Unveiling field-induced single-ion magnetism in pentacoordinate and heptacoordinate cobalt( ii ), dysprosium( iii ) and terbium( iii ) complexes with a tridentate bis(benzimidazole)pyridine ligand

We report the synthesis, crystal structures, and magnetic properties of three Co(II) and two Ln(III) (Dy, Tb) complexes, all containing the tridentate ligand 2,6-bis(3-methoxypropyl-1H-benzimidazol-2-yl)pyridine (L). The pentacoordinate Co(II) complexes [Co(L)Cl$_2$] (1) and [Co(L)Br$_2$] (2) adopt distorted square-pyramidal geometries, while the heptacoordinate complexes [Co(L)(κ$^2$-NO$_3$)$_2$] (3) contain two independent complex molecules with capped trigonal prismatic and pentagonal-bipyramidal shape of polyhedra. Lanthanide complexes [Dy(L)Cl$_3$(H$_2$O)] (4) and [Tb(L)Cl$_3$(H$_2$O)] (5) also adopt pentagonal-bipyramidal coordination environments and display characteristic 4f–4f emission bands in the solid state at 3 K. Ab initio calculations reproduce the observed anisotropy trends and rationalize the magnetic behaviour. Pentacoordinate Co(II) centres require an explicit Griffith–Figgis Hamiltonian approach, while heptacoordinate Co(II) complex 3 exhibits axial anisotropy rationalised by spin Hamiltonian formalism. Analysis of static magnetic properties of complexes 4 and 5 also revealed the notable magnetic anisotropy of the Ln(III) ions. ... mehrAC susceptibility measurements reveal field-supported slow relaxation of magnetisation (SRM) in 1–4, with complexes 1–3 exhibiting single-channel relaxation and 4 displaying two relaxation pathways. In contrast, compound 5 shows no evidence of SRM. Analysis of the temperature dependence of the relaxation times yielded effective energy barrier (U$_{eff}$) values of 15.9(7) K for 1, 25.0(5) K for 2, 27(1) K for 3, and 33(3) K for 4. Comparative analysis across 3d and 4f systems highlights the crucial role of subtle ligand-field distortions and donor charge distribution in governing SIM performance. These findings expand the chemistry of pyridyl-benzimidazole-derived ligands and provide new design principles for both transition-metal and lanthanide single-ion magnets.