Influence Of The Structure Of N_x-Multidentate Ligands On The Catalytic Performance Of Copper And Nickel Complexes For Electrochemical Water Oxidation

The development of clean and sustainable green energy sources to replace conventional fossil fuels is the efficient way to solve energy crisis and CO2 over-emission.Splitting water into hydrogen and oxygen driven by solar energy or electric power from renewable energy sources is an ideal approach to achieve this goal.Because oxygen evolution reaction(OER)proceeds via a four-electron and four-proton transfer process,it requires high energy to drive the reaction;the OER is the rate-limiting step of overall water splitting.Therefore,development of cheap,efficient and robust water oxidation electrocatalysts is the key point for the production of hydrogen from water.Compared to inorganic material catalysts,the advantages of molecular catalysts are their clear structures and the possibility for mechanistic studies.More important,the overpotential,activity and stability of molecular catalysts can be adjusted by reasonably designing the structure of ligands.Systematic studies of molecular catalysts bearing different ligands will give chance to better understand the structure-function relationship of molecular catalysts and hence pave the way to highly performing OER catalysts.This thesis was focused on exploring the relationship between the structure of Nx-multidentate ligands and the catalytic performance of their copper and nickel complexes for electrochemical water oxidation.First,the influence of the dentate number of Nx-multidentate ligands on the catalytic performance of copper complexes was studied for electrochemical water oxidation.Two mononuclear copper complexes,[(L1)Cu](BF4)2(A,L1=N-benzyl-N,N',N'-tris(pyridin-2-ylmethyl)ethylenediamine)and[(L2)Cu(OH2)](BF4)2(B,L2 = N,N'-dibenzyl-N,N'-bis(pyridin-2-ylmethyl)ethylenediamine),were prepared and well characterized.Based on comparative electrochemical studies,the N4-coordinated complex B has a higher catalytic activity than the N5-coordinated complex A for OER in pH 11.5 aqueous solution.The maximal rate constants(kcat)of A and B are 13.1 and 18.7 s-1,respectively.Compared with A bearing a N5 chelating ligand,B coordinated with a N4-ligand and an axial aqua molecule would not suffer from considerable structural change for the formation of the OH?coordinated intermediates,which benefits the water oxidation reaction.Therefore,N4-tetradentate ligands were used in the following work on molecular copper catalysts.In order to explore the influence of the conjugate effect of N4-tetradentate ligands on the catalytic property of copper complexes for electrochemical water oxidation,two new copper complexes,[(L3)Cu(OH2)](BF4)2(C,L3 = N,N'-dimethyl-N,N'-bis(pyridin-2-ylmethyl)-1,2-diaminoethane)and[(L4)Cu(OH2)](BF4)2(D,L4 = 2,7-bis(2-pyridyl)-3,6-diaza-2,6-octadiene),with different conjugacies of ligand backbones were synthesized.Their catalytic performances in electrochemical water oxidation were comparatively studied.Complex D bearing a diimine-dipyridine ligand with better conjugacy displayed an onset overpotential of 306 mV,which is 427 mV lower than that of the analogous complex C containing a diamine-dipyridine ligand for OER at pH 9.0,and the observed rate constant(kobs)of D(50.4 s-1)is about 3.7 times as high as that of C.These results reveal that the conjugate effect of a ligand in molecular catalysts play an important role in the performance of molecular OER catalysts.In the previous studies,the decrease of overpotentials for molecular catalysts by ligand modification was often accompanied by the loss of the catalytic activity.With the strategy of adjusting the conjugate structure of N4-ligands,the enhancement of OER activity of the copper complex simultaneously with the decrease of the overpotential was realized.In the subsequent study,three mononuclear copper complexes,[(L3)Cu(OH2)](BF4)2(C),[(L5)Cu(OH2)](BF4)2(E,L5 =N,N'-bis(pyridin-2-ylmethyl)piperazine),and[(L6)Cu(OH2)](BF4)2(F,L6 = N,N'-bis(pyridin-2-ylmethyl)diazepane),with different backbone rigidities were prepared,to explore the influence of ligand backbone rigidity on the catalytic performance of copper complexes for OER.X-Ray diffraction analysis showed that the complexes have similar distorted square pyramidal geometries,the ligand backbone rigidity apparently influences the coordination bond angles of N4-chelating ligands.Among these copper complexes,E has the strongest backbone rigidity,which leads to easy substitution of the water molecule coordinated at the axial position of E.Electrochemical studies revealed that all three copper complexes were catalytically active for water oxidation reactions in neutral aqueous solutions.Complex E displayed a very low onset overpotential of 358 mV,which is 242 and 292 mV lower than those of C and F,respectively.It is assumed that the low overpotential of E is due to the replacement of the axial aqua molecule by the hydrogen phosphate ion in buffer solutions,which makes the metal center of E more electron richer and hence more easily oxidized to high valence intermediate.To our knowledge,the overpotential of E is the lowest one among the molecule copper catalysts reported to date for OER in neutral aqueous solutions.In anoth respect,C and F with more flexible backbones exhibited better stability than E during long time electrolysis.More interestingly,the backbone rigidity of molecular copper catalysts could also alter their catalytic mechanism.Complexes C and F catalyzed the OER through the mechanism of water nucleophilic attack(WNA),while E catalyzed the OER via the mechanism of the interaction of two metal-oxo units(I2M)owing to the high Lewis acidity of its center metal and the large bond angle between pyridine units and the central copper atom(?NPy-Cu-NPy).To expand the studies on the influence of backbone rigidity of Nx-ligands on the catalytic property of molecular catalysts for OER,two nickel complexes,[(Ll)Ni(OH2)](BF4)2(G)and[(L7)Ni(OH2)](BF4)2(H,L7 =(1R,2R)-N-benzyl-N,N',N'-tris(pyridin-2-ylmethyl)cyclohexane-1,2-diamine),were synthesized and well characterized.They have the same first coordination environment,namely a stable N5-chelating framework together with a labile water molecule at the axial position,but their N5-ligands have different backbones.Comparable studies of the catalytic performances of G and H for electrochemical water oxidation in neutral aqueous solutions revealed that the rigid backbone of N5-chelating ligands has positive influence on the catalytic activity of nickel catalysts.Specifically,H bearing a rigid backbone N5-ligand displayed the kobs of 4.62 S-1,which is higher than that(kobs = 3.06 s-1)obtained for G containing a flexible backbone N5-ligand under the same test conditions.