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(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 x 10(3) M-1 and 1.77 x 10(-3) M-1, respectively, mu = 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 (H(4)cdta). The structure of the meso isomer was solved by a single crystal X-ray analysis using the perchlorate salt [meso-bppH(2)][ClO4](2).2H(2)O.