CRYSTAL-STRUCTURE AND MAGNETIC-PROPERTIES OF [LN2CU4] HEXANUCLEAR CLUSTERS (WHERE LN = TRIVALENT LANTHANIDE) - MECHANISM OF THE GD(III)-CU(II) MAGNETIC INTERACTION

The hexanuclear clusters [Ln2Cu4(fsaaep)4(NO3)6].0.5(CH3OH.H2O) (abbreviated as [Ln2Cu4]) have been synthesized; Ln is a trivalent lanthanide and (fsaaep)2- is the ligand deriving from 3-(N-2-(pyridylethyl)formimidoyl)salicylic acid. The crystal structure of the compound with Ln = Pr has been solved. This compound crystallizes in the orthorhombic system, space group P2(1)2(1)2(1). The lattice parameters are a = 18.746(4) angstrom, b = 24.196(4) angstrom, c = 17.053(4) angstrom, and Z = 4. The structure consists of [Pr2Cu4] entities in which the metal ions form a chair-shaped hexagon. The two praseodymium atoms are located on both sides of a double layer containing the four copper atoms. The copper environments are elongated distorted octahedra, and the praseodymium environments are bicapped square antiprisms. The magnetic susceptibility and the magnetization of [La2Cu4] and [Gd2Cu4] have been investigated. As far as the magnetic properties are concerned, the hexanuclear clusters may be viewed as two independent Cu(II)Ln(III)Cu(II) triads. The magnetic susceptibility data for [La2Cu4] have revealed a weak Cu(II)-Cu(II) interaction through the closed-shell rare earth ion characterized by the interaction parameter J(CuCu) = -3.13 cm-1 (the interaction Hamiltonian being of the form H = -JS(A).S(B)). The magnetic susceptibility data for [Gd2Cu4] have shown that the ground-state spin for the Cu(II)Gd(III)Cu(II) triad is S = 9/2; the Gd(III)-Cu(II) interaction is ferromagnetic with an interaction parameter J(GdCu) = 6.0 cm-1. The field dependence of the magnetization measured at both 2 and 30 K confirms the nature of the ground state and of the Gd(III)-Cu(II) interaction. The heart of the paper is devoted to the mechanism of the Gd(III)-Cu(II) interaction. The ferromagnetic nature of this interaction is attributed to the coupling between the Gd(III)Cu(II) ground configuration and the Gd(II)Cu(III) charge-transfer excited configuration in which an electron is transferred from the singly-occupied 3d-type copper orbital toward an empty 5d-type gadolinium orbital. A semiquantitative estimate of J(GdCu) is performed which agrees fairly well with the value deduced from the magnetic data. It is emphasized that the Gd(III)-Cu(II) interaction is quite peculiar in the sense that owing to the contraction of the 4f orbitals the usual mechanisms involving 4f-3d overlap densities are inoperative.