DESIGN OF A HOMONUCLEAR FERROMAGNETIC CHAIN - STRUCTURES AND MAGNETIC-PROPERTIES OF OXALATO-BRIDGED COPPER(II) COMPLEXES WITH ONE-DIMENSIONAL STRUCTURES

An orthogonality of magnetic orbitals results in the stabilization of a high spin state, that is, a propagation of a ferromagnetic interaction between magnetic centers. Oxalato-bridged one-dimensional copper(II) complexes [Cu(en)2][Cu(ox)2] (1) and [Cu(bpy)(ox)].2H2O (2) were designed for an each magnetic orbital on adjacent units to be orthogonal, which might result in a ferromagnetic intrachain interaction. In the complex 1, the oxalato anions are tridentate ligands, and the terminal oxygen atoms of the [Cu(ox)2]2- unit bridge the adjacent [Cu(en)2]2+ unit from the axial (d(z)2 orbital) position of the cation. The ground states of the cations and anion units are d(x2-y2). Magnetic susceptibility data have revealed that complex 1 is an antiferromagnetic S = 1/2 Heisenberg chain, and exchange coupling constant (J(AF)) and g values were estimated to be -1.95 (1) cm-1 and 2.115 (1), respectively, on the basis of an antiferromagnetic Heisenberg linear chain model. In complex 2, the oxalato anions are centrosymmetric and act as quadridentate bridging ligands, where the oxygen atoms coordinate to each copper atom both from the equatorial (d(x2-y2) orbital) and axial (d(z)2 orbital) positions. Magnetic susceptibility data have shown that complex 2 is the ferromagnetic S = 1/2 Heisenberg chain, and exchange coupling constant (J(F)) and g values were estimated to be 1.22 (4) cm-1 and 2.185 (6), respectively, by using a ferromagnetic Heisenberg linear chain model. A magnetization curve of complex 2 at 2 K exhibited more rapid saturation than the calculated curve for J = 3/2, where J is the angular momentum quantum number. This ferromagnetic behavior can be readily understood by considering the fact that the spin density on the copper atom is spread over the oxalato groups, as confirmed by the all-electron ab initio unrestricted Hartree-Fock calculation for [Cu(ox)2]2-. The spin on the terminal oxygen atom, which coordinates to the adjacent copper atoms from its d(z)2 direction, induces spin on that d(z)2 orbital. The orthogonality of the primary spin orbital (d(x2-y2)) and induced spin orbital (d(z)2) causes the ferromagnetic interaction. The extended Huckel molecular orbital calculation for [Cu2((NH3)8(ox)]2+, which has a coordination geometry similar to that for complex 2, has shown that the HOMO and LUMO are effectively degenerate (separated by 0.02 eV), and this calculation is in good agreement with the experimental result. Crystal data: [Cu(en)2][Cu(ox)2], monoclinic, space group C2/c, a = 14.884 (2) angstrom, b = 12.770 (2) angstrom, c = 7.147 (1) angstrom, beta = 94.83 (2)-degrees, V = 1353.6 (4) angstrom3, Z = 4, and R = 0.026 (R(w) = 0.035) for 1938 data with \F(o)\ > 3-sigma(F(o)).