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Synthesis, Structures, and Magnetic Properties of Tetranuclear CuII−LnIII Complexes

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journal contribution
posted on 06.03.2006, 00:00 by Jean-Pierre Costes, Magali Auchel, Françoise Dahan, Viviane Peyrou, Sergiu Shova, Wolfgang Wernsdorfer
The copper(II)−gadolinium(III) and copper(II)−terbium(III) complexes studied in this report derive from disymmetric trianionic ligands abbreviated H3Li (i = 4−6). These ligands are obtained through reaction of different aldehydes with “half-units” having an amide function, the latter resulting from the monocondensation of different diamines with phenyl 2-hydroxy-3-methoxybenzoate. Upon deprotonation, the Li ligands (i = 4−10) possess an inner N2O2 coordination site with one amido, one imine, and two phenoxo functions, an outer O2O2 or O2O coordination site, and an amido oxygen atom positioned out of these two sites. The trianionic character of such ligands yields original anionic complexes in the presence of copper(II) or nickel(II) ions, with a 1/1 L/M stoichiometry. The crystal and molecular structures of four complexes, two 3d (1, 5) and two 3d−4f (12, 13) complexes, have been determined. Complex 1 crystallizes in the monoclinic space group C2/c:  a = 27.528(2) Å, b = 7.0944(7) Å, c = 22.914(2) Å, β = 92.130(6)°, V = 4471.9(7) Å3, Z = 8 for C21.5H27CuKN2O6.5. Complex 5 crystallizes in the monoclinic space group P21/n (No. 14):  a = 11.0760(9) Å, b = 21.454(2) Å, c = 15.336(1) Å, β = 101.474(1)°, V = 3571.5(5) Å3, Z = 4. Complex 12 crystallizes in the triclinic space group P1̄ (No. 2):  a = 8.682(2) Å, b = 11.848(2) Å, c = 11.928(2) Å, α = 81.77(3)°, β = 89.17(3)°, γ = 85.49(3)°, V = 1210.6(4) Å3, Z = 2 for C20H22CuN5O11Tb. Complex 13 belongs to the monoclinic space group C2/c:  a = 25.475(5)Å, b = 12.934(3)Å, c = 15.023(3) Å, β = 91.06(3)°, V = 4949.0(2)Å3, Z = 8 for C21H25CuN4O12Tb. The structural determinations confirm that the dinuclear entities involved in 12 and 13 are disposed in a head-to-tail arrangement to give tetranuclear complexes in which the copper and lanthanide ions are positioned at the vertexes of a rectangle. In the [Cu−Gd]2 species, there are two different ferromagnetic Cu−Gd interactions. The stronger one is supported by the double phenoxo bridge (CuO2Gd) while the weaker one corresponds to the single amido bridge (Cu−N−C−O−Gd). Replacement of gadolinium ions with anisotropic terbium ions yields tetranuclear entities showing slow relaxation of magnetization and magnetization hysteresis. Detailed relaxation and hysteresis loop studies establish single-molecule magnet (SMM) behavior which is influenced by weak intermolecular interactions.