Complementation and joint contribution of appropriate intramolecular coupling and local ion symmetry to improve magnetic relaxation in a series of dinuclear Dy2 single-molecule magnets

A series of lanthanide binuclear complexes with formula [Dy-2(DMOP)(2)(NTFA)(4)] (1), [Dy-2(DMOP)(2)(NTFA)(4)(CH3OH)(2)center dot 2H(2)O] (1a), [Dy-2(DMOP)(2)(BTFA)(4)] (2), [Dy-2(DMOP)(2)(TTA)(4)] (3), [Y-2(DMOP)(2)(TTA) (4)] (3Y), [Tb-2(DMOP)(2)(NTFA)(4)] (4), [Tb-2(DMOP)(2)(NTFA)(4)(CH3OH)(2)center dot 2H(2)O] (4a), [(Er2)(DMOP)(2)(NTFA)(4)] (5), [Gd-2(DMOP)(2)(NTFA)(4)(CH3OH)(2)center dot 2H(2)O] (6), and [Gd-2(DMOP)(2)(TTA)(4)] (7) (DMOP = 2,6-dimethoxyphen, NTFA = 2-naphtoyltrifluoroacetone, BTFA = benzoyltrifluoroacetone, TTA = thenoyltrifluoroacetone) have been structurally and magnetically characterized. They possess nearly the same dinuclear Ln2 cores bridged by double mu(2)-phenoxyl oxygen atoms and differ in the peripheral coordinated beta-diketonate ligands or coordinated neutral MeOH molecule. The Dy(III)-based derivatives show significant zero-field single-molecule magnet behavior. It is noteworthy that the single-molecule magnet (SMM) performance of 1 was dramatically improved after transforming to 1a with a nearly double energy barrier, which is mostly due to the enhancement of local symmetry around Dy(III) ions in 1a by the introduction of a CH3OH molecule. In addition, the nature of intramolecular Dy-Dy interaction changed from antiferromagnetic to ferromagnetic coupling. The detailed ab initio calculation reveals the alternate domination of the intramolecular exchange coupling and dipole-dipole interactions in 1 and 1a, respectively. Furthermore, improved SMM performance with a hysteresis temperature of 3 K was observed in diluted samples of 3. In the present work, the joint contribution of intramolecular coupling and local symmetry to improve their dynamic magnetic relaxation was unambiguously affirmed.