Pentacoordinate cobalt(ii) single ion magnets with pendant alkyl chains: shall we go for chloride or bromide?

Abstract

Four pentacoordinate complexes, 1–4, of the type [Co(L1)X₂] and [Co(L2)X₂] (where L1 = 2,6-bis(1-octyl-1H-benzimidazol-2-yl)pyridine for 1 and 2, L2 = 2,6-bis(1-dodecyl-1H-benzimidazol-2-yl)-pyridine for 3 and 4; X = Cl⁻ for 1 and 3, X = Br⁻ for 2 and 4) have been synthesized, and their structures have been determined by X-ray analysis. The DC magnetic investigation confirmed the high-spin and anisotropic behavior of the metal centers of the reported compounds, and magnetic data were analyzed with respect to the spin Hamiltonian and the Griffith–Figgis Hamiltonian. CASSCF-NEVPT2 analysis enabled us to identify triaxial magnetic anisotropy for 1 and 2 and axial anisotropy for 3 and 4, and we discuss thoroughly the relationship between geometry and orbital ordering. The g-tensor components of the ground Kramers doublet extracted from EPR spectroscopy are compatible with this prediction of magnetic anisotropy. Simulated FIRMS experimental data of 2, 3 and 4 show very good agreement with theoretical calculations and provide precise values of zero-field splitting. The AC susceptibility measurements confirmed that the reported complexes were field-induced single-ion magnets. The slow relaxation of magnetization in 1 and 2 is mediated through two relaxation channels that are unusually close to each other. On the other hand, complexes 3 and 4 show the single-channel relaxation of magnetization, and their isostructural characteristic allowed us to study the relaxation changes caused solely by the replacement of terminal halido ligands. Finally, a wet lithographic technique has been used to evaluate both the processability of the complexes in solution and the preparation of microstructured films.