Magnetic behaviour vs. structural changes in an isomeric series of binuclear copper(II) complexes: an experimental and theoretical study

In an endeavor to study how a polydentate nitrogen donor ligand affects the magnetic properties of copper(II) methoxybenzoates, three novel complexes of copper(II) were investigated. [Cu2(H2tea)2(o-methoxybenzoate)2], [Cu2(H2tea)2(m-methoxybenzoate)2]·2H2O and [Cu2(H2tea)2(p-methoxybenzoate)2]·2H2O (where H2tea = mono-deprotonated triethanolamine) were synthesized by addition of triethanolamine (H3tea) to the hydrated Cu(o-,m-,p-methoxybenzoates)2. The newly synthesized complexes have been characterized by elemental analyses, spectroscopic techniques (electronic and FT-IR), magnetic moment determination, molar conductance studies, TGA, and single crystal X-ray determination. The experimental characterization was integrated with the ab initio theoretical determination of the magnetic coupling constant value and with the analysis of the correlation between this value and the relevant geometrical parameters. Variable temperature solid state magnetization measurements and ab initio calculations indicate a remarkable ferromagnetic coupling of the unpaired electrons centered on the two Cu atoms for the m- and p-methoxybenzoate complexes (J = 100.9 cm−1), while a non-negligible antiferromagnetic coupling is found for the third complex (J = −83.1 cm−1). This differential behaviour can be rationalized on the basis of the out-of-plane displacement (τ) of the alkoxo group with respect to the molecular Cu2O2 plane. Large τ values prevent an efficient overlap between the O 2p and the magnetic Cu 3dx2−y2, favouring a ferromagnetic coupling between the Cu sites.