Nickel(II) complexes with Schiff-base ligands derived from epimeric pyranose backbones as 2,3-chelators: modeling the coordination chemistry of chitosan

Two mononuclear nickel(II) complexes with Schiff-base ligands derived from the epimeric sugars glucosamine and mannosamine have been synthesized. The X-ray crystal structure reveals a distorted octahedral geometry at the nickel(II) ions with an N4O2 donor set and the rare 2,3-chelation of the donor atoms of the carbohydrate backbone. Upon complexation only the glucopyranose ring maintains the 4C1 chair conformation, whereas the mannopyranose ring adopts the OS5 screw-boat conformation. Dimeric units of complex cations are formed by intermolecular hydrogen bonding which are further assembled by π-stacking affording one-dimensional chains with a twofold screw symmetry.