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)(mu-ac)Cu(mu-ac)CuL1) (1) and ([CuL2)(mu-ac) Cu(mu-ac) CuL2) (2) of the tridentate aminosaccharide-derived Schiff-base ligands H(2)L1 [6-N-(salicylidene)amino-6-deoxy-1,2,3-tri-O-methy-IR alpha-D-glucopyranoside] and H(2)L2 [6-N-(3,5-di-tert-butylsalicylidene) amino-6-deoxy-1,2,3-tri-O-methyl-R-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 Cu-a-Cu-b-Cu-a' 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(SaSb + SbSa')]. For mu-alkoxo-mu-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.