Fluorescence,Thermal Transformation Regulation And Electrocatalytic Properties Of Transition Metal Complexes Based On N-methyl Bisbenzimidazole Ligands

Complexes have become one of the hotspots in inorganic chemistry due to their diverse structures and potential good physical and chemical properties.Scientists have been devoting themselves to the directional design of the structure of functional complexes so as to effectively regulate their functions.The structure of low-nucleus complexes is convenient for design and synthesis,and it is also helpful to study the functional mechanism,so as to better play the application value of low-nucleus complexes.In this paper,a series of transition metal complexes of iron（II）,cobalt（II）,nickel（II）,zinc（II）and Pt（II）based on bidentate chelate ligands were synthesized by solvothermal method using bibenzimidazole and its derivative ligands.By means of X-ray diffraction single crystal technology,X-ray powder diffraction technology,ultraviolet and fluorescence spectrum,electrospray ionization mass spectrometry（ESI-MS）,nuclear magnetic resonance and density functional theory（DFT）and other methods,combined with the relationship between the structure and luminescent properties,the reasons for the changes of UV and fluorescence spectra of the complexes after UV irradiation in liquid state were discussed.The reason may be the cis-trans isomerization of complexes induced by ultraviolet light.Then,combined with TG-MS,the composition of gas escaped from the pyrolysis of these complexes was analyzed,and the possible pyrolysis mechanism of mononuclear and binuclear complexes and the application of pyrolysis products in the electrocatalytic direction were preliminarily explored.The full text is divided into three chapters:The first chapter is an introduction.It introduces the research progress and application of low core complexes in recent years,the principle of electrospray ionization mass spectrometry and TG-MS mass spectrometry in mass spectrometry technology and their applications in the study of complexes.At the same time,we also introduce some related work done by our group using mass spectrometry,and introduce the research progress of the pyrolysis mechanism of complexes and the exploration of their catalytic properties.Finally,the research background and progress of this paper are introduced.In second chapter,trans-1,2-bis（1-R-benzo[d]imidazo-2-yl）ethylene（R=H,L1;R=L1;L1;CH₃,L2）and 1,2-bis（1-R-benzo[d]imidazo[d]imidazo-2-yl）ethane（L’）as ligands,in methanol solvent,Fe Cl₂·6H₂O,Co Cl₂·6H₂O,Ni Cl₂·6H₂O and Zn Cl₂as central metals,at 140℃,1{（cis-L1）Zn Cl₂},2{（cis-L2）Zn Cl₂},3{（cis-L’）Zn Cl₂},4{（cis-L2）Fe Cl₂},5{（cis-L2）Co Cl₂}and6{（cis-L2）Ni Cl₂}.In DMSO solvents,we found that the absorption intensity of these compounds changed with the time of ultraviolet lamp irradiation.For this reason,the mechanism of time-dependent ultraviolet and fluorescence intensity of compounds 1 and 2 in solution was studied in detail.The fluorescence intensity of compounds 1-2 and 4-6 was enhanced by 2.75,5.10,6.54,3.37 and 2.29 times respectively within 1 minute of 365 nm ultraviolet lamp irradiation.First,L1 and Zn Cl₂were tested for PXRD at different reaction temperatures.It was proved that compounds 1 could be obtained only after the reaction temperature reached 140degrees Celsius,indicating that the energy required for the cis configuration of the trans hitchhiker was relatively high.Then,the mass spectra of compound 1 in methanol solvent were measured by electrospray ionization mass spectrometry and time dependent mass spectrometry under light.It can be seen from pressurized mass spectrometry that compound 1 is stable in solution.In addition,with the increase of illumination time,the abundance of mass spectrometry fragment[1+（Solv.）2-Cl]⁺increases,which indicates that（cis-L1）Zn Cl₂can be transformed into trans-isomers under light induction,which is conducive to the coordination of more solvent molecules（including DMSO/H₂O/CH₃OH）with Zn²⁺to form more stable complexes.According to the composition of typical mass spectrum peaks,these possible structural fragments can be fully optimized and their corresponding Gibbs free energy can be calculated.DFT calculations show that compared with cis-L,Zn²⁺tends to be chelated to trans-L,and cis-L is more easily converted to trans-L under light irradiation.Therefore,considering Zn²⁺from the energy point of view is more conducive to the bridge mode with trans-L.All the results are consistent with those of ESI-MS.Finally,the conversion efficiency of photoisomerization is calculated by¹H NMR.From the change of proton peak area,we can calculate that about 15%of trans-L1 is converted to cis-L1.The conversion rate of cis-configuration of compound 1 to trans-configuration of compound 1 is about 71%in the process of photoisomerization.Because the presence of-CH₃in compound 2 hinders the cis-trans conversion,the conversion efficiency of compound 2 is only34%,which is lower than 71%in compound 1.This strategy for immobilizing unstable isomers through chelation can be used in the development of new photoresponsive coordination compounds.In the third chapter,the thermal decomposition process of compounds 4 and 6 in the second chapter is explored.Compounds 4 and 6 were found to have a residual of about 45%after being heated to 800℃,which is much higher than the metal content.Moreover,the weight loss of the single ligand is complete after being heated to 800℃.It is presumed that when the ligand and the metal salt form a mononuclear complex,the thermal stability of the ligand is improved due to the fixed effect of the metal salt on the ligand.Metal salts play a catalytic role,resulting in a series of polymerization reactions of ligands in the heating process,resulting in the formation of carbon materials.Therefore,we try to detect the escaping gases of compounds in the pyrolysis process by TG-MS,and then characterize and analyze the pyrolysis materials by PXRD,Raman,SEM,TEM and XPS,so as to infer the possible pyrolysis mechanism of compounds 4 and 6.Then the electrocatalytic properties of these pyrolytic materials were explored.At the same time,the Pt₂compound was obtained by using N-methyl bisbenzimidazole ethylene ligand（L2）and Pt Cl₄under solvothermal conditions.The residual amount of this compound is about 74%.Therefore,the pyrolysis mechanism of compound 7 was studied by the same method as compound 4 and 6.The difference between the pyrolysis process of 3D metal and precious metal was found by comparing them.Then,compounds 4,6 and Fe_0.2Ni_0.8were pyrolyzed at 800℃to conduct electrocatalytic oxygen evolution reaction（OER）in 1.0 M KOH electrolyte solution.It was found that the performance of Fe_0.2Ni_0.8@NC-800 was significantly better than that of Fe@NC-800 and Ni@NC-800.Its overpotential corresponding to 10 m A cm^-2was only 290 m V,which further demonstrated that heterostructure was helpful to improve the catalytic activity of the materials.In addition,the electrocatalytic hydrogen evolution reaction（HER）performance of the pyrolyzed sample of compound 7 was tested in the electrolyte solutions of 0.5 M H₂SO₄and 1.0M KOH respectively.It was found that Pt@NC-800 had the best performance in both electrolyte solutions.The overpotential corresponding to 10 m A cm^-2in 0.5 M H₂SO₄was 40 m V,the slope of Tafel was 27 m A dec^-1.And in 1.0 M KOH,the overpotential corresponding to 10 m A cm^-2is26 m V and the slope of Tafel is 95 m A dec^-1.Pt@NC-800 has better electrocatalytic hydrogen evolution performance under acidic conditions.