| Single-molecule magnets(SMMs)are a hot topic in the field of molecular magnetism research.Among them,the research related to transition metal single-molecule magnets is of interest.Despite the continuous breakthroughs in the research of low-coordination transition metal single-molecule magnets,they still cannot meet the requirements of realistic applications.The purpose of this paper is to explore the design and synthesis strategies of low coordination transition metal single-molecule magnets(3d-SMMs).several cases of low coordination magnetic complexes were successfully synthesized by using iron,cobalt and chromium ions as paramagnetic centers and β-diketiminate and monodentate amido ligands.The magneto-structural relationship and magnetic relaxation mechanism were discussed by theoretical calculation.The main research content is as follows:1.A Fe(Ⅲ)-based complex[LFe(Cl)2](1)(L=[PhC-(PhCN-Dip)2]-,Dip=2,6-iPr2C6H3)was constructed by combining Fe(Ⅲ)ions and β-diketiminate ligand with the bidentate chelating pattern and dynamic protective effect.Subsequently,a Fe(Ⅱ)complex[LFeCl](2)was obtained by reduction of complex 1.The structural analysis shows that complex 1 exhibits a twisted tetrahedral geometry and complex 2 has a triangular plane geometry,but both complexes belong to mononuclear structure.Combined experiments and ab initio calculations support the observation of field-induced single-ion magnet(SIM)behaviour for 2 with an effective spin-reversal energy barrier of Ueff=40.02 K,as a direct consequence of the large intrinsic magnetic anisotropy(easy axis)arising from out-of-state spin-orbit coupling in the trigonal planar Fe(Ⅱ)complex.Noteworthy butterfly hys-teresis loops were observed from 2 up to 8 K,which represents the first case in Fe(Ⅱ)complex.2.A binuclear complex[LCo(μ-Cl)]2·2C6H14(3)and a monuclear complex[LCoC]Py]·0.5C7H8·0.5C6H14(4)have been constructed by using Co(Ⅱ)ions and β-diketiminate ligand.The analysis of single crystal structure shows that the tetra-coordinate Co(Ⅱ)ions in both complexes exhibit tetrahedral geometry with different structural distortion.Magnetic measurements showed that only complex 4 exhibited a field-induced single-molecule magnet signal with an effective spin reversal energy barrier of 16.27 K,despite the large easy-axis magnetic anisotropy of both compounds revealed by ab initio computational analysis.Dimeric CoⅡ-CoⅡ units in 3 indicate intramolecular antiferromagnetic interaction.Compared to individual Co(Ⅱ)fragment of 3,the first excited quartet state of 4 is strongly destabilized,leading to a smaller unquenched orbital angular momentum and D value.3.A novel CrⅡ-dimeric complex,[CrⅡN(SiiPr3)2(μ-Cl)(THF)]2(5),has been successfully constructed using a bulky silyl-amide ligand.Single-crystal structure analysis reveals complex 5 exhibits a binuclear motif,with a Cr2Cl2 rhombus core,where two equivalent tetra-coordinate CrⅡcentres in the centrosymmetric unit display quasi-square planar geometry.The calculated electronic structure of 5 emphasizes that the unpaired electron is concentrated on the CrⅡ center,giving rise to ferromagnetically coupled ｛Cr2Ⅱ} dimers.The HF-EPR spectroscopy supports that 5 presents a negative D value and large ratio of |D/E|,implying a Ising-type magnetic anisotropy.Ac magnetic susceptibility data unveil 5 features slow dynamic magnetic relaxation typical of single-molecule magnet(SMM)behaviour with an effective energy barrier of 22 K under zero dc field.Moreover,magnetic studies and theoretical calculations point out that a non-negligible ferromagnetic coupling(FMC)exists in the dimeric Cr-Cr units of 5.The coexistence of magnetic anisotropy and FMC contributes to the first example of chromium(Ⅱ)-based SMM in the absence of a dc field. |
PDF Full Text Request