Modification Of The Structures And Properties Of 3d-4f And 4f Single-molecule Magnets Based On Different Strategies

Single-molecule magnets(SMMs)have atttacted a great deal of interest because of their potential applications in diverse fields including untrahigh-densitity information strorage,quantum computation,and molecular spintronics.Lanthanide-based SMMs(Ln-SMMs)have attracted increasing interest recently due to the unquenched orbital angular momentum and large magnetic anisotropy of the lanthanide centers.At present,the researches on Ln-SMMs mainly focus on the construction of SMMs with lagre energy barrier(Ueff)and high blocking temperature(TB)through the modification of the crystal field strength of LnⅢ ions and increase of the magnetic interactions between them.Although both the Ueff and TB values in Ln-SMMs have been enhanced significantly(maximum values so far being 1541 cm-1 and 80 K,respectively),the destination is yet to be reached.In addition,designing and controlling the properties of Ln-SMMs and exploring the relaxation mechanism of different types are also important in the study of SMMs.In this disertation,on the one hand,we have synthesized several different systems of multi-component Ln-SMMs by adjusting the reaction conditions,such as the anions,pH values,and reaction ratios and so on.These compounds show the characteristics of multi-step relaxation processes,chirality,and so on.On the other hand,we designed and synthesized three 3d-4f SMMs with D5h 3d metals.Their magneto-structural relationships were studied in details.The main research results are listed in the following:1.A series of Ln-SMMs have been synthesized and tuned by the anions.They structures and magnetic properties have been characterized.We designed and synthesized a multidentate Schiff base ligand L1 and successfully prepared eight lanthanide compounds(1Dy-8Yb)using this ligand and different anions(NO3-、Cl-、OTf-).By adjusting the molar ratio of the ligand to Dy(NO3)3 in the reaction,mononuclear compound 1Dy and the bicomponent compound 2Dy can be obtained.Single crystal X-ray analysis indicates that DyⅢ ion in 1Dy is in a ten-coordinated polyhedral coordination configuration of Sphenocorona J87.Interestingly,the unit cell of compound 2Dy contains both the component of compound 1by and the anion component of[Dy(NO3)5]2-.Compounds 5Tb-8Yb are isomorphic with 2Dy.The coordination anion OTf-in 3Dy is slightly stronger than the Cl-ion 4Dy.Therefore,the coordination configuration changed from the Spherical capped square antiprism to Muffin.AC measurements showed that 1Dy-4Dy and 7Er display field-induced SMM properties.The differences in the lattice composition and molecular arrangement of 1Dy and 2Dy result in their different relaxation processes.Different coordination anions not only affect the coordination geometry of the lanthanide ions,but also change the molecule arrangements in the lattice.These effects have a significant effect on the dynamic magnetic properties of this system.2.Five multinuclear lanthanide compounds have been synthesized;and the multi-step relaxation processes in these complexes were investigated.Firstly,we synthesized a polydentate Schiff base ligand H5L2.By adjusting the ratio of ligand to lanthanide and the amount of triethylamine,dinuclear compound 9Dy,heptanuclear compound 11Dy and a series of bicomponent compounds 10Dy,12Tb,13Ho were prepared.Complex 10Dy contains not only the basic unit of 9Dy,but also a "disc" like heptanuclear Dye unit.The crystal structure of 11Dy contains also a Dye unit,which is different from the "disc" unit in lODy.Compounds 12Tb and 13Ho are isostructural with 10Dy.Magnetic measurements showed that 9Dy and 11Dy show frequency dependent ac signals at zero dc field in the frequency region of 1 to 10 kHz,while compound 10Dy exhibits a three-step relaxation process.For 12Tb and 13Ho slow magnetic relaxation was observed with or without an applied dc field.For 10Dy,the two-step relaxation process in the frequency region of 0.1～1 kHz is due to the Dye unit,while the relaxation process in the high-frequency region orginates from the dinuclear Dye unit.3.Ln-SMMs with a[2 × 2]grid structure were synthesized and the magnetic coupling and magnetic-structure relationships between spin centers were studied.Four lanthanide compounds of tetranuclear grid-like structures(14Dy,15Tb,16Ho and 17Yb)were obtained through the reaction of the multi-coordinated ligand H4L3 with different lanthanide chlorides in methanol.Because of the difference coordination solvents,coordination anions and configuration of the ligand,the asymmetric unit of 14Dy-16Ho contain two tetranuclear structural units.The LnⅢ ions are mainly octa-and nine-coordinated in these compounds.For compound 17Y6,the asymmetric unit contains only one tetranuclear structural unit.The YbⅢ ions in 17Yb both have a seven-coordinated pentagonal bipyramidal configuration and an eight-coordinated triangular dodecahedral configuration.The AC measurements show that 14Dy is a zero-field SMM;17Yb displays a field-induced slow relaxation behavior;while compounds 15T6 and 16Ho does not exhibit slow relaxation behavior with or without a dc field.Among these compounds,17Yb is a rare example of a multinuclear YbⅢSMM.4.Syntheses,structures,and magnetic properties of three dinuclear heterometallic MⅡLnⅢ complexes,namely,[MⅡLnⅢ(H2L4)(CH3OH)2(NO3)2](NO3)·S(M = Co,Ln = Dy,S=MeOH(1coDy);M = Zn,Ln = Dy,S = MeOH(2ZnDy);M = Co,Ln = Y,S = MeNO2(3coY),H4L4 = 2,6-diacetylpyridine bis[2-(semicarbazono)propionylhydrazone)have been prepared and investigated.Synthesized from the pre-designed multidentate ligand H4L4,which has two different coordination pockets(smaller N3O2 and larger N2O4 pockets)suitable for either a 3d or a 4f metal center,all these complexes have very similar structures,where the MⅡ centres possess a pentagonal bipyramidal geometry and the LnⅢ sites have a tetradecahedron geometry.Magnetic measurements on these compounds revealed the existence of weak ferromagnetic coupling between the Co2+ and Dy3+ centers and the field-induced slow magnetic relaxation of all three complexes.Furthermore,theoretical calculation on all these complexes indicates that although the change of the diamagnetic Zn2+ ion to the paramagnetic Co2+ ion only slightly modifies the local magnetic anisotropy of the Dy3+ ion,the weak Co-Dy magnetic interaction decreases the energy barrier.These compounds are the first systematic results of a heterometallic 3d-4f SMM containing pre-designed PBP 3d metal ions.