Carboxyhydrate Amines A Strong Primary Binding Site May Help in Hydroxyl Group Coordination

In the solid state Y-glycosides (glycosylamines) have been studied by X-ray crystallography. The reaction of diamines, such as ethylenediamine (En) or trimeth-ylenediamine (Tn = 1,3-diaminopropane), with ketose or aldose sugars allows the isolation of compounds which contain the complex units Ni(En)(D-Fru-En)2+ [33], Ni(En)(l-Sor-En)2+ [34], Ni(i-Rha-Tn)2+ [35] or Ni(D-Mal-Tn)2+ [36], where Fru = fructose, Sor = sorbose, Rha = rhamnose, and Mal = maltose. In the first two examples the octahedral coordination sphere of Ni2+ is occupied by the bidentate En and the tetradentate glycosylamine ligand. One nitrogen of En binds to C2 of the sugar unit forming the glycosylamine. This ligand attaches to Ni2+ at four sites, i.e., through the 1- and 3-hydroxyl groups and the two N atoms of the En residue. In the latter two examples two tridentate glycosylamines bind to Ni2+ via their two N atoms and the (C2)OH group [36].

In a circular dichroism study of Ni2+ complexes in solution with ^-glycosylamine ligands formed by 1,3-diaminopropane and pentoses such as d--xylose, D-ribose or D-arabinose, it was concluded that two of the tridentate ^-glycosylamine ligands coordinate to one Ni2+, each ligand being bound in a meridional mode by the primary amino group, the N-glycosidic secondary amino group and the C2 hydroxyl group of the sugar moiety [37]. For the D-ribose derivative this binding mode was confirmed by an X-ray structure analysis [37]. Interestingly, crystalline Ni2+ complexes of dianionic glycopyranoside ligands were obtained by reaction of Ni[tris(2-aminoethyl)amine](OH)2 with methyl-D-glucopyranoside or sucrose [38]. In the latter case (C2)O~ and (C3)O~ chelation occurs in the glucose part of the disaccharide. It is important to note that here deprotonated hydroxyl groups participate in Ni2+ binding. In this context it may also be mentioned that Ni2+ complexes of N,N'-alkylated ethylenediamine are able to catalyze the C2 epimerization of aldoses and ketoses [39].

Potentiometric and spectroscopic studies in aqueous solution (25°C; I = 0.15 M, KNO3) indicate that 2-amino-2-deoxy-D-mannose, 2-amino-2-deoxy-D-galactose, and 2-amino-2-deoxy-D-glucose form Ni2+ complexes which also involve the hydroxyl groups of the sugars [40]. From these and related studies [41] it becomes evident that the amino group is the major donor towards Ni2+ with one of the hydroxyl groups as the second donor which is being deprotonated only at pH > 8.

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