| Different from the classical magnetic materials,single-molecule magnets(SMMs)can exhibit slow magnetization relaxation at the nanometer scale.Their unique magnetic properties make them to have potential applications in high-density information storage,quantum computation and molecular spintronics.As a kind of special single-molecule magnets,single-ion magnets(SIMs)have attracted much attention because their magnetostructural correlation could be easily revealed..Since Long et al.reported the first single-ion magnet based on high spin Fe(Ⅱ)in 2010,the research on single-ion magnets based on 3d transition metal ion has made rapid progress.Theoretically,the SMM behavior,that is slow magnetic relaxation,is often observed in the systems with the ground state spin greater than 1/2.However,few examples of S=1/2 systems have been reported to display slow dynamic relaxation.The research presented in this dissertation concentrates on the magnetic anisotropy and dynamic magnetic properties of several Co(Ⅱ)complexes with different coordination number(4 to 6)and one five-coordinate Cu(Ⅱ)complex.The magnetostructural correlation has been also been probed.The main results are presented here as the following four parts:1.Magnetic anisotropy and slow magnetic relaxation of mononuclear four-coordinate cobalt(Ⅱ)complexes with oxamideThree mononuclear four-coordinate Co(Ⅱ)complexes(HNEt3)2[Co(L1)2]·H2O(1),(HNEt3)2[Co(L2)2]·H2O(2)and(Bu4N)2[Co(L2)2]·H2O(3)(where H2L1=N,N’-bis(p-toluenesulfonyl)oxamide,H2L2=N,N’-bis(methanesulfonyl)oxamide and H2L3=N,N’-diphenyloxamide)have been successfully synthesized by using three different oxamide ligands.Although the steric hindrances of oxamide ligands are different,single crystal structural analyses show that the coordination configurations of three compounds are the distorted tetrahedral geometry.The strong ligand field and axial distortion lead to the large axial anisotropy and small rhombicity.High-field and high-frequency electron paramagnetic resonance(HFEPR)measurement confirm the presence of a very large and negative axial anisotropy.Complexes 1-3 display slow magnetic relaxation in the absence of an applied dc field.In addition,it can be found that complexes 1 and 3 display hysteresis loops with different field sweep rates at 1.8 K,which is rarely observed for Co(Ⅱ)single-ion magnets(SIMs).2.Slow magnetic relaxation of five-coordinate spin-crossover cobalt(Ⅱ)complexesBy using 1,4,7,10-tetramethyl-1,4,4,10-tetra-cyclododecane(12TMC)as a ligand,we synthesized two mononuclear five-coordinate Co(Ⅱ)complexes[Co(12-TMC)(CH3CN)](BF4)2(4)and[Co(12-TMC)(CH3CN)](PF6)2(5)with different anions.The coordination configurations of 4 and 5 are distorted square pyramidal geometry.The direct-current(dc)magnetic data of 4 and 5 display an incomplete spin-crossover behavior nearly at room temperature.Both compounds exhibit the field-induced single-molecule magnet behavior,and the slow magnetic relaxation behavior originates from the low spin state of Co(Ⅱ)ion.By combining the magnetic data and EPR data,the slow magnetic relaxation behavior of both compounds were further confirmed to result from the anisotropy of g factor.The diluted samples 4’ and 5’ in the diamagnetic matrix[Zn(12-TMMC)(CH3CN)](X)2(X=BF4-,6;X=PF6-,7)preserve slow magnetic relaxation,further suggesting that the magnetic dynamics is intrinsic to the individual[Co(12-TMC)(CH3CN)](X)2 molecules.These are the first observations of the coexistence of spin-crossover and slow magnetic relaxation in five-coordinate mononuclear Co(Ⅱ)complexes.3.Coordination geometries and slow magnetic relaxation processes of cobalt(Ⅱ)-halide and-pseudohalide complexesThree mononuclear five-coordinate Co(Ⅱ)complexes[Co(L)X2],where L is a macrocyclic ligand(1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane,12TMC),with different halogenido ions Cl-(8),Br-(9)and I-(10),were successfully synthesized.All three compounds have the distorted square-pyramidal geometry.Positive D parameters expressing an easy-plane magnetic anisotropy were found for all complexes,with the D values in the trend of D(I-<D(Cl-)<D(Br-).Dynamic magnetic properties of three compounds were found to be different Complexes 8 and 9 show field-induced slow magnetic relaxation,while slow magnetic relaxation for 10 is not observed at 1.8 K under different dc external fieldsFour mononuclear Co(Ⅱ)-pseudohalide complexes[Co(12TMC)(NCS)2](11),[Co(12-TMC)(NCSe)2](12),[Co(12-TMC)(N(CN)2)2](13)and[Co(12TMC)(NCO)][B(C6H5)4](14)with different pseudohalido ions have been prepared.Complexes 11,12 and 13 are six-coordinate and present distorted octahedral configurations,while complex 14 is five-coordinate with a distorted square pyramidal configuration.Easy-plane magnetic anisotropy for 11-13 and easy-axis anisotropy for 14 were revealed via the analyses of the direct-current magnetic data.Four complexes display slow magnetic relaxation under an external dc field.However,the relaxation process and mechanism are quite different.Complexes 11 and 12 show two slow relaxation processes under an external dc field,while only one relaxation process occurs in 13 and 14.The Raman-like mechanism is found to be dominant in the studied temperature range in complex 11.But for complexes 12-14,the Raman process is dominant in the low temperature region,while the Orbach mechanism contributes to some extent in the high temperature range.4.Slow magnetic relaxation of a five-coordinate copper(Ⅱ)complexA mononuclear five-coordinate Cu(Ⅱ)complex[Cu(12-TMC)Cl][B(C6H5)4](15)has been synthesized and characterized.Single crystal X-ray diffraction data show that complexe 15 presents a slightly distorted square pyramidal geometry.Complex 15 exhibits slow magnetic relaxation under an external dc field,which is originated from the anisotropy of g factor.The diluted samples 15’ in the diamagnetic matrix[Zn(12-TMC)Cl][B(C6H5)4](16)preserves slow magnetic relaxation,further suggesting that the magnetic dynamics is intrinsic to the individual five-coordinate Cu(Ⅱ)centre.It should be stressed that slow magnetic relaxation has been rarely observed in S=1/2 system. |
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