Metal complexes of carbohydrates: isolation and characterisation of cobalt(III) complexes containing N-substituted glycosylamine ligands derived from ethane-1,2-diamine and glucosamine

Abstract

Reaction between cis- or trans-[Co(en)₂CI₂]⁺(en = ethane-1,2-diamine) and D-glucosamine (2-amino-2-deoxy-D-glucopyranose) in neutral aqueous solution results in a very complicated product mixture including as a major component several isomeric complex ions in which the co-ordination sphere is formally made up of two ethane-1,2-diamine units and one sugar unit. Separation of these complexes by ion-exchange chromatography, and subsequent spectroscopic and structural studies have shown that they are diastereomeric forms in which the co-ordination sphere actually contains one bidentate ethane-1,2-diamine chelate and a multidentate ligand which is an N-(2′-aminoethyl)amino-substituted glycosylamine derived from glucosamine, fructosamine or mannosamine. The presence of the fructosamine moiety reveals that the complex formation reaction is accompanied by the Amadori rearrangement of glucosamine, while the presence of the mannosamine moiety indicates that this rearrangement may be reversible and possibly stereospecific in particular diastereoisomers. Crystal structure determinations have definitively characterised eight of the reaction products as: 1 a complex of the 1-N-(2′-aminoethyl)amino-substituted glucosamine in which the sugar, although chelated to the metal as part of a tridentate ligand, is in its open-chain form and in which the four nitrogen atoms derived from the original two ethane-1,2-diamine units are essentially coplanar; 2 a complex of tridentate 1-N-(2′-aminoethyl)amino-substituted glucosamine in which the sugar has its pyranose form and the chelate edges spanned by the original ethane-1,2-diamine units are in the Δ configuration; 3 a complex of 1-N-(2′-aminoethyl)amino-substituted glucosamine in which the sugar is in its open-chain form and the ligand is bound in a quadridentate manner, with the chelate edges spanned by the original ethane-1,2-diamine units in the Δ configuration; 4 a complex of 1-N-(2′-aminoethyl)amino-substituted mannosamine in which the sugar has a furanose form and is part of a quadridentate ligand, and the chelate edges spanned by the original ethane-1,2-diamine units are in the Δ configuration; 5 a complex of quadridentate 1-(2′-aminoethylamino)-1-deoxy-fructopyranosylamine in which the chelate edges spanned by the original ethane-1,2-diamine units are in the Δ configuration; 6 a complex of quadridentate 1-(2′-aminoethylamino)-1-deoxy-fructopyranosylamine in which the chelate edges spanned by the original ethane-1,2-diamine units are in the Δ configuration; 7 the same complex as 6 but in the form where its co-ordinated sugar hydroxyl group is deprotonated; and 8 a complex of 1-(2′-aminoethylamino)-1-deoxyfructopyranosylamine in which the chelate edges spanned by the original ethane-1,2-diamine units are in the Δ configuration.