Magnetic Anisotropy in Pentacoordinate Ni and Co Complexes: Unraveling Electronic and Geometrical Contributions.

The magnetic properties of the pentacoordinate [M (Me cyclam)N ] (Me cyclam=tetramethylcyclam; N =azido; M=Ni, Co) complexes were investigated. Magnetization and EPR studies indicate that they have an easy plane of magnetization with axial anisotropy parameters D close to 22 and greater than 30 cm for the Ni and Co complexes, respectively. Ab initio calculations reproduced the experimental values of the zero-field splitting parameters and allowed the orientation of the anisotropy tensor axes with respect to the molecular frame to be determined. For M=Ni, the principal anisotropy axis lies along the Ni-N direction perpendicular to the Ni(Me cyclam) mean plane, whereas for M=Co it lies in the Co(Me cyclam) mean plane and thus perpendicular to the Co-N direction. These orientations match one of the possible solutions experimentally provided by single-crystal cantilever torque magnetometry. To rationalize the geometry and its impact on the orientation of the anisotropy tensor axis, calculations were carried out on model complexes [Ni (NCH) ] and [Co (NCH) ] by varying the geometry between square pyramidal and trigonal bipyramidal. The geometry of the complexes was found to be the result of a compromise between the electronic configuration of the metal ion and the structure-orienting effect of the Me cyclam macrocycle. Moreover, the orientation of the anisotropy axes is mainly dependent on the geometry of the complexes.