Structures and Magnetic Properties of Metal Complexes Based on Multidentate Ligands

Author:Zuo Jing Jing

Supervisor:tang jin kui


Degree Year:2019





Molecular-based magnetic materials have been receiving much attention due to their potential applications in ultrahigh-density information memory components and light/heat switches.Lanthanide ions by virtue of large spin ground-state and strong magnetic anisotropy are the best candidates for the construction of high-performance single-molecule magnets(SMMs).In contrast,transition metal ions with 3d4-3d7 electron configuration can involve the interconversion between the low-spin(LS)and high-spin(HS)states under specific conditions,which makes them the ideal spin carriers for synthesizing spin-crossover(SCO)compounds.With these in mind,in this thesis,a series of polynuclear lanthanide-based SMMs and mononuclear SCO compounds with novel structures and functional properties were prepared,and their magneto-structural correlations were systematically investigated.More details are as follows:1.A series of tetranuclear dysprosium compounds(la-3a and lb-3b)were investigated in detail through a combined experimental and theoretical study,which reveals single-ion magnetic anisotropy and magnetic interactions play a crucial role in modulating the magnetic properties of polynuclear lanthanide-based SMMs.Six compounds with similar skeleton but different terminal ligands demonstrate distinct SMM peoperties.In particular,the modulation of the terminal coordination environment around Dyl/Dy2 sites leads to the transformation of relaxation process from single relaxation(1b)to two-step relaxation(2b).Ab initio calculations substantiate that the excellent SMM property of compound lb should mainly profit from strong ferromagnetic interactions between the individual DyⅢ ions,while different single-ion magnetism results in better SMM property of compound 3a than that of 3b.2.With the use of flexible glutarohydrazode-derived bis-tridentate ligand H2L2 we isolated four aesthetically fascinating lanthanide-based hexanuclear circular helicates(4Dy,4Gd;5Dy,5Gd).Magnetic studies indicate the gadolinium analogues exhibit significant cryogenic magnetocaloric effect,while the dysprosium derivatives display slow relaxation of magnetization with rare toroidal magnetic moments.Ab initio calculations further shed light on the origin of such magnetic behaviors arising from both the low and distorted coordination geometries of DyⅢions,leading to highly anisotropic magnetic centers and the presence of antiferromagnetic intramolecular interactions stabilizing a nonmagnetic ground state.3.Self-assembly of a versatile ligand H4L3 by condensation of pyridazine-3,6-dicarbohydrazide and o-vanillin with different dysprosium salts affords five linear helical clusters(6-10).Their formations are largely ascribed to the various and flexible coordination modes of ligand depending on reaction conditions because of the presence of tautomeric maneuver.Detailed magnetic dynamics studies reveal that the compounds 6-8 display slow magnetic relaxation behavior at a zero dc field.It is noteworthy that although the similar structure is present in all Dy6 compounds,a critical difference in the axial coordination environment results in a drastic change from the antiferromagnetic to ferromagnetic interactions,thus leading to the typical zero-field SMM properties in compounds 9 and 10.Especially compound 10 demonstrates a two-step relaxation processes and higher effective energy barrier.4.The reactions between a Schiff-base ligand(Hmph)and different iron(Ⅱ)salts lead to the formation of four mononuclear iron(Ⅲ)compounds(10-14),where the FeⅢ ions are all distorted octahedral with four N and two O atoms.The different counteranions and solvent molecules directly affect the intermolecular interactions,resulting in discriminative SCO properties.Compared with others,compound 11 with a stronger intermolecular interactions features a completely abrupt two-step SCO behavior with a thermal hysteresis of 8 K.