Design of a Binuclear Ni(II) Complex with Large Ising-type Anisotropy and Weak Anti-Ferromagnetic Coupling.

The preparation of a binuclear Ni(II) complex with a pentacoordinate environment using a cryptand organic ligand and the imidazolate bridge is reported. The coordination sphere is close to trigonal bipyramidal (tbp) for one Ni(II) and to square pyramidal (spy) for the other. The use of the imidazolate bridge that undergoes π-π stacking with two benzene rings of the chelating ligand induces steric hindrance that stabilizes the pentacoordinate environment. Magnetic measurements together with theoretical studies of the spin states energy levels allow fitting the data and reveal a large Ising-type anisotropy and a weak anti-ferromagnetic exchange coupling between the metal ions. The magnitude and the nature of the magnetic anisotropy and the difference in anisotropy between the two metal ions are rationalized using wave-function-based calculations. We show that a slight distortion of the coordination sphere of Ni(II) from spy to tbp leads to an Ising-type anisotropy. Broken-symmetry density functional calculations rationalize the weak anti-ferromagnetic exchange coupling through the imidazolate bridge.

Design and Magnetic Properties of a Mononuclear Co(II) Single Molecule Magnet and Its Antiferromagnetically Coupled Binuclear Derivative.

The preparations of related mononuclear and binuclear Co(II) complexes with a quasi-identical local C3v symmetry using a cryptand organic ligand are reported. The mononuclear complex behaves as a single molecule magnet (SMM). A relatively weak antiferromagnetic exchange coupling (J) of the same order of magnitude as the local magnetic anisotropy (D) is determined experimentally and theoretically for the binuclear complex. The weak magnitude of the antiferromagnetic exchange coupling, analyzed using a combination of broken-symmetry density functional theory and wave function based calculations, is ascribed to the weak overlap between the singly occupied orbitals because of the local C3v symmetry of the Co(II) ions; the organic ligand was found to contribute to the exchange coupling as the azido bridge that directly links the Co(II) ions. Calculation of the energy and wave functions of the spin states for the binuclear complex, in the general case, allows analysis of the effect of the |J/D| ratio on the magnetic behavior of the binuclear complex and prediction of the optimum range of values for the complex to behave as two weakly interacting SMMs.

Magnetic Anisotropy in Pentacoordinate NiII and CoII Complexes: Unraveling Electronic and Geometrical Contributions.

The magnetic properties of the pentacoordinate [MII (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-Nazido 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-Nazido 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 [NiII (NCH)5 ]2+ and [CoII (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.