Study On The Synthesis And Electrochemical Hydrogen Evolution Reaction Of Phosphine-Substituted Diiron Dithiolate Complexes Attached Covalently Onto Carbon Nanotubes

In nature,[FeFe]-hydrogenases can efficiently catalyze proton reduction into hydrogen（H₂）,which will provide a good choice for human society to solve the current environmental pollution and energy crisis and to replace the use of traditional fossil energy.To this end,a large number of diiron dithiolate complexes were synthesized for the chemical simulation of the structure and function of[Fe Fe]-hydrogenases,but they still cannot reach the level of the catalytic H₂-producing ability of[Fe Fe]-hydrogenases.This is because in most studies,a lots of diiron dithiolate complexes were designed and synthesized as molecular catalysts,mainly aiming at the chemical simulation of butterfly[2Fe2S]subcluster as the catalytic active center found in enzyme.These molecular catalysts require the catalytic medium to be generally organic solvents,which greatly limits their catalytic H₂-producing ability.In this context,this paper have designed and synthesized two new types of phosphine-substituted diiron dithiolate complexes attached covalently onto carbon nanotubes（CNTs）through the covalent bond formed between mono-or diphosphine ligands and CNTs.This will therefore achieve the efficiently electrochemical hydrogen evolution reaction（HER）in aqueous medium.In this thesis,the completed work is described as follows.（1）Seven novel phosphine-substituted diiron dithiolate complexes were synthesized and characterized,in which the crystal structures of six diiron complexes were determined and analyzed.The electrochemical properties of the representative diiron complexes have been studied.More importantly,the further covalent attachment of the as-prepared diiron dithiolate complexes with CNTs resulted in the successful formation of phosphine-substituted diiron dithiolate complexes supported covalently onto CNTs as new hybrid catalysts.And the electrochemical H₂-generating performances of the as-constructed hybrid catalysts has been studied in aqueous media.（2）Four novel all-carbonyl or monophosphine-substituted diiron dithiolate complexes[Fe₂{（μ-SCH₂）₂N（C₆H₄CH₂CH₂OC（O）CH₂C₆H₄CH₃-4）}（CO）₅L]（1,L=CO and 2-4,L=P（C₆H₄R-4）₃,R=F vs.H vs.Me）were synthesized by the esterification reactions of the N-phenylthanol-containing diiron dithiolate precursors[Fe₂{（μ-SCH₂）₂N（C₆H₄CH₂CH₂OH）}-（CO）₅L]（A,L=CO and B-D,L=P（C₆H₄R-4）₃）with 4-methylphenylacetic acid in THF-DCM（v:v=1:1）as the optimal solvent.Meanwhile,the molecular structures of complexes 1-4 have been determined by elemental analysis,Fourier transform infrared spectroscopy（FT-IR）,nuclear magnetic resonance spectroscopy（NMR）,and X-ray single-crystal diffraction analysis.Additionally,the electrochemical studies of 2-4 using cyclic voltammetry（CV）have shown that they have the ability to catalyze proton reduction into H₂ under acetic acid（Ac OH）as a proton source and acetonitrile as a solvent.Notably,the successful preparation of 1-4 will offer an important synthesis condition for achieving subsequent attachments of the aforementioned diiron dithiolate precursors（A-D）with CNTs through covalent bond.（3）Three novel diphosphine-chelated diiron dithiolate complexes[Fe₂{（μ-SCH₂）₂X}-（CO）₄{k²-（Ph₂PCH₂）₂N（CH₂CH₂OH）}]（5-7,X=NPh vs.CH₂ vs.O）were synthesized by decarbonylation reactions of all-carbonyl diiron dithiolate precursors[Fe₂{（μ-SCH₂）₂X}（CO）₆]（E-G,X=NPh,CH₂,O）and diphosphine ligand（Ph₂PCH₂）₂N（CH₂CH₂OH）in heating toluene.Meanwhile,the molecular structures of complexes 5-7 have been determined by elemental analysis,FT-IR&NMR spectroscopies,and X-ray single-crystal diffraction analysis.Additionally,the electrochemical studies of 5-7 using CV technique have displayed that they have the ability to catalyze proton reduction into H₂ under Ac OH as a proton source and acetonitrile as a solvent.Noticeably,this will provide a necessary prerequisite for the subsequent studies on the structures and electrochemical properties of the aforementioned diiron dithiolate complexes（5-7）supported covalently onto CNTs.