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.