Syntheses And Magneto-Structural Correlations Of(3d/4f)-4f Polynuclear Lanthanide-based Single-Molecule Magnets

Since Single-Molecular Magnets(SMMs)exhibit magnetic bistability at the molecular level and behave the dual characteristics of classical mechanics and quantum mechanics,it is of great significance in both the information storage device applications and the extension of new theories.Polynuclear lanthanide-based SMMs,with multiple metal centers assembled in one unit,exhibits novel structural architecture and peculiar magnetic properties.Relying on the unique exchange coupling and various spin topologies of the polynuclear system,optimizing of the magnetic relaxation under complicated and volatile conditions by molecular design and regulation,as well as understanding of the effects of structural factors on magnetic anisotropy and magnetic exchange coupling can provide new opportunities for the development of lanthanide-based SMMs.In this dissertation,for the purpose of the investigation of the magnetic relaxation behavior of polynuclear lanthanide-based SMMs,a series of novel(3d/4f)-4f polynuclear lanthanide-based complexes have been synthesized by using Schiff bases or phosphine oxide ligands as the main ligands,as well as halogen anions or carboxylate ions with variable coordination modes as the auxiliary ligands.Combined with detailed magnetic analysis and ab initio calculation,the magnetic relaxation mechanism and the magneto-structural correlations of these specific structures are deeply explored.The specific research content is as follows:1.With the compartmental Schiff-base ligand,1-(2-hydroxy-3-methoxybenzylidene)-semicarbazide(H2hms),and acetic acid anion coligands to bond paramagnetic NiII/CoII ions and LnIII ions,a series of novel zigzag-shaped tetranuclear heterometallic complexes 1-6({MII2LnIII2},MII = NiII/CoII,LnIII = DyIII,GdIII and YIII)have been prepared.Ac susceptibility measurements show that both the DyIII-based complexes 1 and 4 exhibit zerofield SMM behavior,and 1 exhibits the absence of the quantum tunnelling of the magnetization(QTM)for the frequency-dependent behavior.Combined with CASSCF calculation,it is shown that the magnetic exchange between 3d and 4f ions can inhibit the QTM process to a large extent.Compare to the antiferromagnetic interaction of Dy-Co in 4,ferromagnetic ones of Dy-Ni in 1 exhibits better frequency dependent behavior with fully suppressed QTM.Remarkable difference in exchange coupling is mainly arise from different anisotropy of 3d metal ions and slightly different bridging parameters.The results provide a new paradigm for understanding the efficient suppression of zero-field QTM by magnetic interaction.And it show that the introduction of paramagnetic 3d metal ions is beneficial to the effective exchange with 4f ions and to the improvement of the magnetic relaxation behavior of DyIII.2.With the compartmental Schiff-base ligand H2 hms and benzoic acid/acetic acid coligands to bond diamagnetic ZnII ion and DyIII ion,two linear-shaped heterometallic trinuclear [Dy Zn2(Hhms)2(RCOO)4]·RCOO(R = C6H5(7)and CH3(8))have been prepared.Magnetic dynamics studies show that 7 and 8 and their diluted samples all show two-step relaxation process.By comparing the dynamic magnetic behavior of diluted and undiluted samples,the dipole interaction between molecules is the main reason for the two-step relaxation of the pure samples.Further structure analysis shows that complex 7 has two crystallographic nonequivalent DyIII centres with the same coordination composition but different coordination orientation of benzoate ligands.Although there is only one asymmetric unit in complex 8,the severely disordered acetate also leads to two different orientations of the molecule.Based on ab initio anaysis,the existence of two nonequivalent magnetic anisotropy units in each complex may be the main reason for the two-step relaxation of diluted samples.Discussion from the intermolecular interaction and structural inequivalence,this work provides a reference model for the study of multiple relaxation mechanism of SMMs.3.The first bromine-bridged dinuclear [Dy(Cy3PO)2(?-Br)(Br)2]2·2C7H8(9)SMM was obtained by the reaction of Cy3 PO with Dy Br3.The asymmetric DyIII centers present attractive structural characteristics of cis/trans stereoisomers,where Dy1 in the trans configuration exhibits a highly axial ligand field environment.The static susceptibility of the isomeric Gd? complex(10)is studied,and the weak magnetic interaction between Gd···Gd is proved by PHI fitting.Furthermore,the static and dynamic magnetic properties of Dy? complex are investigated.Combined with ab initio calculations,the relaxation barriers,g-factors and the difference of the anisotropy axes for two DyIII centres are quantified,and the magnetic exchange coupling mechanism between DyIII ions in cis and trans configuration and the magnetic relaxation path of the molecular are further obtained.Through the above research,the inherent magneto-structural correlations are established.The work provides the new understanding for bromine-bridged lanthanide SMM as well as the polymetallic lanthanide SMMs with the perpendicular anisotropy arrangement.4.Based on the dinuclear system at the third part,a series of iodine-bridged binuclear complexes 11-13 [Ln(Cy3PO)2(?-I)(I)2]2·4C7H8(Ln = GdIII(11),DyIII(12)and ErIII(13))were constructed by introducing weaker coordinating iodine anions with the strong phosphine oxide ligands.In Dy2 complex,the cis/trans arrangement of strong Cy3 PO moieties combined with weak iodine ligands around the DyIII centers produces a SMM behavior with a large anisotropic barrier of ca.1300 K and hysteresis loops up to 16 K.On the basis of dilution studies and ab initio calculations,we demonstrate that such exceptional properties originate from the combination of large single-ion anisotropy at trans site and the cancelation of magnetic interactions through near-orthogonality of the magnetic axes.It is shown that eliciting significant magnetic anisotropy of lanthanide ions is the most crucial factor in the design of high-performance polynuclear lanthanide SMMs.Simultaneously,making full use of the structural feature of cis/trans stereoisomer,the effects of discrepant anisotropy induced by metal ions with distinct 4f-electronic distributions(namely oblate DyIII,prolate ErIII and spherical GdIII)on exchange interaction are investigated by detailed magnetic test and ab initio calculations.