（4）The first new type of monophosphine-substituted diiron dithiolate complexes supported covalently onto CNTs（labeled as hybrids CNT-f-FeP^R,R=F vs.H vs.Me）was successfully prepared by esterification reactions of the above-used diiron dithiolate precursors A-D（herein labeled as FeP^R）with CNTs.Furthermore,the composite structures of hybrids CNT-f-FeP^R have been confirmed by FT-IR,Raman,and X-ray photoelectron spectroscopy（XPS）.In particular,the electrochemical HER properties of CNT-f-FeP^R are studied in 0.1 M H₂SO₄ aqueous solution by means of linear sweep voltammetry（LSV）,CV,and electrochemical impedance（EIS）.This has shown that（i）the CNT-f-FeP^R hybrids all have electrocatalytic HER activity in0.1 M H₂SO₄;however,（ii）in comparison to analogues CNT-f-FeP^H and CNT-f-FeP^Me,hybrid CNT-f-FeP^F exhibits a superior catalytic H₂-producing ability,based on the following facts that their overpotential(η₁₀)at 10 m A cm^-2 current density is found to be CNT-f-FeP^F（457 mV）<CNT-f-FeP^Me（541 mV）<CNT-f-FeP^Me（622 mV）,Tafel slope is found to be CNT-f-FeP^F(67.9mV dec^-1)<CNT-f-FeP^Me(128.3 mV dec^-1)<CNT-f-FeP^H(237.1 mV dec^-1),the electrochemical surface active area(ECSA,C_dl)is estimated to be CNT-f-FeP^F(6.36 mF cm^-2)>CNT-f-FeP^Me(5.28 mF cm^-2)>CNT-f-FeP^H(2.80 mF cm^-2),and the surface charge transfer resistance(R_ct)is estimated to be CNT-f-FeP^F（30.15Ω）<CNT-f-FeP^Me（47.34Ω）<CNT-f-FeP^H（94.19Ω）.Therefore,these results suggest that the introduction of strong electron-withdrawing groups（like R=F）in phosphine ligand may contribute to enhance the electrochemical HER activity of CNT-supported diiron dithiolate complexes as hybrid catalysts in acidic aqueous media.（5）The second new type of diphosphine-substituted diiron dithiolate complexes supported covalently onto CNTs(labeled as hybrids CNT-f-1^XDT,XDT=ADT vs.PDT vs.ODT)was successfully constructed by esterification reactions of the above-prepared diiron dithiolate complexes 5-7(herein labeled as 1^XDT)with CNTs.Furthermore,the composite structures of hybrids CNT-f-1^XDT have been confirmed by Raman and XPS spectroscopies.Especially,the electrochemical HER properties of CNT-f-1^XDT are investigated in both 0.05 M phosphate buffer solution（PBS,p H=7.4）and 0.05 M acetate buffer solution（ABS,p H=4.0）by using LSV,CV,and EIS techniques.It is found that（i）the CNT-f-1^XDT hybrids all show the electrocatalytic HER activity in whether 0.05 M PBS or 0.05 ABS;however,（ii）in contrast to analogues CNT-f-1^PDT and CNT-f-1^ODT,hybrid CNT-f-1^ADT displays a better catalytic H₂-producing ability,based on the following observations that their overpotential(η₁₀)at 10 m A cm^-2 current density follows an order of CNT-f-1^ADT（787 mV in PBS/687mV in ABS）<CNT-f-1^PDT（848 mV in PBS/838 mV in ABS）<CNT-f-1^ODT（909 mV in PBS/1010 mV in ABS）,Tafel slope exhibits an order of CNT-f-1^ADT(162 mV dec^-1 in PBS/140 mV dec^-1 in ABS)<CNT-f-1^PDT(170 mV dec^-1 in PBS/158 mV dec^-1 in ABS)<CNT-f-1^ODT(200 mV dec^-1 in PBS/260 mV dec^-1 in ABS),the estimated C_dl for ECSA follows an order of CNT-f-1^ADT(1.27 mF cm^-2 in PBS/0.79 mF cm^-2 in ABS)>CNT-f-1^PDT(0.57 mF cm^-2 in PBS/0.60 mF cm^-2 in ABS)>CNT-f-1^ODT(0.41 mF cm^-2 in PBS/0.43 mF cm^-2 in ABS),and the calculated R_ctexhibits an order of CNT-f-1^ADT（18.0Ωin PBS/59.5Ωin ABS）<CNT-f-1^PDT（20.2Ωin PBS/81.9Ωin ABS）<CNT-f-1^ODT（36.1Ωin PBS/174.9Ωin ABS）.Thus,these findings indicate that the introduction of the azadithiolate（ADT=（SCH₂）₂NR）unit is able to helpfully improve the electrochemical HER activity of CNT-supported diiron dithiolate complexes as hybrid catalysts in either neutral or acidic aqueous medium.