Studies On Water Oxidation Catalyzed By Ruthenium Complexes Based On Anionic Ligands

Artificial photosynthesis can produce hydrogen and oxygen through the decomposition of water by solar energy,and convert solar energy into chemical energy,which is a main way to solve the energy crisis and develop new energy.In artificial photosynthesis,water oxidation half reaction is crucial,so it is necessary to develop stable and efficient water oxidation catalyst.The water oxidation catalyst of ruthenium complex at the molecular level has very good catalytic performance,so it has become a focus of people's attention.As anionic ligands with negative charge have strong electron donor capacity,they can reduce the redox potential of catalysts and stabilize high-valent intermediates of metal ruthenium,thus improving their catalytic water oxidation performance.However,most of the water oxidation catalysts have the disadvantage of poor water solubility,which will affect the catalytic activity.Based on this,a series of ruthenium water oxidation catalysts based on anionic ligands were designed and synthesized.First,an anionic ligand H₄L?H₄L=4,4',6,6'-tetracarboxy-2,2'-bipyridyl?was synthesized.Two mononuclear ruthenium water oxidation catalysts 1 and 2 were synthesized using this ligand.Because two uncoordinated carboxyl groups can enhance the water solubility of the catalyst,the catalytic effect is improved.The results showed that the TON of catalyst 2 could be as high as 9160,and the TON of catalyst 1 was 5830.The intermolecular hydrophobic interaction and?-?stacking effect in the axial ligand phthalazine in catalyst 2 enhanced the free radical coupling process,thus demonstrating that the catalyst 2 has much higher catalytic activity toward water oxidation than catalyst 1 from reaction mechanismIn addition,two schiff base ligands containing anions were synthesized,and two mononuclear ruthenium water oxidation catalysts 3 and 4 were synthesized using them as a equatorial ligand.The catalytic water oxidation performance was also tested.The results show that these two catalysts have good catalytic properties.The results show that the catalytic cycle number TON of catalyst 4 can be as high as 1150,and the TON of catalyst 3 is 580.The oxygen atom in the methoxy group of the ligand in the catalyst 4 can form a hydrogen bond with the water molecule,thereby enhancing the nucleophilic attack process of the water molecule on the catalytic center Ru and resulting in enhanced performance of the catalytic water oxidation of the catalyst4..However,due to the easy decomposition of schiff base under acidic conditions,the catalyst stability was poor,so it did not achieve a very good catalytic effect.Ru?bpy?₃Cl₂ is a photosensitizer,and the electronic sacrificial agent is Na₂S₂O₈.Under neutral conditions,catalysts 3 and 4 can also catalyze water oxidation under visible light driving.Catalyst 4 has a TON of 168 and catalyst 3 has a TON of only 106.Through the study of the above water oxidation catalyst,it can be found that enhancing the water solubility,intermolecular hydrophobic effect and?-?stacking effect of the catalyst and the group containing molecules capable of forming hydrogen bonds with water molecules can improve the catalytic performance of the catalyst.These results provides a very meaningful reference value for the subsequent design of efficient water oxidation catalysts.