Synthesis, Structural Characterization, and Magnetic Properties of a Copper–Gadolinium Complex Derived from a Hydroxybenzohydrazide Ligand

The reaction of hydroxybenzohydrazide with o-vanillin yields 2-hydroxy-N′-[(2-hydroxy-3-methoxyphenyl)­methylidene]­benzohydrazide (LH3), a ligand that is able to give mononuclear and tetranuclear copper complexes but also to associate copper and gadolinium ions in a Cu2–Gd2 heterotetranuclear complex. This synthesis is successful if the Gd ions, which are acidic in protic solvents, are introduced in a basic methanol solution of the mononuclear copper complex. In the absence of piperidine, the addition of Gd ions to a methanol solution of the mononuclear copper complex only yields a tetranuclear cubane-type copper complex. This work reports on the first structural characterization of a copper–gadolinium complex involving a benzohydrazide ligand. The resulting complex consists of two Cu–Gd pairs linked by a dihydroxo Gd–Gd bridge, in which the Cu and Gd ions are bridged by a nonsymmetric phenoxo–hydroxo bridge. The magnetostructural correlation between the ferromagnetic coupling constant and the hinge angle observed in symmetrical double-phenoxo Cu–Gd bridges remains valid for dissymmetric Cu–Gd bridges and confirms the preponderance of the structural factor over the nature of the bridge. This tetranuclear complex corresponds to two S = 4 units linked through a dihydroxo bridge introducing a weak antiferromagnetic Gd–Gd interaction and impeding the existence of a S = 8 ground state.