Synthetic Chemistry Of Iron-molybdenum Coenzyme-related Iron-sulfur Clusters

The activation of dinitrogen is one of the most challenging molecular activation processes,which has attracted widespread attention from scientists in the past century.The industrial ammonia synthesis based on Haber-Bosch process claims large demand of fossil fuels,to realize the harsh temperature and pressure conditions required for dinitrogen activation,whose energy consumption and carbon dioxide emission are key issues from the viewpoints of energy and environment crisis.On the other hand,the natural nitrogen fixation mediated by nitrogenase is realized with mild conditions,which has therefore attracted significant research interest.However,the key process of biological nitrogen fixation,binding and activation of dinitrogen on the iron-molybdenum cofactor of nitrogenase,remains a mystery,despite the continuous work by biochemists,protein crystallographer and synthetic inorganic chemists.The iron-molybdenum cofactor(Fe Mo-cofactor)is a heterometallic iron-sulfur cluster.After years of investigation,the core structure of Fe Mo-cofactor was determined by protein crystallography and X-ray emission spectroscopy to be[MoFe₈S₉C],which involves a C₃ symmetrical triangular prism-shaped hexanuclear iron substructure incorporating a special?₆-carbide in the center.The up-to-date research in biochemistry,spectroscopy,and computational chemistry still can not provide in-depth information of biological dinitrogen activation.Therefore,synthetic clusters that mimic the topological structure of iron-molybdenum cofactor may provide further structural information and experimental evidences of the mechanism of biological nitrogen fixation,being of great significance for the mechanism studies.Beyond the enormous efforts in the biomimetic synthesis of analogs of Fe Mo-cofactor in the past few decades,the major synthetic challenge still remains to incorporating light 2p atom into the core structure of iron-sulfur clusters,which arises ever since the discoverty of the interstitial light 2p atom in Fe Mo-cofactor.At the same time,the research of the reactivity of iron-sulfur clusters is still at a very preliminary stage.Only very limited cases of iron-sulfur clusters has the ability to activate small molecules at the molecular level,and none of which has close structural similarity with Fe Mo-cofactor.The very rich examples of mononuclear or binuclear metal complexes that activate dinitrogen are often protected by ligand with groups of large steric hinderance.A common feature of such iron complexes among them is that they all exhibit low metal oxidation states(Fe¹⁺,Fe⁰).However,iron-sulfur cluster with low-valent iron are very scarce,especially for high-nuclear iron-sulfur clusters.This thesis aimed to investigate some of the key challenges in the field of nitrogenase related Fe-S clusters,and made some progress in the following aspects:1.A mini review of the process of protein crystal structure research and biochemical research of iron-molybdenum cofactor,combed the milestone work of iron-molybdenum cofactor core structure research,present the scientific process of iron-molybdenum cofactor structure determination;a briefly describing of the discovery of iron-molybdenum cofactor and the possible binding site of small molecules to the cluster.The synthesis of iron-sulfur clusters related to iron-molybdenum cofactors is briefly reviewed,and the incorporation of the light 2p atoms into the core structure of iron-sulfur clusters.The simulation of the partial topological structure of iron-molybdenum cofactors are highlighted.The ligand metathesis strategy plays an important role in the synthesis of iron-sulfur clusters.The synthetic bionic nitrogen-fixing complex system is briefly introduced,and the complete nitrogen fixation cycle catalyzed by the sterically hindered monometallic Mo-based and Fe-based models is described;the molecular activation properties of multinuclear iron clusters are combed;especially the Fe Mo-Cofactor related polynuclear iron-sulfur clusters.2.Synthetic Fe-S cluster incorporating nitride ligand has intrinsic relevance to biological dinitrogen activation mediated by Fe Mo cofactor,which however has never been achieved to date.In this work,two distinct synthetic pathways has been rationalized to install nitride ligands into targeted positions of W-Fe-S cluster,generating the edge-bridged double-cubane clusters[(Tp*)₂W₂Fe₆S₆(?₄-N)₂L₄]^2-(L=Cl or Br)in good yields,each of which comprises a pair of cubic[WFe₃S₃N]²⁺subunits bridged at the Fe-N edges in a head-to-tail mode.M(?)ssbauer study discloses an oxidation states of(W^?)₂(Fe^?)₂(Fe^?)₄ with localized electron distribution on iron centers.Good agreement of M(?)ssbauer data with the empirical linear relationship obtained for[MFe₃S₄]^z clusters indicates that the ligand behavior of nitride in Fe-S cluster regarding electronic contribution is in certain extent similar to that of sulfide.This unique mid-valent Fe-S cluster embedded with?₄-nitride may provide new insight into mimicking the Fe Mo cofactor structure and understanding dinitrogen activation promoted by nitrogenase.3.The heterometallic iron-sulfur cluster compound[(Tp*)MFe₃S₃(?₃-Cl)Cl₃]^2-(M=W,Mo)bridged by halogen atoms is used as the precursor which was incorporated to a large steric hindrance base ligands to construct highly active incomplete cubane type structure cluster{(Tp*)MFe₃S₃[N(TMS)₂]₃}^1-.The cluster is reacted with tetrabutylammonium azide to obtain a tri-linked monoatomic nitrogen bridged cubane type cluster{(Tp*)MFe₃S₃(?₃-N)[N(TMS)₂]₃}^1-.The nitride bridged cluster was methylated by CH₃I to give amide bridged cubane type cluster{(Tp*)MFe₃S₃(?₃-N)[N(TMS)₂]₃}^1-/0 and we found that Mo and W have different oxidation state preferences in this reaction,indicating that Mo-based heterometal iron-sulfur clusters in nitrogenase-related iron-sulfur clusters would be of unique qualities.Phenylhydrazine can be reacted with the incomplete cubane type cluster{(Tp*)MFe₃S₃[N(TMS)₂]₃}^1-to obtaine the molecular-level activation product EBDC type heterometallic iron-sulfur cluster[(Tp*)₂M₂Fe₆S₆(?₄-N)₂(Ph N₂)₄]^2-,the side iron atoms are bonded with rare phenyl-diazene ligand.The disproportionation activation of phenylhydrazine is accompanied by proton transfer progress.The single N-N bond splitting section and proton transformation,which is highly related to the process of biological nitrogen fixation.4.Cubane type Fe-S cluster with terminal group of alkyl phosphorus ligand[(Tp*)WFe₃S₃(?₃-Cl)(PEt₃)₃]¹⁺has been synthesized by using cubane type chloride bridged W-Fe-S cluster.Using this alkyl phosphorus ligand cluster as a precursor,under the condition of extra ferrous chloride and benzophenone ketyl solution as reducing agent by a quantitative control,the high nuclear cluster[(Tp*)₄W₄S₁₂Fe₁₃]is produced by the reduction of the cluster and the precipitation of chloride ions.This cluster has a special structure of[Fe₁₃],the central iron and the surrounding 12 iron directly bonded,which is the first synthesized[Fe₁₃]cluster supported by direct iron-iron interation.The core oxidation state of[Fe₁₃]is positive 16 valence,indicating that the central iron has a low oxidation state with an average of positive 1.23 valence.M(?)ssbauer spectroscopic study shows that the electron distribution of iron at 77K is completely delocalized,which also meets the calculated M(?)ssbauer spectroscopic signal by DFT.The low oxidation state and high nuclear characteristics of this cluster compound show its potential for molecular activation and energy storage,these qualities would be further explored.