Effective tuning of magnetic anisotropy in distorted pentagonal bipyramidal Ni(ii) complexes via substitution of axial coligands

Abstract

A series of Ni(II) complexes with pyridine-based macrocyclic ligand L (3,12,18-triaza-6,9-dioxabicyclo[12.3.1]octadeca-1(18),14,16-triene) with general formula [Ni(L)(X)₂]^0/2+ (X = Br⁻ (1), I⁻ (2), CH₃CN (3), NCS⁻ (4), imidazole (5)) was prepared and thoroughly investigated. X-ray molecular structures confirmed pentagonal bipyramidal geometry for all studied complexes with a strong Jahn–Teller distortion in the pentagonal equatorial plane and significantly elongated Ni–O distance(s) with a decrease of this distortion by varying axial coligands (CH₃CN > Br⁻ > I⁻ > NCS⁻ > imidazole). Direct current magnetic measurements revealed the easy-axis type of magnetic anisotropy with negative as well as positive axial zero-field-splitting parameter D ranging from +6.8 to −14.5 cm⁻¹, which remains not affected in the halogenido series Cl⁻ → Br⁻ → I⁻, but which increases in the series with N-axial ligands in order CH₃CN → NCS⁻ → imidazole. Theoretical calculations helped to elucidate (i) the final coordination numbers 6 + 1 for 1 and 2, and 5 + 2 for 2–5, (ii) the pattern of splitting of d-orbitals, contributions of excited states to the final D-values and their final signs, and (iii) the complexity in the variation of the D and E parameters with elongation of axial bond distances in such strongly distorted systems. The studied complexes did not show any alternating magnetic susceptibility signal, but it was clearly documented that the magnetic anisotropy of the pentagonal bipyramidal Ni(II) complexes can be modulated/tuned by variation of axial coligands. Nevertheless, great care has to be taken for symmetry of the equatorial ligand field.