Crystal Field Engineering via Para-Substituents in Dinuclear Dysprosium Single-Molecule Magnets with an Energy Barrier Exceeding 900 K

Strategic modulation of the local ligand field via substituent effects provides a precise approach to enhancing magnetic anisotropy in lanthanide-based molecular magnets. We report two dinuclear dysprosium(III) complexes, ([Dy2(μ-CA)(bbpen-Cl)2]·MeCN) (1) and ([Dy2(μ-CA)(bbpen)2]·MeCN·2H2O) (2), bridged by a chloranilate (CA2–) ligand. Replacing the electron-withdrawing para-chloro substituent on the phenolate donors in 1 with a hydrogen atom in 2 profoundly tunes the crystal field. This seemingly subtle chemical modification induces a rigorous geometric evolution from a D2d-distorted triangular dodecahedron in 1 toward a more highly symmetric D4d square antiprism in 2. More critically, the removal of the inductive electron-withdrawing effect (-I effect) of the chlorine atoms enhances the electron density on the axial phenolate oxygen atoms in 2, significantly strengthening the axial crystal field. This results in a 72% increase in the effective energy barrier for magnetization reversal, from Ueff/kB = 573(10) K for 1 to Ueff/kB = 987(48) K for 2. Concurrently, under an optimal 2000 Oe DC field, the QTM(quantum tunneling of magnetization) relaxation time is extended by a factor of approximately 12.5 compared to zero field, and the magnetic blocking temperature (TB) is elevated to 6.16 K. Ab initio calculations revealing that the H-substituted framework in 2 not only maintains a higher purity of the Ising-type ground state but also preserves strict axiality across the first three excited states, with the principal magnetic axis reorienting by less than 4° relative to the ground state, effectively quenching thermally assisted QTM (TA-QTM) pathways. Significant transverse character only emerges at a higher-lying excited state whose energy exceeds 900 K. This work unequivocally demonstrates that para-substituent engineering is a potent and rational strategy for surmounting established performance barriers in polynuclear SMMs.