Insight Into The Mechanism Changes Of Water Oxidation Induced By Ligand Modification Of Ru-bda Catalysts

The mechanism of water oxidation is the core of theory system in the homogenous water-oxidation catalysis and plays a critical role in water splitting. In principle, the study on the mechanism of water oxidation is beneficial to the design of the highly efficient, stable and low-costed catalysts. The research of mechanism contributes to achieve the successful reaction of solar hydrogen photo-production by water splitting, and consequently realizes efficient conversion of solar energy into chemical energy.In this thesis, a series of mononuclear Ru-bda complexes [Ru(4-Br-bda)(pic)2] A2? [Ru(4,4'-2Br-bda)(pic)2] A3?[Ru(4-Br-bda)(isoq)2] A5 and [Ru(4-NO2-bda)(pic)2] A6 (H2bda = 2,2'-bpy-6,6'-dicarboxylate ligand; pic= picoline; isoq= isoquinoline) with Br (NO2) substituent on the equatorial ligand of bda ligand were designed and synthesized. Higher oxidation potentials of these catalysts were observed from CVs and DPVs. A notable decrease of reaction rate and catalytic activities promoted the mechanism research of the water oxidation. The kinetic study illustrates that some catalysts molecules modified by bromine substituent would more incline to involve in the water nucleophilic processes. When the NO2 substituent was applied to the bda ligand, the kinetic study of catalyst A6 mirrored that the water nucleophilic mechanism responsible for the 0-0 bond formation. More importantly, we successfully demonstrate that the water oxidation mechanism via nucleophilic attack of a water molecule on the high energy [Ruv= O]+species, a lower-energy pathway of 0-0 bond formation via a [Ru?-OOH] intermediate is firstly reported to be characterized by HRMS and 18O-isotope labelling.Based on the [Ru(bda)(pic)]) monomer, a series of new dinuclear ruthenium catalyst [R1-Ru(bda)(pic)] A7, [R2-Ru(bda)(pic)] A8 and [R3-Ru(bda)(pic)] A9 were prepared and characterized with rigid axial ligand R1-R3 (R1= 4,5-dipyridine-2,7-di-tert-butyl-9,9-dimethylxanthene, R2= 4-pyridine-5-phenyl-2,7-di-tert-butyl-9,9-dimethylxanthene, R3 = 4-pyridine-2,7-di-tert-butyl-9,9-dimethylxanthene). Their catalytic activities towards water oxidation were investigated using a chemical oxidant [Ce(NH4)2(NO3)6] in acidic solution. The TONs of catalysts A7-A9 were 900,500 and 390, respectively. The kinetics of catalytic water oxidation studies showed that the same steric hindrance of the rigidity ligand R1 and R2 promoted the WNA process. The steric hindrance of R3 is considerably smaller than that of R1 and R2. It is inferred that the catalytic mechanism of catalyst A9 involved a bimolecular coupling reaction (I2M) and a water nucleophilic attack reaction (WNA). The key intermediates of catalysts A7 in the catalytic cycle was probed by high resolution mass spectrometry. The R1 of complex A7 allows through hydrogen interaction with the water molecular, which was conducive to the O-O bond information of the catalytic cycle.Based on the [Ru(bda)(pic)2] monomer, a new dinuclear ruthenium catalyst R1-[Ru(bda)(pic)]2 A10 was prepared with rigid axial ligand R1. The electrochemical behavior of complex A10 and catalytic activities of water oxidation were investigated in pH 1.0. The TON of catalyst A10 was 900. The kinetic study exhibits a first-order reaction feature and indicates that O-O bond formation of this dinuclear catalyst follows a WNA pathway.