Li Ying-Ying, Tong Lian-Peng, Liao Rong-Zhen
Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education, Hubei Key Laboratory of Bioinorganic Chemistry and Materia Medica, Hubei Key Laboratory of Materials Chemistry and Service Failure, School of Chemistry and Chemical Engineering , Huazhong University of Science and Technology , Wuhan 430074 , China.
School of Chemistry and Chemical Engineering , Guangzhou University , Guangzhou 510006 , China.
Inorg Chem. 2018 Apr 16;57(8):4590-4601. doi: 10.1021/acs.inorgchem.8b00333. Epub 2018 Mar 30.
The mononuclear [Cl-Fe(dpa)-Cl] (1) complex containing a square planar tetradentate polypyridine ligand has been reported to catalyze water oxidation in pH = 1 aqueous medium with ceric ammonium nitrate (CAN) as a chemical oxidant. The reaction mechanism of the oxygen evolution driven by this catalyst was investigated by means of density functional calculations. The results showed that one chloride ligand of 1 has to exchange with a water molecule to generate 1, [Cl-Fe(dpa)-OH], as the starting species of the catalytic cycle. The initial one-electron oxidation of 1 is coupled with the release of two protons, generating [Cl-Fe(dpa)═O] (2). Another one-electron transfer from 2 leads to the formation of an Fe═O complex [Cl-Fe(dpa)═O] (3), which triggers the critical O-O bond formation. The electronic structure of 3 was found to be very similar to that of the high-valent heme-iron center of P450 enzymes, termed Compound I, in which a π-cation radical ligand is believed to support a formal iron(IV)-oxo core. More importantly, 3 and Compound I share the same tendency toward electrophilic reactions. Two competing pathways were suggested for the O-O bond formation based on the present calculations. One is the nitrate nucleophilic attack on the iron(V)-oxo moiety with a total barrier of 12.3 kcal mol. In this case, nitrate functions as a co-catalyst for the dioxygen formation. The other is the water nucleophilic attack on iron(V)-oxo with a greater barrier of 16.5 kcal mol. In addition, ligand degradation via methyl hydrogen abstraction was found to have a barrier similar to that of the O-O bond formation, while the aromatic carbon hydroxylation has a higher barrier.
据报道，含有平面正方形四齿多吡啶配体的单核[Cl-Fe(dpa)-Cl] (1) 配合物，在pH = 1的水性介质中以硝酸铈铵 (CAN) 作为化学氧化剂催化水氧化。通过密度泛函计算研究了该催化剂驱动的析氧反应机理。结果表明，1的一个氯配体必须与一个水分子交换，生成1, [Cl-Fe(dpa)-OH]，作为催化循环的起始物种。1的初始单电子氧化与两个质子的释放相结合，生成[Cl-Fe(dpa)═O] (2)。来自2的另一次单电子转移导致形成Fe═O配合物[Cl-Fe(dpa)═O] (3)，它触发了关键的O-O键形成。发现3的电子结构与P450酶的高价血红素铁中心（称为化合物I）非常相似，其中π-阳离子自由基配体被认为支持形式上的铁(IV)-氧代核心。更重要的是，3和化合物I在亲电反应方面具有相同的趋势。基于目前的计算，提出了两条竞争的O-O键形成途径。一条是硝酸根对铁(V)-氧代部分的亲核攻击，总势垒为12.3 kcal mol。在这种情况下，硝酸根作为生成双氧的共催化剂。另一条是水对铁(V)-氧代的亲核攻击，势垒更大，为16.5 kcal mol。此外，发现通过甲基氢提取导致的配体降解具有与O-O键形成相似的势垒，而芳族碳羟基化具有更高的势垒。
