Asymmetric sulfur atom coordination in a copper(ii) dipicolylamine (DPA) complex with a thioglycoside ligand

Abstract

The 2,2′-dipicolylamine (DPA)-tethered thioglycoside ligand, N,N-bis(2-pyridylmethyl)-2-aminoethyl 1-deoxy-1-thio-2,3,4,6-tetra-O-acetyl-β-D-glucopyranoside (sL1), has been prepared and its copper(II) complex synthesized. Using copper(II) chloride, the copper complex was isolated as a chloride-bound species formulated as [Cu(sL1)Cl(ClO₄)] (1). The corresponding O-glycoside complex ([Cu(L1)Cl](ClO₄), 2) was also prepared using L1 (N,N-bis(2-pyridylmethyl)-2-aminoethyl 2,3,4,6-tetra-O-acetyl-β-D-glucopyranoside), and both complexes were characterized and compared by means of X-ray crystallography, cyclic voltammetry, electronic absorption and circular dichroism (CD) spectra. Although both complexes exhibited similar copper coordination geometries, the absolute configuration of the O/S chiral center generated by the copper coordination was inverted. The electronic and CD spectra of acetonitrile solutions of 1 and 2 were different likely due to the presence of a copper–sulfur charge-transfer band for 1. Complex 1 also exhibits a large Cotton effect around 700 nm. The corresponding d–d transition of the copper(II) center reveals that the asymmetric copper–sulfur (oxygen) coordination remains even in solution.