Magnetically bistable Co(ii) and Fe(ii) complexes featuring pentyl-decorated pyridyl-benzimidazole ligands: role of isomerism and chirality

Abstract

Three variants of 2-(pyridine-2-yl)-1H-benzimidazole bidentate ligands, with distinct pentyl substituents (n-pentyl (L1), (2S)-2-methylbutyl (L2S), and (2R)-2-methylbutyl (L2R)), were synthesized and employed to prepare magnetically bistable cobalt(II) and iron(II) complexes. The cobalt(II) complexes [Co(L1)(κ²-NO₃)₂(CH₃CN)] (1), [Co(L2S)(κ²-NO₃)₂(H₂O)] (2S), [Co(L2R)(κ²-NO₃)₂(H₂O)] (2R) and [Co(L2)(κ²-NO₃)₂(H₂O)] (2rac) exhibit field supported slow relaxation of magnetisation, while the iron(II) complexes [Fe(L2S)₃](CF₃SO₃)₂·H₂O·CH₃NO₂·C₅H₁₂O (3S) and [Fe(L2S)₃]·1.5H₂O (3R) exhibit thermal spin crossover allocated above room temperature. Structural analysis revealed that the heptacoordinated cobalt(II) complexes adopt a distorted pentagonal bipyramidal geometry, whereas the iron(II) compounds contain hexacoordinated complex cations. Coordination compounds incorporating the chiral ligands L2S and L2R are optically active, with their enantiomeric relationship confirmed by circular dichroism spectroscopy. Computational studies provided insights into electron density distributions and bonding energetics (DFT and QT-AIM), predicted zero-field splitting parameters (CASSCF/NEVPT2), and quantified energies of the d-orbital, ligand field terms and their multiplets (via AILFT). Magnetic investigations of the cobalt(II) complexes yielded experimental ZFS parameters and revealed distinct magnetization relaxation mechanisms: 1 exhibits relaxation governed by a phonon bottle-neck process, whereas 2S and 2R involve a combination of Raman and direct relaxation processes. Notably, compounds 2S and 2R, despite being enantiomers and isostructural, exhibited distinct relaxation dynamics, attributable to differential phonon coupling pathways modulated by the chiral substituents. This is the first report demonstrating enantiomer-dependent slow relaxation of magnetisation behaviour in cobalt(II)-based SMMs. These findings underscore the critical role of stereochemistry in modulating spin dynamics and magnetic bistability, providing design principles for future chiral magnetic materials.