Design, Synthesis And Magnetism Of M₅L₆ Supermolecules With D₃ Symmetry

Recent years obtaining materials with dual functionalities or multifunctionalities attracts considerable attention,e.g.,conductivity and magnetism,magnetic and optical properties,porosity and magnetism,and chirality and magnetism,etc.Particularly,since magnetochiral dichroism(MChD) effect was observed,the design of molecular materials with interesting chirality and magnetism has become an intense new area of investigation at the forefront of supramolecular chemistry,because these materials are not only of academic interests such as asymmetric magnetic anisotropy,magnetic chirality,and magnetochiral dichroism but also have potential for applications as new devices.However, it still remains difficult to predict structures and magnetic behaviors due to the diversity of coordination and the complicated nature of magnetic interactions.As a consequence few examples of this type of magnet exist.So intelligent ligand design and proper metal centers choice are the main keys to the design of such coordination polymers.To be interested in this molecular materials and contribute to magneto-structural work on M₅L₆ systems with D₃ symmetry,we report herein the rational design,synthesis, single-crystal structures,configuration,magnetic behaviors and mechanism of the reaction under hydrothermal conditions of the new family of M₅L₆ isomorphous complexes.The main conclusion is as follows:(â…°) On the basis of pentanuclear cluster with C_3h symmetry,we constructed a new M₅L₆ cluster model with D₃ symmetry firstly and put a planar tridentate ligand, (Z)-1,2-di(1H-benzo[d]imidazol-2-yl)ethenol,which combines a bridging alkoxide oxygen and benzimidazole nitrogen donors together and has never been reported,in our M₅L₆ model.This rigid ligand was capable of controlling the structure of production and transferring the magnetic interactions effectively between metal ions.(â…±) Based on the model,we synthesized the ligand and ten new complexes under microwave and hydrothermal conditions respectively.These complexes were characterized by IR,UV,TG-DTA,EA,ICP and crystal structure,and analysis revealed that they were isomorphous clusters with the same space group P2_1/c and the similar crystal data,as a[13.019(2)～13.330(3)(?)],b[26.132(5)～26.880(2)(?)],c[28.969(2)～29.237(5)(?)], andβ[104.976～106.535(9)°],and can be expressed by M₅L₆·X_nY_4-n·mH₂O(X and Y represented different anions).There were six homopentanuclear clusters with Ni^â…¡and Co^â…¡and four heteropentanuclear clusters with Ni^â…¡-Cu^â…¡and Cu^â… ,Ni^â…¡-Zn^â…¡,Co^â…¡-Zn^â…¡and Fe^â…¡-Zn^â…¡.All of complexes were comprised of a[M₅L₆]⁴⁺ cation,four univalent anions and solvents except for complex 7.The similar cation constructed by six planar ligands around the M₅O₆ core,which were divided into three roughly parallel pairs with interplane distances in the range 3.3-3.5(?),and each pair was approximatively perpendicular to the others.Five metal ions were in trigonal bipyramidal geometry and displayed two different coordination environments:the two apex metal ions laying on the C₃ axis had octahedral coordinated geometries and the other equatorial metal ions had tetrahedral coordinated geometries.The entire complex cation,therefore,had D₃ symmetry.Complex 7 which was different from other structures put up a linear monovalent copper coordinated by a water and one benzimidazole N.Therefore,ignoring the attached monovalent copper atom, the cation of this complex was the similar to other complexes.The X-ray single crystal diffraction and circular dichroism(CD) spectrum both confirmed the chirality of the complexes,which accorded with our project.(â…²) Tubular metal-organic complexes are very rare.Up to now,only one has been described.In my study,tube morphology can be observed in complexes containing nickel(â…¡) ions except for using nickel(â…¡) nitrate as reactant.These tubular crystals showed hollow tetragonal or hexahedral prisms and all formed with an open and a closed terminis. The length of the tubes varied from 250 to 750μm and their diameters were in the range of 30-70μm.The wall thickness(ca.15μm on average) varied greatly.Among the complexes,Ni^â…¡-Cu^â…¡and Cu^â… system had the best image with glazed surface,and others had crackles or finestras.In order to probe the factors which may affect the formation of the tubular crystals,we varied the length of heating time,temperature and the pH.Tubular crystals can be produced with temperatures varying from 423 to 448 K and heating time longer than 92h with pH between 4 and 7.(â…³) The magnetic behaviors of all these M₅L₆ complexes showed dominant antiferromagnetic interactions,which were consistent with large M-O-M bridge angles. But there were also many differences in their magnetic properties.As for simplex Ni systems,the shape and value of the x_mT versus T plot were similar,so they showed the similar antiferromagnetism though with different aions and solvent molecules;when Cu(â…¡) and Cu(â… )joined,because of their magnetic ions and small quantity,the influence was puny and the magnetic behavior was similar to the simplex Ni systems;but when the nonmagnetic ion Zn(â…¡) joined with large proportion,the system exhibited spin-canted antiferromagnetism at low temperature and the critical temperature was at 8K.This phenomenon prompted us that the system doped with nonmagnetic Zn(â…¡) ion may change the magnetism of the complexes.As for simplex Co systems,there were obvious differences in magnetic moment,though they had the similar cations.It may be the results of tiny differences between their main structures and the large anisotropy of Co(â…¡).The Fe^â…¡-Zn^â…¡system had the weak ferromagnetic interactions in the high temperature.These phenomenon that the magnetism of complexes can be changed when the system doped with Zn(â…¡) ion may create a new investigative field of molecular-based ferromagnets.Based on the different distances between Ni(â…¡) ions,we defined three exchange parameters(J) in the systems containing Ni₅O₆ core.Best-fit parameters applying the spin matrix method showed all J negative,which confirmed the overall anti-ferromagnetic exchange within these clusters.DFT study also confirmed this conclusion.(â…´) Because of trying different reactive schemes,we obtained other complexes which were composed of the fragment of ligand(Z)-1,2-di(1H-benzoimidazol-2-yl)ethenol besides the M₅L₆ clusters,so we brought forward the possible mechanism of the reaction under hydrothermal conditions.