Large and negative magnetic anisotropy in pentacoordinate mononuclear Ni(II) Schiff base complexes
A series of pentacoordinate Ni(II) complexes of the general formula [Ni(L5)] (1–6) with various pentadentate Schiff base ligands H2L5 (originating in a condensation of aromatic ortho-hydroxy-aldehydes and aliphatic triamines) was synthesized and characterized by X-ray structure analysis and magnetometry. The alternations of substituents on the H2L parent ligand resulted in the complexes with the geometry varying between the square-pyramid and trigonal-bipyramid. In the compounds whose chromophore geometry is closer to a trigonal-bipyramid, a large and negative uniaxial anisotropy (D = −64 cm−1) was identified. Moreover, the simple linear expression for the axial zero-field splitting (ZFS) parameter, D/cm−1 = 32.7(4.8) − 151(10)τ, was proposed, where τ (in degrees) stands for the Addison parameter. The results of magnetic analysis were also supported by ab initio CASSCF/NEVPT2 calculations of the ZFS splitting parameters D and E, and g tensors. Despite large and negative D-values of the reported compounds, slow relaxation of magnetization was not observed either in zero or non-zero static magnetic field, thus no single-molecule magnetic behaviour was detected.