Studies On Design,synthesis And Magneto-structural Relationships Of Dysprosium-based Single-molecule Magnets With Pseudo-mono-coordinate Linear Ligand Fields

Single-molecule magnets exhibit both macroscopic properties of traditional magnets and microscopic quantum effects such as quantum tunneling and quantum coherence effect,which have potential applications in high density information storage,quantum computing and molecular spin devices.Compared to transition metal ions,lanthanide ions have more single electron numbers and stronger spin-orbit coupling,resulting that lanthanide ions are the best candidate for constructing high-performance single-molecule magnets.In recent years,Dy（Ⅲ）-based single molecule magnets have continuously broke the record of energy barrier(U_eff)and blocking temperature（T_B）,attracting many attentions.However,many high-performance single-molecule magnets are unstable in air,limiting their application.This work proposes a novel“construction of the pseudo-mono-coordinate linear ligand fields”strategy.Based on the high coordinate geometries,the lanthanide-based single-ion magnets with air-stability and high-performance are synthesized by using the bulky polydentate ligands and monodentate ligands with strong donor atoms.Then,these single-ion magnets as the magnetic unit are used to assemble the better-performance dinuclear complexes.This work explores the effects of axial ligands and magnetic coupling on magnetic properties,and investigates the related magneto-structural relationships.The details are as follows:（1）A heptadentate pentapyridyldiamine（BPA-TPA）ligand and fine-tuned axial phenoxy ligands are used to synthesize three triangular dodecahedral Dy（Ⅲ）complexes[Dy（BPA-TPA）（X）]（BPh₄）₂·n CH₂Cl₂（X=Cl^-and n=1,1;X=4-methoxy-PhO^-and n=1,2;X=2,4-dimethyl-PhO^-and n=2,3）.All complexes exhibit triangular dodecahedral geometry.The Dy–N bond lengths for 1-3 exceed 2.45（?）,indicating the weak ligand fields formed by the BPA-TPA ligand.The short Dy–O bonds for 2 and 3 are about 2.1（?）,which forms the strong axial ligand field.Therefore,complexes 2 and 3 have high effective barriers exceeding 400 K and magnetic hysteresis up to 8 K.Due to the long axial Dy–Cl bond,complex 1 is only field-induced single molecule magnet.This work shows the great potential of pseudo-mono-coordinate linear Dy-L ligand fields in the construction of high performance single molecule magnets,and the magnetic properties can be regulated by modification of axial ligands.（2）Based on the above-mentioned strategy,a large steric retarded,seven-dentate,nitrogen-containing organic ligand{[2-{（imidazol-4-yl）methylene}amino]ethyl}amine（L）was designed and synthesized,which was used as a weak ligand to coordinate Dy（Ⅲ）ion,leaving only a small vacancy for a negatively charged pentafluorophenol anion（F₅PhO^-）or2,6-dichloro-4-nitrophenol anion（2,6-dichloro-4-nitro-PhO^-）moiety to occupy,resulting in two air-stable eight-coordinate mononuclear Dy（Ⅲ）complexes[Dy（L）（F₅PhO）]（BPh₄）₂（4）and[Dy（L）（2,6-dichloro-4-nitro-PhO）]（BPh₄）₂（5）.The crystal structure shows that the two complexes are also triangular dodecahedral configurations.Complex 4 and 5 exhibits typical field-induced single-molecule magnet behavior with the U_eff values of 68.6 and 34.6 K,respectively.Because the axial ligand contains F,Cl and nitro substituents with strong electron absorption ability,the negative charge of complex 4 and 5 is reduced along the axis,resulting in the low energy barriers of both complexes.（3）Using a promising design concept in which Ising-type magnetic anisotropy combines with collinear magnetic interaction,a novel hydroquinone-bridged dinuclear dysprosium（Ⅲ）single-molecule magnet[Dy₂（BPA-TPA）₂（C₆H₄O₂）]（BPh₄）₄·4CH₃CN（6）based on a polydentate ligand BPA-TPA has been made,exhibiting a high energy barrier of 615 K.Meanwhile,the blocking temperature（T_B）of 6 exceeds 16 K with remanence at zero field,which holds the highest T_B for all air-stable dinuclear Dy（Ⅲ）SMMs.However,the Y（Ⅲ）-diluted analogue 6@Y shows that butterfly-shaped hysteresis loops are closed at zero field.This discrepancy clearly demonstrates the positive effect of axial magnetic interaction between the Dy dimer.The fascinating behavior is induced by the pseudo linear Dy–O ligand field and collinear weak intramolecular antiferromagnetic coupling,as revealed by ab initio calculations.