Optimization and expansion of the Schiff base [Zn–Dy] unit to enhance the performance of single molecule magnetic materials

Abstract

Five Zn(II)–Dy(III) single molecule magnetic (SMM) materials with the molecular formulae [pyZnL¹Dy(acac)₂]PF₆ (1), [MApy-(ZnL¹Dy(acac)₂)]PF₆ (2), [bipy-(ZnL¹Dy(acac)₂)₂]PF₆·C₆H₁₄ (3), [bipy-(ZnL²Dy(acac)₂)₂]PF₆ (4), and [bpe-(ZnL²Dy(acac)₂)₂]PF₆ (5) (H₂L¹ = N,N′-bis(3-methoxysalicylidene)-1,2-phenylenediamine, H₂L² = N,N′-bis(3-methoxysalicylidene)propane-1,3-diamine, py = pyridine, MApy = m-aminopyridine, bipy = 4,4′-bipyridine, bpe = 1,2-di(4-pyridyl)ethylene, acac = acetylacetone) have been structurally and magnetically characterized. They possess a nearly identical [Zn(II)–Dy(III)] core bridged by phenoxy O of two compartmental Schiff base ligands. The substitutable monodentate pyridine-N allows to modulate the coordination environment of Zn(II) ions while keeping Dy(III) ions intact. Interestingly, the remote modulation on Zn(II) beyond the first coordination sphere of Dy(III) ions has a significant impact on the dynamic magnetic relaxation performance. More importantly, two [Zn–Dy] units were expanded to new dimers bridged by double pyridine-N linkers with different lengths, which leads to more pronounced magnetic relaxation performance with 3.5 K hysteresis temperature, albeit low local symmetry exists in the system. In combination with the magnetic easy axis analysis based on electrostatic potential and careful inspection of the local structure environment around Zn(II) and Dy(III) ions, it is revealed that the promotion of SMM properties arises from the favorable distribution of coordination atoms surrounding the Dy(III) ions.