Synthesis Ni/Fe Heterobimetallic Complexes Based Pyralate Pincer Ligand And Redox Properties Study

Nitrogen cycle is the material cycle of an ecosystem that describes the transformation process between nitrogen and nitrogenous compounds in nature.The N₂ gas is a very stable compound due to the strength of the triple bond between the nitrogen atoms,and it requires a large amount of energy to break this bond.To date,the industy Haber-Bosch process is the most way to active the dnitrogen molecule,which cause worsening environmental pollution.Nitrogenase in plants can catalyze the activation of nitrogen under normal temperature and pressure to achieve nitrogen to ammonia conversion,which has attracted the attention of many researchers.Moreover,simulating biological enzymes to immobilize and activate nitrogen has been a hot research topic for researchers at home and abroad.And most active site of the biological enzymes contains two or more metals,such as nitrogenase（Fe/Fe,Fe/V,and Fe/Mo cofactors）;Ni/Fe hydrogenase enzyme,carbon monoxide dehydrogenase（nickel iron）etc.,which have attracted widespread attention from chemists to focus on the polynuclear organometallic compounds.However,owing to unreproducibility,low yiled and high reactivity of the heteronuclear and polynuclear compounds,results in this chemistry has huge gap behind the mononuclear and homo-bimetallic chemistry.Inspired by the nitrogenase enzyme and nickel iron hydrogenase core,combined with the existing problems in heteronuclear compounds,this thesis attempts to synthesize Ni/Fe heterobimetallic compounds,hoping to achieve the fixation and activation of inert molecules such as dinitrogen,carbon monoxide and nitric oxide,in the Ni/Fe heterobimetallic complex.In this thesis,a new“two-in-one”pincer ligand scaffold consists of two PNN pincer-type subunits and a 3,5-substituted pyrazole as a bridging unit used for heterobimetallic compound 1（[LNi（μ-Br）Fe（Br）]OTf）])construction.The designed product LNi（μ-Br）Fe（Br）]OTf)]was synthesized in gramme scale through the priority of metal salt and base,solvent volume,reaction time and so on.Based on the Ni/Fe heterobimetallic compound 1,the nitrogen fixation and activation were studied from the heterometallic hydrides and azide complexes.Therefore,two parts of research works were carried out.（1）Syntheise,Structure and Redox propeities study of the nickel/iron heterobimetallic complex.In the first part of this thesis,organic experiments were conducted to synthesize the“two-in-one”PNP pince ligand as supports for the construction of Ni/Fe heterobimetallic compounds.The heterobimetallic Ni/Fe complex were prepared in good yield by the rate-determining step.Analytical methods such as SXRD,NMR,ESI-MS,UV-Vis,IR,CV,SQUID and M（?）ssbauer give insight into the geometric and electrionic structures of these complexes.The M（?）ssbauer spectrum result of complex 1 indicates that the iron（Ⅱ）ion is in a high spin state of S=2.The results of electrochemical cyclic voltammetry（CV）show that nickel/iron heteronuclear compounds can undergo reversible redox proces.The versitle redox Fe site in the heterobimetallic were attempted by the redox reagents.In this chapter,we mainly study how to synthesize nickel/iron heteronuclear compounds directionally and repetitively,and investigate the interaction between metal ligands and the transition of metal valence states.This provides a theoretical basis for the subsequent study of small molecule catalytic activation of nickel/iron heteronuclear compounds.（2）The nitrogen fixation and activation in the Ni/Fe heterobimeallic complex were studied by the reductive elimination H₂ in Ni/Fe metal hydride complex or N₂ released from metal azide complex.According to the mechanism of nitrogen activation,metal hydrides can do nitrogen fixation from the reductive elimination H₂ without strong reducing agents.Therefore,experiments were conductued to synthesize Ni/Fe metal hydrides from complex 1.Due to the high activity of Fe（Ⅱ）site in the Ni/Fe complex,the two hydrides followed reductive elimination H₂ to form a Ni（Ⅱ）/Fe（0）complex.Because of the demetallation with Fe（0）site,a new mononuclear pyrazlated nickel hydride compound is generated.In order to further study the catalytic dinitrogen by synthesized nickel/iron heteronuclear compounds,an attempt was made to synthesize metal nitride triple bond compounds,an intermediate of nitrogen activtion.Using nickel/iron azide compound as precursor,attempts to use light,heating,and other methods did not cause the N₂ release from azide group.Subsequently,an attempt was made to use an oxidant,silver trifluorosulfonate,to oxidize nickel/iron azides to obtain trivalent iron azides,to explore whether they are more conducive to the decomposition of azides.In the process of silver-oxidation of sulfonate,whether at low temperature or at room temperature,the replacement reaction of iron ions and anion will occur,indicating that the metal iron ion in the compound is high and suspended.This chapter mainly constructs metal hydrogen,metal nitrogen multi-key compounds,thereby exploring the catalytic activity of nitrogen nitrogen nitrogen nitrogen.