Stereochemistry of sugar units in glycosylamine ligands of octahedral complexes derived from tris(trimethylenediamine)-nickel(II) and natural aldohexoses

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Abstract
Some natural aldohexoses reacted with [Ni(tn)3]2+(tn = trimethylenediamine) to yield octahedral nickel(II) complexes containing glycosylamine ligands. The complexes were characterized by elemental analyses, magnetic moments, electronic and CD spectra. The molecular structure of [Ni(L-Rha-tn)2]Br2·2H2O·MeOH [L-Rha-tn = 1-(3-aminopropylamino)-1,6-dideoxy-L-mannose]1 has been determined by single-crystal X-ray techniques: orthorhombic, space group P212121, a= 12.373(1), b= 21.614(2), c= 11.272(1)Å, least-squares refinement led to a final R factor of 0.044 using 2657 reflections having Fo > 3σ(Fo). From X-ray crystallographic studies of glycosylamine complexes, two typical co-ordination structures of the aldose unit have been found in 1 and [Ni(D-GlcN-en)2]2+[D-GlcN-en = 2-amino-1-(2-aminoethylamino)-1,2-dideoxy-D-glucose], which can be discriminated by the two geometrical isomeric forms arising from the two substituents on C1 and C2 of the pyranoside ring, namely cis and trans. The sugar unit of 1 has cis geometry containing equatorial and axial substituents on C1 and C2 of the pyranoside ring. In contrast, the sugar unit of the D-GlcN-en complex has trans geometry at these positions, which are both equatorial. Assuming the 4C1(chair) conformers to be the most stable and based on some stereochemical considerations of both the sugars and the chelate rings, the structures of the sugar units in the glycosylamine complexes from all natural aldohexoses can be classified into either of the above two types, the only difference being the relative absolute configuration at C2 and C5. This classification is well correlated with the CD spectral patterns obtained in methanolic solutions.

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