Bis-(μ-saccharide-C-2-oxo) dinuclear Cu(II) complexes of 4,6-O-butylidene/ethylidene-N-(α-hydroxynaphthylidene/o-hydroxybenzylidene/5-bromo-o-hydroxybenzylidene)-β-D-glucopyranosylamine: structural aspects and data correlations

Abstract

A total of five dinuclear copper complexes were synthesized using 4,6-O-butylidene/ethylidene-N-(α-hydroxynaphthylidene/o-hydroxybenzylidene/5-bromo-o-hydroxybenzylidene)-β-D-glucopyranosylamine. Upon recrystallisation from different solvents, viz., dmso/MeOH/pyridine, seven different dinuclear copper complexes were generated, wherein the geometry around one or both of the copper centers changes from square planar to square pyramidal due to the binding of solvent molecule as the fifth ligand. The ligands and their complexes were characterized by elemental analysis, ¹H and ¹³C NMR, FT-IR, FABMS, UV-Vis, optical rotation, CD and magnetic susceptibility measurements. The 3D structures of all the seven complexes were established by single crystal XRD. All the complexes are neutral and dinuclear with the metal to the glycosylamine ratio being 1:1. Each glycosylamine acts as tridentate with di-negative charge and bridges between the two copper centers through the C-2-oxo group of the saccharide part and further the coupling between the copper centers is antiferromagnetic. At least four different types of Cu₂O₂²⁺ core structures were identified depending upon the presence or absence of a fifth ligand at the Cu(II) center. The β-⁴C₁-pyranose form of the glycosylamine is retained even in complexes. The dinuclear complex is stabilized through intra-complex hydrogen bond interaction. The inter-molecular C-H···O interactions are manifested in the formation of a helical structure where the water molecules occupy the cavity. The structural diversity observed in the complexes and several data correlations are discussed in detail.