Heteronuclear Bimetallic Ni-M Dithiolato Complexes:Synthesis And Reactivity Studies

In nature,solar energy is fixed through photosynthesis accompanied by the conversion of water to oxygen,meanwhile releasing protons?H+?and electron?e-?.Hydrogenase can catalyze H+ reduction to generate H₂.in mild condition,which realizes the storage of energy?e-?.Above all,hydrogenase also has the activity of H₂ oxidation and H₂ splitting to supply energy for the biological growth.According to the kind of metal center,there are three major classes of hydrogenases:mononuclear[Fe]-hydrogenase,dinuclear[F eF e]-hydrogenase and heteronuclear[NiFe]-hydrogenase.Interestingly,[FeFe]-hydrogenase is shown to mediate the conversion of protons?H+?and electrons?e-?to dihydrogen?H₂?at highest rates?TOF=6000?9000 s-1?,while[NiFe]-hydrogenase does best in oxidizing H₂ with a TOF of 700 s-1.Studies on the structures and functions of the active sites in hydrogenase have significant value in the development of the high-efficiency H₂-energy techniques.The reported works almost focus on the H+ reduction by[FeFe]-hydrogenase mimic,however there is lack of[NiFe]-hydrogenase mimic which can be used for H₂ splitting.It has a challenge for us to study the heteronuclear mimics because heteronuclear[NiFe]models are more difficult to synthesis than[FeFe]ones,In this thesis,a series of heteronuclear[Ni-M]complexes were synthesized.We systematically studied the structures and properties of[NiCu],[NiFe],[NiCo]systems by FT-IR,1HNMR,31P NMR,GC-MS,elemental analysis,electrochemical analysis and X-ray diffraction.On this basis,we explored the functions of these mimics including electrochemical H+ reduction,H₂ splitting,and discussed the mechanism of H₂ generation.The details were obtained as follows:1.[?dppe?Ni??-pdt?FeCp*?CO?]BF₄ isomers were synthesized by reacting Ni?pdt??dppe?with[Cp*Fe?CO??NCMe?2]BF₄?Cp*= C5Me5-?.The crystal structures of Ni?Fe? complexes[?dppe?Ni??-pdt?FeCp*?CO?]BF₄?1:1?have been determined by X-ray single crystal diffraction:the CO ligand of complex 1?CO?+ coordinates to the top of S-Fe-S plane and that of 1'?CO?+ coordinates below the S-Fe-S plane.They have the same formula weight and the planar geometry as Ni unit.1?CO?+ can be converted to 1'?CO?+ by heating the solution.Interestingly,1'?C0?+ is oxidized to form Ni?Fe? complex 1'?CO?2+.According to the dynamic FT-IR and UV-vis data,1'?CO?2+ converts slowly to a new species 1?CO?2+?kobs=1.07 × 10-3 s-1?.Then 1?CO?2+ can be reduced to 1?CO?+ by Cp*2Fe,the CO ligand of complex 1?CO?2+coordinates above the S-Fe-S plane.The EPR data and the crystal structure of 2?CO?2+ prove that redox process can realize the rotation of CO ligand around Fe center.Based on the cycle of redox,we have estimated the difference of gibbs free energy between[1'?CO?]+ and[1?CO?]+ is 2.6 kcal/mol.We evaluate the electrocatalytic proton reduction of[?dppe?Ni??-pdt?FeCp*?CO?]BF₄?TOF?761 s-1?.Also,the increment of the current intensity appears at the second reduction process which is assigned to EECC mechanism:the catalyst first gets two electrons to form Ni0Fe? state and then get two H+ to release H₂.2.[?dppe?Ni??-pdt?Cu?P-P?]BF₄ complexes were successfully prepared by reacting Ni?pdt??dppe?and Cu precursors with diphosphine ligand?[?P-P?Cu?NCMe?2]BF₄?,The Ni?Cu? compounds[?dppe?Ni??-pdt?Cu?P-P?]BF₄ include complex 3?P-P = dppe?,complex 4?P-P = 1,2-Ph2PC6H4PPh2?and complex 5?P-P = 1,2-Cy2PC2H4PCy2?.X-ray crystal structures display the[?dppe?Ni?pdt?]framework possesses a square-planar geometry and[?pdt?Cu?P-P?]framework adopts a distorted tetrahedral geometry.Low-temperature NMR indicates the two P atoms convert dynamically around Cu center,the rigidity of P-P ligand on Cu can affect the turnstile rotation for Cu?P-P?.DFT studies prove that Ni atom is the electron-deficient center which can easily combine with H-,while Cu atom has no activity as a cofactor,which is similar to the active site of[NiFe]-hydrogenase,Ni?S-Cys?4 has the redox activity but there is no change in Fe?CN?2?CO?unit.In the reaction between[NiCu]complex and H-,a[NiHCu]0 intermediate can be produced and the Ni? center obtains two electrons to afford the known Ni?0?compound[Ni?dppe?2]with the release of H₂.Ni?pdt??dppe?combines with Ag+ to get an unique heteronuclear compound[?dppe?Ni??-pdt?Ag]²⁺,Ni remains the square-planar geometry and the structure of Ag-S is also in planar geometry.3.[?dppe?Ni??-pdt??X?CoCp*]n+.[?dppe?Ni??-pdt??Cl?CoCp*]+?[6C1]+?was prepared by reacting Ni?pdt??dppe?and[Cp*CoCl]₂ precursor.The Cl" in Ni?Co?complex[6C1]+ can be replaced by H-to form[Ni?Co?]hydride[6H]+.The H/D exchange of 6[H]+ and D2O can occur in DCM or MeCN to produce deuterated 6[D]+.To study the acidity of the new[NiCo]hydride,we found the balance between CH3COOH and complex 6[H]+ by screening experiment.The range of pKa of 18.5?22.3 is calculated by equation.The cyclic voltammogram of 6[H]+ consists of a reversible reduction.Upon the addition of Cl2CHCOOH,the cathodic current increases,coinciding with the hydride reduction,which indicates the reduction catalysis of proton.The value of ic/ip begins to plateau at?218 equiv?90 ?L?of acid and the turnover frequency?TOF?was estimated to be 244 s-1.Based on the electrochemical data above,it is more likely that the catalysis proceeds through an ECEC mechanism:the reduced 6[H]0 reacts with H+ to form H₂-coordinated complex;after the release of H₂,Ni?Co? complex[?dppe?Ni??-pdt?CoCp*]+?6+?was reduced to Ni?Co? complex 6;then 6 was protonated by H+ to get hydride 6[H]+.Luckily,we successfully get the intermediates([6H]+,6 and MeCN-coordinated Ni?Co? complex[6?NCMe?]²⁺)measured by X-ray diffraction,1H?31P NMR to better study the process of electrochemical H+ reduction.Further,we conducted the reaction of 6[MeCN]²⁺and H₂ with the presence of CH3ONa to form 6[H]+.This study has given us more insights into the activity of[NiCo]systems in H₂ generation and H₂ splitting,and it is of great significance to realize an efficient H cycle for a green and sustainable future.