Study On The Synthesis And Electrochemical Properties Of[FeFe]-hydrogenase Mimics With Substituted Oxadithiolate Bridge

As the consumption of energy and the depletion of fossil fuels have posed a great threat to the world's energy and economic security,hydrogen(H2)that is considered as a new type of environmentally-friendly and renewable energy carrier has attracted considerable attention.In the preparation and application of H2,researchers discovered a family of highly efficient metalloenzyme for catalytic H2 evolution,named hydrogenases.They can not only lay a solid foundation for improving biological hydrogen production systems but also can be listed as one of the substitutes for noble-metal catalyst platinum.Notably,natural[FeFe]-hydrogenases with catalytic active[2Fe2S]butterfly cluster show the excellent catalytic H2 production ability,thus the chemical simulation of structure and function of[FeFe]-hydrogenase active site is widely studied.In this context,a new series of[FeFe]-hydrogenase mimics with substituted oxadithiolate bridges are designed and prepared through both the introduction of different substituents into the oxadithiolate bridges and the replacement of iron carbonyls with different phosphines in all-carbonyl diiron dithiolate complexes in this thesis.This purpose is as follows:if the rotation of one carbonyl of the aforementioned mimics could happen due to the steric hindrance between the substituents attached to dithiolate bridges and the phosphines coordinated to iron core;afterward,the rotated state featuring one bridging carbonyl and a vacant coordination site will be formed,which may be considered to be an ideal structural criterion of hydrogenase-inspired biomimetic catalysts for efficient H2 evolution.Therefore,this thesis has achieved the following meaningful results:? In this thesis,a total of seventeen[FeFe]-hydrogenase mimics and their relevant protonated products bearing substituted oxadithiolate bridges were synthesized.Furthermore,their molecular structures have fully characterized by elemental analysis,fourier transform infrared spectroscopy(FT-IR)and nuclear magnetic resonance(NMR)spectroscopies,as well as X-ray crystal diffraction analysis.Meanwhile,the redox properties and electrocatalytic H2 evolution abilities of some representative models are studied by using cyclic voltammetry(CV)and controlled electrolysis potential(CPE).? Two new types of all-carbonyl[FeFe]-hydrogenase models with mono-and disubstituted oxadithiolate bridges,namely[Fe2(?-Rodt)(CO)6](Rodt=SCH(R)OCH2 and R=Ph,Me,H)and[Fe2(?-R2odt)(CO)6](R2odt=SCH(R)OCH(R)and R=Ph,C6H4OH),were designed and synthesized through the introduction of different substituents into the oxadithiolate bridge for the first time so as to construct the asymmetric structure of diiron cores.All the as-prepared models are characterized by elemental analysis,FT-IR,and 1H-NMR spectroscopies.In order to explore the effect of different substituents of dithiolate bridges on the electrochemical performances of all-carbonyl[FeFe]-hydrogenase mimics,the electrochemistry of the as-obtained models are investigated in the absence and presence of trifluoroacetic acid(TFA)by using CV and CPE techniques.This result suggests that the substituents of dithiolate bridges have a certain role in the redox properties and electrocatalytic H2 production abilities of all-carbonyl biomimetic models.? To obtain the as-expected rotated-state structure,the PPh3 mono-and disubstituted[FeFe]-hydrogenase models with monosubstituted oxadithiolate bridges,namely[Fe2(?-Rodt)(CO)6-n(PPh3)n](R=Ph,Me,H and n=1,2),were obtained through the carbonyl replacements of the monosubstituted oxadithiolate-containing all-carbonyl precursors by monophosphine PPh3,All the as-prepared mimics are characterized by elemental analysis,FT-IR as well as NMR(1H,31P)spectroscopies,and by X-ray crystallography for some representative models.This finding indicates that these PPh3-substituted models didn't show the formation of the rotated-state unit in solid state.In order to investigate the impact of substituent nature of the dithiolate bridge and the substitution amounts of the PPh3 ligand on the proton-capturing and catalytic proton reduction ability of monophosphine-substituted[FeFe]-hydrogenase mimics,the chemical protonations and electrochemical processes of the above monophosphine-substituted models are studied in the presence of TFA and acetic acid(HOAc)by using spectroscopic monitor and electrochemical method.This outcome demonstrates that their protonation processes are mainly affected by the strength of the added acid while the unasymmetry of monosubstituted dithiolate bridge has a certain influence on their redox properties and electrocatalytic H2 production activities.? To further obtain the as-expected rotated-state structure,the diphosphine-chelated[FeFe]-hydrogenase models with disubstituted oxadithiolate bridges,namely[Fe2(?-R2odt)(CO)4(?2-L)][R=Ph,H and L=(Ph2P)2NBn(PNP),(Ph2PCH2)2NBn(PCNCP),(Ph2PCH2)2CH2(DPPP)],were afforded through the carbonyl substitution reactions of the disubstituted oxadithiolate-containing all-carbonyl precursors and diphosphines(L).All the as-prepared models have been characterized by elemental analysis,FT-IR and NMR(1H,31P)spectroscopies,as well as X-ray crystallography.This result suggests that for the PCNCP-or DPPP-chelated models,the semi-bridging carbonyl is observed in solid state but the rotated-state structure isn't formed.In order to explore the influence of substituents(R)linked to dithiolate bridge and the diphosphine ligands(L)on the proton-capturing and electrochemical catalytic proton reduction ability of diphosphine-chelated[FeFe]-hydrogenase mimics,the chemical protonations and electrochemical behaviors of the representative diphosphine-substituted models are investigated in the absence and presence of TFA by using spectroscopic monitor and electrochemical method.This finding exhibits that the redox properties of diphosphine-chelated models are primarily tuned by the electronic effects of the diphosphine ligands(L)whereas their electrocatalytic H2 evolution abilities are determined by the structural factors of the diphosphines(L)and the substituents(R).