Synthesis,Characterization And Properties Of [2Fe2S] Hydrogenase Models Containing Ferrocenylamino Phosphine Ligands

Increasing energy crisis and environment pollution prompt people to develop new energy other than fossil oil.Hydrogen has received considerable attention due to high energy density and its friendship to the environment after combustion.Biological hydrogen production is one of the effective ways to develop and utilize hydrogen energy.In recent decades,the study found that[Fe Fe]-hydrogenase in microorganisms can reversibly catalyze the proton reduction for hydrogen evolution.The high efficiency and special structure of the[Fe Fe]-hydrogenase active site have attracted a great attention.Through the structure and function simulation of hydrogenase,people can make the catalytic menchanism for hydrogenase clear.It’s center structure mainly includes:a binuclear iron subsite[Fe₂S₂]at which the catalytic reacation take place,and a redox-active[4Fe4S]cubane that serves as a conduit for shuttling electrons,and an regulate ligands of electron cloud density（CN^-,CO）.In order to better simulate the active central structure of natural[Fe Fe]-hydrogenase,in this paper,phosphine ligands containing ferrocene amino group were introduced into the structure of the（μ-pdt）Fe₂（CO）₆（1）,resulting in four diiron disulfide complexes containing ferrocene amino phosphine ligand.These models were characterized in detail.At the same time,the protonation reaction of these models were studied by NMR and electrochemical tests.The content of this paper can be listed as follows:In the dissertation,three monosubstituted models[（μ-pdt）Fe₂（CO）₅L](μ-pdt=-S（CH₂）₃S-,L=Fc N^PhP,（2）;Fc N^2-PyP,（3）;Fc N^4-PyP,（4）;Fc=ferrocene)and one disubstituted model（5）[（μ-pdt）Fe₂（CO）₄（κ²-PNP）]（PNP=Fc NP^P）were synthesized.They were characterized by MS,FTIR,NMR spectroscopy.Their crystal structures are determined by X-ray single crystal diffraction,while their electrochemical properties are explored by cyclic voltammetry（CV）.The protonation of these complexes was studied by using CH₃COOH and HOTf,and tracked by ¹H NMR and ³¹P NMR,while the electrochemical behaviors under different concentration acid were tested by cyclic voltammetry.In the presence of weak CH₃COOH,the active hydrogen signal was not detected by ¹H NMR,indicating that there was no protonation.However,the proton reduction process can be observed electrochemically catalyzed by these[2Fe2S]models in the presence of acid,while it is observed that the reduction peak current of unsymmetrical disubstituted complex 5 is higher than monosubstituted complexes.It indicates that the higher electron density in diiron core induced by the bidentate chelation coordination results in better the electrocatalytic activities.In the presence of the medium strong acid HOTf,the active hydrogen signal was detected by ¹H NMR,indicating that the protonation has occurred easier.When adding 1 equivalent trifluoromesulfonic acid to these complexes at room temperature,the signals of active terminal hydrogen,stableμ-hydrogen and the protonation on nitrogen atoms can be detected in complex2,3,5,but only the terminal hydrogen was detected in complex 4.It indicated that the synergistic effect of phosphine and nitrogen atoms as well as the distance between the intramolecular nitrogen bases and the proximal Fe atom could have important influence on the stability of terminal hydrogen.With the concentration increase of HOTf,the signal of terminal hydrogen disappeared,and the protonated products exist as the modes[H_NH_μ]²⁺and[H_{Py N}H_μ]²⁺.It is worth mentioned that the formation of terminal hydrogen is of great significance to the research menchanism.In the present of HOTf,the first reduction potential of monosubstituted complexes show the anodic shifts 300-470 m V,and the disubstituted complexes show the anodic shifts 910 m V,which belongs to the reduction peak of doubly protonated products.Its protonated products have low overpotentials and show good catalytic proton reduction.This also indicates that the larger the electron density of the active center is,the better the protonation is.After preliminary study,the possible catalytic mechanism of hydrogen production under different acid conditions was proposed.