Water Oxidation Catalyzed By Mononuclear Ruthenium Complexes Containing Electronegative Ligands

The critical factor of artificial photosynthesis is preparation of high-efficient and robust water oxidation catalysts(WOCs),so far,extensive attention has been paid on WOCs based on ruthenium complexes due to their low overpotential and high activity.In this thesis,homogeneous water oxidation and heterogeneous electrocatalytic water splitting based on mononuclear ruthenium catalysts,which contain electronegative ligands,were mainly researched.Firstly,three mononuclear ruthenium WOCs Rul,Ru2 and Ru5 containing electronegative tridentate ligands,which are composed of pyridyl,benzimidazoyl and carboxyl,weresynthesized.Eletrochemical studies and homogeneous water oxidation results show that catalyst Ru1,containing the most electronegative ligand,has the lowest onset potential(1.4 V vs.NHE,pH = 1)and the highest activity for chemical water oxidation(TON=2100,TOF = 0.21 s-1).Dynamic light scattering demonstrates that there is no RuOx nanoparticles produced during the long-term reaction time.Kinetics studies and the intermidiates detection confirm the O-O bond formation mechanism involved water nucleophilic attack.More impressively,for ionic catalyst Ru5,the capture of high valence intermidiate RuV =O using HRMS provides a strong evidence.Additionally,based on tridentate electronegative ligands(3,6-di-tert-butyl-9H-carbazole-1,8-dicarboxylic acid),two mononuclear ruthenium WOCs with different auxiliary ligands(4-picoline for Ru3;isoquinoline for Ru4)were prepared and structurally identified.Electrochemical measurements using cyclic voltammetry(CV)and differential pulse polarography(DPV)show that a distinct variation in the auxiliary ligands has no influence on the electrochemical properties of such ruthenium complexes,and the onset potential for water oxidation of both catalysts is 1.23 V vs.NHE(pH = 7),displaying the potential for visible-light driven water oxidation.Catalyst Ru3 was found to be high active for Ce?-driven water oxidation,with a high TON of 950,however,the TON of catalyst Ru4 was found to be only 90.The mechanism studies demonstrate water nucleophilic attack is involved in the water oxidation catalytical circle.And then,the crystal structures indicate a strong ?-?stacking interaction between isoquinoline molecules,which is adverse to ligand exchange of auxiliary ligand with water,leading to a low catalytical activity of Ru4.Furthermore,photocatalysis measurements reveal catalyst Ru3 is stable for light-driven water oxidation.Finally,a mononuclear ruthenium WOC Ru6 with two long carbon chains was successfully synthesized.In order to carry out electrocatalytic water splitting,molecular catalyst Ru6 was immobilized on reusable glassy carbon(GC)electrode with multiwalled carbon nanotubes(MWCNTsCOOH)by noncovalent interaction,then obtained a Ru6/MWCNTsCOOH/GC anode,which was found to be highly efficient for electrocatalytic water oxidation.The water oxidation overpotential of Ru6/MWCNTsCOOH/GC anode is as low as 380 mV,sustained current density of 1.25 mA/cm2 was achieved at a relatively low applied potential of 1.4 V vs.NHE in buffered neutral aqueous solution.The long-term electrocatalytic measurement over 5 hours gave a total TON of 186000 and an average TOF of 10.2 s-1.