| This paper focuses on exploring influence factors, the synthesis strategy and theoretical basis of the single molecule magnets (SMMs) to obtain high blocking temperature (TB) and large energy barrier to magnetization reversal (Ueff)-In present, the best candidate of these molecular materials is Dy (?) and Er (?) ions due to the inherent strong spin-orbital coupling effect and hence very large magnetic anisotropy of the 6H15/2 and 4H15/2 ground state. Herein, we obtained a serious of mononuclear [LnN2O6] (Ln= Dy (?) or Er (?)) complexes based on different ?-diketonate ligands and auxiliary Ligands, exhibiting approximately square-antiprismatic (SAP) N2O6 coordination environment with D44 axial symmetry. Based on counter cations, mononuclear [DyOg] dysprosium(?) complexes were synthesized. The effects of solvent effect, configuration and counter cations on the magnetic dynamics of mononuclear single molecule magnets were discussed. Additionally, a nine-coordinated dysprosium (?) compound with the oxalate-bridged dysprosium (?) layer was obtained, exhibiting two slow magnetic relaxation processes and a Dy2 single-molecule magnet (SMM) with the benzoate anions and phenol-O- bridging groups was synthesized and characterized. The structure-property relationship of some selected Dy2 paradigmatic compounds was further discussed. The specific contents are as follows:1. Five mononuclear dysprosium (?) complexes were synthesized based on three ?-diketonate ligands,4,4,4-Trifluoro-l-(4-methylphenyl)-1,3-butanedione (tfmb), 4,4,4-Trifluoro-1-(4-fluorophenyl)-1,3-butanedione (tffb) and 4,4,4-Trifluoro-l-(2-naphthyl)-1,3-butanedione (tfnb) as well as auxiliary ligands, 1,10-phenanthroline (Phen),5-Nitro-1,10-phenanthroline (5-NO2-Phen), DMF and 2,2'-bipyridine (bpy), namely, [Dy(5-NO2-Phen)(tfmb)3] (1), [Dy(DMF)2(tfmb)3] (2) and [Dy(bpy)2(tfnb)3]-(1,4-dioxane)1/2 (3), [Dy(bpy)2(tffb)3]-(1,4-dioxane)1/3 (4) and [Dy(Phen)2(tffb)3] (5). In 1-5, the Dy (?) ions exhibit approximately square-antiprismatic (SAP) N2O6 coordination environment with D4d axial symmetry. According to the Dy (?) coordination spheres of 1-5, these complexes are slightly distorted with the following order:3 >2>4=1>5. The shortest intermolecular distances between Dy (?) ions are 10.035 A for 1,8.750 A for 2,10.035 A for 3,9.019 A for 4 and 9.679 A for 5. The magnetic properties of 1-5 have been investigated, displaying weak out-of-phase AC signals under a zero-DC field except 5. With an applied DC field of 1200 Oe, the quantum tunnelling of the magnetization was suppressed in 1-5, giving effective energy barrier (?E/kB) with a following order:5 (91.70 K)> 4 (87.31 K)> 1 (82.93 K)> 2 (55.36 K)> 3 (38.66 K). Interestingly, for frequency dependence of the out-of-phase (?") of the AC susceptibility of 2, two slow relaxation of the magnetization processes were observed under the applied magnetic field of 1200 Oe, corresponding to the fast relaxation phase (FR) and slow relaxation phase (SR), respectively. The structure-property relationship of some Dy (?) based SIMs with SAP configuration was further discussed.2. Two groups of mononuclear SMMs with square-antiprismatic (SAP) N2O6 coordination environment of Dy (?) ion, formulated as [Dy(Phen)(tfmb)3] (6) and [Dy(Phen)(tfmb)3]-0.5(1,4-dioxane) (7) as well as Dy(bpy)(tfmb)3] (8) and [Dy(bpy)(tfmb)3]0.5C4H8O2 (9) were obtained, therein, complexes 6 and 8 were transformed to 7 and 9 in 1,4-dioxane through the process of dissolution and precipitation, respectively. Structural analysis shows different configurations of 7(D2a) and 8(D4d). Complex 9 is more closer to the ideal D4d symmetry than 1. Additionally, the different types of weak interactions ?-? stacking and hydrogen bonds exist in 6-9. Magnetic studies indicate that complexes 7 and 9 exhibit more high barrier heights (AE/ks) than complexes 6 and 8, respectively, revealing that the subtle structural changes associated with guest solvent result in drastic different effective relaxation barriers.3. Three mononuclear Dysprosium (?) complexes, [Hthyl3N][Dy(tffb)4] (10, Hthyl3N= triethylamine) [Butyl4N][Dy(tfnb)4] (11, Butyl4NBr=Tetra-n-butylammonium bromide) and [Hthyl3N][Dy(dbma)4] (12) were synthesized using three kinds of ?-diketonate ligands and counter cations. The magnetic studies show that these complexes exhibits the field induced mononuclear SMMs behaviors. We discussed structure-property relationship based on the different counter cations.4. We synthesized two mononuclear Er (?) complexes, [Er(Phen)(tfmb)3] (13) and [Er(Phen)(tfnb)3] (14). Both complexes have similar square anti-prismatic coordination geometries around the lanthanide but show distinct magnetic properties.5. A 2D oxalate-bridged dysprosium (?) layer was obtained, formula as [Dy(C2O4)1.5(H2O)3]n·2nH2O (15), in which the nine-coordinated Dy (?) ions reside in a slightly distorted tricapped trigonal prism. Dy (?) ions are connected by double bridging carboxylate groups (double ?2-?O, O) to form a 2D layer, consisting of puckered six-membered rings [Dy(C2O4)]6. Magnetic studies revealed that two slow relaxation of the magnetization processes occurred in 15 under an applied magnetic field of 700 Oe, with energy barriers (?E/kB?) of 3.8 and 10.7 K and pre-exponential factors (?0) of 4.3 × 10-2 and 4.39×-6 s for the fast relaxation phase (FR) and slow relaxation phase (SR), respectively. It is significant to ensure the possibility of synthesizing and tuning the properties of the single-ion magnetic behavior of Dy (?) ion in Dy-MOFs in an easy manner. A Dy2 single-molecule magnet, namely [Dy2(H3L)2(PhCOO)4]·4H2O (16), was obtained from the reaction of Dy(PhCOO)3 with 1,5-Bis(2-hydroxy-3-methoxybenzylidene)carbonohydrazide (H4L). Each Dy(?) ion is located in the chelating pocket [DyOsN] with a tricapped trigonal prism configuration. The Dy (?) centers are bridged by the benzoate anions with ?2:?1,?2 coordination mode and the phenol-O- groups in the form of ?1:?2, respectively. The structure-property relationship of some selected Dy2 paradigmatic compounds was further discussed. |
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