Absence of a magnetic interaction in a dinuclear copper complex? The case of a crossed axial–equatorial oxalate coordination mode

DFT calculations show that the triplet and singlet states of an oxalate-bridged dinuclear copper complex have the same energy, in agreement with the presence of non-coupled magnetic CuII centers. A simple and easy preparation of the hydrogen oxalate ligand (HOOCCOO)−, with only one deprotonated carboxylic function, is described. 13C NMR spectroscopy furnishes a straightforward characterization of this interesting ligand. This ligand coordinates in its monodeprotonated form to a CuII complex with a free coordination position, yielding a complex that presents a very weak antiferromagnetic interaction. In a subsequent step, the second deprotonation allows the isolation of an oxalato-bridged dinuclear CuII complex, which is characterized by structural determination, the copper ions being pentacoordinate in an equivalent centrosymmetry-related environment. DFT calculations show that the magnetic interaction through the oxalato bridge is strictly equal to 0cm−1, demonstrating that the singlet and triplet states of this oxalato-bridged dinuclear CuII complex have the same energy. This dinuclear complex presents a rhombic EPR spectrum, with three well-defined g values, at room temperature as well as at 100K.