Syntheses,Structures And Magnetic Properties Of High-nuclearity Lanthanide-chromium Metal Clusters Based On Carboxylic Acid Ligand

Due to its existence for many years,the chemistry of lanthanide-transition metal clusters is no longer an emerging discipline,but the researchers'enthusiasm for high-nuclearity lanthanide-transition metal clusters is still unabated.Specifically,resarchers focused initially on their beautiful and nearly perfect molecular structures(such as Keplerate-type Ln₂₀Ni₃₀,nesting doll-like Ln₅₄Ni₅₄ and Ln₆₀Ni₇₆),later on exploration and promotion of the properties in optics,magnetism and catalysis,and recently on the assembly mechanism.These reports indicate that high-nuclearity lanthanide-transition metal clusters are still an interesting research field for further investigation.Compared with other frequently reported mature lanthanide-transition metal clusters,there are relatively few reports on lanthanide-chromium(Ln-Cr)clusters,especially those with more than ten metals.However,the inert nature of Cr³⁺ions makes it difficult to combine with Ln³⁺ions,suggesting the necessity of a suitable ligand or synthetic strategy for constructing high-nuclearity Ln-Cr clusters.In this work,we select different types of functional carboxylic acid ligands such as simple carboxylic acid ligands(2-aminoisobutyric acid,2-pyrazinecarboxylic acid)and multidentate chelating carboxylic acid ligands(1-acetylpyrrole Alkane-2-carboxylic acid,2,2-dimethylolpropionic acid,2,2-thiodiacetic acid)and obtain series of Ln-Cr metal clusters by ligand-controlled hydrolysis and anion template methods.And the unique structure and magnetic properties are investigated.The detail findings are following:1.Synthesis,Structures and Magnetic Properties of Ln-Cr Metal Clusters Based on Ligands of Amino Acids and Their Derivatives.1)Using 2-aminoisobutyric acid as the simple amino acid ligand,two Ln-Cr metal clusters Ln₃Cr₂(Ln=Y/Gd)featuring the triangular bipyramid configuration were obtained by ligand-controlled hydrolysis method.2)Using 2-pyrazinecarboxylic acid as the amino acid derivative,two Ln-Cr metal clusters Ln₇Cr₃(Ln=Gd/Dy)featuring double triangular bipyramid configuration were obtained by ligand-controlled hydrolysis method.Magnetic studies show that compound Gd₇Cr₃ has a large magnetocaloric effect,and Dy₇Cr₃ display a slow magnetic relaxation behavior.2.Synthesis,Structures and Magnetic Properties of Ln-Cr Metal Clusters Based on Ligands of Multidentate Carboxylic Acid.1)Two Ln-Cr metal clusters Ln₄Cr₂(Ln=Gd/Dy)were obtained using 1-acetylpyrrolidine-2-carboxylic acid as the amino acid derivative ligand.Structure analysis shows compounds Ln₄Cr₂ were made of two cubane units sharing faces.Magnetic studies show that compounds Gd₄Cr₂ and Dy₄Cr₂ both exhibit weak antiferromagnetic interactions.2)Two two-dimensional layer metal frameworks 2D-Ln₄Cr₄(Ln=Gd/Dy)featuring Ln-Cr clusters of{Ln₄Cr₄}as metal nodes were obtained under the hydrothermal reaction of high temperature and high pressure by using 2,2-thiodiacetic acid as the multidentate chelate ligand.Magnetic studies show that compound 2D-Gd₄Cr₂has a considerable magnetocaloric effect at low temperatures,and 2D-Dy₄Cr₄ has a significant single-molecule magnet behavior.3)We selected 2,2-dimethylolpropionic acid with small steric hindrance and multidentate coordination sites as the ligand,and obtained two Ln-Cr metal clusters Ln₅Cr₂(Ln=Gd/Dy).When Cl^-and Cl O₄^-as a mixed anion template were introduced,two wheel-shaped high-nuclear Ln-Cr metal clusters Ln₈Cr₄(Ln=Gd/Dy)were obtained.Magnetic calculations show that compound Gd₅Cr₂ has weak ferromagnetic coupling of Gd···Cr and weak antiferromagnetic coupling of Gd···Gd,compound Gd₈Cr₄ has antiferromagnetic coupling of Gd···Cr and Gd···Gd.Magnetocaloric effect studies show that compounds Gd₅Cr₂ and Gd₈Cr₄ have high magnetic entropy changes under low temperature and high field,which are a potential low-temperature magnetic refrigeration material.