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-1-benzimidazol-2-yl)pyridine (L). The pentacoordinate Co(II) complexes [Co(L)Cl] (1) and [Co(L)Br] (2) adopt distorted square-pyramidal geometries, while the heptacoordinate complexes [Co(L)(κ-NO)] (3) contain two independent complex molecules with capped trigonal prismatic and pentagonal-bipyramidal shape of polyhedra. Lanthanide complexes [Dy(L)Cl(HO)] (4) and [Tb(L)Cl(HO)] (5) also adopt pentagonal-bipyramidal coordination environments and display characteristic 4f-4f emission bands in the solid state at 3 K. 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. AC 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 () 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.