-[Cu(bpp)(CO₃)] (1.02 × 10³ M^–1 and 1.77 × 10³ M^–1, respectively, <i>µ</i>= 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(<i>meso</i>-bpp)(H₂O)]²⁺ and <i>rac</i>-[Cu(bpp)(H₂O)]²⁺ is separated by elution from SP Sephadex C-25, either as hydroxo or carbonato derivatives. <i>rac</i>-[Cu(bpp)(H₂O)]²⁺ is then resolved into the enantiomers [Cu(<i>S,S</i>-bpp)(H₂O)]²⁺ and [Cu(<i>R,R</i>-bpp)(H₂O)]²⁺, again on SP Sephadex C-25, by means of L-(+)-tartrate as chiral eluent. The three stereoisomers, <i>meso</i>-bpp, (<i>S,S</i>)-bpp and (R,R)-bpp are liberated from the corresponding copper(II) complexes by ligand displacement using <i>trans</i>-1,2-diaminocyclohexane-<i>N,N,N’,N’</i>-tetraacetic acid (H₄cdta). The structure of the <i>meso</i> isomer was solved by a single crystal X-ray analysis using the perchlorate salt [<i>meso</i>-bppH₂][ClO₄]₂•2H₂O.