Bidentate palladium(II) chelation by the common aldoses

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).

A prominent metal-binding site in the aqueous solution equilibrium of an aldose is provided by the furanose isomer in its 1,3-bonding mode. Since this binding mode does not lead to typical signal patterns in NMR spectra, structural work on single crystals was necessary to confirm this form of aldose chelating. The figure shows l-threose adopting this mode, but pentoses and hexoses also ligate that way to palladium(II) centres. 1,3-Bonding galactofuranose thus is the major isomer in an equimolar aqueous solution of the palladium reagent [PdII{(R,R)-chxn}(OH)2] (chxn = 1,2-diaminocyclohexane) and d-galactose.Figure optionsDownload as PowerPoint slide