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

Three cubane copper(II) clusters, namely [Cu-4(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 Xray 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 mu(3)-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 Peen 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: J(1)=38.5 and J(2)=-18cm(-1) for 1 with a triplet (S=1) ground state and quintet (S=2) lowest excited state; J(1)=14.7 and J(2)=-18.4 cm(-1) for 2 with a triplet ground state and singlet (S=0) lowest excited state; and J(1)=33.3 and J(2)=-15.6 cm(-1) for 3 with a triplet ground state and quintet lowest excited state, where J(1) and J(2) are two different exchange pathways in the cubane {Cu4O4} core. The crystal structures of 2.6H(2)O and 3.2H(2)O center dot THF show the presence of channels containing the lattice solvent molecules.