单核钌催化剂化学催化和光催化水氧化反应

合成了一系列含有不同对位取代基团的吡啶轴向配体的单核钌化合物Ru(bda)(pic)₂ (H₂bda=2,2''-联吡啶-6,6''-二羧酸; pic=对甲基吡啶),对化合物的结构进行了核磁、质谱和X射线单晶衍射表征,并在中性和酸性条件下研究了这些化合物的电化学性质.以硝酸铈铵为氧化剂,对催化剂的催化活性进行了测试,并以Ru(bpy)₃]²⁺为光敏剂,S₂O₈^2-为电子牺牲剂,在三组分体系中考察了这些化合物的光催化活性.研究发现,在化学法水氧化反应中,化合物1由于其轴向配体4,4''-联吡啶在酸性条件下能够发生质子化,从而增强了吸电子效应,因此表现出最高的催化活性,催化循环数达到4000.在光催化水氧化反应中,化合物2因其轴向配体具有最强的吸电子能力而表现出最高的催化活性,反应2h的催化循环数达到270.结果表明,轴向配体的吸电子能力明显提高了这类Ru催化剂催化水氧化反应活性.

Chemical and photocatalytic water oxidation by mononuclear Ru catalysts

Four mononuclear Ru complexes with different substituents on the para position of the pyridine ligand of Ru(bda)(pic)₂ (H₂bda=2,2''-bipyridine-6,6''-dicarboxylic acid; pic=picoline) were synthesized and characterized by ¹H nuclear magnetic resonance or X-ray crystallography. The electrochemical properties of this series of compounds in acidic and neutral conditions were studied by cyclic voltammetry. Their catalytic activity towards water oxidation was investigated using a chemical oxidant (Ce(NH₄)₂(NO₃)₆] (Ce^IV) in acidic solution, or driven by visible light in a three-component system containing a photosensitizer (Ru(bpy)₃]²⁺) and an electron acceptor (S₂O₈^2-). For the chemical water oxidation, complex 1 was found to be the most effective, exhibiting a turnover number (TON) of up to 4000. The pyridine substituent at the 4-position in 1 may be protonated giving an intensive electron-withdrawing effect. Complex 2 bears the most electron-withdrawing trifluoromethyl group under neutral conditions and showed the highest photocatalytic activity with a TON of 270 over 2 h. It was concluded that the more electron-withdrawing substituents led to higher activity towards oxygen evolution for this type of Ru catalysts in the oxidation of water.