Study On The Structure,Assembly Process And Magnetic Regulation Of 3d Metal Cubic Clusters Based On Hydroxymethyl Benzimidazole Ligands

Single molecule magnetisms are a phenomenon of magnetic moment blocking in a single molecular cluster,which show great potential applications in quantum computing,high density information storage and so on.It has always been one of the challenging topics in the field of molecular based magnetic materials that how to construct directly single molecule magnetisms,and explore the relationship between cluster to cluster.In this thesis,we choose the key ligands with that have the coordinating atoms pointing on one side of the molecule to synthesize a series of polynuclear cluster by solvothermal with the Fe²⁺and Co²⁺metal salt.Through ligand modification,different solvents and different coordination anions were selected to influence the interrelation between cluster units and cluster units,and then to regulate the macroscopic magnetic behavior of molecular cluster complexes on micro and nano scale.This paper is divided into three chapters:The first chapter introduces the research background and current research focus of this paper,focusing on the experimental examples and theoretical progress of single-molecule-magnetism and the latest developments.At the same time,the strategies and methods of regulating the behavior of single-molecule-magnetism are introduced.The application of ESI-MS in solution coordination chemistry and the new ideas and strategies for the study of the assembly process of clusters are also introduced.In chapter 2,we report that a clear dependence on the ligand has been observed for the magnetic properties of a closely related series of Co（Ⅱ）cubane structures,viz.[Co₄（mbm or bm）₄（ROH）₄X₄]（R=-CH₃ or-CH₂CH₃,X=Br or Cl）,1-MeOH-Br,1-EtOH-Br,2-MeOH-Br,2-EtOH-Br,1-MeOH-Cl,1-EtOH-Cl,2-MeOH-Cl and 2-EtOH-Cl,where 1=[Co₄（mbm）₄Br₄],2=[Co₄（bm）₄Br₄],bm=（1H-benzo[d]imidazol-2-yl）methanolate and mbm=1-methyl-bm.The[Co₄（OR）₄]cubane core consists of octahedral Co^Ⅱ center is chelated by the alkoxide oxygen and imidazole nitrogen atoms from mono-anionic bm or mbm,and coordinated by methanol/alcohol and bromine.Interestingly,electrospray ionization mass spectrometry（ESI-MS）indicates that 1-MeOH and 2-MeOH are unstable in methanol and transformed to the butterfly[Co₄L₆]²⁺and get its structure[Co₄（mbm）₆Br₂]·3CH₃OH for 1-MeOH-Br,the solution of 1-MeOH-Br for five days later is transformed to the discotic[Co₇（mbm）₁₂]²⁺and get its structure[Co₇（mbm）₁₂]·Br₂,but 1-EtOH and 2-EtOH are stable in ethanol.Their magnetic susceptibilities suggest ferromagnetic coupling between nearest cobalt centers to give a theoretically S=4×3/2 ground state with considerable magneto-crystalline.The packing and intermolecular interactions appear to influence the geometry of the cubes and thus the anisotropy of cobalt which leads to different blocking temperature（T_B）.Consequently,the compounds with mbm,1-MeOH-Br and 1-EtOH-Br,exhibit T_B>2 K as shown by the relaxation of magnetization in zero applied dc-field where the barriers U_eff/k _B are respectively 27 and 21 K,and relaxation timeτ₀=1.3×10^-9 and 9.7×10^-9 s.The results of structure to structure analysis showed that the magnetic properties of Co（Ⅱ）cubic structure were obviously dependent on the ligand.These findings reaffirm the subtle dependence of single-molecule-magnetism on coordination geometry and intermolecular interaction.In chapter 3,we report a very rare observation of a single crystal-to-single crystal post-synthetic modification of a non-porous molecular solid in air.Fe₄^Ⅱ（mbm）₄Cl₄（MeOH）₄（1）,（mbm=（1-methyl-1H-benzo[d]imidazol-2-yl）methanolate）,synthesized solvothermally as light yellow rhombic crystals from FeCl₂·4H₂O,and Hmbm neutralized with triethylamine in methanol,has a distorted Fe₄O₄ cubic core.On exposure of the crystals to the atmosphere its color changes gradually to opaque black,but without apparent change of crystal size and shape.After 2 days the black crystal diffracts just as well and its structure reveals a chemical reaction to a new compound,[Fe₄^Ⅲ（mbm）₄Cl₄（OH）₄]·2H₂O（1-2d）,also having the cubic core.Four changes were noted;an exchange of methanol to hydroxide,oxidation of Fe（Ⅱ）to Fe（Ⅲ）with the expected changes in bond parameters,change of stereochemistry by rearrangement of the Cl atom via a possible five-coordinated intermediate and hydration.The presence of cluster was confirmed by ESI-MS of the dissolved crystals in CH₃CN.Theoretical calculations reveal that the stability of 1-2d is less than 1,only when the voltage is added it can break into different peaks.The data of ESI-MS reveals that there are no peaks associated with 1 in the mother liquor,but get other peaks which are[Fe₂^Ⅱ（C₉H₉N₂O）₃]⁺and[Fe₇^Ⅲ（C₉H₉N₂O）₉（O）₃（CH₃O）₃Cl]²⁺.A light yellow clear hexagonal columnar crystal Fe7L9（3）was obtained by volatilizing mother liquor,the molecular formula is[Fe₇^Ⅲ（C₉H₉N₂O）₉（O）₃（CH₃O）₃Cl]·（CH₃O）₂·5CH₃OH.Increasing the amount of triethylamine in the reaction solution to obtain a brown yellow block butterfly structure compound Fe4L6（2）,the molecular formula is[Fe₄（C₉H₉N₂O）₆Cl₂]·H₂O·2CH₃OH.