Bidentate palladium(II) chelation by the common aldoses Original Research Article

The [PdII{(R,R)-chxn}(OH)2] reagent (chxn = 1,2-diaminocyclohexane) is introduced as a metal probe for the detection of the bidentate chelating sites of a glycose. Two moles of hydroxide per mole palladium support double deprotonation of potentially chelating diol functions at a glycose’s backbone. The individual chelating sites are detected using one- and two-dimensional NMR techniques. At equimolar amounts of palladium(II) and aldose, the metal-binding sites include mostly the hydroxy function at the anomeric carbon atom. Chelators are derived from both the pyranose and the furanose isomers. Most pyranose-based chelators form five-membered chelate rings by using their 1,2-diol function. Though 1,2-diolate bonding is also common to the furanoses, the formation of six-membered chelate rings by 1,3-bonding is more significant for them. Metal-excess conditions provoke mostly bis-bidentate 1,2;3,4-chelation but unusual isomers form also: thus d-xylose is dimetallated in its all-axial β-pyranose form, and erythrose’s dimetallation results in the formation of two isomers of a metal derivative of the open-chain hydrate. The spectroscopic results are supported by crystal-structure determinations on [Pd{(R,R)-chxn}(α-d-Xylp1,2H−2-κO1,2)]·H2O (Xyl = xylose), [Pd{(R,R)-chxn}(α-d-Ribp1,2H−2-κO1,2)]·2.25H2O (Rib = ribose), [Pd{(R,R)-chxn}(α-l-Thrf1,3H−2-κO1,3)]·2H2O (Thr = threose) and [Pd{(R,R)-chxn}(α-d-Eryf1,3H−2-κO1,3)]·3H2O (Ery = erythrose).