Magnetostructural studies on ferromagnetically coupled copper(II) cubanes of schiff-base ligands

Abstract

Three cubane copper(II) clusters, namely [Cu₄(HL')₄] (1), [Cu₄L₂(OH)₂] (2), and [Cu₄L₂(OMe)₂] (3), of two pentadentate Schiff-base ligands N,N'-(2-hydroxypropane-1,3-diyl)bis(acetylacetoneimine) (H₃L_') and N,N'-(2-hydroxypropane-1,3-diyl)bis(salicylaldimine) (H₃L), are prepared, structurally characterized by X-ray crystallography, and their variable-temperature magnetic properties studied. Complex 1 has a metal-to-ligand stoichiometry of 1:1 and it crystallizes in the cubic space group P43n with a structure that consists of a tetranuclear core with metal centers linked by a μ₃-alkoxo oxygen atom to form a cubic arrangement of the metal and oxygen atoms. Each ligand displays a tridentate binding mode which means that a total of eight pendant binding sites remain per cubane molecule. Complexes [Cu₄L₂(OH)₂] (2) and [Cu₄L₂(OMe)₂] (3) crystallize in the orthorhombic space group Pccn and have a cubane structure that is formed by the self-assembly of two {Cu₂L}⁺ units. The variable-temperature magnetic susceptibility data in the range 300-18 K show ferromagnetic exchange interactions in the complexes. Along with the ferromagnetic exchange pathway, there is also a weak antiferromagnetic exchange between the copper centers. The theoretical fitting of the magnetic data gives the following parameters: J₁=38.5 and J₂=-18 cm^-1 for 1 with a triplet (S=1) ground state and quintet (S=2) lowest excited state; J₁=14.7 and J₂=-18.4 cm^-1 for 2 with a triplet ground state and singlet (S=0) lowest excited state; and J₁=33.3 and J₂=-15.6 cm^-1 for 3 with a triplet ground state and quintet lowest excited state, where J₁ and J₂ are two different exchange pathways in the cubane {Cu₄O₄} core. The crystal structures of 2·6H₂O and 3·2H₂O·THF show the presence of channels containing the lattice solvent molecules.