Aqueous Ethylenediamine Dihydroxo Palladium(II): A Coordinating Agent for Low- and High-Molecular Weight Carbohydrates

Abstract

An aqueous solution of [(en)Pd(OH)2] (Pd-en) forms 1,2and 1,3-diolato complexes on reaction with polyols. Three crystalline palladium(ii) complexes have been isolated and characterized with anhydroerythritol (AnEryt), 1,6-anhydro-b-d-glucose (1,6AnGlc, levoglucosan), and sucrose (Suc) as the polyol components, namely [(en)Pd(AnErytHy2)] 4 H2O (1), [(en)Pd(1,6AnGlc2,4Hy2)] H2O (2) and [(en)2Pd2(Suc1,3,3',4'Hy4)] 11 H2O (3). Pd-en acts as a coordinating solvent for cellulose; the polysaccharide anhydroglucose units (GlcC) form [(en)Pd(GlcC2,3Hy2)] entities on dissolution. The palladium-binding sites of the saccharides may be recognized by 13C NMR spectroscopy in the case of 1,2-diolic chelator groups, which exhibit a downfield, coordination-induced shift of the two diolato carbon atoms of about 10 ppm. Slightly yellow, viscous solutions of cellulose in Pd-en, which are largely insensitive to oxidative degradation, were characterized by light scattering (LS). Analysis of the chain conformation by LS supports the significance of chain stiffening by intramolecular interresidue hydrogen bonds of the type O6' ± H O2y in the molecularly dispersed and entirely metallated cellulose strands. The flexibility and energy of the hydrogen bond were investigated by density functional theory (DFT) calculations.