Base-Free Construction of Dinuclear Lanthanide(III) Complexes with Unusual Coordination Modes: Magneto-Structural and Theoretical Insights into Single-Molecule Magnet Behavior

Abstract

Three isostructural dinuclear lanthanide complexes, [Ln2(μ-HL)2(HL)2(NO3)2]·4MeCN (Ln = DyIII (1), ErIII (2), YbIII (3)), were synthesized under base-free conditions using the o-vanillin- and 2-aminophenol-based Schiff base H2L. This ligand exhibits two distinct protonation-dependent coordination modes that have not been previously shown in this class of ligands. The simultaneous presence of monoanionic HL and monoanionic zwitterionic HL coordination leads to an unusual combination of chelating and asymmetric bridging features, generating a flexible and moderately axial LnO7N environment. All three complexes display field-induced single-molecule magnet behavior, and complex 1 additionally shows measurable zero-field slow relaxation. Detailed ac susceptibility studies indicate that their relaxation dynamics are dominated by Raman processes, with additional Orbach relaxation in 2 and a direct process in 3. We further combine dc and ac magnetic data with ab initio calculations, providing insight into the relaxation dynamics and the contributions of single-ion anisotropy and weak exchange interactions, if any. The combined structural, magnetic, and theoretical analyses demonstrate that base-free synthesis allows controlled ligand protonation, which in turn modulates crystal field effects and governs the magnetic relaxation pathways in these dinuclear lanthanide systems.