Cooperative"Metal-Ligand" Strategy In Hydrogenase And Nitrogenase Synthetic Models

In nature,H2O was converted to O2 through photosynthesis with the release of proton(H+)and electron(e-).Under mild conditions hydrogenases and nitrogenases in microorganisms utilize H+ and e-to produce H2 and NH3,respectively.In the active sites of these metalloenzymes,ligands and metals work in a synergistic way to catalyze the reactions,and this strategy is called "Metal-ligand Cooperation(MLC)".[FeFe]H2ase can efficiently and reversibly catalyze H2 production and oxidation,and the rate of H2 production reaches 6000-9000 s-1.Though nitrogenase can catalyzes N2 fixation under mild conditions,the production of NH3 in industry requires high temperature and pressure.Therefore,it is of significant interest to model the active sites of these metalloenzymes and investigate the cooperative catalysis mechanisms for the development of energy storage and transfer based on abundant-metal catalysts.The work presented in this thesis focused on the key intermediates in catalytic cycles of[FeFe]-H2ase and the mechanisms of "Metal-ligand Cooperation" in active sites of metalloenzyme.A series of iron complexes bearing pentamethylcyclopentadiene ligand(Cp*)were developed and the H+reduction and N2 activation of theses complexes were studied,which laid a foundation for further understanding of the catalytic mechanisms of metalloenzymes.The results were discussed as follows:1.The diiron terminal hydride(Fe-Fe-H)complex is regarded as the key intermediate in the interconversion of H2 and H+mediated by[FeFe]-H2ase,while the reported diiron hydride complexes are mostly bridged hydrides(Fe-H-Fe).Therefore,it is of great significance to develop the synthetic Fe-Fe-H complex relevant to the[FeFe]-H2ase active site.Chapter II presented a stable Fe-Fe-H complexes[Cp*Fe(pdt)Fe(dppe)(CO)(H)]0([1(t-H]0).Unlike previous examples of terminal diiron hydrides,[1(t-H)]0 does not isomerize to the bridging hydride[1(μ-H)]0.Oxidation of[1(r-H)]0 gives[1(t-H]+,which isomerized to Fe-H-Fe complex[1(μ-H)]+in solution.Reduction of[1(μ-H)]+afforded the netural Fe-H-Fe complex[1(μ-H)]0,which can converse to[1(t-H)]0.Protonation of[1(t-H)]0 in MeCN solution affords H2 even with weak acids via hydride transfer.In contrast,protonation of[1(μ-H)]0 yields 0.5 equiv of H2 and[1(μ-H)]+,indicating the inertness of μ-H ligand.The part of this thesis indicated that the Fe-Fe-H complex can be stabilized by the anionic ligand Cp*and terminal hydride is more reactive than that of bridging hydride.2.Based on the coorperetive "Fe-S" strategy,a novel diiron complex(μH)Fe2(pdt)(dppbz)(CO)2(SR)([2(μ-H)]0)with terminal RS-ligand was synthesized.The terminal thiolate in[2(μ-H)]0 underwent protonation,affording a "thiol-hydride"complex[2(μ-H)H]+.It was found that[2(μ-H)H]+ can not only achieve intramolecular thiol-hydride coupling to release H2,but also occur H/D exchange reactions with D2,D2O or CD3OD to generate[2(μ-D)D]+.Isotopic exchange in[2(μ-D)H]+ was driven by an equilibrium isotope effect(Keq=2.36),and D was more favor to combine with RS-,resulting in[2(μ-H)D]+.[2(μ-H)H]+ also catalyzed H/D scrambling between H2 and D2O or CD3OD to produce HD and D2.The reactions based on such a "proton-hydride"model opened a new direction in the studying of Fe-H-Fe complexes activity and provided insights into the reversible heterolytic cleavage of H2 by H2ase.3.The last chapter focused on exploring the mechanism of N-N bond cleavage in N2 reduction.A serie of iron complexes[Cp*Fe(P-X)]+/0(X=S,NH,NH2)were synthesized,and their stoichiometric reactions with N2H4 and N2 were studied.It was found that[Cp*Fe(P-NH)]+ and[Cp*Fe(P-S)]0 react with 0.5 equiv.N2H4 to achieve N-N bond homolysis,forming the amino complex[Cp*Fe(P-NH)NH2]+ and[Cp*Fe(PS)NH2]0,respectively.The latter can be further protonated to generate NH3 complex[Cp*Fe(P-S)NH3]+.While the N-N bond of N2H4 was hetero-cleaved in the reaction between[Cp*Fe(P-NH2)N2]+and N2H4(1 equiv.),yielding[Cp*Fe(P-NH)NH2]+ and NH3,by synergistic transfer of proton in NH2 ligand.N-N hetero-cleavage of N2H4 can also abtained by[Cp*Fe(P-S)]+,which generate[Cp*Fe(P(=NH)-S)NH3]+ via the synergy of S group.Therefore,this work achieved the heterolysis and homolysis of N-N bonds based on the cooperative metal-ligand and bimetallic strategies,which laid the foundation for further modeling the function of nitrogenase.