Trinuclear Copper(II) Complexes Derived from Schiff-Base Ligands Based on a 6-Amino-6-deoxyglucopyranoside:  Structural and Magnetic Characterization

The trinuclear copper(II) complexes ([CuL1)(μ-ac)Cu(μ-ac)CuL1) (1) and ([CuL2)(μ-ac)Cu(μ-ac)CuL2) (2) of the tridentate aminosaccharide-derived Schiff-base ligands H2 L1 [6-N-(salicylidene)amino-6-deoxy-1,2,3-tri-O-methyl-α- d-glucopyranoside] and H2 L2 [6-N-(3,5-di-tert-butylsalicylidene)amino-6-deoxy-1,2,3-tri-O-methyl-α- d-glucopyranoside] were synthesized and structurally characterized. The trinuclear complex units can be described as two terminal copper−ligand moieties bridged by a central copper acetate moiety, with the Cu centers arranged in a triangular fashion. IR and UV/vis spectroscopic studies strongly indicate that the trinuclear structure is maintained in a methanolic solution. The temperature dependence of the magnetic susceptibility of both complexes shows a moderate antiferromagnetic coupling and can be well interpreted by applying a symmetric Cua−Cub−Cua ‘ model with linear spin topology. The fit of the magnetic data affords coupling constants J of −34 and −24 cm-1 for 1 and 2, respectively [H = −J(S a S b + S b S a ‘)]. For μ-alkoxo-μ-acetato-bridged copper(II) complexes with a large dihedral angle between the adjacent coordination planes, as found in 1 and 2, such an antiferromagnetic coupling is unusual. However, density functional theory calculations of 2 using BP86, B3LYP*, and B3LYP density functionals confirmed a symmetric doublet ground state.