Stereochemistry of nickel(II) complexes with N-glycosylamine ligands from 1,3-diaminopropane and aldopentoses. Correlation between configurational structures and circular dichroism spectra†

Abstract

Reactions of [Ni(tn)₃]X₂·2H₂O (tn = 1,3-diaminopropane, X = Cl or Br) with aldopentoses afforded a series of mononuclear nickel(II) complexes with N-glycosylamine ligands, [Ni(aldose-tn)₂]X₂ [aldose-tn = 1-(N-aldosyl)amino-3-aminopropane, aldose = D-xylose (D-Xyl) 1, D-lyxose (D-Lyx) 2, D-ribose (D-Rib) 3, or D-arabinose (D-Ara) 4], which were characterized by elemental analysis, electronic absorption and circular dichroism (CD) spectroscopy, and X-ray crystallography. Compound 4b, [Ni(D-Ara-tn)₂]Br₂·2H₂O, was shown by an X-ray analysis to have a C₂ symmetrical mononuclear nickel(II) structure ligated by two tridentate N-glycosylamine ligands, 1-amino-3-(N-D-arabinosyl)propane. The two N-glycosylamines are co-ordinated to the metal through the primary amino and N-glycosidic secondary amino groups and the C-2 hydroxy group of the sugar moiety in a meridional mode, resulting in a Λ-C₂-helical configuration around the metal centre. The sugar rings adopt an unusual α-¹C₄ chair conformation and the sugar–chelate ring conformation is δ. The co-ordination behavior of D-ribose was confirmed by an X-ray analysis of an analogous compound, [Ni(D-Rib-men)₂]Br₂·2CH₃OH 5 [D-Rib-men = 1-methylamino-2-(N-D-ribosyl)aminoethane], derived from the reaction of [Ni(men)₃]Br₂ with D-ribose [men = 1-amino-2-(N-methylamino)ethane]. The D-ribose moiety forms an α-N-glycosidic bond with the primary amino group of men, and adopts an α-⁴C₁ chair form, the sugar–chelate ring conformation being δ. The CD spectra of 1–4 were measured in the region 9000–50 000 cm^–1 and clearly indicated the structural features of the complexes; the Cotton effects at around 10 000 cm^–1 (the first absorption band of the d–d transitions) reflected the conformation of the sugar chelate or the absolute configuration of the N-glycosidic nitrogen atom, and those at around 40 000–46 000 cm^–1 (charge-transfer bands) demonstrated the C₂ chiral configuration around the metal centre. These assignments were also confirmed by the CD spectra of known compounds.