Easy-axis magnetic anisotropy in tetragonally elongated cobalt(ii) complexes beyond the spin-Hamiltonian formalism

Abstract

Two hexacoordinate Co(II) complexes [Co(hfac)₂(etpy)₂] (1) and [Co(hfac)₂(bzpyCl)₂] (2) were synthesized and spectrally and structurally characterized. The {CoO₄N₂} chromophore adopts a geometry of the elongated tetragonal bipyramid with a small o-rhombic component. This less common arrangement causes the magnetic data to need be analysed using the Griffith-Figgis model, instead of the commonly used spin-Hamiltonian with zero-field splitting parameters D and E. In the case of the elongated bipyramid for d⁷ complexes, the source of the magnetic anisotropy of an easy-axis type is the axial crystal field splitting Δ_ax. The ab initio CASSCF calculations followed by the NEVPT2 module confirm that the ground electronic term is quasi-degenerate owing to the splitting of the ⁴E_g (D_4h) mother term. The lowest spin–orbit multiplets appear as four Kramers doublets belonging to the Γ₅ irreducible representation of the double point group D₂′. They exhibit a serious mixing of the |±1/2〉 and |±3/2〉 spins which reflects a sizable effect of the spin–orbit coupling. Both complexes exhibit field-supported slow magnetic relaxation governed by the Raman process.