Synthesis And Characterizations Of Ru(Ⅱ) Complexes Containing Tetradentate Ligands Hbpp For Catalytic Water Oxidation

In nature, The most important processes such as solar energy absorption, water oxidation,electron and proton transfer proceed in the donor side of PSâ…¡(Photosystemâ…¡), which supplyraw materials and energy for sustain all lifeforms on earth. In order to construct an artificialphotosystem to realize light-induced water oxidation, substantial efforts have been paid tomimic the structure and function of the donor side of PSâ…¡. Recently, some dinuclear Ru(â…¡)complexes with higher stability capable of catalyzing water oxidation to evolve oxygen werereported, in which two Ru(â…¡) metals were bridged by planar polydentate ligands such as3,5-bis(2-pyridyl)-1H-pyrazole (Hbpp). In this thesis, some new mono-and dinuclear Ru(â…¡)complexes containing ligands Hbpp were designed and synthesized, aiming for mimicking theelectron transfer process from Tyr_z to P₆₈₀⁺ assisted by a vicinal histidine His190 anddeveloping more effective OEC(Oxygen Evolving Complex) model complexes respectively.First, ligands 2-5 were prepared upon the introduction of different electron-withdrawingsubstitutes Cl, Br, I, NO₂ to Hbpp (1). And then ligands 1-5 were coordinated to complexRu(bpy)₂Cl₂ to give mononuclear Ru(â…¡) complexes 1a-5a respectively. Through themeasurements of their photophysical, electrochemical and acid-base properties, the influencesof the substituents Cl, Br, I, NO₂ to the mononulear Ru(â…¡) complex were determined. Inaddtion, by the study of nanosecond laser flash photolysis, complex 1a acting as a mimic ofP₆₈₀ can transfer one electron to the acceptor MV²⁺ after being excited. Aiming to furthersynthesize a triad model complex containing P₆₈₀, Tyr_z and His190, we introduceddiazo-2-naphthol group to Hbpp and obtained ligand 14 and its corresponding mononuclearRu(â…¡) complexe 14a. In the structure of 14a, two strong intramolecular hydrogen bonds(formed from the same hydrogen of the naphtholic OH with the bridging diazo nitrogen andthe pyridine nitrogen) were observed by ¹H NMR and X-ray crystallography. According to theelectrochemical data, this intramolecular H-bonding network made the oxidation potential ofnaphthol decreased so dramatically from 1.14 V to 0.25 V (vs Fc/Fc⁺) that the photogeneratedRu^â…¢in complex 14a can oxidize the hydrogen bonded naphthol moiety thermodynamicallythrough the intramolecular electron transfer. Afterward, intramolecular electron transfer in14a was studied in the presence of the acceptor MV²⁺ in acetonitrile by nanosecond laser flashphotolysis. From the results, we can suggest that a light-induced electron transfer processfrom donor to acceptor occurred in this model system. In this model, the polypyridyl Ru(â…¡)moiety acts as a mimic of P₆₈₀, the diazo and pyridine moieties could mimic the function ofHis190, and the naphthol behaves as Tyr_z. The intramolecular H-bonding network should beresponsible for the fast electron transfer rate＞9×10⁷ s^-1. Complex 15b have been reported capable of catalyzing water oxidation in the literature,in which the two metals were connected through one planar tetradentate ligand Hbpp and one(μ-OAc) bridge. To investigate the electron-withdrawing effects on the catalytic activity ofOEC model complex 15b, we tried to prepare some derivatives of complex 15b using ligands2-5 as bridging ligand respectively. Unfortunately, no target complexes were obtained becauseof the low yields and difficult separation. In order to synthesize a integrated model systemcontaining P₆₈₀, Tyr_z, His190 and OEC successfully in the future work, we proposed tointroduce diazo-2-naphthol moiety as a mimic of Tyr_z and His190 to OEC model complex15b. Complex 14b with (μ-Cl) bridge between the two Ru metals was obtained firstly. Due tothe unsuccessful separation from 14b, the final target complex with(μ-OAc) bridge couldn'tbe obtained. Consequently, the photophysical, electrochemical properties and catalytic activityof complex 14b were investigated. The results showed that complex 14b still possessed thecatalytic activity, but its catalytic rate was much lower than that of 15b.To confirm the key role of (μ-Cl) or (μ-OAc) bridge in the structure of this kind ofmodel complexes, five fully coordinated dinuclear Ru(â…¡) complexes 1b-5b were prepared byusing four 2,2'-bipyridine ligands and one bridging ligand(among ligands 1-5). Without the(μ-Cl) or (μ-OAc) bridge between the two metal center, aqua ligands can not coordinate to themetal center by substitution reaction. Then we studied the catalytic activity for wateroxidation of lb-5b in the presence of oxidant Ce⁴⁺ in pH=1 solution respectively, but nocatalytic activity was found. Therefore, we can conclude that the existence of (μ-Cl) or(μ-OAc) bridge between two Ru metals is necessary for catalyzing water oxidation.Furthermore, we also studied the effects of different electron-withdrawing groups Cl, Br, I,NO₂ on the dinuclear complex through the studies of photophysical, electrochemicalproperties of complexes 1b-5b.In summary, eight Hbpp derivatives, six mononuclear and six dinuclear rutheniumcomplexes were synthesized. All these novel compounds have been characterized by ¹H NMR,HRMS, and elemental analysis. Among them, the structures of ligand 12, complexes 2a-5a,14a, 1b-5b and 14b were further confirmed by X-ray crystallography.