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

The magnetic properties of the pentacoordinate [M^II (Me₄ cyclam)N₃ ]⁺ (Me₄ cyclam=tetramethylcyclam; N₃ =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^-1 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_azido direction perpendicular to the Ni(Me₄ cyclam) mean plane, whereas for M=Co it lies in the Co(Me₄ cyclam) mean plane and thus perpendicular to the Co-N_azido 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^II (NCH)₅ ]²⁺ and [Co^II (NCH)₅ ]²⁺ 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 Me₄ cyclam macrocycle. Moreover, the orientation of the anisotropy axes is mainly dependent on the geometry of the complexes.