Synthese And Single-molecule Magnet Properties Of Dinuclear Lanthanide-carboxylate Coordination Compounds

Single molecular magnet(SMM)is composed of a relatively independent and magnetically non-interacting single molecule,which can show slow magnetic relaxation behavior within a certain temperature range and can be magnetized as a magnet.The magnetization is derived from the single molecule,rather than resulted from a large number of spin carriers in a crystal through magnetic interactions and long-range magnetic ordered for a conventional magnet.Similar to a conventional magnet,each discrete single molecule can be as an independent magnet.Thus SMMs have potential applications in high-density information storage devices,quantum computing and spin electronics.Lanthanide coordination compounds is one of the important kind of molecule-based magnetic materials.The lanthanide ions,especially for heavy lanthanide ions such as Dy?,Tb?,and Er?,usually exhibit a large ground-state spin and a significant spin-orbit coupling that generates strong magnetic anisotropy,which leads to an energy barrier opposing the reversal of the magnetization,resulting in slow magnetization relaxation for a SMM.Thus the lanthanide ions becoming the favourable candidates for constructing of the SMMs.In this thesis,the rigid carboxylate ligands with large backbone are selected for bridging the LnIII ions to form the Ln2 dimer through the carboxyl groups,which will air-stable materials due to the strong oxygen-affinity of the lanthanide ions.The central Ln2 core is protected by the peripheral carboxylate ligands with large skeleton,isolating the dimers from each other effectively.As a result,the magnetic coupling between molecules is avoid and favored for a magnetic relaxation.The 9-anthracenecarboxylic acid and 2-phenyl-4-quinolinecarboxylic acid organic ligands were selected as bridging ligands and the N-containing chelating ligands were served as the terminal ligands for these lanthanide coordination compounds.Ten dinuclear lanthanide coordination compounds were provided in this thesis and were characterized by single-crystal X-ray diffraction,infrared spectroscopy and thermogravimetric analysis.The magnetic properties of the lanthanide coordination compounds have been investigated and discussed in detail.Four chapters are presented in this thesis.The first chapter briefly introduces the history of traditional magnetic materials and the concept and development of new SMMs materials.The research ideal for this thesis is presented in this chapter.In the second chapter,seven dinuclear lanthanide coordination compounds with the 9-anthracenecarboxylate ligand have been synthesized under solvothermal reactions.The 9-anthracenecarboxylate(anthc)ligands bridge two DyIII ions to form a dinuclear compound of Dy2(?2-anthc)4(anthc)2(2,2'-bpy)2(1)with the terminal N-containing ligands of chelating 2,2'-bipyridyl(2,2'-bpy).Interestingly,compound 1 displays a slow magnetic relaxation in the absence of the direct-current(dc)filed,which is a typical SMM.It is a rare example of the lanthanide-carboxylate SMMs because most of the reported lanthanide-carboxylate compounds showed no slow magnetic relaxation or are field-induced magnetic relaxation.Theoretical calculations revealed that the DyIII ions in the dinuclear molecule exhibit strong anisotropy crystal field,leading to a strong easy-axis for the DyIII ions,which resulted in the SMM behavior of compound 1.Remarkably,two other isostructural Dy-compounds of 1,Dy2(?2-anthc)4(anthc)2(phen)2(2)and Dy2(?2-anthc)4(anthc)2(4,7-dm-phen)2(3),were obtained.If we take compound 1 as the parent structure,compounds 2 and 3 can be obtained just by sequentially modifying the terminal 2,2'-bpy ligand with substituent groups.Thus,these compounds have the similar Dy2(?2-anthc)4(anthc)2(2,2'-bpy)2 molecular structures wherein the DyIII ions have a very similar first coordination sphere,only differing in the outer 2,2'-bpy moiety.From 1 to 3,an increase in electron density on nitrogen atoms that form the five-atom hard plane of a DyIII center associated with three carboxylate oxygen atoms,is observed as the phenyl and methyl substituent are added sequentially,which leads to the slight decrease of the energy barrier(Ueff(1)> Ueff(2)> Ueff(3))in this Dy2 system.As a result,we presents a rational method to modulate the dynamic magnetic relaxation of the lanthanide SMMs through fine-tuning electrostatic potential of the atoms on the hard plane.Two other isostructural compounds of 2,namely,Tb2(?2-anthc)4(anthc)2(phen)2(4)and Er2(?2-anthc)4(anthc)2(phen)2(5),were obtained for studing the magnetic dynamic behavior of the different lanthanide centers under the similar crystal field.However,no slow relaxation phenomenon in the absence of an applied dc field was observed for the terbium and erbium analogue 2,which may result from a fast magnetization relaxation.The difference in the magnetic behavior between 2 and 4 and 5 in the present study may be related to the crystal field splitting energy diagrams of these ions.With the first coordination sphere of DyIII centers being fixed from 1 to 3,the magnetic dynamic behaviour was modulated by sequentially modifying the electrostatic potential of the nitrogen atoms of the terminal N-containing ligands.Thus the novel dinuclear Dy-compounds without the terminal N-containing ligands in the similar Dy2 system will show different magnetic behavior.Under similar reactions without the N-containing ligand,two compounds of Dy2(?2-anthc)4(anthc)2(DMF)2(6)and Dy2(?2-anthc)2(anthc)4(H2O)2(DMF)2(7)were isolated.The two compounds have the similar dinuclear structures to compounds 1–5.But the coordination number of 8 and the coordination environments of the DyIII centers are different from those compounds 1–5.Magnetic measurements showed that compounds 6 and 7 exhibit no slow magnetic relaxation without the static magnetic filed,revealing the DyIII ion shows the strong magnetic easy-axis only in the suitable crystal fields.Three Dy-compounds,Dy2(?2-phqc)4(phqc)2(2,2?-bpy)2(8),Er2(?2-phqc)4(phqc)2(2,2?-bpy)2(9)and Tb2(?2-phqc)4(phqc)2(2,2?-bpy)2(10),constructed from the 2-phenyl-4-quinolinecarboxylate and 2,2'-bpy ligands were presented in the third chapter.All these three compounds have a dinuclear Ln2 structuture with the lanthanide centers have a coordination number of 8 and a [Ln O6N2] coordination sphere.Magnetic measurements showed that compounds 8 – 10 exhibit no slow magnetic relaxation without a dc applied filed,revealing the magnetic behavior of the lanthanide ions is significantly affected by their crystal fields.The last chapter of the thesis presents the summary of this work.