Johansson Adam Johannes, Noack Holger, Siegbahn Per E M, Xue Genqiang, Que Lawrence
Department of Physics, Stockholm University, SE-106 91, Stockholm, Sweden.
Dalton Trans. 2009 Sep 14(34):6741-50. doi: 10.1039/b907263b. Epub 2009 Jul 14.
The biomimetic diiron complex Fe(III)Fe(IV)(mu-O)(2)(5-Me(3)-TPA)(2)(3) (TPA = tris(2-pyridylmethyl)amine) has been found to be capable of oxidizing 9,10-dihydroanthracene in a solution of acetonitrile. Addition of water up to 1 M makes the reaction 200 times faster, suggesting that the water molecule in some way activates the catalyst for more efficient substrate oxidation. It is proposed that the enhanced reactivity results from the coordination of a water molecule to the iron(III) half of the complex, converting the bis-mu-oxo structure of the diiron complex to a ring-opened form where one of the bridging oxo groups is transformed into a terminal oxo group on iron(IV). The suggested mechanism is supported by DFT (B3LYP) calculations and transition state theory. Two different computational models of the diiron complex are used to model the hydroxylation of cyclohexane to cyclohexanol. Model has a bis-mu-oxo diiron core (diamond core) while model represents the "open core" analogue with one bridging mu-oxo group, a terminal oxo ligand on iron(IV), and a water molecule coordinated to iron(III). The computational results clearly suggest that the terminal oxo group is more reactive than the bridging oxo group. The free energy of activation is 7.0 kcal mol(-1) lower for the rate limiting step when the oxidant has a terminal oxo group than when both oxo groups are bridging the irons.
已发现仿生二铁配合物Fe(III)Fe(IV)(μ-O)₂(5-Me₃-TPA)₂₃（TPA = 三(2-吡啶甲基)胺）能够在乙腈溶液中氧化9,10-二氢蒽。加入高达1 M的水会使反应速度加快200倍，这表明水分子以某种方式激活了催化剂，使其能更高效地氧化底物。有人提出，反应活性增强是由于水分子与配合物中铁(III)部分配位，将二铁配合物的双μ-氧结构转变为开环形式，其中一个桥连氧原子基团转化为铁(IV)上的端基氧原子基团。密度泛函理论（DFT，B3LYP）计算和过渡态理论支持了所提出的机理。使用二铁配合物的两种不同计算模型来模拟环己烷羟基化为环己醇的过程。模型具有双μ-氧二铁核心（菱形核心），而模型代表“开放核心”类似物，具有一个桥连μ-氧基团、铁(IV)上的一个端基氧配体以及一个与铁(III)配位的水分子。计算结果清楚地表明，端基氧原子基团比桥连氧原子基团更具反应活性。当氧化剂具有端基氧原子基团时，限速步骤的活化自由能比两个氧原子基团都桥连铁时低7.0 kcal mol⁻¹。
