High-Coordinate Mononuclear Ln(III) Complexes: Synthetic Strategies and Magnetic Properties

A highly anisotropic family of hexagonal bipyramidal Dy(III) unsaturated 18-crown-6 complexes exceeding the blockade barrier over 2700 K: a computational exploration

In the present work, we have explored a series of unsaturated hexa-18-crown-6 (U18C6) ligands towards designing highly anisotropic Dy(III) based single-ion magnets (SIMs) with the general formula [Dy(U18C6)X2]+ (where U18C6 = [C12H12O6] (1), [C12H12S6] (2), [C12H12Se6] (3), [C12H12O4S2] (4), [C12H12O4Se2] (5) and X = F, Cl, Br, I, OtBu and OSiPh3). By analysing the electronic structure, bonding and magnetic properties, we find that the U18C6 ligands prefer stabilising the highly symmetric eight-coordinated hexagonal bipyramidal geometry (HBPY-8), which is the source of the near-Ising type anisotropy in all the [Dy(U18C6)X2]+ complexes. Moreover, the ability of sulfur/selenium substituted U18C6 ligands to stabilize the highly anisotropic HBPY-8 geometry makes them more promising towards engineering the equatorial ligand field compared to substituted saturated 18C6 ligands where the exodentate arrangement of the S lone pairs results in low symmetry. Magnetic relaxation analysis predicts a record barrier height over 2700 K for [Dy(C12H12O6)F2]+ and [Dy(C12H12S6)X2]+ (where X = F, OtBu and OSiPh3) complexes, nearly 23% higher than those of the top performing Dy(III) based SIMs in the literature.

Synthesis, Structure, and Zero-Field SMM Behavior of Homometallic Dy2, Dy4, and Dy6 Complexes

The synthesis, structure, and magnetic properties of three Dy^III complexes of different nuclearity, [Dy₂(H₂L)₂(NO₃)] [NO₃]·2H₂O·CH₃OH (1), [Dy₄(HL)₂(piv)₄(OH)₂] (2), and [Dy₆(H₂L)₃(μ₃-OH)(μ₃-CO₃)₃(CH₃OH)₄(H₂O)₈] 5Cl·3H₂O (3) [(H₄L) = 6-((bis(2-hydroxyethyl)amino)-N'-(2-hydroxybenzylidene)picolinohydrazide)], are described. This variety of complexes with the same ligand could be obtained by playing with the metal-to-ligand molar ratio, the type of Dy^III salt, the kind of base, and the presence/absence of coligand. 1 is a dinuclear complex, while 2 is a tetranuclear assembly with a butterfly-shaped topology. 3 is a homometallic hexanuclear complex that exhibits a propeller-shaped topology. Interestingly, in this complex 3, three atmospheric carbon dioxide molecules are trapped in the form of carbonate ions, which assist in holding the hexanuclear complex together. All of the complexes reveal a slow relaxation of magnetization even in zero applied field. Complex 1 is a zero-field SMM with an effective energy barrier (U_eff) of magnetization reversal equal to 87(1) K and a relaxation time of τ₀ = 6.4(3) × 10^-9 s. Under an applied magnetic field of 0.1 T, these parameters change to U_eff = 101(3) K, τ₀ = 2.5(1) × 10^-9 s. Complex 2 shows zero-field SMM behavior with U_eff = 31(2) K, τ₀ = 4.2(1) × 10^-7 s or τ₀₁ = 2(1) × 10^-7 s, U_eff1 = 37(8) K, τ₀₂ = 5(6) × 10^-5 s, and U_eff2 = 8(4) by considering two Orbach relaxation processes, while 3, also a zero-field SMM, shows a double relaxation of magnetization [U_eff1 = 62.4(3) K, τ₀₁ = 4.6(3) × 10^-8 s, and U_eff1 = 2(1) K, τ₀₂ = 4.6(2) × 10^-5 s]. The ab initio calculations indicated that in these complexes, the Kramer's ground doublet is characterized by an axial g-tensor with the prevalence of the m_J = ±15/2 component, as well as that due to the weak magnetic coupling between the metal centers, the magnetic relaxation, which is dominated by the single Dy^III centers rather than by the exchange-coupled states, takes place via Raman/Orbach or TA-QTM. Moreover, theoretical calculations support a toroidal magnetic state for complex 2.

Terminal-fluoride-coordinated air-stable chiral dysprosium single-molecule magnets

Terminal fluoride ligands generate strong magnetic anisotropy in air-stable chiral dysprosium enantiomers supported by a bulky equatorial macrocycle, exhibiting a typical zero-field single-molecule magnet behaviour.