Trans Effect On Catalytic Activity Of Ruthenium-Based Water Oxidation Catalysts

The use of solar energy to split water into hydrogen and using hydrogen energy to replace fossil fuels is one of the most pressing and challenging issues today.Water splitting reaction includes mainly two half reactions: water oxidation and proton reduction.Water oxidation involves multi-step electron transfer,proton transfer and the chemical bond cleavage/formation.The difficulty of water splitting is that water oxidation reaction requires large energy in thermodynamics.Therefore,an efficient and stable catalyst is needed to reduce the overpotential in order to efficiently use sunlight to split water.In previous studies,it was found that negatively charged ligands can increase the rate of ligand exchange,thus increasing the activity of the ruthenium water oxidation catalyst.In order to fully exert the influence of the ligand on the activity of the transition metal center we propose,in this thesis,to use the classical trans effect to activate water molecules bound to the metal center,thereby accelerating the ligand exchange rate and improving the catalytic performance.Herein,two catalyst systems were designed.In system one,4-hydroxypyridine-2,6-dicarboxylic acid(hdc^2-)was used as a ligand to prepare Ru-based water oxidation catalyst（WOC）where the hydroxyl group of hdc^2-endows strong electron-donating ability to the pyridine ring due to the p-p conjugation,which improves the electron donating ability of the ligand and enhances the trans effect.A family of Ru-hdc complexes,[Ru（hdc）（py-R）₃]（py-R = 4-pyridyl ligands;R = OMe,Ru1;Me,Ru2;H,Ru3;COOMe,Ru4）with electron-donating and-withdrawing groups were prepared and fully characterized.Electrochemical water oxidation experiments demonstrated that（i）the redox potentials of Ru^III/II are sensitive to the substituents,and stronger electron-donating groups give lower oxidation potentials and（ii）the catalytic current for water oxidation correlates well with their electron-donating ability of pyridyl ligands,Ru1 > Ru2 > Ru3 > Ru4.A similar trend for the performance of Ce^IV-driven water oxidation was also observed.In system two,a series of Ru-based water oxidation catalysts with tridentate ligands containing amides was synthesized.By changing the position of the negatively charged amide N,we studied the trans effect on the catalytic activity of ruthenium catalysts.Complexes Ru5 and Ru6 were synthesized using N-（quinolin-8-yl）picolinamide（L1）as equatorial ligand.Complex Ru7 was synthesized using N-phenyl-[2,2’-bipyridin]-6-amine（L2）as equatorial ligand.In a homogeneous system with Ce^IV as the oxidant,the TON of the complex Ru6 > Ru5 > Ru7 was measured,confirming that the trans effect can reduce the oxidation potential of the catalysts,accelerate the exchange rate of the ligand,and improve the catalytic performance of the catalyst.