Antiferromagnetic Interactions in Copper(II) µ‐Oxalato Dinuclear Complexes: The Role of the Counterion

We report the preparation, crystal structure determination, magnetic properties and DFT calculations of five oxalato-bridged dicopper(II) complexes of formula [Cu-2(bpy)(2)(H2O)(2)(C2O4)](CF3SO3)(2) (1), [Cu-2(bpy)(2)(C2O4)](PF6)(2) (2), [Cu-2(bpy)(2)(C2O4)](ClO4)(2) (3), [Cu-2(bpy)(2)Cl-2(C2O4)]center dot H2O (4) and [Cu-2(bpy)(2)(NO2)(2)(C2O4)] (5) (bpy = 2,2'-bipyridine and C2O42- = oxalate). Compounds 1, 2, 4 and 5 crystallize in the monoclinic system and 3 crystallizes in the triclinic system. The oxalate ligands in 1-5 adopt the bis-bidentate coordination mode and the two bpy molecules act as terminal ligands. The coordination of the counterions and the surroundings of the copper(II) ions differentiate the five compounds. The four nearest neighbours of copper(II) in 1-4 are roughly in the plane of the CuC2O4Cu framework, whereas they are in an almost perpendicular plane in 5. Using the isotropic Hamiltonian H = -J S-1.S-2, where S-1 and S-2 are the spin quantum operators for Cu-1 and Cu-2; J is - 384 cm(-1) for 1, - 392 cm(-1) for 2 and -387 cm(-1) for 3, slightly decreasing to - 328 cm(-1) for 4 and falling to -14 cm(-1) for 5. The influence of the anions on the magnetic properties of this family of compounds is explained by the changes in the overlap of the magnetic orbitals through the oxalate bridge. DFT calculations reproduce well the experimental values of J and provide an illustration of the magnetic orbitals.