Complexation of amino sugars with cobalt(III) bis (phenanthroline)

Abstract

The I3- salts soluble in acetone and sparingly so in methanol, and I3- inhibits formation of Co(II) so that minor decomposition of acomplex does not broaden its 1H NMR signals. The (phen)2 complex of Co(III) with D-glucosamine (1) has been isolated as the sulfate and its stereochemistry examined [6], and there is limited evidence on the corresponding complex with D-mannosamine, (2)[5]. We have now extended the investigation of the corresponding complexes with D-galactosamine (·). These amino sugars are α- and β-mixtures in the 4C1 conformation (Scheme 1)[1,7]. There is extensive evidence that formation of complexes from these, and similar ligands preferentially involves, cis, ax-eq groups a[7,8]. Therefore if the I-OH (O-) and 2-NH2 groups are involved we would expect complexation (2), and α-glucosamine (1), β-mannosamine (2), and α-galactosamine (3). The anomeric composition of the sugar residue can, in principle, be determined by 1H NMR spectroscopy. Coordination to metal cations increases acid dissociation constants of hydroxyl groups by 6-8 log units[3,10], depending upon the metal ion and solvent. Therefore the complexes with Co(III) dicationic in mildly basic aqueous solution, but the analytical composition often corresponds to the existence of a trication due to ion-pairing and hydrogen-bonding in the crystals [3,4,6]. Octahedral complexes can have the ∆ -or Λ-configuration at Co(III) (Scheme2) and it can be determined from the signs and sequence of the CD or ORD signals, and we used the first method [I I.]