Study On The Small Bite-angle Diphosphines-substituted [FeFe]-Hydrogenase Models And Related Hybrids

In nature,[FeFe]-hydrogenases have the highly efficient ability to catalyze the reduction of protons into hydrogen（H₂）that is regarded as one of the most potential renewable energy fuels.Therefore,it has become a research hotspot for scientists to investigate the structural and function simulation of[FeFe]-hydrogenases active site.In this context,a series of new phosphine-substituted[FeFe]-hydrogenase models synthesized and their molecular structures as well as catalytic properties are studied in this thesis.The meaningful results are described as follows:（1）In this thesis,28 novel phosphine-substituted diiron dithiolate complexes as[FeFe]-hydrogenase models were prepared.All the model complexes have been characterized by elemental analysis,FT-IR,¹H-NMR,and ³¹P{¹H}-NMR,in which 15 complexes are further determined by single-crystal X-ray diffraction analysis.The electrochemical properties of representative complexes are evaluated by using cyclic voltammetry（CV）.Additionally,the self-assembly between model complex and carbon dots are simply performed to explore the effect of carbon dots on the catalytic performance of diiron model complex.（2）In the first section of the second chapter,ten novel PNP-chelate diiron model complexes Fe₂(μ-adt ^NPh)（CO）₄{k²-（Ph₂P）₂NR}（1-5）with an azadithiolate bridge and Fe₂（μ-edt）（CO）₄{k²-（Ph₂P）₂NR}（6-10）with an ethandithiolate bridge were synthesized through CO-substitution reactions of all-carbonyl precursor Fe₂（μ-xdt）（CO）₆[xdt=adt^NPh（SCH₂N（Ph）CH₂S）and edt（SCH₂CH₂S）]with small-bite diphosphine ligands PNP[PNP=（Ph₂P）₂NR,R=CMe₃,CH₂CHMe₂,（CH₂）₃Me,CH₂)₃Si（OEt）₃,（CH₂）₃NMe₂]in the presence of Me₃NO·2H₂O and UV irradiation.The crystal structures of complexes 1,2,6,and 9 are further determined by X-ray crystallography.The electrochemical properties of representative models 2 and 7 are studied and compared by CV method,indicating that they are eletrocatalytically active for proton reduction to H₂ in the presence of acetic acid（HOAc）as a proton source.Relative to the edt counterpart 7,the adt^NPh complex 2 shows a more positive reduction potential,lower overpotential,and greater turnover frequency,strongly supporting the important role of the azadithiolate cofactor（NH）in the natural[FeFe]-hydrogenases.（3）In the second section of the second chapter,seven new phosphine-monosubstituted diiron model complexes Fe₂(μ-adt^NPh)（CO）₅{k¹-Ph₂P（R）}with azadithiolate bridge（R=NHC₆H₅,11;NHC₆H₄Me-p,12;NHC₆H₄OMe-p,13;NHC₆H₄CO₂Me-p,14;NHC₆H₄Cl-p,15;CH₂PPh₂,16;CH₂Ph,17）were prepared by the oxidative decarbonylations between all-carbonyl precursor Fe₂(μ-adt^NPh)（CO）₆ and several phosphine ligands such as PNP,dppm,Ph₂P（CH₂Ph）.The crystal structures of complexes 11,12,14,and 17 are further confirmed by X-ray crystallography.The electrochemical behaviors of model complexes 11-15 and reference17 are studied by CV technique,suggesting that complexes 11-15 display a little lower electrocatalytic H₂ evolution ability in contrast to 17 under acetic acid（HOAc）as a proton source.（4）In the first section of the third chapter,six new PNP-chelate and–bridge diiron model complexes Fe₂（μ-odt）（CO）₄{k²-（Ph₂P）₂NR}（18-20）and Fe₂（μ-odt）（CO）₄{μ-（Ph₂P）₂NR}（21-23）with an oxadithiolate bridge were respectively prepared by CO-substitution reactions of all-carbonyl precursor Fe₂（μ-odt）（CO）₆[odt=SCH₂OCH₂S]with small bite-angle diphosphine ligands PNP[PNP=（Ph₂P）₂NR,R=（CH₂）₃Me,（CH₂）₃NMe₂,（CH₂）₃Si（OEt）₃)under Me₃NO·2H₂O and reflux.The crystal structures of complexes 18,21,22,and 23 are further determined by X-ray crystallography.The electrochemical properties of representative models18 and 21 are investigated through CV,showing that the chelate complex 18 has a much higher turnover frequency and similar overpotential relative to the bridge complex 21 under whether strong acid（TFA）or weak acid（HOAc）as proton sources.This CV result strongly supports the theoretical prediction that asymmetrically-substituted diiron model complexes would be considered as an ideal molecular catalyst.（5）In the second section of the third chapter,the selective CO-substitution reactions of all-CO precursor Fe₂（μ-odt）（CO）₆ with two small bite-angle diphosphine ligands such as（Ph₂P）₂CH₂（PCP）versus（Ph₂P）₂NBuⁱ（PNP）are carried out under different reaction conditions.The results is as follows:i)with Me₃NO·2H₂O,the reaction of Fe₂（μ-odt）（CO）₆ and the PCP ligand produced diiron monodentate complex Fe₂（μ-odt）（CO）₅（k¹-PCP）（24）whereas the similar reaction with the PNP ligand produced diiron chelate complex Fe₂（μ-odt）（CO）₄（k²-PNP）（25）;ii)using UV irradiation in toluene emitting 365 nm,the treatments of Fe₂（μ-odt）（CO）₆ and the PCP or PNP ligands resulted in the respective formation of diiron bridge complex Fe₂（μ-odt）（CO）₄（μ-PCP）（26）and the chelate complex 25;and iii)using refluxing,the toluene or xylene solutions of Fe₂（μ-odt）（CO）₆ and the PCP or PNP ligands produced the bridge complexes26 and Fe₂（μ-odt）（CO）₄（μ-PNP）（27）,respectively.The crystal structures of complexes 24,25,and 26 are further determined by X-ray crystallography.The electrochemical performances of representative models 25-27 are studied by means of CV,indicating that they are all active for electrocatalytic proton reduction to hydrogen and the chelate complex 25 shows a lower overpotential as well as greater turnover frequency under trifluoroaetic acid（TFA）as a proton source.（6）In the fourth chapter,the self-assembly between diiron model complex Fe₂（μ-SCH₂）₂N（CH₂CH₂SO₃H）（CO）₆（28）and functionalized carbon dots CQDs-COONa are preliminarily explored by using UV-vis and fluorescence emission spectroscopies,showing that the fluorescence quenching is observed.This result well indicates that an efficient electron transfer from the excited carbon dots to the[2Fe2S]cluster can take place in the self-assembly system mentioned above,which is an important step required for the catalytic proton reduction to H₂ by the light-driven biomimetic models.