Synthesis, Structures, and Magnetic Properties of Tetranuclear Cu^II−Ln^III\nComplexes

Abstract

The copper(II)−gadolinium(III) and copper(II)−terbium(III) complexes studied in this report derive from disymmetric\ntrianionic ligands abbreviated H₃L<i>ⁱ</i> (<i>i</i> = 4−6). These ligands are obtained through reaction of different aldehydes\nwith “half-units” having an amide function, the latter resulting from the monocondensation of different diamines with\nphenyl 2-hydroxy-3-methoxybenzoate. Upon deprotonation, the L<i>ⁱ</i> ligands (<i>i</i> = 4−10) possess an inner N₂O₂\ncoordination site with one amido, one imine, and two phenoxo functions, an outer O₂O₂ or O₂O coordination site,\nand an amido oxygen atom positioned out of these two sites. The trianionic character of such ligands yields\noriginal anionic complexes in the presence of copper(II) or nickel(II) ions, with a 1/1 L/M stoichiometry. The crystal\nand molecular structures of four complexes, two 3d (<b>1</b>, <b>5</b>) and two 3d−4f (<b>12</b>, <b>13</b>) complexes, have been determined.\nComplex <b>1</b> crystallizes in the monoclinic space group <i>C</i>2/<i>c</i>: <i>a</i> = 27.528(2) Å, <i>b</i> = 7.0944(7) Å, <i>c</i> = 22.914(2) Å,\nβ = 92.130(6)°, <i>V</i> = 4471.9(7) ų, <i>Z</i> = 8 for C_21.5H₂₇CuKN₂O_6.5. Complex <b>5 </b>crystallizes in the monoclinic space\ngroup <i>P</i>2₁/<i>n</i> (No. 14): <i>a</i> = 11.0760(9) Å, <i>b</i> = 21.454(2) Å, <i>c</i> = 15.336(1) Å, β = 101.474(1)°, <i>V</i> = 3571.5(5) ų,\n<i>Z</i> = 4. Complex <b>12</b> crystallizes in the triclinic space group <i>P</i>1̄ (No. 2): <i>a</i> = 8.682(2) Å, <i>b</i> = 11.848(2) Å, <i>c</i> =\n11.928(2) Å, α = 81.77(3)°, β = 89.17(3)°, γ = 85.49(3)°, <i>V</i> = 1210.6(4) ų, <i>Z</i> = 2 for C₂₀H₂₂CuN₅O₁₁Tb.\nComplex <b>13 </b>belongs to the monoclinic space group <i>C</i>2/<i>c</i>: <i>a</i> = 25.475(5)Å, <i>b</i> = 12.934(3)Å, <i>c</i> = 15.023(3) Å, β\n= 91.06(3)°, <i>V</i> = 4949.0(2)ų, <i>Z</i> = 8 for C₂₁H₂₅CuN₄O₁₂Tb. The structural determinations confirm that the dinuclear\nentities involved in <b>12 </b>and <b>13</b> are disposed in a head-to-tail arrangement to give tetranuclear complexes in which\nthe copper and lanthanide ions are positioned at the vertexes of a rectangle. In the [Cu−Gd]₂ species, there are\ntwo different ferromagnetic Cu−Gd interactions. The stronger one is supported by the double phenoxo bridge\n(CuO₂Gd) while the weaker one corresponds to the single amido bridge (Cu−N−C−O−Gd). Replacement of gadolinium\nions with anisotropic terbium ions yields tetranuclear entities showing slow relaxation of magnetization and\nmagnetization hysteresis. Detailed relaxation and hysteresis loop studies establish single-molecule magnet (SMM)\nbehavior which is influenced by weak intermolecular interactions.