Tri- and Tetranuclear Copper(II) Complexes Consisting of Mononuclear Cu(II) Chiral Building Blocks with a Sugar-Derived Schiff's Base Ligand

Abstract

A new sugar-derived Schiff's base ligand N-(3-tert-butyl-2-hydroxybenzylidene)-4,6-O-ethylidene-β-d-glucopyranosylamine (H3L1) has been developed which afforded the coordinatively labile, alcoholophilic trinuclear Cu(II) complex [Cu3(L1)2(CH3OH)(H2O)] (1). Complex 1 has been further used in the synthesis of a series of alcohol-bound complexes with a common formula of [Cu3(L1)2(ROH)2] (R = Me (2), Et (3), nPr (4), nBu (5), nOct (6)). X-ray structural analyses of complexes 2−6 revealed the collinearity of trinuclear copper(II) centers with Cu−Cu−Cu angles in the range of 166−172°. The terminal and central coppers are bound with NO3 and O4 atoms, respectively, and exhibit square-planar geometry. The trinuclear structures of 2−6 can be viewed as the two {Cu(L1)}- fragments capture a copper(II) ion in the central position, which is further stabilized by a hydrogen-bonding interaction between the alcohol ligands and the sugar C-3 alkoxo group. Complex 2 exhibits a strong antiferromagnetic interaction between the Cu(II) ions (J = −238 cm-1). Diffusion of methanol into a solution of complex 1 in a chloroform/THF mixed solvent afforded the linear trinuclear complex [Cu3(L1)2(CH3OH)2(THF)2] (7). The basic structure of 7 is identical to complex 2; however, THF binding about the terminal coppers (Cu−OTHF = 2.394(7) and 2.466(7) Å) has introduced the square-pyramidal geometry, indicating that the planar trinuclear complexes 2−6 are coordinatively unsaturated and the terminal metal sites are responsible for further ligations. In the venture of proton-transfer reactions, a successful proton transfer onto the saccharide C-3 alkoxo group has been achieved using 4,6-O-ethylidene-d-glucopyranose, resulting in the self-assembled tetranuclear complex, [Cu4(HL1)4] (8), consisting of the mononuclear Cu(II) chiral building blocks, {Cu(HL1)}.