Crystal Field Tuningvia Ligand Substitution at Axialand Equatorial Sites: Engineering Pentagonal Bipyramidal Dy(III) Complexeswith Enhanced Single-Molecule Magnet Performance

Abstract A series of isostructural mononuclear Dy(III) complexes with the general formula [Dy(n-bmbpen-X)Cl]·mCH3CN (n = 3, m = 0, X = F (1), Cl (2), Br (3); n = 4, m = 2, X = F (4)) were synthesized using hexadentate ligands (n-H2bmbpen-X = N,N′-bis(n-methyl-2-hydroxybenzyl)-N,N′-bis(5-X-2-methylpyridyl)ethylenediamine). Single-crystal X-ray diffraction analysis confirms that all complexes adopt a seven-coordinate pentagonal bipyramidal (PBP) geometry with quasi-D5h symmetry. Magnetic susceptibility measurements demonstrate that all four complexes exhibit slow magnetic relaxation characteristic of single-molecule magnets (SMMs) under zero applied dc field, with effective energy barriers (Ueff) of 708.1 K (1), 531.6 K (2), 464.3 K (3), and 852.8 K (4). Open magnetic hysteresis loops are observed up to 11, 10, 9, and 14 K for 1–4, respectively. Structural analysis suggests two potential regulatory pathways: (i) The meta-methyl isomer 4 mitigates proximal axial steric repulsion, affording shorter Dy–O bonds and a wider O–Dy–O angle compared to ortho-methyl analogues; (ii) A fluorine-specific C–H···Cl hydrogen-bonding interaction appears to resolve the counterintuitive equatorial Dy–N bond shortening trend in the ortho-methyl series, likely via outward displacement of the equatorial Cl– and compensatory framework contraction. Ab initio calculations support that these geometric and electronic perturbations may reduce detrimental nondiagonal crystal field (CF) terms and suppress quantum tunnelling of magnetization (QTM). This work highlights positional isomerism as a viable parameter for CF tuning and proposes a bivariate modulation strategy combining axial steric adjustment with targeted equatorial electronic modification. These findings may offer useful insights for the rational design of air-stable SMMs, complementing existing ligand engineering approaches.